03Pendulum swing

<svg width="0" height="0" style="position:absolute" aria-hidden="true">
  <defs>
    <symbol id="ico-grab" viewBox="0 0 256 256" fill="currentColor"><g><path d="M208 108v44a80 80 0 0 1-160 0v-12a20 20 0 0 1 20-20h20V92a20 20 0 0 1 40 0a20 20 0 0 1 40 0v16a20 20 0 0 1 40 0" opacity=".2"/><path d="M188 80a27.8 27.8 0 0 0-13.36 3.4a28 28 0 0 0-46.64-11A28 28 0 0 0 80 92v20H68a28 28 0 0 0-28 28v12a88 88 0 0 0 176 0v-44a28 28 0 0 0-28-28m12 72a72 72 0 0 1-144 0v-12a12 12 0 0 1 12-12h12v24a8 8 0 0 0 16 0V92a12 12 0 0 1 24 0v28a8 8 0 0 0 16 0V92a12 12 0 0 1 24 0v28a8 8 0 0 0 16 0v-12a12 12 0 0 1 24 0Z"/></g></symbol>
    <symbol id="ico-reset" viewBox="0 0 256 256" fill="currentColor"><g><path d="M232 136a104 104 0 1 1-38.17-80.55L208 60v-4a8 8 0 0 1 16 0v34a8 8 0 0 1-8 8h-34a8 8 0 0 1 0-16h7.6l-8.89-7.4A88 88 0 1 0 216 136a8 8 0 0 1 16 0Z" opacity=".2"/><path d="M232 136a104 104 0 1 1-38.17-80.55L208 60v-4a8 8 0 0 1 16 0v34a8 8 0 0 1-8 8h-34a8 8 0 0 1 0-16h7.6l-8.89-7.4A88 88 0 1 0 216 136a8 8 0 0 1 16 0Z"/></g></symbol>
  </defs>
</svg>
<main class="stage">
<div class="pd is-demo" id="pd">
  <p class="pd__hint"><svg class="pd__ico" aria-hidden="true"><use href="#ico-grab"/></svg><span>Pull the bob and let go, and the angle turns into height so it keeps swinging on its own</span></p>
  <div class="pd__field" id="field" role="application" aria-label="Pendulum test board" tabindex="0">
    <span class="pd__pivot" aria-hidden="true"></span>
    <div class="pd__arm" id="arm">
      <span class="pd__rod" aria-hidden="true"></span>
      <button class="pd__bob" id="bob" type="button" aria-label="Pendulum bob">
        <svg class="pd__bobIco" aria-hidden="true"><use href="#ico-grab"/></svg>
      </button>
    </div>
    <p class="pd__angle" aria-live="polite"><span class="pd__angleVal" id="angleText">0 deg</span><span class="pd__angleLoop" aria-hidden="true"><i>26 deg</i><i>0 deg</i><i>26 deg</i><i>0 deg</i></span></p>
  </div>
  <div class="pd__bar">
    <button class="pd__pull" id="pull" type="button">
      <svg class="pd__ico" aria-hidden="true"><use href="#ico-grab"/></svg><span>Release at 30 deg</span>
    </button>
    <button class="pd__reset" id="reset" type="button">
      <svg class="pd__ico" aria-hidden="true"><use href="#ico-reset"/></svg><span>Stop</span>
    </button>
  </div>
</div>
</main>

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

// 진자 흔들기 — 각가속도 -(g/L)sinθ - c·ω 의 준암시적 오일러 적분. 각도가 곧 위치에너지다.
// 회전축은 고정점(위쪽 경첩)이므로 rotate 의 transform-origin 을 맨 위 변에 둔다 (blog-demo 규칙 8 "붙는 지점" 예외).
// 격자 미리보기(.is-demo)는 감쇠 없는 이상 진자(에너지 보존)의 사인 흔들림을 키프레임으로 재생한다 —
// 주기의 시작과 끝이 같은 각도라 경계가 닫혀 있다. 사람이 추를 잡으면 JS 가 .is-demo 를 떼고 진짜 적분으로 간다.

