03摩擦減衰 — 毎フレーム積分して自ら止まる盤

<main class="stage">
<div class="fr" id="fr">
  <p class="fr__hint">rAFループでdtを測り、角度 += 角速度·dt、角速度 ×= 摩擦^dt を積分します</p>
  <div class="fr__row">
    <div class="fr__field" id="field" role="application" aria-label="摩擦減衰実験盤" tabindex="0">
      <canvas class="fr__cv" id="cv"></canvas>
    </div>
    <div class="fr__side">
      <b class="fr__num" id="coef">0.30</b>
      <span class="fr__unit">摩擦係数 — 毎秒角速度に掛ける</span>
      <div class="fr__sw" id="sw" role="group" aria-label="摩擦係数を選ぶ"></div>
      <p class="fr__meta">停止まで <b class="fr__count" id="sec">0.0</b><span class="fr__metaUnit">秒で停止</span></p>
      <p class="fr__meta">同じはじき(16 rad/s)で係数だけ変わります</p>
    </div>
  </div>
</div>
</main>

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

// 마찰 감쇠(03) — 움직임은 전부 캔버스 rAF 루프다. CSS @keyframes 를 두지 않는다.
// 무대는 주황 무대의 파스텔 변형 — 무대 주황 토큰(채도 1.0) 대신 같은 색상의 연한
// 피치 톤(채도 .43 · 명도 .84)을 쓴다(BD26). 칸 여섯 색도 채도 .40 이하 파스텔.

$duration: $dur-loop;
$easing: $ease-out;
$color: $subject-blue;

@include stage(#f2d8c8);

.fr {
  --w1: #b3bfd4;   // 파랑빛 파스텔 — 1번 칸
  --w2: #d9c48f;   // 골드 파스텔 — 2번 칸
  --w3: #a9c4b0;   // 세이지 파스텔 — 3번 칸
  --w4: #c2a8d1;   // 플럼 파스텔 — 4번 칸
  --w5: #d8a3b4;   // 로즈 파스텔 — 5번 칸
  --w6: #9fc6c2;   // 틸 파스텔 — 6번 칸
  --ink: #{$subject-ink};
  --cream: #{$subject-cream};

  display: flex; flex-direction: column; gap: $sp-2;
  width: 100%; max-width: 420px;
}

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

.fr__row { display: flex; gap: $sp-2; align-items: stretch; }

.fr__field {
  position: relative; overflow: hidden;
  flex: 0 0 auto;
  width: clamp(176px, 46vw, 232px);
  height: clamp(176px, 46vw, 232px);
  border-radius: $r-card;
  background: rgba($subject-ink, .05);
  box-shadow: inset 0 0 0 1px rgba($subject-ink, .16);
  cursor: grab; touch-action: none;
}
.fr__field:active { cursor: grabbing; }
.fr__field:focus-visible { outline: 2px solid $color; outline-offset: 2px; }

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

.fr__side {
  flex: 1; min-width: 0;
  display: flex; flex-direction: column; gap: $sp-1;
  padding-top: $sp-1;
}
.fr__num {
  font-weight: 800; font-size: clamp(20px, 6vw, 30px); line-height: 1;
  color: $subject-blue-deep; font-variant-numeric: tabular-nums;
}
.fr__unit { font-weight: 600; font-size: clamp(9px, 2.4vw, 11px); line-height: 1.4; color: rgba($subject-ink, .82); }

.fr__sw { display: flex; gap: $sp-1; margin-top: $sp-1; }
.fr__btn {
  border: 0; cursor: pointer;
  border-radius: $r-xs;
  padding: $sp-1 $sp-2;
  background: rgba($subject-ink, .10);
  color: $subject-ink;
  font-weight: 700; font-size: 11px; line-height: 1.2;
  font-family: inherit;
}
.fr__btn.is-on { background: $subject-blue-deep; color: $subject-cream; }
.fr__btn:focus-visible { outline: 2px solid $color; outline-offset: 2px; }

