06속도 값만큼 길어지는 공의 잔상

<main class="stage">
<div class="vd is-demo" id="vdRoot">
  <canvas class="vd__canvas" id="vdCanvas" width="480" height="300"></canvas>
  <p class="vd__caption">속도 값만큼 길어지는 공의 잔상</p>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;
@include stage($stage-yellow);

.vd {
  position: relative;
  width: min(92vw, calc(160vh - 12.8vw));
  aspect-ratio: 480 / 300;
  margin: 0 auto;
  border-radius: $r-card;
  overflow: hidden;
  box-shadow: $shadow-raised;
}

.vd__canvas {
  display: block;
  width: 100%;
  height: 100%;
}

.vd__caption {
  position: absolute;
  left: $sp-3;
  bottom: $sp-3;
  max-width: 72%;
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(255, 247, 230, .85);
  color: $subject-ink;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  pointer-events: none;
}

@media (max-width: 340px) {
  .vd__caption { font-size: 10px; padding: $sp-1 $sp-2; }
}

@media (prefers-reduced-motion: reduce) {
  .vd { cursor: default; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.vd {
  position: relative;
  width: min(92vw, 160vh - 12.8vw);
  aspect-ratio: 480/300;
  margin: 0 auto;
  border-radius: 16px;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.vd__canvas {
  display: block;
  width: 100%;
  height: 100%;
}

.vd__caption {
  position: absolute;
  left: 12px;
  bottom: 12px;
  max-width: 72%;
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(255, 247, 230, 0.85);
  color: #17141a;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  pointer-events: none;
}

@media (max-width: 340px) {
  .vd__caption {
    font-size: 10px;
    padding: 4px 8px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .vd {
    cursor: default;
  }
}

(function () {
  var root = document.getElementById('vdRoot');
  var canvas = document.getElementById('vdCanvas');
  var ctx = canvas.getContext('2d');
  var wrap = canvas.parentElement;

  var LW = 320, LH = 200, WALL = 6;
  var PADDLE_H = 8, PADDLE_Y = LH - 18, PADDLE_W = 46;
  var BALL_R = 4;
  var BRICK_COLS = 6, BRICK_ROWS = 3, BRICK_W = 44, BRICK_H = 12, BRICK_GAP = 4, BRICK_TOP = 22;
  var BASE_SPEED = 130;
  var BIAS_PERIOD = 560;
  var STAGE_BG = '#17141a';
  var BRICK_ROW_COLORS = ['#2f6df6', '#ff9ebb', '#4cd4a6'];
  var PADDLE_COLOR = '#fff7e6';
  var BALL_COLOR = '#fff7e6';
  var FLASH_LIFE = 0.4, FLASH_MAX_R = 130;

  function seedRand(seed) {
    var s = seed >>> 0;
    return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
  }
  var rand = seedRand(41006);

  function clamp(v, a, b) { return v < a ? a : (v > b ? b : v); }
  function aabb(ax, ay, aw, ah, bx, by, bw, bh) { return ax < bx + bw && ax + aw > bx && ay < by + bh && ay + ah > by; }

  var PADDLE_LERP = 0.32;
  var MAX_REFLECT_ANGLE = Math.PI / 180 * 48;
  var SPEED_STEP = 14;
  var SPEED_MAX = 210;

  function buildBricks() {
    var totalW = BRICK_COLS * BRICK_W + (BRICK_COLS - 1) * BRICK_GAP;
    var startX = (LW - totalW) / 2;
    var arr = [];
    for (var r = 0; r < BRICK_ROWS; r += 1) {
      for (var c = 0; c < BRICK_COLS; c += 1) {
        arr.push({ x: startX + c * (BRICK_W + BRICK_GAP), y: BRICK_TOP + r * (BRICK_H + BRICK_GAP), w: BRICK_W, h: BRICK_H, alive: true, hits: 0 });
      }
    }
    return arr;
  }
  var bricks = buildBricks();
  var cleared = 0;
  var trail = []; var TRAIL_MAX = 16;

  function rebuildLevel() {
    bricks = buildBricks();
    cleared = 0;
    trail = [];
  }

