04ビート反応パーティクル

<svg width="0" height="0" style="position:absolute" aria-hidden="true">
  <defs>
    <symbol id="ico-waveform" viewBox="0 0 256 256" fill="currentColor"><path d="M208 96v64H48V96Z" opacity=".2"/><path d="M56 96v64a8 8 0 0 1-16 0V96a8 8 0 0 1 16 0m32-72a8 8 0 0 0-8 8v192a8 8 0 0 0 16 0V32a8 8 0 0 0-8-8m40 32a8 8 0 0 0-8 8v128a8 8 0 0 0 16 0V64a8 8 0 0 0-8-8m40 32a8 8 0 0 0-8 8v64a8 8 0 0 0 16 0V96a8 8 0 0 0-8-8m40-16a8 8 0 0 0-8 8v96a8 8 0 0 0 16 0V72a8 8 0 0 0-8-8"/></symbol>
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    <symbol id="ico-pause" viewBox="0 0 256 256" fill="currentColor"><path d="M208 48v160a8 8 0 0 1-8 8h-40a8 8 0 0 1-8-8V48a8 8 0 0 1 8-8h40a8 8 0 0 1 8 8M96 40H56a8 8 0 0 0-8 8v160a8 8 0 0 0 8 8h40a8 8 0 0 0 8-8V48a8 8 0 0 0-8-8" opacity=".2"/><path d="M200 32h-40a16 16 0 0 0-16 16v160a16 16 0 0 0 16 16h40a16 16 0 0 0 16-16V48a16 16 0 0 0-16-16m0 176h-40V48h40zM96 32H56a16 16 0 0 0-16 16v160a16 16 0 0 0 16 16h40a16 16 0 0 0 16-16V48a16 16 0 0 0-16-16m0 176H56V48h40z"/></symbol>
  </defs>
</svg>
<main class="stage">
<div class="bp is-demo" id="bp">
  <p class="bp__hint"><svg class="bp__ico" aria-hidden="true"><use href="#ico-waveform"/></svg><span>再生を押すと実際の音で動きます</span></p>
  <div class="bp__field" id="field" aria-hidden="true"><div class="bp__core" id="core"><svg class="bp__coreIco" aria-hidden="true"><use href="#ico-waveform"/></svg></div></div>
  <div class="bp__foot">
    <button class="bp__play" id="play" type="button" aria-pressed="false">
      <svg class="bp__ico bp__ico--play" aria-hidden="true"><use href="#ico-play"/></svg>
      <svg class="bp__ico bp__ico--pause" aria-hidden="true"><use href="#ico-pause"/></svg>
      <span id="playLabel">再生</span>
    </button>
    <p class="bp__count">
      <span class="bp__countLabel">拾ったビート</span>
      <b class="bp__countBox">
        <span class="bp__countText" id="beatText" aria-live="polite">0</span>
        <span class="bp__loop" aria-hidden="true"><i>1</i><i>3</i><i>5</i><i>7</i></span>
      </b>
    </p>
  </div>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;

// 비트 반응 입자 — 저음 에너지가 자기 이동 평균을 넘는 순간(=비트)에만 겹 하나가 터진다.
// 코어는 늘 그 자리에 있는다 — 입자의 퇴장이 주인공이 되면 대표 이미지가 빈 화면이 된다(255 실측).
// 미리보기(.is-demo)는 2초에 네 번 비트가 도는 CSS 루프. 재생을 누르면 진짜 판정이 겹에 불을 붙인다.