.fr__meta { margin: $sp-1 0 0; font-size: clamp(9px, 2.4vw, 11px); line-height: 1.5; color: rgba($subject-ink, .82); }
.fr__meta b { font-weight: 700; color: $subject-blue-deep; font-variant-numeric: tabular-nums; margin-right: $sp-1; }
.fr__metaUnit { color: rgba($subject-ink, .72); }

@media (max-width: 340px) {
  .fr__hint { font-size: 8px; line-height: 1.3; }
  .fr { gap: $sp-1; }
  .fr__field { width: 150px; height: 150px; }
  .fr__num { font-size: 16px; }
  .fr__meta { margin-top: 0; font-size: 8px; }
  .fr__btn { padding: $sp-1; font-size: 9px; }
}

// reduce — 루프가 안 돈다. 감쇠 곡선 전체를 미리 계산해 둔 정지 한 장이 남는다.
@media (prefers-reduced-motion: reduce) {
  .fr.is-still .fr__field { cursor: default !important; }
  .fr.is-still .fr__btn { cursor: default; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #f2d8c8;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.fr {
  --w1: #b3bfd4;
  --w2: #d9c48f;
  --w3: #a9c4b0;
  --w4: #c2a8d1;
  --w5: #d8a3b4;
  --w6: #9fc6c2;
  --ink: #17141a;
  --cream: #fff7e6;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 420px;
}

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

.fr__row {
  display: flex;
  gap: 8px;
  align-items: stretch;
}

.fr__field {
  position: relative;
  overflow: hidden;
  flex: 0 0 auto;
  width: clamp(176px, 46vw, 232px);
  height: clamp(176px, 46vw, 232px);
  border-radius: 16px;
  background: rgba(23, 20, 26, 0.05);
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.16);
  cursor: grab;
  touch-action: none;
}

.fr__field:active {
  cursor: grabbing;
}

.fr__field:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 2px;
}

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

.fr__side {
  flex: 1;
  min-width: 0;
  display: flex;
  flex-direction: column;
  gap: 4px;
  padding-top: 4px;
}

.fr__num {
  font-weight: 800;
  font-size: clamp(20px, 6vw, 30px);
  line-height: 1;
  color: #0a4fe8;
  font-variant-numeric: tabular-nums;
}

.fr__unit {
  font-weight: 600;
  font-size: clamp(9px, 2.4vw, 11px);
  line-height: 1.4;
  color: rgba(23, 20, 26, 0.82);
}

.fr__sw {
  display: flex;
  gap: 4px;
  margin-top: 4px;
}

.fr__btn {
  border: 0;
  cursor: pointer;
  border-radius: 4px;
  padding: 4px 8px;
  background: rgba(23, 20, 26, 0.1);
  color: #17141a;
  font-weight: 700;
  font-size: 11px;
  line-height: 1.2;
  font-family: inherit;
}

.fr__btn.is-on {
  background: #0a4fe8;
  color: #fff7e6;
}

.fr__btn:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 2px;
}

.fr__meta {
  margin: 4px 0 0;
  font-size: clamp(9px, 2.4vw, 11px);
  line-height: 1.5;
  color: rgba(23, 20, 26, 0.82);
}

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

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

@media (max-width: 340px) {
  .fr__hint {
    font-size: 8px;
    line-height: 1.3;
  }
  .fr {
    gap: 4px;
  }
  .fr__field {
    width: 150px;
    height: 150px;
  }
  .fr__num {
    font-size: 16px;
  }
  .fr__meta {
    margin-top: 0;
    font-size: 8px;
  }
  .fr__btn {
    padding: 4px;
    font-size: 9px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .fr.is-still .fr__field {
    cursor: default !important;
  }
  .fr.is-still .fr__btn {
    cursor: default;
  }
}

