03Drag inertia — tracking pointer velocity, then decaying it with an exponential glide

<main class="stage">
<div class="di" id="di">
  <p class="di__hint">From the moment you release, velocity is multiplied by 0.90 every frame so it glides like ice and stops</p>
  <div class="di__wrap" id="diWrap" role="img" aria-label="Scene of a slider gliding on inertia, decelerating, and stopping">
    <input class="di__range" id="diRange" type="range" min="0" max="100" step="1" value="50" aria-label="Value slider">
  </div>
  <p class="di__meta"><b class="di__tag" id="diVel">0.00</b><span class="di__unit">Velocity (decay coefficient 0.90)</span></p>
</div>
</main>

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

// 드래그 관성(03) — 위치는 JS 가 매초 DECAY^12 배(변형 0.90 기준)로 감쇠하는
// 속도를 적분해 계산한다. 트랙 색만 --pct 로 채운다.

$duration: $dur-base;
$easing: $ease-out;
$color: $subject-cream;

@include stage($stage-orange);

$track-h: 8px;
$thumb-d: 20px;

.di {
  display: flex; flex-direction: column; align-items: center; gap: $sp-3;
  width: 100%; max-width: 360px;
}

.di__hint {
  margin: 0; font-weight: 700; font-size: clamp(10px, 2.8vw, 12px); line-height: 1.35;
  color: $subject-ink; text-align: center;
}

.di__wrap {
  position: relative; width: 100%; height: 40px;
  display: flex; align-items: center;
  --pct: 50;
}

.di__range {
  appearance: none; -webkit-appearance: none;
  width: 100%; height: $thumb-d; margin: 0; background: transparent;
  touch-action: none;
}
.di__range:focus-visible { outline: 2px solid $subject-ink; outline-offset: 4px; border-radius: $r-pill; }

.di__range::-webkit-slider-runnable-track {
  height: $track-h; border-radius: $r-pill;
  background: linear-gradient(to right, $color calc(var(--pct) * 1%), rgba($subject-ink, .22) calc(var(--pct) * 1%));
}
.di__range::-webkit-slider-thumb {
  appearance: none; -webkit-appearance: none;
  width: $thumb-d; height: $thumb-d; border-radius: 50%;
  background: $subject-ink; box-shadow: $shadow-press;
  margin-top: calc(($track-h - $thumb-d) / 2);
}
.di__range::-moz-range-track {
  height: $track-h; border-radius: $r-pill; background: rgba($subject-ink, .22);
}
.di__range::-moz-range-progress {
  height: $track-h; border-radius: $r-pill; background: $color;
}
.di__range::-moz-range-thumb {
  width: $thumb-d; height: $thumb-d; border: 0; border-radius: 50%;
  background: $subject-ink; box-shadow: $shadow-press;
}

.di__meta { margin: 0; font-size: clamp(9px, 2.4vw, 11px); color: rgba($subject-ink, .82); }
.di__tag { font-weight: 700; color: $subject-blue-deep; font-variant-numeric: tabular-nums; margin-right: $sp-1; }
.di__unit { color: rgba($subject-ink, .72); }

@media (max-width: 340px) {
  .di { gap: $sp-2; max-width: 280px; }
  .di__hint { font-size: 8px; }
  .di__meta { font-size: 7px; }
}

@media (prefers-reduced-motion: reduce) {
  .di__range { transition: none; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.di {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
  width: 100%;
  max-width: 360px;
}

.di__hint {
  margin: 0;
  font-weight: 700;
  font-size: clamp(10px, 2.8vw, 12px);
  line-height: 1.35;
  color: #17141a;
  text-align: center;
}

.di__wrap {
  position: relative;
  width: 100%;
  height: 40px;
  display: flex;
  align-items: center;
  --pct: 50;
}

.di__range {
  appearance: none;
  -webkit-appearance: none;
  width: 100%;
  height: 20px;
  margin: 0;
  background: transparent;
  touch-action: none;
}

.di__range:focus-visible {
  outline: 2px solid #17141a;
  outline-offset: 4px;
  border-radius: 999px;
}

.di__range::-webkit-slider-runnable-track {
  height: 8px;
  border-radius: 999px;
  background: linear-gradient(to right, #fff7e6 calc(var(--pct) * 1%), rgba(23, 20, 26, 0.22) calc(var(--pct) * 1%));
}

.di__range::-webkit-slider-thumb {
  appearance: none;
  -webkit-appearance: none;
  width: 20px;
  height: 20px;
  border-radius: 50%;
  background: #17141a;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  margin-top: -6px;
}

.di__range::-moz-range-track {
  height: 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.22);
}

.di__range::-moz-range-progress {
  height: 8px;
  border-radius: 999px;
  background: #fff7e6;
}

.di__range::-moz-range-thumb {
  width: 20px;
  height: 20px;
  border: 0;
  border-radius: 50%;
  background: #17141a;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
}

.di__meta {
  margin: 0;
  font-size: clamp(9px, 2.4vw, 11px);
  color: rgba(23, 20, 26, 0.82);
}