$duration: $dur-loop;
$easing: ease-in-out;   // 사인파의 4분의 한 구간마다 가감속이 있어 구간 이징으로 표본화한다
$color: $stage-orange;

@include stage($stage-orange);

.pd { display: flex; flex-direction: column; gap: $sp-3; width: 100%; max-width: 424px; }
.pd__hint {
  display: flex; align-items: center; gap: $sp-1; margin: 0;
  font-weight: 700; font-size: clamp(10px, 2.8vw, 12px); color: $subject-ink;
}
.pd__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $subject-ink; }

.pd__field {
  position: relative; height: 176px; border-radius: $r-card;
  // 판을 이만큼 어둡게 해야 위의 크림 각도 글자가 WCAG AA(4.5:1)를 넘는다 — .18 에서는 4.23:1 이었다
  background: rgba($subject-ink, .42);
  box-shadow: inset 0 0 0 1px rgba($subject-cream, .22);
  cursor: grab; overflow: hidden; touch-action: none;
}
.pd__field:focus-visible { outline: 2px solid $subject-cream; outline-offset: 2px; }

.pd__pivot {
  position: absolute; left: 50%; top: 0; width: 20px; height: 20px;
  transform: translateX(-50%); border-radius: 50%;
  background: $subject-cream; box-shadow: $shadow-press;
}
// 팔 전체가 고정점을 축으로 돈다 — transform-origin 을 맨 위로 (붙는 지점)
.pd__arm {
  position: absolute; left: 50%; top: 10px; width: 0; height: 118px;
  transform-origin: 50% 0;
}
.pd__rod {
  position: absolute; left: -1px; top: 0; width: 2px; height: 100%;
  background: rgba($subject-cream, .8);
}
.pd__bob {
  position: absolute; left: -16px; bottom: -12px; width: 32px; height: 32px;
  display: grid; place-items: center; border: 0; border-radius: 50%;
  background: $subject-cream; color: $stage-orange; cursor: grab; padding: 0;
  box-shadow: $shadow-press; touch-action: none;
}
.pd__bob:active { cursor: grabbing; }
.pd__bobIco { width: 16px; height: 16px; pointer-events: none; }
.pd__angle {
  position: absolute; right: $sp-2; bottom: 0; margin: 0;
  font-weight: 800; font-size: clamp(10px, 2.6vw, 11px); color: $subject-cream;
}

.pd__bar { display: flex; align-items: center; justify-content: flex-end; gap: $sp-2; flex-wrap: wrap; }
.pd__pull, .pd__reset {
  display: inline-flex; align-items: center; gap: $sp-1; border: 0; border-radius: $r-pill;
  padding: $sp-1 $sp-3; background: $subject-cream; color: $subject-ink; cursor: pointer;
  font-family: inherit; font-weight: 800; font-size: clamp(10px, 2.6vw, 11px); line-height: 1;
}

// ── 격자 미리보기 루프 ─────────────────────────────────────────────
// 에너지 보존(감쇠 0)의 등진폭 사인 흔들림 — 2초에 왕복 한 번.
// 각도 텍스트는 사인과 같은 주기로 세 값을 steps 로 돌며 컷 교체된다.
.pd.is-demo .pd__arm {
  animation-name: pdSwing; animation-duration: $duration; animation-timing-function: $easing;
  animation-iteration-count: infinite;
}
@keyframes pdSwing {
  0%   { transform: rotate(26deg); }
  25%  { transform: rotate(0deg); }
  50%  { transform: rotate(-26deg); }
  75%  { transform: rotate(0deg); }
  100% { transform: rotate(26deg); }
}
// 각도 값 교체 — 미리보기 전용 겹칠 텍스트(291 막대 스펙트럼의 대역 이름 패턴).
// 본문 값은 color: transparent 로 공간만 유지한다 (274 실측: display:none 은 폭까지 없앤다)
.pd__angleLoop { position: absolute; inset: 0; display: grid; place-items: center end; visibility: hidden; }
.pd__angleLoop i { grid-area: 1 / 1; font-style: normal; visibility: hidden; }
.pd.is-demo .pd__angleVal { color: transparent; }
.pd.is-demo .pd__angleLoop { visibility: visible; }
.pd.is-demo .pd__angleLoop i {
  animation-name: pdAngle; animation-duration: $duration; animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.pd.is-demo .pd__angleLoop i:nth-child(2) { animation-delay: 500ms; }
.pd.is-demo .pd__angleLoop i:nth-child(3) { animation-delay: 1000ms; }
.pd.is-demo .pd__angleLoop i:nth-child(4) { animation-delay: 1500ms; }
@keyframes pdAngle {
  0%, 24%  { visibility: visible; }
  25%, 100% { visibility: hidden; }
}

