#+TITLE: Chronometrist - an extensible time tracker for Emacs
#+SUBTITLE: Developer manual and program source
#+AUTHOR: contrapunctus
#+TODO: TODO TEST WIP EXTEND CLEANUP FIXME HACK REVIEW |
#+PROPERTY: header-args :tangle yes :load yes :comments link
#+INFOJS_OPT: view:info toc:nil

* Introduction
This is a book about Chronometrist, a time tracker for Emacs, written by a humble hobbyist hacker. It also happens to contain the canonical copy of the source code, and can be loaded as an Emacs Lisp program using the =literate-elisp= library.

I hope this book—when completed—passes Tim Daly's [[https://www.youtube.com/watch?v=Av0PQDVTP4A&t=8m52s]["Hawaii Test"]], in which a programmer with no knowledge of this program whatsover can read this book end-to-end, and come out as much of an expert in its maintenance as the original author.

—contrapunctus

* Explanation
:PROPERTIES:
:DESCRIPTION: The design, the implementation, and a little history
:END:
** Why I wrote Chronometrist
It probably started off with a desire for introspection and self-awareness - what did I do all day? How much time have I spent doing X? It is also a tool to help me stay focused on a single task at a time, and to not go overboard and work on something for 7 hours straight.

At first I tried an Android application, "A Time Tracker". The designs of Chronometrist's main buffer and the =chronometrist-report= buffer still resemble that of A Time Tracker. However, every now and then I'd forget to start or stop tracking. Each time I did, I had to open an SQLite database and edit UNIX timestamps to correct it, which was not fun :\

Later, I discovered John Wiegley's =timeclock=. It turned out that since it was an Emacs extension (actually, a part of Emacs), I was more likely to use it. Chronometrist started out as an "A Time Tracker"-like UI for =timeclock=, moving to an s-expression backend later.

Quite recently, after around two years of Chronometrist developement and use, I discovered that Org mode has a time tracking feature, too. Even though I've embraced some of [[#explanation-literate-programming][the joys of Org]], I'm not quite at ease with the idea of storing data in a complex text format with only one complete implementation.

** Design goals
:PROPERTIES:
:DESCRIPTION: Some vague objectives which guided the project
:END:
1. Don't make assumptions about the user's profession
   - e.g. timeclock seems to assume you're using it for a 9-to-5/contractor job
2. Incentivize use
   * Hooks allow the time tracker to automate tasks and become a useful part of your workflow
3. Make it easy to edit data using existing, familiar tools
   * We don't use an SQL database, where changing a single field is tricky [fn:1]
   * We use a text file containing s-expressions (easy for humans to read and write)
   * We use ISO-8601 for timestamps (easy for humans to read and write) rather than UNIX epoch time
4. Reduce human errors in tracking
5. Have a useful, informative, interactive interface
6. Support mouse and keyboard use equally

[fn:1] I still have doubts about this. Having SQL as a query language would be very useful in perusing the stored data. Maybe we should have tried to create a companion mode to edit SQL databases interactively?

** Terminology
:PROPERTIES:
:DESCRIPTION: Explanation of some terms used later
:END:
For lack of a better term, events are how we refer to time intervals. They are stored as plists; each contains at least a =:name "<name>"=, a =:start "<iso-timestamp>"=, and (except in case of an ongoing task) a =:stop "<iso-timestamp>"=.

+ row :: a row of a table in a =tabulated-list-mode= buffer; an element of =tabulated-list-entries=.
+ schema :: the column descriptor of a table in a =tabulated-list-mode= buffer; the value of =tabulated-list-format=.

See also [[#explanation-time-formats][Currently-Used Time Formats]]

** Overview
At its most basic, we read data from a [[#program-backend][plain text file]] containing Lisp plists, store it in a [[#program-data-structures][hash table]], and [[#program-frontend-chronometrist][display it]] as a [[elisp:(find-library "tabulated-list-mode")][=tabulated-list-mode=]] buffer. When the file is changed—whether by the program or the user—we [[refresh-file][update the hash table]] and the buffer.

In addition, we implement a [[#program-pretty-printer][plist pretty-printer]] and some migration commands. This repository also contains an extension for [[file:chronometrist-key-values.org][attaching arbitrary metadata]] to time intervals, and there is a extension for [[https://tildegit.org/contrapunctus/chronometrist-goal][time goals and alerts]] in a separate repository.

** Optimization
It is of great importance that Chronometrist be responsive -
+ A responsive program is more likely to be used; recall our design goal of 'incentivizing use'.
+ Being an Emacs program, freezing the UI for any human-noticeable length of time is unacceptable - it prevents the user from working on anything in their environment.
Thus, I have considered various optimization strategies, and so far implemented two.

*** Prevent excess creation of file watchers
One of the earliest 'optimizations' of great importance turned out to simply be a bug - turns out, if you run an identical call to [[elisp:(describe-function 'file-notify-add-watch)][=file-notify-add-watch=]] twice, you create /two/ file watchers and your callback will be called /twice./ We were creating a file watcher /each time the =chronometrist= command was run./ 🤦 This was causing humongous slowdowns each time the file changed. 😅
+ It was fixed in v0.2.2 by making the watch creation conditional, using [[* fs-watch][=chronometrist--fs-watch=]] to store the watch object.

*** Preserve hash table state for some commands
NOTE - this has been replaced with a more general optimization - see next section.

The next one was released in v0.5. Till then, any time the [[* chronometrist-file][=chronometrist-file=]] was modified, we'd clear the [[* chronometrist-events][=chronometrist-events=]] hash table and read data into it again. The reading itself is nearly-instant, even with ~2 years' worth of data [fn:2] (it uses Emacs' [[elisp:(describe-function 'read)][=read=]], after all), but the splitting of [[#explanation-midnight-spanning-intervals][midnight-spanning events]] is the real performance killer.

After the optimization...
1. Two backend functions ([[* new][=chronometrist-sexp-new=]] and [[* replace-last][=chronometrist-sexp-replace-last=]]) were modified to set a flag (=chronometrist--inhibit-read-p=) before saving the file.
2. If this flag is non-nil, [[* refresh-file][=chronometrist-refresh-file=]] skips the expensive calls to =chronometrist-events-populate=, =chronometrist-tasks-from-table=, and =chronometrist-tags-history-populate=, and resets the flag.
3. Instead, the aforementioned backend functions modify the relevant variables - =chronometrist-events=, =chronometrist-task-list=, and =chronometrist-tags-history= - via...
   * =chronometrist-events-add= / =chronometrist-events-replace-last=
   * =chronometrist-task-list-add=, and
   * =chronometrist-tags-history-add= / =chronometrist-tags-history-replace-last=, respectively.

There are still some operations which [[* refresh-file][=chronometrist-refresh-file=]] runs unconditionally - which is to say there is scope for further optimization, if or when required.

[fn:2] As indicated by exploratory work in the =parsimonious-reading= branch, where I made a loop to only =read= and collect s-expressions from the file. It was near-instant...until I added event splitting to it.

*** Determine type of change made to file
Most changes, whether made through user-editing or by Chronometrist commands, happen at the end of the file. We try to detect the kind of change made - whether the last expression was modified, removed, or whether a new expression was added to the end - and make the corresponding change to =chronometrist-events=, instead of doing a full parse again (=chronometrist-events-populate=). The increase in responsiveness has been significant.

When =chronometrist-refresh-file= is run by the file system watcher, it uses  =chronometrist-file-hash= to assign indices and a hash to =chronometrist--file-state=. The next time the file changes, =chronometrist-file-change-type= compares this state to the current state of the file to determine the type of change made.

Challenges -
1. Correctly detecting the type of change
2. Updating =chronometrist-task-list= and the Chronometrist buffer, when a new task is added or the last interval for a task is removed (v0.6.4)
3. Handling changes made to an active interval after midnight
   * use the date from the plist's =:start= timestamp instead of the date today
   * =:append= - normally, add to table; for spanning intervals, invalid operation
   * =:modify= - normally, replace in table; for spanning intervals, split and replace
   * =:remove= - normally, remove from table; for spanning intervals, split and remove

Effects on the task list
1. When a plist is added, the =:name= might be new, in which case we need to add it to the task list.
2. When the last plist is modified, the =:name= may have changed -
   1. the =:name= might be new and require addition to the task list.
   2. the old plist may have been the only plist for the old =:name=, so we need to check if there are any other plists with the old =:name=. If there are none, the old =:name= needs to be removed from the task list.
3. When the last plist is removed, it may have been the only plist for the old =:name=, so we need to check if there are any other plists with the old =:name=. If there are none, the old =:name= needs to be removed from the task list.

** Midnight-spanning intervals
:PROPERTIES:
:DESCRIPTION: Events starting on one day and ending on another
:CUSTOM_ID: explanation-midnight-spanning-intervals
:END:
A unique problem in working with Chronometrist, one I had never foreseen, was tasks which start on one day and end on another. For instance, you start working on something at 2021-01-01T23:00 hours and stop on 2021-01-02T01:00.

These mess up data consumption in all sorts of unforeseen ways, especially interval calculations and acquiring intervals for a specific date. In case of two of the most common operations throughout the program -
1. finding the intervals recorded on a given date -
2. finding the time spent on a task on a given day - if the day's intervals used for this contain a midnight-spanning interval, you'll have inaccurate results - it will include yesterday's time from the interval as well as today's.

There are a few different approaches of dealing with them. (Currently, Chronometrist uses #3.)
*** Check the code of the first event of the day (timeclock format)
:PROPERTIES:
:DESCRIPTION: When the code of the first event in the day is "o", it's a midnight-spanning event.
:END:
+ Advantage - very simple to detect
+ Disadvantage - "in" and "out" events must be represented separately
*** Split them at the file level
+ Advantage - operation is performed only once for each such event + simpler data-consuming code + reduced post-parsing load.
+ What happens when the user changes their day-start-time? The split-up events are now split wrongly, and the second event may get split /again./

Possible solutions -
1. Add function to check if, for two events A and B, the :stop of A is the same as the :start of B, and that all their other tags are identical. Then we can re-split them according to the new day-start-time.
2. Add a :split tag to split events. It can denote that the next event was originally a part of this one.
3. Re-check and update the file when the day-start-time changes.
   - Possible with ~add-variable-watcher~ or ~:custom-set~ in Customize (thanks bpalmer)
*** Split them at the hash-table-level
Handled by ~chronometrist-sexp-events-populate~
+ Advantage - simpler data-consuming code.
*** Split them at the data-consumer level (e.g. when calculating time for one day/getting events for one day)
+ Advantage - reduced repetitive post-parsing load.

** Point restore behaviour
:PROPERTIES:
:DESCRIPTION: The desired behaviour of point in Chronometrist
:END:
After hacking, always test for and ensure the following -
1. Toggling the buffer via =chronometrist= / =chronometrist-report= / =chronometrist-statistics= should preserve point
2. The timer function should preserve point when the buffer is current
3. The timer function should preserve point when the buffer is not current, but is visible in another window
4. The next/previous week keys and buttons should preserve point.

** chronometrist-report date range logic
:PROPERTIES:
:DESCRIPTION: Deriving dates in the current week
:END:
A quick description, starting from the first time [[* chronometrist-report][=chronometrist-report=]] is run in an Emacs session -
1. We get the current date as a ts struct, using =chronometrist-date=.
2. The variable =chronometrist-report-week-start-day= stores the day we consider the week to start with. The default is "Sunday".

   We check if the date from #2 is on the week start day, else decrement it till we are, using =(chronometrist-report-previous-week-start)=.
3. We store the date from #3 in the global variable =chronometrist-report--ui-date=.
4. By counting up from =chronometrist-report--ui-date=, we get dates for the days in the next 7 days using =(chronometrist-report-date->dates-in-week)=. We store them in =chronometrist-report--ui-week-dates=.

   The dates in =chronometrist-report--ui-week-dates= are what is finally used to query the data displayed in the buffer.
5. To get data for the previous/next weeks, we decrement/increment the date in =chronometrist-report--ui-date= by 7 days and repeat the above process (via =(chronometrist-report-previous-week)= / =(chronometrist-report-next-week)=).

** Literate programming
:PROPERTIES:
:CUSTOM_ID: explanation-literate-programming
:END:
The shift from a bunch of Elisp files to a single Org literate program was born out of frustration with programs stored as text files, which are expensive to restructure (especially in the presence of a VCS). While some dissatisfactions remain, I generally prefer the outcome - tree and source-block folding, tags, properties, and =org-match= have made it trivial to get different views of the program, and literate programming may allow me to express the "explanation" documentation in the same context as the program, without having to try to link between documentation and source.
*** Tangling
At first, I tried tangling. Back when I used =benchmark.el= to test it, =org-babel-tangle= took about 30 seconds to tangle this file. Thus, I wrote a little sed one-liner (in the file-local variables) to do the tangling, which was nearly instant. It emitted anything between lines matching the exact strings ="#+BEGIN_SRC emacs-lisp"= and ="#+END_SRC"= -

#+BEGIN_SRC org :tangle no
# eval: (progn (make-local-variable 'after-save-hook) (add-hook 'after-save-hook (lambda () (start-process-shell-command "chronometrist-sed-tangle" "chronometrist-sed-tangle" "sed -n -e '/#+BEGIN_SRC emacs-lisp$/,/#+END_SRC$/{//!p;};/#+END_SRC/i\\ ' chronometrist.org | sed -E 's/^ +$//' > chronometrist.el"))))
#+END_SRC

*** literate-elisp-load
Later, we switched from tangling to using the =literate-elisp= package to loading this Org file directly - a file =chronometrist.el= would be used to load =chronometrist.org=.

#+BEGIN_SRC emacs-lisp :tangle no :load no
(literate-elisp-load
 (format "%schronometrist.org" (file-name-directory load-file-name)))
#+END_SRC

This way, source links (e.g. help buffers, stack traces) would lead to this Org file, and this documentation was available to each user, within the comfort of their Emacs. The presence of the =.el= file meant that users of =use-package= did not need to make any changes to their configuration.

*** Reject modernity, return to tangling
For all its benefits, the previous approach broke autoloads and no sane way could be devised to make them work, so back we came to tangling. =org-babel-tangle-file= seems to be quicker when run as a Git pre-commit hook - a few seconds' delay before I write a commit message.

Certain tools like =checkdoc= remain a pain to use with any kind of literate program. This will probably continue to be the case until these tools are fixed or extended.

*** Definition metadata
Each definition has its own heading. The type of definition is stored in tags -
1. custom group
2. [custom|hook|internal] variable
3. keymap (use variable instead?)
4. macro
5. function
   * does not refer to external state
   * primarily used for the return value
6. reader
   * reads external state without modifying it
   * primarily used for the return value
7. writer
   * modifies external state, namely a data structure or file
   * primarily used for side-effects
8. procedure
   * any other impure function
   * usually affects the display
   * primarily used for side-effects
9. major/minor mode
10. command

Further details are stored in properties -
1. :INPUT: (for functions)
2. :VALUE: list|hash table|...
   * for functions, this is the return value
3. :STATE: <external file or data structure read or written to>
*** TODO Issues [40%]
1. [X] When opening this file, Emacs may freeze at the prompt for file-local variable values; if so, C-g will quit the prompt, and permanently marking them as safe will make the freezing stop. [fn:3]
2. [ ] I like =visual-fill-column-mode= for natural language, but I don't want it applied to code blocks. =polymode.el= may hold answers.
3. [X] Is there a tangling solution which requires only one command (e.g. currently we use two =sed= s) but is equally fast? [fn:3]
   * Perhaps we can get rid of the requirement of adding newlines after each source block, and add the newlines ourselves. That gives us control, and also makes it possible to insert Org text in the middle of a definition without unnecessary newlines.
4. [ ] =nameless-insert-name= does not work in source blocks.
5. [ ] Some source blocks don't get syntax highlighted.
   * A workaround is to press =M-o M-o=

[fn:3] No longer a problem since we switched to =literate-elisp=
** Currently-Used Time Formats
:PROPERTIES:
:CUSTOM_ID: explanation-time-formats
:END:

*** ts
  ts.el struct
  * Used by nearly all internal functions
*** iso-timestamp
  ="YYYY-MM-DDTHH:MM:SSZ"=
  * Used in the s-expression file format
  * Read by chronometrist-sexp-events-populate
  * Used in the plists in the chronometrist-events hash table values
*** iso-date
  ="YYYY-MM-DD"=
  * Used as hash table keys in chronometrist-events - can't use ts structs for keys, you'd have to make a hash table predicate which uses ts=
*** seconds
  integer seconds as duration
  * Used for most durations
  * May be changed to floating point to allow larger durations. The minimum range of =most-positive-fixnum= is 536870911, which seems to be enough to represent durations of 17 years.
  * Used for update intervals (chronometrist-update-interval, chronometrist-change-update-interval)
*** minutes
  integer minutes as duration
  * Used by [[https://tildegit.org/contrapunctus/chronometrist-goal][chronometrist-goal]] (chronometrist-goals-list, chronometrist-get-goal) - minutes seems like the ideal unit for users to enter
*** list-duration
  =(hours minute seconds)=
  * Only returned by =chronometrist-seconds-to-hms=, called by =chronometrist-format-time=

** Backend protocol
:PROPERTIES:
:CREATED:  2022-01-05T19:00:12+0530
:END:
The protocol as of now, with remarks -
1. =backend-run-assertions (backend)=
2. =view-backend (backend)=
3. =edit-backend (backend)= - these two would make sense if there was only one way to view/edit a backend. But if we want [[file:TODO.org::#viewing-editing-frontends][viewing/editing frontends]], there would be many.
4. =backend-empty-p (backend)=
5. =backend-modified-p (backend)=
6. =create-file (backend &optional file)=
7. =latest-date-records (backend)=
8. =insert (backend plist)=
9. =remove-last (backend)=
10. =latest-record (backend)=
11. =task-records-for-date (backend task date-ts)=
12. =replace-last (backend plist)=
13. =to-file (input-hash-table output-backend output-file)=
14. =on-add (backend)=
15. =on-modify (backend)=
16. =on-remove (backend)=
17. =on-change (backend)=
18. =verify (backend)=
19. =on-file-path-change (backend old-path new-path)=
20. =reset-backend (backend)= - probably rename to "initialize"
21. =memory-layer-empty-p (backend)= - needs a more generic name; perhaps "initialized-p", to go with #20
22. =to-hash-table (backend)=
23. =to-list (backend)=

There are many operations which are file-oriented, whereas I have tried to treat files as implementation details. =create-file=, for instance, is used by =to-file=; I could make creation of files implicit by moving it into =initialize-instance=, but that would mean creation of files in =to-file= would require creation of a backend object. That seems to me to be an abuse of implicit behaviour; and what would backends which are not file-backed do in =to-file=, then? There's probably a way to do it, but I had other things I preferred to tackle first.

** generic cl-loop interface for iterating over records
Of all the ways to work with Chronometrist data, both as part of the program and as part of my occasional "queries", my favorite was to use =cl-loop=.

First, there was the =chronometrist-loop-file= macro, which handled the sole backend at that time - the plist backend. It took care of the common logic (=read= ing each plist in the file, checking loop termination conditions), and let the client code specify (with the terseness of =cl-loop=) what they wanted to do with the data.

During the migration to the CLOS-based backend design began the quest to make  =chronometrist-loop-file= work with generic backends - it eventually became =chronometrist-loop-records= and =chronometrist-loop-days=. The idea was to call a generic function (=chronometrist-record-iterator= and =chronometrist-day-iterator=) which would return a new record on each call. Internal state of each of these generic functions was stored in backend slots. No list would be built up, unless the client code specified an accumulation clause.

Most recently, gilberth of #lispcafe suggested an alternate approach - trying to build a list of records first, and using =cl-loop= (or any other iteration mechanism) on that. Testing the two approaches yielded a clear advantage for this new suggestion. The test was to generate key-values suitable for completion history from my full Chronometrist data to date (the plist backend had ~6.6k plists in a 1.2M file), using almost identical =cl-loop= client code for both cases. Here was the output from =(benchmark 1 ...)= -

| approach | backend     | benchmark output                                 |
|----------+-------------+--------------------------------------------------|
| current  | plist       | "Elapsed time: 5.322056s (0.709023s in 4 GCs)"   |
| current  | plist-group | "Elapsed time: 107.159170s (1.064125s in 6 GCs)" |
| new      | plist       | "Elapsed time: 0.559264s (0.172344s in 1 GCs)"   |
| new      | plist-group | "Elapsed time: 0.671106s (0.179435s in 1 GCs)"   |

In addition, with this approach, client code can use any kind of iteration constructs they fancy - not just =cl-loop= but also =dolist=, higher-order functions (including those from =dash= and =seq=), =loopy=, etc.

The macro still exists in its non-generic form as =chronometrist-loop-sexp-file=, providing a common way to loop over s-expressions in a text file, used by =chronometrist-to-list= in both backends and =chronometrist-to-hash-table= in the plist group backend.

* How-to guides for maintainers
** How to set up Emacs to contribute
# Different approaches to this setup -
# 1. Document it and let contributors do it voluntarily.
#    * Downside - if a contributor does not do it, it can lead to inconsistency and an additional headache for the maintainer.
# 2. Put it in file variables and/or =.dir-locals.el=.
#    * Downside - if the contributor does not have a dependency installed (e.g. nameless), Emacs will create an error. That's not the UX I want for someone opening the file for the first time - it does not tell them that they need to install something.
#      + Technically, a friendlier error message could be displayed. But you want to read/edit a document and it asks you to install something first...kind of a flow breaker.
# 3. Use a tool which installs developement dependencies for you, e.g. Cask, Eldev.
#    * May look into this in the future. But as of now I don't even foresee any contributors 😓

1. Install [[https://github.com/Malabarba/Nameless][nameless-mode]] for easier reading of Emacs Lisp code, and [[https://github.com/jingtaozf/literate-elisp][literate-elisp]] to load this file directly without tangling.

   #+BEGIN_SRC emacs-lisp :tangle no :load no
     (mapcar #'package-install '(nameless literate-elisp))
   #+END_SRC

2. Create a =.dir-locals-2.el= in the project root, containing -

      #+BEGIN_SRC emacs-lisp :tangle no :load no
        ((org-mode
          .
          ((eval . (nameless-mode))
           (eval . (progn
             (make-local-variable 'after-save-hook)
             ;; you can't `defun' in one `eval' and use the
             ;; function in another `eval', apparently
             (add-hook
              'after-save-hook
              (defun chronometrist-tangle ()
                (interactive)
                (compile
                 (mapconcat #'shell-quote-argument
                            `("emacs" "-q" "-Q" "--batch"
                              "--eval=(require 'ob-tangle)"
                              ,(format "--eval=(org-babel-tangle-file \"%s\")"
                                       (buffer-file-name)))
                            " ")))))))))
      #+END_SRC

3. Set up compiling, linting, and testing with =makem.sh=. First, define this command -

   #+BEGIN_SRC emacs-lisp :tangle no :load no
   (defun run-makem ()
     (interactive)
     (cd (locate-dominating-file default-directory "makem.sh"))
     (compile "./makem.sh compile lint test-ert"))
   #+END_SRC

   Then, run it after staging the files -
   #+BEGIN_SRC emacs-lisp :tangle no :load no
   (add-hook 'magit-post-stage-hook #'run-makem)
   #+END_SRC

   Or after tangling ends -
   #+BEGIN_SRC emacs-lisp :tangle no :load no
     (add-hook 'org-babel-post-tangle-hook #'run-makem)
   #+END_SRC

** How to tangle this file
Use =org-babel= (=org-babel-tangle= / =org-babel-tangle-file=), /not/ =literate-elisp-tangle=. The file emitted by the latter does not contain comments - thus, it does not contain library headers or abide by =checkdoc='s comment conventions.

