06Vortex decay

<main class="stage stage--ink">
<div class="fa" id="fa">
  <p class="fa__hint">
    <svg class="fa__ico" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M128 216a88 88 0 1 1 88-88 8 8 0 0 1-16 0 72 72 0 1 0-72 72 8 8 0 0 1 0 16Z" opacity=".2"/><path d="M128 24a104.2 104.2 0 0 0-73.5 30.5 8 8 0 0 0 11.3 11.3A88 88 0 1 1 40 128a8 8 0 0 0-16 0 104 104 0 1 0 104-104Zm0 40a64 64 0 1 0 64 64 8 8 0 0 0-16 0 48 48 0 1 1-48-48 8 8 0 0 0 0-16Z"/></g></svg>
    <span>The core spins together like a rigid body while the outer rim slows with distance, and viscosity fades the whole thing out over time</span>
  </p>
  <div class="fa__field" id="field" role="application" aria-label="Vortex decay board" tabindex="0">
    <canvas class="fa__cv" id="cv"></canvas>
    <b class="fa__badge" aria-hidden="true">32<span class="fa__badgeUnit">grid</span></b>
  </div>
</div>
</main>

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

// 소용돌이 감쇠 — 움직임은 캔버스 rAF 루프 안에 있다. CSS 는 무대·판·배지만 맡는다. @keyframes 없음.
// 염료 팔레트는 항목01 과 같은 파스텔 3단(하늘색 → 라일락 → 산호)을 재사용한다 — 잉크 무대 위 대비 7.6~13.0.

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

@include stage($stage-ink);

.fa {
  --dye-0: #9ac2df;
  --dye-1: #b19edb;
  --dye-2: #e0a3b8;

  display: flex; flex-direction: column; gap: $sp-2;
  width: 100%; max-width: 424px;
}

.fa__hint {
  display: flex; align-items: center; gap: $sp-1; margin: 0;
  font-weight: 700; font-size: clamp(10px, 2.8vw, 12px); line-height: 1.35;
  color: $subject-cream;
}
.fa__ico { width: 16px; height: 16px; flex: 0 0 auto; color: #9ac2df; }

.fa__field {
  position: relative; height: 208px;
  border-radius: $r-card; overflow: hidden;
  background: $stage-ink;
  box-shadow: inset 0 0 0 1px rgba($subject-cream, .14);
  cursor: crosshair; touch-action: none;
}
.fa__field:focus-visible { outline: 2px solid #9ac2df; outline-offset: 2px; }

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

.fa__badge {
  position: absolute; right: $sp-2; top: $sp-2;
  display: flex; align-items: baseline; gap: $sp-1;
  padding: $sp-1 $sp-2; border-radius: $r-pill;
  background: rgba($subject-ink, .78);
  color: $subject-cream; font-weight: 700; font-size: 11px;
}
.fa__badgeUnit { font-weight: 700; }

@media (max-width: 340px) {
  .fa { gap: $sp-1; }
  .fa__field { height: 124px; }
  .fa__badge { font-size: 10px; padding: 0 $sp-1; }
}

// reduce — 루프가 멈춘 한 장이 남으므로 커서 신호만 바꾼다.
@media (prefers-reduced-motion: reduce) {
  .fa.is-still .fa__field { cursor: default !important; }
}

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);
}

.fa {
  --dye-0: #9ac2df;
  --dye-1: #b19edb;
  --dye-2: #e0a3b8;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.fa__hint {
  display: flex;
  align-items: center;
  gap: 4px;
  margin: 0;
  font-weight: 700;
  font-size: clamp(10px, 2.8vw, 12px);
  line-height: 1.35;
  color: #fff7e6;
}

.fa__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #9ac2df;
}

.fa__field {
  position: relative;
  height: 208px;
  border-radius: 16px;
  overflow: hidden;
  background: #17141a;
  box-shadow: inset 0 0 0 1px rgba(255, 247, 230, 0.14);
  cursor: crosshair;
  touch-action: none;
}

.fa__field:focus-visible {
  outline: 2px solid #9ac2df;
  outline-offset: 2px;
}

