feat(scala): Scala.js browser platform, snake and pong pages, build-web.sh

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

scala/web/BrowserPlatform.scala

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+package snake
+package web
+
+import scala.scalajs.js
+import scala.scalajs.js.Dynamic.global
+import scala.scalajs.js.typedarray.Uint32Array
+
+/**
+ * Browser glue: turns a canvas into a Platform. Impure by nature; not part of the exercises' logic.
+ * `js.Dynamic` lets us call browser APIs by name without type facades (and without extra libraries).
+ */
+final class BrowserPlatform(canvas: js.Dynamic) extends Platform:
+  private val ctx = canvas.getContext("2d")
+  private val window = global.window
+
+  // Keys that scroll the page should only steer the game.
+  private val prevent = Set("ArrowUp", "ArrowDown", "ArrowLeft", "ArrowRight", " ")
+
+  private def listen(kind: String, cb: String => Unit): Unit =
+    val handler: js.Function1[js.Dynamic, Unit] = e =>
+      val key = e.key.asInstanceOf[String]
+      if prevent(key) then e.preventDefault()
+      cb(key)
+    window.addEventListener(kind, handler)
+
+  def fillRect(x: Int, y: Int, w: Int, h: Int, color: String): Unit =
+    ctx.fillStyle = color
+    ctx.fillRect(x, y, w, h)
+
+  def fillText(x: Int, y: Int, text: String, color: String, size: Int, align: String): Unit =
+    ctx.fillStyle = color
+    ctx.font = s"${size}px monospace"
+    ctx.textAlign = align
+    ctx.fillText(text, x, y)
+
+  def randomSeed(): Long =
+    val buf = new Uint32Array(1)
+    global.crypto.getRandomValues(buf)
+    buf(0).toLong
+
+  // t is the number of milliseconds since the first animation frame.
+  def startClock(onTime: Long => Unit): Unit =
+    def frame(start: Option[Double]): js.Function1[Double, Unit] = now =>
+      val t0 = start.getOrElse(now)
+      onTime((now - t0).toLong)
+      window.requestAnimationFrame(frame(Some(t0)))
+    window.requestAnimationFrame(frame(None))
+
+  def onKeyDown(cb: String => Unit): Unit = listen("keydown", cb)
+  def onKeyUp(cb: String => Unit): Unit = listen("keyup", cb)

scala/web/Main.scala

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+package snake
+package web
+
+import scala.scalajs.js.Dynamic.global
+
+// Two programs, one runtime: build-web.sh picks the entry point with --main-class.
+
+private def canvas() = global.document.getElementById("game")
+
+/** scala/index.html: Snake. */
+object SnakeMain:
+  def main(args: Array[String]): Unit = Runtime.runApp(App.main, BrowserPlatform(canvas()))
+
+/** scala/pong.html: Pong (m11). */
+object PongMain:
+  def main(args: Array[String]): Unit = Runtime.runApp(pong.PongApp.pongMain, BrowserPlatform(canvas()))