* The Program
** Library headers and commentary
Library headers are necessary once more, since we're back to tangling instead of =literate-elisp-load=.

Once, for sake of neatness, I made the value of =Package-Requires:= multiline -
#+BEGIN_SRC emacs-lisp :tangle no :load no
;; Package-Requires: ((emacs "25.1")
;;                    (dash "2.16.0")
;;                    (seq "2.20")
;;                    (ts "0.2"))
#+END_SRC
But I discovered that if I do that, =package-lint= says - =error: Couldn't parse "Package-Requires" header: End of file during parsing=.

#+BEGIN_SRC emacs-lisp :comments no
;;; chronometrist.el --- A time tracker with a nice interface -*- lexical-binding: t; -*-

;; Author: contrapunctus <xmpp:contrapunctus@jabjab.de>
;; Maintainer: contrapunctus <xmpp:contrapunctus@jabjab.de>
;; Keywords: calendar
;; Homepage: https://tildegit.org/contrapunctus/chronometrist
;; Package-Requires: ((emacs "27.1") (dash "2.16.0") (seq "2.20") (ts "0.2"))
;; Version: 0.9.0

;; This is free and unencumbered software released into the public domain.
;;
;; Anyone is free to copy, modify, publish, use, compile, sell, or
;; distribute this software, either in source code form or as a compiled
;; binary, for any purpose, commercial or non-commercial, and by any
;; means.
;;
;; For more information, please refer to <https://unlicense.org>
#+END_SRC

"Commentary" is displayed when the user clicks on the package's entry in =M-x list-packages=.

#+BEGIN_SRC emacs-lisp :comments no
;;; Commentary:
;;
;; A time tracker in Emacs with a nice interface

;; Largely modelled after the Android application, [A Time Tracker](https://github.com/netmackan/ATimeTracker)

;; * Benefits
;;   1. Extremely simple and efficient to use
;;   2. Displays useful information about your time usage
;;   3. Support for both mouse and keyboard
;;   4. Human errors in tracking are easily fixed by editing a plain text file
;;   5. Hooks to let you perform arbitrary actions when starting/stopping tasks

;; * Limitations
;;   1. No support (yet) for adding a task without clocking into it.
;;   2. No support for concurrent tasks.

;; ## Comparisons
;; ### timeclock.el
;; Compared to timeclock.el, Chronometrist
;; * stores data in an s-expression format rather than a line-based one
;; * supports attaching tags and arbitrary key-values to time intervals
;; * has commands to shows useful summaries
;; * has more hooks

;; ### Org time tracking
;; Chronometrist and Org time tracking seem to be equivalent in terms of capabilities, approaching the same ends through different means.
;; * Chronometrist doesn't have a mode line indicator at the moment. (planned)
;; * Chronometrist doesn't have Org's sophisticated querying facilities. (an SQLite backend is planned)
;; * Org does so many things that keybindings seem to necessarily get longer. Chronometrist has far fewer commands than Org, so most of the keybindings are single keys, without modifiers.
;; * Chronometrist's UI makes keybindings discoverable - they are displayed in the buffers themselves.
;; * Chronometrist's UI is cleaner, since the storage is separate from the display. It doesn't show tasks as trees like Org, but it uses tags and key-values to achieve that. Additionally, navigating a flat list takes fewer user operations than navigating a tree.
;; * Chronometrist data is just s-expressions (plists), and may be easier to parse than a complex text format with numerous use-cases.

;; For information on usage and customization, see https://tildegit.org/contrapunctus/chronometrist or the included manual.org
#+END_SRC

** Dependencies
#+BEGIN_SRC emacs-lisp :comments no
;;; Code:
;; This file was automatically generated from chronometrist.org
(require 'dash)
(require 'ts)

(require 'cl-lib)
(require 'seq)
(require 'filenotify)
(require 'subr-x)
(require 'parse-time)
(require 'eieio)

(eval-when-compile
  (defvar chronometrist-mode-map)
  (require 'subr-x))
#+END_SRC

** Common definitions
*** custom group                                             :custom:group:
#+BEGIN_SRC emacs-lisp
(defgroup chronometrist nil
  "An extensible time tracker."
  :group 'applications)
#+END_SRC

*** format-time                                                  :function:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-format-duration (seconds &optional (blank (make-string 3 ?\s)))
  "Format SECONDS as a string suitable for display in Chronometrist buffers.
SECONDS must be a positive integer.

BLANK is a string to display in place of blank values. If not
supplied, 3 spaces are used."
  (-let [(h m s) (chronometrist-seconds-to-hms seconds)]
    (if (and (zerop h) (zerop m) (zerop s))
        (concat (make-string 7 ?\s) "-")
      (let ((h (if (zerop h) blank (format "%2d:" h)))
            (m (cond ((and (zerop h) (zerop m))  blank)
                     ((zerop h)  (format "%2d:" m))
                     (t  (format "%02d:" m))))
            (s (if (and (zerop h) (zerop m))
                   (format "%2d" s)
                 (format "%02d" s))))
        (concat h m s)))))
#+END_SRC
*** file-empty-p                                                   :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-file-empty-p (file)
  "Return t if FILE is empty."
  (zerop (nth 7 (file-attributes file))))
#+END_SRC

*** format-keybinds                                              :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-format-keybinds (command map &optional firstonly)
  "Return the keybindings for COMMAND in MAP as a string.
If FIRSTONLY is non-nil, return only the first keybinding found."
  (if firstonly
      (key-description
       (where-is-internal command map firstonly))
    (->> (where-is-internal command map)
         (mapcar #'key-description)
         (-take 2)
         (-interpose ", ")
         (apply #'concat))))
#+END_SRC
*** day-start-time                                        :custom:variable:
[[* events-maybe-split][=chronometrist-events-maybe-split=]] refers to this, but I'm not sure this has the desired effect at the moment—haven't even tried using it.
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-day-start-time "00:00:00"
  "The time at which a day is considered to start, in \"HH:MM:SS\".

The default is midnight, i.e. \"00:00:00\"."
  :type 'string)
#+END_SRC
*** week-start-day                                        :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-report-week-start-day "Sunday"
  "The day used for start of week by `chronometrist-report'."
  :type 'string)
#+END_SRC
*** weekday-number-alist                                  :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-report-weekday-number-alist
  '(("Sunday"    . 0)
    ("Monday"    . 1)
    ("Tuesday"   . 2)
    ("Wednesday" . 3)
    ("Thursday"  . 4)
    ("Friday"    . 5)
    ("Saturday"  . 6))
  "Alist in the form (\"NAME\" . NUMBER), where \"NAME\" is the name of a weekday and NUMBER its associated number."
  :type 'alist)
#+END_SRC
*** previous-week-start                                            :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-previous-week-start (ts)
  "Find the previous `chronometrist-report-week-start-day' from TS.
Return a ts struct for said day's beginning.

If the day of TS is the same as the
`chronometrist-report-week-start-day', return TS.

TS must be a ts struct (see `ts.el')."
  (cl-loop with week-start = (alist-get chronometrist-report-week-start-day
                                        chronometrist-report-weekday-number-alist
                                        nil nil #'equal)
    until (= week-start (ts-dow ts))
    do (ts-decf (ts-day ts))
    finally return ts))
#+END_SRC
*** plist-remove                                                 :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-plist-remove (plist &rest keys)
  "Return PLIST with KEYS and their associated values removed."
  (let ((keys (--filter (plist-member plist it) keys)))
    (mapc (lambda (key)
            (let ((pos (seq-position plist key)))
              (setq plist (append (seq-take plist pos)
                                  (seq-drop plist (+ 2 pos))))))
          keys)
    plist))
#+END_SRC

**** tests
#+BEGIN_SRC emacs-lisp :tangle chronometrist-tests.el :load test
(ert-deftest chronometrist-plist-remove ()
  (should
   (equal (chronometrist-plist-remove '(:a 1 :b 2 :c 3 :d 4) :a)
          '(:b 2 :c 3 :d 4)))
  (should
   (equal (chronometrist-plist-remove '(:a 1 :b 2 :c 3 :d 4) :b)
          '(:a 1 :c 3 :d 4)))
  (should
   (equal (chronometrist-plist-remove '(:a 1 :b 2 :c 3 :d 4) :c)
          '(:a 1 :b 2 :d 4)))
  (should
   (equal (chronometrist-plist-remove '(:a 1 :b 2 :c 3 :d 4) :d)
          '(:a 1 :b 2 :c 3)))
  (should
   (equal (chronometrist-plist-remove '(:a 1 :b 2 :c 3 :d 4) :a :b)
          '(:c 3 :d 4)))
  (should
   (equal (chronometrist-plist-remove '(:a 1 :b 2 :c 3 :d 4) :a :d)
          '(:b 2 :c 3)))
  (should
   (equal (chronometrist-plist-remove '(:a 1 :b 2 :c 3 :d 4) :c :d)
          '(:a 1 :b 2)))
  (should (equal
           (chronometrist-plist-remove '(:a 1 :b 2 :c 3 :d 4) :a :b :c :d)
           nil))
  (should
   (equal (chronometrist-plist-remove '(:a 1 :b 2 :c 3 :d 4) :d :a)
          '(:b 2 :c 3))))

#+END_SRC

*** plist-key-values                                             :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-plist-key-values (plist)
  "Return user key-values from PLIST."
  (chronometrist-plist-remove plist :name :tags :start :stop))
#+END_SRC

*** plist-p                                                      :function:
[[file:../tests/chronometrist-tests.org::#tests-common-plist-p][tests]]

#+BEGIN_SRC emacs-lisp
(defun chronometrist-plist-p (list)
  "Return non-nil if LIST is a property list, i.e. (:KEYWORD VALUE ...)"
  (when list
    (while (consp list)
      (setq list (if (and (keywordp (cl-first list)) (consp (cl-rest list)))
                     (cddr list)
                   'not-plist)))
    (null list)))
#+END_SRC

*** plist type                                                       :type:
#+BEGIN_SRC emacs-lisp
(cl-deftype chronometrist-plist ()
  '(satisfies chronometrist-plist-p)
  '(satisfies (lambda (plist)
                (and (plist-get plist :name)
                     (plist-get plist :start)))))
#+END_SRC

*** delete-list                                                    :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-sexp-delete-list (&optional arg)
  "Delete ARG lists after point.
Return new position of point."
  (let ((point-1 (point)))
    ;; try to preserve the file-local variable prop line in case this
    ;; is run from the start of buffer
    (while (forward-comment 1) nil)
    (forward-sexp (or arg 1))
    (delete-region point-1 (point))
    (point)))
#+END_SRC

*** make-hash-table                                              :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-make-hash-table ()
  "Return an empty hash table with `equal' as test."
  (make-hash-table :test #'equal))
#+END_SRC

*** current-task                                            :reader:method:
[[file:../tests/chronometrist-tests.org::#tests-common-current-task][tests]]

#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-current-task (&optional (backend (chronometrist-active-backend)))
  "Return the name of the active task as a string, or nil if not clocked in."
  (let ((last-event (chronometrist-latest-record backend)))
    (if (plist-member last-event :stop)
        nil
      (plist-get last-event :name))))
#+END_SRC

*** install-directory                                            :variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-install-directory
  (when load-file-name
    (file-name-directory load-file-name))
  "Directory where Chronometrist has been installed.")
#+END_SRC

*** doc-paths                                                    :variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-doc-paths '(:lp "chronometrist.org")
  "Plist of names of Chronometrist's documentation files.")
#+END_SRC

*** open-literate-source                                          :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-open-literate-source ()
  "Visit the Org literate program for Chronometrist."
  (interactive)
  (find-file (concat chronometrist-install-directory (plist-get chronometrist-doc-paths :lp))))
#+END_SRC

*** debug logging
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-debug-enable nil
  "Whether to log debugging messages."
  :type 'boolean
  :group 'chronometrist)

(defcustom chronometrist-debug-buffer "*chronometrist-debug*"
  "Name of buffer to log debug messages to."
  :type 'string
  :group 'chronometrist)

(define-derived-mode chronometrist-debug-log-mode special-mode "debug-log")

(defun chronometrist-debug-message (format-string &rest args)
  "Log a debug message to `chronometrist-debug-buffer'.
FORMAT-STRING and ARGS are passed to `format'."
  (when chronometrist-debug-enable
    (with-current-buffer (get-buffer-create chronometrist-debug-buffer)
      (goto-char (point-max))
      (chronometrist-debug-log-mode)
      (let ((inhibit-read-only t))
        (insert
         (apply #'format
                (concat (format-time-string "[%T] ")
                        format-string)
                args)
         "\n")))))
#+END_SRC

** Data structures
:PROPERTIES:
:CUSTOM_ID: program-data-structures
:END:
Reading directly from the file could be difficult, especially when your most common query is "get all intervals recorded on <date>" [fn:4] - and so, we maintain the hash table =chronometrist-events=, where each key is a date in the ISO-8601 format. The plists in this hash table are free of [[#explanation-midnight-spanning-intervals][midnight-spanning intervals]], making code which consumes it easier to write.

The data from =chronometrist-events= is used by most (all?) interval-consuming functions, but is never written to the user's file itself.

[fn:4] it might be the case that the [[#program-backend][file format]] is not suited to our most frequent operation...
*** reset                                                         :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-reset ()
  "Reset Chronometrist's internal state."
  (interactive)
  (chronometrist-debug-message "[Command] reset")
  (chronometrist-reset-backend (chronometrist-active-backend))
  (chronometrist-refresh))
#+END_SRC

*** apply-time                                                   :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-apply-time (time timestamp)
  "Return TIMESTAMP with time modified to TIME.
TIME must be a string in the form \"HH:MM:SS\"

TIMESTAMP must be a time string in the ISO-8601 format.

Return value is a ts struct (see `ts.el')."
  (-let [(h m s) (mapcar #'string-to-number (split-string time ":"))]
    (ts-apply :hour h :minute m :second s
              (chronometrist-iso-to-ts timestamp))))
#+END_SRC
**** tests
#+BEGIN_SRC emacs-lisp :tangle chronometrist-tests.el :load test
(ert-deftest chronometrist-apply-time ()
  (should
   (equal (ts-format "%FT%T%z" (chronometrist-apply-time "01:02:03" "2021-02-17T01:20:18+0530"))
          "2021-02-17T01:02:03+0530")))
#+END_SRC

*** split-plist                                                  :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-split-plist (plist)
  "Return a list of two split plists if PLIST spans a midnight, else nil."
  (when (plist-get plist :stop)
    (let ((split-time (chronometrist-split-time (plist-get plist :start)
                                           (plist-get plist :stop)
                                           chronometrist-day-start-time)))
      (when split-time
        (-let* (((&plist :start start-1 :stop stop-1) (cl-first  split-time))
                ((&plist :start start-2 :stop stop-2) (cl-second split-time))
                ;; `plist-put' modifies lists in-place. The resulting bugs
                ;; left me puzzled for a while.
                (event-1      (cl-copy-list plist))
                (event-2      (cl-copy-list plist)))
          (list (-> event-1
                    (plist-put :start start-1)
                    (plist-put :stop  stop-1))
                (-> event-2
                    (plist-put :start start-2)
                    (plist-put :stop  stop-2))))))))
#+END_SRC

**** tests
#+BEGIN_SRC emacs-lisp :tangle chronometrist-tests.el :load test
(ert-deftest chronometrist-split-plist ()
  (should
   (null (chronometrist-split-plist
          '(:name  "Task"
                   :start "2021-02-17T01:33:12+0530"
                   :stop  "2021-02-17T01:56:08+0530"))))
  (should
   (equal (chronometrist-split-plist
           '(:name  "Guitar"
                    :tags  (classical warm-up)
                    :start "2021-02-12T23:45:21+0530"
                    :stop  "2021-02-13T00:03:46+0530"))
          '((:name "Guitar"
                   :tags (classical warm-up)
                   :start "2021-02-12T23:45:21+0530"
                   :stop "2021-02-13T00:00:00+0530")
            (:name "Guitar"
                   :tags (classical warm-up)
                   :start "2021-02-13T00:00:00+0530"
                   :stop "2021-02-13T00:03:46+0530")))))
#+END_SRC

*** events-update                                                  :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-events-update (plist hash-table &optional replace)
  "Return HASH-TABLE with PLIST added as the latest interval.
If REPLACE is non-nil, replace the last interval with PLIST."
  (let* ((date (->> (plist-get plist :start)
                    (chronometrist-iso-to-ts )
                    (ts-format "%F" )))
         (events-today (gethash date hash-table)))
    (--> (if replace (-drop-last 1 events-today) events-today)
         (append it (list plist))
         (puthash date it hash-table))
    hash-table))
#+END_SRC

*** last-date                                                      :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-events-last-date (hash-table)
  "Return an ISO-8601 date string for the latest date present in `chronometrist-events'."
  (--> (hash-table-keys hash-table)
       (last it)
       (car it)))
#+END_SRC

*** events-last                                                    :reader:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-events-last (&optional (backend (chronometrist-active-backend)))
  "Return the last plist from `chronometrist-events'."
  (let* ((hash-table (chronometrist-backend-hash-table backend))
         (last-date  (chronometrist-events-last-date hash-table)))
    (--> (gethash last-date hash-table)
         (last it)
         (car it))))
#+END_SRC
*** events-subset                                                  :reader:
:PROPERTIES:
:VALUE:    hash table
:CUSTOM_ID: program-data-structures-events-subset
:END:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-events-subset (start end hash-table)
  "Return a subset of HASH-TABLE.
The subset will contain values between dates START and END (both
inclusive).

START and END must be ts structs (see `ts.el'). They will be
treated as though their time is 00:00:00."
  (let ((subset (chronometrist-make-hash-table))
        (start  (chronometrist-date-ts start))
        (end    (chronometrist-date-ts end)))
    (maphash (lambda (key value)
               (when (ts-in start end (chronometrist-iso-to-ts key))
                 (puthash key value subset)))
             hash-table)
    subset))
#+END_SRC

*** task-time-one-day                                              :reader:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-task-time-one-day (task &optional (date (chronometrist-date-ts)) (backend (chronometrist-active-backend)))
  "Return total time spent on TASK today or on DATE, an ISO-8601 date.
The return value is seconds, as an integer."
  (let ((task-events (chronometrist-task-records-for-date backend task date)))
    (if task-events
        (->> (chronometrist-events-to-durations task-events)
             (-reduce #'+)
             (truncate))
      ;; no events for this task on DATE, i.e. no time spent
      0)))
#+END_SRC

*** active-time-on                                                 :reader:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-task-list)
(cl-defun chronometrist-active-time-on (&optional (date (chronometrist-date-ts)))
  "Return the total active time today, or on DATE.
Return value is seconds as an integer."
  (->> (--map (chronometrist-task-time-one-day it date) (chronometrist-task-list))
       (-reduce #'+)
       (truncate)))
#+END_SRC

*** count-active-days                                            :function:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-statistics-count-active-days (task table)
  "Return the number of days the user spent any time on TASK.
  TABLE must be a hash table - if not supplied, `chronometrist-events' is used.

  This will not return correct results if TABLE contains records
which span midnights."
  (cl-loop for events being the hash-values of table
    count (seq-find (lambda (event)
                      (equal task (plist-get event :name)))
                    events)))
#+END_SRC

*** task-list                                                    :variable:
:PROPERTIES:
:VALUE:    list
:END:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-task-list nil
  "List of tasks used by `chronometrist'.
Value may be either nil or a list of strings.

If nil, the task list is generated from user data in
`chronometrist-file' and stored in the task-list slot of the
active backend."
  :type '(choice (repeat string) nil)
  :group 'chronometrist)
#+END_SRC

** Time functions
*** iso-to-ts                                                    :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-iso-to-ts (timestamp)
  "Convert TIMESTAMP to a TS struct. (see `ts.el')
TIMESTAMP must be an ISO-8601 timestamp, as handled by
`parse-iso8601-time-string'."
  (-let [(second minute hour day month year dow _dst utcoff)
         (decode-time
          (parse-iso8601-time-string timestamp))]
    (ts-update
     (make-ts :hour hour :minute minute :second second
              :day day   :month month   :year year
              :dow dow   :tz-offset utcoff))))

#+END_SRC
**** tests
#+BEGIN_SRC emacs-lisp :tangle chronometrist-tests.el :load test
(ert-deftest chronometrist-iso-to-ts ()
  (should (ts= (chronometrist-iso-to-ts "2021-01-01")
               (make-ts :year 2021 :month 1  :day 1
                        :hour 0    :minute 0 :second 0)))
  (should (ts= (chronometrist-iso-to-ts "2021-01-01T01:01:01")
               (make-ts :year 2021 :month 1  :day 1
                        :hour 1    :minute 1 :second 1))))

#+END_SRC
*** events-to-durations                                          :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-events-to-durations (events)
  "Convert EVENTS into a list of durations in seconds.
EVENTS must be a list of valid Chronometrist property lists (see
`chronometrist-file').

Return 0 if EVENTS is nil."
  (if events
      (cl-loop for plist in events collect
        (let* ((start-ts (chronometrist-iso-to-ts
                          (plist-get plist :start)))
               (stop-iso (plist-get plist :stop))
               ;; Add a stop time if it does not exist.
               (stop-ts  (if stop-iso
                             (chronometrist-iso-to-ts stop-iso)
                           (ts-now))))
          (ts-diff stop-ts start-ts)))
    0))
#+END_SRC
*** date-iso                                                     :function:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-date-iso (&optional (ts (ts-now)))
  (ts-format "%F" ts))
#+END_SRC

*** date-ts                                                      :function:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-date-ts (&optional (ts (ts-now)))
  "Return a ts struct representing the time 00:00:00 on today's date.
If TS is supplied, use that date instead of today.
TS should be a ts struct (see `ts.el')."
  (ts-apply :hour 0 :minute 0 :second 0 ts))
#+END_SRC

*** format-time-iso8601                                          :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-format-time-iso8601 (&optional unix-time)
  "Return current date and time as an ISO-8601 timestamp.
Optional argument UNIX-TIME should be a time value (see
`current-time') accepted by `format-time-string'."
  (format-time-string "%FT%T%z" unix-time))

;; Note - this assumes that an event never crosses >1 day. This seems
;; sufficient for all conceivable cases.
#+END_SRC
*** FIXME split-time                                               :reader:
It does not matter here that the =:stop= dates in the returned plists are different from the =:start=, because =chronometrist-events-populate= uses only the date segment of the =:start= values as hash table keys. (The hash table keys form the rest of the program's notion of "days", and that of which plists belong to which day.)

