04A Countdown Where the Burning Point Walks Along the Fuse Length and Throws Sparks

<main class="stage">
  <div class="cd is-demo" id="cd" data-demo="1">
    <div class="cd__view cd__view--scene">
      <canvas class="cd__cv" data-r="cv" role="img" aria-label="A flame burning along a fuse"></canvas>
      <p class="cd__read cd__read--corner" role="timer"><b data-r="num">1.5</b><span>s</span></p>
    </div>
    <div class="cd__bar">
      <button class="cd__btn" type="button" data-r="toggle">Pause</button>
      <button class="cd__btn" type="button" data-r="reset">Reset</button>
    </div>
    <p class="cd__sr" aria-live="polite" data-r="status"></p>
  </div>
</main>

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

// 핵심: 남은 시간 하나(경과는 rAF 타임스탬프 누적)에서 모양을 계산해 그린다. CSS 는 모양과 색만 맡는다.

$bg: $stage-ink;
$fg: $subject-cream;
$btn: rgba($subject-cream, .14);
@include stage($bg);
.cd {
  --view-h: min(calc(100vh - 2 * clamp(8px, 4vw, 24px) - 48px), 196px);
  position: relative;
  box-sizing: border-box;
  width: 100%;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-2;
  color: $fg;
  font-size: 14px;
  line-height: 20px;
  font-weight: 600;
}
.cd__view { position: relative; }
.cd__view--scene { width: 100%; height: var(--view-h); overflow: hidden; border-radius: $r-card; background: mix($stage-ink, $subject-lilac, 88%); }
.cd__cv { display: block; width: 100%; height: 100%; }
.cd__read { margin: 0; display: flex; align-items: baseline; gap: $sp-1; font-variant-numeric: tabular-nums; }
.cd__read b { font-size: 28px; line-height: 32px; font-weight: 800; }
.cd__read--center { position: absolute; left: 50%; top: 50%; transform: translate(-50%, -50%); }
.cd__read--corner { position: absolute; left: $sp-2; top: $sp-2; }
.cd__bar { display: flex; gap: $sp-2; }
.cd__btn {
  box-sizing: border-box;
  height: 36px;
  padding: 0 $sp-4;
  border: 0;
  border-radius: $r-control;
  background: $btn;
  color: $fg;
  font-size: 14px;
  line-height: 20px;
  font-weight: 700;
  cursor: pointer;
}
.cd__btn:focus-visible { outline: 2px solid $subject-lilac; outline-offset: 2px; }
.cd__sr { position: absolute; width: 1px; height: 1px; margin: 0; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }
@media (prefers-reduced-motion: reduce) {
  .cd__strip { filter: none !important; }
  .cd__leaf { display: none !important; }
}
.cd {
  --flame: #{mix($stage-yellow, $subject-cream, 60%)};
  --hot: #{$subject-pink};
  --rope: #{$subject-cream};
  --bomb: #{$subject-lilac};
}

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);
}

.cd {
  --view-h: min(calc(100vh - 2 * clamp(8px, 4vw, 24px) - 48px), 196px);
  position: relative;
  box-sizing: border-box;
  width: 100%;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
  color: #fff7e6;
  font-size: 14px;
  line-height: 20px;
  font-weight: 600;
}

.cd__view {
  position: relative;
}

.cd__view--scene {
  width: 100%;
  height: var(--view-h);
  overflow: hidden;
  border-radius: 16px;
  background: rgb(16.6431372549%, 14.431372549%, 20.9725490196%);
}

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

.cd__read {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 4px;
  font-variant-numeric: tabular-nums;
}

.cd__read b {
  font-size: 28px;
  line-height: 32px;
  font-weight: 800;
}

.cd__read--center {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}

.cd__read--corner {
  position: absolute;
  left: 8px;
  top: 8px;
}

.cd__bar {
  display: flex;
  gap: 8px;
}

.cd__btn {
  box-sizing: border-box;
  height: 36px;
  padding: 0 16px;
  border: 0;
  border-radius: 12px;
  background: rgba(255, 247, 230, 0.14);
  color: #fff7e6;
  font-size: 14px;
  line-height: 20px;
  font-weight: 700;
  cursor: pointer;
}

