02A Flip Digit Card Whose Shadow Turns With the Hinge Angle

<main class="stage">
  <div class="cd is-demo" id="cd" data-demo="1">
    <div class="cd__view">
      <div class="cd__cards" data-r="cards" role="timer" aria-label="Time left flip cards"></div>
    </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-paper;
$fg: $subject-ink;
$btn: rgba($subject-ink, .1);
$card-bg: $subject-ink;
$card-fg: $subject-cream;
@include stage($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__view { display: grid; place-items: center; }
.cd__cards { display: flex; gap: $sp-3; --cw: clamp(72px, 20vw, 108px); }
.cd__card {
  position: relative;
  width: var(--cw);
  height: calc(var(--cw) * 1.4);
  transform-style: preserve-3d;
}
.cd__half {
  position: absolute;
  left: 0;
  width: 100%;
  height: 50%;
  overflow: hidden;
  background: $card-bg;
  color: $card-fg;
  backface-visibility: hidden;
}
.cd__half--top { top: 0; border-radius: $r-sm $r-sm 0 0; transform-origin: 50% 100%; border-bottom: 1px solid rgba($subject-cream, .12); }
.cd__half--bot { bottom: 0; border-radius: 0 0 $r-sm $r-sm; transform-origin: 50% 0; }
.cd__num {
  position: absolute;
  left: 0;
  width: 100%;
  height: 200%;
  display: grid;
  place-items: center;
  font-size: calc(var(--cw) * .98);
  font-weight: 800;
  line-height: 1;
  font-variant-numeric: tabular-nums;
}
.cd__half--top .cd__num { top: 0; }
.cd__half--bot .cd__num { top: -100%; }
.cd__shade { position: absolute; inset: 0; background: $stage-ink; opacity: 0; pointer-events: none; }
.cd__leaf { z-index: 2; }
.cd__leaf--bot { transform: rotateX(180deg); }

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffffff;
  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: #17141a;
  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(23, 20, 26, 0.1);
  color: #17141a;
  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__view {
  display: grid;
  place-items: center;
}

.cd__cards {
  display: flex;
  gap: 12px;
  --cw: clamp(72px, 20vw, 108px);
}

.cd__card {
  position: relative;
  width: var(--cw);
  height: calc(var(--cw) * 1.4);
  transform-style: preserve-3d;
}

.cd__half {
  position: absolute;
  left: 0;
  width: 100%;
  height: 50%;
  overflow: hidden;
  background: #17141a;
  color: #fff7e6;
  backface-visibility: hidden;
}

.cd__half--top {
  top: 0;
  border-radius: 8px 8px 0 0;
  transform-origin: 50% 100%;
  border-bottom: 1px solid rgba(255, 247, 230, 0.12);
}

.cd__half--bot {
  bottom: 0;
  border-radius: 0 0 8px 8px;
  transform-origin: 50% 0;
}

.cd__num {
  position: absolute;
  left: 0;
  width: 100%;
  height: 200%;
  display: grid;
  place-items: center;
  font-size: calc(var(--cw) * 0.98);
  font-weight: 800;
  line-height: 1;
  font-variant-numeric: tabular-nums;
}

.cd__half--top .cd__num {
  top: 0;
}

.cd__half--bot .cd__num {
  top: -100%;
}

.cd__shade {
  position: absolute;
  inset: 0;
  background: #17141a;
  opacity: 0;
  pointer-events: none;
}