#+BEGIN_SRC emacs-lisp
(defun chronometrist-split-time (start-time stop-time day-start-time)
  "If START-TIME and STOP-TIME intersect DAY-START-TIME, split them into two intervals.
START-TIME and STOP-TIME must be ISO-8601 timestamps e.g. \"YYYY-MM-DDTHH:MM:SSZ\".

DAY-START-TIME must be a string in the form \"HH:MM:SS\" (see
`chronometrist-day-start-time')

Return a list in the form
\((:start START-TIME
  :stop <day-start time on initial day>)
 (:start <day start time on second day>
  :stop STOP-TIME))"
  ;; FIXME - time zones are ignored; may cause issues with
  ;; time-zone-spanning events

  ;; The time on which the first provided day starts (according to `chronometrist-day-start-time')
  (let* ((stop-ts         (chronometrist-iso-to-ts stop-time))
         (first-day-start (chronometrist-apply-time day-start-time start-time))
         (next-day-start  (ts-adjust 'hour 24 first-day-start)))
    ;; Does the event stop time exceed the next day start time?
    (when (ts< next-day-start stop-ts)
      (let ((split-time (ts-format "%FT%T%z" next-day-start)))
        (list `(:start ,start-time :stop ,split-time)
              `(:start ,split-time :stop ,stop-time))))))
#+END_SRC

**** tests
#+BEGIN_SRC emacs-lisp :tangle chronometrist-tests.el :load test
(ert-deftest chronometrist-split-time ()
  (should
   (null
    (chronometrist-split-time "2021-02-17T01:33:12+0530"
                 "2021-02-17T01:56:08+0530"
                 "00:00:00")))
  (should
   (equal
    (chronometrist-split-time "2021-02-19T23:45:36+0530"
                 "2021-02-20T00:18:40+0530"
                 "00:00:00")
    '((:start "2021-02-19T23:45:36+0530"
              :stop "2021-02-20T00:00:00+0530")
      (:start "2021-02-20T00:00:00+0530"
              :stop "2021-02-20T00:18:40+0530"))))
  (should
   (equal
    (chronometrist-split-time "2021-02-19T23:45:36+0530"
                 "2021-02-20T03:18:40+0530"
                 "01:20:30")
    '((:start "2021-02-19T23:45:36+0530"
              :stop "2021-02-20T01:20:30+0530")
      (:start "2021-02-20T01:20:30+0530"
              :stop "2021-02-20T03:18:40+0530")))))
#+END_SRC

*** seconds-to-hms                                               :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-seconds-to-hms (seconds)
  "Convert SECONDS to a vector in the form [HOURS MINUTES SECONDS].
SECONDS must be a positive integer."
  (let* ((seconds (truncate seconds))
         (s       (% seconds 60))
         (m       (% (/ seconds 60) 60))
         (h       (/ seconds 3600)))
    (list h m s)))
#+END_SRC
*** interval                                                     :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-interval (event)
  "Return the period of time covered by EVENT as a time value.
EVENT should be a plist (see `chronometrist-file')."
  (let ((start (plist-get event :start))
        (stop  (plist-get event :stop)))
    (time-subtract (parse-iso8601-time-string stop)
                   (parse-iso8601-time-string start))))
#+END_SRC
*** format-duration-long                                         :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-format-duration-long (seconds)
  "Return SECONDS as a human-friendly duration string.
e.g. \"2 hours, 10 minutes\". SECONDS must be an integer. If
SECONDS is less than 60, return a blank string."
  (let* ((hours         (/ seconds 60 60))
         (minutes       (% (/ seconds 60) 60))
         (hour-string   (if (= 1 hours) "hour" "hours"))
         (minute-string (if (= 1 minutes) "minute" "minutes")))
    (cond ((and (zerop hours) (zerop minutes)) "")
          ((zerop hours)
           (format "%s %s" minutes minute-string))
          ((zerop minutes)
           (format "%s %s" hours hour-string))
          (t (format "%s %s, %s %s"
                     hours hour-string
                     minutes minute-string)))))
#+END_SRC
**** tests
#+BEGIN_SRC emacs-lisp :tangle ../tests/chronometrist-tests.el :load test
(ert-deftest chronometrist-format-duration-long ()
  (should (equal (chronometrist-format-duration-long 5) ""))
  (should (equal (chronometrist-format-duration-long 65) "1 minute"))
  (should (equal (chronometrist-format-duration-long 125) "2 minutes"))

  (should (equal (chronometrist-format-duration-long 3605) "1 hour"))
  (should (equal (chronometrist-format-duration-long 3660) "1 hour, 1 minute"))
  (should (equal (chronometrist-format-duration-long 3725) "1 hour, 2 minutes"))

  (should (equal (chronometrist-format-duration-long 7200) "2 hours"))
  (should (equal (chronometrist-format-duration-long 7260) "2 hours, 1 minute"))
  (should (equal (chronometrist-format-duration-long 7320) "2 hours, 2 minutes")))
#+END_SRC
** Plist pretty-printing
:PROPERTIES:
:CUSTOM_ID: program-pretty-printer
:END:
=pp.el=, part of Emacs, doesn't align plist keys along the same column. =ppp.el= (available on MELPA) doesn't align plist /values/ along the same column; also, it's GPL, and I try to avoid GPL dependencies. And thus, I wrote this ad hoc plist pretty-printer.

1. [X] work recursively for plist/alist values
2. [X] Add variable (to chronometrist-sexp.el) to set pretty-printing function. Default to ppp.el if found, fallback to internal Emacs pretty printer, and let users set their own pretty printing function.
3. [ ] Fix alignment of alist dots
   * While also handling alist members which are proper lists

The suggested way of debugging this pretty printer is to
1. edebug =chronometrist-pp-buffer= (and others if desired),
2. insert an s-expression in a blank =emacs-lisp-mode= buffer, ensuring it does not contain newlines (other than in strings),
3. place point at the beginning of the buffer, and
4. =M-: (chronometrist-pp-buffer) RET=.

*** normalize-whitespace                                           :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-pp-normalize-whitespace ()
  "Remove whitespace following point, and insert a space.
Point is placed at the end of the space."
  (when (looking-at "[[:blank:]]+")
    (delete-region (match-beginning 0) (match-end 0))
    (insert " ")))
#+END_SRC
*** column                                                         :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-pp-column ()
  "Return column point is on, as an integer.
0 means point is at the beginning of the line."
  (- (point) (point-at-bol)))
#+END_SRC
*** pair-p                                                       :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-pp-pair-p (cons)
  "Return non-nil if CONS is a pair, i.e. (CAR . CDR)."
  (and (listp cons) (not (listp (cdr cons)))))
#+END_SRC
*** alist-p                                                      :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-pp-alist-p (list)
  "Return non-nil if LIST is an association list.
If even a single element of LIST is a pure cons cell (as
determined by `chronometrist-pp-pair-p'), this function
considers it an alist."
  (when (listp list)
    (cl-loop for elt in list thereis (chronometrist-pp-pair-p elt))))
#+END_SRC
*** plist-group-p                                                :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-plist-group-p (list)
  "Return non-nil if LIST is in the form \(ATOM PLIST+\)."
  (and (consp list)
       (not (consp (cl-first list)))
       (cl-rest list)
       (seq-every-p #'chronometrist-plist-p (cl-rest list))))
#+END_SRC

*** longest-keyword-length                                         :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-pp-longest-keyword-length ()
  "Find the length of the longest keyword in a plist.
This assumes there is a single plist in the current buffer, and
that point is after the first opening parenthesis."
  (save-excursion
    (cl-loop with sexp
      while (setq sexp (ignore-errors (read (current-buffer))))
      when (keywordp sexp)
      maximize (length (symbol-name sexp)))))
#+END_SRC
*** indent-sexp                                                  :function:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-pp-indent-sexp (sexp &optional (right-indent 0))
  "Return a string indenting SEXP by RIGHT-INDENT spaces."
  (format (concat "% -" (number-to-string right-indent) "s")
          sexp))
#+END_SRC
*** buffer                                                         :writer:
It might help to make =in-sublist= an integer representing depth, instead of a boolean. But at the moment, it's getting the job done.

This procedure runs in a buffer containing a single s-expression, entirely on one line - which is why reaching the line-end is a termination condition.

#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-pp-buffer (&optional in-sublist)
  "Recursively indent the alist, plist, or a list of plists after point.
The list must be on a single line, as emitted by `prin1'.

IN-SUBLIST, if non-nil, means point is inside an inner list."
  (if (not (looking-at-p (rx (or ")" line-end))))
      (let ((sexp (save-excursion
                    (read (current-buffer)))))
          (cond
           ((chronometrist-plist-p sexp)
            (chronometrist-pp-buffer-plist in-sublist)
            ;; we want to continue, in case we were inside a sublist
            (chronometrist-pp-buffer in-sublist))
           ((chronometrist-plist-group-p sexp)
            (chronometrist-pp-buffer-plist-group in-sublist)
            (chronometrist-pp-buffer in-sublist))
           ((chronometrist-pp-alist-p sexp)
            (chronometrist-pp-buffer-alist)
            (unless in-sublist (chronometrist-pp-buffer)))
           ((chronometrist-pp-pair-p sexp)
            (forward-sexp)
            (chronometrist-pp-buffer in-sublist))
           ((listp sexp)
            (down-list)
            (chronometrist-pp-buffer t)
            (up-list))
           ;; atoms and other values
           (t (forward-sexp)
              (chronometrist-pp-buffer in-sublist))))
    ;; we're before a ) - is it a lone paren on its own line?
    (let ((point (point))
          (bol   (point-at-bol)))
      (goto-char bol)
      (if (string-match "^[[:blank:]]*$" (buffer-substring bol point))
          ;; join the ) to the previous line by deleting the newline and whitespace
          (delete-region (1- bol) point)
        (goto-char point)))))
#+END_SRC

*** buffer-plist                                                   :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-pp-buffer-plist (&optional in-sublist)
  "Indent a single plist after point."
  (down-list)
  (let ((left-indent  (1- (chronometrist-pp-column)))
        (right-indent (chronometrist-pp-longest-keyword-length))
        (first-p t) sexp)
    (while (not (looking-at-p ")"))
      (chronometrist-pp-normalize-whitespace)
      (setq sexp (save-excursion (read (current-buffer))))
      (cond ((keywordp sexp)
             (chronometrist-sexp-delete-list)
             (insert (if first-p
                         (progn (setq first-p nil) "")
                       (make-string left-indent ?\s))
                     (chronometrist-pp-indent-sexp sexp right-indent)))
            ;; not a keyword = a value
            ((chronometrist-plist-p sexp)
             (chronometrist-pp-buffer-plist))
            ((and (listp sexp)
                  (not (chronometrist-pp-pair-p sexp)))
             (chronometrist-pp-buffer t)
             (insert "\n"))
            (t (forward-sexp)
               (insert "\n"))))
    (when (bolp) (delete-char -1))
    (up-list)
    ;; we have exited the plist, but might still be in a list with more plists
    (unless (eolp) (insert "\n"))
    (when in-sublist
      (insert (make-string (1- left-indent) ?\s)))))
#+END_SRC

*** buffer-plist-group                                             :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-pp-buffer-plist-group (&optional _in-sublist)
  (down-list)
  (forward-sexp)
  (default-indent-new-line)
  (chronometrist-pp-buffer t))
#+END_SRC

*** buffer-alist                                                   :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-pp-buffer-alist ()
  "Indent a single alist after point."
  (down-list)
  (let ((indent (chronometrist-pp-column)) (first-p t) sexp)
    (while (not (looking-at-p ")"))
      (setq sexp (save-excursion (read (current-buffer))))
      (chronometrist-sexp-delete-list)
      (insert (if first-p
                  (progn (setq first-p nil) "")
                (make-string indent ?\s))
              (format "%S\n" sexp)))
    (when (bolp) (delete-char -1))
    (up-list)))
#+END_SRC

*** to-string                                                      :reader:
:PROPERTIES:
:STATE: emacs-lisp-mode-syntax-table
:END:

#+BEGIN_SRC emacs-lisp
(defun chronometrist-pp-to-string (object)
  "Convert OBJECT to a pretty-printed string."
  (with-temp-buffer
    (lisp-mode-variables nil)
    (set-syntax-table emacs-lisp-mode-syntax-table)
    (let ((print-quoted t))
      (prin1 object (current-buffer)))
    (goto-char (point-min))
    (chronometrist-pp-buffer)
    (buffer-string)))
#+END_SRC

*** plist-pp                                                       :reader:
#+NAME: plist-pp
#+BEGIN_SRC emacs-lisp
(defun chronometrist-plist-pp (object &optional stream)
  "Pretty-print OBJECT and output to STREAM (see `princ')."
  (princ (chronometrist-pp-to-string object)
         (or stream standard-output)))
#+END_SRC
** Backends
:PROPERTIES:
:CUSTOM_ID: program-backend
:END:
*** chronometrist-file                                                 :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-file
  (locate-user-emacs-file "chronometrist")
  "Name (without extension) and full path of the Chronometrist database."
  :type 'file)

(defun chronometrist-file-variable-watcher (_symbol newval _operation _where)
  "Update slots of the active backend when `chronometrist-file' is changed.
For SYMBOL, NEWVAL, OPERATION, and WHERE, see `add-variable-watcher'."
  (chronometrist-on-file-path-change (chronometrist-active-backend) chronometrist-file newval))

(add-variable-watcher 'chronometrist-file #'chronometrist-file-variable-watcher)
#+END_SRC

*** protocol
**** backend                                                       :class:
The backend may use no files, a single file, or multiple files. Thus, =chronometrist-backend= makes no reference to files, and [[#file-backend-mixin][=chronometrist-file-backend-mixin=]] may be used by single file backends.

#+BEGIN_SRC emacs-lisp
(defclass chronometrist-backend ()
  ((task-list :initform nil
              :initarg :task-list
              :accessor chronometrist-backend-task-list)))
#+END_SRC

**** backends-alist                                             :variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-backends-alist nil
  "Alist of Chronometrist backends.
Each element must be in the form `(KEYWORD TAG OBJECT)', where
TAG is a string used as a tag in customization, and OBJECT is an
EIEIO object such as one returned by `make-instance'.")
#+END_SRC

**** active-backend                                      :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-active-backend :plist
  "The backend currently in use.
Value must be a keyword corresponding to a key in
`chronometrist-backends-alist'."
  :type `(choice
          ,@(cl-loop for elt in chronometrist-backends-alist
              collect `(const :tag ,(cl-second elt) ,(cl-first elt)))))
#+END_SRC

**** active-backend                                               :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-active-backend ()
  "Return an object representing the currently active backend."
  (cl-second (alist-get chronometrist-active-backend chronometrist-backends-alist)))
#+END_SRC

**** switch-backend                                              :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-switch-backend ()
  (interactive)
  (chronometrist-debug-message "[Command] switch-backend")
  (let* ((prompt (format "Switch to backend (current - %s): "
                         chronometrist-active-backend))
         (choice (chronometrist-read-backend-name prompt
                                     chronometrist-backends-alist
                                     (lambda (keyword)
                                       (not (eq chronometrist-active-backend
                                                keyword)))
                                     t)))
    (setq chronometrist-active-backend choice)
    (chronometrist-reset-backend (chronometrist-active-backend))
    ;; timer function is backend-dependent
    (chronometrist-force-restart-timer)))
#+END_SRC

**** register-backend                                             :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-register-backend (keyword tag object)
  "Add backend to `chronometrist-backends-alist'.
For values of KEYWORD, TAG, and OBJECT, see `chronometrist-backends-alist'.

If a backend with KEYWORD already exists, the existing entry will
be replaced."
  (setq chronometrist-backends-alist
        (assq-delete-all keyword chronometrist-backends-alist))
  (add-to-list 'chronometrist-backends-alist
               (list keyword tag object)
               t))
#+END_SRC

**** read-backend-name                                         :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-read-backend-name (prompt backend-alist
                                  &optional predicate return-keyword)
  "Prompt user for a Chronometrist backend name.
BACKEND-ALIST should be an alist similar to `chronometrist-backends-alist'.

RETURN-KEYWORD, if non-nil, means return only the keyword of the
selected backend; otherwise, return the CLOS object for the
backend.

PROMPT and PREDICATE have the same meanings as in
`completing-read'."
  (let ((backend-keyword
         (read
          (completing-read prompt
                           (cl-loop for list in backend-alist
                             collect (cl-first list))
                           predicate t))))
    (if return-keyword
        backend-keyword
      (cl-second (alist-get backend-keyword backend-alist)))))
#+END_SRC

**** task-list                                                  :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-task-list ()
  "Return the list of tasks to be used.
If `chronometrist-task-list' is non-nil, return its value; else,
return a list of tasks from the active backend."
  (let ((backend (chronometrist-active-backend)))
    (with-slots (task-list) backend
      (or chronometrist-task-list
          task-list
          (setf task-list (chronometrist-list-tasks backend))))))
#+END_SRC

**** list-tasks                                                 :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-list-tasks (backend)
  "Return all tasks recorded in BACKEND as a list of strings."
  (cl-loop for plist in (chronometrist-to-list backend)
    collect (plist-get plist :name) into names
    finally return
    (sort (cl-remove-duplicates names :test #'equal)
          #'string-lessp)))
#+END_SRC

**** run-assertions                                     :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-backend-run-assertions (backend)
  "Check common preconditions for any operations on BACKEND.
Signal errors for any unmet preconditions.")
#+END_SRC

**** view-backend                                       :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-view-backend (backend)
  "Open BACKEND for interactive viewing.")
#+END_SRC

**** edit-backend                                       :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-edit-backend (backend)
  "Open BACKEND for interactive editing.")
#+END_SRC

**** backend-empty-p                                    :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-backend-empty-p (backend)
  "Return non-nil if BACKEND contains no records, else nil.")
#+END_SRC

**** backend-modified-p                                 :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-backend-modified-p (backend)
  "Return non-nil if BACKEND is being modified.
For instance, a file-based backend could be undergoing editing by
a user.")
#+END_SRC

**** file operations
***** create-file                                      :generic:function:
[[file:../tests/chronometrist-tests.org::#tests-backend-create-file][tests]]

#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-create-file (backend &optional file)
  "Create file associated with BACKEND.
Use FILE as a path, if provided.
Return path of new file if successfully created, and nil if it already exists.")
#+END_SRC

***** latest-date-records                              :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-latest-date-records (backend)
  "Return intervals of latest day in BACKEND as a tagged list (\"DATE\" PLIST*).
Return nil if BACKEND contains no records.")
#+END_SRC

***** insert                                           :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-insert (backend plist)
  "Insert PLIST as new record in BACKEND.
Return non-nil if record is inserted successfully.")

(cl-defmethod chronometrist-insert :before ((_backend t) plist &key &allow-other-keys)
  (unless (cl-typep plist 'chronometrist-plist)
    (error "Not a valid plist: %S" plist)))
#+END_SRC

***** remove-last                                      :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-remove-last (backend)
  "Remove last record from BACKEND.
Return non-nil if record is successfully removed.
Signal an error if there is no record to remove.")
#+END_SRC

***** latest-record                                    :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-latest-record (backend)
  "Return the latest entry from BACKEND as a plist, or nil if BACKEND contains no records.
Return value may be active, i.e. it may or may not have a :stop key-value.")
#+END_SRC

***** task-records-for-date                            :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-task-records-for-date (backend task date-ts)
  "From BACKEND, return records for TASK on DATE-TS as a list of plists.
DATE-TS must be a `ts.el' struct.

Return nil if BACKEND contains no records.")

(cl-defmethod chronometrist-task-records-for-date :before ((_backend t) task date-ts &key &allow-other-keys)
  (unless (cl-typep task 'string)
    (error "task %S is not a string" task))
  (unless (cl-typep date-ts 'ts)
    (error "date-ts %S is not a `ts' struct" date-ts)))
#+END_SRC

***** replace-last                                     :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-replace-last (backend plist)
  "Replace last record in BACKEND with PLIST.
Return non-nil if successful.")

(cl-defmethod chronometrist-replace-last :before ((_backend t) plist &key &allow-other-keys)
  (unless (cl-typep plist 'chronometrist-plist)
    (error "Not a valid plist: %S" plist)))
#+END_SRC

***** to-file                                          :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-to-file (input-hash-table output-backend output-file)
  "Save data from INPUT-HASH-TABLE to OUTPUT-FILE, in OUTPUT-BACKEND format.
Any existing data in OUTPUT-FILE is overwritten.")
#+END_SRC

***** on-add                                           :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-on-add (backend)
  "Function called when data is added to BACKEND.
This may happen within Chronometrist (e.g. via
`chronometrist-insert') or outside it (e.g. a user editing the
backend file).

NEW-DATA is the data that was added.")
#+END_SRC

***** on-modify                                        :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-on-modify (backend)
  "Function called when data in BACKEND is modified (rather than added or removed).
This may happen within Chronometrist (e.g. via
`chronometrist-replace-last') or outside it (e.g. a user editing
the backend file).

OLD-DATA and NEW-DATA is the data before and after the changes,
respectively.")
#+END_SRC

***** on-remove                                        :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-on-remove (backend)
  "Function called when data is removed from BACKEND.
This may happen within Chronometrist (e.g. via
`chronometrist-remove-last') or outside it (e.g. a user editing
the backend file).

OLD-DATA is the data that was modified.")
#+END_SRC

***** on-change                                        :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-on-change (backend)
  "Function to be run when BACKEND changes on disk.
This may happen within Chronometrist (e.g. via
`chronometrist-insert') or outside it (e.g. a user editing the
backend file).")
#+END_SRC

***** verify                                           :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-verify (backend)
  "Check BACKEND for errors in data.
Return nil if no errors are found.

