feat(fsharp): m11 pong capstone (model, rules, view, app) with tests

Co-Authored-By: Claude Opus 5.5 <[email protected]>

fsharp/solutions/Pong/App.fs

@@ -0,0 +1,36 @@
+/// m11 pong app: the same wiring as App.fs (streams in, drawings out), reusing the course's modules.
+module Snake.Pong.App
+
+open Snake
+open Snake.IO
+open Snake.Stream
+open Snake.Pong.Model
+open Snake.Pong.Rules
+open Snake.Pong.View
+
+let FRAME_MS_PONG = 30L
+
+let private paddleKey (key: string) : Paddle option =
+    match key with
+    | "ArrowUp" | "w" | "W" -> Some Paddle.Up
+    | "ArrowDown" | "s" | "S" -> Some Paddle.Down
+    | _ -> None
+
+let pongKey (isDown: bool) (key: string) : PongAction option =
+    match paddleKey key with
+    | Some p -> Some(if isDown then PongAction.Press p else PongAction.Release p)
+    | None -> if isDown && (key = "r" || key = "R") then Some PongAction.Restart else None
+
+let pongActions: Stream<PongAction> =
+    Stream.merge
+        (Stream.merge
+            (Stream.interval FRAME_MS_PONG |> Stream.map (fun _ -> PongAction.Frame))
+            (Stream.keyDowns |> Stream.choose (pongKey true)))
+        (Stream.keyUps |> Stream.choose (pongKey false))
+
+let pongStates (seed: uint32) (actions: Stream<PongAction>) : Stream<PongState> =
+    actions |> Stream.scan pongUpdate (initialPong seed) |> Stream.startWith (initialPong seed)
+
+let pongMain () : IO<Stream<IO<unit>>> =
+    IO.randomSeed ()
+    |> IO.map (fun seed -> pongStates seed pongActions |> Stream.map (fun state -> IO.drawScene (pongView state)))

fsharp/solutions/Pong/Model.fs

@@ -0,0 +1,75 @@
+/// m11 pong model: the same ideas as the snake model, for a different game.
+module Snake.Pong.Model
+
+open Snake.Model
+
+/// The ball's direction of travel, one cell per frame on each axis.
+type Velocity = { Dx: int; Dy: int }
+
+/// Which of the player's two keys are being held down right now.
+type Held = { Up: bool; Down: bool }
+
+type Score = { Player: int; Cpu: int }
+
+// RequireQualifiedAccess: write `PongStatus.Playing`, `Paddle.Up`, `PongAction.Frame`, so these cases never get
+// mixed up with the snake's `Playing`, `Up` and `Frame`.
+[<RequireQualifiedAccess>]
+type PongStatus =
+    | Playing
+    | Over
+
+/// The two keys that move the player's paddle.
+[<RequireQualifiedAccess>]
+type Paddle =
+    | Up
+    | Down
+
+/// A whole match. Positions are in cells: `Ball` is a cell of the 80x50 court; `Player` and `Cpu` are the y of the
+/// top cell of each paddle. `Seed` is the unsigned 32-bit random seed used to pick the direction of each serve.
+type PongState =
+    { Ball: Point
+      Vel: Velocity
+      Player: int
+      Cpu: int
+      Held: Held
+      Score: Score
+      Status: PongStatus
+      Seed: uint32
+      Frame: int }
+
+/// Everything that can happen in a match, as data.
+[<RequireQualifiedAccess>]
+type PongAction =
+    | Press of Paddle
+    | Release of Paddle
+    | Frame
+    | Restart
+
+// The court, in cells.
+let W = 80
+let H = 50
+
+/// Pixels per cell.
+let PCELL = 8
+
+/// Paddle height in cells.
+let PADDLE = 8
+
+/// Column of the player's paddle (left) and of the cpu's paddle (right).
+let PLAYER_X = 2
+let CPU_X = 77
+
+/// Points needed to win the match.
+let WIN = 5
+
+let initialPong (seed: uint32) : PongState =
+    { Ball = { X = 40; Y = 25 }
+      Vel = { Dx = 1; Dy = 1 }
+      Player = 21
+      Cpu = 21
+      Held = { Up = false; Down = false }
+      // `Score.Player` says which record is meant: PongState has Player and Cpu fields too.
+      Score = { Score.Player = 0; Cpu = 0 }
+      Status = PongStatus.Playing
+      Seed = seed
+      Frame = 0 }

fsharp/solutions/Pong/Rules.fs

@@ -0,0 +1,93 @@
+/// 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
+    | PongAction.Restart -> initialPong state.Seed
+    | PongAction.Frame -> frame state

fsharp/solutions/Pong/View.fs

@@ -0,0 +1,24 @@
+/// m11 pong view: the same Scene shapes as the snake view, so the same drawScene and runtime draw it.
+module Snake.Pong.View
+
+open Snake.View
+open Snake.Pong.Model
+
+let private text x y str size = Text(x, y, str, "#ffffff", size, "center")
+
+let private paddle column y = Rect(column * PCELL, y * PCELL, PCELL, PADDLE * PCELL, "#ffffff")
+
+let private overlay (state: PongState) : Scene =
+    match state.Status with
+    | PongStatus.Over ->
+        [ text 320 200 (if state.Score.Player > state.Score.Cpu then "You Win!" else "CPU Wins") 32
+          text 320 230 "Press R to restart" 16 ]
+    | PongStatus.Playing -> []
+
+let pongView (state: PongState) : Scene =
+    [ Rect(0, 0, 640, 400, "#111111")
+      paddle PLAYER_X state.Player
+      paddle CPU_X state.Cpu
+      Rect(state.Ball.X * PCELL, state.Ball.Y * PCELL, PCELL, PCELL, "#ffeb3b")
+      text 320 30 $"{state.Score.Player}   {state.Score.Cpu}" 24 ]
+    @ overlay state

fsharp/solutions/Snake.fsproj

@@ -18,5 +18,9 @@
     <Compile Include="Input.fs" />
     <Compile Include="App.fs" />
     <Compile Include="Runtime.fs" />
+    <Compile Include="Pong/Model.fs" />
+    <Compile Include="Pong/Rules.fs" />
+    <Compile Include="Pong/View.fs" />
+    <Compile Include="Pong/App.fs" />
   </ItemGroup>
 </Project>