Remove mutable state from day 8 of 2024.

bin/day2408.ml

@@ -0,0 +1,88 @@
+type map = { map : string; width : int; height : int }
+
+let map_of_file fname =
+  let strs = Aoc.strings_of_file fname in
+  let width = String.length (List.hd strs) in
+  let map = List.fold_left ( ^ ) "" strs in
+  let height = String.length map / width in
+  { map; width; height }
+
+let map_length map = String.length map.map
+let map_get_by_idx map idx = map.map.[idx]
+let map_pos_of_idx map idx = (idx mod map.width, idx / map.width)
+
+let map_is_valid_pos map (x, y) =
+  x >= 0 && x < map.width && y >= 0 && y < map.height
+
+module CharMap = Map.Make (Char)
+
+(** [get_station_indices map] returns a list of pairs mapping station ID to the
+    indices in [map] where there is a station with that ID. *)
+let get_station_indices map =
+  let rec impl acc idx =
+    if idx >= map_length map then acc
+    else if map_get_by_idx map idx = '.' then impl acc (idx + 1)
+    else
+      let station = map_get_by_idx map idx in
+      let update_fn lst =
+        match lst with None -> Some [ idx ] | Some t -> Some (idx :: t)
+      in
+      impl (CharMap.update station update_fn acc) (idx + 1)
+  in
+  impl CharMap.empty 0 |> CharMap.to_list
+
+(** Generate antinodes for part 1. *)
+let get_antinodes1 acc _ (px, py) (px', py') =
+  let dx = px' - px in
+  let dy = py' - py in
+  (px - dx, py - dy) :: (px' + dx, py' + dy) :: acc
+
+(** [add_antinodes lst map pos vel] adds antinodes at [pos + n * vel] to [lst]
+    for all non-negative [n] that are valid positions in [map]. *)
+let rec add_antinodes lst map (x, y) (dx, dy) =
+  if map_is_valid_pos map (x, y) then
+    add_antinodes ((x, y) :: lst) map (x + dx, y + dy) (dx, dy)
+  else lst
+
+(** Generate antinodes for part 2. *)
+let get_antinodes2 acc map (px, py) (px', py') =
+  let dx = px' - px in
+  let dy = py' - py in
+  let acc' = add_antinodes acc map (px, py) (dx, dy) in
+  let acc'' = add_antinodes acc' map (px, py) (-dx, -dy) in
+  acc''
+
+(** [process_stations map fn stations] generates a list of all antinodes for the
+    stations in [stations] on the map [map]. [fn acc map p p'] is called to
+    generate the antinode list for each pair of stations [p] and [p']. It should
+    add the positions of antinodes to the list [acc]. *)
+let process_stations map fn stations =
+  let rec impl2 acc p t =
+    match t with
+    | [] -> acc
+    | h :: t -> impl2 (fn acc map p (map_pos_of_idx map h)) p t
+  in
+  let rec impl acc = function
+    | [] -> acc
+    | h :: t -> impl (impl2 acc (map_pos_of_idx map h) t) t
+  in
+  List.map (impl []) stations
+
+(** [part antifn map station_indices] process all the stations in
+    station_indices calling [antifn acc map p p'] on all stations. Here [acc] is
+    a list of antinodes which [antifn] should update and return, [p] and [p']
+    are positions of stations to generate antinodes for. *)
+let part antifn map =
+  get_station_indices map
+  |> List.map snd (* we do not care about the station IDs *)
+  |> process_stations map antifn
+  |> List.concat
+  |> List.filter (map_is_valid_pos map)
+  |> List.sort_uniq Stdlib.compare
+  |> List.length
+
+let _ =
+  Aoc.main map_of_file
+    [
+      (string_of_int, part get_antinodes1); (string_of_int, part get_antinodes2);
+    ]

bin/dune

@@ -1,4 +1,12 @@
 (executables
- (public_names day2401 day2402 day2403 day2404 day2405 day2406 day2407)
- (names day2401 day2402 day2403 day2404 day2405 day2406 day2407)
+ (public_names
+  day2401
+  day2402
+  day2403
+  day2404
+  day2405
+  day2406
+  day2407
+  day2408)
+ (names day2401 day2402 day2403 day2404 day2405 day2406 day2407 day2408)
  (libraries str aoc))