// 마찰 감쇠(03) — requestAnimationFrame 루프에서 dt 를 재고 각도 += 각속도·dt,
// 각속도 ×= 마찰^dt 를 적분하면 튕김 직후엔 빠르다가 지수곡선으로 느려진다.
// 우측 그래프는 그 적분을 실시간으로 그린다 — 판의 각속도와 그래프의 기울기가 같다.
(function () {
  var root = document.getElementById('fr');
  var field = document.getElementById('field');
  var cv = document.getElementById('cv');
  var coefEl = document.getElementById('coef');
  var secEl = document.getElementById('sec');
  var swEl = document.getElementById('sw');
  if (!root || !field || !cv || !coefEl || !secEl || !swEl) return;
  var ctx = cv.getContext('2d');
  var css = getComputedStyle(root);
  var PAL = [];
  for (var i = 1; i <= 6; i++) PAL.push(css.getPropertyValue('--w' + i).trim());
  var INK = css.getPropertyValue('--ink').trim();
  var CREAM = css.getPropertyValue('--cream').trim();
  var TAU = Math.PI * 2;
  var N = 6, SEG = TAU / N;
  var COEFS = [0.08, 0.18, 0.38];    // 낮음·보통·높음 — 3段階 버튼
  var ci = 1;
  var FLICK = 16;                    // 시연 튕김은 항상 같다 — 계수만 바뀐다
  var STOP = 0.5;
  var ang = -0.8, angularVelocity = 0, t0 = 0, hist = [], settled = false, settleAt = 0, dpr = 1;
  var MXW = 3.2;                     // 그래프에 담는 시야 (초) — 한 시행이 창을 가득 채운다
  var sweep = -1;                    // 재시작 대기 중 그래프를 훑는 스캔라인 (0~1)

  function fit() {
    var w = field.clientWidth, h = field.clientHeight;
    dpr = Math.min(2, window.devicePixelRatio || 1);
    cv.width = Math.round(w * dpr); cv.height = Math.round(h * dpr);
  }

  function draw() {
    var w = cv.width / dpr, h = cv.height / dpr;
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    ctx.clearRect(0, 0, w, h);
    var cx = w * 0.28, cy = h * 0.5, R = Math.min(w * 0.56, h * 0.94) / 2 - 4;
    // 판
    ctx.save(); ctx.translate(cx, cy); ctx.rotate(ang);
    for (var k = 0; k < N; k++) {
      var a0 = k * SEG;
      ctx.beginPath(); ctx.moveTo(0, 0);
      ctx.arc(0, 0, R, a0, a0 + SEG); ctx.closePath();
      ctx.fillStyle = PAL[k]; ctx.fill();
      ctx.strokeStyle = 'rgba(23,20,26,.20)'; ctx.lineWidth = 1; ctx.stroke();
    }
    ctx.beginPath(); ctx.arc(0, 0, 11, 0, TAU);
    ctx.fillStyle = CREAM; ctx.fill(); ctx.strokeStyle = 'rgba(23,20,26,.35)'; ctx.stroke();
    // 정지 뒤 허브 펄스 — 대기 중에도 판이 산다
    if (sweep >= 0) {
      ctx.beginPath(); ctx.arc(0, 0, 11 + sweep * 16, 0, TAU);
      ctx.strokeStyle = 'rgba(23,20,26,' + (1 - sweep) * 0.5 + ')';
      ctx.lineWidth = 2; ctx.stroke();
    }
    ctx.restore();
    ctx.beginPath();
    ctx.moveTo(cx - 6, cy - R - 8); ctx.lineTo(cx + 6, cy - R - 8); ctx.lineTo(cx, cy - R + 8);
    ctx.closePath(); ctx.fillStyle = INK; ctx.fill();
    // 감쇠 그래프 — 우측 패널. x = 경과 초, y = 각속도 (0~FLICK). 판 옆 자리를 최소화한다
    var gx = w * 0.56, gw = w - gx - 8, gy = 10, gh = h - 20;
    ctx.strokeStyle = 'rgba(23,20,26,.30)'; ctx.lineWidth = 1;
    ctx.beginPath(); ctx.moveTo(gx, gy); ctx.lineTo(gx, gy + gh); ctx.lineTo(gx + gw, gy + gh); ctx.stroke();
    // 보조 눈금 — 25%·50%·75%
    for (var gI = 1; gI <= 3; gI++) {
      var yy = gy + gh * (1 - gI / 4);
      ctx.beginPath(); ctx.moveTo(gx, yy); ctx.lineTo(gx + 4, yy);
      ctx.strokeStyle = 'rgba(23,20,26,.25)'; ctx.stroke();
    }
    if (hist.length > 1) {
      ctx.beginPath();
      for (var j = 0; j < hist.length; j++) {
        var px = gx + (hist[j].t / MXW) * gw;
        var py = gy + gh * (1 - hist[j].v / (FLICK + 1));
        if (j === 0) ctx.moveTo(px, py); else ctx.lineTo(px, py);
      }
      ctx.strokeStyle = INK; ctx.lineWidth = 2; ctx.stroke();
      // 현재 점
      var lp = hist[hist.length - 1];
      ctx.beginPath();
      ctx.arc(gx + (lp.t / MXW) * gw, gy + gh * (1 - lp.v / (FLICK + 1)), 3, 0, TAU);
      ctx.fillStyle = INK; ctx.fill();
    }
    // 스캔라인 — 재시작 대기 중 시야를 훑는다. 그래프가 다시 그려질 시점을 예고한다
    if (sweep >= 0) {
      var sx = gx + sweep * gw;
      ctx.beginPath(); ctx.moveTo(sx, gy); ctx.lineTo(sx, gy + gh);
      ctx.strokeStyle = 'rgba(23,20,26,.4)'; ctx.lineWidth = 1; ctx.stroke();
      ctx.beginPath(); ctx.arc(sx, gy + gh, 3, 0, TAU);
      ctx.fillStyle = INK; ctx.fill();
    }
  }