$duration: $dur-quick;
$easing: $ease-pop;
$color: $stage-yellow;

@include stage($stage-ink);

.bp { display: flex; flex-direction: column; align-items: center; gap: $sp-2; width: 100%; }
.bp__hint {
  display: flex; align-items: center; gap: $sp-1; margin: 0;
  font-weight: 700; font-size: clamp(11px, 3vw, 12px); color: $subject-cream;
}
.bp__ico { width: 16px; height: 16px; color: $color; }

// --rest 는 점이 쉬는 반경, --fly 는 비트에 터지는 반경. 좁은 화면에서는 이 값만 줄인다(루프는 그대로).
.bp__field { position: relative; width: 172px; height: 132px; --rest: 52px; --fly: 84px; }
.bp__ring { position: absolute; inset: 0; }
.bp__dot {
  position: absolute; left: 50%; top: 50%; width: 8px; height: 8px; margin: -4px 0 0 -4px;
  border-radius: 50%; background: rgba($subject-cream, .38);
  transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(1);
}
// 코어 — 입자가 흩어져도 화면의 주인공으로 남는다. 커질 땐 자기 중심에서(규칙 8).
.bp__core {
  position: absolute; left: 50%; top: 50%; width: 52px; height: 52px; margin: -26px 0 0 -26px;
  display: grid; place-items: center; border-radius: 50%;
  background: #211d26; border: 4px solid $color;
}
.bp__coreIco { width: 20px; height: 20px; color: $color; }

// 실제 비트가 오면 JS 가 클래스를 붙인다 — 일회성 transform 이지 루프가 아니다
.bp__ring--hit .bp__dot { animation: bpBurst $easing 480ms 1 both; }
.bp__core--hit { animation: bpThump $easing 480ms 1 both; }
@keyframes bpBurst {
  0%   { transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(1); opacity: 1; }
  100% { transform: rotate(var(--a)) translateY(calc(-1 * var(--fly))) scale(.4); opacity: 0; }
}
@keyframes bpThump {
  0%  { transform: scale(1); }
  35% { transform: scale(1.18); }
  100% { transform: scale(1); }
}

.bp__foot { display: flex; align-items: center; justify-content: center; gap: $sp-4; width: 100%; }
.bp__play {
  display: inline-flex; align-items: center; gap: $sp-1;
  border: 0; border-radius: $r-pill; padding: $sp-2 $sp-4;
  background: $color; color: $subject-ink; cursor: pointer;
  font-family: inherit; font-weight: 800; font-size: clamp(11px, 3vw, 12px); line-height: 1;
}
.bp__ico--pause { display: none; }
.bp__play[aria-pressed="true"] .bp__ico--play { display: none; }
.bp__play[aria-pressed="true"] .bp__ico--pause { display: inline-block; }
.bp__play .bp__ico { color: $subject-ink; }

.bp__count { margin: 0; display: flex; align-items: center; gap: $sp-2; }
.bp__countLabel { font-weight: 700; font-size: clamp(10px, 2.6vw, 11px); color: rgba($subject-cream, .82); }
.bp__countBox {
  position: relative; display: grid; place-items: center;
  min-width: 28px; height: 20px; border-radius: $r-xs; padding: 0 $sp-1;
  background: rgba($subject-cream, .16); color: $subject-cream;
  font-weight: 800; font-size: clamp(11px, 3vw, 12px); line-height: 1;
}
.bp__loop { position: absolute; inset: 0; display: grid; place-items: center; visibility: hidden; }
.bp__loop i { grid-area: 1 / 1; font-style: normal; visibility: hidden; }