If an error is found, return (LINE-NUMBER . COLUMN-NUMBER) for file-based backends.")
#+END_SRC

***** on-file-path-change                              :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-on-file-path-change (backend old-path new-path)
  "Function run when the value of `chronometrist-file' is changed.
OLD-PATH and NEW-PATH are the old and new values of
`chronometrist-file', respectively.")
#+END_SRC

**** memory operations
***** reset-backend                                    :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-reset-backend (backend)
  "Reset data structures for BACKEND.")
#+END_SRC

***** to-hash-table                                    :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-to-hash-table (backend)
  "Return data in BACKEND as a hash table in chronological order.
Hash table keys are ISO-8601 date strings. Hash table values are
lists of records, represented by plists. Both hash table keys and
hash table values must be in chronological order.")
#+END_SRC

***** to-list                                          :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-to-list (backend)
  "Return all records in BACKEND as a list of plists.")
#+END_SRC

***** memory-layer-empty-p                             :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-memory-layer-empty-p (backend)
  "Return non-nil if memory layer of BACKEND contains no records, else nil.")
#+END_SRC

**** extended protocol (unimplemented)
These can be implemented in terms of the minimal protocol above.

***** active-days                                      :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-active-days (backend task &key start end)
  "From BACKEND, return number of days on which TASK had recorded time.")
#+END_SRC

***** count-records                                    :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defgeneric chronometrist-count-records (backend)
  "Return number of records in BACKEND.")
#+END_SRC

*** Common definitions for s-expression backends
**** file-backend-mixin                                            :mixin:
:PROPERTIES:
:CUSTOM_ID: file-backend-mixin
:END:

#+BEGIN_SRC emacs-lisp
(defclass chronometrist-file-backend-mixin ()
  ((path :initform nil
    :initarg :path
    :accessor chronometrist-backend-path
    :custom 'string
    :documentation
    "Path to backend file, without extension.")
   (extension :initform nil
    :initarg :extension
    :accessor chronometrist-backend-ext
    :custom 'string
    :documentation
    "Extension of backend file.")
   (file :initarg :file
         :initform nil
         :accessor chronometrist-backend-file
         :custom 'string
         :documentation "Full path to backend file, with extension.")
   (hash-table :initform (chronometrist-make-hash-table)
               :initarg :hash-table
               :accessor chronometrist-backend-hash-table)
   (file-watch :initform nil
               :initarg :file-watch
               :accessor chronometrist-backend-file-watch
               :documentation "Filesystem watch object, as returned by `file-notify-add-watch'."))
  :documentation "Mixin for backends storing data in a single file.")
#+END_SRC

**** setup-file-watch                                             :writer:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-setup-file-watch (&optional (callback #'chronometrist-refresh-file))
  "Arrange for CALLBACK to be called when the backend file changes."
  (with-slots (file file-watch) (chronometrist-active-backend)
    (unless file-watch
      (setq file-watch
            (file-notify-add-watch file '(change) callback)))))
#+END_SRC

**** edit-backend                                                 :method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-edit-backend ((backend chronometrist-file-backend-mixin))
  (find-file-other-window (chronometrist-backend-file backend))
  (goto-char (point-max)))
#+END_SRC

**** initialize-instance                                          :method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod initialize-instance :after ((backend chronometrist-file-backend-mixin)
                                          &rest _initargs)
  "Initialize FILE based on PATH and EXTENSION."
  (with-slots (path extension file) backend
    (when (and path extension (not file))
      (setf file (concat path "." extension)))))
#+END_SRC

**** reset-backend                                         :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-reset-backend ((backend chronometrist-file-backend-mixin))
  (with-slots (hash-table file-watch
               rest-start rest-end rest-hash
               file-length last-hash) backend
    (chronometrist-reset-task-list backend)
    (when file-watch
      (file-notify-rm-watch file-watch))
    (setf hash-table  (chronometrist-to-hash-table backend)
          file-watch  nil
          rest-start  nil
          rest-end    nil
          rest-hash   nil
          file-length nil
          last-hash   nil)
    (chronometrist-setup-file-watch)))
#+END_SRC

**** backend-empty-p                                       :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-backend-empty-p ((backend chronometrist-file-backend-mixin))
  (with-slots (file) backend
      (or (not (file-exists-p file))
          (chronometrist-file-empty-p file))))
#+END_SRC

**** memory-layer-empty-p                                  :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-memory-layer-empty-p ((backend chronometrist-file-backend-mixin))
  (with-slots (hash-table) backend
    (zerop (hash-table-count hash-table))))
#+END_SRC

**** backend-modified-p                                    :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-backend-modified-p ((backend chronometrist-file-backend-mixin))
  (with-slots (file) backend
    (buffer-modified-p
     (get-buffer-create
      (find-file-noselect file)))))
#+END_SRC

**** on-file-path-change                                :generic:function:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-on-file-path-change ((backend chronometrist-file-backend-mixin) _old-path new-path)
  (with-slots (path extension file) backend
    (setf path new-path
          file (concat path "." extension))))
#+END_SRC

**** elisp-sexp-backend                                            :class:
#+BEGIN_SRC emacs-lisp
(defclass chronometrist-elisp-sexp-backend (chronometrist-backend chronometrist-file-backend-mixin)
  ((rest-start :initarg :rest-start
               :initform nil
               :accessor chronometrist-backend-rest-start
               :documentation "Integer denoting start of first s-expression in file.")
   (rest-end :initarg :rest-end
             :initform nil
             :accessor chronometrist-backend-rest-end
             :documentation "Integer denoting end of second-last s-expression in file.")
   (rest-hash :initarg :rest-hash
              :initform nil
              :accessor chronometrist-backend-rest-hash
              :documentation "Hash of content between rest-start and rest-end.")
   (file-length :initarg :file-length
                :initform nil
                :accessor chronometrist-backend-file-length
                :documentation "Integer denoting length of file, as returned by `(point-max)'.")
   (last-hash :initarg :last-hash
              :initform nil
              :accessor chronometrist-backend-last-hash
              :documentation "Hash of content between rest-end and file-length."))
  :documentation "Base class for any text file backend which stores s-expressions readable by Emacs Lisp.")
#+END_SRC

**** sexp-mode                                                :major:mode:
#+BEGIN_SRC emacs-lisp
(define-derived-mode chronometrist-sexp-mode
  ;; fundamental-mode
  emacs-lisp-mode
  "chronometrist-sexp")
#+END_SRC

**** create-file                                           :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-create-file ((backend chronometrist-elisp-sexp-backend) &optional file)
  (let ((file (or file (chronometrist-backend-file backend))))
    (unless (file-exists-p file)
      (with-current-buffer (find-file-noselect file)
        (erase-buffer)
        (goto-char (point-min))
        (insert ";;; -*- mode: chronometrist-sexp; -*-\n\n")
        (write-file file))
      file)))
#+END_SRC

**** in-file                                                       :macro:
#+BEGIN_SRC emacs-lisp
(defmacro chronometrist-sexp-in-file (file &rest body)
  "Run BODY in a buffer visiting FILE, restoring point afterwards."
  (declare (indent defun) (debug t))
  `(with-current-buffer (find-file-noselect ,file)
     (save-excursion ,@body)))
#+END_SRC
**** pre-read-check                                            :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-sexp-pre-read-check (buffer)
  "Return non-nil if there is an s-expression before point in BUFFER.
Move point to the start of this s-expression."
  (with-current-buffer buffer
    (and (not (bobp))
         (backward-list)
         (or (not (bobp))
             (not (looking-at-p "^[[:blank:]]*;"))))))
#+END_SRC

**** loop-sexp-file                                                :macro:
#+BEGIN_SRC emacs-lisp
(defmacro chronometrist-loop-sexp-file (_for sexp _in file &rest loop-clauses)
  "`cl-loop' LOOP-CLAUSES over s-expressions in FILE.
SEXP is bound to each s-expressions in reverse order (last
expression first)."
  (declare (indent defun) (debug 'cl-loop))
  `(chronometrist-sexp-in-file ,file
     (goto-char (point-max))
     (cl-loop with ,sexp
       while (and (chronometrist-sexp-pre-read-check (current-buffer))
                  (setq ,sexp (ignore-errors (read (current-buffer))))
                  (backward-list))
       ,@loop-clauses)))
#+END_SRC

**** backend-empty-p                                       :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-backend-empty-p ((backend chronometrist-elisp-sexp-backend))
  (chronometrist-sexp-in-file (chronometrist-backend-file backend)
    (goto-char (point-min))
    (not (ignore-errors
           (read (current-buffer))))))
#+END_SRC

**** indices and hashes
#+BEGIN_SRC emacs-lisp
(defun chronometrist-rest-start (file)
  (chronometrist-sexp-in-file file
    (goto-char (point-min))
    (forward-list)
    (backward-list)
    (point)))

(defun chronometrist-rest-end (file)
  (chronometrist-sexp-in-file file
    (goto-char (point-max))
    (backward-list 2)
    (forward-list)
    (point)))

(defun chronometrist-file-length (file)
  (chronometrist-sexp-in-file file (point-max)))
#+END_SRC

**** file-hash                                                    :reader:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-file-hash (start end &optional (file (chronometrist-backend-file (chronometrist-active-backend))))
  "Calculate hash of `chronometrist-file' between START and END."
  (chronometrist-sexp-in-file file
    (secure-hash 'sha1
                 (buffer-substring-no-properties start end))))
#+END_SRC

***** tests
#+BEGIN_SRC emacs-lisp :tangle chronometrist-tests.el :load test
(ert-deftest file-hash ()
  (-let* ((file chronometrist-test-file)
          ((last-start last-end)
           (chronometrist-file-hash :before-last nil nil file))
          ((rest-start rest-end rest-hash)
           (chronometrist-file-hash nil :before-last t file)))
    (message "chronometrist - file-hash test - file path is %s" file)
    (should (= 1 rest-start))
    (should (= 1254 rest-end))
    (should (= 1256 last-start))
    (should (= 1426 last-end))))
#+END_SRC
**** file-change-type                                             :reader:
+ rest-start - start of first sexp
+ rest-end - end of second last sexp
+ file-length - end of file

+ rest-hash - hash of content between rest-start and rest-end
+ last-hash - hash of content between rest-end and file-length

+ ht-last-sexp - last sexp in memory
+ file-sexp-after-rest - sexp after rest-end
+ file-last-sexp - last sexp in file

| situation    | rest-hash | last-hash | file-sexp-after-rest | file-last-sexp                          | file-length                  |
|--------------+-----------+-----------+----------------------+-----------------------------------------+------------------------------|
| no change    | same      | same      | same as ht-last-sexp | same as ht-last-sexp and file-last-sexp | same                         |
| append       | same      | same      | -                    | (new s-expression)                      | always greater               |
| modify       | same      | changed   | changed              | changed                                 | may be smaller               |
| remove       | same      | changed   | nil                  | same as second last sexp                | always smaller               |
| other change | changed   | -         |                      | -                                       | may be smaller than rest-end |

We avoid comparing s-expressions in the file with the contents of the hash table, since the last s-expression might be represented differently in the hash tables of different elisp-sexp backends. Additionally, in =:modify= as well as =nil= situations, there is no s-expression after old-file-length.

#+BEGIN_SRC emacs-lisp
(defun chronometrist-file-change-type (backend)
  "Determine the type of change made to BACKEND's file.
    Return
    :append  if a new s-expression was added to the end,
    :modify  if the last s-expression was modified,
    :remove  if the last s-expression was removed,
        nil  if the contents didn't change, and
          t  for any other change."
  (with-slots
      (file file-watch
            ;; The slots contain the old state of the file.
            hash-table
            rest-start rest-end rest-hash
            file-length last-hash) backend
    (let* ((new-length    (chronometrist-file-length file))
           (new-rest-hash (when (and (>= new-length rest-start)
                                     (>= new-length rest-end))
                            (chronometrist-file-hash rest-start rest-end file)))
           (new-last-hash (when (and (>= new-length rest-end)
                                     (>= new-length file-length))
                            (chronometrist-file-hash rest-end file-length file))))
      ;; (chronometrist-debug-message "File indices - old rest-start: %s rest-end: %s file-length: %s new-length: %s"
      ;;          rest-start rest-end file-length new-length)
      (cond ((and (= file-length new-length)
                  (equal rest-hash new-rest-hash)
                  (equal last-hash new-last-hash))
             nil)
            ((or (< new-length rest-end) ;; File has shrunk so much that we cannot compare rest-hash.
                 (not (equal rest-hash new-rest-hash)))
             t)
            ;; From here on, it is implicit that the change has happened at the end of the file.
            ((and (< file-length new-length) ;; File has grown.
                  (equal last-hash new-last-hash))
             :append)
            ((and (< new-length file-length) ;; File has shrunk.
                  (not (chronometrist-sexp-in-file file
                         (goto-char rest-end)
                         (ignore-errors
                           (read (current-buffer)))))) ;; There is no sexp after rest-end.
             :remove)
            (t :modify)))))
#+END_SRC

***** tests
#+BEGIN_SRC emacs-lisp :tangle chronometrist-tests.el :load test
(ert-deftest chronometrist-file-change-type ()
  (with-slots (file hash-table file-state) chronometrist-plist-test-backend
    (let* ((b chronometrist-plist-test-backend)
           (test-contents (with-current-buffer (find-file-noselect file)
                            (buffer-substring (point-min) (point-max)))))
      (chronometrist-reset-backend b)
      (setf file-state
            (list :last (chronometrist-file-hash :before-last nil)
                  :rest (chronometrist-file-hash nil :before-last t)))
      (unwind-protect
          (progn
            (should
             (eq nil (chronometrist-file-change-type file-state)))
            (should
             (eq :append
                 (progn
                   (chronometrist-insert chronometrist-plist-test-backend
                             '(:name "Append Test"
                                     :start "2021-02-01T13:06:46+0530"
                                     :stop "2021-02-01T13:06:49+0530"))
                   (chronometrist-tests--change-type-and-update file-state file))))
            (should
             (eq :modify
                 (progn
                   (chronometrist-replace-last chronometrist-plist-test-backend
                                   '(:name "Modify Test"
                                           :tags (some tags)
                                           :start "2021-02-01T13:06:46+0530"
                                           :stop "2021-02-01T13:06:49+0530"))
                   (chronometrist-tests--change-type-and-update file-state file))))
            (should
             (eq :remove
                 (progn
                   (chronometrist-sexp-in-file file
                     (goto-char (point-max))
                     (backward-list 1)
                     (chronometrist-sexp-delete-list 1)
                     (save-buffer))
                   (chronometrist-tests--change-type-and-update file-state file))))
            (should
             (eq t
                 (progn
                   (chronometrist-sexp-in-file file
                     (goto-char (point-min))
                     (chronometrist-plist-pp '(:name "Other Change Test"
                                         :start "2021-02-02T17:39:40+0530"
                                         :stop "2021-02-02T17:39:44+0530")
                                 (current-buffer))
                     (save-buffer))
                   (chronometrist-tests--change-type-and-update file-state file)))))
        (with-current-buffer
            (find-file-noselect file)
          (delete-region (point-min) (point-max))
          (insert test-contents)
          (save-buffer))
        (chronometrist-reset-backend b)))))
#+END_SRC

**** reset-task-list                                              :writer:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-reset-task-list (backend)
  "Regenerate BACKEND's task list from its data.
Only takes effect if `chronometrist-task-list' is nil (i.e. the
user has not defined their own task list)."
  (unless chronometrist-task-list
    (setf (chronometrist-backend-task-list backend) (chronometrist-list-tasks backend))))
#+END_SRC

**** add-to-task-list                                             :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-add-to-task-list (task backend)
  "Add TASK to BACKEND's task list, if it is not already present.
Only takes effect if `chronometrist-task-list' is nil (i.e. the
user has not defined their own task list)."
  (with-slots (task-list) backend
    (unless (and (not chronometrist-task-list)
                 (cl-member task task-list :test #'equal))
      (setf task-list
            (sort (cons task task-list)
                  #'string-lessp)))))
#+END_SRC

**** remove-from-task-list                                        :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-remove-from-task-list (task backend)
  "Remove TASK from BACKEND's task list if necessary.
TASK is removed if it does not occur in BACKEND's hash table, or
if it only occurs in the newest plist of the same.

Only takes effect if `chronometrist-task-list' is nil (i.e. the
user has not defined their own task list).

Return new value of BACKEND's task list, or nil if
unchanged."
  (with-slots (hash-table task-list) backend
    (unless chronometrist-task-list
      (let (;; number of plists in hash table
            (ht-plist-count (cl-loop with count = 0
                              for intervals being the hash-values of hash-table
                              do (cl-loop for _interval in intervals
                                   do (cl-incf count))
                              finally return count))
            ;; index of first occurrence of TASK in hash table, or nil if not found
            (ht-task-first-result (cl-loop with count = 0
                                    for intervals being the hash-values of hash-table
                                    when (cl-loop for interval in intervals
                                           do (cl-incf count)
                                           when (equal task (plist-get interval :name))
                                           return t)
                                    return count)))
        (when (or (not ht-task-first-result)
                  (= ht-task-first-result ht-plist-count))
          ;; The only interval for TASK is the last expression
          (setf task-list (remove task task-list)))))))
#+END_SRC

**** on-change                                             :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-on-change ((backend chronometrist-elisp-sexp-backend) fs-event)
  "Function called when BACKEND file is changed.
This may happen within Chronometrist (through the backend
protocol) or outside it (e.g. a user editing the backend file).

FS-EVENT is the event passed by the `filenotify' library (see `file-notify-add-watch')."
  (with-slots (file hash-table file-watch
                    rest-start rest-end rest-hash
                    file-length last-hash) backend
    (-let* (((_ action _ _) fs-event)
            (file-state-bound-p (and rest-start rest-end rest-hash
                                     file-length last-hash))
            (change      (when file-state-bound-p
                           (chronometrist-file-change-type backend)))
            (reset-watch-p (or (eq action 'deleted)
                               (eq action 'renamed))))
      (chronometrist-debug-message "[Method] on-change: file change type %s" change)
      ;; If only the last plist was changed, update hash table and
      ;; task list, otherwise clear and repopulate hash table.
      (cond ((or reset-watch-p
                 (not file-state-bound-p) ;; why?
                 (eq change t))
             (chronometrist-reset-backend backend))
            (file-state-bound-p
             (pcase change
               ;; A new s-expression was added at the end of the file
               (:append (chronometrist-on-add backend))
               ;; The last s-expression in the file was changed
               (:modify (chronometrist-on-modify backend))
               ;; The last s-expression in the file was removed
               (:remove (chronometrist-on-remove backend))
               ((pred null) nil))))
      (setf rest-start  (chronometrist-rest-start file)
            rest-end    (chronometrist-rest-end file)
            file-length (chronometrist-file-length file)
            last-hash   (chronometrist-file-hash rest-end file-length file)
            rest-hash   (chronometrist-file-hash rest-start rest-end file)))))
#+END_SRC

*** plist backend
In this format, user data is stored as Elisp plists in a plain text file. A basic plist in this file looks like this -

#+BEGIN_SRC emacs-lisp :tangle no :load no
(:name "Task Name"
 [:keyword <value>]*
 :start "<ISO-8601 date-time>"
 :stop "<ISO-8601 date-time>")
#+END_SRC

=:name= and =:start= are essential. =:stop= may be missing if the task is currently active.

The reasons I like this format are -
1. Users can browse and edit the data using the Emacs setup they are accustomed to. We get things like swiper, undo-tree, and a host of other features for free.
2. It is trivial to parse using the =read= built-in to Emacs.
   * =chronometrist-loop-file= is provided as an additional convenience, to iterate through each expression in the file.
3. It is easy to diff and version control.

**** tests
:PROPERTIES:
:header-args:   :tangle chronometrist-tests.el
:END:

#+BEGIN_SRC emacs-lisp :tangle chronometrist-tests.el :load test
(defvar chronometrist-plist-test-backend
  (make-instance 'chronometrist-plist-backend
                 :file (make-temp-file "chronometrist-plist-test-" nil ".sexp")))

(let ((file (chronometrist-backend-file chronometrist-plist-test-backend)))
  (with-current-buffer (find-file-noselect file)
    (mapcar
     (lambda (plist)
       ;; to use this, we'd have to move `chronometrist-plist-pp' before this
       ;; definition, and I'm perfectly content with where it is
       ;; right now
       (chronometrist-plist-pp plist (current-buffer))
       (insert "\n\n")
       ;; (print plist) (princ "\n")
       )
     '((:name "Programming"
              :start "2018-01-01T00:00:00+0530"
              :stop  "2018-01-01T01:00:00+0530")
       (:name "Swimming"
              :start "2018-01-01T02:00:00+0530"
              :stop  "2018-01-01T03:00:00+0530")
       (:name "Cooking"
              :start "2018-01-01T04:00:00+0530"
              :stop  "2018-01-01T05:00:00+0530")
       (:name "Guitar"
              :start "2018-01-01T06:00:00+0530"
              :stop  "2018-01-01T07:00:00+0530")
       (:name "Cycling"
              :start "2018-01-01T08:00:00+0530"
              :stop  "2018-01-01T09:00:00+0530")
       (:name "Programming"
              :start "2018-01-02T23:00:00+0530"
              :stop  "2018-01-03T01:00:00+0530")
       (:name "Cooking"
              :start "2018-01-03T23:00:00+0530"
              :stop  "2018-01-04T01:00:00+0530")
       (:name "Programming"
              :tags      (bug-hunting)
              :project   "Chronometrist"
              :component "goals"
              :start     "2020-05-09T20:03:25+0530"
              :stop      "2020-05-09T20:05:55+0530")
       (:name "Arrangement/new edition"
              :tags     (new edition)
              :song     "Songs of Travel"
              :composer "Vaughan Williams, Ralph"
              :start    "2020-05-10T00:04:14+0530"
              :stop     "2020-05-10T00:25:48+0530")
       (:name "Guitar"
              :tags  (classical warm-up)
              :start "2020-05-10T15:41:14+0530"
              :stop  "2020-05-10T15:55:42+0530")
       (:name "Guitar"
              :tags  (classical solo)
              :start "2020-05-10T16:00:00+0530"
              :stop  "2020-05-10T16:30:00+0530")
       (:name "Programming"
              :tags  (reading)
              :book  "Smalltalk-80: The Language and Its Implementation"
              :start "2020-05-10T16:33:17+0530"
              :stop  "2020-05-10T17:10:48+0530")))))
#+END_SRC

1. [X] finding the test input file
   * =buffer-file-name= returns nil when Emacs is run in batch mode; I've tried using =(concat (or (ignore-errors (file-name-directory (buffer-file-name))) default-directory) "test.sexp")=, but that resulted in ="~/.emacs.d/test.sexp"= being used instead, for some reason.
   * maybe we can store the test file contents in a string instead, and create a temporary test file using =make-temp-file=?

