/// m05 game: one `update` turns every action into a new state; replaying a game is a fold over its actions.
/// m10 adds pause, restart, a frame counter that makes the snake speed up as the score grows, and a turn queue.
module Snake.Game

open Snake.Model

/// What `update` does with `Turn dir` from m10 on. With no turn pending (`NextDir = Dir`) it is the m03 `Rules.turn`.
/// With a turn pending, a key that is neither `NextDir` nor its opposite is remembered in `Queued`
/// (replacing an earlier one) and taken on the step after; any other key changes nothing.
let steer (state: GameState) (dir: Direction) : GameState =
    if state.Status <> Playing then state
    elif state.NextDir = state.Dir then Rules.turn state dir
    elif dir <> state.NextDir && dir <> opposite state.NextDir then { state with Queued = Some dir }
    else state

// After a step: a game still in play takes the queued turn, if there is one. It is always a legal turn, because the
// step just made `Dir` equal to the old `NextDir`, and `Queued` is neither that nor its opposite.
let private takeQueued (state: GameState) : GameState =
    match state.Queued with
    | Some d when state.Status = Playing -> { state with NextDir = d; Queued = None }
    | _ -> state

// One game step as `update` takes it: `Spawn.tick`, then the queued turn.
let private step (state: GameState) : GameState = takeQueued (Spawn.tick state)

/// 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. `Frame` counts the frames since the snake last moved; when it reaches
// `movesEvery Score` the count starts again and the game takes a step. Counting from the last move
// (not from the start of the game) keeps every step exactly `movesEvery` frames long, also right after a speed-up.
let private frame (state: GameState) : GameState =
    if state.Status <> Playing then
        state
    else
        let n = state.Frame + 1
        if n >= movesEvery state.Score then step { state with Frame = 0 } else { state with Frame = n }

// 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 -> step state
    | Turn dir -> steer 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