  // 계수 버튼 3개
  var btns = [];
  for (var b = 0; b < COEFS.length; b++) {
    (function (idx) {
      var el = document.createElement('button');
      el.type = 'button';
      el.className = 'fr__btn' + (idx === ci ? ' is-on' : '');
      el.textContent = String(COEFS[idx]);
      el.addEventListener('click', function () {
        ci = idx;
        coefEl.textContent = COEFS[idx].toFixed(2);
        for (var q = 0; q < btns.length; q++) btns[q].className = 'fr__btn' + (q === idx ? ' is-on' : '');
        relaunch();
      });
      btns.push(el); swEl.appendChild(el);
    })(b);
  }

  var drag = { on: false, lastA: 0, lastT: 0, v: 0 };
  function angleAt(e) {
    var b = field.getBoundingClientRect();
    return Math.atan2(e.clientY - (b.top + b.height / 2), e.clientX - (b.left + b.width / 2));
  }
  field.addEventListener('pointerdown', function (e) {
    drag.on = true; drag.lastA = angleAt(e); drag.lastT = performance.now(); drag.v = 0;
    if (field.setPointerCapture) { try { field.setPointerCapture(e.pointerId); } catch (err) { /* 캡처 실패는 회전에 영향 없다 */ } }
  });
  field.addEventListener('pointermove', function (e) {
    if (!drag.on) return;
    var a = angleAt(e), now = performance.now();
    var dA = a - drag.lastA;
    if (dA > Math.PI) dA -= TAU;
    if (dA < -Math.PI) dA += TAU;
    ang += dA;
    var dt = Math.max(1, now - drag.lastT) / 1000;
    drag.v = 0.65 * drag.v + 0.35 * (dA / dt);
    drag.lastA = a; drag.lastT = now;
  });
  function endDrag() {
    if (!drag.on) return;
    drag.on = false;
    var v = Math.max(-24, Math.min(24, drag.v));
    start(Math.abs(v) < 1 ? FLICK : v);
  }
  field.addEventListener('pointerup', endDrag);
  field.addEventListener('pointercancel', endDrag);
  field.addEventListener('keydown', function (e) {
    if (e.key === ' ' || e.key === 'Enter') { e.preventDefault(); start(FLICK); }
  });