@media (max-width: 340px) {
  .pd { gap: $sp-2; }
  .pd__field { height: 104px; }
  .pd__arm { height: 64px; top: 8px; }
  .pd__bob { width: 26px; height: 26px; left: -13px; bottom: -8px; }
  .pd__bobIco { width: 13px; height: 13px; }
}

// reduce 에서는 수직으로 멈춘 한 장을 둔다
@media (prefers-reduced-motion: reduce) {
  .pd.is-demo .pd__arm { animation: none !important; transform: rotate(0deg) !important; }
  .pd.is-demo .pd__angleVal { color: $subject-cream !important; }
  .pd.is-demo .pd__angleLoop { visibility: hidden !important; }
  .pd.is-demo .pd__angleLoop i { animation: none !important; }
}

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);
}

.pd {
  display: flex;
  flex-direction: column;
  gap: 12px;
  width: 100%;
  max-width: 424px;
}

.pd__hint {
  display: flex;
  align-items: center;
  gap: 4px;
  margin: 0;
  font-weight: 700;
  font-size: clamp(10px, 2.8vw, 12px);
  color: #17141a;
}

.pd__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #17141a;
}

.pd__field {
  position: relative;
  height: 176px;
  border-radius: 16px;
  background: rgba(23, 20, 26, 0.42);
  box-shadow: inset 0 0 0 1px rgba(255, 247, 230, 0.22);
  cursor: grab;
  overflow: hidden;
  touch-action: none;
}

.pd__field:focus-visible {
  outline: 2px solid #fff7e6;
  outline-offset: 2px;
}

.pd__pivot {
  position: absolute;
  left: 50%;
  top: 0;
  width: 20px;
  height: 20px;
  transform: translateX(-50%);
  border-radius: 50%;
  background: #fff7e6;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
}

.pd__arm {
  position: absolute;
  left: 50%;
  top: 10px;
  width: 0;
  height: 118px;
  transform-origin: 50% 0;
}

.pd__rod {
  position: absolute;
  left: -1px;
  top: 0;
  width: 2px;
  height: 100%;
  background: rgba(255, 247, 230, 0.8);
}

.pd__bob {
  position: absolute;
  left: -16px;
  bottom: -12px;
  width: 32px;
  height: 32px;
  display: grid;
  place-items: center;
  border: 0;
  border-radius: 50%;
  background: #fff7e6;
  color: #ff4d1f;
  cursor: grab;
  padding: 0;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  touch-action: none;
}

.pd__bob:active {
  cursor: grabbing;
}

.pd__bobIco {
  width: 16px;
  height: 16px;
  pointer-events: none;
}

.pd__angle {
  position: absolute;
  right: 8px;
  bottom: 0;
  margin: 0;
  font-weight: 800;
  font-size: clamp(10px, 2.6vw, 11px);
  color: #fff7e6;
}

.pd__bar {
  display: flex;
  align-items: center;
  justify-content: flex-end;
  gap: 8px;
  flex-wrap: wrap;
}

