01ドラッグを離すと慣性でさらに滑るカルーセル

<main class="stage">
<section class="carousel is-demo" id="carousel" role="region" aria-roledescription="carousel" aria-label="カードカルーセル">
  <div class="viewport" id="viewport">
    <div class="track" id="track">
      <div class="card card--0"><span class="num">1</span></div>
      <div class="card card--1"><span class="num">2</span></div>
      <div class="card card--2"><span class="num">3</span></div>
      <div class="card card--3"><span class="num">4</span></div>
      <div class="card card--4"><span class="num">5</span></div>
    </div>
  </div>
  <button type="button" class="navBtn navBtn--prev" id="prevBtn" aria-label="前のカード">
    <svg viewBox="0 0 256 256" width="18" height="18" aria-hidden="true"><g fill="currentColor"><path d="M112 56v144l-72-72Z" opacity=".2"/><path d="M216 120h-96V56a8 8 0 0 0-13.66-5.66l-72 72a8 8 0 0 0 0 11.32l72 72A8 8 0 0 0 120 200v-64h96a8 8 0 0 0 0-16m-112 60.69L51.31 128L104 75.31Z"/></g></svg>
  </button>
  <button type="button" class="navBtn navBtn--next" id="nextBtn" aria-label="次のカード">
    <svg viewBox="0 0 256 256" width="18" height="18" aria-hidden="true"><g fill="currentColor"><path d="m216 128l-72 72V56Z" opacity=".2"/><path d="m221.66 122.34l-72-72A8 8 0 0 0 136 56v64H40a8 8 0 0 0 0 16h96v64a8 8 0 0 0 13.66 5.66l72-72a8 8 0 0 0 0-11.32M152 180.69V75.31L204.69 128Z"/></g></svg>
  </button>
  <div class="readout" id="readout" aria-live="polite"></div>
</section>
</main>

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

// 변수 3개
$duration: $dur-base;
$easing: $ease-out;
$color: $subject-blue-deep;

// 계산 겹: index.html 의 settle() 이 포인터 릴리스 속도에 감쇠 계수를 매 프레임 곱해
// pos·velocity 를 산술로 굳힌다 — CSS 트랜지션이 아니라 requestAnimationFrame 물리다.
@include stage($stage-ink);

.carousel {
  position: relative;
  width: 300px;
}

.viewport {
  position: relative;
  width: 300px;
  height: 168px;
  border-radius: $r-card;
  overflow: hidden;
  box-shadow: $shadow-raised;
  background: $stage-ink;
  outline: none;
}

.track {
  display: flex;
  height: 100%;
  will-change: transform;
  touch-action: none;
  cursor: grab;

  &.isDragging { cursor: grabbing; }
}

.card {
  flex: 0 0 200px;
  height: 100%;
  margin-right: $sp-4;
  border-radius: $r-card;
  display: grid;
  place-items: center;

  &:last-child { margin-right: 0; }

  .num {
    font-weight: 700;
    font-size: 56px;
    line-height: 1;
  }
}

.card--0 { background: linear-gradient(160deg, $subject-blue-deep, #475690); .num { color: $subject-cream; } }
.card--1 { background: linear-gradient(160deg, $subject-pink, #d97fa0); .num { color: $subject-ink; } }
.card--2 { background: linear-gradient(160deg, $subject-mint, #63b299); .num { color: $subject-ink; } }
.card--3 { background: linear-gradient(160deg, $subject-lilac, #bdb3da); .num { color: $subject-ink; } }
.card--4 { background: linear-gradient(160deg, $subject-sky, #9cb8ce); .num { color: $subject-ink; } }

.navBtn {
  position: absolute;
  top: 50%;
  margin-top: -18px;
  width: 36px;
  height: 36px;
  border-radius: $r-pill;
  border: none;
  display: grid;
  place-items: center;
  background: rgba($stage-ink, .6);
  color: $subject-cream;
  cursor: pointer;

  &:focus-visible { outline: 2px solid $color; outline-offset: 2px; }
}
.navBtn--prev { left: -($sp-3); }
.navBtn--next { right: -($sp-3); }