  var flashes = [];
  function spawnFlash(x, y) { flashes.push({ x: x, y: y, t: 0.001 }); }
  function stepFlashes(dt) {
    for (var i = flashes.length - 1; i >= 0; i -= 1) {
      flashes[i].t += dt;
      if (flashes[i].t > FLASH_LIFE) flashes.splice(i, 1);
    }
  }
  function drawFlashes() {
    for (var i = 0; i < flashes.length; i += 1) {
      var f = flashes[i];
      var p = f.t / FLASH_LIFE;
      var r = 14 + FLASH_MAX_R * p;
      var a = (1 - p) * 0.42;
      var g = ctx.createRadialGradient(f.x, f.y, 0, f.x, f.y, r);
      g.addColorStop(0, 'rgba(255,247,230,' + a + ')');
      g.addColorStop(0.6, 'rgba(255,209,63,' + (a * 0.6) + ')');
      g.addColorStop(1, 'rgba(255,209,63,0)');
      ctx.fillStyle = g;
      ctx.beginPath();
      ctx.arc(f.x, f.y, r, 0, Math.PI * 2);
      ctx.fill();
    }
  }

  var paddle = { x: LW / 2 - PADDLE_W / 2, w: PADDLE_W, target: LW / 2 };
  var ball = { x: LW / 2, y: PADDLE_Y - 40, vx: 0, vy: 0 };
  function resetBall() {
    var ang = rand() * 0.5 - 0.25;
    ball.x = LW / 2; ball.y = PADDLE_Y - 40;
    ball.vx = Math.sin(ang) * BASE_SPEED;
    ball.vy = -Math.cos(ang) * BASE_SPEED;
  }
  resetBall();

  var pointerActive = false;
  var keys = { left: false, right: false };
  var biasSchedule = [0, 0.75, -0.75, 0.35, -0.35, 0.6];
  function autoplay(tMs) {
    if (pointerActive || reduce) return;
    var slot = Math.floor(tMs / BIAS_PERIOD) % biasSchedule.length;
    var bias = biasSchedule[slot] * (paddle.w / 2);
    paddle.target = ball.x + bias;
  }

  function paddleStep(dt) {
    if (keys.left) paddle.target -= 160 * dt;
    if (keys.right) paddle.target += 160 * dt;
    paddle.target = clamp(paddle.target, WALL + paddle.w / 2, LW - WALL - paddle.w / 2);
    var goal = paddle.target - paddle.w / 2;
    var k = 1 - Math.pow(1 - PADDLE_LERP, dt * 60);
    paddle.x += (goal - paddle.x) * k;
    paddle.x = clamp(paddle.x, WALL, LW - WALL - paddle.w);
  }

  function onWallHit(side) { spawnFlash(ball.x, ball.y);  }
  function mirrorWallBounce(side) {
    ball.vx = side === 'left' ? Math.abs(ball.vx) : -Math.abs(ball.vx);
    onWallHit(side);
  }
  function onPaddleHit() {
    var hitPos = clamp((ball.x - (paddle.x + paddle.w / 2)) / (paddle.w / 2), -1, 1);
    var angle = hitPos * MAX_REFLECT_ANGLE;
    var speed = Math.sqrt(ball.vx * ball.vx + ball.vy * ball.vy);
    ball.vx = Math.sin(angle) * speed;
    ball.vy = -Math.abs(Math.cos(angle)) * speed;
    ball.y = PADDLE_Y - BALL_R - 0.5;
    spawnFlash(ball.x, PADDLE_Y);
    var sp = Math.min(Math.sqrt(ball.vx * ball.vx + ball.vy * ball.vy) + SPEED_STEP, SPEED_MAX);
    var ang2 = Math.atan2(ball.vx, -ball.vy);
    ball.vx = Math.sin(ang2) * sp;
    ball.vy = -Math.abs(Math.cos(ang2)) * sp;
  }
  function onLevelClear() { spawnFlash(LW / 2, LH / 2); rebuildLevel(); }
  function onBrickHit(b) {
    spawnFlash(b.x + b.w / 2, b.y + b.h / 2);
    b.alive = false; cleared += 1;
    if (cleared >= bricks.length) { onLevelClear(); }
  }
  function onMiss() {
    spawnFlash(ball.x, LH - 4);
    trail.length = 0;
    resetBall();
  }