.cd__btn:focus-visible {
  outline: 2px solid #b9a0ff;
  outline-offset: 2px;
}

.cd__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: 0;
  overflow: hidden;
  clip: rect(0 0 0 0);
  white-space: nowrap;
}

@media (prefers-reduced-motion: reduce) {
  .cd__strip {
    filter: none !important;
  }
  .cd__leaf {
    display: none !important;
  }
}
.cd {
  --flame: rgb(100%, 88.1568627451%, 50.9019607843%);
  --hot: #ff9ebb;
  --rope: #fff7e6;
  --bomb: #b9a0ff;
}

(function () {
    const root = document.getElementById("cd");
    if (!root) return;
    function cx(s) {
        return s;
    }
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let running = demo;
    let elapsed = 0;
    let lastTs = -1;
    let lastDraw = -1e9;
    let raf = 0;
    const hooks = { stop: () => { } };
    const offs = [];
    const BURN_MS = 1500;
    const HOLD_MS = 300;
    const EMBER_N = 56;
    const SPARK_SPREAD = 1.1;
    let totalMs = BURN_MS;
    const cv = q('[data-r="cv"]');
    const ctx2d = cv.getContext("2d");
    if (!ctx2d)
        return () => { };
    const g = ctx2d;
    let cw = 1;
    let ch = 1;
    let dpr = 1;
    // 캔버스 안쪽 해상도를 화면 크기와 devicePixelRatio 에 맞춘다. 리사이즈마다 Reset 호출된다
    function fitCanvas() {
        const r = cv.getBoundingClientRect();
        dpr = Math.min(2, window.devicePixelRatio || 1);
        cw = Math.max(1, Math.round(r.width));
        ch = Math.max(1, Math.round(r.height));
        cv.width = Math.round(cw * dpr);
        cv.height = Math.round(ch * dpr);
        g.setTransform(dpr, 0, 0, dpr, 0, 0);
    }
    function cssVar(name) {
        return getComputedStyle(root).getPropertyValue(name).trim();
    }
    function parseColor(h) {
        // CSS 변수 값은 #rrggbb 로도, rgb(r, g, b) 로도, rgb(100%, 50%, 0%) 로도 나올 수 있다
        const m6 = /^#([0-9a-f]{6})$/i.exec(h);
        if (m6) {
            const n = parseInt(m6[1], 16);
            return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
        }
        const m = /([0-9.]+)(%?)[ ,]+([0-9.]+)(%?)[ ,]+([0-9.]+)(%?)/.exec(h);
        if (!m)
            return [255, 255, 255];
        const ch = (v, pct) => Math.round(pct ? (Number(v) * 255) / 100 : Number(v));
        return [ch(m[1], m[2]), ch(m[3], m[4]), ch(m[5], m[6])];
    }
    function rgba(c, a) {
        return "rgba(" + c[0] + "," + c[1] + "," + c[2] + "," + a.toFixed(3) + ")";
    }
    function q(sel) {
        return root.querySelector(sel);
    }
    function on(el, type, fn) {
        el.addEventListener(type, fn);
        offs.push(() => el.removeEventListener(type, fn));
    }
    function clamp(v, lo, hi) {
        return Math.max(lo, Math.min(hi, v));
    }
    // 지수 접근: 프레임률이 달라도 같은 속도가 되도록 dt 를 60fps 프레임 수로 환산해 쓴다
    function approach(x, target, per, dt) {
        return x + (target - x) * (1 - Math.pow(1 - per, dt * 60));
    }
    const toggle = q('[data-r="toggle"]');
    const resetBtn = q('[data-r="reset"]');
    const status = q('[data-r="status"]');
    // 남은 시간. 경과는 rAF 타임스탬프 차이를 누적한 값이라 시계를 따로 읽지 않는다
    function remainMs() {
        return clamp(totalMs - elapsed, 0, totalMs);
    }
    function syncButtons() {
        toggle.textContent = running ? "Pause" : "Start";
        toggle.setAttribute("aria-pressed", running ? "true" : "false");
    }
    // 스크린리더 안내는 멈출 때와 끝날 때만 갱신한다. 시연 루프 중에는 읽지 않는다
    function say(msg) {
        if (!demo)
            status.textContent = msg;
    }
    function stopDemo() {
        if (!demo)
            return;
        demo = false;
        root.classList.remove("is-demo");
        root.removeAttribute("data-demo");
        hooks.stop();
    }
    function restart() {
        elapsed = 0;
        resetState();
    }
    function update(dt) {
        tick(dt);
        draw();
    }
    function frame(ts) {
        raf = requestAnimationFrame(frame);
        const raw = lastTs < 0 ? 0 : ts - lastTs;
        lastTs = ts;
