functional-programming-snake/fsharp/solutions/Pong/Rules.fs

/// m11 pong rules: one pure `pongUpdate`, built from small steps that each return a new piece of state.
module Snake.Pong.Rules

open Snake
open Snake.Model
open Snake.Pong.Model

let clamp (lo: int) (hi: int) (x: int) : int = min hi (max lo x)

let private clampPaddle y = clamp 0 (H - PADDLE) y

let private movePlayer (state: PongState) : int =
    clampPaddle (state.Player + (if state.Held.Down then 1 else 0) - (if state.Held.Up then 1 else 0))

// The cpu chases the middle of its paddle toward the ball, one cell every other frame.
let private moveCpu (state: PongState) (frame: int) : int =
    if frame % 2 = 0 then
        clampPaddle (state.Cpu + sign (state.Ball.Y - PADDLE / 2 - state.Cpu))
    else
        state.Cpu

let private hits (paddle: int) (y: int) = paddle <= y && y < paddle + PADDLE

type private Scorer =
    | PlayerScored
    | CpuScored

// Where the ball goes next, given the paddles' new positions, and who scored, if anyone.
let private moveBall (state: PongState) (player: int) (cpu: int) : Point * Velocity * Scorer option =
    let ball, vel = state.Ball, state.Vel
    let wall = ball.Y + vel.Dy < 0 || ball.Y + vel.Dy >= H
    let dy = if wall then -vel.Dy else vel.Dy
    let ny = ball.Y + dy
    let nx = ball.X + vel.Dx

    let bounce =
        (nx = PLAYER_X && vel.Dx < 0 && hits player ny) || (nx = CPU_X && vel.Dx > 0 && hits cpu ny)

    let scorer =
        if nx < 0 then Some CpuScored
        elif nx >= W then Some PlayerScored
        else None

    ({ X = (if bounce then ball.X else nx); Y = ny }, { Dx = (if bounce then -vel.Dx else vel.Dx); Dy = dy }, scorer)

// After a point the ball restarts in the middle, heading toward whoever conceded; the seed picks up or down.
let private serve (state: PongState) (scorer: Scorer) : PongState =
    let r, seed = Random.run state.Seed Random.nextUint32

    let score =
        match scorer with
        | PlayerScored -> { state.Score with Player = state.Score.Player + 1 }
        | CpuScored -> { state.Score with Cpu = state.Score.Cpu + 1 }

    { state with
        Ball = { X = 40; Y = 25 }
        Vel = { Dx = (if scorer = CpuScored then -1 else 1); Dy = (if r % 2u = 0u then 1 else -1) }
        Score = score
        Seed = seed
        Status = if max score.Player score.Cpu >= WIN then PongStatus.Over else PongStatus.Playing }

let private frame (state: PongState) : PongState =
    if state.Status <> PongStatus.Playing then
        state
    else
        let n = state.Frame + 1
        let player = movePlayer state
        let cpu = moveCpu state n
        let ball, vel, scorer = moveBall state player cpu

        let next =
            { state with
                Ball = ball
                Vel = vel
                Player = player
                Cpu = cpu
                Frame = n }

        match scorer with
        | Some side -> serve next side
        | None -> next

let private setHeld (state: PongState) (key: Paddle) (value: bool) : PongState =
    match key with
    | Paddle.Up -> { state with Held = { state.Held with Up = value } }
    | Paddle.Down -> { state with Held = { state.Held with Down = value } }

let pongUpdate (state: PongState) (action: PongAction) : PongState =
    match action with
    | PongAction.Press key -> setHeld state key true
    | PongAction.Release key -> setHeld state key false
    // A new match, but the keys are still where the player's fingers are.
    | PongAction.Restart -> { initialPong state.Seed with Held = state.Held }
    | PongAction.Frame -> frame state