  function start(v) {
    angularVelocity = v; t0 = 0; hist = [{ t: 0, v: Math.abs(v) }]; settled = false; sweep = -1;
    secEl.textContent = '0.0';
  }
  function relaunch() { start(FLICK); }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var last = performance.now();
  function step(now) {
    var dt = Math.min(64, now - last) / 1000; last = now;
    if (!drag.on) {
      if (angularVelocity !== 0) {
        ang += angularVelocity * dt;
        var before = Math.abs(angularVelocity);
        angularVelocity *= Math.pow(COEFS[ci], dt);
        t0 += dt;
        if (Math.abs(angularVelocity) < STOP) {
          angularVelocity = 0; settled = true; settleAt = t0;
          secEl.textContent = settleAt.toFixed(1);
          hist.push({ t: t0, v: 0 });
        } else {
          hist.push({ t: t0, v: Math.abs(angularVelocity) });
        }
        if (hist.length > 200) hist.shift();
      } else if (settled && t0 > settleAt + 1.0) {
        relaunch();
      } else if (!settled) { relaunch(); }
      if (settled) {
        t0 += dt;
        sweep = Math.max(0, Math.min(1, (t0 - settleAt) / 1.0));
      }
    }
    draw();
    requestAnimationFrame(step);
  }

  fit();
  window.addEventListener('resize', fit);
  if (reduce) {
    // 정지 한 장 — 계수 0.18 로 시뮬레이션한 감쇠 곡선 전체를 미리 계산해 그린다
    var vv = FLICK, tt = 0, rr = [];
    rr.push({ t: 0, v: vv });
    while (Math.abs(vv) >= STOP && tt < MXW) { vv *= Math.pow(0.18, 1 / 60); tt += 1 / 60; rr.push({ t: tt, v: Math.abs(vv) }); }
    hist = rr; ang = -0.8 + FLICK * 0.35; sweep = 0.55;
    draw();
    root.classList.add('is-still');
  } else {
    start(FLICK);
    requestAnimationFrame(step);
  }
})();

// SpinWheelFrictionDecel — vanilla 판(run/332/demo/03-spin-wheel-friction-decel/index.html) 의 React 포트.
// rAF 루프에서 dt 를 재고 각도 += 각속도·dt, 각속도 ×= 계수^dt 를 적분한다. 우측 그래프는
// 그 적분을 실시간 꺾은선으로 그린다 — 판의 각속도와 그래프 기울기가 같다. 튕김 세기(FLICK=16)는
// 항상 같고 계수 3단계(낮음·보통·높음)만 바뀐다. 정지 뒤 1초 뒤 스캔라인이 훑고 자동 재시작한다.
// 움직임은 전부 캔버스 rAF 루프가 그리므로 CSS 애니메이션이 없다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties, PointerEvent } from "react";
import styles from "./SpinWheelFrictionDecel.module.scss";

// demo.py 의 ambient react 스텁은 KeyboardEvent 를 export 하지 않는다 — 최소 자체 타입으로 대체
type KeyEvt = { key: string; preventDefault(): void };

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

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

const TAU = Math.PI * 2;
const N = 6, SEG = TAU / N;
const COEFS = [0.08, 0.18, 0.38];   // 낮음·보통·높음 — 3단계 버튼
const FLICK = 16;                   // 시연 튕김은 항상 같다 — 계수만 바뀐다
const STOP = 0.5;
const MXW = 3.2;                    // 그래프에 담는 시야(초) — 한 시행이 창을 가득 채운다
const PAL = ["#b3bfd4", "#d9c48f", "#a9c4b0", "#c2a8d1", "#d8a3b4", "#9fc6c2"];
const INK = "#17141a";
const CREAM = "#fff7e6";
const HINT = "rAF 루프에서 dt 를 재고 각도 += 각속도·dt, 각속도 ×= 마찰^dt 를 적분합니다";
const ARIA_FIELD = "마찰 감쇠 실험판";
const ARIA_SW = "마찰 계수 고르기";
const UNIT = "마찰 계수 — 매초 각속도에 곱한다";
const META_LABEL = "정지까지 ";
const META_UNIT = "초 소요";
const META_FIXED = "같은 튕김(16 rad/s)으로 계수만 바뀝니다";