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

.fa__badge {
  position: absolute;
  right: 8px;
  top: 8px;
  display: flex;
  align-items: baseline;
  gap: 4px;
  padding: 4px 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.78);
  color: #fff7e6;
  font-weight: 700;
  font-size: 11px;
}

.fa__badgeUnit {
  font-weight: 700;
}

@media (max-width: 340px) {
  .fa {
    gap: 4px;
  }
  .fa__field {
    height: 124px;
  }
  .fa__badge {
    font-size: 10px;
    padding: 0 4px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .fa.is-still .fa__field {
    cursor: default !important;
  }
}

// 소용돌이 감쇠 — 초기화 시점에 랭킨 소용돌이(Rankine vortex) 속도장을 딱 한 번 심는다.
// 중심 반경(r_core) 안쪽은 강체 회전(속도 ∝ 거리), 바깥쪽은 속도 ∝ 1/거리로 줄어든다.
// 이후엔 연속 주입 없이 약한 확산 + 매 프레임 곱셈 감쇠만 걸어 소용돌이가 스스로 흐려져 사라지게 둔다.
// 감쇠가 끝나면 자동으로 다시 심어 루프를 반복한다 — 클릭하면 그 자리에 즉시 새로 심어진다.
var root = document.getElementById('fa');
var field = document.getElementById('field');
var cv = document.getElementById('cv');
var ctx = cv.getContext('2d', { willReadFrequently: true });

var css = getComputedStyle(root);
var PAL0 = css.getPropertyValue('--dye-0').trim();
var PAL1 = css.getPropertyValue('--dye-1').trim();
var PAL2 = css.getPropertyValue('--dye-2').trim();

var N = 32;
var SIZE = (N + 2) * (N + 2);
var IX = function (x, y) { return x + (N + 2) * y; };
var u = new Float32Array(SIZE), v = new Float32Array(SIZE);
var u0 = new Float32Array(SIZE), v0 = new Float32Array(SIZE);
var d = new Float32Array(SIZE), d0 = new Float32Array(SIZE);

var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
var W = 0, H = 0, raf = 0;

var R_CORE = N * 0.17;      // 강체 회전 반경 — 이 안쪽은 각속도가 일정하다
var OMEGA = 1.35;           // 중심 각속도 — 값이 클수록 처음에 세게 돈다
var LIFE = 110;             // 씨드 뒤 이 스텝(≈3.7초 @30fps) 이 지나면 자동으로 다시 심는다
var lifeStep = 0;
var cycle = 0;

function fit() {
  var r = field.getBoundingClientRect();
  W = r.width; H = r.height;
  cv.width = Math.round(W); cv.height = Math.round(H);
}

// 01 과 동일한 후방 이류(semi-Lagrangian).
function advect(b, dt) {
  var src = b === 'u' ? u0 : (b === 'v' ? v0 : d0);
  var dst = b === 'u' ? u : (b === 'v' ? v : d);
  for (var j = 1; j <= N; j++) {
    for (var i = 1; i <= N; i++) {
      var x = i - dt * N * u0[IX(i, j)];
      var y = j - dt * N * v0[IX(i, j)];
      if (x < 0.5) x = 0.5; else if (x > N + 0.5) x = N + 0.5;
      var i0 = x | 0; var i1 = i0 + 1;
      if (y < 0.5) y = 0.5; else if (y > N + 0.5) y = N + 0.5;
      var j0 = y | 0; var j1 = j0 + 1;
      var s1 = x - i0, s0 = 1 - s1, t1 = y - j0, t0 = 1 - t1;
      dst[IX(i, j)] = s0 * (t0 * src[IX(i0, j0)] + t1 * src[IX(i0, j1)]) +
                      s1 * (t0 * src[IX(i1, j0)] + t1 * src[IX(i1, j1)]);
    }
  }
}