.cd__leaf {
  z-index: 2;
}

.cd__leaf--bot {
  transform: rotateX(180deg);
}

(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 COUNT_FROM = 12;
    const DEMO_SECOND_MS = 150;
    const SECOND_MS = 1000;
    const HOLD_MS = 300;
    const FLIP_MS = 140;
    const STIFFNESS = 2200;
    const SHADE_MAX = 0.6;
    const DEPTH_PX = 360;
    let secondMs = demo ? DEMO_SECOND_MS : SECOND_MS;
    let totalMs = COUNT_FROM * secondMs;
    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);
    });
    const DAMPING = 2 * 0.55 * Math.sqrt(STIFFNESS);
    const cardsEl = q('[data-r="cards"]');
    function leaf(cls) {
        const el = document.createElement("div");
        el.className = cx("cd__half " + cls);
        const num = document.createElement("span");
        num.className = cx("cd__num");
        const shade = document.createElement("i");
        shade.className = cx("cd__shade");
        el.appendChild(num);
        el.appendChild(shade);
        return { el, num, shade };
    }
    function makeCard() {
        const el = document.createElement("div");
        el.className = cx("cd__card");
        el.style.perspective = DEPTH_PX + "px";
        const top = leaf("cd__half--top");
        const bot = leaf("cd__half--bot");
        const leafTop = leaf("cd__half--top cd__leaf cd__leaf--top");
        const leafBot = leaf("cd__half--bot cd__leaf cd__leaf--bot");
        [top, bot, leafTop, leafBot].forEach((l) => el.appendChild(l.el));
        cardsEl.appendChild(el);
        return { el, top, bot, leafTop, leafBot, cur: 0, nxt: 0, flipping: false, x: 0, v: 0, age: 0 };
    }
    const cards = [makeCard(), makeCard()];
    function setNums(c, topV, botV, ltV, lbV) {
        c.top.num.textContent = String(topV);
        c.bot.num.textContent = String(botV);
        c.leafTop.num.textContent = String(ltV);
        c.leafBot.num.textContent = String(lbV);
    }
    // 숫자가 바뀌면 접기를 새로 시작한다. 이전 접기가 아직 돌고 있으면 그 끝 값으로 먼저 굳힌다
    function setTarget(c, d) {
        const shown = c.flipping ? c.nxt : c.cur;
        if (shown === d)
            return;
        if (c.flipping)
            c.cur = c.nxt;
        c.nxt = d;
        if (reduce) {
            c.cur = d;
            c.flipping = false;
            return;
        }
        c.flipping = true;
        c.x = 0;
        c.v = 0;
        c.age = 0;
    }
    function place(c) {
        if (!c.flipping) {
            setNums(c, c.cur, c.cur, c.cur, c.cur);
            c.leafTop.el.style.display = "none";
            c.leafBot.el.style.display = "none";
            c.top.shade.style.opacity = "0";
            c.bot.shade.style.opacity = "0";
            return;
        }
        // 접힌 각도 θ (도). 스프링이 1 을 살짝 넘으면 θ 도 180 을 넘어 한 번 튕긴다
        const th = clamp(c.x, 0, 1.12) * 180;
        const s = SHADE_MAX * Math.abs(Math.sin((th * Math.PI) / 180));
        setNums(c, c.nxt, c.cur, c.cur, c.nxt);
        c.leafTop.el.style.display = th < 90 ? "block" : "none";
        c.leafBot.el.style.display = th >= 90 ? "block" : "none";
        c.leafTop.el.style.transform = "rotateX(" + (-th).toFixed(2) + "deg)";
        c.leafBot.el.style.transform = "rotateX(" + (180 - th).toFixed(2) + "deg)";
        // 두 반쪽과 종이 자신의 그림자 농도 = SHADE_MAX × sin θ
        c.top.shade.style.opacity = s.toFixed(3);
        c.bot.shade.style.opacity = s.toFixed(3);
        c.leafTop.shade.style.opacity = (s * 0.7).toFixed(3);
        c.leafBot.shade.style.opacity = (s * 0.7).toFixed(3);
    }
    function shownCount() {
        return Math.ceil(remainMs() / secondMs - 1e-6);
    }
    function resize() { }
    function resetState() {
        cards.forEach((c) => {
            c.flipping = false;
            c.x = 0;
            c.v = 0;
        });
    }
    hooks.stop = () => {
        elapsed *= SECOND_MS / secondMs;
        secondMs = SECOND_MS;
        totalMs = COUNT_FROM * secondMs;
    };
    function tick(dt) {
        const n = shownCount();
        setTarget(cards[0], Math.floor(n / 10));
        setTarget(cards[1], n % 10);
        cards.forEach((c) => {
            if (!c.flipping)
                return;
            c.age += dt * 1000;
            let left = dt;
            while (left > 0) {
                const h = Math.min(left, 1 / 240);
                c.v += (-STIFFNESS * (c.x - 1) - DAMPING * c.v) * h;
                c.x += c.v * h;
                left -= h;
            }
            if (c.age >= FLIP_MS) {
                c.cur = c.nxt;
                c.flipping = false;
            }
        });
    }
    function draw() {
        cards.forEach(place);
    }
    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());
    };
})();

// FlipDigitHingeShadow - vanilla 판(02-cd-flip-digit-hinge-shadow) 의 React 포트. 같은 계산을 useEffect 안에서 돌린다.
import { useEffect, useRef } from "react";
import styles from "./FlipDigitHingeShadow.module.scss";

