02A Scene Where Pulling the Paper Below the Perforation and Letting Go Tears It Off Past a Set Distance and Springs It Back Short of It

<main class="stage">
<div class="rc is-demo" id="rc">
  <canvas class="rc__canvas" data-role="canvas" width="480" height="300" tabindex="0" aria-label="Dragging the paper below the perforation: past the set distance it tears off, short of it a spring pulls it back"></canvas>
  <p class="rc__status" data-role="status" aria-live="polite">A new receipt came out</p>
  <div class="rc__tools">
    <button class="rc__btn" data-role="tear" type="button">Tear</button>
    <button class="rc__btn" data-role="soft" type="button">Tug lightly</button>
  </div>
</div>
</main>

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

.rc {
  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;
  background: $subject-ink;
}

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

.rc__canvas:focus-visible {
  outline: 2px solid rgba(255, 255, 255, .85);
  outline-offset: -6px;
}

.rc__status {
  position: absolute;
  left: $sp-3;
  bottom: $sp-3;
  max-width: 46%;
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-control;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font-weight: 500;
  font-size: 11px;
  line-height: 1.4;
  pointer-events: none;
}

.rc__tools {
  position: absolute;
  right: $sp-3;
  bottom: $sp-3;
  display: flex;
  gap: $sp-2;
}

.rc__btn {
  margin: 0;
  padding: $sp-1 $sp-3;
  border: 0;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font: inherit;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  cursor: pointer;
}

.rc__btn[aria-pressed="true"] {
  background: $subject-mint;
}

.rc__btn:focus-visible {
  outline: 2px solid rgba(255, 255, 255, .95);
  outline-offset: 2px;
}

.rc__canvas { touch-action: none; }

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

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

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);
}

.rc {
  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);
  background: #17141a;
}

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

.rc__canvas:focus-visible {
  outline: 2px solid rgba(255, 255, 255, 0.85);
  outline-offset: -6px;
}

.rc__status {
  position: absolute;
  left: 12px;
  bottom: 12px;
  max-width: 46%;
  margin: 0;
  padding: 4px 12px;
  border-radius: 12px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font-weight: 500;
  font-size: 11px;
  line-height: 1.4;
  pointer-events: none;
}

.rc__tools {
  position: absolute;
  right: 12px;
  bottom: 12px;
  display: flex;
  gap: 8px;
}

.rc__btn {
  margin: 0;
  padding: 4px 12px;
  border: 0;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font: inherit;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  cursor: pointer;
}

.rc__btn[aria-pressed=true] {
  background: #4cd4a6;
}

.rc__btn:focus-visible {
  outline: 2px solid rgba(255, 255, 255, 0.95);
  outline-offset: 2px;
}