// ── 격자 미리보기 루프 ─────────────────────────────────────────────
// 2초에 비트 네 번(2Hz) — 4번은 점멸이 아니라 파동이다(점들은 부드러운 감쇠 곡선으로 사라진다).
// 겹 넷은 서로 500ms 간격. animation 단축을 안 쓰고 낱개 속성만(240 실측).
.bp.is-demo .bp__dot {
  animation-name: bpLoop; animation-duration: $dur-loop; animation-timing-function: $easing;
  animation-iteration-count: infinite;
  animation-delay: calc(var(--i) * -500ms);
}
// 미리보기에서 쉬는 상태의 점은 옅은 헤일로 — 폭 0 이 아니라 반투명 링으로 대기한다
@keyframes bpLoop {
  0%   { transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(.92); opacity: .5; }
  2%   { transform: rotate(var(--a)) translateY(calc(-1 * var(--rest) - 2px)) scale(1.5); opacity: 1; }
  30%  { transform: rotate(var(--a)) translateY(calc(-1 * var(--fly))) scale(.4); opacity: 0; }
  46%  { transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(.92); opacity: .5; }
  100% { transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(.92); opacity: .5; }
}
.bp.is-demo .bp__core {
  animation-name: bpCoreLoop; animation-duration: 500ms; animation-timing-function: $easing;
  animation-iteration-count: infinite;
}
@keyframes bpCoreLoop {
  0%  { transform: scale(1); }
  22% { transform: scale(1.16); }
  100% { transform: scale(1); }
}
// 잡은 비트 수도 1 → 3 → 5 → 7 로 컷. 지연 구간에 fill-mode 를 두지 않는다(작성 상태가 이미 숨김).
.bp.is-demo .bp__countText { color: transparent; }
.bp.is-demo .bp__loop { visibility: visible; }
.bp.is-demo .bp__loop i {
  animation-name: bpBeatNum; animation-duration: $dur-loop; animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.bp.is-demo .bp__loop i:nth-child(1) { animation-delay: 0ms; }
.bp.is-demo .bp__loop i:nth-child(2) { animation-delay: 500ms; }
.bp.is-demo .bp__loop i:nth-child(3) { animation-delay: 1000ms; }
.bp.is-demo .bp__loop i:nth-child(4) { animation-delay: 1500ms; }
@keyframes bpBeatNum {
  0%, 24% { visibility: visible; }
  25%, 100% { visibility: hidden; }
}

@media (max-width: 340px) {
  .bp { gap: 0; }
  .bp__field { width: 140px; height: 104px; --rest: 40px; --fly: 58px; }
  .bp__dot { width: 6px; height: 6px; margin: -3px 0 0 -3px; }
  .bp__core { width: 44px; height: 44px; margin: -22px 0 0 -22px; }
  .bp__foot { gap: $sp-2; }
  .bp__play { padding: $sp-1 $sp-3; }
}

// reduce 에서는 루프와 실제 반응을 다 끈다 — 코어와 헤일로만 남아 화면이 비지 않는다
@media (prefers-reduced-motion: reduce) {
  .bp.is-demo .bp__dot,
  .bp.is-demo .bp__core,
  .bp.is-demo .bp__loop i { animation: none !important; }
  .bp.is-demo .bp__countText { color: $subject-cream !important; }
  .bp.is-demo .bp__loop { visibility: hidden !important; }
  .bp__ring--hit .bp__dot,
  .bp__core--hit { animation: none !important; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.bp {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
  width: 100%;
}

.bp__hint {
  display: flex;
  align-items: center;
  gap: 4px;
  margin: 0;
  font-weight: 700;
  font-size: clamp(11px, 3vw, 12px);
  color: #fff7e6;
}

.bp__ico {
  width: 16px;
  height: 16px;
  color: #ffd23f;
}

.bp__field {
  position: relative;
  width: 172px;
  height: 132px;
  --rest: 52px;
  --fly: 84px;
}

.bp__ring {
  position: absolute;
  inset: 0;
}

.bp__dot {
  position: absolute;
  left: 50%;
  top: 50%;
  width: 8px;
  height: 8px;
  margin: -4px 0 0 -4px;
  border-radius: 50%;
  background: rgba(255, 247, 230, 0.38);
  transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(1);
}

.bp__core {
  position: absolute;
  left: 50%;
  top: 50%;
  width: 52px;
  height: 52px;
  margin: -26px 0 0 -26px;
  display: grid;
  place-items: center;
  border-radius: 50%;
  background: #211d26;
  border: 4px solid #ffd23f;
}

.bp__coreIco {
  width: 20px;
  height: 20px;
  color: #ffd23f;
}

.bp__ring--hit .bp__dot {
  animation: bpBurst cubic-bezier(0.2, 1.4, 0.4, 1) 480ms 1 both;
}