export default function SpinWheelFrictionDecel({
  duration = "2s",
  easing = "ease-out",
  color = "#2f6df6"
}: SpinWheelFrictionDecelProps) {
  const fieldRef = useRef<HTMLDivElement | null>(null);
  const cvRef = useRef<HTMLCanvasElement | null>(null);
  const secRef = useRef<HTMLSpanElement | null>(null);
  const ctxRef = useRef<CanvasRenderingContext2D | null>(null);
  const [ci, setCi] = useState(1);
  const ciRef = useRef(1) as Box<number>;   // step() 클로저 안에서 최신 값을 바로 읽기 위한 거울
  const stRef = useRef({
    ang: -0.8, vel: 0, t0: 0, hist: [{ t: 0, v: FLICK }] as HistPt[],
    settled: false, settleAt: 0, sweep: -1, dpr: 1,
    drag: { on: false, lastA: 0, lastT: 0, v: 0 }
  }) as Box<{
    ang: number; vel: number; t0: number; hist: HistPt[];
    settled: boolean; settleAt: number; sweep: number; dpr: number;
    drag: { on: boolean; lastA: number; lastT: number; v: number };
  }>;
  const lastRef = useRef(0) as Box<number>;
  const rafRef = useRef(0) as Box<number>;
  const [still, setStill] = useState(false);

  function reduced(): boolean {
    return window.matchMedia("(prefers-reduced-motion: reduce)").matches;
  }

  function fit(): void {
    const st = stRef.current;
    const field = fieldRef.current, cv = cvRef.current;
    if (!field || !cv) return;
    const ctx = cv.getContext("2d");
    if (!ctx) return;
    st.dpr = Math.min(2, window.devicePixelRatio || 1);
    cv.width = Math.round(field.clientWidth * st.dpr);
    cv.height = Math.round(field.clientHeight * st.dpr);
    ctxRef.current = ctx;
  }