.di__tag {
  font-weight: 700;
  color: #0a4fe8;
  font-variant-numeric: tabular-nums;
  margin-right: 4px;
}

.di__unit {
  color: rgba(23, 20, 26, 0.72);
}

@media (max-width: 340px) {
  .di {
    gap: 8px;
    max-width: 280px;
  }
  .di__hint {
    font-size: 8px;
  }
  .di__meta {
    font-size: 7px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .di__range {
    transition: none;
  }
}

// 드래그 관성(03) — 놓는 순간의 속도를 저장해 두고, 매 프레임 velocity *= 0.90 을
// 곱해 위치에 더한다. 속도가 문턱 밑으로 떨어지면 정지, 트랙 끝에서는 클램프한다.
// 실제 드래그가 없는 동안은 시드 난수(mulberry32)로 결정적인 "놓는 순간" 을 만든다.
(function () {
  var root = document.getElementById('di');
  var wrap = document.getElementById('diWrap');
  var range = document.getElementById('diRange');
  var velEl = document.getElementById('diVel');
  if (!root || !wrap || !range || !velEl) return;

  var MIN = 0, MAX = 100;
  var DECAY = 0.90;       // "매 프레임" 감쇠 계수 (변형: 0.75) — 기준 프레임률 12fps
  var BASE_FPS = 12;
  var DECAY_PER_SEC = Math.pow(DECAY, BASE_FPS); // 실제 rAF 주사율과 무관하게 같은 감쇠가 되도록 시간 기준으로 환산
  var STOP_THRESHOLD = 0.03;
  var KICK_EVERY_MS = 650;

  function mulberry32(seed) {
    return function () {
      seed |= 0; seed = (seed + 0x6D2B79F5) | 0;
      var t = Math.imul(seed ^ (seed >>> 15), 1 | seed);
      t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
      return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
    };
  }
  var rand = mulberry32(42);

  var pos = 50;
  var velocity = 0;
  var lastKick = 0;

  function paint() {
    var pct = (pos - MIN) / (MAX - MIN) * 100;
    wrap.style.setProperty('--pct', pct.toFixed(2));
    range.setAttribute('aria-valuenow', String(Math.round(pos)));
    velEl.textContent = velocity.toFixed(2);
  }
  paint();

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

  var dragging = false, lastX = 0, lastT = 0;
  function pointerDown(e) {
    dragging = true; velocity = 0;
    lastX = e.clientX; lastT = performance.now();
  }
  function pointerMove(e) {
    if (!dragging) return;
    var now = performance.now();
    var dt = Math.max(now - lastT, 1);
    var dx = e.clientX - lastX;
    var trackW = wrap.getBoundingClientRect().width || 1;
    velocity = (dx / trackW) * 100 / (dt / 1000); // 퍼센트/s
    lastX = e.clientX; lastT = now;
  }
  function pointerUp() { dragging = false; }
  range.addEventListener('pointerdown', pointerDown);
  range.addEventListener('pointermove', pointerMove);
  window.addEventListener('pointerup', pointerUp);
  range.addEventListener('input', function () { if (dragging) pos = Number(range.value); });

  if (reduce) {
    pos = 50; velocity = 0;
    range.value = String(pos);
    paint();
  } else {
    var last = performance.now();
    (function step(now) {
      var dt = now - last; last = now;
      if (!dragging) {
        if (now - lastKick > KICK_EVERY_MS && Math.abs(velocity) < STOP_THRESHOLD) {
          velocity = (rand() - 0.5) * 160; // 결정적 "놓는 순간" 속도(퍼센트/s)
          lastKick = now;
        }
        if (Math.abs(velocity) >= STOP_THRESHOLD) {
          pos += velocity * (dt / 1000);
          velocity *= Math.pow(DECAY_PER_SEC, dt / 1000);
          if (pos < MIN) { pos = MIN; velocity = 0; }
          if (pos > MAX) { pos = MAX; velocity = 0; }
          if (Math.abs(velocity) < STOP_THRESHOLD) velocity = 0;
          range.value = String(Math.round(pos));
        }
      }
      paint();
      requestAnimationFrame(step);
    })(last);
  }
})();

// RangeSliderDragInertiaLerp — vanilla 판(run/334/demo/03-range-slider-drag-inertia-lerp/index.html)
// 의 React 포트. 놓는 순간의 속도를 저장해 두고, 매 프레임 velocity *= 0.90 을 곱해
// 위치에 더한다. 속도가 문턱 밑으로 떨어지면 정지, 트랙 끝에서는 클램프한다. 실제 드래그가
// 없는 동안은 시드 난수(mulberry32)로 결정적인 "놓는 순간" 을 만든다.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./RangeSliderDragInertiaLerp.module.scss";

export type RangeSliderDragInertiaLerpProps = {
  duration?: string;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };

const HINT = "놓는 순간부터 속도에 0.90 을 매 프레임 곱해 얼음판처럼 미끄러지다 멈춥니다";
const ARIA_SCENE = "관성으로 미끄러지다 감속해 멈추는 슬라이더 장면";
const ARIA_RANGE = "값 슬라이더";
const UNIT = "속도 (감쇠 계수 0.90)";