        if (running) {
            elapsed += raw;
            if (demo) {
                const cycle = totalMs + HOLD_MS;
                if (elapsed >= cycle) {
                    elapsed %= cycle;
                    resetState();
                }
            }
            else if (elapsed >= totalMs) {
                elapsed = totalMs;
                running = false;
                say("Time is up");
                syncButtons();
            }
        }
        if (reduce && ts - lastDraw < 250)
            return;
        lastDraw = ts;
        update(Math.min(raw, 50) / 1000);
    }
    on(toggle, "click", () => {
        stopDemo();
        if (!running && elapsed >= totalMs)
            restart();
        running = !running;
        say(running ? "" : "Paused");
        syncButtons();
        update(0);
    });
    on(resetBtn, "click", () => {
        stopDemo();
        running = false;
        restart();
        say("");
        syncButtons();
        update(0);
    });
    // 그림 좌표계는 220 × 100. 실제 화면 크기는 맞는 배율로 곱한다
    const LW = 220;
    const LH = 100;
    // 도화선 경로: 곡선 위 점을 직접 샘플링하고 점마다 누적 길이와 법선을 둔다
    function buildPath() {
        const N = 180;
        const pts = [];
        let len = 0;
        for (let i = 0; i <= N; i++) {
            const t = i / N;
            const x = 14 + t * 172;
            const y = 50 + 30 * Math.sin(t * Math.PI * 3.4) * (1 - 0.2 * t);
            if (i > 0)
                len += Math.hypot(x - pts[i - 1].x, y - pts[i - 1].y);
            pts.push({ x, y, nx: 0, ny: 0, s: len });
        }
        for (let i = 0; i <= N; i++) {
            const a = pts[Math.max(0, i - 1)];
            const b = pts[Math.min(N, i + 1)];
            const tx = b.x - a.x;
            const ty = b.y - a.y;
            const m = Math.hypot(tx, ty) || 1;
            pts[i].nx = -ty / m;
            pts[i].ny = tx / m;
        }
        return pts;
    }
    const PATH = buildPath();
    const PATH_LEN = PATH[PATH.length - 1].s;
    // 길이 s 에 해당하는 점(경로 위 선형 보간)
    function pointAt(s) {
        const t = clamp(s, 0, PATH_LEN);
        let lo = 0;
        let hi = PATH.length - 1;
        while (hi - lo > 1) {
            const mid = (lo + hi) >> 1;
            if (PATH[mid].s <= t)
                lo = mid;
            else
                hi = mid;
        }
        const a = PATH[lo];
        const b = PATH[hi];
        const f = b.s > a.s ? (t - a.s) / (b.s - a.s) : 0;
        return { x: a.x + (b.x - a.x) * f, y: a.y + (b.y - a.y) * f, nx: a.nx + (b.nx - a.nx) * f, ny: a.ny + (b.ny - a.ny) * f, s: t };
    }
    // 불똥 입자 풀: 고정 길이 배열을 돌려 쓴다
    const ex = new Float32Array(EMBER_N);
    const ey = new Float32Array(EMBER_N);
    const evx = new Float32Array(EMBER_N);
    const evy = new Float32Array(EMBER_N);
    const elife = new Float32Array(EMBER_N);
    const EMBER_LIFE = 0.5;
    let seed = 7;
    let emitAcc = 0;
    let slot = 0;
    function rand() {
        seed = (Math.imul(seed, 1664525) + 1013904223) >>> 0;
        return seed / 4294967296;
    }
    let cFlame = [255, 233, 159];
    let cHot = [255, 158, 187];
    let cRope = [255, 247, 230];
    let cBomb = [185, 160, 255];
    function resize() {
        fitCanvas();
        cFlame = parseColor(cssVar("--flame"));
        cHot = parseColor(cssVar("--hot"));
        cRope = parseColor(cssVar("--rope"));
        cBomb = parseColor(cssVar("--bomb"));
    }
    const num = q('[data-r="num"]');
    function burnS() {
        return PATH_LEN * (1 - remainMs() / totalMs);
    }
    function resetState() {
        elife.fill(0);
        emitAcc = 0;
        seed = 7;
    }
    function emit(p) {
        const side = rand() < 0.5 ? -1 : 1;
        const th = (rand() - 0.5) * 2 * SPARK_SPREAD;
        const c = Math.cos(th);
        const sn = Math.sin(th);
        const dx = p.nx * side;
        const dy = p.ny * side;