.bp__core--hit {
  animation: bpThump cubic-bezier(0.2, 1.4, 0.4, 1) 480ms 1 both;
}

@keyframes bpBurst {
  0% {
    transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(1);
    opacity: 1;
  }
  100% {
    transform: rotate(var(--a)) translateY(calc(-1 * var(--fly))) scale(0.4);
    opacity: 0;
  }
}
@keyframes bpThump {
  0% {
    transform: scale(1);
  }
  35% {
    transform: scale(1.18);
  }
  100% {
    transform: scale(1);
  }
}
.bp__foot {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 16px;
  width: 100%;
}

.bp__play {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  border: 0;
  border-radius: 999px;
  padding: 8px 16px;
  background: #ffd23f;
  color: #17141a;
  cursor: pointer;
  font-family: inherit;
  font-weight: 800;
  font-size: clamp(11px, 3vw, 12px);
  line-height: 1;
}

.bp__ico--pause {
  display: none;
}

.bp__play[aria-pressed=true] .bp__ico--play {
  display: none;
}

.bp__play[aria-pressed=true] .bp__ico--pause {
  display: inline-block;
}

.bp__play .bp__ico {
  color: #17141a;
}

.bp__count {
  margin: 0;
  display: flex;
  align-items: center;
  gap: 8px;
}

.bp__countLabel {
  font-weight: 700;
  font-size: clamp(10px, 2.6vw, 11px);
  color: rgba(255, 247, 230, 0.82);
}

.bp__countBox {
  position: relative;
  display: grid;
  place-items: center;
  min-width: 28px;
  height: 20px;
  border-radius: 4px;
  padding: 0 4px;
  background: rgba(255, 247, 230, 0.16);
  color: #fff7e6;
  font-weight: 800;
  font-size: clamp(11px, 3vw, 12px);
  line-height: 1;
}

.bp__loop {
  position: absolute;
  inset: 0;
  display: grid;
  place-items: center;
  visibility: hidden;
}

.bp__loop i {
  grid-area: 1/1;
  font-style: normal;
  visibility: hidden;
}

.bp.is-demo .bp__dot {
  animation-name: bpLoop;
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 1.4, 0.4, 1);
  animation-iteration-count: infinite;
  animation-delay: calc(var(--i) * -500ms);
}

@keyframes bpLoop {
  0% {
    transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(0.92);
    opacity: 0.5;
  }
  2% {
    transform: rotate(var(--a)) translateY(calc(-1 * var(--rest) - 2px)) scale(1.5);
    opacity: 1;
  }
  30% {
    transform: rotate(var(--a)) translateY(calc(-1 * var(--fly))) scale(0.4);
    opacity: 0;
  }
  46% {
    transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(0.92);
    opacity: 0.5;
  }
  100% {
    transform: rotate(var(--a)) translateY(calc(-1 * var(--rest))) scale(0.92);
    opacity: 0.5;
  }
}
.bp.is-demo .bp__core {
  animation-name: bpCoreLoop;
  animation-duration: 500ms;
  animation-timing-function: cubic-bezier(0.2, 1.4, 0.4, 1);
  animation-iteration-count: infinite;
}

@keyframes bpCoreLoop {
  0% {
    transform: scale(1);
  }
  22% {
    transform: scale(1.16);
  }
  100% {
    transform: scale(1);
  }
}
.bp.is-demo .bp__countText {
  color: transparent;
}

.bp.is-demo .bp__loop {
  visibility: visible;
}

.bp.is-demo .bp__loop i {
  animation-name: bpBeatNum;
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}