  function ballStep(dt) {
    ball.x += ball.vx * dt;
    ball.y += ball.vy * dt;
    if (ball.x - BALL_R <= WALL) { ball.x = WALL + BALL_R; mirrorWallBounce('left'); }
    else if (ball.x + BALL_R >= LW - WALL) { ball.x = LW - WALL - BALL_R; mirrorWallBounce('right'); }
    if (ball.y - BALL_R <= WALL) { ball.y = WALL + BALL_R; ball.vy = Math.abs(ball.vy); }
    if (ball.vy > 0 && aabb(ball.x - BALL_R, ball.y - BALL_R, BALL_R * 2, BALL_R * 2, paddle.x, PADDLE_Y, paddle.w, PADDLE_H)) {
      onPaddleHit();
    }
    for (var i = 0; i < bricks.length; i += 1) {
      var b = bricks[i];
      if (b.alive && aabb(ball.x - BALL_R, ball.y - BALL_R, BALL_R * 2, BALL_R * 2, b.x, b.y, b.w, b.h)) {
        ball.vy = -ball.vy;
        onBrickHit(b);
        break;
      }
    }
    if (ball.y - BALL_R > LH) onMiss();
  }

  function pushTrail() { trail.push({ x: ball.x, y: ball.y }); }
  function stepExtra(dt) {
    pushTrail();
    var speedNow = Math.sqrt(ball.vx * ball.vx + ball.vy * ball.vy);
    var count = Math.round(clamp((speedNow - BASE_SPEED) / (SPEED_MAX - BASE_SPEED), 0, 1) * (TRAIL_MAX - 3)) + 3;
    while (trail.length > count) trail.shift();
  }

  function update(dt, tMs) {
    autoplay(tMs);
    paddleStep(dt);
    ballStep(dt);
    stepFlashes(dt);
    stepExtra(dt);
  }

  function drawCourt() {
    ctx.fillStyle = STAGE_BG;
    ctx.fillRect(0, 0, LW, LH);
  }
  function drawBrick(b) {
    ctx.fillStyle = BRICK_ROW_COLORS[Math.floor((b.y - BRICK_TOP) / (BRICK_H + BRICK_GAP)) % BRICK_ROW_COLORS.length];
    ctx.fillRect(b.x, b.y, b.w, b.h);
  }
  function drawBricks() {
    for (var i = 0; i < bricks.length; i += 1) {
      if (bricks[i].alive) drawBrick(bricks[i]);
    }
  }
  function drawPaddle() {
    ctx.fillStyle = PADDLE_COLOR;
    var r = 3;
    ctx.beginPath();
    ctx.moveTo(paddle.x + r, PADDLE_Y);
    ctx.arcTo(paddle.x + paddle.w, PADDLE_Y, paddle.x + paddle.w, PADDLE_Y + PADDLE_H, r);
    ctx.arcTo(paddle.x + paddle.w, PADDLE_Y + PADDLE_H, paddle.x, PADDLE_Y + PADDLE_H, r);
    ctx.arcTo(paddle.x, PADDLE_Y + PADDLE_H, paddle.x, PADDLE_Y, r);
    ctx.arcTo(paddle.x, PADDLE_Y, paddle.x + paddle.w, PADDLE_Y, r);
    ctx.closePath();
    ctx.fill();
  }
  function drawBall() {
    ctx.fillStyle = BALL_COLOR;
    ctx.beginPath();
    ctx.arc(ball.x, ball.y, BALL_R, 0, Math.PI * 2);
    ctx.fill();
  }
  function drawExtraBelow() {
    for (var i = 0; i < trail.length; i += 1) {
      var t = trail[i];
      var a = (i + 1) / trail.length;
      ctx.fillStyle = 'rgba(110,200,255,' + (a * 0.5) + ')';
      ctx.beginPath();
      ctx.arc(t.x, t.y, BALL_R * (0.4 + a * 0.6), 0, Math.PI * 2);
      ctx.fill();
    }
  }
  function drawExtraAbove() {}

  var cw = 480, ch = 300, sx = 1, sy = 1;
  function fit() {
    var r = wrap.getBoundingClientRect();
    cw = Math.max(1, Math.round(r.width));
    ch = Math.max(1, Math.round(r.height));
    canvas.width = cw; canvas.height = ch;
    sx = cw / LW; sy = ch / LH;
    render();
  }