.readout {
  position: absolute;
  left: $sp-3;
  bottom: $sp-3;
  padding: $sp-1 $sp-2;
  border-radius: $r-pill;
  background: rgba($stage-ink, .6);
  color: rgba($subject-cream, .9);
  font-size: 11px;
  font-weight: 600;
  line-height: 1;
  font-variant-numeric: tabular-nums;
}

@media (max-width: 340px) {
  .carousel { width: 220px; }
  .viewport { width: 220px; height: 106px; }
  .card { flex-basis: 148px; margin-right: $sp-2; .num { font-size: 36px; } }
  .navBtn { width: 28px; height: 28px; margin-top: -14px; }
  .readout { font-size: 9px; }
}

@media (prefers-reduced-motion: reduce) {
  // 실사용 경로 적용 — 드래그·키보드·버튼 플릭은 index.html 이 그대로 물리 계산을 돌린다.
  // scss 만: 포커스 아웃라인 전환만 꺼서 잔여 트랜지션을 없앤다.
  .navBtn { transition: none; }
}

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);
}

.carousel {
  position: relative;
  width: 300px;
}

.viewport {
  position: relative;
  width: 300px;
  height: 168px;
  border-radius: 16px;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  background: #17141a;
  outline: none;
}

.track {
  display: flex;
  height: 100%;
  will-change: transform;
  touch-action: none;
  cursor: grab;
}
.track.isDragging {
  cursor: grabbing;
}

.card {
  flex: 0 0 200px;
  height: 100%;
  margin-right: 16px;
  border-radius: 16px;
  display: grid;
  place-items: center;
}
.card:last-child {
  margin-right: 0;
}
.card .num {
  font-weight: 700;
  font-size: 56px;
  line-height: 1;
}

.card--0 {
  background: linear-gradient(160deg, #0a4fe8, #475690);
}
.card--0 .num {
  color: #fff7e6;
}

.card--1 {
  background: linear-gradient(160deg, #ff9ebb, #d97fa0);
}
.card--1 .num {
  color: #17141a;
}

.card--2 {
  background: linear-gradient(160deg, #4cd4a6, #63b299);
}
.card--2 .num {
  color: #17141a;
}

.card--3 {
  background: linear-gradient(160deg, #b9a0ff, #bdb3da);
}
.card--3 .num {
  color: #17141a;
}

.card--4 {
  background: linear-gradient(160deg, #6ec8ff, #9cb8ce);
}
.card--4 .num {
  color: #17141a;
}

.navBtn {
  position: absolute;
  top: 50%;
  margin-top: -18px;
  width: 36px;
  height: 36px;
  border-radius: 999px;
  border: none;
  display: grid;
  place-items: center;
  background: rgba(23, 20, 26, 0.6);
  color: #fff7e6;
  cursor: pointer;
}
.navBtn:focus-visible {
  outline: 2px solid #0a4fe8;
  outline-offset: 2px;
}

.navBtn--prev {
  left: -12px;
}

.navBtn--next {
  right: -12px;
}

.readout {
  position: absolute;
  left: 12px;
  bottom: 12px;
  padding: 4px 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.6);
  color: rgba(255, 247, 230, 0.9);
  font-size: 11px;
  font-weight: 600;
  line-height: 1;
  font-variant-numeric: tabular-nums;
}

@media (max-width: 340px) {
  .carousel {
    width: 220px;
  }
  .viewport {
    width: 220px;
    height: 106px;
  }
  .card {
    flex-basis: 148px;
    margin-right: 8px;
  }
  .card .num {
    font-size: 36px;
  }
  .navBtn {
    width: 28px;
    height: 28px;
    margin-top: -14px;
  }
  .readout {
    font-size: 9px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .navBtn {
    transition: none;
  }
}