.bp.is-demo .bp__loop i:nth-child(1) {
  animation-delay: 0ms;
}

.bp.is-demo .bp__loop i:nth-child(2) {
  animation-delay: 500ms;
}

.bp.is-demo .bp__loop i:nth-child(3) {
  animation-delay: 1000ms;
}

.bp.is-demo .bp__loop i:nth-child(4) {
  animation-delay: 1500ms;
}

@keyframes bpBeatNum {
  0%, 24% {
    visibility: visible;
  }
  25%, 100% {
    visibility: hidden;
  }
}
@media (max-width: 340px) {
  .bp {
    gap: 0;
  }
  .bp__field {
    width: 140px;
    height: 104px;
    --rest: 40px;
    --fly: 58px;
  }
  .bp__dot {
    width: 6px;
    height: 6px;
    margin: -3px 0 0 -3px;
  }
  .bp__core {
    width: 44px;
    height: 44px;
    margin: -22px 0 0 -22px;
  }
  .bp__foot {
    gap: 8px;
  }
  .bp__play {
    padding: 4px 12px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bp.is-demo .bp__dot,
  .bp.is-demo .bp__core,
  .bp.is-demo .bp__loop i {
    animation: none !important;
  }
  .bp.is-demo .bp__countText {
    color: #fff7e6 !important;
  }
  .bp.is-demo .bp__loop {
    visibility: hidden !important;
  }
  .bp__ring--hit .bp__dot,
  .bp__core--hit {
    animation: none !important;
  }
}

var root = document.getElementById('bp');
var field = document.getElementById('field');
var core = document.getElementById('core');
var playBtn = document.getElementById('play');
var playLabel = document.getElementById('playLabel');
var beatText = document.getElementById('beatText');
var RING_COUNT = 4, DOT_COUNT = 8;
var rings = [];
// 겹 넷 × 점 여덟. 겹마다 시작 각을 11.25도 민다 — 쉬는 상태에서는 점 서른두 개가 동그란 헤일로가 되고,
// 터질 때는 겹마다 다른 방향으로 흩어진다. --i 는 겹 번호이자 미리보기 루프의 음수 지연 배수다.
for (var r = 0; r < RING_COUNT; r++) {
  var ring = document.createElement('div');
  ring.className = 'bp__ring';
  for (var d = 0; d < DOT_COUNT; d++) {
    var dot = document.createElement('i');
    dot.className = 'bp__dot';
    dot.style.setProperty('--a', (((d * 45 + r * 11.25) % 360)).toFixed(2) + 'deg');
    dot.style.setProperty('--i', String(r));
    ring.appendChild(dot);
  }
  field.insertBefore(ring, core);
  rings.push(ring);
}
var ctx = null, analyser = null, freq = null, timer = 0, playing = false;
var bassAvg = 0, lastBeatAt = 0, beats = 0, ringTurn = 0;
function buildAudio() {
  ctx = new (window.AudioContext || window.webkitAudioContext)();
  analyser = ctx.createAnalyser();
  analyser.fftSize = 128;
  analyser.smoothingTimeConstant = 0.5;
  freq = new Uint8Array(analyser.frequencyBinCount);
  var out = ctx.createGain();
  out.gain.value = 0.08;
  out.connect(analyser);
  analyser.connect(ctx.destination);
  // 킥 — 낮은 사인파를 0.5초마다 한 번씩 쳐서 2초에 네 번 비트(120BPM). 低音 대역을 명확히 치고 빠진다.
  var kick = ctx.createOscillator();
  var kickAmp = ctx.createGain();
  kick.type = 'sine';
  kick.frequency.value = 70;
  kickAmp.gain.value = 0;
  for (var b = 0; b < 128; b++) {
    var at = ctx.currentTime + b * 0.5;
    kickAmp.gain.setValueAtTime(1, at);
    kickAmp.gain.exponentialRampToValueAtTime(0.004, at + 0.3);
  }
  kick.connect(kickAmp); kickAmp.connect(out); kick.start();
  // 패드 — "평균보다 큰 순간"이 성립하려면 바닥 에너지가 필요하다. 킥이 없는 사이 저음의 평균을 만든다.
  [110, 164.81, 220].forEach(function (hz) {
    var pad = ctx.createOscillator();
    var padAmp = ctx.createGain();
    pad.type = 'sawtooth';
    pad.frequency.value = hz;
    padAmp.gain.value = 0.035;
    pad.connect(padAmp); padAmp.connect(out); pad.start();
  });
}
function fire(el, hitClass) {
  el.classList.remove(hitClass);
  void el.offsetWidth; // 같은 클래스를 다시 걸어 일회성 애니메이션을 처음부터 돌린다
  el.classList.add(hitClass);
}
function draw() {
  analyser.getByteFrequencyData(freq);
  var bass = 0;
  for (var k = 0; k < 4; k++) { bass += freq[k]; }
  bass = bass / (4 * 255);
  var now = performance.now();
  // 비트 판정: 低音 에너지가 직전 구간의 이동 평균 × 1.3 을 넘는 순간이다.
  // 고정 문턱(예: "에너지 0.5 이상")은 곡이 조용해지면 비트를 아예 못 잡고 시끄러워지면 매 순간을 비트로 만든다 —
  // 평균 대비 계수여야 셈여림이 바뀌어도 비트를 놓치지 않는다. 220ms 최소 간격은 한 번 친 킥을 두 번 세는 것을 막는다.
  if (bass > bassAvg * 1.3 && bass > 0.14 && now - lastBeatAt > 220) {
    lastBeatAt = now;
    beats += 1;
    beatText.textContent = String(beats);
    fire(rings[ringTurn % RING_COUNT], 'bp__ring--hit');
    fire(core, 'bp__core--hit');
    ringTurn += 1;
  }
  bassAvg += (bass - bassAvg) * 0.12;
  timer = requestAnimationFrame(draw);
}
playBtn.addEventListener('click', function () {
  root.classList.remove('is-demo');
  if (!ctx) { buildAudio(); }
  playing = !playing;
  playBtn.setAttribute('aria-pressed', playing ? 'true' : 'false');
  playLabel.textContent = playing ? '停止' : '再生';
  // 자동재생 정책: AudioContext 는 사용자 제스처 안에서 resume() 해야 소리가 난다.
  if (playing) { ctx.resume(); timer = requestAnimationFrame(draw); }
  else { ctx.suspend(); cancelAnimationFrame(timer); }
});