Boilerplate for updating state between file operations in tests.
#+BEGIN_SRC emacs-lisp :load test
(defmacro chronometrist-tests--change-type-and-update (state file)
  `(prog1 (chronometrist-file-change-type ,state)
     (setq ,state
           (list :last (chronometrist-file-hash :before-last nil nil ,file)
                 :rest (chronometrist-file-hash nil :before-last t ,file)))))
#+END_SRC
**** backend                                                       :class:
#+BEGIN_SRC emacs-lisp
(defclass chronometrist-plist-backend (chronometrist-elisp-sexp-backend)
  ((extension :initform "plist"
              :accessor chronometrist-backend-ext
              :custom 'string)))

(chronometrist-register-backend
 :plist "Store records as plists."
 (make-instance 'chronometrist-plist-backend :path chronometrist-file))
#+END_SRC

**** pretty-print-function                               :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-sexp-pretty-print-function #'chronometrist-plist-pp
  "Function used to pretty print plists in `chronometrist-file'.
Like `pp', it must accept an OBJECT and optionally a
STREAM (which is the value of `current-buffer')."
  :type 'function
  :group 'chronometrist)
#+END_SRC

**** latest-date-records                                   :reader:method:
In this backend, it's easier to implement this in terms of [[#program-backend-plist-latest-record][=chronometrist-latest-record=]] than the other way round.

#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-latest-date-records ((backend chronometrist-plist-backend))
  (chronometrist-backend-run-assertions backend)
  (with-slots (hash-table) backend
    (when-let*
        ((latest-date (chronometrist-events-last-date hash-table))
         (records     (gethash latest-date hash-table)))
      (cons latest-date records))))
#+END_SRC

**** to-hash-table                                         :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-to-hash-table ((backend chronometrist-plist-backend))
  (chronometrist-sexp-in-file (chronometrist-backend-file backend)
    (goto-char (point-min))
    (let ((table (chronometrist-make-hash-table))
          expr pending-expr)
      (while (or pending-expr
                 (setq expr (ignore-errors (read (current-buffer)))))
        ;; find and split midnight-spanning events during deserialization itself
        (let* ((split-expr (chronometrist-split-plist expr))
               (new-value  (cond (pending-expr
                                  (prog1 pending-expr
                                    (setq pending-expr nil)))
                                 (split-expr
                                  (setq pending-expr (cl-second split-expr))
                                  (cl-first split-expr))
                                 (t expr)))
               (new-value-date (--> (plist-get new-value :start)
                                    (substring it 0 10)))
               (existing-value (gethash new-value-date table)))
          (puthash new-value-date
                   (if existing-value
                       (append existing-value
                               (list new-value))
                     (list new-value))
                   table)))
      table)))
#+END_SRC

**** insert                                                :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-insert ((backend chronometrist-plist-backend) plist &key (save t))
  (chronometrist-backend-run-assertions backend)
  (chronometrist-debug-message "[Method] insert: %s" plist)
  (chronometrist-sexp-in-file (chronometrist-backend-file backend)
    (goto-char (point-max))
    ;; If we're adding the first s-exp in the file, don't add a
    ;; newline before it
    (unless (bobp) (insert "\n"))
    (unless (bolp) (insert "\n"))
    (funcall chronometrist-sexp-pretty-print-function plist (current-buffer))
    (when save (save-buffer))
    t))
#+END_SRC

**** remove-last                                           :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-remove-last ((backend chronometrist-plist-backend))
  (chronometrist-debug-message "[Method] remove-last")
  (chronometrist-backend-run-assertions backend)
  (when (chronometrist-backend-empty-p backend)
  (error "chronometrist-remove-last has nothing to remove in %s"
         (eieio-object-class-name backend)))
  (chronometrist-sexp-in-file (chronometrist-backend-file backend)
    (goto-char (point-max))
    ;; this condition should never really occur, since we insert a
    ;; file local variable prop line when the file is created...
    (unless (and (bobp) (bolp)) (insert "\n"))
    (backward-list 1)
    (chronometrist-sexp-delete-list)))
#+END_SRC

**** reindent-buffer                                             :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-sexp-reindent-buffer ()
  "Reindent the current buffer.
This is meant to be run in `chronometrist-file' when using an s-expression backend."
  (interactive)
  (chronometrist-debug-message "[Command] reindent-buffer")
  (let (expr)
    (goto-char (point-min))
    (while (setq expr (ignore-errors (read (current-buffer))))
      (backward-list)
      (chronometrist-sexp-delete-list)
      (when (looking-at "\n*")
        (delete-region (match-beginning 0) (match-end 0)))
      (funcall chronometrist-sexp-pretty-print-function expr (current-buffer))
      (insert "\n")
      (unless (eobp) (insert "\n")))))
#+END_SRC

**** to-file                                               :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-to-file (hash-table (backend chronometrist-plist-backend) file)
  (delete-file file)
  (chronometrist-create-file backend file)
  (chronometrist-reset-backend backend)             ; possibly to ensure BACKEND is up to date
  (chronometrist-sexp-in-file file
    (goto-char (point-max))
    (cl-loop
      for date in (sort (hash-table-keys hash-table) #'string-lessp) do
      (cl-loop for plist in (gethash date hash-table) do
        (insert (chronometrist-plist-pp plist) "\n\n"))
      finally do (save-buffer))))
#+END_SRC

**** to-list                                               :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-to-list ((backend chronometrist-plist-backend))
  (chronometrist-backend-run-assertions backend)
  (chronometrist-loop-sexp-file for expr in (chronometrist-backend-file backend) collect expr))
#+END_SRC

**** on-add                                                :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-on-add ((backend chronometrist-plist-backend))
  "Function run when a new plist is added at the end of a
`chronometrist-plist-backend' file."
  (with-slots (hash-table) backend
    (-let [(new-plist &as &plist :name new-task) (chronometrist-latest-record backend)]
      (setf hash-table (chronometrist-events-update new-plist hash-table))
      (chronometrist-add-to-task-list new-task backend))))
#+END_SRC

**** on-modify                                             :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-on-modify ((backend chronometrist-plist-backend))
  "Function run when the newest plist in a
`chronometrist-plist-backend' file is modified."
  (with-slots (hash-table) backend
    (-let (((new-plist &as &plist :name new-task) (chronometrist-latest-record backend))
           ((&plist :name old-task) (chronometrist-events-last backend)))
      (setf hash-table (chronometrist-events-update new-plist hash-table t))
      (chronometrist-remove-from-task-list old-task backend)
      (chronometrist-add-to-task-list new-task backend))))
#+END_SRC

**** on-remove                                             :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-on-remove ((backend chronometrist-plist-backend))
  "Function run when the newest plist in a
`chronometrist-plist-backend' file is deleted."
  (with-slots (hash-table) backend
    (-let (((&plist :name old-task) (chronometrist-events-last))
           (date  (chronometrist-events-last-date hash-table)))
      ;; `chronometrist-remove-from-task-list' checks the hash table to determine
      ;; if `chronometrist-task-list' is to be updated. Thus, the hash table must
      ;; not be updated until the task list is.
      (chronometrist-remove-from-task-list old-task backend)
      (--> (gethash date hash-table)
           (-drop-last 1 it)
           (setf (gethash date hash-table) it)))))
#+END_SRC

**** extended protocol
***** latest-record                                       :reader:method:
:PROPERTIES:
:CUSTOM_ID: program-backend-plist-latest-record
:END:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-latest-record ((backend chronometrist-plist-backend))
  (chronometrist-backend-run-assertions backend)
  (chronometrist-sexp-in-file (chronometrist-backend-file backend)
    (goto-char (point-max))
    (backward-list)
    (ignore-errors (read (current-buffer)))))
#+END_SRC

***** task-records-for-date                               :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-task-records-for-date ((backend chronometrist-plist-backend) task date-ts)
  (chronometrist-backend-run-assertions backend)
  (let* ((date    (chronometrist-date-iso date-ts))
         (records (gethash date (chronometrist-backend-hash-table backend))))
    (cl-loop for record in records
      when (equal task (plist-get record :name))
      collect record)))
#+END_SRC

***** replace-last                                        :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-replace-last ((backend chronometrist-plist-backend) plist)
  (chronometrist-debug-message "[Method] replace-last with %s" plist)
  (chronometrist-sexp-in-file (chronometrist-backend-file backend)
    (goto-char (chronometrist-remove-last backend))
    (funcall chronometrist-sexp-pretty-print-function plist (current-buffer))
    (save-buffer)
    t))
#+END_SRC

***** count-records                                       :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-count-records ((backend chronometrist-plist-backend))
  (chronometrist-sexp-in-file (chronometrist-backend-file backend)
    (goto-char (point-min))
    (cl-loop with count = 0
      while (ignore-errors (read (current-buffer)))
      do (cl-incf count)
      finally return count)))
#+END_SRC

*** plist group backend
This is largely like the plist backend, but plists are grouped by date by wrapping them in a tagged list -

#+BEGIN_SRC emacs-lisp :tangle no :load no
("<ISO-8601 date>"
 (:name "Task Name"
  [:keyword <value>]*
  :start "<ISO-8601 time>"
  :stop "<ISO-8601 time>")
 ...)
#+END_SRC

This makes it easy and computationally cheap to perform our most common query - getting the plists on a given day. [[#explanation-midnight-spanning-intervals][Midnight-spanning intervals]] are split in the file itself. The downside is that the user, if editing it by hand, must take care to split the intervals.

Note that migrating from the plist backend to the plist group backend is inherently likely to result in more plists compared to the source, as each midnight-spanning plist is split into two.

Concerns specific to the plist group backend -
1. the last plist may be split across two days. In such a situation -
   * changing key-values for the last plist would only apply to the most recent one
   * +deleting the last plist via =chronometrist-remove-last= would only delete the recent part of the split plist+ fixed
   * resetting is unaffected, since that only applies to the last interval, whether or not it's a split plist
   * restarting is unaffected, since that only applies to the active interval, and split intervals are always inactive ones

**** backend                                                       :class:
#+BEGIN_SRC emacs-lisp
(defclass chronometrist-plist-group-backend (chronometrist-elisp-sexp-backend)
  ((extension :initform "plg"
              :accessor chronometrist-backend-ext
              :custom 'string)))

(chronometrist-register-backend
 :plist-group "Store records as plists grouped by date."
 (make-instance 'chronometrist-plist-group-backend
                :path chronometrist-file))
#+END_SRC

**** backward-read-sexp                                           :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-backward-read-sexp (buffer)
  (backward-list)
  (save-excursion (read buffer)))
#+END_SRC

**** run-assertions                                        :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-backend-run-assertions ((backend chronometrist-file-backend-mixin))
  (with-slots (file) backend
    (unless (file-exists-p file)
      (error "Backend file %S does not exist" file))))
#+END_SRC

**** latest-date-records                                   :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-latest-date-records ((backend chronometrist-plist-group-backend))
  (chronometrist-backend-run-assertions backend)
  (chronometrist-sexp-in-file (chronometrist-backend-file backend)
    (goto-char (point-max))
    (ignore-errors
      (chronometrist-backward-read-sexp (current-buffer)))))
#+END_SRC

**** HACK insert                                           :writer:method:
<<hack-note-plist-group-insert>>
We just want to insert a plist, but as a hack to avoid updating the pretty-printer to handle indentation of plists being inserted into an outer list, we =append= the plist to a plist group and insert/replace the plist group instead.

Situations -
1. new inactive day-crossing record
   1. first record - split, insert into two new groups
   2. not first record - split, insert into existing group + new group
2. new active record, or new non-day-crossing inactive record
   1. first record - insert into new plist group
   2. not first record
      1. latest recorded date = today - insert into existing group
      2. insert into new group

#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-insert ((backend chronometrist-plist-group-backend) plist &key (save t))
  (cl-check-type plist chronometrist-plist)
  (chronometrist-debug-message "[Method] insert: %S" plist)
  (chronometrist-backend-run-assertions backend)
  (if (not plist)
      (error "%s" "`chronometrist-insert' was called with an empty plist")
    (chronometrist-sexp-in-file (chronometrist-backend-file backend)
      (-let* (((plist-1 plist-2)   (chronometrist-split-plist plist))
              ;; Determine if we need to insert a new plist group
              (latest-plist-group  (chronometrist-latest-date-records backend))
              (backend-latest-date (cl-first latest-plist-group))
              (date-today          (chronometrist-date-iso))
              (insert-new-group    (not (equal date-today backend-latest-date)))
              (start-date          (cl-first (split-string (plist-get plist :start) "T")))
              (new-plist-group-1   (if latest-plist-group
                                       (append latest-plist-group
                                               (list (or plist-1 plist)))
                                     (list start-date (or plist-1 plist))))
              (new-plist-group-2   (when (or plist-2 insert-new-group)
                                     (list date-today (or plist-2 plist)))))
        (goto-char (point-max))
        (when (not latest-plist-group)
          ;; first record
          (while (forward-comment 1) nil))
        (if (and plist-1 plist-2)
            ;; inactive, day-crossing record
            (progn
              (when latest-plist-group
                ;; not the first record
                (chronometrist-sexp-pre-read-check (current-buffer))
                (chronometrist-sexp-delete-list))
              (funcall chronometrist-sexp-pretty-print-function new-plist-group-1 (current-buffer))
              (dotimes (_ 2) (default-indent-new-line))
              (funcall chronometrist-sexp-pretty-print-function new-plist-group-2 (current-buffer)))
          ;; active, or non-day-crossing inactive record
          ;; insert into new group
          (if (or (not latest-plist-group) ;; first record
                  insert-new-group)
              (progn
                (default-indent-new-line)
                (funcall chronometrist-sexp-pretty-print-function new-plist-group-2 (current-buffer)))
            ;; insert into existing group
            (chronometrist-sexp-pre-read-check (current-buffer))
            (chronometrist-sexp-delete-list)
            (funcall chronometrist-sexp-pretty-print-function new-plist-group-1 (current-buffer))))
        (when save (save-buffer))
        t))))
#+END_SRC

**** plists-split-p                                             :function:
[[file:../tests/chronometrist-tests.org::#tests-common-plists-split-p][tests]]

#+BEGIN_SRC emacs-lisp
(defun chronometrist-plists-split-p (old-plist new-plist)
  "Return t if OLD-PLIST and NEW-PLIST are split plists.
Split plists means the :stop time of old-plist must be the same as
the :start time of new-plist, and they must have identical
keyword-values (except :start and :stop)."
  (-let* (((&plist :stop  old-stop)  old-plist)
          ((&plist :start new-start) new-plist)
          (old-stop-unix     (parse-iso8601-time-string old-stop))
          (new-start-unix    (parse-iso8601-time-string new-start))
          (old-plist-no-time (chronometrist-plist-remove old-plist :start :stop))
          (new-plist-no-time (chronometrist-plist-remove new-plist :start :stop)))
    (and (time-equal-p old-stop-unix
                       new-start-unix)
         (equal old-plist-no-time
                new-plist-no-time))))
#+END_SRC

**** last-two-split-p                                          :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-last-two-split-p (file)
  "Return non-nil if the latest two plists in FILE are split.
FILE must be a file containing plist groups, as created by
`chronometrist-plist-backend'.

Return value is either a list in the form
(OLDER-PLIST NEWER-PLIST), or nil."
  (chronometrist-sexp-in-file file
    (let* ((newer-group (progn (goto-char (point-max))
                               (backward-list)
                               (read (current-buffer))))
           (older-group (and (= 2 (length newer-group))
                             (backward-list 2)
                             (read (current-buffer))))
           ;; in case there was just one plist-group in the file
           (older-group (unless (equal older-group newer-group)
                          older-group))
           (newer-plist (cl-second newer-group))
           (older-plist (cl-first (last older-group))))
      (when (and older-plist newer-plist
                 (chronometrist-plists-split-p older-plist newer-plist))
        (list older-plist newer-plist)))))
#+END_SRC

**** plist-unify                                                :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-plist-unify (old-plist new-plist)
  "Return a plist with the :start of OLD-PLIST and the :stop of NEW-PLIST."
  (let ((old-plist-wo-time (chronometrist-plist-remove old-plist :start :stop))
        (new-plist-wo-time (chronometrist-plist-remove new-plist :start :stop)))
    (cond ((not (and old-plist new-plist)) nil)
          ((equal old-plist-wo-time new-plist-wo-time)
           (let ((plist (cl-copy-list old-plist)))
             (plist-put plist :stop (plist-get new-plist :stop))))
          (t (error "Attempt to unify plists with non-identical key-values")))))
#+END_SRC

**** remove-last                                           :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-remove-last ((backend chronometrist-plist-group-backend) &key (save t))
  (with-slots (file) backend
    (chronometrist-sexp-in-file file
      (goto-char (point-max))
      (when (chronometrist-backend-empty-p backend)
        (error "chronometrist-remove-last has nothing to remove in %s"
               (eieio-object-class-name backend)))
      (when (chronometrist-last-two-split-p file) ;; cannot be checked after changing the file
        ;; latest plist-group has only one plist, which is split - delete the group
        (backward-list)
        (chronometrist-sexp-delete-list))
      ;; remove the last plist in the last plist-group
      ;; if the plist-group has only one plist, delete the group
      (let ((plist-group (save-excursion (backward-list)
                                         (read (current-buffer)))))
        (if (= 2 (length plist-group))
            (progn (backward-list)
                   (chronometrist-sexp-delete-list))
          (down-list -1)
          (backward-list)
          (chronometrist-sexp-delete-list)
          (join-line))
        (when save (save-buffer))
        t))))
#+END_SRC

**** to-list                                               :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-to-list ((backend chronometrist-plist-group-backend))
  (chronometrist-backend-run-assertions backend)
  (chronometrist-loop-sexp-file for expr in (chronometrist-backend-file backend)
    append (reverse (cl-rest expr))))
#+END_SRC

**** to-hash-table                                         :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-to-hash-table ((backend chronometrist-plist-group-backend))
  (with-slots (file) backend
    (chronometrist-loop-sexp-file for plist-group in file
      with table = (chronometrist-make-hash-table) do
      (puthash (cl-first plist-group) (cl-rest plist-group) table)
      finally return table)))
#+END_SRC

**** to-file                                               :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-to-file (hash-table (backend chronometrist-plist-group-backend) file)
  (cl-check-type hash-table hash-table)
  (delete-file file)
  (chronometrist-create-file backend file)
  (chronometrist-reset-backend backend)
  (chronometrist-sexp-in-file file
    (goto-char (point-max))
    (cl-loop for date being the hash-keys of hash-table
      using (hash-values plists) do
      (insert
       (chronometrist-plist-pp (apply #'list date plists))
       "\n")
      finally do (save-buffer))))
#+END_SRC

**** on-add                                                :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-on-add ((backend chronometrist-plist-group-backend))
  "Function run when a new plist-group is added at the end of a
`chronometrist-plist-group-backend' file."
  (with-slots (hash-table) backend
    (-let [(date plist) (chronometrist-latest-date-records backend)]
      (puthash date plist hash-table)
      (chronometrist-add-to-task-list (plist-get plist :name) backend))))
#+END_SRC

**** on-modify                                             :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-on-modify ((backend chronometrist-plist-group-backend))
  "Function run when the newest plist-group in a
`chronometrist-plist-group-backend' file is modified."
  (with-slots (hash-table) backend
    (-let* (((date . plists) (chronometrist-latest-date-records backend))
            (old-date        (chronometrist-events-last-date hash-table))
            (old-plists      (gethash old-date hash-table)))
      (puthash date plists hash-table)
      (cl-loop for plist in old-plists
        do (chronometrist-remove-from-task-list (plist-get plist :name) backend))
      (cl-loop for plist in plists
        do (chronometrist-add-to-task-list (plist-get plist :name) backend)))))
#+END_SRC

**** on-remove                                             :writer:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-on-remove ((backend chronometrist-plist-group-backend))
  "Function run when the newest plist-group in a
`chronometrist-plist-group-backend' file is deleted."
  (with-slots (hash-table) backend
    (-let* ((old-date        (chronometrist-events-last-date hash-table))
            (old-plists      (gethash old-date hash-table)))
      (cl-loop for plist in old-plists
        do (chronometrist-remove-from-task-list (plist-get plist :name) backend))
      (puthash old-date nil hash-table))))
#+END_SRC

**** verify                                                :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-verify ((backend chronometrist-plist-group-backend))
  (with-slots (file hash-table) backend
    ;; incorrectly ordered groups check
    (chronometrist-loop-sexp-file for group in file
      with old-date-iso with old-date-unix
      with new-date-iso with new-date-unix
      ;; while (not (bobp))
      do (setq new-date-iso  (cl-first group)
               new-date-unix (parse-iso8601-time-string new-date-iso))
      when (and old-date-unix
                (time-less-p old-date-unix
                             new-date-unix))
      do (cl-return (format "%s appears before %s on line %s"
                            new-date-iso old-date-iso (line-number-at-pos)))
      else do (setq old-date-iso  new-date-iso
                    old-date-unix new-date-unix)
      finally return "Yay, no errors! (...that I could find 💀)")))
#+END_SRC

**** extended protocol
***** latest-record                                       :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-latest-record ((backend chronometrist-plist-group-backend))
  (cl-first (last (chronometrist-latest-date-records backend))))
#+END_SRC

***** task-records-for-date                               :reader:method:
#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-task-records-for-date ((backend chronometrist-plist-group-backend)
                                      task date-ts)
  (cl-check-type task string)
  (cl-check-type date-ts ts)
  (chronometrist-backend-run-assertions backend)
  (cl-loop for plist in (gethash (chronometrist-date-iso date-ts)
                                 (chronometrist-backend-hash-table backend))
    when (equal task (plist-get plist :name))
    collect plist))
#+END_SRC

***** TODO active-days                           :reader:method:NOEXPORT:
#+BEGIN_SRC emacs-lisp :tangle no
(cl-defmethod chronometrist-active-days ((backend chronometrist-plist-group-backend) task &key start end)
  (cl-check-type task string)
  (chronometrist-backend-run-assertions backend))
#+END_SRC

***** replace-last                                        :writer:method:
=chronometrist-replace-last= is what is used for clocking out, so we split midnight-spanning intervals in this operation.

We apply the same hack as in the [[<<hack-note-plist-group-insert>>][insert]] method, removing and inserting the plist group instead of just the specific plist, to avoid having to update the pretty printer.