.rc__canvas {
  touch-action: none;
}

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

(function () {
    const root = document.getElementById("rc");
    if (!root) return;
    const W = 480;
    const H = 300;
    const FIXED_DT = 16;
    // 파스텔 팔레트 — 어두운 무대 위에 밝은 종이를 얹어 명도 차로 읽힌다
    const BG = [29, 26, 44];
    const PAPER = [255, 248, 234];
    const INK = [58, 48, 76];
    const MINT = [126, 226, 192];
    const SKY = [124, 196, 255];
    const LILAC = [190, 164, 255];
    const PINK = [255, 160, 190];
    const PEACH = [255, 208, 168];
    function clamp(v, lo, hi) { return v < lo ? lo : (v > hi ? hi : v); }
    function lerp(a, b, k) { return a + (b - a) * k; }
    function smooth(x) { const c = clamp(x, 0, 1); return c * c * (3 - 2 * c); }
    function rgba(c, a) {
        return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + clamp(a, 0, 1).toFixed(3) + ')';
    }
    function mix(a, b, k) { return [lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)]; }
    // 시드 고정 난수 — 같은 시드면 언제나 같은 수열 (선형 합동 방식)
    function lcg(seed) {
        let s = seed >>> 0;
        return function () {
            s = (Math.imul(s, 1664525) + 1013904223) >>> 0;
            return s / 4294967296;
        };
    }
    const canvasEl = root.querySelector('[data-role="canvas"]');
    const statusEl = root.querySelector('[data-role="status"]');
    const ctxRaw = canvasEl ? canvasEl.getContext('2d') : null;
    if (!canvasEl || !ctxRaw || !statusEl) {
        return;
    }
    const cv = canvasEl;
    const ctx = ctxRaw;
    const status = statusEl;
    const reduceQuery = window.matchMedia('(prefers-reduced-motion: reduce)');
    function fitCanvas() {
        const dpr = window.devicePixelRatio || 1;
        cv.width = Math.round(W * dpr);
        cv.height = Math.round(H * dpr);
        ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    }
    function onResize() {
        fitCanvas();
        if (reduceQuery.matches) {
            draw();
        }
    }
    fitCanvas();
    window.addEventListener('resize', onResize);
    // 화면 좌표(CSS px) → 장면 좌표(480x300). 리사이즈 뒤에도 늘 현재 크기로 다시 잰다
    function toScene(ev) {
        const r = cv.getBoundingClientRect();
        return [((ev.clientX - r.left) / r.width) * W, ((ev.clientY - r.top) / r.height) * H];
    }
    const TEAR_PX = 60;
    const GRAVITY = 2600;
    const SPRING_K = 190;
    const TOOTH_PX = 10;
    const DAMP = 17;
    const STRIP_W = 210;
    const TEAR_Y = 92;
    const PIECE_H = 132;
    const MAX_PULL = 170;
    const DEMO_A = 36;
    const DEMO_B = 96;
    const TXT_PULL = 'Pulling';
    const TXT_TORN = 'Torn off';
    const TXT_BACK = 'Sprang back';
    const TXT_READY = 'A new receipt came out';
    const LINES = ['Moonlight Shop', 'Milk', 'Bread', 'Apples', 'Total'];
    const PRICES = ['', '1,200', '2,800', '3,500', '7,500'];
    const tearBtn = root.querySelector('[data-role="tear"]');
    const softBtn = root.querySelector('[data-role="soft"]');
    // 상태: idle 대기 · drag 끌림 · back 스프링 복귀 · fall 낙하 · feed 새 종이
    let state = 'idle';
    let pull = 0;
    let vel = 0;
    let fallY = 0;
    let fallV = 0;
    let fallRot = 0;
    let fallVR = 0;
    let feed = 1;
    let clock = 0;
    let grabbed = false;
    let grabY = 0;
    let fake = false;
    let fakeT = 0;
    let fakeTarget = 0;
    let holdAt = 0;
    let autoCount = 0;
    let userTouched = false;
    const toothBreak = [];
    const rnd = lcg(461);
    for (let i = 0; i < 40; i += 1) {
        toothBreak.push(0.45 + 0.55 * rnd());
    }
    function teethCount() { return Math.floor(STRIP_W / TOOTH_PX); }
    // 끈 거리에 따라 이가 하나씩 끊긴다 — 이마다 끊기는 거리가 다르다
    function intactTeeth(d) {
        let n = 0;
        for (let i = 0; i < teethCount(); i += 1) {
            if (d < TEAR_PX * toothBreak[i % 40]) {
                n += 1;
            }
        }
        return n;
    }
    function release() {
        grabbed = false;
        fake = false;
        if (pull >= TEAR_PX) {
            state = 'fall';
            fallY = pull;
            fallV = 120;
            fallRot = 0;
            fallVR = 0.9;
            status.textContent = TXT_TORN;
        }
        else {
            state = 'back';
            vel = 0;
            status.textContent = TXT_BACK;
        }
    }
    function startFake(target) {
        if (state !== 'idle') {
            return;
        }
        fake = true;
        fakeT = 0;
        fakeTarget = target;
        state = 'drag';
