bugfix: don't query network while panning around

I'm taking this opportunity to significantly clean up the data flow.
It's easy to forget quite what A and B are supposed to do, and so to
pile on additional responsibilities on them:
  * A rebuilds Surface
  * B updates just a few attributes within Surface without structurally
    modifying it

In particular, A is not intended to modify the _inputs_ that go into
rebuilding Surface, in this app Nodes.

Perhaps I shouldn't call it A, but M or something to leave lots of space
for phases before A. For now, hopefully a comment will suffice.

3 major code paths:
  - on.initialize as well as C-v
    initialize Input_filename (v11) or Url (v12)
    initialize Nodes and Root
    initialize Cursor_node
    A()
    ensure cursor in viewport
  - on pan:
    A()
    In particular, don't touch Cursor_node

0009-on.code_change

@@ -1,4 +1,7 @@
 on.code_change = function()
 	print('code changed')
+	load_nodes_from_url()
+	set_cursor(ml_from_url(Url).id)
 	A()  -- just in case we edited something
+	ensure_cursor_node_within_viewport()
 end

0012-on.initialize

@@ -4,7 +4,10 @@ on.initialize = function(arg)
 		-- commandline arg is a url
 		if #arg > 0 then
 			Url = arg[1]
+			load_nodes_from_url()
+			set_cursor(ml_from_url(Url).id)
 			A()
+			ensure_cursor_node_within_viewport()
 		end
 	else
 		-- LÖVE pre-v12 doesn't have https
@@ -15,14 +18,12 @@ on.initialize = function(arg)
 			error('When running with LÖVE v11, please pass in a file containing a Mastodon thread, and optionally an id within it to start out focused on.\n\nThis app currently needs to be invoked from a terminal.')
 		end
 		Input_filename = arg[1]
-		A()
+		load_nodes_from_input_file()
 		Cursor_node = Root
 		if #arg >= 2 then
-			local initial_id = arg[2]
-			if Nodes[initial_id] then
-				Cursor_node = Nodes[initial_id]
-			end
+			set_cursor(arg[2])
 		end
+		A()
 		ensure_cursor_node_within_viewport()
 	end
 end

0023-on.keychord_press

@@ -24,7 +24,10 @@ on.keychord_press = function(chord, key)
 			if cb:match('^%s*https://[^%s]*%s*$') then
 				print('clipboard contains a URL')
 				Url = trim(cb)
+				load_nodes_from_url()
+				set_cursor(ml_from_url(Url).id)
 				A()
+				ensure_cursor_node_within_viewport()
 			end
 		end
 	elseif chord == 'C-up' then

0028-A

@@ -1,34 +1,6 @@
 A = function()
-	print('A')
-	if Major_version >= 12 then
-		-- reload Url from network
-		if Url then
-			-- indicate some progress
-			-- load can take some time, and it's synchronous, so we'll draw to screen right here
-			love.graphics.clear(love.graphics.getBackgroundColor())
-			love.graphics.print('loading '..Url..'...', 5,5)
-			love.graphics.present()
-			local initial_ml = ml_from_url(Url)
-			local thread_data = try_load_nodes_from_url(initial_ml)
-			render_thread_to_surface(thread_data)
-			Cursor_node = Root
-			if Nodes[initial_ml.id] then
-				Cursor_node = Nodes[initial_ml.id]
-			end
-			ensure_cursor_node_within_viewport()
-		end
-		B()
-	else
-		-- LÖVE pre-v12 has no https
-		-- reload same thread from file
-		if Input_filename then
-			love.graphics.setFont(love.graphics.newFont(scale(20)))  -- editor objects implicitly depend on current font
-			local f = io.open(Input_filename)
-			assert(f)
-			local thread_data = json.decode(f:read('*a'))
-			f:close()
-			render_thread_to_surface(thread_data)
-		end
-		B()
-	end
+	-- load Nodes to Surface
+	love.graphics.setFont(love.graphics.newFont(scale(20)))  -- editor objects implicitly depend on current font
+	render_nodes_to_surface()
+	B()
 end

0039-ensure_cursor_node_within_viewport

@@ -25,4 +25,4 @@ ensure_cursor_node_within_viewport = function()
 	if dirty then
 		B()
 	end
-end
+end