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

export default function FlipDigitHingeShadow() {
  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 COUNT_FROM = 12;
    const DEMO_SECOND_MS = 150;
    const SECOND_MS = 1000;
    const HOLD_MS = 300;
    const FLIP_MS = 140;
    const STIFFNESS = 2200;
    const SHADE_MAX = 0.6;
    const DEPTH_PX = 360;
    let secondMs = demo ? DEMO_SECOND_MS : SECOND_MS;
    let totalMs = COUNT_FROM * secondMs;
    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);
    });
    type Leaf = { el: HTMLElement; num: HTMLElement; shade: HTMLElement };
    type Card = {
        el: HTMLElement;
        top: Leaf;
        bot: Leaf;
        leafTop: Leaf;
        leafBot: Leaf;
        cur: number;
        nxt: number;
        flipping: boolean;
        x: number;
        v: number;
        age: number;
    };
    const DAMPING = 2 * 0.55 * Math.sqrt(STIFFNESS);
    const cardsEl = q<HTMLElement>('[data-r="cards"]');
    function leaf(cls: string): Leaf {
        const el = document.createElement("div");
        el.className = cx("cd__half " + cls);
        const num = document.createElement("span");
        num.className = cx("cd__num");
        const shade = document.createElement("i");
        shade.className = cx("cd__shade");
        el.appendChild(num);
        el.appendChild(shade);
        return { el, num, shade };
    }
    function makeCard(): Card {
        const el = document.createElement("div");
        el.className = cx("cd__card");
        el.style.perspective = DEPTH_PX + "px";
        const top = leaf("cd__half--top");
        const bot = leaf("cd__half--bot");
        const leafTop = leaf("cd__half--top cd__leaf cd__leaf--top");
        const leafBot = leaf("cd__half--bot cd__leaf cd__leaf--bot");
        [top, bot, leafTop, leafBot].forEach((l) => el.appendChild(l.el));
        cardsEl.appendChild(el);
        return { el, top, bot, leafTop, leafBot, cur: 0, nxt: 0, flipping: false, x: 0, v: 0, age: 0 };
    }
    const cards: Card[] = [makeCard(), makeCard()];
    function setNums(c: Card, topV: number, botV: number, ltV: number, lbV: number): void {
        c.top.num.textContent = String(topV);
        c.bot.num.textContent = String(botV);
        c.leafTop.num.textContent = String(ltV);
        c.leafBot.num.textContent = String(lbV);
    }
    // 숫자가 바뀌면 접기를 새로 시작한다. 이전 접기가 아직 돌고 있으면 그 끝 값으로 먼저 굳힌다
    function setTarget(c: Card, d: number): void {
        const shown = c.flipping ? c.nxt : c.cur;
        if (shown === d) return;
        if (c.flipping) c.cur = c.nxt;
        c.nxt = d;
        if (reduce) {
            c.cur = d;
            c.flipping = false;
            return;
        }
        c.flipping = true;
        c.x = 0;
        c.v = 0;
        c.age = 0;
    }
    function place(c: Card): void {
        if (!c.flipping) {
            setNums(c, c.cur, c.cur, c.cur, c.cur);
            c.leafTop.el.style.display = "none";
            c.leafBot.el.style.display = "none";
            c.top.shade.style.opacity = "0";
            c.bot.shade.style.opacity = "0";
            return;
        }
        // 접힌 각도 θ (도). 스프링이 1 을 살짝 넘으면 θ 도 180 을 넘어 한 번 튕긴다
        const th = clamp(c.x, 0, 1.12) * 180;
        const s = SHADE_MAX * Math.abs(Math.sin((th * Math.PI) / 180));
        setNums(c, c.nxt, c.cur, c.cur, c.nxt);
        c.leafTop.el.style.display = th < 90 ? "block" : "none";
        c.leafBot.el.style.display = th >= 90 ? "block" : "none";
        c.leafTop.el.style.transform = "rotateX(" + (-th).toFixed(2) + "deg)";
        c.leafBot.el.style.transform = "rotateX(" + (180 - th).toFixed(2) + "deg)";
        // 두 반쪽과 종이 자신의 그림자 농도 = SHADE_MAX × sin θ
        c.top.shade.style.opacity = s.toFixed(3);
        c.bot.shade.style.opacity = s.toFixed(3);
        c.leafTop.shade.style.opacity = (s * 0.7).toFixed(3);
        c.leafBot.shade.style.opacity = (s * 0.7).toFixed(3);
    }
    function shownCount(): number {
        return Math.ceil(remainMs() / secondMs - 1e-6);
    }
    function resize(): void {}
    function resetState(): void {
        cards.forEach((c) => {
            c.flipping = false;
            c.x = 0;
            c.v = 0;
        });
    }
    hooks.stop = () => {
        elapsed *= SECOND_MS / secondMs;
        secondMs = SECOND_MS;
        totalMs = COUNT_FROM * secondMs;
    };
    function tick(dt: number): void {
        const n = shownCount();
        setTarget(cards[0], Math.floor(n / 10));
        setTarget(cards[1], n % 10);
        cards.forEach((c) => {
            if (!c.flipping) return;
            c.age += dt * 1000;
            let left = dt;
            while (left > 0) {
                const h = Math.min(left, 1 / 240);
                c.v += (-STIFFNESS * (c.x - 1) - DAMPING * c.v) * h;
                c.x += c.v * h;
                left -= h;
            }
            if (c.age >= FLIP_MS) {
                c.cur = c.nxt;
                c.flipping = false;
            }
        });
    }
    function draw(): void {
        cards.forEach(place);
    }
    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")}>
      <div className={cx("cd__cards")} data-r="cards" role="timer" aria-label="남은 시간 카드"></div>
      </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>
  );
}