#+BEGIN_SRC emacs-lisp
(cl-defmethod chronometrist-replace-last ((backend chronometrist-plist-group-backend) plist)
  (cl-check-type plist chronometrist-plist)
  (when (chronometrist-backend-empty-p backend)
    (error "No record to replace in %s" (eieio-object-class-name backend)))
  (chronometrist-sexp-in-file (chronometrist-backend-file backend)
    (chronometrist-remove-last backend :save nil)
    (chronometrist-insert backend plist :save nil)
    (save-buffer)
    t))
#+END_SRC

***** count-records                              :reader:method:NOEXPORT:
#+BEGIN_SRC emacs-lisp :tangle no
(cl-defmethod chronometrist-count-records ((backend chronometrist-plist-group-backend)))
#+END_SRC

** Migration
*** remove-prefix
#+BEGIN_SRC emacs-lisp
(defun chronometrist-remove-prefix (string)
  (replace-regexp-in-string "^chronometrist-" "" string))
#+END_SRC

*** migrate
#+BEGIN_SRC emacs-lisp
(defun chronometrist-migrate ()
  "Convert from one Chronometrist backend to another."
  (interactive)
  (let* ((input-backend
          (chronometrist-read-backend-name "Backend to convert: "
                              chronometrist-backends-alist))
         (input-file-suggestion (chronometrist-backend-file input-backend))
         (input-file (read-file-name "File to convert: " nil
                                     input-file-suggestion t
                                     input-file-suggestion))
         (output-backend (chronometrist-read-backend-name
                          "Backend to write: "
                          chronometrist-backends-alist
                          (lambda (keyword)
                            (not (equal (cl-second
                                         (alist-get keyword chronometrist-backends-alist))
                                        input-backend)))))
         (output-file-suggestion (chronometrist-backend-file output-backend))
         (output-file (read-file-name "File to write: " nil nil nil
                                      output-file-suggestion))
         (input-backend-name  (chronometrist-remove-prefix
                               (symbol-name
                                (eieio-object-class-name input-backend))))
         (output-backend-name (chronometrist-remove-prefix
                               (symbol-name
                                (eieio-object-class-name output-backend))))
         (confirm (yes-or-no-p
                   (format "Convert %s (%s) to %s (%s)? "
                           input-file
                           input-backend-name
                           output-file
                           output-backend-name)))
         (confirm-exists
          (if (and confirm
                   (file-exists-p output-file)
                   (not (chronometrist-file-empty-p output-file)))
              (yes-or-no-p
               (format "Overwrite existing non-empty file %s ?"
                       output-file))
            t)))
    (if (and confirm confirm-exists)
        (chronometrist-to-file (chronometrist-backend-hash-table input-backend)
                  output-backend
                  output-file)
      (message "Conversion aborted."))))
#+END_SRC
*** table                                                        :variable:
:PROPERTIES:
:VALUE:    hash table
:END:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-migrate-table (make-hash-table))
#+END_SRC
*** EXTEND populate                                                :writer:
:PROPERTIES:
:STATE: chronometrist-migrate-table
:END:

1. [ ] support other timeclock codes - currently only "i" and "o" are supported.

#+BEGIN_SRC emacs-lisp
(defun chronometrist-migrate-populate (in-file)
  "Read data from IN-FILE to `chronometrist-migrate-table'.
IN-FILE should be a file in the format supported by timeclock.el.
See `timeclock-log-data' for a description."
  (clrhash chronometrist-migrate-table)
  (with-current-buffer (find-file-noselect in-file)
    (save-excursion
      (goto-char (point-min))
      (let ((key-counter 0))
        (while (not (eobp))
          (let* ((event-string (buffer-substring-no-properties (point-at-bol)
                                                               (point-at-eol)))
                 (event-list   (split-string event-string "[ /:]"))
                 (code         (cl-first event-list))
                 (date-time    (--> (seq-drop event-list 1)
                                    (seq-take it 6)
                                    (mapcar #'string-to-number it)
                                    (reverse it)
                                    (apply #'encode-time it)
                                    (chronometrist-format-time-iso8601 it)))
                 (project-or-comment
                  (replace-regexp-in-string
                   (rx (and (or "i" "o") " "
                            (and (= 4 digit) "/" (= 2 digit) "/" (= 2 digit) " ")
                            (and (= 2 digit) ":" (= 2 digit) ":" (= 2 digit))
                            (opt " ")))
                   ""
                   event-string)))
            (pcase code
              ("i"
               (cl-incf key-counter)
               (puthash key-counter
                        `(:name ,project-or-comment :start ,date-time)
                        chronometrist-migrate-table))
              ("o"
               (--> (gethash key-counter chronometrist-migrate-table)
                    (append it
                            `(:stop ,date-time)
                            (when (and (stringp project-or-comment)
                                       (not
                                        (string= project-or-comment "")))
                              `(:comment ,project-or-comment)))
                    (puthash key-counter it chronometrist-migrate-table)))))
          (forward-line)
          (goto-char (point-at-bol))))
      nil)))
#+END_SRC

*** timelog-file-to-sexp-file                                      :writer:
#+BEGIN_SRC emacs-lisp
(defvar timeclock-file)

(defun chronometrist-migrate-timelog-file-to-sexp-file (&optional in-file out-file)
  "Migrate your existing `timeclock-file' to the Chronometrist file format.
IN-FILE and OUT-FILE, if provided, are used as input and output
file names respectively."
  (interactive `(,(if (featurep 'timeclock)
                      (read-file-name (concat "timeclock file (default: "
                                              timeclock-file
                                              "): ")
                                      user-emacs-directory
                                      timeclock-file t)
                    (read-file-name (concat "timeclock file: ")
                                    user-emacs-directory
                                    nil t))
                 ,(read-file-name (concat "Output file (default: "
                                          (locate-user-emacs-file "chronometrist.sexp")
                                          "): ")
                                  user-emacs-directory
                                  (locate-user-emacs-file "chronometrist.sexp"))))
  (when (if (file-exists-p out-file)
            (yes-or-no-p (concat "Output file "
                                 out-file
                                 " already exists - overwrite? "))
          t)
    (let ((output (find-file-noselect out-file)))
      (with-current-buffer output
        (erase-buffer)
        (chronometrist-migrate-populate in-file)
        (maphash (lambda (_key value)
                   (chronometrist-plist-pp value output)
                   (insert "\n\n"))
                 chronometrist-migrate-table)
        (save-buffer)))))
#+END_SRC
*** check                                                          :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-migrate-check ()
  "Offer to import data from `timeclock-file' if `chronometrist-file' does not exist."
  (when (and (bound-and-true-p timeclock-file)
             (not (file-exists-p chronometrist-file)))
    (if (yes-or-no-p (format (concat "Chronometrist v0.3+ uses a new file format;"
                                     " import data from %s ? ")
                             timeclock-file))
        (chronometrist-migrate-timelog-file-to-sexp-file timeclock-file chronometrist-file)
      (message "You can migrate later using `chronometrist-migrate-timelog-file-to-sexp-file'."))))
#+END_SRC
** Timer
Instead of the Emacs convention of pressing ~g~ to update, we keep buffers updated with a timer.

Note - sometimes, when hacking or dealing with errors, timers may result in subtle bugs which are very hard to debug. Using =chronometrist-force-restart-timer= or restarting Emacs can fix them, so try that as a first sanity check.

*** update-interval                                       :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-update-interval 5
  "How often the `chronometrist' buffer should be updated, in seconds.

This is not guaranteed to be accurate - see (info \"(elisp)Timers\")."
  :type 'integer)
#+END_SRC
*** timer-object                                        :internal:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist--timer-object nil)
#+END_SRC
*** timer-hook                                       :hook:custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-timer-hook nil
  "Functions run by `chronometrist-timer'."
  :type '(repeat function))
#+END_SRC

*** start-timer                                                   :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-start-timer ()
  (setq chronometrist--timer-object
        (run-at-time t chronometrist-update-interval #'chronometrist-timer)))
#+END_SRC

*** stop-timer                                                    :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-stop-timer ()
  "Stop the timer for Chronometrist buffers."
  (interactive)
  (cancel-timer chronometrist--timer-object)
  (setq chronometrist--timer-object nil))
#+END_SRC

*** maybe-start-timer                                             :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-maybe-start-timer (&optional interactive-test)
  "Start `chronometrist-timer' if `chronometrist--timer-object' is non-nil.
INTERACTIVE-TEST is used to determine if this has been called
interactively."
  (interactive "p")
  (unless chronometrist--timer-object
    (chronometrist-start-timer)
    (when interactive-test
      (message "Timer started."))
    t))
#+END_SRC

*** force-restart-timer                                           :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-force-restart-timer ()
  "Restart the timer for Chronometrist buffers."
  (interactive)
  (when chronometrist--timer-object
    (cancel-timer chronometrist--timer-object))
  (chronometrist-start-timer))
#+END_SRC

*** change-update-interval                                        :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-change-update-interval (arg)
  "Change the update interval for Chronometrist buffers.

ARG should be the new update interval, in seconds."
  (interactive "NEnter new interval (in seconds): ")
  (cancel-timer chronometrist--timer-object)
  (setq chronometrist-update-interval arg
        chronometrist--timer-object nil)
  (chronometrist-maybe-start-timer))
#+END_SRC

*** timer                                                        :function:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-buffer-name)
(defun chronometrist-timer ()
  "Refresh Chronometrist and related buffers.
Buffers will be refreshed only if they are visible, the user is
clocked in to a task, and the active backend is not being
modified."
  ;; No need to update the buffer if there is no active task, or if
  ;; the file is being edited by the user. (The file may be in an
  ;; invalid state, and reading it then may result in a read error.)
  ;; Check for buffer modification first, since `chronometrist-current-task' may
  ;; access the file (causing an error).
  (when (and (not (chronometrist-backend-modified-p (chronometrist-active-backend)))
             (chronometrist-current-task))
    (when (get-buffer-window chronometrist-buffer-name)
      (chronometrist-refresh))
    (run-hooks 'chronometrist-timer-hook)))
#+END_SRC

** Frontends
All four of these use [[info:elisp#Tabulated List Mode][=(info "(elisp)Tabulated List Mode")=]]. Each of them also contains a "-print-non-tabular" function, which prints the non-tabular parts of the buffer.

1. [ ] There is some duplication between the four frontend commands, e.g. all four act as toggles for their respective buffers, point preservation, etc.
*** Chronometrist
:PROPERTIES:
:CUSTOM_ID: program-frontend-chronometrist
:END:

**** TODO [33%]
1. [X] Define hooks with defcustom instead of defvar
2. [ ] Change abnormal hooks to normal hooks
3. [ ] midnight-spanning plist not displayed (may have to do with partial updates)
**** buffer-name                                         :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-buffer-name "*Chronometrist*"
  "The name of the buffer created by `chronometrist'."
  :type 'string)
#+END_SRC
**** hide-cursor                                         :custom:variable:
I have not yet gotten this to work as well as I wanted.

#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-hide-cursor nil
  "If non-nil, hide the cursor and only highlight the current line in the `chronometrist' buffer."
  :type 'boolean)
#+END_SRC
**** activity-indicator                                  :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-activity-indicator "*"
  "How to indicate that a task is active.
Can be a string to be displayed, or a function which returns this string.
The default is \"*\""
  :type '(choice string function))
#+END_SRC
**** point                                             :internal:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist--point nil)
#+END_SRC
**** open-log                                                    :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-open-log (&optional _button)
  "Open `chronometrist-file' in another window.

Argument _BUTTON is for the purpose of using this command as a
button action."
  (interactive)
  (chronometrist-edit-backend (chronometrist-active-backend)))
#+END_SRC

**** task-active-p                                                :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-task-active-p (task)
  "Return t if TASK is currently clocked in, else nil."
  (equal (chronometrist-current-task) task))
#+END_SRC

**** activity-indicator                                        :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-activity-indicator ()
  "Return a string to indicate that a task is active.
See custom variable `chronometrist-activity-indicator'."
  (if (functionp chronometrist-activity-indicator)
      (funcall chronometrist-activity-indicator)
    chronometrist-activity-indicator))
#+END_SRC

**** run-transformers                                           :function:
Used by [[* row-transformers][=chronometrist-row-transformers=]] and [[* schema-transformers][=chronometrist-schema-transformers=]] to remove the need for Chronometrist to know about extensions like =chronometrist-goal=.
#+BEGIN_SRC emacs-lisp
(defun chronometrist-run-transformers (transformers arg)
  "Run TRANSFORMERS with ARG.
TRANSFORMERS should be a list of functions (F₁ ... Fₙ), each of
which should accept a single argument.

Call F₁ with ARG, with each following function being called with
the return value of the previous function.

Return the value returned by Fₙ."
  (if transformers
      (dolist (fn transformers arg)
        (setq arg (funcall fn arg)))
    arg))
#+END_SRC

**** TODO schema                                         :custom:variable:
1. Define custom =:type=

#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-schema
  '[("#" 3 t) ("Task" 25 t) ("Time" 10 t) ("Active" 10 t)]
  "Vector specifying schema of `chronometrist' buffer.
See `tabulated-list-format'."
  :type '(vector))
#+END_SRC
**** chronometrist-mode-hook                                             :hook:normal:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-mode-hook nil
  "Normal hook run at the very end of `chronometrist-mode'.")
#+END_SRC
**** schema-transformers                              :extension:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-schema-transformers nil
  "List of functions to transform `chronometrist-schema'.
This is called with `chronometrist-run-transformers' in `chronometrist-mode', which see.

Extensions using `chronometrist-schema-transformers' to
increase the number of columns will also need to modify the value
of `tabulated-list-entries' by using
`chronometrist-row-transformers'.")
#+END_SRC

**** row-transformers                                 :extension:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-row-transformers nil
  "List of functions to transform each row of `tabulated-list-entries'.
This is called with `chronometrist-run-transformers' in `chronometrist-rows', which see.

Extensions using `chronometrist-row-transformers' to increase
the number of columns will also need to modify the value of
`tabulated-list-format' by using
`chronometrist-schema-transformers'.")
#+END_SRC

**** before-in-functions                                   :hook:abnormal:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-before-in-functions nil
  "Functions to run before a task is clocked in.
Each function in this hook must accept a single argument, which
is the name of the task to be clocked-in.

The commands `chronometrist-toggle-task-button',
`chronometrist-add-new-task-button', `chronometrist-toggle-task',
and `chronometrist-add-new-task' will run this hook."
  :type '(repeat function))
#+END_SRC

**** after-in-functions                                    :hook:abnormal:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-after-in-functions nil
  "Functions to run after a task is clocked in.
Each function in this hook must accept a single argument, which
is the name of the task to be clocked-in.

The commands `chronometrist-toggle-task-button',
`chronometrist-add-new-task-button', `chronometrist-toggle-task',
and `chronometrist-add-new-task' will run this hook."
  :type '(repeat function))
#+END_SRC

**** before-out-functions                                  :hook:abnormal:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-before-out-functions nil
  "Functions to run before a task is clocked out.
Each function in this hook must accept a single argument, which
is the name of the task to be clocked out of.

The task will be stopped only if all functions in this list
return a non-nil value."
  :type '(repeat function))
#+END_SRC

**** after-out-functions                                   :hook:abnormal:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-after-out-functions nil
  "Functions to run after a task is clocked out.
Each function in this hook must accept a single argument, which
is the name of the task to be clocked out of."
  :type '(repeat function))
#+END_SRC

**** file-change-hook                                        :hook:normal:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-file-change-hook nil
  "Functions to be run after `chronometrist-file' is changed on disk."
  :type '(repeat function))
#+END_SRC

**** rows                                                      :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-rows ()
  "Return rows to be displayed in the buffer created by `chronometrist', in the format specified by `tabulated-list-entries'."
  (cl-loop with index = 1
    for task in (-sort #'string-lessp (chronometrist-task-list)) collect
    (let* ((index       (number-to-string index))
           (task-button `(,task action chronometrist-toggle-task-button
                                follow-link t))
           (task-time   (chronometrist-format-duration (chronometrist-task-time-one-day task)))
           (indicator   (if (chronometrist-task-active-p task)
                            (chronometrist-activity-indicator) "")))
      (--> (vector index task-button task-time indicator)
           (list task it)
           (chronometrist-run-transformers chronometrist-row-transformers it)))
    do (cl-incf index)))
#+END_SRC

**** task-at-point                                             :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-task-at-point ()
  "Return the task at point in the `chronometrist' buffer, or nil if there is no task at point."
  (save-excursion
    (beginning-of-line)
    (when (re-search-forward "[0-9]+ +" nil t)
      (get-text-property (point) 'tabulated-list-id))))
#+END_SRC

**** goto-last-task                                            :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-goto-last-task ()
  "In the `chronometrist' buffer, move point to the line containing the last active task."
  (let* ((latest-record (chronometrist-latest-record (chronometrist-active-backend)))
         (name (plist-get latest-record :name)))
    (goto-char (point-min))
    (re-search-forward name nil t)
    (beginning-of-line)))
#+END_SRC

**** CLEANUP print-non-tabular                                 :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-print-non-tabular ()
  "Print the non-tabular part of the buffer in `chronometrist'."
  (with-current-buffer chronometrist-buffer-name
    (let ((inhibit-read-only t) (w "\n    "))
      (goto-char (point-max))
      (--> (chronometrist-active-time-on)
           (chronometrist-format-duration it)
           (format "%s%- 26s%s" w "Total" it)
           (insert it)))))
#+END_SRC

**** goto-nth-task                                             :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-goto-nth-task (n)
  "Move point to the line containing the Nth task.
Return the task at point, or nil if there is no corresponding
task. N must be a positive integer."
  (goto-char (point-min))
  (when (re-search-forward (format "^%d" n) nil t)
    (beginning-of-line)
    (chronometrist-task-at-point)))
#+END_SRC

**** refresh                                                   :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-refresh (&optional _ignore-auto _noconfirm)
  "Refresh the `chronometrist' buffer, without re-reading `chronometrist-file'.
The optional arguments _IGNORE-AUTO and _NOCONFIRM are ignored,
and are present solely for the sake of using this function as a
value of `revert-buffer-function'."
  (let* ((window (get-buffer-window chronometrist-buffer-name t))
         (point  (window-point window)))
    (when window
      (with-current-buffer chronometrist-buffer-name
        (tabulated-list-print t nil)
        (chronometrist-print-non-tabular)
        (chronometrist-maybe-start-timer)
        (set-window-point window point)))))
#+END_SRC

**** refresh-file                                                 :writer:
=chronometrist-file-change-type= must be run /before/ we update =chronometrist--file-state= (the latter represents the old state of the file, which =chronometrist-file-change-type= compares with the newer current state).

#+BEGIN_SRC emacs-lisp
(defun chronometrist-refresh-file (fs-event)
  "Procedure run when `chronometrist-file' changes.
Re-read `chronometrist-file', update caches, and
refresh the `chronometrist' buffer."
  (run-hooks 'chronometrist-file-change-hook)
  ;; (message "chronometrist - file %s" fs-event)
  (chronometrist-on-change (chronometrist-active-backend) fs-event)
  (chronometrist-refresh))
#+END_SRC

**** query-stop                                                :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-query-stop ()
  "Ask the user if they would like to clock out."
  (let ((task (chronometrist-current-task)))
    (and task
         (yes-or-no-p (format "Stop tracking time for %s? " task))
         (chronometrist-out))
    t))
#+END_SRC
**** chronometrist-in                                                        :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-in (task &optional _prefix)
  "Clock in to TASK; record current time in `chronometrist-file'.
TASK is the name of the task, a string. PREFIX is ignored."
  (interactive "P")
  (let ((plist `(:name ,task :start ,(chronometrist-format-time-iso8601))))
    (chronometrist-insert (chronometrist-active-backend) plist)
    (chronometrist-refresh)))
#+END_SRC
**** chronometrist-out                                                       :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-out (&optional _prefix)
  "Record current moment as stop time to last s-exp in `chronometrist-file'.
PREFIX is ignored."
  (interactive "P")
  (let* ((latest (chronometrist-latest-record (chronometrist-active-backend)))
         (plist  (plist-put latest :stop (chronometrist-format-time-iso8601))))
    (chronometrist-replace-last (chronometrist-active-backend) plist)))
#+END_SRC
**** run-functions-and-clock-in                                   :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-run-functions-and-clock-in (task)
  "Run hooks and clock in to TASK."
  (run-hook-with-args 'chronometrist-before-in-functions task)
  (chronometrist-in task)
  (run-hook-with-args 'chronometrist-after-in-functions task))
#+END_SRC
**** run-functions-and-clock-out                                  :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-run-functions-and-clock-out (task)
  "Run hooks and clock out of TASK."
  (when (run-hook-with-args-until-failure 'chronometrist-before-out-functions task)
    (chronometrist-out)
    (run-hook-with-args 'chronometrist-after-out-functions task)))
#+END_SRC
**** chronometrist-mode-map                                                   :keymap:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map (kbd "a")          #'chronometrist-add-new-task)
    (define-key map (kbd "RET")        #'chronometrist-toggle-task)
    (define-key map (kbd "M-RET")      #'chronometrist-toggle-task-no-hooks)
    (define-key map [mouse-1]          #'chronometrist-toggle-task)
    (define-key map [mouse-3]          #'chronometrist-toggle-task-no-hooks)
    (define-key map (kbd "<C-return>") #'chronometrist-restart-task)
    (define-key map (kbd "<C-M-return>") #'chronometrist-extend-task)
    (define-key map (kbd "D")          #'chronometrist-discard-active)
    (define-key map (kbd "d")          #'chronometrist-details)
    (define-key map (kbd "r")          #'chronometrist-report)
    (define-key map (kbd "l")          #'chronometrist-open-log)
    (define-key map (kbd "G")          #'chronometrist-reset)
    (define-key map (kbd "T")          #'chronometrist-force-restart-timer)
    map)
  "Keymap used by `chronometrist-mode'.")
#+END_SRC
**** chronometrist-menu                                                         :menu:
#+BEGIN_SRC emacs-lisp
(easy-menu-define chronometrist-menu chronometrist-mode-map
  "Chronometrist mode menu."
  '("Chronometrist"
    ["Start a new task" chronometrist-add-new-task]
    ["Toggle task at point" chronometrist-toggle-task]
    ["Toggle task without running hooks" chronometrist-toggle-task-no-hooks]
    ["Discard and restart active task" chronometrist-restart-task]
    ["Discard and restart without running hooks" (chronometrist-restart-task t)
     :keys "\\[universal-argument] \\[chronometrist-restart-task]"]
    ["Extend time for last completed task" chronometrist-extend-task]
    ["Extend time without running hooks" (chronometrist-extend-task t)
     :keys "\\[universal-argument] \\[chronometrist-extend-task]"]
    ["Discard active interval" chronometrist-discard-active]
    "----"
    ["View details of today's data" chronometrist-details]
    ["View weekly report" chronometrist-report]
    ["View/edit log file" chronometrist-open-log]
    ["View/edit literate source" chronometrist-open-literate-source]
    "----"
    ["Restart timer" chronometrist-force-restart-timer]
    ["Reset state" chronometrist-reset]
    ["Import/export data" chronometrist-migrate]))
#+END_SRC
**** chronometrist-mode                                                   :major:mode:
#+BEGIN_SRC emacs-lisp
(define-derived-mode chronometrist-mode tabulated-list-mode "Chronometrist"
  "Major mode for `chronometrist'."
  (make-local-variable 'tabulated-list-format)
  (--> (chronometrist-run-transformers chronometrist-schema-transformers chronometrist-schema)
    (setq tabulated-list-format it))
  (make-local-variable 'tabulated-list-entries)
  (setq tabulated-list-entries 'chronometrist-rows)
  (make-local-variable 'tabulated-list-sort-key)
  (setq tabulated-list-sort-key '("Task" . nil))
  (tabulated-list-init-header)
  (setq revert-buffer-function #'chronometrist-refresh)
  (run-hooks 'chronometrist-mode-hook))
#+END_SRC
**** toggle-task-button                                           :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-toggle-task-button (_button)
  "Button action to toggle a task.
Argument _BUTTON is for the purpose of using this as a button
action, and is ignored."
  (when current-prefix-arg
    (chronometrist-goto-nth-task (prefix-numeric-value current-prefix-arg)))
  (let ((current  (chronometrist-current-task))
        (at-point (chronometrist-task-at-point)))
    ;; clocked in + point on current    = clock out
    ;; clocked in + point on some other task = clock out, clock in to task
    ;; clocked out = clock in
    (when current
      (chronometrist-run-functions-and-clock-out current))
    (unless (equal at-point current)
      (chronometrist-run-functions-and-clock-in at-point))))
#+END_SRC
**** add-new-task-button                                          :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-add-new-task-button (_button)
  "Button action to add a new task.
Argument _BUTTON is for the purpose of using this as a button
action, and is ignored."
  (let ((current (chronometrist-current-task)))
    (when current
      (chronometrist-run-functions-and-clock-out current))
    (let ((task (read-from-minibuffer "New task name: " nil nil nil nil nil t)))
      (chronometrist-run-functions-and-clock-in task))))
#+END_SRC
**** toggle-task                                                 :command:
#+BEGIN_SRC emacs-lisp
;; TODO - if clocked in and point not on a task, just clock out
(defun chronometrist-toggle-task (&optional prefix inhibit-hooks)
  "Start or stop the task at point.