(function () {
  var STEP_MS = 16.6667;
  // 관성 감쇠 계수 — 프레임(16.6667ms)마다 이 비율만큼 속도가 깎인다. 낮출수록 손을 뗀 뒤
  // 카드가 더 오래·더 멀리 미끄러진다(변형 1). 높이면 짧게 미끄러지다 빨리 멈춘다.
  var MOMENTUM_FRICTION = 0.10;
  var VELOCITY_STOP = 0.02;      // px/ms 미만이면 정지로 본다
  var SAMPLE_WINDOW_MS = 110;    // 이 시간 안의 포인터 표본만 속도 계산에 쓴다
  var KEY_FLICK_VELOCITY = 0.62; // 화살표 키 한 번 = 이 속도의 플릭
  var DEMO_FLICKS = [-0.85, 0.5, -0.62, 0.78]; // 자동 시연이 번갈아 쏘는 플릭 속도(px/ms) — 크기가 다르므로 미끄러지는 거리도 다르다
  var DEMO_HOLD_MS = 260;

  var carousel = document.getElementById('carousel');
  var viewport = document.getElementById('viewport');
  var track = document.getElementById('track');
  var readout = document.getElementById('readout');
  var prevBtn = document.getElementById('prevBtn');
  var nextBtn = document.getElementById('nextBtn');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var TOTAL = 5;
  var LABEL_PREFIX = 'カード ';
  var LABEL_SEP = ' / ';

  var pos = 0;          // 현재 translateX (px)
  var velocity = 0;      // px/ms
  var minX = 0, maxX = 0;
  var dragging = false;
  var samples = [];
  var settleFrame = null;
  var demoTimers = [];

  function bounds() {
    maxX = 0;
    minX = Math.min(0, viewport.clientWidth - track.scrollWidth);
  }
  function clamp(v, lo, hi) { return Math.min(hi, Math.max(lo, v)); }

  function paint() {
    track.style.transform = 'translate3d(' + pos.toFixed(1) + 'px,0,0)';
    var step = track.scrollWidth / TOTAL;
    var idx = clamp(Math.round(-pos / step), 0, TOTAL - 1);
    readout.textContent = LABEL_PREFIX + (idx + 1) + LABEL_SEP + TOTAL;
  }

  function stopSettle() {
    if (settleFrame) { cancelAnimationFrame(settleFrame); settleFrame = null; }
  }

  function human() {
    if (carousel.classList.contains('is-demo')) {
      carousel.classList.remove('is-demo');
      demoTimers.forEach(function (t) { clearTimeout(t); });
      demoTimers = [];
    }
  }

  // 관성 감쇠 루프 — 매 프레임 pos += velocity*dt, velocity *= (1-MOMENTUM_FRICTION)^(dt/16.6667).
  // 경계에 닿으면 그 자리에서 속도를 0으로 끊는다(되튕김은 이 항목의 역할이 아니다 — 02가 담당).
  function settle() {
    var last = performance.now();
    function frame(now) {
      var dt = Math.min(40, now - last);
      last = now;
      pos = clamp(pos + velocity * dt, minX, maxX);
      var decay = Math.pow(1 - MOMENTUM_FRICTION, dt / STEP_MS);
      velocity *= decay;
      if (pos === minX || pos === maxX) velocity = 0;
      paint();
      if (Math.abs(velocity) > VELOCITY_STOP) {
        settleFrame = requestAnimationFrame(frame);
      } else {
        velocity = 0;
        settleFrame = null;
      }
    }
    settleFrame = requestAnimationFrame(frame);
  }

  function pushSample(t, x) {
    samples.push({ t: t, x: x });
    while (samples.length && t - samples[0].t > SAMPLE_WINDOW_MS) samples.shift();
  }

  function releaseVelocity() {
    if (samples.length < 2) return 0;
    var a = samples[0], b = samples[samples.length - 1];
    var dt = b.t - a.t;
    if (dt <= 0) return 0;
    return (b.x - a.x) / dt;
  }

