2024 day 8 part 2.

bin/day2408.ml

@@ -11,13 +11,10 @@ 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_get_pos map pos = map_get_by_idx map (map_idx_of_pos map pos)*)
 
 let map_is_valid_pos map (x, y) =
   x >= 0 && x < map.width && y >= 0 && y < map.height
 
-(*let map_is_valid_idx map idx = idx >= 0 && idx < map_length map*)
-
 let get_station_indices map =
   let arr = Array.make 256 [] in
   let rec impl idx =
@@ -30,46 +27,51 @@ let get_station_indices map =
   in
   impl 0
 
-let get_antinodes map station_idxs =
-  let rec impl2 acc (px, py) = function
+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
-        impl2 ((px - dx, py - dy) :: (hx + dx, hy + dy) :: acc) (px, py) t
-  in
-  let rec impl acc = function
+      get_antinodes1 ((px - dx, py - dy) :: (hx + dx, hy + dy) :: acc) map p t
+
+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 -> impl (impl2 acc (map_pos_of_idx map h) t) t
-  in
-  let lst = impl [] station_idxs |> List.filter (map_is_valid_pos map) in
-  let lst2 = lst |> List.map (map_idx_of_pos map) in
-  lst2
+  | h :: t ->
+      let hx, hy = map_pos_of_idx map h in
+      let dx = hx - px in
+      let dy = hy - 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
 
-let process_stations map stations = Array.map (get_antinodes map) stations
+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 string_replace str pos c =
-  assert (pos < String.length str);
-  String.sub str 0 pos ^ String.make 1 c
-  ^ String.sub str (pos + 1) (String.length str - pos - 1)
-
-let map_print map =
-  let rec impl off = if off < map_length map then impl (off + map.width) in
-  impl 0
-
-let rec add_stations map = function
-  | [] -> map
-  | h :: t -> add_stations { map with map = string_replace map.map h '#' } t
-
-let part1 map =
-  let lst =
-    get_station_indices map |> process_stations map
+let part antifn map =
+  get_station_indices map
+  |> process_stations map antifn
   |> Array.fold_left List.append []
   |> List.sort_uniq Stdlib.compare
-  in
-  let map' = add_stations map lst in
-  map_print map';
-  List.length lst
+  |> List.length
 
-let _ = Aoc.main map_of_file [ (string_of_int, part1) ]
+let _ =
+  Aoc.main map_of_file
+    [
+      (string_of_int, part get_antinodes1); (string_of_int, part get_antinodes2);
+    ]