const MIN = 0, MAX = 100;
const DECAY = 0.90; // "매 프레임" 감쇠 계수 (변형: 0.75) — 기준 프레임률 12fps
const BASE_FPS = 12;
const DECAY_PER_SEC = Math.pow(DECAY, BASE_FPS); // 실제 rAF 주사율과 무관하게 같은 감쇠가 되도록 시간 기준으로 환산
const STOP_THRESHOLD = 0.03;
const KICK_EVERY_MS = 650;

function mulberry32(seed: number): () => number {
  let s = seed;
  return function () {
    s |= 0;
    s = (s + 0x6D2B79F5) | 0;
    let t = Math.imul(s ^ (s >>> 15), 1 | s);
    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}

export default function RangeSliderDragInertiaLerp({
  duration = "300ms",
  easing = "ease-out",
  color = "#fff7e6"
}: RangeSliderDragInertiaLerpProps) {
  const wrapRef = useRef<HTMLDivElement | null>(null);
  const rangeRef = useRef<HTMLInputElement | null>(null);
  const velRef = useRef<HTMLElement | null>(null);
  const rafRef = useRef(0) as Box<number>;

  useEffect(() => {
    const wrap = wrapRef.current;
    const range = rangeRef.current;
    const velEl = velRef.current;
    if (!wrap || !range || !velEl) return;

    const rand = mulberry32(42);

    let pos = 50;
    let velocity = 0;
    let lastKick = 0;

    function paint(): void {
      const pct = ((pos - MIN) / (MAX - MIN)) * 100;
      wrap!.style.setProperty("--pct", pct.toFixed(2));
      range!.setAttribute("aria-valuenow", String(Math.round(pos)));
      velEl!.textContent = velocity.toFixed(2);
    }
    paint();

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

    let dragging = false, lastX = 0, lastT = 0;
    function pointerDown(e: PointerEvent): void {
      dragging = true;
      velocity = 0;
      lastX = e.clientX;
      lastT = performance.now();
    }
    function pointerMove(e: PointerEvent): void {
      if (!dragging) return;
      const now = performance.now();
      const dt = Math.max(now - lastT, 1);
      const dx = e.clientX - lastX;
      const trackW = wrap!.getBoundingClientRect().width || 1;
      velocity = ((dx / trackW) * 100) / (dt / 1000); // 퍼센트/초
      lastX = e.clientX;
      lastT = now;
    }
    function pointerUp(): void {
      dragging = false;
    }
    function onInput(): void {
      if (dragging) pos = Number(range!.value);
    }
    range.addEventListener("pointerdown", pointerDown);
    range.addEventListener("pointermove", pointerMove);
    window.addEventListener("pointerup", pointerUp);
    range.addEventListener("input", onInput);

    if (reduce) {
      pos = 50;
      velocity = 0;
      range.value = String(pos);
      paint();
    } else {
      let last = performance.now();
      const step = (now: number): void => {
        const dt = now - last;
        last = now;
        if (!dragging) {
          if (now - lastKick > KICK_EVERY_MS && Math.abs(velocity) < STOP_THRESHOLD) {
            velocity = (rand() - 0.5) * 160; // 결정적 "놓는 순간" 속도(퍼센트/초)
            lastKick = now;
          }
          if (Math.abs(velocity) >= STOP_THRESHOLD) {
            pos += velocity * (dt / 1000);
            velocity *= Math.pow(DECAY_PER_SEC, dt / 1000);
            if (pos < MIN) { pos = MIN; velocity = 0; }
            if (pos > MAX) { pos = MAX; velocity = 0; }
            if (Math.abs(velocity) < STOP_THRESHOLD) velocity = 0;
            range!.value = String(Math.round(pos));
          }
        }
        paint();
        rafRef.current = requestAnimationFrame(step);
      };
      rafRef.current = requestAnimationFrame(step);
    }

    return () => {
      cancelAnimationFrame(rafRef.current);
      range.removeEventListener("pointerdown", pointerDown);
      range.removeEventListener("pointermove", pointerMove);
      window.removeEventListener("pointerup", pointerUp);
      range.removeEventListener("input", onInput);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <main className={styles.stage}>
      <div
        className={styles.di}
        style={{ "--duration": duration, "--easing": easing, "--color": color } as Vars}
      >
        <p className={styles.hint}>{HINT}</p>
        <div className={styles.wrap} ref={wrapRef} role="img" aria-label={ARIA_SCENE}>
          <input
            className={styles.range}
            ref={rangeRef}
            type="range"
            min={0}
            max={100}
            step={1}
            defaultValue={50}
            aria-label={ARIA_RANGE}
          />
        </div>
        <p className={styles.meta}>
          <b className={styles.tag} ref={velRef}>0.00</b>
          <span className={styles.unit}>{UNIT}</span>
        </p>
      </div>
    </main>
  );
}