  function onPointerDown(e) {
    human();
    stopSettle();
    dragging = true;
    velocity = 0;
    samples = [];
    track.classList.add('isDragging');
    viewport.setPointerCapture(e.pointerId);
    pushSample(performance.now(), e.clientX);
    carousel._originPos = pos;
    carousel._startX = e.clientX;
  }
  function onPointerMove(e) {
    if (!dragging) return;
    var dx = e.clientX - carousel._startX;
    pos = clamp(carousel._originPos + dx, minX, maxX);
    pushSample(performance.now(), e.clientX);
    paint();
  }
  function onPointerUp(e) {
    if (!dragging) return;
    dragging = false;
    track.classList.remove('isDragging');
    try { viewport.releasePointerCapture(e.pointerId); } catch (err) {}
    velocity = releaseVelocity();
    settle();
  }
  function flick(v) {
    human();
    stopSettle();
    velocity = v;
    settle();
  }
  function onKeydown(e) {
    if (e.key === 'ArrowLeft') { flick(-KEY_FLICK_VELOCITY); e.preventDefault(); }
    else if (e.key === 'ArrowRight') { flick(KEY_FLICK_VELOCITY); e.preventDefault(); }
  }

  viewport.addEventListener('pointerdown', onPointerDown);
  viewport.addEventListener('pointermove', onPointerMove);
  viewport.addEventListener('pointerup', onPointerUp);
  viewport.addEventListener('pointercancel', onPointerUp);
  viewport.addEventListener('keydown', onKeydown);
  viewport.tabIndex = 0;
  prevBtn.addEventListener('click', function () { flick(-KEY_FLICK_VELOCITY); });
  nextBtn.addEventListener('click', function () { flick(KEY_FLICK_VELOCITY); });

  // reduce: 실사용 경로 적용 — 드래그·키보드·버튼 플릭은 위 리스너가 그대로 물리 계산을 돌린다.
  // 아래 demoCycle() 은 시연 게이트만이고 reduce 에서는 돌지 않는다.
  function demoCycle(i) {
    if (!carousel.classList.contains('is-demo')) return;
    velocity = DEMO_FLICKS[i % DEMO_FLICKS.length];
    stopSettle();
    settle();
    demoTimers.push(setTimeout(function () {
      demoTimers.push(setTimeout(function () { demoCycle(i + 1); }, DEMO_HOLD_MS));
    }, 40));
  }

  function init() {
    bounds();
    paint();
    if (!reduce) demoCycle(0);
  }
  window.addEventListener('resize', bounds);
  init();
})();

import { useEffect, useRef } from 'react';
import styles from './CarouselDragMomentum.module.scss';

// 상수 — vanilla demo 01 (run/384/demo/01-carousel-slide-drag-momentum/index.html) 과 동일
const STEP_MS = 16.6667;
// 관성 감쇠 계수 — 프레임(16.6667ms)마다 이 비율만큼 속도가 깎인다. 낮출수록 손을 뗀 뒤
// 카드가 더 오래·더 멀리 미끄러진다(변형 1). 높이면 짧게 미끄러지다 빨리 멈춘다.
const MOMENTUM_FRICTION = 0.10;
const VELOCITY_STOP = 0.02;      // px/ms 미만이면 정지로 본다
const SAMPLE_WINDOW_MS = 110;    // 이 시간 안의 포인터 표본만 속도 계산에 쓴다
const KEY_FLICK_VELOCITY = 0.62; // 화살표 키 한 번 = 이 속도의 플릭
const DEMO_FLICKS = [-0.85, 0.5, -0.62, 0.78]; // 자동 시연이 번갈아 쏘는 플릭 속도(px/ms)
const DEMO_HOLD_MS = 260;
const TOTAL = 5;
const LABEL_PREFIX = '카드 ';
const LABEL_SEP = ' / ';

export default function CarouselDragMomentum() {
  const carouselRef = useRef<HTMLElement>(null);

  useEffect(() => {
    const carousel = carouselRef.current;
    if (!carousel) return undefined;
    const viewport = carousel.querySelector<HTMLDivElement>('.' + styles.viewport);
    const track = carousel.querySelector<HTMLDivElement>('.' + styles.track);
    const readout = carousel.querySelector<HTMLDivElement>('.' + styles.readout);
    const prevBtn = carousel.querySelector<HTMLButtonElement>('.' + styles['navBtn--prev']);
    const nextBtn = carousel.querySelector<HTMLButtonElement>('.' + styles['navBtn--next']);
    if (!viewport || !track || !readout || !prevBtn || !nextBtn) return undefined;

