Day 4!

AdventOfCode2024/bin/main.ml

@@ -13,7 +13,7 @@ let read_lines name =
 let () =
   print_endline "\nAdvent of Code 2024";
   let part1, part2 =
-    read_lines "./inputs/03.txt" |> AdventOfCode2024.Day_03.solve
+    read_lines "./inputs/04.txt" |> AdventOfCode2024.Day_04.solve
   in
   Printf.printf "Part 1: %s\nPart 2: %s\n" (string_of_int part1)
     (string_of_int part2)

AdventOfCode2024/lib/day_04.ml

@@ -0,0 +1,93 @@
+let check_at grid x y =
+  if x < 0 || x >= Array.length grid.(0) || y < 0 || y >= Array.length grid then
+    None
+  else Some grid.(y).(x)
+
+let get_next_char c =
+  match c with
+  | 'M' -> 'A'
+  | 'A' -> 'S'
+  | _ -> raise (Invalid_argument "cant put this here")
+
+let rec check_work_in_direction grid x y starting_char direction path =
+  let dx, dy = direction in
+  let grid_char = check_at grid (x + dx) (y + dy) in
+  match grid_char with
+  | None -> 0
+  | Some c ->
+      if c != starting_char then 0
+      else if c == 'S' then 1
+      else
+        check_work_in_direction grid (x + dx) (y + dy)
+          (get_next_char starting_char)
+          direction
+          ([ (x + dx, y + dy) ] @ path)
+
+let is_char op cc = match op with None -> false | Some c -> c == cc
+
+let check_word_part2 grid x y =
+  let grid_char = check_at grid x y in
+  match grid_char with
+  | None -> false
+  | Some 'A' ->
+      let top_left = check_at grid (x - 1) (y - 1) in
+      let top_right = check_at grid (x + 1) (y - 1) in
+      let bottom_left = check_at grid (x - 1) (y + 1) in
+      let bottom_right = check_at grid (x + 1) (y + 1) in
+
+      let is_back_diagal_mas =
+        (is_char top_left 'S' && is_char bottom_right 'M')
+        || (is_char top_left 'M' && is_char bottom_right 'S')
+      in
+
+      let is_forward_diagal_mas =
+        (is_char top_right 'S' && is_char bottom_left 'M')
+        || (is_char top_right 'M' && is_char bottom_left 'S')
+      in
+
+      is_back_diagal_mas && is_forward_diagal_mas
+  | Some _ -> false
+
+let check_word grid x y =
+  let grid_char = check_at grid x y in
+  let curried_check = check_work_in_direction grid x y 'M' in
+  match grid_char with
+  | None -> 0
+  | Some 'X' ->
+      curried_check (-1, -1) [ (x, y) ]
+      + curried_check (0, -1) [ (x, y) ]
+      + curried_check (1, -1) [ (x, y) ]
+      + curried_check (-1, 0) [ (x, y) ]
+      + curried_check (1, 0) [ (x, y) ]
+      + curried_check (-1, 1) [ (x, y) ]
+      + curried_check (0, 1) [ (x, y) ]
+      + curried_check (1, 1) [ (x, y) ]
+  | Some _ -> 0
+
+let solve lines =
+  let grid =
+    Array.init (List.length lines) (fun i ->
+        Array.init
+          (String.length (List.nth lines 0))
+          (fun j -> String.get (List.nth lines i) j))
+  in
+
+  let x_list = List.init (Array.length grid.(0)) (fun i -> i) in
+  let y_list = List.init (Array.length grid) (fun i -> i) in
+
+  let part1 =
+    List.fold_left
+      (fun acc y ->
+        acc + List.fold_left (fun acc x -> acc + check_word grid x y) 0 x_list)
+      0 y_list
+  in
+  let part2 =
+    List.fold_left
+      (fun acc y ->
+        acc
+        + List.fold_left
+            (fun acc x -> acc + if check_word_part2 grid x y then 1 else 0)
+            0 x_list)
+      0 y_list
+  in
+  (part1, part2)