        status.textContent = TXT_PULL;
    }
    function step() {
        const dt = FIXED_DT / 1000;
        clock += FIXED_DT;
        if (fake) {
            fakeT += FIXED_DT;
            const k = smooth(fakeT / 520);
            pull = fakeTarget * k;
            if (fakeT >= 700) {
                release();
            }
        }
        else if (state === 'idle' && !grabbed && clock > 200 && clock >= holdAt && !userTouched) {
            autoCount += 1;
            startFake(autoCount % 2 === 1 ? DEMO_A : DEMO_B);
        }
        if (state === 'back') {
            vel += (-SPRING_K * pull - DAMP * vel) * dt;
            pull += vel * dt;
            if (Math.abs(pull) < 0.4 && Math.abs(vel) < 4) {
                pull = 0;
                state = 'idle';
                holdAt = clock + 260;
            }
        }
        else if (state === 'fall') {
            fallV += GRAVITY * dt;
            fallY += fallV * dt;
            fallRot += fallVR * dt * 3;
            if (fallY > H + 40) {
                state = 'feed';
                feed = 0;
                pull = 0;
                status.textContent = TXT_READY;
            }
        }
        else if (state === 'feed') {
            feed = Math.min(1, feed + FIXED_DT / 520);
            if (feed >= 1) {
                state = 'idle';
                holdAt = clock + 380;
            }
        }
    }
    function settle() {
        state = 'idle';
        pull = 0;
        feed = 1;
    }
    function easeOutBack(k) {
        const c = clamp(k, 0, 1) - 1;
        return 1 + 2.2 * c * c * c + 1.2 * c * c;
    }
    function drawPiece(x, top, rot) {
        ctx.save();
        ctx.translate(x + STRIP_W / 2, top + PIECE_H / 2);
        ctx.rotate(rot);
        ctx.translate(-STRIP_W / 2, -PIECE_H / 2);
        ctx.fillStyle = rgba(BG, 0.45);
        ctx.fillRect(6, 8, STRIP_W, PIECE_H);
        ctx.fillStyle = rgba(PAPER, 1);
        ctx.fillRect(0, 0, STRIP_W, PIECE_H);
        ctx.fillStyle = rgba(INK, 0.08);
        for (let y = 4; y < PIECE_H; y += 8) {
            ctx.fillRect(0, y, STRIP_W, 1);
        }
        // 톱니 아래 가장자리
        ctx.fillStyle = rgba(PAPER, 1);
        ctx.beginPath();
        ctx.moveTo(0, PIECE_H);
        for (let i = 0; i <= teethCount(); i += 1) {
            ctx.lineTo(i * TOOTH_PX, PIECE_H + (i % 2 === 0 ? 0 : 6));
        }
        ctx.lineTo(STRIP_W, PIECE_H);
        ctx.closePath();
        ctx.fill();
        ctx.font = '600 12px "Noto Sans KR", ui-monospace, monospace';
        for (let i = 0; i < LINES.length; i += 1) {
            const y = 24 + i * 23;
            ctx.fillStyle = rgba(INK, 1);
            ctx.textAlign = i === 0 ? 'center' : 'left';
            ctx.fillText(LINES[i], i === 0 ? STRIP_W / 2 : 18, y);
            if (PRICES[i]) {
                ctx.textAlign = 'right';
                ctx.fillText(PRICES[i], STRIP_W - 18, y);
            }
        }
        ctx.restore();
    }
    function draw() {
        ctx.fillStyle = rgba(BG, 1);
        ctx.fillRect(0, 0, W, H);
        const x = (W - STRIP_W) / 2;
        // 위쪽 종이 — 인쇄기 틈에서 늘어진다
        ctx.fillStyle = rgba(mix(BG, LILAC, 0.3), 1);
        ctx.fillRect(x - 24, 0, STRIP_W + 48, 14);
        ctx.fillStyle = rgba(BG, 0.5);
        ctx.fillRect(x + 6, 14, STRIP_W, TEAR_Y - 14);
        ctx.fillStyle = rgba(PAPER, 1);
        ctx.fillRect(x, 14, STRIP_W, TEAR_Y - 14);
        ctx.fillStyle = rgba(INK, 0.08);
        for (let y = 18; y < TEAR_Y; y += 8) {
            ctx.fillRect(x, y, STRIP_W, 1);
        }
        // 위쪽 조각의 절취선 톱니
        const n = teethCount();
        ctx.fillStyle = rgba(PAPER, 1);
        ctx.beginPath();
        ctx.moveTo(x, TEAR_Y);
        for (let i = 0; i <= n; i += 1) {
            ctx.lineTo(x + i * TOOTH_PX, TEAR_Y + (i % 2 === 0 ? 0 : 5));
        }
        ctx.lineTo(x + STRIP_W, TEAR_Y);
        ctx.closePath();
        ctx.fill();
        // 새 종이는 위쪽 조각 뒤에서 미끄러져 나온다
        let top = TEAR_Y + pull;
        if (state === 'feed') {
            top = TEAR_Y - (1 - easeOutBack(feed)) * PIECE_H;
        }
        if (state === 'fall') {
            top = TEAR_Y + fallY;
        }
        drawPiece(x, top, state === 'fall' ? fallRot * 0.35 : 0);
        // 아직 이어진 이 — 끈 사이 틈에 남은 종이 섬유
        if (state === 'drag' || state === 'back') {
            const keep = intactTeeth(pull);
            const gap = Math.max(pull, 0);
            for (let i = 0; i < n; i += 1) {
                if (pull >= TEAR_PX * toothBreak[i % 40]) {
