package snake

/**
 * m05 game: one `update` turns every action into a new state; replaying a game is a fold over its actions.
 * m10 adds pause, restart and a frame clock that speeds up as the score grows.
 */
object Game:

  /** How many frames pass between game steps: 8 at first, one fewer every 3 points, never fewer than 2. */
  def movesEvery(score: Int): Int = math.max(2, 8 - score / 3)

  // One animation frame: count it, and every `movesEvery(score)`-th frame run a game step.
  private def frame(state: GameState): GameState =
    if state.status != Status.Playing then state
    else
      val counted = state.copy(frame = state.frame + 1)
      if counted.frame % movesEvery(counted.score) == 0 then Spawn.tick(counted) else counted

  // Pause and resume only make sense in those two statuses; a finished game stays finished.
  private def togglePause(state: GameState): GameState = state.status match
    case Status.Playing               => state.copy(status = Status.Paused)
    case Status.Paused                => state.copy(status = Status.Playing)
    case Status.GameOver | Status.Won => state

  // Every case is listed (no `case _`): adding an Action later makes the compiler point here.
  def update(state: GameState, action: Action): GameState = action match
    case Action.Tick        => Spawn.tick(state)
    case Action.Turn(dir)   => Rules.turn(state, dir)
    case Action.TogglePause => togglePause(state)
    case Action.Restart     => Spawn.newGame(state.seed)
    case Action.Frame       => frame(state)

  def replay(state: GameState, actions: List[Action]): GameState = actions.foldLeft(state)(update)

  // scanLeft is foldLeft that keeps every intermediate result.
  def history(state: GameState, actions: List[Action]): List[GameState] = actions.scanLeft(state)(update)