/// 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.
module Snake.Game

open Snake.Model

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

// One animation frame: count it, and every `movesEvery score`-th frame run a game step.
let private frame (state: GameState) : GameState =
    if state.Status <> Playing then
        state
    else
        let counted = { state with 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.
let private togglePause (state: GameState) : GameState =
    match state.Status with
    | Playing -> { state with Status = Paused }
    | Paused -> { state with Status = Playing }
    | GameOver
    | Won -> state

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

let replay (state: GameState) (actions: Action list) : GameState = List.fold update state actions

// List.scan is List.fold that keeps every intermediate result (the start state included).
let history (state: GameState) (actions: Action list) : GameState list = List.scan update state actions