                    continue;
                }
                ctx.fillStyle = rgba(PAPER, 0.92);
                ctx.fillRect(x + i * TOOTH_PX + 2, TEAR_Y + 3, TOOTH_PX - 4, gap);
            }
            if (keep === 0) {
                ctx.fillStyle = rgba(PEACH, 0.6);
                ctx.fillRect(x, TEAR_Y + 2, STRIP_W, 1);
            }
        }
        // 기준선 — 이 거리를 넘기고 놓으면 찢어진다
        ctx.fillStyle = rgba(MINT, 0.5);
        ctx.fillRect(x - 30, TEAR_Y + TEAR_PX + 6, 22, 2);
        // 가짜 포인터 — 자동 시연에서 스스로 끈다
        if (fake || grabbed) {
            const cy = TEAR_Y + pull + PIECE_H - 24;
            ctx.fillStyle = rgba(SKY, 0.85);
            ctx.beginPath();
            ctx.arc(x + STRIP_W / 2, cy, 11, 0, Math.PI * 2);
            ctx.fill();
            ctx.strokeStyle = rgba(PAPER, 0.95);
            ctx.lineWidth = 2;
            ctx.stroke();
        }
    }
    // 움직임 줄이기 — 보간 없이 결과 상태로 바로 놓는다
    function finishNow() {
        if (state === 'fall') {
            status.textContent = TXT_TORN;
        }
        if (state === 'back') {
            status.textContent = TXT_BACK;
        }
        settle();
        draw();
    }
    cv.addEventListener('pointerdown', function (ev) {
        if (state !== 'idle' && state !== 'drag') {
            return;
        }
        const p = toScene(ev);
        if (p[1] < TEAR_Y + pull - 4) {
            return;
        }
        userTouched = true;
        fake = false;
        grabbed = true;
        grabY = p[1] - pull;
        state = 'drag';
        cv.setPointerCapture(ev.pointerId);
        status.textContent = TXT_PULL;
        if (reduceQuery.matches) {
            draw();
        }
    });
    cv.addEventListener('pointermove', function (ev) {
        if (!grabbed) {
            return;
        }
        pull = clamp(toScene(ev)[1] - grabY, 0, MAX_PULL);
        if (reduceQuery.matches) {
            draw();
        }
    });
    function endGrab() {
        if (!grabbed) {
            return;
        }
        release();
        if (reduceQuery.matches) {
            finishNow();
        }
    }
    cv.addEventListener('pointerup', endGrab);
    cv.addEventListener('pointercancel', endGrab);
    cv.addEventListener('keydown', function (ev) {
        if (ev.key === 'ArrowDown') {
            ev.preventDefault();
            userTouched = true;
            fake = false;
            if (state === 'idle' || state === 'drag') {
                state = 'drag';
                grabbed = false;
                pull = Math.min(MAX_PULL, pull + 12);
                status.textContent = TXT_PULL;
                if (reduceQuery.matches) {
                    draw();
                }
            }
        }
        else if (ev.key === 'ArrowUp') {
            ev.preventDefault();
            if (state === 'drag') {
                pull = Math.max(0, pull - 12);
                if (reduceQuery.matches) {
                    draw();
                }
            }
        }
        else if (ev.key === 'Enter') {
            ev.preventDefault();
            if (state === 'drag') {
                release();
                if (reduceQuery.matches) {
                    finishNow();
                }
            }
        }
    });
    function triggerPull(target) {
        userTouched = true;
        if (reduceQuery.matches) {
            pull = target;
            state = 'drag';
            release();
            finishNow();
            return;
        }
        if (state === 'idle') {
            startFake(target);
        }
    }
    if (tearBtn) {
        tearBtn.addEventListener('click', function () { triggerPull(TEAR_PX + 30); });
    }
    if (softBtn) {
        softBtn.addEventListener('click', function () { triggerPull(TEAR_PX * 0.5); });
    }
    let rafId = 0;
    let lastTs = -1;
    let acc = 0;
    function frame(ts) {
        if (lastTs < 0) {
            lastTs = ts;
        }
        acc += Math.min(ts - lastTs, 100);
        lastTs = ts;
        let n = 0;
        while (acc >= FIXED_DT && n < 6) {
            step();
            acc -= FIXED_DT;
            n += 1;
        }
        draw();
        rafId = window.requestAnimationFrame(frame);
    }
    if (reduceQuery.matches) {
        settle();
        draw();
    }
    else {
        rafId = window.requestAnimationFrame(frame);
    }
    return function () {
        window.cancelAnimationFrame(rafId);
        window.removeEventListener('resize', onResize);
    };
})();