  function render() {
    ctx.setTransform(1, 0, 0, 1, 0, 0);
    ctx.clearRect(0, 0, cw, ch);
    ctx.save();
    ctx.scale(sx, sy);
    drawCourt();
    drawFlashes();
    drawBricks();
    drawExtraBelow();
    drawPaddle();
    drawBall();
    drawExtraAbove();
    ctx.restore();
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var playing = !reduce;
  if (reduce) { root.classList.remove('is-demo'); }

  function calcFromEvent(e) {
    var r = wrap.getBoundingClientRect();
    var t0 = (e.touches && e.touches[0]) ? e.touches[0] : e;
    var relX = (t0.clientX - r.left) / r.width * LW;
    paddle.target = clamp(relX, WALL + paddle.w / 2, LW - WALL - paddle.w / 2);
  }
  wrap.addEventListener('pointerdown', function (e) { pointerActive = true; calcFromEvent(e); if (reduce) { paddleStep(0); render(); } });
  wrap.addEventListener('pointermove', function (e) { pointerActive = true; calcFromEvent(e); if (reduce) { paddleStep(0); render(); } });
  window.addEventListener('pointerup', function () { pointerActive = false; });
  wrap.addEventListener('pointerleave', function () { pointerActive = false; });

  window.addEventListener('keydown', function (e) {
    if (e.key === 'ArrowLeft') { keys.left = true; pointerActive = true; }
    else if (e.key === 'ArrowRight') { keys.right = true; pointerActive = true; }
    else return;
    if (reduce) { paddleStep(0.05); render(); }
  });
  window.addEventListener('keyup', function (e) {
    if (e.key === 'ArrowLeft') keys.left = false;
    else if (e.key === 'ArrowRight') keys.right = false;
    if (!keys.left && !keys.right) pointerActive = false;
  });

  var lastTs = null;
  function tick(ts) {
    if (lastTs === null) lastTs = ts;
    var dt = Math.min((ts - lastTs) / 1000, 0.05);
    lastTs = ts;
    if (playing) update(dt, ts);
    render();
    requestAnimationFrame(tick);
  }
  fit();
  window.addEventListener('resize', fit);
  render();
  if (playing) requestAnimationFrame(tick);
})();

import { useEffect, useRef } from "react";
import styles from "./BrickBreakerTrailSpeedAfterimage.module.scss";

export default function BrickBreakerTrailSpeedAfterimage() {
  const rootRef = useRef<HTMLDivElement>(null);
  const canvasRef = useRef<HTMLCanvasElement>(null);

  useEffect(() => {
    const root = rootRef.current;
    const canvas = canvasRef.current;
    if (!root || !canvas) return;
    const ctx = canvas.getContext("2d");
    if (!ctx) return;
    const wrap = canvas.parentElement as HTMLElement;

    const LW = 320, LH = 200, WALL = 6;
    const PADDLE_H = 8, PADDLE_Y = LH - 18, PADDLE_W = 46;
    const BALL_R = 4;
    const BRICK_COLS = 6, BRICK_ROWS = 3, BRICK_W = 44, BRICK_H = 12, BRICK_GAP = 4, BRICK_TOP = 22;
    const BASE_SPEED = 130;
    const BIAS_PERIOD = 560;
    const STAGE_BG = "#17141a";
    const BRICK_ROW_COLORS = ["#2f6df6", "#ff9ebb", "#4cd4a6"];
    const PADDLE_COLOR = "#fff7e6";
    const BALL_COLOR = "#fff7e6";
    const FLASH_LIFE = 0.4, FLASH_MAX_R = 130;

    function seedRand(seed: number) {
      let s = seed >>> 0;
      return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
    }
    const rand = seedRand(41006);

    function clamp(v: number, a: number, b: number): number { return v < a ? a : (v > b ? b : v); }
    function aabb(ax: number, ay: number, aw: number, ah: number, bx: number, by: number, bw: number, bh: number): boolean {
      return ax < bx + bw && ax + aw > bx && ay < by + bh && ay + ah > by;
    }

    const PADDLE_LERP = 0.32;
    const MAX_REFLECT_ANGLE = Math.PI / 180 * 48;
    const SPEED_STEP = 14;
    const SPEED_MAX = 210;