// 약한 점성 확산 — 회전 에너지를 이웃 칸으로 새어 나가게 해 소용돌이 팔이 무뎌지며 사라지게 한다.
function diffuse(b, k, dt) {
  var src = b === 'u' ? u : (b === 'v' ? v : d);
  var a = dt * k * N * N;
  for (var iter = 0; iter < 2; iter++) {
    for (var j = 1; j <= N; j++) {
      for (var i = 1; i <= N; i++) {
        var idx = IX(i, j);
        src[idx] = (src[idx] + a * (src[IX(i - 1, j)] + src[IX(i + 1, j)] + src[IX(i, j - 1)] + src[IX(i, j + 1)])) / (1 + 4 * a);
      }
    }
  }
}

function bounds() {
  for (var i = 1; i <= N; i++) {
    d[IX(i, 0)] = d[IX(i, 1)] * 0.5; d[IX(i, N + 1)] = d[IX(i, N)] * 0.5;
    u[IX(i, 0)] = u[IX(i, 1)] * 0.4; u[IX(i, N + 1)] = u[IX(i, N)] * 0.4;
    v[IX(i, 0)] = v[IX(i, 1)] * 0.4; v[IX(i, N + 1)] = v[IX(i, N)] * 0.4;
  }
  for (var j = 1; j <= N; j++) {
    d[IX(0, j)] = d[IX(1, j)] * 0.5; d[IX(N + 1, j)] = d[IX(N, j)] * 0.5;
    u[IX(0, j)] = u[IX(1, j)] * 0.4; u[IX(N + 1, j)] = u[IX(N, j)] * 0.4;
    v[IX(0, j)] = v[IX(1, j)] * 0.4; v[IX(N + 1, j)] = v[IX(N, j)] * 0.4;
  }
}

// 랭킨 소용돌이 심기 — 필드를 전부 비우고 새 소용돌이 하나만 남긴다(연속 주입이 아니라 일회성 초기화).
function seedVortex(cx, cy, dir) {
  u.fill(0); v.fill(0); d.fill(0);
  u0.fill(0); v0.fill(0); d0.fill(0);
  for (var j = 1; j <= N; j++) {
    for (var i = 1; i <= N; i++) {
      var dx = i - cx, dy = j - cy;
      var r = Math.sqrt(dx * dx + dy * dy) || 0.0001;
      var vTheta = r <= R_CORE ? OMEGA * r : OMEGA * (R_CORE * R_CORE) / r;
      var idx = IX(i, j);
      // 접선 방향(반시계=dir 1, 시계=dir -1): (-dy/r, dx/r)
      u[idx] = dir * (-dy / r) * vTheta;
      v[idx] = dir * (dx / r) * vTheta;
      // 밀도는 소용돌이 팔 모양을 보이려고 코어 주변 링 모양으로 심는다
      var ring = Math.exp(-Math.pow((r - R_CORE * 1.15) / (R_CORE * 0.85), 2));
      d[idx] = ring * 1.4;
    }
  }
  lifeStep = 0;
}

function step(dt) {
  u0.set(u); v0.set(v); d0.set(d);
  advect('u', dt); u0.set(u);
  advect('v', dt); v0.set(v);
  advect('d', dt);
  diffuse('u', 0.0016, dt);
  diffuse('v', 0.0016, dt);
  diffuse('d', 0.0016, dt);
  bounds();
  // 매 프레임 곱셈 감쇠 — 연속 주입 없이 이것만으로 소용돌이가 시간이 갈수록 흐려진다
  var decay = 0.9905;
  for (var k = 0; k < SIZE; k++) { u[k] *= decay; v[k] *= decay; d[k] *= 0.988; }
  lifeStep++;
  if (lifeStep >= LIFE) {
    cycle++;
    var dir = cycle % 2 === 0 ? 1 : -1;
    var cx = N * 0.5 + (cycle % 3 - 1) * N * 0.05;
    var cy = N * 0.5 + ((cycle + 1) % 3 - 1) * N * 0.05;
    seedVortex(cx, cy, dir);
  }
}

