package snake

/** m03 rules: the pure game rules. Each function takes a state and returns a new one. */
object Rules:

  def move(p: Point, dir: Direction): Point =
    val d = Model.delta(dir)
    Point(p.x + d.x, p.y + d.y)

  def inBounds(state: GameState, p: Point): Boolean =
    p.x >= 0 && p.x < state.width && p.y >= 0 && p.y < state.height

  // Validate against `dir` (last moved), not `nextDir`: Up then Left within one tick must not fold the snake back on itself.
  def turn(state: GameState, dir: Direction): GameState =
    if state.status == Status.Playing && dir != Model.opposite(state.dir) then state.copy(nextDir = dir)
    else state

  def advance(state: GameState): GameState =
    if state.status != Status.Playing then state
    else
      val d = state.nextDir
      val head = move(state.snake.head, d)
      val eating = state.apple.contains(head)
      // When not eating, the tail leaves its cell this step, so the head may move into it.
      val body = if eating then state.snake else Util.dropLast(state.snake)
      if !inBounds(state, head) || Util.contains(body, head, _ == _) then
        state.copy(dir = d, status = Status.GameOver)
      else
        state.copy(
          snake = head :: body,
          dir = d,
          apple = if eating then None else state.apple,
          score = state.score + (if eating then 1 else 0)
        )