    type Brick = { x: number; y: number; w: number; h: number; alive: boolean; hits: number };
    function buildBricks(): Brick[] {
      const totalW = BRICK_COLS * BRICK_W + (BRICK_COLS - 1) * BRICK_GAP;
      const startX = (LW - totalW) / 2;
      const arr: Brick[] = [];
      for (let r = 0; r < BRICK_ROWS; r += 1) {
        for (let c = 0; c < BRICK_COLS; c += 1) {
          arr.push({ x: startX + c * (BRICK_W + BRICK_GAP), y: BRICK_TOP + r * (BRICK_H + BRICK_GAP), w: BRICK_W, h: BRICK_H, alive: true, hits: 0 });
        }
      }
      return arr;
    }
    let bricks: Brick[] = buildBricks();
    let cleared = 0;
    type Pt = { x: number; y: number };
    let trail: Pt[] = [];
    const TRAIL_MAX = 16;

    function rebuildLevel(): void {
      bricks = buildBricks();
      cleared = 0;
      trail = [];
    }

    type Flash = { x: number; y: number; t: number };
    const flashes: Flash[] = [];
    function spawnFlash(x: number, y: number): void { flashes.push({ x: x, y: y, t: 0.001 }); }
    function stepFlashes(dt: number): void {
      for (let i = flashes.length - 1; i >= 0; i -= 1) {
        flashes[i].t += dt;
        if (flashes[i].t > FLASH_LIFE) flashes.splice(i, 1);
      }
    }
    function drawFlashes(): void {
      for (let i = 0; i < flashes.length; i += 1) {
        const f = flashes[i];
        const p = f.t / FLASH_LIFE;
        const r = 14 + FLASH_MAX_R * p;
        const a = (1 - p) * 0.42;
        const g = ctx!.createRadialGradient(f.x, f.y, 0, f.x, f.y, r);
        g.addColorStop(0, "rgba(255,247,230," + a + ")");
        g.addColorStop(0.6, "rgba(255,209,63," + (a * 0.6) + ")");
        g.addColorStop(1, "rgba(255,209,63,0)");
        ctx!.fillStyle = g;
        ctx!.beginPath();
        ctx!.arc(f.x, f.y, r, 0, Math.PI * 2);
        ctx!.fill();
      }
    }

    const paddle = { x: LW / 2 - PADDLE_W / 2, w: PADDLE_W, target: LW / 2 };
    const ball = { x: LW / 2, y: PADDLE_Y - 40, vx: 0, vy: 0 };
    function resetBall(): void {
      const ang = rand() * 0.5 - 0.25;
      ball.x = LW / 2; ball.y = PADDLE_Y - 40;
      ball.vx = Math.sin(ang) * BASE_SPEED;
      ball.vy = -Math.cos(ang) * BASE_SPEED;
    }
    resetBall();

    let pointerActive = false;
    const keys = { left: false, right: false };
    const biasSchedule = [0, 0.75, -0.75, 0.35, -0.35, 0.6];
    function autoplay(tMs: number): void {
      if (pointerActive || reduce) return;
      const slot = Math.floor(tMs / BIAS_PERIOD) % biasSchedule.length;
      const bias = biasSchedule[slot] * (paddle.w / 2);
      paddle.target = ball.x + bias;
    }

    function paddleStep(dt: number): void {
      if (keys.left) paddle.target -= 160 * dt;
      if (keys.right) paddle.target += 160 * dt;
      paddle.target = clamp(paddle.target, WALL + paddle.w / 2, LW - WALL - paddle.w / 2);
      const goal = paddle.target - paddle.w / 2;
      const k = 1 - Math.pow(1 - PADDLE_LERP, dt * 60);
      paddle.x += (goal - paddle.x) * k;
      paddle.x = clamp(paddle.x, WALL, LW - WALL - paddle.w);
    }

