type dir =
  | Left
  | Right
  | Down
  | Up

type background =
  | Grass
  | Water
  | Black

type position = int * int * dir

type t =
  { tiles : background array array
  ; width : int
  ; height : int
  }

let init () =
  let width = 1000 in
  let height = 1000 in
  let tiles =
    Array.init width (fun _x ->
        Array.init height (fun _y ->
            if Random.int 1000 <= 42 then Water else Grass ) )
  in
  { tiles; width; height }

let get_tile_kind ~x ~y map =
  try map.tiles.(x).(y) with Invalid_argument _ -> Black

let check_move map entity_pos dir =
  let x, y, current_dir = entity_pos in
  let x, y =
    if current_dir <> dir then (x, y)
    else
      match dir with
      | Left -> (x - 1, y)
      | Right -> (x + 1, y)
      | Down -> (x, y + 1)
      | Up -> (x, y - 1)
  in
  match get_tile_kind ~x ~y map with
  | Black | Water -> Error "invalid terrain"
  | Grass -> Ok (x, y, dir)