// RecTearOffPerforationDragSpring - vanilla 판(02-rec-tear-off-perforation-drag-spring) 의 React 포트. 같은 계산을 useEffect 안에서 돌린다.
import { useEffect, useRef } from "react";
import styles from "./RecTearOffPerforationDragSpring.module.scss";

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

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

  useEffect(() => {
    const rootEl = rootRef.current;
    if (!rootEl) return;
    const root: HTMLElement = rootEl;
    type RGB = [number, number, number];
    const W = 480;
    const H = 300;
    const FIXED_DT = 16;
    // 파스텔 팔레트 — 어두운 무대 위에 밝은 종이를 얹어 명도 차로 읽힌다
    const BG: RGB = [29, 26, 44];
    const PAPER: RGB = [255, 248, 234];
    const INK: RGB = [58, 48, 76];
    const MINT: RGB = [126, 226, 192];
    const SKY: RGB = [124, 196, 255];
    const LILAC: RGB = [190, 164, 255];
    const PINK: RGB = [255, 160, 190];
    const PEACH: RGB = [255, 208, 168];
    function clamp(v: number, lo: number, hi: number): number { return v < lo ? lo : (v > hi ? hi : v); }
    function lerp(a: number, b: number, k: number): number { return a + (b - a) * k; }
    function smooth(x: number): number { const c = clamp(x, 0, 1); return c * c * (3 - 2 * c); }
    function rgba(c: RGB, a: number): string {
        return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + clamp(a, 0, 1).toFixed(3) + ')';
    }
    function mix(a: RGB, b: RGB, k: number): RGB { return [lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)]; }
    // 시드 고정 난수 — 같은 시드면 언제나 같은 수열 (선형 합동 방식)
    function lcg(seed: number): () => number {
        let s = seed >>> 0;
        return function (): number {
            s = (Math.imul(s, 1664525) + 1013904223) >>> 0;
            return s / 4294967296;
        };
    }
    const canvasEl = root.querySelector('[data-role="canvas"]') as HTMLCanvasElement | null;
    const statusEl = root.querySelector('[data-role="status"]') as HTMLElement | null;
    const ctxRaw = canvasEl ? canvasEl.getContext('2d') : null;
    if (!canvasEl || !ctxRaw || !statusEl) { return; }
    const cv: HTMLCanvasElement = canvasEl;
    const ctx: CanvasRenderingContext2D = ctxRaw;
    const status: HTMLElement = statusEl;
    const reduceQuery = window.matchMedia('(prefers-reduced-motion: reduce)');
    function fitCanvas(): void {
        const dpr = window.devicePixelRatio || 1;
        cv.width = Math.round(W * dpr);
        cv.height = Math.round(H * dpr);
        ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    }
    function onResize(): void {
        fitCanvas();
        if (reduceQuery.matches) { draw(); }
    }
    fitCanvas();
    window.addEventListener('resize', onResize);
    // 화면 좌표(CSS px) → 장면 좌표(480x300). 리사이즈 뒤에도 늘 현재 크기로 다시 잰다
    function toScene(ev: PointerEvent): [number, number] {
        const r = cv.getBoundingClientRect();
        return [((ev.clientX - r.left) / r.width) * W, ((ev.clientY - r.top) / r.height) * H];
    }
    const TEAR_PX = 60;
    const GRAVITY = 2600;
    const SPRING_K = 190;
    const TOOTH_PX = 10;
    const DAMP = 17;
    const STRIP_W = 210;
    const TEAR_Y = 92;
    const PIECE_H = 132;
    const MAX_PULL = 170;
    const DEMO_A = 36;
    const DEMO_B = 96;
    const TXT_PULL = '당기는 중';
    const TXT_TORN = '찢어졌다';
    const TXT_BACK = '되돌아갔다';
    const TXT_READY = '새 영수증이 나왔다';
    const LINES: string[] = ['달빛 상점', '우유', '식빵', '사과', '합계'];
    const PRICES: string[] = ['', '1,200', '2,800', '3,500', '7,500'];
    const tearBtn = root.querySelector('[data-role="tear"]') as HTMLButtonElement | null;
    const softBtn = root.querySelector('[data-role="soft"]') as HTMLButtonElement | null;
    // 상태: idle 대기 · drag 끌림 · back 스프링 복귀 · fall 낙하 · feed 새 종이
    let state = 'idle';
    let pull = 0;
    let vel = 0;
    let fallY = 0;
    let fallV = 0;
    let fallRot = 0;
    let fallVR = 0;
    let feed = 1;
    let clock = 0;
    let grabbed = false;
    let grabY = 0;
    let fake = false;
    let fakeT = 0;
    let fakeTarget = 0;
    let holdAt = 0;
    let autoCount = 0;
    let userTouched = false;
    const toothBreak: number[] = [];
    const rnd = lcg(461);
    for (let i = 0; i < 40; i += 1) { toothBreak.push(0.45 + 0.55 * rnd()); }
    function teethCount(): number { return Math.floor(STRIP_W / TOOTH_PX); }