    function onWallHit(side: string): void { spawnFlash(ball.x, ball.y);  }
    function mirrorWallBounce(side: string): void {
      ball.vx = side === "left" ? Math.abs(ball.vx) : -Math.abs(ball.vx);
      onWallHit(side);
    }
    function onPaddleHit(): void {
      const hitPos = clamp((ball.x - (paddle.x + paddle.w / 2)) / (paddle.w / 2), -1, 1);
      const angle = hitPos * MAX_REFLECT_ANGLE;
      const speed = Math.sqrt(ball.vx * ball.vx + ball.vy * ball.vy);
      ball.vx = Math.sin(angle) * speed;
      ball.vy = -Math.abs(Math.cos(angle)) * speed;
      ball.y = PADDLE_Y - BALL_R - 0.5;
      spawnFlash(ball.x, PADDLE_Y);
      const sp = Math.min(Math.sqrt(ball.vx * ball.vx + ball.vy * ball.vy) + SPEED_STEP, SPEED_MAX);
      const ang2 = Math.atan2(ball.vx, -ball.vy);
      ball.vx = Math.sin(ang2) * sp;
      ball.vy = -Math.abs(Math.cos(ang2)) * sp;
    }
    function onLevelClear(): void { spawnFlash(LW / 2, LH / 2); rebuildLevel(); }
    function onBrickHit(b: Brick): void {
      spawnFlash(b.x + b.w / 2, b.y + b.h / 2);
      b.alive = false; cleared += 1;
      if (cleared >= bricks.length) { onLevelClear(); }
    }
    function onMiss(): void {
      spawnFlash(ball.x, LH - 4);
      trail.length = 0;
      resetBall();
    }

    function ballStep(dt: number): void {
      ball.x += ball.vx * dt;
      ball.y += ball.vy * dt;
      if (ball.x - BALL_R <= WALL) { ball.x = WALL + BALL_R; mirrorWallBounce("left"); }
      else if (ball.x + BALL_R >= LW - WALL) { ball.x = LW - WALL - BALL_R; mirrorWallBounce("right"); }
      if (ball.y - BALL_R <= WALL) { ball.y = WALL + BALL_R; ball.vy = Math.abs(ball.vy); }
      if (ball.vy > 0 && aabb(ball.x - BALL_R, ball.y - BALL_R, BALL_R * 2, BALL_R * 2, paddle.x, PADDLE_Y, paddle.w, PADDLE_H)) {
        onPaddleHit();
      }
      for (let i = 0; i < bricks.length; i += 1) {
        const b = bricks[i];
        if (b.alive && aabb(ball.x - BALL_R, ball.y - BALL_R, BALL_R * 2, BALL_R * 2, b.x, b.y, b.w, b.h)) {
          ball.vy = -ball.vy;
          onBrickHit(b);
          break;
        }
      }
      if (ball.y - BALL_R > LH) onMiss();
    }

    function pushTrail(): void { trail.push({ x: ball.x, y: ball.y }); }
    function stepExtra(dt: number): void {
      pushTrail();
      const speedNow = Math.sqrt(ball.vx * ball.vx + ball.vy * ball.vy);
      const count = Math.round(clamp((speedNow - BASE_SPEED) / (SPEED_MAX - BASE_SPEED), 0, 1) * (TRAIL_MAX - 3)) + 3;
      while (trail.length > count) trail.shift();
    }

    function update(dt: number, tMs: number): void {
      autoplay(tMs);
      paddleStep(dt);
      ballStep(dt);
      stepFlashes(dt);
      stepExtra(dt);
    }

    function drawCourt(): void {
      ctx!.fillStyle = STAGE_BG;
      ctx!.fillRect(0, 0, LW, LH);
    }
    function drawBrick(b: Brick): void {
      ctx!.fillStyle = BRICK_ROW_COLORS[Math.floor((b.y - BRICK_TOP) / (BRICK_H + BRICK_GAP)) % BRICK_ROW_COLORS.length];
      ctx!.fillRect(b.x, b.y, b.w, b.h);
    }
    function drawBricks(): void {
      for (let i = 0; i < bricks.length; i += 1) {
        if (bricks[i].alive) drawBrick(bricks[i]);
      }
    }
    function drawPaddle(): void {
      ctx!.fillStyle = PADDLE_COLOR;
      const r = 3;
      ctx!.beginPath();
      ctx!.moveTo(paddle.x + r, PADDLE_Y);
      ctx!.arcTo(paddle.x + paddle.w, PADDLE_Y, paddle.x + paddle.w, PADDLE_Y + PADDLE_H, r);
      ctx!.arcTo(paddle.x + paddle.w, PADDLE_Y + PADDLE_H, paddle.x, PADDLE_Y + PADDLE_H, r);
      ctx!.arcTo(paddle.x, PADDLE_Y + PADDLE_H, paddle.x, PADDLE_Y, r);
      ctx!.arcTo(paddle.x, PADDLE_Y, paddle.x + paddle.w, PADDLE_Y, r);
      ctx!.closePath();
      ctx!.fill();
    }
    function drawBall(): void {
      ctx!.fillStyle = BALL_COLOR;
      ctx!.beginPath();
      ctx!.arc(ball.x, ball.y, BALL_R, 0, Math.PI * 2);
      ctx!.fill();
    }
    function drawExtraBelow(): void {
      for (let i = 0; i < trail.length; i += 1) {
        const t = trail[i];
        const a = (i + 1) / trail.length;
        ctx!.fillStyle = "rgba(110,200,255," + (a * 0.5) + ")";
        ctx!.beginPath();
        ctx!.arc(t.x, t.y, BALL_R * (0.4 + a * 0.6), 0, Math.PI * 2);
        ctx!.fill();
      }
    }
    function drawExtraAbove(): void {}