  function draw(): void {
    const st = stRef.current;
    const ctx = ctxRef.current;
    if (!ctx) return;
    const w = cvRef.current!.width / st.dpr, h = cvRef.current!.height / st.dpr;
    ctx.setTransform(st.dpr, 0, 0, st.dpr, 0, 0);
    ctx.clearRect(0, 0, w, h);
    const cx = w * 0.28, cy = h * 0.5, R = Math.min(w * 0.56, h * 0.94) / 2 - 4;
    ctx.save(); ctx.translate(cx, cy); ctx.rotate(st.ang);
    for (let k = 0; k < N; k++) {
      const a0 = k * SEG;
      ctx.beginPath(); ctx.moveTo(0, 0);
      ctx.arc(0, 0, R, a0, a0 + SEG); ctx.closePath();
      ctx.fillStyle = PAL[k]; ctx.fill();
      ctx.strokeStyle = "rgba(23,20,26,.20)"; ctx.lineWidth = 1; ctx.stroke();
    }
    ctx.beginPath(); ctx.arc(0, 0, 11, 0, TAU);
    ctx.fillStyle = CREAM; ctx.fill(); ctx.strokeStyle = "rgba(23,20,26,.35)"; ctx.stroke();
    if (st.sweep >= 0) {
      ctx.beginPath(); ctx.arc(0, 0, 11 + st.sweep * 16, 0, TAU);
      ctx.strokeStyle = "rgba(23,20,26," + (1 - st.sweep) * 0.5 + ")";
      ctx.lineWidth = 2; ctx.stroke();
    }
    ctx.restore();
    ctx.beginPath();
    ctx.moveTo(cx - 6, cy - R - 8); ctx.lineTo(cx + 6, cy - R - 8); ctx.lineTo(cx, cy - R + 8);
    ctx.closePath(); ctx.fillStyle = INK; ctx.fill();
    // 감쇠 그래프 — 우측 패널. x = 경과 초, y = 각속도(0~FLICK)
    const gx = w * 0.56, gw = w - gx - 8, gy = 10, gh = h - 20;
    ctx.strokeStyle = "rgba(23,20,26,.30)"; ctx.lineWidth = 1;
    ctx.beginPath(); ctx.moveTo(gx, gy); ctx.lineTo(gx, gy + gh); ctx.lineTo(gx + gw, gy + gh); ctx.stroke();
    for (let gI = 1; gI <= 3; gI++) {
      const yy = gy + gh * (1 - gI / 4);
      ctx.beginPath(); ctx.moveTo(gx, yy); ctx.lineTo(gx + 4, yy);
      ctx.strokeStyle = "rgba(23,20,26,.25)"; ctx.stroke();
    }
    const hist = st.hist;
    if (hist.length > 1) {
      ctx.beginPath();
      for (let j = 0; j < hist.length; j++) {
        const px = gx + (hist[j].t / MXW) * gw;
        const py = gy + gh * (1 - hist[j].v / (FLICK + 1));
        if (j === 0) ctx.moveTo(px, py); else ctx.lineTo(px, py);
      }
      ctx.strokeStyle = INK; ctx.lineWidth = 2; ctx.stroke();
      const lp = hist[hist.length - 1];
      ctx.beginPath();
      ctx.arc(gx + (lp.t / MXW) * gw, gy + gh * (1 - lp.v / (FLICK + 1)), 3, 0, TAU);
      ctx.fillStyle = INK; ctx.fill();
    }
    if (st.sweep >= 0) {
      const sx = gx + st.sweep * gw;
      ctx.beginPath(); ctx.moveTo(sx, gy); ctx.lineTo(sx, gy + gh);
      ctx.strokeStyle = "rgba(23,20,26,.4)"; ctx.lineWidth = 1; ctx.stroke();
      ctx.beginPath(); ctx.arc(sx, gy + gh, 3, 0, TAU);
      ctx.fillStyle = INK; ctx.fill();
    }
  }

  function start(v: number): void {
    const st = stRef.current;
    st.vel = v; st.t0 = 0; st.hist = [{ t: 0, v: Math.abs(v) }]; st.settled = false; st.sweep = -1;
    const sec = secRef.current;
    if (sec) sec.textContent = "0.0";
  }
  function relaunch(): void { start(FLICK); }

  function angleAt(e: PointerEvent<HTMLDivElement>): number {
    const b = e.currentTarget.getBoundingClientRect();
    return Math.atan2(e.clientY - (b.top + b.height / 2), e.clientX - (b.left + b.width / 2));
  }
  function onPointerDown(e: PointerEvent<HTMLDivElement>): void {
    const st = stRef.current;
    st.drag.on = true; st.drag.lastA = angleAt(e); st.drag.lastT = performance.now(); st.drag.v = 0;
    try { e.currentTarget.setPointerCapture(e.pointerId); } catch { /* 캡처 실패는 회전에 영향 없다 */ }
  }
  function onPointerMove(e: PointerEvent<HTMLDivElement>): void {
    const st = stRef.current;
    if (!st.drag.on) return;
    const a = angleAt(e), now = performance.now();
    let dA = a - st.drag.lastA;
    if (dA > Math.PI) dA -= TAU;
    if (dA < -Math.PI) dA += TAU;
    st.ang += dA;
    const dt = Math.max(1, now - st.drag.lastT) / 1000;
    st.drag.v = 0.65 * st.drag.v + 0.35 * (dA / dt);
    st.drag.lastA = a; st.drag.lastT = now;
  }
  function endDrag(): void {
    const st = stRef.current;
    if (!st.drag.on) return;
    st.drag.on = false;
    const v = Math.max(-24, Math.min(24, st.drag.v));
    start(Math.abs(v) < 1 ? FLICK : v);
  }
  function onKeyDown(e: KeyEvt): void {
    if (e.key === " " || e.key === "Enter") { e.preventDefault(); start(FLICK); }
  }
  function onCoefClick(idx: number): void {
    ciRef.current = idx;
    setCi(idx);
    relaunch();
  }