    let pos = 0;          // 현재 translateX (px)
    let velocity = 0;      // px/ms
    let minX = 0, maxX = 0;
    let dragging = false;
    let samples: { t: number; x: number }[] = [];
    let settleFrame = 0;
    let demoTimers: number[] = [];
    let originPos = 0, startX = 0;

    function clamp(v: number, lo: number, hi: number) { return Math.min(hi, Math.max(lo, v)); }

    function bounds() {
      maxX = 0;
      minX = Math.min(0, viewport!.clientWidth - track!.scrollWidth);
    }

    function paint() {
      track!.style.transform = 'translate3d(' + pos.toFixed(1) + 'px,0,0)';
      const step = track!.scrollWidth / TOTAL;
      const idx = clamp(Math.round(-pos / step), 0, TOTAL - 1);
      readout!.textContent = LABEL_PREFIX + (idx + 1) + LABEL_SEP + TOTAL;
    }

    function stopSettle() {
      if (settleFrame) { cancelAnimationFrame(settleFrame); settleFrame = 0; }
    }

    function human() {
      if (carousel!.classList.contains(styles['is-demo'])) {
        carousel!.classList.remove(styles['is-demo']);
        demoTimers.forEach((t) => window.clearTimeout(t));
        demoTimers = [];
      }
    }

    // 관성 감쇠 루프 — 매 프레임 pos += velocity*dt, velocity *= (1-MOMENTUM_FRICTION)^(dt/16.6667).
    // 경계에 닿으면 그 자리에서 속도를 0으로 끊는다(되튕김은 이 항목의 역할이 아니다 — 02가 담당).
    function settle() {
      let last = performance.now();
      function frame(now: number) {
        const dt = Math.min(40, now - last);
        last = now;
        pos = clamp(pos + velocity * dt, minX, maxX);
        const decay = Math.pow(1 - MOMENTUM_FRICTION, dt / STEP_MS);
        velocity *= decay;
        if (pos === minX || pos === maxX) velocity = 0;
        paint();
        if (Math.abs(velocity) > VELOCITY_STOP) {
          settleFrame = requestAnimationFrame(frame);
        } else {
          velocity = 0;
          settleFrame = 0;
        }
      }
      settleFrame = requestAnimationFrame(frame);
    }

    function pushSample(t: number, x: number) {
      samples.push({ t, x });
      while (samples.length && t - samples[0].t > SAMPLE_WINDOW_MS) samples.shift();
    }

    function releaseVelocity() {
      if (samples.length < 2) return 0;
      const a = samples[0], b = samples[samples.length - 1];
      const dt = b.t - a.t;
      if (dt <= 0) return 0;
      return (b.x - a.x) / dt;
    }

    function onPointerDown(e: PointerEvent) {
      human();
      stopSettle();
      dragging = true;
      velocity = 0;
      samples = [];
      track!.classList.add(styles.isDragging);
      viewport!.setPointerCapture(e.pointerId);
      pushSample(performance.now(), e.clientX);
      originPos = pos;
      startX = e.clientX;
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      const dx = e.clientX - startX;
      pos = clamp(originPos + dx, minX, maxX);
      pushSample(performance.now(), e.clientX);
      paint();
    }
    function onPointerUp(e: PointerEvent) {
      if (!dragging) return;
      dragging = false;
      track!.classList.remove(styles.isDragging);
      try { viewport!.releasePointerCapture(e.pointerId); } catch { /* 캡처가 이미 풀린 경우 */ }
      velocity = releaseVelocity();
      settle();
    }
    function flick(v: number) {
      human();
      stopSettle();
      velocity = v;
      settle();
    }
    function onKeydown(e: KeyboardEvent) {
      if (e.key === 'ArrowLeft') { flick(-KEY_FLICK_VELOCITY); e.preventDefault(); }
      else if (e.key === 'ArrowRight') { flick(KEY_FLICK_VELOCITY); e.preventDefault(); }
    }
    function onPrev() { flick(-KEY_FLICK_VELOCITY); }
    function onNext() { flick(KEY_FLICK_VELOCITY); }
    function onResize() { bounds(); }