        const sp = 28 + 60 * rand();
        ex[slot] = p.x;
        ey[slot] = p.y;
        evx[slot] = (dx * c - dy * sn) * sp - 14;
        evy[slot] = (dx * sn + dy * c) * sp - 10;
        elife[slot] = EMBER_LIFE * (0.6 + 0.4 * rand());
        slot = (slot + 1) % EMBER_N;
    }
    function tick(dt) {
        if (reduce)
            return;
        const p = pointAt(burnS());
        if (running && remainMs() > 0) {
            emitAcc += dt * (EMBER_N / EMBER_LIFE) * 0.8;
            while (emitAcc >= 1) {
                emit(p);
                emitAcc -= 1;
            }
        }
        for (let i = 0; i < EMBER_N; i++) {
            if (elife[i] <= 0)
                continue;
            elife[i] -= dt;
            evy[i] += 120 * dt;
            evx[i] *= Math.pow(0.9, dt * 60);
            ex[i] += evx[i] * dt;
            ey[i] += evy[i] * dt;
        }
    }
    function draw() {
        const sc = Math.min(cw / LW, ch / LH);
        const ox = (cw - LW * sc) / 2;
        const oy = (ch - LH * sc) / 2;
        g.clearRect(0, 0, cw, ch);
        g.save();
        g.translate(ox, oy);
        g.scale(sc, sc);
        const s = burnS();
        const p = pointAt(s);
        const burning = remainMs() > 0;
        // 불빛이 번지는 배경
        const gl = g.createRadialGradient(p.x, p.y, 0, p.x, p.y, 64);
        gl.addColorStop(0, rgba(cHot, burning ? 0.42 : 0.12));
        gl.addColorStop(1, rgba(cHot, 0));
        g.fillStyle = gl;
        g.fillRect(-ox / sc, -oy / sc, cw / sc, ch / sc);
        g.lineCap = "round";
        g.lineJoin = "round";
        // 탄 부분(재)
        g.strokeStyle = rgba(cRope, 0.2);
        g.lineWidth = 2.4;
        g.beginPath();
        g.moveTo(PATH[0].x, PATH[0].y);
        for (let i = 1; i < PATH.length && PATH[i].s <= s; i++)
            g.lineTo(PATH[i].x, PATH[i].y);
        g.lineTo(p.x, p.y);
        g.stroke();
        // 남은 도화선
        g.strokeStyle = rgba(cRope, 0.95);
        g.lineWidth = 3.4;
        g.setLineDash([4, 2.4]);
        g.beginPath();
        g.moveTo(p.x, p.y);
        for (let i = 0; i < PATH.length; i++)
            if (PATH[i].s > s)
                g.lineTo(PATH[i].x, PATH[i].y);
        g.stroke();
        g.setLineDash([]);
        // 폭탄
        const end = PATH[PATH.length - 1];
        g.fillStyle = rgba(cBomb, 1);
        g.beginPath();
        g.arc(end.x + 14, end.y, 13, 0, Math.PI * 2);
        g.fill();
        g.fillStyle = rgba(cRope, 0.35);
        g.beginPath();
        g.arc(end.x + 10, end.y - 4, 4, 0, Math.PI * 2);
        g.fill();
        // 불똥
        if (!reduce) {
            for (let i = 0; i < EMBER_N; i++) {
                if (elife[i] <= 0)
                    continue;
                const a = clamp(elife[i] / EMBER_LIFE, 0, 1);
                g.fillStyle = rgba(a > 0.5 ? cFlame : cHot, a);
                g.beginPath();
                g.arc(ex[i], ey[i], 0.8 + 1.6 * a, 0, Math.PI * 2);
                g.fill();
            }
        }
        // 불꽃 머리
        const flick = reduce ? 1 : 1 + 0.18 * Math.sin(elapsed * 0.05) + 0.1 * Math.sin(elapsed * 0.13);
        const fl = g.createRadialGradient(p.x, p.y, 0, p.x, p.y, 9 * flick);
        fl.addColorStop(0, rgba([255, 255, 255], 1));
        fl.addColorStop(0.4, rgba(cFlame, 0.95));
        fl.addColorStop(1, rgba(cHot, 0));
        g.fillStyle = fl;
        g.beginPath();
        g.arc(p.x, p.y, 9 * flick, 0, Math.PI * 2);
        g.fill();
        g.restore();
        num.textContent = (remainMs() / 1000).toFixed(1);
    }
    const ro = new ResizeObserver(() => {
        resize();
        draw();
    });
    ro.observe(root);
    offs.push(() => ro.disconnect());
    resize();
    resetState();
    syncButtons();
    update(0);
    raf = requestAnimationFrame(frame);
    return () => {
        cancelAnimationFrame(raf);
        offs.forEach((f) => f());
    };
})();