    // 끈 거리에 따라 이가 하나씩 끊긴다 — 이마다 끊기는 거리가 다르다
    function intactTeeth(d: number): number {
        let n = 0;
        for (let i = 0; i < teethCount(); i += 1) { if (d < TEAR_PX * toothBreak[i % 40]) { n += 1; } }
        return n;
    }
    function release(): void {
        grabbed = false;
        fake = false;
        if (pull >= TEAR_PX) {
            state = 'fall';
            fallY = pull;
            fallV = 120;
            fallRot = 0;
            fallVR = 0.9;
            status.textContent = TXT_TORN;
        } else {
            state = 'back';
            vel = 0;
            status.textContent = TXT_BACK;
        }
    }
    function startFake(target: number): void {
        if (state !== 'idle') { return; }
        fake = true;
        fakeT = 0;
        fakeTarget = target;
        state = 'drag';
        status.textContent = TXT_PULL;
    }
    function step(): void {
        const dt = FIXED_DT / 1000;
        clock += FIXED_DT;
        if (fake) {
            fakeT += FIXED_DT;
            const k = smooth(fakeT / 520);
            pull = fakeTarget * k;
            if (fakeT >= 700) { release(); }
        } else if (state === 'idle' && !grabbed && clock > 200 && clock >= holdAt && !userTouched) {
            autoCount += 1;
            startFake(autoCount % 2 === 1 ? DEMO_A : DEMO_B);
        }
        if (state === 'back') {
            vel += (-SPRING_K * pull - DAMP * vel) * dt;
            pull += vel * dt;
            if (Math.abs(pull) < 0.4 && Math.abs(vel) < 4) {
                pull = 0;
                state = 'idle';
                holdAt = clock + 260;
            }
        } else if (state === 'fall') {
            fallV += GRAVITY * dt;
            fallY += fallV * dt;
            fallRot += fallVR * dt * 3;
            if (fallY > H + 40) {
                state = 'feed';
                feed = 0;
                pull = 0;
                status.textContent = TXT_READY;
            }
        } else if (state === 'feed') {
            feed = Math.min(1, feed + FIXED_DT / 520);
            if (feed >= 1) { state = 'idle'; holdAt = clock + 380; }
        }
    }
    function settle(): void {
        state = 'idle';
        pull = 0;
        feed = 1;
    }
    function easeOutBack(k: number): number {
        const c = clamp(k, 0, 1) - 1;
        return 1 + 2.2 * c * c * c + 1.2 * c * c;
    }
    function drawPiece(x: number, top: number, rot: number): void {
        ctx.save();
        ctx.translate(x + STRIP_W / 2, top + PIECE_H / 2);
        ctx.rotate(rot);
        ctx.translate(-STRIP_W / 2, -PIECE_H / 2);
        ctx.fillStyle = rgba(BG, 0.45);
        ctx.fillRect(6, 8, STRIP_W, PIECE_H);
        ctx.fillStyle = rgba(PAPER, 1);
        ctx.fillRect(0, 0, STRIP_W, PIECE_H);
        ctx.fillStyle = rgba(INK, 0.08);
        for (let y = 4; y < PIECE_H; y += 8) { ctx.fillRect(0, y, STRIP_W, 1); }
        // 톱니 아래 가장자리
        ctx.fillStyle = rgba(PAPER, 1);
        ctx.beginPath();
        ctx.moveTo(0, PIECE_H);
        for (let i = 0; i <= teethCount(); i += 1) { ctx.lineTo(i * TOOTH_PX, PIECE_H + (i % 2 === 0 ? 0 : 6)); }
        ctx.lineTo(STRIP_W, PIECE_H);
        ctx.closePath();
        ctx.fill();
        ctx.font = '600 12px "Noto Sans KR", ui-monospace, monospace';
        for (let i = 0; i < LINES.length; i += 1) {
            const y = 24 + i * 23;
            ctx.fillStyle = rgba(INK, 1);
            ctx.textAlign = i === 0 ? 'center' : 'left';
            ctx.fillText(LINES[i], i === 0 ? STRIP_W / 2 : 18, y);
            if (PRICES[i]) {
                ctx.textAlign = 'right';
                ctx.fillText(PRICES[i], STRIP_W - 18, y);
            }
        }
        ctx.restore();
    }
    function draw(): void {
        ctx.fillStyle = rgba(BG, 1);
        ctx.fillRect(0, 0, W, H);
        const x = (W - STRIP_W) / 2;
        // 위쪽 종이 — 인쇄기 틈에서 늘어진다
        ctx.fillStyle = rgba(mix(BG, LILAC, 0.3), 1);
        ctx.fillRect(x - 24, 0, STRIP_W + 48, 14);
        ctx.fillStyle = rgba(BG, 0.5);
        ctx.fillRect(x + 6, 14, STRIP_W, TEAR_Y - 14);
        ctx.fillStyle = rgba(PAPER, 1);
        ctx.fillRect(x, 14, STRIP_W, TEAR_Y - 14);
        ctx.fillStyle = rgba(INK, 0.08);
        for (let y = 18; y < TEAR_Y; y += 8) { ctx.fillRect(x, y, STRIP_W, 1); }