// BeatParticles — vanilla 판(run/291/demo/04-audio-viz-beat-particles/index.html) 의 React 포트.
// 저음 에너지가 자기 이동 평균을 넘는 순간(비트)에 겹 하나가 터진다. 코어는 늘 그 자리에 남는다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./BeatParticles.module.scss";

export type BeatParticlesProps = {
  /** 격자 미리보기용 2초 자동 루프. 재생을 누르면 꺼진다 */
  demo?: boolean;
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type DotVars = CSSProperties & { "--a"?: string; "--i"?: string };
/** ambient 타입에 useRef<T> 의 null 합집합이 없어 로컬로 좁힌다 */
type Box<T> = { current: T };

const RING_COUNT = 4;
const DOT_COUNT = 8;

export default function BeatParticles({
  demo = true,
  duration = 220,
  easing = "cubic-bezier(.2, 1.4, .4, 1)",
  color = "#ffd23f",
}: BeatParticlesProps) {
  const [live, setLive] = useState(demo);
  const [playing, setPlaying] = useState(false);
  const [beats, setBeats] = useState(0);
  const [hitRing, setHitRing] = useState(-1);
  const [coreHit, setCoreHit] = useState(false);
  const ctxBox = useRef(null) as Box<AudioContext | null>;
  const nodeBox = useRef(null) as Box<AnalyserNode | null>;
  const frameBox = useRef(0) as Box<number>;
  const avgBox = useRef(0) as Box<number>;
  const lastBeatBox = useRef(0) as Box<number>;
  const turnBox = useRef(0) as Box<number>;
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  useEffect(() => {
    return () => {
      cancelAnimationFrame(frameBox.current);
      if (ctxBox.current) {
        ctxBox.current.close();
      }
    };
  }, []);