.pd__pull, .pd__reset {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  border: 0;
  border-radius: 999px;
  padding: 4px 12px;
  background: #fff7e6;
  color: #17141a;
  cursor: pointer;
  font-family: inherit;
  font-weight: 800;
  font-size: clamp(10px, 2.6vw, 11px);
  line-height: 1;
}

.pd.is-demo .pd__arm {
  animation-name: pdSwing;
  animation-duration: 2s;
  animation-timing-function: ease-in-out;
  animation-iteration-count: infinite;
}

@keyframes pdSwing {
  0% {
    transform: rotate(26deg);
  }
  25% {
    transform: rotate(0deg);
  }
  50% {
    transform: rotate(-26deg);
  }
  75% {
    transform: rotate(0deg);
  }
  100% {
    transform: rotate(26deg);
  }
}
.pd__angleLoop {
  position: absolute;
  inset: 0;
  display: grid;
  place-items: center end;
  visibility: hidden;
}

.pd__angleLoop i {
  grid-area: 1/1;
  font-style: normal;
  visibility: hidden;
}

.pd.is-demo .pd__angleVal {
  color: transparent;
}

.pd.is-demo .pd__angleLoop {
  visibility: visible;
}

.pd.is-demo .pd__angleLoop i {
  animation-name: pdAngle;
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}

.pd.is-demo .pd__angleLoop i:nth-child(2) {
  animation-delay: 500ms;
}

.pd.is-demo .pd__angleLoop i:nth-child(3) {
  animation-delay: 1000ms;
}

.pd.is-demo .pd__angleLoop i:nth-child(4) {
  animation-delay: 1500ms;
}

@keyframes pdAngle {
  0%, 24% {
    visibility: visible;
  }
  25%, 100% {
    visibility: hidden;
  }
}
@media (max-width: 340px) {
  .pd {
    gap: 8px;
  }
  .pd__field {
    height: 104px;
  }
  .pd__arm {
    height: 64px;
    top: 8px;
  }
  .pd__bob {
    width: 26px;
    height: 26px;
    left: -13px;
    bottom: -8px;
  }
  .pd__bobIco {
    width: 13px;
    height: 13px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .pd.is-demo .pd__arm {
    animation: none !important;
    transform: rotate(0deg) !important;
  }
  .pd.is-demo .pd__angleVal {
    color: #fff7e6 !important;
  }
  .pd.is-demo .pd__angleLoop {
    visibility: hidden !important;
  }
  .pd.is-demo .pd__angleLoop i {
    animation: none !important;
  }
}

var root = document.getElementById('pd');
var field = document.getElementById('field');
var arm = document.getElementById('arm');
var bob = document.getElementById('bob');
var angleText = document.getElementById('angleText');
var reduce = window.matchMedia('(prefers-reduced-motion: reduce)');
// G 는 px/s² 로 02 중력 드롭과 같은 눈금이고 RAD 는  deg 변환 상수다.
// -(g/L)sinθ 는 라디안/s² 이라  deg 단위인 angle·avel 에 그대로 더하면 57배 작아진다 —
// 그 상태로는 30도에서 놓아도 6초에 0.9도밖에 안 움직여 추가 멈춰 있는 것처럼 보인다
var G = 1800, LEN = 118, DAMP = 0.4, RAD = 180 / Math.PI;
var angle = 0, avel = 0, dragging = false, last = 0, running = false;