        // 위쪽 조각의 절취선 톱니
        const n = teethCount();
        ctx.fillStyle = rgba(PAPER, 1);
        ctx.beginPath();
        ctx.moveTo(x, TEAR_Y);
        for (let i = 0; i <= n; i += 1) { ctx.lineTo(x + i * TOOTH_PX, TEAR_Y + (i % 2 === 0 ? 0 : 5)); }
        ctx.lineTo(x + STRIP_W, TEAR_Y);
        ctx.closePath();
        ctx.fill();
        // 새 종이는 위쪽 조각 뒤에서 미끄러져 나온다
        let top = TEAR_Y + pull;
        if (state === 'feed') { top = TEAR_Y - (1 - easeOutBack(feed)) * PIECE_H; }
        if (state === 'fall') { top = TEAR_Y + fallY; }
        drawPiece(x, top, state === 'fall' ? fallRot * 0.35 : 0);
        // 아직 이어진 이 — 끈 사이 틈에 남은 종이 섬유
        if (state === 'drag' || state === 'back') {
            const keep = intactTeeth(pull);
            const gap = Math.max(pull, 0);
            for (let i = 0; i < n; i += 1) {
                if (pull >= TEAR_PX * toothBreak[i % 40]) { continue; }
                ctx.fillStyle = rgba(PAPER, 0.92);
                ctx.fillRect(x + i * TOOTH_PX + 2, TEAR_Y + 3, TOOTH_PX - 4, gap);
            }
            if (keep === 0) { ctx.fillStyle = rgba(PEACH, 0.6); ctx.fillRect(x, TEAR_Y + 2, STRIP_W, 1); }
        }
        // 기준선 — 이 거리를 넘기고 놓으면 찢어진다
        ctx.fillStyle = rgba(MINT, 0.5);
        ctx.fillRect(x - 30, TEAR_Y + TEAR_PX + 6, 22, 2);
        // 가짜 포인터 — 자동 시연에서 스스로 끈다
        if (fake || grabbed) {
            const cy = TEAR_Y + pull + PIECE_H - 24;
            ctx.fillStyle = rgba(SKY, 0.85);
            ctx.beginPath();
            ctx.arc(x + STRIP_W / 2, cy, 11, 0, Math.PI * 2);
            ctx.fill();
            ctx.strokeStyle = rgba(PAPER, 0.95);
            ctx.lineWidth = 2;
            ctx.stroke();
        }
    }
    // 움직임 줄이기 — 보간 없이 결과 상태로 바로 놓는다
    function finishNow(): void {
        if (state === 'fall') { status.textContent = TXT_TORN; }
        if (state === 'back') { status.textContent = TXT_BACK; }
        settle();
        draw();
    }
    cv.addEventListener('pointerdown', function (ev: PointerEvent): void {
        if (state !== 'idle' && state !== 'drag') { return; }
        const p = toScene(ev);
        if (p[1] < TEAR_Y + pull - 4) { return; }
        userTouched = true;
        fake = false;
        grabbed = true;
        grabY = p[1] - pull;
        state = 'drag';
        cv.setPointerCapture(ev.pointerId);
        status.textContent = TXT_PULL;
        if (reduceQuery.matches) { draw(); }
    });
    cv.addEventListener('pointermove', function (ev: PointerEvent): void {
        if (!grabbed) { return; }
        pull = clamp(toScene(ev)[1] - grabY, 0, MAX_PULL);
        if (reduceQuery.matches) { draw(); }
    });
    function endGrab(): void {
        if (!grabbed) { return; }
        release();
        if (reduceQuery.matches) { finishNow(); }
    }
    cv.addEventListener('pointerup', endGrab);
    cv.addEventListener('pointercancel', endGrab);
    cv.addEventListener('keydown', function (ev: KeyboardEvent): void {
        if (ev.key === 'ArrowDown') {
            ev.preventDefault();
            userTouched = true;
            fake = false;
            if (state === 'idle' || state === 'drag') {
                state = 'drag';
                grabbed = false;
                pull = Math.min(MAX_PULL, pull + 12);
                status.textContent = TXT_PULL;
                if (reduceQuery.matches) { draw(); }
            }
        } else if (ev.key === 'ArrowUp') {
            ev.preventDefault();
            if (state === 'drag') { pull = Math.max(0, pull - 12); if (reduceQuery.matches) { draw(); } }
        } else if (ev.key === 'Enter') {
            ev.preventDefault();
            if (state === 'drag') { release(); if (reduceQuery.matches) { finishNow(); } }
        }
    });
    function triggerPull(target: number): void {
        userTouched = true;
        if (reduceQuery.matches) {
            pull = target;
            state = 'drag';
            release();
            finishNow();
            return;
        }
        if (state === 'idle') { startFake(target); }
    }
    if (tearBtn) { tearBtn.addEventListener('click', function (): void { triggerPull(TEAR_PX + 30); }); }
    if (softBtn) { softBtn.addEventListener('click', function (): void { triggerPull(TEAR_PX * 0.5); }); }