// FuseBurnSparkTrail - vanilla 판(04-cd-fuse-burn-spark-trail) 의 React 포트. 같은 계산을 useEffect 안에서 돌린다.
import { useEffect, useRef } from "react";
import styles from "./FuseBurnSparkTrail.module.scss";

const cx = (s: string): string => s.split(" ").map((n) => styles[n] ?? n).join(" ");

export default function FuseBurnSparkTrail() {
  const rootRef = useRef<HTMLDivElement | null>(null);

  useEffect(() => {
    const rootEl = rootRef.current;
    if (!rootEl) return;
    const root: HTMLElement = rootEl;
    const reduce: boolean = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo: boolean = !reduce;
    let running: boolean = demo;
    let elapsed = 0;
    let lastTs = -1;
    let lastDraw = -1e9;
    let raf = 0;
    const hooks: { stop: () => void } = { stop: () => {} };
    const offs: Array<() => void> = [];
    const BURN_MS = 1500;
    const HOLD_MS = 300;
    const EMBER_N = 56;
    const SPARK_SPREAD = 1.1;
    let totalMs = BURN_MS;
    const cv = q<HTMLCanvasElement>('[data-r="cv"]');
    const ctx2d = cv.getContext("2d");
    if (!ctx2d) return () => {};
    const g: CanvasRenderingContext2D = ctx2d;
    let cw = 1;
    let ch = 1;
    let dpr = 1;
    // 캔버스 안쪽 해상도를 화면 크기와 devicePixelRatio 에 맞춘다. 리사이즈마다 다시 호출된다
    function fitCanvas(): void {
        const r = cv.getBoundingClientRect();
        dpr = Math.min(2, window.devicePixelRatio || 1);
        cw = Math.max(1, Math.round(r.width));
        ch = Math.max(1, Math.round(r.height));
        cv.width = Math.round(cw * dpr);
        cv.height = Math.round(ch * dpr);
        g.setTransform(dpr, 0, 0, dpr, 0, 0);
    }
    function cssVar(name: string): string {
        return getComputedStyle(root).getPropertyValue(name).trim();
    }
    function parseColor(h: string): number[] {
        // CSS 변수 값은 #rrggbb 로도, rgb(r, g, b) 로도, rgb(100%, 50%, 0%) 로도 나올 수 있다
        const m6 = /^#([0-9a-f]{6})$/i.exec(h);
        if (m6) {
            const n = parseInt(m6[1], 16);
            return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
        }
        const m = /([0-9.]+)(%?)[ ,]+([0-9.]+)(%?)[ ,]+([0-9.]+)(%?)/.exec(h);
        if (!m) return [255, 255, 255];
        const ch = (v: string, pct: string): number => Math.round(pct ? (Number(v) * 255) / 100 : Number(v));
        return [ch(m[1], m[2]), ch(m[3], m[4]), ch(m[5], m[6])];
    }
    function rgba(c: number[], a: number): string {
        return "rgba(" + c[0] + "," + c[1] + "," + c[2] + "," + a.toFixed(3) + ")";
    }
    function q<T extends Element>(sel: string): T {
        return root.querySelector(sel) as T;
    }
    function on(el: EventTarget, type: string, fn: (e: Event) => void): void {
        el.addEventListener(type, fn);
        offs.push(() => el.removeEventListener(type, fn));
    }
    function clamp(v: number, lo: number, hi: number): number {
        return Math.max(lo, Math.min(hi, v));
    }
    // 지수 접근: 프레임률이 달라도 같은 속도가 되도록 dt 를 60fps 프레임 수로 환산해 쓴다
    function approach(x: number, target: number, per: number, dt: number): number {
        return x + (target - x) * (1 - Math.pow(1 - per, dt * 60));
    }
    const toggle = q<HTMLButtonElement>('[data-r="toggle"]');
