1 files changed, 84 insertions, 0 deletions
diff --git a/2024/OCaml/lib/day_04/part_01.ml b/2024/OCaml/lib/day_04/part_01.ml
new file mode 100644
index 0000000..63c1cfd
--- /dev/null
+++ b/2024/OCaml/lib/day_04/part_01.ml
@@ -0,0 +1,84 @@
+(* Day 4: Part 1 *)
+
+type character = { index : int; character : char }
+type direction = N | NE | E | SE | S | SW | W | NW
+
+let all_directions = [ N; NE; E; SE; S; SW; W; NW ]
+let needle = "XMAS"
+
+(** [calculate_row_offset lines] is the offset used to move between the rows of the concatenated variant of [lines] as though it were a grid.
+    Requires: values of [lines] all have the same length *)
+let calculate_row_offset lines =
+  match lines with [] -> 0 | h :: _ -> String.length h
+
+let join_lines lines =
+  let concat_trim acc line = acc ^ String.trim line in
+  List.fold_left concat_trim "" lines
+
+let explode s = List.init (String.length s) (String.get s)
+let is_in_bounds length index = index >= 0 && index < length
+
+let get_at_index line index =
+  let length = String.length line in
+  if not (is_in_bounds length index) then "."
+  else
+    let character = String.get line index in
+    Char.escaped character
+
+let get_direction_index row_offset direction index =
+  match direction with
+  | N -> index - row_offset
+  | NE -> index - row_offset + 1
+  | E -> index + 1
+  | SE -> index + row_offset + 1
+  | S -> index + row_offset
+  | SW -> index + row_offset - 1
+  | W -> index - 1
+  | NW -> index - row_offset - 1
+
+let get_direction_string get_char get_offset_index index =
+  let first = get_char index in
+  let second_index = get_offset_index index in
+  let second = get_char second_index in
+  let third_index = get_offset_index second_index in
+  let third = get_char third_index in
+  let fourth_index = get_offset_index third_index in
+  let fourth = get_char fourth_index in
+  first ^ second ^ third ^ fourth
+
+let check_direction get_from_line get_offset_index index direction =
+  get_direction_string get_from_line (get_offset_index direction) index = needle
+
+let calculate_num_of_matches line row_offset character =
+  let get_from_line_at_index = get_at_index line in
+  let get_offset_direction_index = get_direction_index row_offset in
+  let check_direction_from_index =
+    check_direction get_from_line_at_index get_offset_direction_index
+      character.index
+  in
+  all_directions
+  |> List.map check_direction_from_index
+  |> List.map (fun x -> match x with true -> 1 | false -> 0)
+  |> List.fold_left ( + ) 0
+
+(* Tried:
+   2544 -> too high
+   1272 (previous halved) -> too low *)
+let solve lines =
+  let row_offset = calculate_row_offset lines in
+  let line = join_lines lines in
+  let chars = explode line in
+  let characters = List.mapi (fun i c -> { index = i; character = c }) chars in
+  let xs =
+    List.filter
+      (fun c -> match c.character with 'X' -> true | _ -> false)
+      characters
+  in
+  let calculate_num_of_surrounding_matches_in_line =
+    calculate_num_of_matches line row_offset
+  in
+  let num_of_matches =
+    List.map calculate_num_of_surrounding_matches_in_line xs
+    |> List.fold_left ( + ) 0
+  in
+  num_of_matches