    viewport.addEventListener('pointerdown', onPointerDown);
    viewport.addEventListener('pointermove', onPointerMove);
    viewport.addEventListener('pointerup', onPointerUp);
    viewport.addEventListener('pointercancel', onPointerUp);
    viewport.addEventListener('keydown', onKeydown);
    prevBtn.addEventListener('click', onPrev);
    nextBtn.addEventListener('click', onNext);
    window.addEventListener('resize', onResize);

    const reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

    // reduce: 실사용 경로 적용 — 드래그·키보드·버튼 플릭은 위 리스너가 그대로 물리 계산을 돌린다.
    // 아래 demoCycle() 은 시연 게이트만이고 reduce 에서는 돌지 않는다.
    function demoCycle(i: number) {
      if (!carousel!.classList.contains(styles['is-demo'])) return;
      velocity = DEMO_FLICKS[i % DEMO_FLICKS.length];
      stopSettle();
      settle();
      demoTimers.push(window.setTimeout(() => {
        demoTimers.push(window.setTimeout(() => demoCycle(i + 1), DEMO_HOLD_MS));
      }, 40));
    }

    bounds();
    paint();
    if (!reduce) demoCycle(0);

    return () => {
      viewport.removeEventListener('pointerdown', onPointerDown);
      viewport.removeEventListener('pointermove', onPointerMove);
      viewport.removeEventListener('pointerup', onPointerUp);
      viewport.removeEventListener('pointercancel', onPointerUp);
      viewport.removeEventListener('keydown', onKeydown);
      prevBtn.removeEventListener('click', onPrev);
      nextBtn.removeEventListener('click', onNext);
      window.removeEventListener('resize', onResize);
      demoTimers.forEach((t) => window.clearTimeout(t));
      stopSettle();
    };
  }, []);

  return (
    <main className={styles.stage}>
      <section
        className={styles.carousel + ' ' + styles['is-demo']}
        ref={carouselRef}
        role="region"
        aria-roledescription="carousel"
        aria-label="카드 캐러셀"
      >
        <div className={styles.viewport} tabIndex={0}>
          <div className={styles.track}>
            <div className={styles.card + ' ' + styles['card--0']}><span className={styles.num}>1</span></div>
            <div className={styles.card + ' ' + styles['card--1']}><span className={styles.num}>2</span></div>
            <div className={styles.card + ' ' + styles['card--2']}><span className={styles.num}>3</span></div>
            <div className={styles.card + ' ' + styles['card--3']}><span className={styles.num}>4</span></div>
            <div className={styles.card + ' ' + styles['card--4']}><span className={styles.num}>5</span></div>
          </div>
        </div>
        <button type="button" className={styles.navBtn + ' ' + styles['navBtn--prev']} aria-label="이전 카드">
          <svg viewBox="0 0 256 256" width={18} height={18} aria-hidden="true"><g fill="currentColor"><path d="M112 56v144l-72-72Z" opacity=".2" /><path d="M216 120h-96V56a8 8 0 0 0-13.66-5.66l-72 72a8 8 0 0 0 0 11.32l72 72A8 8 0 0 0 120 200v-64h96a8 8 0 0 0 0-16m-112 60.69L51.31 128L104 75.31Z" /></g></svg>
        </button>
        <button type="button" className={styles.navBtn + ' ' + styles['navBtn--next']} aria-label="다음 카드">
          <svg viewBox="0 0 256 256" width={18} height={18} aria-hidden="true"><g fill="currentColor"><path d="m216 128l-72 72V56Z" opacity=".2" /><path d="m221.66 122.34l-72-72A8 8 0 0 0 136 56v64H40a8 8 0 0 0 0 16h96v64a8 8 0 0 0 13.66 5.66l72-72a8 8 0 0 0 0-11.32M152 180.69V75.31L204.69 128Z" /></g></svg>
        </button>
        <div className={styles.readout} aria-live="polite"></div>
      </section>
    </main>
  );
}