Remove mutable state from day 8 of 2024.

bin/day2408.ml

@@ -10,63 +10,74 @@ let map_of_file fname =
 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_idx_of_pos map (x, y) = x + (y * 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 arr = Array.make 256 [] in
-  let rec impl idx =
-    if idx >= map_length map then arr
-    else if map_get_by_idx map idx = '.' then impl (idx + 1)
+  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 = int_of_char (map_get_by_idx map idx) in
-      arr.(station) <- idx :: arr.(station);
-      impl (idx + 1)
+      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 0
+      impl (CharMap.update station update_fn acc) (idx + 1)
+  in
+  impl CharMap.empty 0 |> CharMap.to_list
 
-let rec get_antinodes1 acc map p =
-  let px, py = map_pos_of_idx map p in
-  function
-  | [] -> acc
-  | h :: t ->
-      let hx, hy = map_pos_of_idx map h in
-      let dx = hx - px in
-      let dy = hy - py in
-      get_antinodes1 ((px - dx, py - dy) :: (hx + dx, hy + dy) :: acc) map p t
+(** 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
 
-let rec get_antinodes2 acc map p =
-  let px, py = map_pos_of_idx map p in
-  function
-  | [] -> acc
-  | h :: t ->
-      let hx, hy = map_pos_of_idx map h in
-      let dx = hx - px in
-      let dy = hy - py in
+(** 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
-      get_antinodes2 acc'' map p t
+  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 impl acc = function [] -> acc | h :: t -> impl (fn acc map h t) t in
-  Array.map
-    (fun x ->
-      impl [] x
-      |> List.filter (map_is_valid_pos map)
-      |> List.map (map_idx_of_pos map))
-    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
-  |> Array.fold_left List.append []
+  |> List.concat
+  |> List.filter (map_is_valid_pos map)
   |> List.sort_uniq Stdlib.compare
   |> List.length