function paint() {
  arm.style.transform = 'rotate(' + angle.toFixed(2) + 'deg)';
  var deg = Math.round(Math.abs(angle));
  if (angleText.textContent !== deg + ' deg') { angleText.textContent = deg + ' deg'; }
}
function step(now) {
  var dt = Math.min((now - last) / 1000, 0.033);
  last = now;
  // 각가속도 = -(g/L)·sinθ - Damping. 각도가 높이(에너지)로 바뀌는 게 진자의 물리다
  avel += (-(G / LEN) * Math.sin(angle / RAD) * RAD - DAMP * avel) * dt;
  angle += avel * dt;
  paint();
  if (Math.abs(angle) < 0.2 && Math.abs(avel) < 0.6) {
    angle = 0; avel = 0; paint(); running = false; return;
  }
  requestAnimationFrame(step);
}
function start() {
  root.classList.remove('is-demo');
  if (reduce.matches) { angle = 0; avel = 0; paint(); return; }
  if (!running) { running = true; last = performance.now(); requestAnimationFrame(step); }
}
function angleFromEvent(e) {
  var r = field.getBoundingClientRect();
  var px = e.clientX - (r.left + r.width / 2);
  var py = e.clientY - r.top;
  var deg = Math.atan2(px, py) * 180 / Math.PI;
  return Math.max(-75, Math.min(75, deg));
}
bob.addEventListener('pointerdown', function (e) {
  e.preventDefault();
  dragging = true; root.classList.remove('is-demo');
  angle = angleFromEvent(e); avel = 0; paint();
  bob.setPointerCapture(e.pointerId);
});
bob.addEventListener('pointermove', function (e) {
  if (!dragging) { return; }
  angle = angleFromEvent(e); avel = 0; paint();
});
bob.addEventListener('pointerup', function () {
  if (!dragging) { return; }
  dragging = false;
  if (reduce.matches) { angle = 0; avel = 0; paint(); return; }
  // 놓는 순간의 각도가 위치에너지 — 여기서부터 진동이 시작된다
  if (!running) { running = true; last = performance.now(); requestAnimationFrame(step); }
});
field.addEventListener('keydown', function (e) {
  var JUMP = 15;
  if (e.key === 'ArrowLeft') { angle = Math.max(-75, angle - JUMP); }
  else if (e.key === 'ArrowRight') { angle = Math.min(75, angle + JUMP); }
  else if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); start(); return; }
  else { return; }
  e.preventDefault();
  root.classList.remove('is-demo'); avel = 0; paint();
});
document.getElementById('pull').addEventListener('click', function () { angle = 30; avel = 0; start(); });
document.getElementById('reset').addEventListener('click', function () { angle = 0; avel = 0; running = false; paint(); });
paint();

// PendulumSwing — vanilla 판(run/292/demo/03-physics-pendulum-swing/index.html) 의 React 포트.
// 각가속도 = -(G/LEN)·sin(각도) - DAMP·각속도 의 준암시적 오일러 적분. 각도가 곧 위치에너지다.
// 추를 잡고 있는 동안에는 포인터 각도를 그대로 쓰고, 놓는 순간의 각도에서 진동이 시작된다.
// 값·수치는 vanilla 와 같다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties, PointerEvent as ReactPointerEvent } from "react";
import styles from "./PendulumSwing.module.scss";

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

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
// useRef 는 T | null 을 돌려준다. 처음부터 값이 있고 절대 비지 않는 상자는 이렇게 좁혀 쓴다
type Box<T> = { current: T };
// React 의 KeyboardEvent 제네릭이 없는 환경이라 필요한 것만 로컬로 선언한다
type KeyEvt = { key: string; preventDefault(): void };

// G 는 px/s² 로 02 중력 드롭과 같은 눈금이고 RAD 는 도 변환 상수다.
// -(g/L)sinθ 는 라디안/s² 이라 도 단위인 angle·avel 에 그대로 더하면 57배 작아진다 —
// 그 상태로는 30도에서 놓아도 6초에 0.9도밖에 안 움직여 추가 멈춰 있는 것처럼 보인다
const G = 1800;
const LEN = 118;
const DAMP = 0.4;
const RAD = 180 / Math.PI;
const JUMP = 15;    // 방향키 한 번에 더하는 각도
const LIMIT = 75;   // 잡아 당길 수 있는 최대 각도
const FIELD_LABEL = "진자 실험판";