  function buildAudio() {
    const ctx = new AudioContext();
    const analyser = ctx.createAnalyser();
    analyser.fftSize = 128;
    analyser.smoothingTimeConstant = 0.5;
    const out = ctx.createGain();
    out.gain.value = 0.08;
    out.connect(analyser);
    analyser.connect(ctx.destination);
    // 킥 — 낮은 사인파를 0.5초마다 한 번씩 쳐서 2초에 네 번 비트(120BPM).
    const kick = ctx.createOscillator();
    const kickAmp = ctx.createGain();
    kick.type = "sine";
    kick.frequency.value = 70;
    kickAmp.gain.value = 0;
    for (let b = 0; b < 128; b++) {
      const at = ctx.currentTime + b * 0.5;
      kickAmp.gain.setValueAtTime(1, at);
      kickAmp.gain.exponentialRampToValueAtTime(0.004, at + 0.3);
    }
    kick.connect(kickAmp);
    kickAmp.connect(out);
    kick.start();
    // 패드 — "평균보다 큰 순간"이 성립하려면 바닥 에너지가 필요하다.
    [110, 164.81, 220].forEach((hz) => {
      const pad = ctx.createOscillator();
      const padAmp = ctx.createGain();
      pad.type = "sawtooth";
      pad.frequency.value = hz;
      padAmp.gain.value = 0.035;
      pad.connect(padAmp);
      padAmp.connect(out);
      pad.start();
    });
    ctxBox.current = ctx;
    nodeBox.current = analyser;
  }

  function draw() {
    const analyser = nodeBox.current;
    if (!analyser) {
      return;
    }
    const freq = new Uint8Array(analyser.frequencyBinCount);
    analyser.getByteFrequencyData(freq);
    let bass = 0;
    for (let k = 0; k < 4; k++) {
      bass += freq[k];
    }
    bass = bass / (4 * 255);
    const now = performance.now();
    // 비트 판정: 저음 에너지가 직전 구간의 이동 평균 × 1.3 을 넘는 순간이다(고정 문턱은 셈여림에 약하다).
    if (bass > avgBox.current * 1.3 && bass > 0.14 && now - lastBeatBox.current > 220) {
      lastBeatBox.current = now;
      turnBox.current += 1;
      setBeats(turnBox.current);
      setHitRing(turnBox.current % RING_COUNT);
      setCoreHit(true);
      window.setTimeout(() => setCoreHit(false), 460);
    }
    avgBox.current += (bass - avgBox.current) * 0.12;
    frameBox.current = requestAnimationFrame(draw);
  }

  function toggle() {
    setLive(false);
    if (!ctxBox.current) {
      buildAudio();
    }
    const ctx = ctxBox.current;
    if (!ctx) {
      return;
    }
    const next = !playing;
    setPlaying(next);
    // 자동재생 정책: AudioContext 는 사용자 제스처 뒤 resume() 해야 소리가 난다.
    if (next) {
      ctx.resume();
      frameBox.current = requestAnimationFrame(draw);
    } else {
      ctx.suspend();
      cancelAnimationFrame(frameBox.current);
    }
  }