0043-render_nodes_to_surface

@@ -1,5 +1,4 @@
-render_thread_to_surface = function(thread_data)
-	-- side effects: Nodes, Root
+render_nodes_to_surface = function()
 	-- design constraints:
 	--   trees only, no graphs or DAGs
 	--   parents above children
@@ -12,33 +11,6 @@ render_thread_to_surface = function(thread_data)
 	Surface = {}
 	-- we're going to be computing box heights
 	love.graphics.setFont(love.graphics.newFont(scale(20)))
-	-- compute mapping from ids to nodes
-	Nodes = {}  -- id to object
-	Nodes[thread_data.id] = thread_data
-	for _,x in ipairs(thread_data.descendants) do
-		Nodes[x.id] = x
-	end
-	Root = thread_data
-	-- compute children
-	for _,x in pairs(Nodes) do
-		parent_id = x.in_reply_to_id
-		if x.id ~= Root.id then
-			assert(parent_id)
-			local parent = Nodes[parent_id]
-			if parent then  -- watch out for private posts
-				if parent.children == nil then
-					parent.children = {}
-				end
-				table.insert(parent.children, x.id)
-			end
-		end
-	end
-	-- sort children by time
-	for _,x in pairs(Nodes) do
-		if x.children then
-			table.sort(x.children)
-		end
-	end
 	-- compute number of tracks needed
 	for _,x in pairs(Nodes) do
 		if x.ntracks == nil then
@@ -48,4 +20,4 @@ render_thread_to_surface = function(thread_data)
 	-- prepare the tracks
 	-- each track is 600px + 20px of gutter between nodes
 	render_node_and_descendants(Root.id, --[[y]] 0, --[[xlo]] 0, --[[xhi]] 620 * Root.ntracks)
-end
+end

0049-load_nodes_from_url

@@ -0,0 +1,19 @@
+load_nodes_from_url = function()
+	-- side effects: Nodes, Root, Cursor_node
+	-- indicate some progress
+	-- load can take some time, and it's synchronous, so we'll draw to screen right here
+	love.graphics.clear(love.graphics.getBackgroundColor())
+	love.graphics.print('loading '..Url..'...', 5,5)
+	love.graphics.present()
+	-- load JSON from Url
+	local initial_ml = ml_from_url(Url)
+	local thread = get_toot(initial_ml)
+	if thread.ancestors and #thread.ancestors > 0 then
+		-- redo with oldest ancestor
+		love.graphics.clear(love.graphics.getBackgroundColor())
+		love.graphics.print('reloading from earliest ancestor '..thread.ancestors[1].url..'...', 5,5)
+		love.graphics.present()
+		thread = get_toot(ml_from_url(thread.ancestors[1].url))
+	end
+	load_nodes_from_json(thread, initial_ml.id)
+end

0049-try_load_nodes_from_url

@@ -1,11 +0,0 @@
-try_load_nodes_from_url = function(initial_ml)
-	local result = get_toot(initial_ml)
-	if result.ancestors and #result.ancestors > 0 then
-		-- redo with oldest ancestor
-		love.graphics.clear(love.graphics.getBackgroundColor())
-		love.graphics.print('reloading from earliest ancestor '..result.ancestors[1].url..'...', 5,5)
-		love.graphics.present()
-		result = get_toot(ml_from_url(result.ancestors[1].url))
-	end
-	return result
-end

0053-load_nodes_from_input_file

@@ -0,0 +1,9 @@
+load_nodes_from_input_file = function()
+	-- side effects: Nodes, Root, Cursor_node
+	-- load JSON from Input_filename
+	local f = io.open(Input_filename)
+	assert(f)
+	local thread = json.decode(f:read('*a'))
+	f:close()
+	load_nodes_from_json(thread)
+end

0054-load_nodes_from_json

@@ -0,0 +1,31 @@
+load_nodes_from_json = function(thread_data, cursor_id)
+	-- massage JSON into a table from id to node
+	Nodes = {}  -- id to object
+	Nodes[thread_data.id] = thread_data
+	for _,x in ipairs(thread_data.descendants) do
+		Nodes[x.id] = x
+	end
+	-- initialize Root
+	Root = thread_data
+	-- compute children
+	for _,x in pairs(Nodes) do
+		parent_id = x.in_reply_to_id
+		if x.id ~= Root.id then
+			assert(parent_id)
+			local parent = Nodes[parent_id]
+			if parent then  -- watch out for private posts
+				if parent.children == nil then
+					parent.children = {}
+				end
+				table.insert(parent.children, x.id)
+			end
+		end
+	end
+	-- sort children by time
+	for _,x in pairs(Nodes) do
+		if x.children then
+			table.sort(x.children)
+		end
+	end
+end
+

0055-set_cursor

@@ -0,0 +1,7 @@
+set_cursor = function(cursor_id)
+	-- position Cursor on the specific toot at Url
+	Cursor_node = Root
+	if Nodes[cursor_id] then
+		Cursor_node = Nodes[cursor_id]
+	end
+end

Manual_tests.md

@@ -8,6 +8,13 @@ record those here.
   - from defaults
     - default_map absent/present
 
+  - run app using LÖVE v11 and `lua unfurl.lua` shim. Ensure cursor is on correct node and visible.
+  - run app using LÖVE v12. Ensure cursor is on correct node and visible.
+    Paste in a new thread using `ctrl+v`. Ensure cursor is on correct node and visible.
+  - pan around. Ensure cursor node can go out of viewport.
+  - Press `ctrl`+arrow keys. Ensure cursor moves to correct node and becomes
+    visible.
+
 * How the screen looks. Our tests use a level of indirection to check text and
   graphics printed to screen, but not the precise pixels they translate to.
     - where exactly the cursor is drawn to highlight a given character