export default function PendulumSwing({
  duration = 2000,
  easing = "ease-in-out",
  color = "#ff4d1f",
}: PendulumSwingProps) {
  const fieldRef = useRef<HTMLDivElement | null>(null);
  const armRef = useRef<HTMLDivElement | null>(null);
  const angleRef = useRef<number>(0) as Box<number>;
  const avelRef = useRef<number>(0) as Box<number>;
  const draggingRef = useRef<boolean>(false) as Box<boolean>;
  const lastRef = useRef<number>(0) as Box<number>;
  const rafRef = useRef<number>(0) as Box<number>;
  const runningRef = useRef<boolean>(false) as Box<boolean>;
  const shownRef = useRef<number>(0) as Box<number>;   // 화면에 찍힌 각도 — 같은 값이면 다시 그리지 않는다
  const [shown, setShown] = useState(0);
  const [demo, setDemo] = useState(true);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

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

  function paint(): void {
    const arm = armRef.current;
    if (arm) arm.style.transform = "rotate(" + angleRef.current.toFixed(2) + "deg)";
    const deg = Math.round(Math.abs(angleRef.current));
    if (shownRef.current !== deg) { shownRef.current = deg; setShown(deg); }
  }

  function step(now: number): void {
    const dt = Math.min((now - lastRef.current) / 1000, 0.033);
    lastRef.current = now;
    // 각가속도 = -(g/L)·sinθ - 감쇠. 각도가 높이(에너지)로 바뀌는 게 진자의 물리다
    avelRef.current += (-(G / LEN) * Math.sin(angleRef.current / RAD) * RAD - DAMP * avelRef.current) * dt;
    angleRef.current += avelRef.current * dt;
    paint();
    if (Math.abs(angleRef.current) < 0.2 && Math.abs(avelRef.current) < 0.6) {
      angleRef.current = 0; avelRef.current = 0;
      paint();
      runningRef.current = false;
      return;
    }
    rafRef.current = requestAnimationFrame(step);
  }

  function start(): void {
    setDemo(false);
    if (reduced()) { angleRef.current = 0; avelRef.current = 0; paint(); return; }
    if (!runningRef.current) {
      runningRef.current = true;
      lastRef.current = performance.now();
      rafRef.current = requestAnimationFrame(step);
    }
  }

  function angleFromEvent(event: ReactPointerEvent<HTMLButtonElement>): number {
    const field = fieldRef.current;
    if (!field) return angleRef.current;
    const r = field.getBoundingClientRect();
    const px = event.clientX - (r.left + r.width / 2);
    const py = event.clientY - r.top;
    const deg = (Math.atan2(px, py) * 180) / Math.PI;
    return Math.max(-LIMIT, Math.min(LIMIT, deg));
  }

  useEffect(() => {
    paint();
    return () => cancelAnimationFrame(rafRef.current);
  }, []);

  function onBobPointerDown(event: ReactPointerEvent<HTMLButtonElement>): void {
    event.preventDefault();
    draggingRef.current = true;
    setDemo(false);
    angleRef.current = angleFromEvent(event);
    avelRef.current = 0;
    paint();
    event.currentTarget.setPointerCapture(event.pointerId);
  }

  function onBobPointerMove(event: ReactPointerEvent<HTMLButtonElement>): void {
    if (!draggingRef.current) return;
    angleRef.current = angleFromEvent(event);
    avelRef.current = 0;
    paint();
  }

  function onBobPointerUp(): void {
    if (!draggingRef.current) return;
    draggingRef.current = false;
    if (reduced()) { angleRef.current = 0; avelRef.current = 0; paint(); return; }
    // 놓는 순간의 각도가 위치에너지 — 여기서부터 진동이 시작된다
    if (!runningRef.current) {
      runningRef.current = true;
      lastRef.current = performance.now();
      rafRef.current = requestAnimationFrame(step);
    }
  }

  function onKeyDown(event: KeyEvt): void {
    if (event.key === "ArrowLeft") angleRef.current = Math.max(-LIMIT, angleRef.current - JUMP);
    else if (event.key === "ArrowRight") angleRef.current = Math.min(LIMIT, angleRef.current + JUMP);
    else if (event.key === "Enter" || event.key === " ") { event.preventDefault(); start(); return; }
    else return;
    event.preventDefault();
    setDemo(false);
    avelRef.current = 0;
    paint();
  }