function render() {
  var img = ctx.createImageData(cv.width, cv.height);
  var px = img.data;
  var cellW = cv.width / N, cellH = cv.height / N;
  var pal = [PAL0, PAL1, PAL2];
  for (var py = 0; py < cv.height; py++) {
    var gy = Math.min(N - 1, (py / cellH) | 0);
    for (var pxx = 0; pxx < cv.width; pxx++) {
      var gx = Math.min(N - 1, (pxx / cellW) | 0);
      var q = d[IX(gx + 1, gy + 1)];
      var o = (py * cv.width + pxx) * 4;
      if (q <= 0) { px[o] = 23; px[o + 1] = 20; px[o + 2] = 26; px[o + 3] = 255; continue; }
      var band = q > 0.62 ? 2 : (q > 0.3 ? 1 : 0);
      var c = pal[band];
      var rr = parseInt(c.slice(1, 3), 16), gg = parseInt(c.slice(3, 5), 16), bb = parseInt(c.slice(5, 7), 16);
      var a = Math.min(1, q);
      px[o] = 23 + (rr - 23) * a; px[o + 1] = 20 + (gg - 20) * a; px[o + 2] = 26 + (bb - 26) * a; px[o + 3] = 255;
    }
  }
  ctx.putImageData(img, 0, 0);
}

function frame() {
  step(1 / 30);
  render();
  raf = requestAnimationFrame(frame);
}

// 실제 사용 — 클릭한 자리에 즉시 새 소용돌이를 심는다(같은 seedVortex 를 자동 루프와 공유).
field.addEventListener('pointerdown', function (e) {
  var r = field.getBoundingClientRect();
  var gx = 1 + ((e.clientX - r.left) / r.width * (N - 1));
  var gy = 1 + ((e.clientY - r.top) / r.height * (N - 1));
  cycle++;
  seedVortex(gx, gy, cycle % 2 === 0 ? 1 : -1);
  if (reduce) render();
});
field.addEventListener('keydown', function (e) {
  if (e.key !== 'Enter' && e.key !== ' ') return;
  e.preventDefault();
  cycle++;
  seedVortex(N * 0.5, N * 0.5, cycle % 2 === 0 ? 1 : -1);
  if (reduce) render();
});
window.addEventListener('resize', function () { fit(); if (reduce) render(); });

fit();
seedVortex(N * 0.5, N * 0.5, 1);
if (reduce) {
  // reduce — 40 걸음만 앞으로 감아 감쇠가 진행 중인 한 장을 그린다(빈 화면도, 갓 심은 화면도 아니다)
  for (var k = 0; k < 40; k++) step(1 / 30);
  render();
} else {
  raf = requestAnimationFrame(frame);
}

// VortexDecay — vanilla 판(run/308/demo/06-fluid-vortex-decay/index.html) 의 React 포트.
// 초기화 때 랭킨 소용돌이 속도장을 한 번 심고 점성 감쇠만 두면 시간이 갈수록 퍼져 사라지는 소용돌이가 관찰된다.
// 격자 필드는 ref 상자 안에서만 산다 — React 상태로 두면 리렌더가 매 프레임 일어나 60fps 예산을 못 맞춘다.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./VortexDecay.module.scss";

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

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type KeyEvt = { key: string; preventDefault(): void };
type PtrEvt = { clientX: number; clientY: number };

const N = 32;
const SIZE = (N + 2) * (N + 2);
function IX(x: number, y: number): number { return x + (N + 2) * y; }

const R_CORE = N * 0.17;
const OMEGA = 1.35;
const LIFE = 110;

export default function VortexDecay({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#2f6df6",
}: VortexDecayProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const fieldRef = useRef<HTMLDivElement | null>(null);
  const cvRef = useRef<HTMLCanvasElement | null>(null);
  const ctxRef = useRef<CanvasRenderingContext2D | null>(null);
  const palRef = useRef(["#9ac2df", "#b19edb", "#e0a3b8"]) as Box<string[]>;

  const u = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const v = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const u0 = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const v0 = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const d = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const d0 = useRef(new Float32Array(SIZE)) as Box<Float32Array>;

  const lifeStepRef = useRef(0) as Box<number>;
  const cycleRef = useRef(0) as Box<number>;
  const rafRef = useRef(0) as Box<number>;

  const vars: Vars = { "--duration": duration, "--easing": easing, "--color": color };

