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)