    let cw = 480, ch = 300, sx = 1, sy = 1;
    function fit(): void {
      const r = wrap.getBoundingClientRect();
      cw = Math.max(1, Math.round(r.width));
      ch = Math.max(1, Math.round(r.height));
      canvas!.width = cw; canvas!.height = ch;
      sx = cw / LW; sy = ch / LH;
      render();
    }

    function render(): void {
      ctx!.setTransform(1, 0, 0, 1, 0, 0);
      ctx!.clearRect(0, 0, cw, ch);
      ctx!.save();
      ctx!.scale(sx, sy);
      drawCourt();
      drawFlashes();
      drawBricks();
      drawExtraBelow();
      drawPaddle();
      drawBall();
      drawExtraAbove();
      ctx!.restore();
    }

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const playing = !reduce;
    if (reduce) { root.classList.remove("is-demo"); }

    function calcFromEvent(e: PointerEvent): void {
      const r = wrap.getBoundingClientRect();
      const relX = ((e.clientX - r.left) / r.width) * LW;
      paddle.target = clamp(relX, WALL + paddle.w / 2, LW - WALL - paddle.w / 2);
    }
    function onPointerDown(e: PointerEvent): void { pointerActive = true; calcFromEvent(e); if (reduce) { paddleStep(0); render(); } }
    function onPointerMove(e: PointerEvent): void { pointerActive = true; calcFromEvent(e); if (reduce) { paddleStep(0); render(); } }
    function onPointerUp(): void { pointerActive = false; }
    function onPointerLeave(): void { pointerActive = false; }
    wrap.addEventListener("pointerdown", onPointerDown);
    wrap.addEventListener("pointermove", onPointerMove);
    window.addEventListener("pointerup", onPointerUp);
    wrap.addEventListener("pointerleave", onPointerLeave);

    function onKeyDown(e: KeyboardEvent): void {
      if (e.key === "ArrowLeft") { keys.left = true; pointerActive = true; }
      else if (e.key === "ArrowRight") { keys.right = true; pointerActive = true; }
      else return;
      if (reduce) { paddleStep(0.05); render(); }
    }
    function onKeyUp(e: KeyboardEvent): void {
      if (e.key === "ArrowLeft") keys.left = false;
      else if (e.key === "ArrowRight") keys.right = false;
      if (!keys.left && !keys.right) pointerActive = false;
    }
    window.addEventListener("keydown", onKeyDown);
    window.addEventListener("keyup", onKeyUp);

    let lastTs: number | null = null;
    let raf = 0;
    function tick(ts: number): void {
      if (lastTs === null) lastTs = ts;
      const dt = Math.min((ts - lastTs) / 1000, 0.05);
      lastTs = ts;
      if (playing) update(dt, ts);
      render();
      raf = requestAnimationFrame(tick);
    }
    fit();
    window.addEventListener("resize", fit);
    render();
    if (playing) raf = requestAnimationFrame(tick);

    return () => {
      cancelAnimationFrame(raf);
      window.removeEventListener("resize", fit);
      window.removeEventListener("keydown", onKeyDown);
      window.removeEventListener("keyup", onKeyUp);
      window.removeEventListener("pointerup", onPointerUp);
      wrap.removeEventListener("pointerdown", onPointerDown);
      wrap.removeEventListener("pointermove", onPointerMove);
      wrap.removeEventListener("pointerleave", onPointerLeave);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={`${styles.vd} is-demo`} ref={rootRef}>
        <canvas className={styles.vd__canvas} ref={canvasRef} width={480} height={300} />
        <p className={styles.vd__caption}>속도 값만큼 길어지는 공의 잔상</p>
      </div>
    </main>
  );
}