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

  function fit(): void {
    const field = fieldRef.current;
    const cv = cvRef.current;
    if (!field || !cv) return;
    const r = field.getBoundingClientRect();
    cv.width = Math.round(r.width);
    cv.height = Math.round(r.height);
  }

  function advect(b: "u" | "v" | "d", dt: number): void {
    const src = b === "u" ? u0.current : b === "v" ? v0.current : d0.current;
    const dst = b === "u" ? u.current : b === "v" ? v.current : d.current;
    const uu = u0.current, vv = v0.current;
    for (let j = 1; j <= N; j++) {
      for (let i = 1; i <= N; i++) {
        let x = i - dt * N * uu[IX(i, j)];
        let y = j - dt * N * vv[IX(i, j)];
        if (x < 0.5) x = 0.5; else if (x > N + 0.5) x = N + 0.5;
        const i0 = x | 0, i1 = i0 + 1;
        if (y < 0.5) y = 0.5; else if (y > N + 0.5) y = N + 0.5;
        const j0 = y | 0, j1 = j0 + 1;
        const s1 = x - i0, s0 = 1 - s1, t1 = y - j0, t0 = 1 - t1;
        dst[IX(i, j)] = s0 * (t0 * src[IX(i0, j0)] + t1 * src[IX(i0, j1)]) +
                        s1 * (t0 * src[IX(i1, j0)] + t1 * src[IX(i1, j1)]);
      }
    }
  }

  function diffuse(b: "u" | "v" | "d", k: number, dt: number): void {
    const src = b === "u" ? u.current : b === "v" ? v.current : d.current;
    const a = dt * k * N * N;
    for (let iter = 0; iter < 2; iter++) {
      for (let j = 1; j <= N; j++) {
        for (let i = 1; i <= N; i++) {
          const idx = IX(i, j);
          src[idx] = (src[idx] + a * (src[IX(i - 1, j)] + src[IX(i + 1, j)] + src[IX(i, j - 1)] + src[IX(i, j + 1)])) / (1 + 4 * a);
        }
      }
    }
  }

  function bounds(): void {
    const dd = d.current, uu = u.current, vv = v.current;
    for (let i = 1; i <= N; i++) {
      dd[IX(i, 0)] = dd[IX(i, 1)] * 0.5; dd[IX(i, N + 1)] = dd[IX(i, N)] * 0.5;
      uu[IX(i, 0)] = uu[IX(i, 1)] * 0.4; uu[IX(i, N + 1)] = uu[IX(i, N)] * 0.4;
      vv[IX(i, 0)] = vv[IX(i, 1)] * 0.4; vv[IX(i, N + 1)] = vv[IX(i, N)] * 0.4;
    }
    for (let j = 1; j <= N; j++) {
      dd[IX(0, j)] = dd[IX(1, j)] * 0.5; dd[IX(N + 1, j)] = dd[IX(N, j)] * 0.5;
      uu[IX(0, j)] = uu[IX(1, j)] * 0.4; uu[IX(N + 1, j)] = uu[IX(N, j)] * 0.4;
      vv[IX(0, j)] = vv[IX(1, j)] * 0.4; vv[IX(N + 1, j)] = vv[IX(N, j)] * 0.4;
    }
  }

  function seedVortex(cx: number, cy: number, dir: number): void {
    const uu = u.current, vv = v.current, dd = d.current;
    const u0v = u0.current, v0v = v0.current, d0v = d0.current;
    uu.fill(0); vv.fill(0); dd.fill(0);
    u0v.fill(0); v0v.fill(0); d0v.fill(0);
    for (let j = 1; j <= N; j++) {
      for (let i = 1; i <= N; i++) {
        const dx = i - cx, dy = j - cy;
        const r = Math.sqrt(dx * dx + dy * dy) || 0.0001;
        const vTheta = r <= R_CORE ? OMEGA * r : OMEGA * (R_CORE * R_CORE) / r;
        const idx = IX(i, j);
        uu[idx] = dir * (-dy / r) * vTheta;
        vv[idx] = dir * (dx / r) * vTheta;
        const ring = Math.exp(-Math.pow((r - R_CORE * 1.15) / (R_CORE * 0.85), 2));
        dd[idx] = ring * 1.4;
      }
    }
    lifeStepRef.current = 0;
  }