  const cls = [styles.root, demo ? styles.isDemo : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <svg width="0" height="0" style={{ position: "absolute" }} aria-hidden="true">
        <defs>
          <symbol id="ico-grab" viewBox="0 0 256 256" fill="currentColor"><g><path d="M208 108v44a80 80 0 0 1-160 0v-12a20 20 0 0 1 20-20h20V92a20 20 0 0 1 40 0a20 20 0 0 1 40 0v16a20 20 0 0 1 40 0" opacity=".2"/><path d="M188 80a27.8 27.8 0 0 0-13.36 3.4a28 28 0 0 0-46.64-11A28 28 0 0 0 80 92v20H68a28 28 0 0 0-28 28v12a88 88 0 0 0 176 0v-44a28 28 0 0 0-28-28m12 72a72 72 0 0 1-144 0v-12a12 12 0 0 1 12-12h12v24a8 8 0 0 0 16 0V92a12 12 0 0 1 24 0v28a8 8 0 0 0 16 0V92a12 12 0 0 1 24 0v28a8 8 0 0 0 16 0v-12a12 12 0 0 1 24 0Z"/></g></symbol>
          <symbol id="ico-reset" viewBox="0 0 256 256" fill="currentColor"><g><path d="M232 136a104 104 0 1 1-38.17-80.55L208 60v-4a8 8 0 0 1 16 0v34a8 8 0 0 1-8 8h-34a8 8 0 0 1 0-16h7.6l-8.89-7.4A88 88 0 1 0 216 136a8 8 0 0 1 16 0Z" opacity=".2"/><path d="M232 136a104 104 0 1 1-38.17-80.55L208 60v-4a8 8 0 0 1 16 0v34a8 8 0 0 1-8 8h-34a8 8 0 0 1 0-16h7.6l-8.89-7.4A88 88 0 1 0 216 136a8 8 0 0 1 16 0Z"/></g></symbol>
        </defs>
      </svg>
      <div className={cls} style={vars}>
        <p className={styles.hint}>
          <svg className={styles.ico} aria-hidden="true"><use href="#ico-grab" /></svg>
          <span>추를 잡아 당겼다 놓으면 각도가 높이로 바뀌어 스스로 흔들립니다</span>
        </p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label={FIELD_LABEL}
          tabIndex={0}
          onKeyDown={onKeyDown}
        >
          <span className={styles.pivot} aria-hidden="true"></span>
          <div className={styles.arm} ref={armRef}>
            <span className={styles.rod} aria-hidden="true"></span>
            <button
              className={styles.bob}
              type="button"
              aria-label="진자 추"
              onPointerDown={onBobPointerDown}
              onPointerMove={onBobPointerMove}
              onPointerUp={onBobPointerUp}
            >
              <svg className={styles.bobIco} aria-hidden="true"><use href="#ico-grab" /></svg>
            </button>
          </div>
          <p className={styles.angle} aria-live="polite">
            <span className={styles.angleVal}>{shown}도</span>
            <span className={styles.angleLoop} aria-hidden="true"><i>26도</i><i>0도</i><i>26도</i><i>0도</i></span>
          </p>
        </div>
        <div className={styles.bar}>
          <button
            className={styles.pull}
            type="button"
            onClick={() => { angleRef.current = 30; avelRef.current = 0; start(); }}
          >
            <svg className={styles.ico} aria-hidden="true"><use href="#ico-grab" /></svg><span>30도에서 놓기</span>
          </button>
          <button
            className={styles.reset}
            type="button"
            onClick={() => { angleRef.current = 0; avelRef.current = 0; runningRef.current = false; paint(); }}
          >
            <svg className={styles.ico} aria-hidden="true"><use href="#ico-reset" /></svg><span>멈추기</span>
          </button>
        </div>
      </div>
    </main>
  );
}