    const resetBtn = q<HTMLButtonElement>('[data-r="reset"]');
    const status = q<HTMLElement>('[data-r="status"]');
    // 남은 시간. 경과는 rAF 타임스탬프 차이를 누적한 값이라 시계를 따로 읽지 않는다
    function remainMs(): number {
        return clamp(totalMs - elapsed, 0, totalMs);
    }
    function syncButtons(): void {
        toggle.textContent = running ? "멈춤" : "시작";
        toggle.setAttribute("aria-pressed", running ? "true" : "false");
    }
    // 스크린리더 안내는 멈출 때와 끝날 때만 갱신한다. 시연 루프 중에는 읽지 않는다
    function say(msg: string): void {
        if (!demo) status.textContent = msg;
    }
    function stopDemo(): void {
        if (!demo) return;
        demo = false;
        root.classList.remove("is-demo");
        root.removeAttribute("data-demo");
        hooks.stop();
    }
    function restart(): void {
        elapsed = 0;
        resetState();
    }
    function update(dt: number): void {
        tick(dt);
        draw();
    }
    function frame(ts: number): void {
        raf = requestAnimationFrame(frame);
        const raw = lastTs < 0 ? 0 : ts - lastTs;
        lastTs = ts;
        if (running) {
            elapsed += raw;
            if (demo) {
                const cycle = totalMs + HOLD_MS;
                if (elapsed >= cycle) {
                    elapsed %= cycle;
                    resetState();
                }
            } else if (elapsed >= totalMs) {
                elapsed = totalMs;
                running = false;
                say("시간이 다 됐습니다");
                syncButtons();
            }
        }
        if (reduce && ts - lastDraw < 250) return;
        lastDraw = ts;
        update(Math.min(raw, 50) / 1000);
    }
    on(toggle, "click", () => {
        stopDemo();
        if (!running && elapsed >= totalMs) restart();
        running = !running;
        say(running ? "" : "멈췄습니다");
        syncButtons();
        update(0);
    });
    on(resetBtn, "click", () => {
        stopDemo();
        running = false;
        restart();
        say("");
        syncButtons();
        update(0);
    });
    // 그림 좌표계는 220 × 100. 실제 화면 크기는 맞는 배율로 곱한다
    const LW = 220;
    const LH = 100;
    type Pt = { x: number; y: number; nx: number; ny: number; s: number };
    // 도화선 경로: 곡선 위 점을 직접 샘플링하고 점마다 누적 길이와 법선을 둔다
    function buildPath(): Pt[] {
        const N = 180;
        const pts: Pt[] = [];
        let len = 0;
        for (let i = 0; i <= N; i++) {
            const t = i / N;
            const x = 14 + t * 172;
            const y = 50 + 30 * Math.sin(t * Math.PI * 3.4) * (1 - 0.2 * t);
            if (i > 0) len += Math.hypot(x - pts[i - 1].x, y - pts[i - 1].y);
            pts.push({ x, y, nx: 0, ny: 0, s: len });
        }
        for (let i = 0; i <= N; i++) {
            const a = pts[Math.max(0, i - 1)];
            const b = pts[Math.min(N, i + 1)];
            const tx = b.x - a.x;
            const ty = b.y - a.y;
            const m = Math.hypot(tx, ty) || 1;
            pts[i].nx = -ty / m;
            pts[i].ny = tx / m;
        }
        return pts;
    }
    const PATH = buildPath();
    const PATH_LEN = PATH[PATH.length - 1].s;
    // 길이 s 에 해당하는 점(경로 위 선형 보간)
    function pointAt(s: number): Pt {