If there is no task at point, do nothing.

With numeric prefix argument PREFIX, toggle the Nth task in
the buffer. If there is no corresponding task, do nothing.

If INHIBIT-HOOKS is non-nil, the hooks
`chronometrist-before-in-functions',
`chronometrist-after-in-functions',
`chronometrist-before-out-functions', and
`chronometrist-after-out-functions' will not be run."
  (interactive "P")
  (chronometrist-debug-message "[Command] toggle-task %s" (if inhibit-hooks "(without hooks)" ""))
  (let* ((empty-file   (chronometrist-backend-empty-p (chronometrist-active-backend)))
         (nth          (when prefix (chronometrist-goto-nth-task prefix)))
         (at-point     (chronometrist-task-at-point))
         (target       (or nth at-point))
         (current      (chronometrist-current-task))
         (in-function  (if inhibit-hooks #'chronometrist-in  #'chronometrist-run-functions-and-clock-in))
         (out-function (if inhibit-hooks #'chronometrist-out #'chronometrist-run-functions-and-clock-out)))
    ;; do not run hooks - chronometrist-add-new-task will do it
    (cond (empty-file (chronometrist-add-new-task))
          ;; What should we do if the user provides an invalid
          ;; argument? Currently - nothing.
          ((and prefix (not nth)))
          (target ;; do nothing if there's no task at point
           ;; clocked in + target is current = clock out
           ;; clocked in + target is some other task = clock out, clock in to task
           ;; clocked out = clock in
           (when current
             (funcall out-function current))
           (unless (equal target current)
             (funcall in-function target))))))
#+END_SRC

**** toggle-task-no-hooks                                        :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-toggle-task-no-hooks (&optional prefix)
  "Like `chronometrist-toggle-task', but don't run hooks.

With numeric prefix argument PREFIX, toggle the Nth task. If
there is no corresponding task, do nothing."
  (interactive "P")
  (chronometrist-toggle-task prefix t))
#+END_SRC

**** add-new-task                                                :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-add-new-task ()
  "Add a new task."
  (interactive)
  (chronometrist-debug-message "[Command] add-new-task")
  (chronometrist-add-new-task-button nil))
#+END_SRC

**** restart-task                                                :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-restart-task (&optional inhibit-hooks)
  "Change the start time of the active task to the current time.
`chronometrist-before-in-functions' and
`chronometrist-after-in-functions' are run again, unless
INHIBIT-HOOKS is non-nil or prefix argument is supplied.

Has no effect if no task is active."
  (interactive "P")
  (chronometrist-debug-message "[Command] restart-task")
  (if (chronometrist-current-task)
      (let* ((latest (chronometrist-latest-record (chronometrist-active-backend)))
             (plist  (plist-put latest :start (chronometrist-format-time-iso8601)))
             (task   (plist-get plist :name)))
        (unless inhibit-hooks
         (run-hook-with-args 'chronometrist-before-in-functions task))
        (chronometrist-replace-last (chronometrist-active-backend) plist)
        (unless inhibit-hooks
         (run-hook-with-args 'chronometrist-after-in-functions task)))
    (message "Can only restart an active task - use this when clocked in.")))
#+END_SRC

**** extend-task                                                 :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-extend-task (&optional inhibit-hooks)
  "Change the stop time of the last task to the current time.
`chronometrist-before-out-functions' and
`chronometrist-after-out-functions' are run again, unless
INHIBIT-HOOKS is non-nil or prefix argument is supplied.

Has no effect if a task is active."
  (interactive "P")
  (chronometrist-debug-message "[Command] extend-task")
  (if (chronometrist-current-task)
      (message "Cannot extend an active task - use this after clocking out.")
    (let* ((latest (chronometrist-latest-record (chronometrist-active-backend)))
           (plist  (plist-put latest :stop (chronometrist-format-time-iso8601)))
           (task   (plist-get plist :name)))
      (unless inhibit-hooks
         (run-hook-with-args-until-failure 'chronometrist-before-out-functions task))
      (chronometrist-replace-last (chronometrist-active-backend) plist)
      (unless inhibit-hooks
        (run-hook-with-args 'chronometrist-after-out-functions task)))))
#+END_SRC

**** discard-active                                              :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-discard-active ()
  "Remove active interval from the active backend."
  (interactive)
  (chronometrist-debug-message "[Command] discard-active")
  (let ((backend (chronometrist-active-backend)))
    (if (chronometrist-current-task backend)
        (chronometrist-remove-last backend)
      (message "Nothing to discard - use this when clocked in."))))
#+END_SRC

**** chronometrist                                               :command:
#+BEGIN_SRC emacs-lisp
;;;###autoload
(defun chronometrist (&optional arg)
  "Display the user's tasks and the time spent on them today.
If numeric argument ARG is 1, run `chronometrist-report'; if 2,
run `chronometrist-statistics'."
  (interactive "P")
  (chronometrist-migrate-check)
  (let* ((buffer (get-buffer-create chronometrist-buffer-name))
         (w      (save-excursion
                   (get-buffer-window chronometrist-buffer-name t)))
         (backend (chronometrist-active-backend)))
    (cond
     (arg (cl-case arg
            (1 (chronometrist-report))
            (2 (chronometrist-statistics))))
     (w (with-current-buffer buffer
          (setq chronometrist--point (point))
          (kill-buffer chronometrist-buffer-name)))
     (t (with-current-buffer buffer
          (cond ((chronometrist-backend-empty-p backend)
                 ;; database is empty
                 (chronometrist-create-file backend)
                 (let ((inhibit-read-only t))
                   (erase-buffer)
                   (insert "Welcome to Chronometrist! Hit RET to ")
                   (insert-text-button "start a new task."
                                       'action #'chronometrist-add-new-task-button
                                       'follow-link t)
                   (chronometrist-mode)
                   (switch-to-buffer buffer)))
                (t (chronometrist-mode)
                   (when chronometrist-hide-cursor
                     (make-local-variable 'cursor-type)
                     (setq cursor-type nil)
                     (hl-line-mode))
                   (switch-to-buffer buffer)
                   (when (chronometrist-memory-layer-empty-p backend)
                     (chronometrist-reset-backend backend))
                   (chronometrist-refresh)
                   (if chronometrist--point
                       (goto-char chronometrist--point)
                     (chronometrist-goto-last-task))))
          (chronometrist-setup-file-watch))))))
#+END_SRC

*** Report
**** TODO [0%]
1. [ ] preserve point when clicking buttons
**** report                                                 :custom:group:
#+BEGIN_SRC emacs-lisp
(defgroup chronometrist-report nil
  "Weekly report for the `chronometrist' time tracker."
  :group 'chronometrist)
#+END_SRC
**** buffer-name                                         :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-report-buffer-name "*Chronometrist-Report*"
  "The name of the buffer created by `chronometrist-report'."
  :type 'string)
#+END_SRC
**** ui-date                                           :internal:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-report--ui-date nil
  "The first date of the week displayed by `chronometrist-report'.
A value of nil means the current week. Otherwise, it must be a
date in the form \"YYYY-MM-DD\".")
#+END_SRC
**** ui-week-dates                                     :internal:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-report--ui-week-dates nil
  "List of dates currently displayed by `chronometrist-report'.
Each date is a list containing calendrical information (see (info \"(elisp)Time Conversion\"))")
#+END_SRC
**** point                                             :internal:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-report--point nil)
#+END_SRC
**** date-to-dates-in-week                                      :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-report-date-to-dates-in-week (first-date-in-week)
  "Return a list of dates in a week, starting from FIRST-DATE-IN-WEEK.
Each date is a ts struct (see `ts.el').

FIRST-DATE-IN-WEEK must be a ts struct representing the first date."
  (cl-loop for i from 0 to 6 collect
           (ts-adjust 'day i first-date-in-week)))
#+END_SRC
**** date-to-week-dates                                         :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-report-date-to-week-dates ()
  "Return dates in week as a list.
Each element is a ts struct (see `ts.el').

The first date is the first occurrence of
`chronometrist-report-week-start-day' before the date specified in
`chronometrist-report--ui-date' (if non-nil) or the current date."
  (->> (or chronometrist-report--ui-date (chronometrist-date-ts))
       (chronometrist-previous-week-start)
       (chronometrist-report-date-to-dates-in-week)))
#+END_SRC
**** rows                                                      :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-report-rows ()
  "Return rows to be displayed in the `chronometrist-report' buffer."
  (cl-loop
    ;; `chronometrist-report-date-to-week-dates' uses today if chronometrist-report--ui-date is nil
    with week-dates = (setq chronometrist-report--ui-week-dates
                            (chronometrist-report-date-to-week-dates))
    for task in (chronometrist-task-list) collect
    (let* ((durations        (--map (chronometrist-task-time-one-day task (chronometrist-date-ts it))
                                    week-dates))
           (duration-strings (mapcar #'chronometrist-format-duration durations))
           (total-duration   (->> (-reduce #'+ durations)
                                  (chronometrist-format-duration)
                                  (vector))))
      (list task
            (vconcat
             (vector task)
             duration-strings ;; vconcat converts lists to vectors
             total-duration)))))
#+END_SRC
**** print-keybind                                             :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-report-print-keybind (command &optional description firstonly)
  "Insert one or more keybindings for COMMAND into the current buffer.
DESCRIPTION is a description of the command.

If FIRSTONLY is non-nil, insert only the first keybinding found."
  (insert "\n    "
          (chronometrist-format-keybinds command firstonly)
          " - "
          (if description description "")))
#+END_SRC
**** CLEANUP print-non-tabular                                 :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-report-print-non-tabular ()
  "Print the non-tabular part of the buffer in `chronometrist-report'."
  (let* ((inhibit-read-only t)
         (w "\n    ")
         (ui-week-dates-ts  (mapcar #'chronometrist-date-ts chronometrist-report--ui-week-dates))
         (total-time-daily  (mapcar #'chronometrist-active-time-on
                                    ui-week-dates-ts)))
    (goto-char (point-min))
    (insert (make-string 25 ?\s))
    (insert (mapconcat (lambda (ts)
                         (ts-format "%F" ts))
                       (chronometrist-report-date-to-week-dates)
                       " "))
    (insert "\n")
    (goto-char (point-max))
    (insert w (format "%- 21s" "Total"))
    (->> (mapcar #'chronometrist-format-duration total-time-daily)
         (--map (format "% 9s  " it))
         (apply #'insert))
    (->> (-reduce #'+ total-time-daily)
         (chronometrist-format-duration)
         (format "% 13s")
         (insert))
    (insert "\n" w)
    (insert-text-button "<<" 'action #'chronometrist-report-previous-week 'follow-link t)
    (insert (format "% 4s" " "))
    (insert-text-button ">>" 'action #'chronometrist-report-next-week 'follow-link t)
    (insert "\n")
    (chronometrist-report-print-keybind 'chronometrist-report-previous-week)
    (insert-text-button "previous week" 'action #'chronometrist-report-previous-week 'follow-link t)
    (chronometrist-report-print-keybind 'chronometrist-report-next-week)
    (insert-text-button "next week" 'action #'chronometrist-report-next-week 'follow-link t)
    (chronometrist-report-print-keybind 'chronometrist-open-log)
    (insert-text-button "open log file" 'action #'chronometrist-open-log 'follow-link t)))
#+END_SRC
**** REVIEW refresh                                            :procedure:
Merge this into `chronometrist-refresh-file', while moving the -refresh call to the call site?

#+BEGIN_SRC emacs-lisp
(defun chronometrist-report-refresh (&optional _ignore-auto _noconfirm)
  "Refresh the `chronometrist-report' buffer, without re-reading `chronometrist-file'."
  (let* ((w (get-buffer-window chronometrist-report-buffer-name t))
         (p (point)))
    (with-current-buffer chronometrist-report-buffer-name
      (tabulated-list-print t nil)
      (chronometrist-report-print-non-tabular)
      (chronometrist-maybe-start-timer)
      (set-window-point w p))))
#+END_SRC
**** refresh-file                                                 :writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-report-refresh-file (fs-event)
  "Re-read `chronometrist-file' and refresh the `chronometrist-report' buffer."
  (run-hooks 'chronometrist-file-change-hook)
  (chronometrist-on-change (chronometrist-active-backend) fs-event)
  (chronometrist-report-refresh))
#+END_SRC
**** report-mode-map                                              :keymap:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-report-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map (kbd "l") #'chronometrist-open-log)
    (define-key map (kbd "b") #'chronometrist-report-previous-week)
    (define-key map (kbd "f") #'chronometrist-report-next-week)
    ;; Works when number of tasks < screen length; after that, you
    ;; probably expect mousewheel to scroll up/down, and
    ;; alt-mousewheel or something for next/previous week. For now,
    ;; I'm assuming most people won't have all that many tasks - I've
    ;; been using it for ~2 months and have 18 tasks, which are
    ;; still just half the screen on my 15" laptop. Let's see what
    ;; people say.
    (define-key map [mouse-4] #'chronometrist-report-next-week)
    (define-key map [mouse-5] #'chronometrist-report-previous-week)
    map)
  "Keymap used by `chronometrist-report-mode'.")
#+END_SRC
**** report-mode                                              :major:mode:
#+BEGIN_SRC emacs-lisp
(define-derived-mode chronometrist-report-mode tabulated-list-mode "Chronometrist-Report"
  "Major mode for `chronometrist-report'."
  (make-local-variable 'tabulated-list-format)
  (setq tabulated-list-format [("Task"   25 t)
                               ("Sunday"    10 t)
                               ("Monday"    10 t)
                               ("Tuesday"   10 t)
                               ("Wednesday" 10 t)
                               ("Thursday"  10 t)
                               ("Friday"    10 t)
                               ("Saturday"  10 t :pad-right 5)
                               ("Total"     12 t)])
  (make-local-variable 'tabulated-list-entries)
  (setq tabulated-list-entries 'chronometrist-report-rows)
  (make-local-variable 'tabulated-list-sort-key)
  (setq tabulated-list-sort-key '("Task" . nil))
  (tabulated-list-init-header)
  (chronometrist-maybe-start-timer)
  (add-hook 'chronometrist-timer-hook
            (lambda ()
              (when (get-buffer-window chronometrist-report-buffer-name)
                (chronometrist-report-refresh))))
  (setq revert-buffer-function #'chronometrist-report-refresh)
  (chronometrist-setup-file-watch))
#+END_SRC

**** chronometrist-report                                        :command:
#+BEGIN_SRC emacs-lisp
;;;###autoload
(defun chronometrist-report (&optional keep-date)
  "Display a weekly report of the data in `chronometrist-file'.
If a buffer called `chronometrist-report-buffer-name' already
exists and is visible, kill the buffer.

If KEEP-DATE is nil (the default when not supplied), set
`chronometrist-report--ui-date' to nil and display data from the
current week. Otherwise, display data from the week specified by
`chronometrist-report--ui-date'."
  (interactive)
  (chronometrist-migrate-check)
  (let ((buffer (get-buffer-create chronometrist-report-buffer-name)))
    (with-current-buffer buffer
      (cond ((and (get-buffer-window chronometrist-report-buffer-name)
                  (not keep-date))
             (setq chronometrist-report--point (point))
             (kill-buffer buffer))
            (t (unless keep-date
                 (setq chronometrist-report--ui-date nil))
               (chronometrist-create-file (chronometrist-active-backend))
               (chronometrist-report-mode)
               (switch-to-buffer buffer)
               (chronometrist-report-refresh-file nil)
               (goto-char (or chronometrist-report--point 1)))))))
#+END_SRC
**** report-previous-week                                        :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-report-previous-week (arg)
  "View the previous week's report.
With prefix argument ARG, move back ARG weeks."
  (interactive "P")
  (let ((arg (if (and arg (numberp arg))
                 (abs arg)
               1)))
    (setq chronometrist-report--ui-date
          (ts-adjust 'day (- (* arg 7))
                     (if chronometrist-report--ui-date
                         chronometrist-report--ui-date
                       (ts-now)))))
  (setq chronometrist-report--point (point))
  (kill-buffer)
  (chronometrist-report t))
#+END_SRC
**** report-next-week                                            :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-report-next-week (arg)
  "View the next week's report.
With prefix argument ARG, move forward ARG weeks."
  (interactive "P")
  (let ((arg (if (and arg (numberp arg))
                 (abs arg)
               1)))
    (setq chronometrist-report--ui-date
          (ts-adjust 'day (* arg 7)
                     (if chronometrist-report--ui-date
                         chronometrist-report--ui-date
                       (ts-now))))
    (setq chronometrist-report--point (point))
    (kill-buffer)
    (chronometrist-report t)))
#+END_SRC
*** Statistics
**** statistics                                             :custom:group:
#+BEGIN_SRC emacs-lisp
(defgroup chronometrist-statistics nil
  "Statistics buffer for the `chronometrist' time tracker."
  :group 'chronometrist)
#+END_SRC
**** buffer-name                                         :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-statistics-buffer-name "*Chronometrist-Statistics*"
  "The name of the buffer created by `chronometrist-statistics'."
  :type 'string)
#+END_SRC
**** ui-state                                          :internal:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-statistics--ui-state nil
  "Stores the display state for `chronometrist-statistics'.

This must be a plist in the form (:MODE :START :END).

:MODE is either 'week, 'month, 'year, 'full, or 'custom.

'week, 'month, and 'year mean display statistics
weekly/monthly/yearly respectively.

'full means display statistics for all available data at once.

'custom means display statistics from an arbitrary date range.

:START and :END are the start and end of the date range to be
displayed. They must be ts structs (see `ts.el').")
#+END_SRC
**** point                                             :internal:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-statistics--point nil)
#+END_SRC
**** mode-map                                                     :keymap:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-statistics-mode-map)
#+END_SRC
**** count-average-time-spent                                   :function:
#+BEGIN_SRC emacs-lisp
(cl-defun chronometrist-statistics-count-average-time-spent (task &optional (backend (chronometrist-active-backend)))
  "Return the average time the user has spent on TASK in BACKEND."
  (cl-loop with days = 0
    with events-in-day
    for date being the hash-keys of (chronometrist-backend-hash-table backend)
    when (setq events-in-day (chronometrist-task-records-for-date backend task date))
    do (cl-incf days) and
    collect
    (-reduce #'+ (chronometrist-events-to-durations events-in-day))
    into per-day-time-list
    finally return
    (if per-day-time-list
        (/ (-reduce #'+ per-day-time-list) days)
      0)))
#+END_SRC
**** rows-internal                                                :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-statistics-rows-internal (table)
  "Helper function for `chronometrist-statistics-rows'.

It simply operates on the entire hash table TABLE (see
`chronometrist-to-hash-table' for table format), so ensure that TABLE is
reduced to the desired range using
`chronometrist-events-subset'."
  (cl-loop for task in (chronometrist-task-list) collect
    (let* ((active-days    (chronometrist-statistics-count-active-days task table))
           (active-percent (cl-case (plist-get chronometrist-statistics--ui-state :mode)
                             ('week (* 100 (/ active-days 7.0)))))
           (active-percent (if (zerop active-days)
                               (format "    % 6s" "-")
                             (format "    %05.2f%%" active-percent)))
           (active-days    (format "% 5s"
                                   (if (zerop active-days)
                                       "-"
                                     active-days)))
           (average-time   (->> (chronometrist-statistics-count-average-time-spent task table)
                             (chronometrist-format-duration)
                             (format "% 5s")))
           (content        (vector task active-days active-percent average-time)))
      (list task content))))
#+END_SRC
**** TEST rows                                                    :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-statistics-rows ()
  "Return rows to be displayed in the buffer created by `chronometrist-statistics'."
  ;; We assume that all fields in `chronometrist-statistics--ui-state' are set, so they must
  ;; be changed by the view-changing functions.
  (with-slots (hash-table) (chronometrist-active-backend)
    (cl-case (plist-get chronometrist-statistics--ui-state :mode)
      ('week
       (let* ((start (plist-get chronometrist-statistics--ui-state :start))
              (end   (plist-get chronometrist-statistics--ui-state :end))
              (ht    (chronometrist-events-subset start end hash-table)))
         (chronometrist-statistics-rows-internal ht)))
      (t ;; `chronometrist-statistics--ui-state' is nil, show current week's data
       (let* ((start (chronometrist-previous-week-start (chronometrist-date-ts)))
              (end   (ts-adjust 'day 7 start))
              (ht    (chronometrist-events-subset start end hash-table)))
         (setq chronometrist-statistics--ui-state `(:mode week :start ,start :end ,end))
         (chronometrist-statistics-rows-internal ht))))))
#+END_SRC
**** print-keybind                                             :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-statistics-print-keybind (command &optional description firstonly)
  "Insert the keybindings for COMMAND.
If DESCRIPTION is non-nil, insert that too.
If FIRSTONLY is non-nil, return only the first keybinding found."
  (insert "\n    "
          (chronometrist-format-keybinds command
                             chronometrist-statistics-mode-map
                             firstonly)
          " - "
          (if description description "")))
#+END_SRC
**** print-non-tabular                                         :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-statistics-print-non-tabular ()
  "Print the non-tabular part of the buffer in `chronometrist-statistics'."
  (let ((w "\n    ")
        (inhibit-read-only t))
    (goto-char (point-max))
    (insert w)
    (insert-text-button (cl-case (plist-get chronometrist-statistics--ui-state :mode)
                          ('week "Weekly view"))
                        ;; 'action #'chronometrist-report-previous-week ;; TODO - make interactive function to accept new mode from user
                        'follow-link t)
    (insert ", from")
    (insert
     (format " %s to %s\n"
             (ts-format "%F" (plist-get chronometrist-statistics--ui-state :start))
             (ts-format "%F" (plist-get chronometrist-statistics--ui-state :end))))))
#+END_SRC
**** refresh                                                   :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-statistics-refresh (&optional _ignore-auto _noconfirm)
  "Refresh the `chronometrist-statistics' buffer.
This does not re-read `chronometrist-file'.