    let rafId = 0;
    let lastTs = -1;
    let acc = 0;
    function frame(ts: number): void {
        if (lastTs < 0) { lastTs = ts; }
        acc += Math.min(ts - lastTs, 100);
        lastTs = ts;
        let n = 0;
        while (acc >= FIXED_DT && n < 6) {
            step();
            acc -= FIXED_DT;
            n += 1;
        }
        draw();
        rafId = window.requestAnimationFrame(frame);
    }
    if (reduceQuery.matches) {
        settle();
        draw();
    } else {
        rafId = window.requestAnimationFrame(frame);
    }
    return function (): void {
        window.cancelAnimationFrame(rafId);
        window.removeEventListener('resize', onResize);
    };

  }, []);

  return (
    <div className={cx("rc") + " is-demo"} ref={rootRef}>
        <canvas className={cx("rc__canvas")} data-role="canvas" width={480} height={300} tabIndex={0} aria-label="절취선 아래 종이를 끌어 내려 기준 거리를 넘기면 찢어지고 못 넘기면 스프링으로 돌아오는 장면"></canvas>
        <p className={cx("rc__status")} data-role="status" aria-live="polite">새 영수증이 나왔다</p>
        <div className={cx("rc__tools")}>
          <button className={cx("rc__btn")} data-role="tear" type="button">찢기</button>
          <button className={cx("rc__btn")} data-role="soft" type="button">살짝 당기기</button>
        </div>
    </div>
  );
}