        const t = clamp(s, 0, PATH_LEN);
        let lo = 0;
        let hi = PATH.length - 1;
        while (hi - lo > 1) {
            const mid = (lo + hi) >> 1;
            if (PATH[mid].s <= t) lo = mid;
            else hi = mid;
        }
        const a = PATH[lo];
        const b = PATH[hi];
        const f = b.s > a.s ? (t - a.s) / (b.s - a.s) : 0;
        return { x: a.x + (b.x - a.x) * f, y: a.y + (b.y - a.y) * f, nx: a.nx + (b.nx - a.nx) * f, ny: a.ny + (b.ny - a.ny) * f, s: t };
    }
    // 불똥 입자 풀: 고정 길이 배열을 돌려 쓴다
    const ex = new Float32Array(EMBER_N);
    const ey = new Float32Array(EMBER_N);
    const evx = new Float32Array(EMBER_N);
    const evy = new Float32Array(EMBER_N);
    const elife = new Float32Array(EMBER_N);
    const EMBER_LIFE = 0.5;
    let seed = 7;
    let emitAcc = 0;
    let slot = 0;
    function rand(): number {
        seed = (Math.imul(seed, 1664525) + 1013904223) >>> 0;
        return seed / 4294967296;
    }
    let cFlame = [255, 233, 159];
    let cHot = [255, 158, 187];
    let cRope = [255, 247, 230];
    let cBomb = [185, 160, 255];
    function resize(): void {
        fitCanvas();
        cFlame = parseColor(cssVar("--flame"));
        cHot = parseColor(cssVar("--hot"));
        cRope = parseColor(cssVar("--rope"));
        cBomb = parseColor(cssVar("--bomb"));
    }
    const num = q<HTMLElement>('[data-r="num"]');
    function burnS(): number {
        return PATH_LEN * (1 - remainMs() / totalMs);
    }
    function resetState(): void {
        elife.fill(0);
        emitAcc = 0;
        seed = 7;
    }
    function emit(p: Pt): void {
        const side = rand() < 0.5 ? -1 : 1;
        const th = (rand() - 0.5) * 2 * SPARK_SPREAD;
        const c = Math.cos(th);
        const sn = Math.sin(th);
        const dx = p.nx * side;
        const dy = p.ny * side;
        const sp = 28 + 60 * rand();
        ex[slot] = p.x;
        ey[slot] = p.y;
        evx[slot] = (dx * c - dy * sn) * sp - 14;
        evy[slot] = (dx * sn + dy * c) * sp - 10;
        elife[slot] = EMBER_LIFE * (0.6 + 0.4 * rand());
        slot = (slot + 1) % EMBER_N;
    }
    function tick(dt: number): void {
        if (reduce) return;
        const p = pointAt(burnS());
        if (running && remainMs() > 0) {
            emitAcc += dt * (EMBER_N / EMBER_LIFE) * 0.8;
            while (emitAcc >= 1) {
                emit(p);
                emitAcc -= 1;
            }
        }
        for (let i = 0; i < EMBER_N; i++) {
            if (elife[i] <= 0) continue;
            elife[i] -= dt;
            evy[i] += 120 * dt;
            evx[i] *= Math.pow(0.9, dt * 60);
            ex[i] += evx[i] * dt;
            ey[i] += evy[i] * dt;
        }
    }
    function draw(): void {
        const sc = Math.min(cw / LW, ch / LH);
        const ox = (cw - LW * sc) / 2;
        const oy = (ch - LH * sc) / 2;
        g.clearRect(0, 0, cw, ch);
        g.save();
        g.translate(ox, oy);
        g.scale(sc, sc);
        const s = burnS();
        const p = pointAt(s);
        const burning = remainMs() > 0;
        // 불빛이 번지는 배경
        const gl = g.createRadialGradient(p.x, p.y, 0, p.x, p.y, 64);