  const cls = [styles.root, live ? styles.isDemo : ""].filter(Boolean).join(" ");
  const rings = Array.from({ length: RING_COUNT }, (_, r) => (
    <div className={[styles.ring, hitRing === r ? styles.ringHit : ""].join(" ")} key={r}>
      {Array.from({ length: DOT_COUNT }, (_, d) => {
        const dotVars: DotVars = {
          "--a": (((d * 45 + r * 11.25) % 360)).toFixed(2) + "deg",
          "--i": String(r),
        };
        return <i className={styles.dot} key={d} style={dotVars} />;
      })}
    </div>
  ));
  return (
    <main className={styles.stage} style={vars}>
      <svg width="0" height="0" style={{ position: "absolute" }} aria-hidden="true">
        <defs>
          <symbol id="ico-waveform" viewBox="0 0 256 256" fill="currentColor">
            <path d="M208 96v64H48V96Z" opacity=".2" />
            <path d="M56 96v64a8 8 0 0 1-16 0V96a8 8 0 0 1 16 0m32-72a8 8 0 0 0-8 8v192a8 8 0 0 0 16 0V32a8 8 0 0 0-8-8m40 32a8 8 0 0 0-8 8v128a8 8 0 0 0 16 0V64a8 8 0 0 0-8-8m40 32a8 8 0 0 0-8 8v64a8 8 0 0 0 16 0V96a8 8 0 0 0-8-8m40-16a8 8 0 0 0-8 8v96a8 8 0 0 0 16 0V72a8 8 0 0 0-8-8" />
          </symbol>
          <symbol id="ico-play" viewBox="0 0 256 256" fill="currentColor">
            <path d="M228.23 134.69L84.15 222.81A8 8 0 0 1 72 216.12V39.88a8 8 0 0 1 12.15-6.69l144.08 88.12a7.82 7.82 0 0 1 0 13.38" opacity=".2" />
            <path d="M232.4 114.49L88.32 26.35a16 16 0 0 0-16.2-.3A15.86 15.86 0 0 0 64 39.87v176.26A15.94 15.94 0 0 0 80 232a16.07 16.07 0 0 0 8.36-2.35l144.04-88.14a15.88 15.88 0 0 0 0-27.02M80 215.94V40l143.83 88Z" />
          </symbol>
          <symbol id="ico-pause" viewBox="0 0 256 256" fill="currentColor">
            <path d="M208 48v160a8 8 0 0 1-8 8h-40a8 8 0 0 1-8-8V48a8 8 0 0 1 8-8h40a8 8 0 0 1 8 8M96 40H56a8 8 0 0 0-8 8v160a8 8 0 0 0 8 8h40a8 8 0 0 0 8-8V48a8 8 0 0 0-8-8" opacity=".2" />
            <path d="M200 32h-40a16 16 0 0 0-16 16v160a16 16 0 0 0 16 16h40a16 16 0 0 0 16-16V48a16 16 0 0 0-16-16m0 176h-40V48h40zM96 32H56a16 16 0 0 0-16 16v160a16 16 0 0 0 16 16h40a16 16 0 0 0 16-16V48a16 16 0 0 0-16-16m0 176H56V48h40z" />
          </symbol>
        </defs>
      </svg>
      <div className={cls}>
        <p className={styles.hint}>
          <svg className={styles.ico} aria-hidden="true">
            <use href="#ico-waveform" />
          </svg>
          <span>재생을 누르면 실제 소리로 움직입니다</span>
        </p>
        <div className={styles.field} aria-hidden="true">
          {rings}
          <div className={[styles.core, coreHit ? styles.coreHit : ""].join(" ")}>
            <svg className={styles.coreIco} aria-hidden="true">
              <use href="#ico-waveform" />
            </svg>
          </div>
        </div>
        <div className={styles.foot}>
          <button className={styles.play} type="button" aria-pressed={playing} onClick={toggle}>
            <svg className={[styles.ico, playing ? styles.icoHidden : ""].join(" ")} aria-hidden="true">
              <use href="#ico-play" />
            </svg>
            <svg className={[styles.ico, playing ? "" : styles.icoHidden].join(" ")} aria-hidden="true">
              <use href="#ico-pause" />
            </svg>
            <span>{playing ? "정지" : "재생"}</span>
          </button>
          <p className={styles.count}>
            <span className={styles.countLabel}>잡은 비트</span>
            <b className={styles.countBox}>
              <span className={styles.countText} aria-live="polite">
                {beats}
              </span>
              <span className={styles.loop} aria-hidden="true">
                <i>1</i>
                <i>3</i>
                <i>5</i>
                <i>7</i>
              </span>
            </b>
          </p>
        </div>
      </div>
    </main>
  );
}