The optional arguments _IGNORE-AUTO and _NOCONFIRM are ignored,
and are present solely for the sake of using this function as a
value of `revert-buffer-function'."
  (let* ((w (get-buffer-window chronometrist-statistics-buffer-name t))
         (p (point)))
    (with-current-buffer chronometrist-statistics-buffer-name
      (tabulated-list-print t nil)
      (chronometrist-statistics-print-non-tabular)
      (chronometrist-maybe-start-timer)
      (set-window-point w p))))
#+END_SRC
**** mode-map                                                     :keymap:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-statistics-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map (kbd "l") #'chronometrist-open-log)
    (define-key map (kbd "b") #'chronometrist-statistics-previous-range)
    (define-key map (kbd "f") #'chronometrist-statistics-next-range)
    map)
  "Keymap used by `chronometrist-statistics-mode'.")
#+END_SRC
**** statistics-mode                                          :major:mode:
#+BEGIN_SRC emacs-lisp
(define-derived-mode chronometrist-statistics-mode tabulated-list-mode "Chronometrist-Statistics"
  "Major mode for `chronometrist-statistics'."
  (make-local-variable 'tabulated-list-format)
  (setq tabulated-list-format
        [("Task"              25 t)
         ("Active days"       12 t)
         ("%% of days active" 17 t)
         ("Average time"      12 t)
         ;; ("Current streak"    10 t)
         ;; ("Last streak"       10 t)
         ;; ("Longest streak"    10 t)
         ])
  (make-local-variable 'tabulated-list-entries)
  (setq tabulated-list-entries 'chronometrist-statistics-rows)
  (make-local-variable 'tabulated-list-sort-key)
  (setq tabulated-list-sort-key '("Task" . nil))
  (tabulated-list-init-header)
  ;; (chronometrist-maybe-start-timer)
  (add-hook 'chronometrist-timer-hook
            (lambda ()
              (when (get-buffer-window chronometrist-statistics-buffer-name)
                (chronometrist-statistics-refresh))))
  (setq revert-buffer-function #'chronometrist-statistics-refresh)
  (chronometrist-setup-file-watch))
#+END_SRC

**** chronometrist-statistics                                    :command:
#+BEGIN_SRC emacs-lisp
;;;###autoload
(defun chronometrist-statistics (&optional preserve-state)
  "Display statistics for Chronometrist data.
If a buffer called `chronometrist-statistics-buffer-name' already
exists and is visible, kill the buffer.

If PRESERVE-STATE is nil (the default when not supplied), display
data from the current week. Otherwise, display data from the week
specified by `chronometrist-statistics--ui-state'."
  (interactive)
  (chronometrist-migrate-check)
  (let* ((buffer     (get-buffer-create chronometrist-statistics-buffer-name))
         (today      (chronometrist-date-ts))
         (week-start (chronometrist-previous-week-start today))
         (week-end   (ts-adjust 'day 6 week-start)))
    (with-current-buffer buffer
      (cond ((get-buffer-window chronometrist-statistics-buffer-name)
             (kill-buffer buffer))
            (t ;; (delete-other-windows)
             (unless preserve-state
               (setq chronometrist-statistics--ui-state `(:mode week
                                         :start ,week-start
                                         :end   ,week-end)))
             (chronometrist-create-file (chronometrist-active-backend))
             (chronometrist-statistics-mode)
             (switch-to-buffer buffer)
             (chronometrist-statistics-refresh))))))
#+END_SRC
**** previous-range                                              :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-statistics-previous-range (arg)
  "View the statistics in the previous time range.
If ARG is a numeric argument, go back that many times."
  (interactive "P")
  (let* ((arg   (if (and arg (numberp arg))
                    (abs arg)
                  1))
         (start (plist-get chronometrist-statistics--ui-state :start)))
    (cl-case (plist-get chronometrist-statistics--ui-state :mode)
      ('week
       (let* ((new-start (ts-adjust 'day (- (* arg 7)) start))
              (new-end   (ts-adjust 'day +6 new-start)))
         (plist-put chronometrist-statistics--ui-state :start new-start)
         (plist-put chronometrist-statistics--ui-state :end   new-end))))
    (setq chronometrist-statistics--point (point))
    (kill-buffer)
    (chronometrist-statistics t)))
#+END_SRC
**** next-range                                                  :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-statistics-next-range (arg)
  "View the statistics in the next time range.
If ARG is a numeric argument, go forward that many times."
  (interactive "P")
  (let* ((arg   (if (and arg (numberp arg))
                    (abs arg)
                  1))
         (start (plist-get chronometrist-statistics--ui-state :start)))
    (cl-case (plist-get chronometrist-statistics--ui-state :mode)
      ('week
       (let* ((new-start (ts-adjust 'day (* arg 7) start))
              (new-end   (ts-adjust 'day 6 new-start)))
         (plist-put chronometrist-statistics--ui-state :start new-start)
         (plist-put chronometrist-statistics--ui-state :end   new-end))))
    (setq chronometrist-statistics--point (point))
    (kill-buffer)
    (chronometrist-statistics t)))
#+END_SRC
*** Details
=chronometrist= displays the total time spent on a task - but what were the details? That's where =chronometrist-details= comes in - to display details of recorded time intervals for a given day, in a format terser and more informative than the plists in the file.

1. [X] Handle active task (no =:stop=).
2. Update data with timer
3. Permit forward/backward scrolling through dates + input a specific date.
4. [X] Display key-values and tags
   * make it possible to create columns using keys
5. [X] Remove outer parentheses from tags

**** details                                                :custom:group:
#+BEGIN_SRC emacs-lisp
(defgroup chronometrist-details nil
  "Details buffer for the `chronometrist' time tracker."
  :group 'chronometrist)
#+END_SRC

**** buffer-name-base                                    :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-details-buffer-name-base "chronometrist-details"
  "Name of buffer created by `chronometrist-details'."
  :type 'string)

#+END_SRC

**** buffer-name                                                  :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-buffer-name (&optional suffix)
  "Return buffer name based on `chronometrist-details-buffer-name-base' and SUFFIX."
  (if suffix
      (format "*%s_%s*" chronometrist-details-buffer-name-base suffix)
    (format "*%s*" chronometrist-details-buffer-name-base)))

#+END_SRC
**** display-tags                                        :custom:variable:
If the value of this variable is a function and the string it returns contains a newline, the results may be undesirable...but hardly unrecoverable, so try it and see, if you wish.

#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-details-display-tags "%s"
  "How to display tags in `chronometrist-details' buffers.
Value can be
nil, meaning do not display tags, or
a format string consuming a single argument passed to `format', or
a function of one argument (the tags, as a list of symbols),
which must return the string to be displayed.

To disable display of tags, customize `chronometrist-details-schema'."
  :type '(choice nil string function))
#+END_SRC

**** display-key-values                                  :custom:variable:
If the value of this variable is a function and the string it returns contains a newline, the results may be undesirable...but hardly unrecoverable, so try it and see, if you wish.

#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-details-display-key-values "%s"
  "How to display tags in `chronometrist-details' buffers.
Value can be
nil, meaning do not display key-values, or
a format string consuming a single argument passed to `format', or
a function of one argument (the full interval plist),
which must return the string to be displayed.

To disable display of key-values, set this to nil and customize
`chronometrist-details-schema'."
  :type '(choice nil string function))
#+END_SRC

**** time-format-string                                  :custom:variable:
#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-details-time-format-string "%H:%M"
  "String specifying time format in `chronometrist-details' buffers.
See `format-time-string'."
  :type 'string)
#+END_SRC

**** FIXME schema                                        :custom:variable:
This was originally called =chronometrist-details-table-format=, but "schema" is both shorter and a term I'm more familiar with.

1. Index column does not sort correctly with 10 or more rows - see =tabulated-list-format=

#+BEGIN_SRC emacs-lisp
(defcustom chronometrist-details-schema
  [("#" 3 (lambda (row-1 row-2)
            (< (car row-1)
               (car row-2))))
   ("Task" 20 t)
   ("Tags" 20 t)
   ("Details" 45 t)
   ("Duration" 20 t :right-align t :pad-right 3)
   ("Time" 10 t)]
  "Vector specifying format of `chronometrist-details' buffer.
See `tabulated-list-format'."
  :type '(vector))
#+END_SRC

**** schema-transformers                              :extension:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-details-schema-transformers nil
  "List of functions to transform `chronometrist-details-schema' (which see).
This is passed to `chronometrist-run-transformers', which see.

Extensions adding to this list to increase the number of columns
will also need to modify the value of `tabulated-list-entries' by
using `chronometrist-details-row-transformers'.")
#+END_SRC

**** rows-helper                                                  :reader:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-rows-helper (list)
  "Return LIST as a string to be inserted in a `chronometrist-details' buffer.
LIST is either tags (a list of symbols) or a plist."
  (let (contents custom)
    (if (chronometrist-plist-p list)
        (setq custom   chronometrist-details-display-key-values
              contents (seq-remove #'keywordp
                                   (chronometrist-plist-key-values list)))
      (setq custom   chronometrist-details-display-tags
            contents list))
    (if (and contents custom)
        (pcase custom
          ((pred stringp)
           (--> (flatten-list contents)
             (seq-remove #'keywordp it)
             (mapconcat
              (lambda (elt) (format custom elt))
              it ", ")))
          ((pred functionp)
           (funcall custom list)))
      "")))
#+END_SRC

***** tests
#+BEGIN_SRC emacs-lisp :tangle chronometrist-tests.el :load test
(ert-deftest chronometrist-details-row-helper ()
  (let ((tags  '(a b c))
        (plist '(:a 1 :b 2 :c 3)))
    (let ((chronometrist-details-display-tags nil)
          (chronometrist-details-display-key-values nil))
      (should (equal (chronometrist-details-rows-helper tags)  ""))
      (should (equal (chronometrist-details-rows-helper plist) "")))
    (let ((chronometrist-details-display-tags "%s")
          (chronometrist-details-display-key-values "%s"))
      (should (equal (chronometrist-details-rows-helper nil) ""))
      (should (equal (chronometrist-details-rows-helper nil) ""))
      (should (equal (chronometrist-details-rows-helper tags)
                     "a b c"))
      (should (equal (chronometrist-details-rows-helper plist)
                     "1 2 3")))))
#+END_SRC

**** row-transformers                                 :extension:variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-details-row-transformers nil
  "List of functions to transform each row of `chronometrist-details-rows'.
This is passed to `chronometrist-run-transformers', which see.

Extensions adding to this list to increase the number of columns
will also need to modify the value of `tabulated-list-format' by
using `chronometrist-details-schema-transformers'.")
#+END_SRC

**** map                                                          :keymap:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-details-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map (kbd "s r") #'chronometrist-details-set-range)
    (define-key map (kbd "s f") #'chronometrist-details-set-filter)
    (define-key map (kbd "r") #'chronometrist-report)
    (define-key map (kbd "l") #'chronometrist-open-log)
    (define-key map (kbd "G") #'chronometrist-reset)
    map))

#+END_SRC
**** chronometrist-details-menu                                     :menu:
#+BEGIN_SRC emacs-lisp
(easy-menu-define chronometrist-details-menu chronometrist-details-mode-map
  "Menu for `chronometrist-details'."
  '("Details"
    ["Set date/time range" chronometrist-details-set-range]
    ["Set interval filter" chronometrist-details-set-filter]
    ["View weekly report" chronometrist-report]
    ["View/edit log file" chronometrist-open-log]
    ["Reset state" chronometrist-reset]))
#+END_SRC
**** chronometrist-details-mode                               :major:mode:
#+BEGIN_SRC emacs-lisp
(define-derived-mode chronometrist-details-mode tabulated-list-mode "Details"
  "Major mode for `chronometrist-details'."
  (make-local-variable 'tabulated-list-format)
  (--> (chronometrist-run-transformers chronometrist-details-schema-transformers chronometrist-details-schema)
    (setq tabulated-list-format it))
  (make-local-variable 'tabulated-list-entries)
  (setq tabulated-list-entries #'chronometrist-details-rows)
  (make-local-variable 'tabulated-list-sort-key)
  (tabulated-list-init-header)
  (run-hooks 'chronometrist-mode-hook))
#+END_SRC

**** details-setup-buffer                                      :procedure:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-setup-buffer (buffer-or-name)
  "Enable `chronometrist-details-mode' in BUFFER-OR-NAME and switch to it.
BUFFER-OR-NAME must be an existing buffer."
  (with-current-buffer buffer-or-name
    (switch-to-buffer buffer-or-name)
    (chronometrist-details-mode)
    (tabulated-list-print)))

#+END_SRC
**** chronometrist-details                                       :command:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details ()
  "Display details of time tracked over a period of time."
  (interactive)
  (let* ((buffer (get-buffer-create (chronometrist-details-buffer-name)))
         (window (save-excursion
                   (get-buffer-window buffer t))))
    (cond (window (kill-buffer buffer))
          (t (chronometrist-details-setup-buffer buffer)))))
#+END_SRC

**** range                                                      :variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-details-range nil
  "Time range for intervals displayed by `chronometrist-details'.
Values can be one of -
nil - no range. Display all intervals for today.
An ISO date string - display intervals for this date.
A cons cell in the form (BEGIN . END), where BEGIN and END are
ISO date strings (inclusive) or date-time strings (\"BEGIN\"
inclusive, \"END\" exclusive) - display intervals in this
range.")
(make-variable-buffer-local 'chronometrist-details-range)

#+END_SRC

**** iso-date-p                                                 :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-iso-date-p (string)
  "Return non-nil if STRING is a date in the ISO-8601 format."
  (string-match-p
   (rx (and string-start
            (>= 1 num) "-" (= 2 num) "-" (= 2 num)
            string-end))
   string))

#+END_SRC
**** intervals-for-range                                          :reader:
This is basically like [[#program-data-structures-events-subset][chronometrist-events-subset]], but returns a list instead of a hash table. Might replace one with the other in the future.

#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-intervals-for-range (range table)
  "Return intervals for RANGE from TABLE.
RANGE must be a time range as specified by `chronometrist-details-range'.

TABLE must be a hash table as returned by
`chronometrist-to-hash-table'."
  (pcase range
    ('nil
     (gethash (format-time-string "%F") table))
    ((pred stringp)
     (gethash range table))
    (`(,begin . ,end)
     ;; `chronometrist-iso-to-ts' also accepts ISO dates
     (let ((begin-ts (chronometrist-iso-to-ts begin))
           (end-ts   (chronometrist-iso-to-ts end)))
       (if (and (chronometrist-iso-date-p begin) (chronometrist-iso-date-p end))
           (cl-loop while (not (ts> begin-ts end-ts))
             append (gethash (ts-format "%F" begin-ts) table)
             do (ts-adjustf begin-ts 'day 1))
         (cl-loop while (not (ts> begin-ts end-ts))
           append
           (cl-loop for plist in (gethash (ts-format "%F" begin-ts) table)
             when
             (let ((start-ts (chronometrist-iso-to-ts (plist-get plist :start)))
                   (stop-ts  (chronometrist-iso-to-ts (plist-get plist :stop))))
               (and (ts>= start-ts begin-ts)
                    (ts<= stop-ts end-ts)))
             collect plist)
           do (ts-adjustf begin-ts 'day 1)))))))

;; (chronometrist-details-intervals-for-range nil chronometrist-events)
;; (chronometrist-details-intervals-for-range "2021-06-01" chronometrist-events)
;; (chronometrist-details-intervals-for-range '("2021-06-01" . "2021-06-03") chronometrist-events)
;; (chronometrist-details-intervals-for-range '("2021-06-02T01:00+05:30" . "2021-06-02T03:00+05:30") chronometrist-events)
#+END_SRC

**** input-to-value                                             :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-input-to-value (input)
  "Return INPUT as a value acceptable to `chronometrist-details-range'."
  (pcase input
    ('nil nil)
    (`(,date) date)
    (`(,begin ,end)
     (let* ((ht-keys      (hash-table-keys
                           (chronometrist-backend-hash-table (chronometrist-active-backend))))
            (date-p       (seq-find #'chronometrist-iso-date-p input))
            (begin-date   (car ht-keys))
            (begin-iso-ts (ts-format
                           "%FT%T%z" (chronometrist-iso-to-ts begin-date)))
            (end-date     (car (last ht-keys)))
            (end-iso-ts   (chronometrist-format-time-iso8601))
            (begin (if (equal begin "begin")
                       (if date-p begin-date begin-iso-ts)
                     begin))
            (end   (if (equal end "end")
                       (if date-p end-date end-iso-ts)
                     end)))
       (cons begin end)))
    (_ (error "Unsupported range %S" input))))
#+END_SRC

**** set-range                                            :command:writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-set-range ()
  "Prompt user for range for current `chronometrist-details' buffer."
  (interactive)
  (let* ((hash-table (chronometrist-backend-hash-table (chronometrist-active-backend)))
         (input (completing-read-multiple
                 (concat "Range (blank, ISO-8601 date, "
                         "or two ISO-8601 dates/timestamps): ")
                 (append '("begin" "end")
                         (reverse (hash-table-keys hash-table)))
                 nil nil (pcase chronometrist-details-range
                           ('nil nil)
                           ((pred stringp)
                            (format "%s" chronometrist-details-range))
                           (`(,begin . ,end)
                            (format "%s,%s" begin end)))
                 'chronometrist-details-range-history))
         (new-value (chronometrist-details-input-to-value input))
         (buffer-name (pcase new-value
                        (`(,begin . ,end)
                         (chronometrist-details-buffer-name (format "%s_%s" begin end)))
                        ((pred stringp)
                         (chronometrist-details-buffer-name new-value)))))
    (chronometrist-details-setup-buffer (get-buffer-create buffer-name))
    (with-current-buffer buffer-name
      (setq-local chronometrist-details-range new-value)
      (tabulated-list-revert))))
#+END_SRC

**** filter                                                     :variable:
#+BEGIN_SRC emacs-lisp
(defvar chronometrist-details-filter nil
  "Parameters to filter intervals displayed by `chronometrist-details'.
Values can be one of -
nil - no filter. Display all intervals in the given time range.
A list of keywords - display intervals containing all given keywords.
A plist - display intervals containing all given keyword-values.
A predicate of one argument (the interval plist) - display all
intervals for which the predicate returns non-nil.")
(make-variable-buffer-local 'chronometrist-details-filter)
#+END_SRC

**** filter-match-p                                             :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-filter-match-p (plist filter)
  "Return PLIST if it matches FILTER.
FILTER must be a filter specifier as described by
`chronometrist-details-filter'."
  (cond ((null filter) plist)
        ((seq-every-p #'keywordp filter)
         (when (--every-p (plist-get plist it) filter)
           plist))
        ((chronometrist-plist-p filter)
         (when (cl-loop for (keyword value) on filter by #'cddr
                 always (equal (plist-get plist keyword) value))
           plist))
        ((functionp filter)
         (when (funcall filter plist) plist))
        (t (error "Invalid filter %S" filter))))
#+END_SRC

**** set-filter                                           :command:writer:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-set-filter ()
  "Prompt user for filter for current `chronometrist-details' buffer."
  (interactive)
  (let* ((input (read-from-minibuffer
                 (concat "Filter (blank, a list of keywords, "
                         "a plist, or a predicate): ")
                 nil nil nil 'chronometrist-details-filter-history
                 (pcase chronometrist-details-filter
                   ('nil "")
                   ((pred consp) (format "%S" chronometrist-details-filter)))))
         (sexp (ignore-errors (read input))))
    (cond ((equal input "") (setq-local chronometrist-details-filter nil))
          ((consp sexp)     (setq-local chronometrist-details-filter sexp))
          (t (error "Unsupported filter %S" input)))
    (tabulated-list-revert)))
#+END_SRC

**** intervals                                                  :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-intervals (range filter backend)
  "Return plists matching RANGE and FILTER from BACKEND.
For values of RANGE, see `chronometrist-details-range'. For
values of FILTER, see `chronometrist-details-filter'. TABLE must
be a hash table as returned by `chronometrist-to-hash-table'."
  (cl-loop for plist in (chronometrist-details-intervals-for-range range (chronometrist-backend-hash-table backend))
    when (chronometrist-details-filter-match-p plist filter)
    collect plist))
#+END_SRC

**** rows                                                       :function:
#+BEGIN_SRC emacs-lisp
(defun chronometrist-details-rows ()
  "Return rows to be displayed in the `chronometrist-details' buffer.
Return value is a list as specified by `tabulated-list-entries'."
  (cl-loop with index = 1
    for plist in (chronometrist-details-intervals chronometrist-details-range chronometrist-details-filter (chronometrist-active-backend))
    collect
    (-let* (((&plist :name name :tags tags :start start :stop stop) plist)
            ;; whether tags or key-values are actually displayed is handled later
            (tags       (chronometrist-details-rows-helper tags))
            (key-values (chronometrist-details-rows-helper plist))
            ;; resetting seconds with `ts-apply' is necessary to
            ;; prevent situations like "1 hour  from 00:08 to 01:09"
            (start   (ts-apply :second 0 (chronometrist-iso-to-ts start)))
            (stop    (ts-apply :second 0 (if stop
                                             (chronometrist-iso-to-ts stop)
                                           (ts-now))))
            (interval      (floor (ts-diff stop start)))
            (index-string  (format "%s" index))
            (duration      (chronometrist-format-duration-long interval))
            (timespan (format "from %s to %s"
                              (ts-format chronometrist-details-time-format-string
                                         start)
                              (ts-format chronometrist-details-time-format-string
                                         stop))))
      (--> (vconcat (vector index-string name)
                    (when chronometrist-details-display-tags (vector tags))
                    (when chronometrist-details-display-key-values (vector key-values))
                    (vector duration timespan))
        (list index it)
        (chronometrist-run-transformers chronometrist-details-row-transformers it)))
    do (cl-incf index)))
#+END_SRC

** Provide
#+BEGIN_SRC emacs-lisp :comments no
(provide 'chronometrist)

;;; chronometrist.el ends here
#+END_SRC

** Local variables                                                :NOEXPORT:
Evaluate this to be able to insert the package prefix via =nameless-insert-name=, at the cost of having all =nameless-aliases= break (= less readable code).

#+BEGIN_SRC emacs-lisp :load no :tangle no
(setq nameless-current-name "chronometrist")
#+END_SRC