  function step(now: number): void {
    const st = stRef.current;
    const dt = Math.min(64, now - lastRef.current) / 1000; lastRef.current = now;
    const sec = secRef.current;
    if (!st.drag.on) {
      if (st.vel !== 0) {
        st.ang += st.vel * dt;
        st.vel *= Math.pow(COEFS[ciRef.current], dt);
        st.t0 += dt;
        if (Math.abs(st.vel) < STOP) {
          st.vel = 0; st.settled = true; st.settleAt = st.t0;
          if (sec) sec.textContent = st.settleAt.toFixed(1);
          st.hist.push({ t: st.t0, v: 0 });
        } else {
          st.hist.push({ t: st.t0, v: Math.abs(st.vel) });
        }
        if (st.hist.length > 200) st.hist.shift();
      } else if (st.settled && st.t0 > st.settleAt + 1.0) {
        relaunch();
      } else if (!st.settled) { relaunch(); }
      if (st.settled) {
        st.t0 += dt;
        st.sweep = Math.max(0, Math.min(1, (st.t0 - st.settleAt) / 1.0));
      }
    }
    draw();
    rafRef.current = requestAnimationFrame(step);
  }

  useEffect(() => {
    fit();
    function onResize(): void { fit(); }
    window.addEventListener("resize", onResize);
    if (reduced()) {
      // 정지 한 장 — 계수 0.18 로 시뮬레이션한 감쇠 곡선 전체를 미리 계산해 그린다
      let vv = FLICK, tt = 0;
      const rr: HistPt[] = [{ t: 0, v: vv }];
      while (Math.abs(vv) >= STOP && tt < MXW) { vv *= Math.pow(0.18, 1 / 60); tt += 1 / 60; rr.push({ t: tt, v: Math.abs(vv) }); }
      const st = stRef.current;
      st.hist = rr; st.ang = -0.8 + FLICK * 0.35; st.sweep = 0.55;
      draw();
      setStill(true);
    } else {
      start(FLICK);
      lastRef.current = performance.now();
      rafRef.current = requestAnimationFrame(step);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  const rootCls = still ? styles.fr + " " + styles.isStill : styles.fr;
  return (
    <main className={styles.stage}>
      <div className={rootCls} style={{ "--duration": duration, "--easing": easing, "--color": color } as Vars}>
        <p className={styles.hint}>{HINT}</p>
        <div className={styles.row}>
          <div
            className={styles.field}
            ref={fieldRef}
            role="application"
            aria-label={ARIA_FIELD}
            tabIndex={0}
            onKeyDown={onKeyDown}
            onPointerDown={onPointerDown}
            onPointerMove={onPointerMove}
            onPointerUp={endDrag}
            onPointerCancel={endDrag}
          >
            <canvas className={styles.cv} ref={cvRef}></canvas>
          </div>
          <div className={styles.side}>
            <b className={styles.num}>{COEFS[ci].toFixed(2)}</b>
            <span className={styles.unit}>{UNIT}</span>
            <div className={styles.sw} role="group" aria-label={ARIA_SW}>
              {COEFS.map((c, idx) => (
                <button
                  key={idx}
                  type="button"
                  className={idx === ci ? styles.btn + " " + styles.isOn : styles.btn}
                  onClick={() => onCoefClick(idx)}
                >
                  {c}
                </button>
              ))}
            </div>
            <p className={styles.meta}>{META_LABEL}<b className={styles.count} ref={secRef}>0.0</b><span className={styles.metaUnit}>{META_UNIT}</span></p>
            <p className={styles.meta}>{META_FIXED}</p>
          </div>
        </div>
      </div>
    </main>
  );
}