        gl.addColorStop(0, rgba(cHot, burning ? 0.42 : 0.12));
        gl.addColorStop(1, rgba(cHot, 0));
        g.fillStyle = gl;
        g.fillRect(-ox / sc, -oy / sc, cw / sc, ch / sc);
        g.lineCap = "round";
        g.lineJoin = "round";
        // 탄 부분(재)
        g.strokeStyle = rgba(cRope, 0.2);
        g.lineWidth = 2.4;
        g.beginPath();
        g.moveTo(PATH[0].x, PATH[0].y);
        for (let i = 1; i < PATH.length && PATH[i].s <= s; i++) g.lineTo(PATH[i].x, PATH[i].y);
        g.lineTo(p.x, p.y);
        g.stroke();
        // 남은 도화선
        g.strokeStyle = rgba(cRope, 0.95);
        g.lineWidth = 3.4;
        g.setLineDash([4, 2.4]);
        g.beginPath();
        g.moveTo(p.x, p.y);
        for (let i = 0; i < PATH.length; i++) if (PATH[i].s > s) g.lineTo(PATH[i].x, PATH[i].y);
        g.stroke();
        g.setLineDash([]);
        // 폭탄
        const end = PATH[PATH.length - 1];
        g.fillStyle = rgba(cBomb, 1);
        g.beginPath();
        g.arc(end.x + 14, end.y, 13, 0, Math.PI * 2);
        g.fill();
        g.fillStyle = rgba(cRope, 0.35);
        g.beginPath();
        g.arc(end.x + 10, end.y - 4, 4, 0, Math.PI * 2);
        g.fill();
        // 불똥
        if (!reduce) {
            for (let i = 0; i < EMBER_N; i++) {
                if (elife[i] <= 0) continue;
                const a = clamp(elife[i] / EMBER_LIFE, 0, 1);
                g.fillStyle = rgba(a > 0.5 ? cFlame : cHot, a);
                g.beginPath();
                g.arc(ex[i], ey[i], 0.8 + 1.6 * a, 0, Math.PI * 2);
                g.fill();
            }
        }
        // 불꽃 머리
        const flick = reduce ? 1 : 1 + 0.18 * Math.sin(elapsed * 0.05) + 0.1 * Math.sin(elapsed * 0.13);
        const fl = g.createRadialGradient(p.x, p.y, 0, p.x, p.y, 9 * flick);
        fl.addColorStop(0, rgba([255, 255, 255], 1));
        fl.addColorStop(0.4, rgba(cFlame, 0.95));
        fl.addColorStop(1, rgba(cHot, 0));
        g.fillStyle = fl;
        g.beginPath();
        g.arc(p.x, p.y, 9 * flick, 0, Math.PI * 2);
        g.fill();
        g.restore();
        num.textContent = (remainMs() / 1000).toFixed(1);
    }
    const ro = new ResizeObserver(() => {
        resize();
        draw();
    });
    ro.observe(root);
    offs.push(() => ro.disconnect());
    resize();
    resetState();
    syncButtons();
    update(0);
    raf = requestAnimationFrame(frame);
    return () => {
        cancelAnimationFrame(raf);
        offs.forEach((f) => f());
    };
  }, []);

  return (
    <div className={cx("cd") + " is-demo"} id="cd" data-demo="1" ref={rootRef}>
      <div className={cx("cd__view cd__view--scene")}>
      <canvas className={cx("cd__cv")} data-r="cv" role="img" aria-label="도화선을 따라 불이 타들어 가는 장면"></canvas>
      <p className={cx("cd__read cd__read--corner")} role="timer"><b data-r="num">1.5</b><span>초</span></p>
      </div>
      <div className={cx("cd__bar")}>
        <button className={cx("cd__btn")} type="button" data-r="toggle">멈춤</button>
        <button className={cx("cd__btn")} type="button" data-r="reset">다시</button>
      </div>
      <p className={cx("cd__sr")} aria-live="polite" data-r="status"></p>
    </div>
  );
}