Day 14 in OCaml

14/dune

@@ -0,0 +1,2 @@
+(executable
+  (name solution))

14/dune-project

@@ -0,0 +1 @@
+(lang dune 2.9)

14/example_input

@@ -0,0 +1,18 @@
+NNCB
+
+CH -> B
+HH -> N
+CB -> H
+NH -> C
+HB -> C
+HC -> B
+HN -> C
+NN -> C
+BH -> H
+NC -> B
+NB -> B
+BN -> B
+BB -> N
+BC -> B
+CC -> N
+CN -> C

14/solution.ml

@@ -0,0 +1,68 @@
+let read_lines path =
+  let lines = ref [] in
+  let chan = open_in path in
+  try
+    while true; do
+      lines := input_line chan :: !lines
+    done; !lines
+  with End_of_file ->
+    close_in chan;
+    List.rev !lines
+
+let h_mod tbl k f default =
+  let new_value =
+    if Hashtbl.mem tbl k
+    then Hashtbl.find tbl k |> f
+    else default
+  in Hashtbl.replace tbl k new_value
+
+let parse input =
+  let template = List.hd input in
+  let mappings = input |> List.tl |> List.tl in
+  let counts = Hashtbl.create (List.length mappings) in
+  let table = Hashtbl.create (List.length mappings) in
+  let h_template = Hashtbl.create (String.length template) in
+  let add_mapping line =
+    Hashtbl.add table (line.[0], line.[1]) line.[6]
+  in
+  for i = 0 to (String.length template) - 2 do
+    h_mod h_template (template.[i], template.[i+1]) ((+) 1) 1;
+  done;
+  String.iter (fun c -> h_mod counts c ((+) 1) 1) template;
+  List.iter add_mapping mappings;
+  (h_template, counts, table)
+
+let step pairs counts mappings =
+  let tmp_pairs = Hashtbl.copy pairs in
+  let expand_pair pair count =
+    let (a, b) = pair in
+    let next = Hashtbl.find mappings pair in
+    h_mod counts next ((+) count) count;
+    h_mod pairs (a, next) ((+) count) count;
+    h_mod pairs (next, b) ((+) count) count;
+    h_mod pairs pair (fun x -> x - count) 0;
+  in Hashtbl.iter expand_pair tmp_pairs
+
+let rec times n f =
+  if n > 0
+  then (f (); times (n - 1) f)
+
+let list_agg f l =
+  List.fold_left f (List.hd l) l
+
+let solution steps template counts mappings =
+  let counts = Hashtbl.copy counts in
+  let template = Hashtbl.copy template in
+  times steps (fun () -> step template counts mappings);
+  let value_counts = counts |> Hashtbl.to_seq_values |> List.of_seq in
+  let highest = list_agg max value_counts in
+  let lowest = list_agg min value_counts in
+  highest - lowest
+
+let part1 = solution 10
+let part2 = solution 40
+
+let input = read_lines Sys.argv.(1)
+let (h_template, counts, mappings) = parse input
+let () = Printf.printf "Part 1: %d\n" (part1 h_template counts mappings)
+let () = Printf.printf "Part 2: %d\n" (part2 h_template counts mappings)