  function step(dt: number): void {
    u0.current.set(u.current); v0.current.set(v.current); d0.current.set(d.current);
    advect("u", dt); u0.current.set(u.current);
    advect("v", dt); v0.current.set(v.current);
    advect("d", dt);
    diffuse("u", 0.0016, dt);
    diffuse("v", 0.0016, dt);
    diffuse("d", 0.0016, dt);
    bounds();
    const uu = u.current, vv = v.current, dd = d.current;
    const decay = 0.9905;
    for (let k = 0; k < SIZE; k++) { uu[k] *= decay; vv[k] *= decay; dd[k] *= 0.988; }
    lifeStepRef.current++;
    if (lifeStepRef.current >= LIFE) {
      cycleRef.current++;
      const c = cycleRef.current;
      const dir = c % 2 === 0 ? 1 : -1;
      const cx = N * 0.5 + (c % 3 - 1) * N * 0.05;
      const cy = N * 0.5 + ((c + 1) % 3 - 1) * N * 0.05;
      seedVortex(cx, cy, dir);
    }
  }

  function render(): void {
    const ctx = ctxRef.current, cv = cvRef.current;
    if (!ctx || !cv) return;
    const img = ctx.createImageData(cv.width, cv.height);
    const px = img.data;
    const cellW = cv.width / N, cellH = cv.height / N;
    const pal = palRef.current;
    const dd = d.current;
    for (let py = 0; py < cv.height; py++) {
      const gy = Math.min(N - 1, (py / cellH) | 0);
      for (let pxx = 0; pxx < cv.width; pxx++) {
        const gx = Math.min(N - 1, (pxx / cellW) | 0);
        const q = dd[IX(gx + 1, gy + 1)];
        const o = (py * cv.width + pxx) * 4;
        if (q <= 0) { px[o] = 23; px[o + 1] = 20; px[o + 2] = 26; px[o + 3] = 255; continue; }
        const band = q > 0.62 ? 2 : q > 0.3 ? 1 : 0;
        const c = pal[band];
        const rr = parseInt(c.slice(1, 3), 16), gg = parseInt(c.slice(3, 5), 16), bb = parseInt(c.slice(5, 7), 16);
        const a = Math.min(1, q);
        px[o] = 23 + (rr - 23) * a; px[o + 1] = 20 + (gg - 20) * a; px[o + 2] = 26 + (bb - 26) * a; px[o + 3] = 255;
      }
    }
    ctx.putImageData(img, 0, 0);
  }

  function frame(): void {
    step(1 / 30);
    render();
    rafRef.current = requestAnimationFrame(frame);
  }

  function onDown(e: PtrEvt): void {
    const field = fieldRef.current;
    if (!field) return;
    const r = field.getBoundingClientRect();
    const gx = 1 + (e.clientX - r.left) / r.width * (N - 1);
    const gy = 1 + (e.clientY - r.top) / r.height * (N - 1);
    cycleRef.current++;
    seedVortex(gx, gy, cycleRef.current % 2 === 0 ? 1 : -1);
    if (reduced()) render();
  }

  function onKey(e: KeyEvt): void {
    if (e.key !== "Enter" && e.key !== " ") return;
    e.preventDefault();
    cycleRef.current++;
    seedVortex(N * 0.5, N * 0.5, cycleRef.current % 2 === 0 ? 1 : -1);
    if (reduced()) render();
  }

  function onResize(): void { fit(); if (reduced()) render(); }

  useEffect(() => {
    const cv = cvRef.current;
    if (!cv) return;
    ctxRef.current = cv.getContext("2d", { willReadFrequently: true });
    if (!ctxRef.current) return;
    const root = rootRef.current;
    if (root) {
      const css = getComputedStyle(root);
      const p0 = css.getPropertyValue("--dye-0").trim();
      const p1 = css.getPropertyValue("--dye-1").trim();
      const p2 = css.getPropertyValue("--dye-2").trim();
      if (p0 && p1 && p2) palRef.current = [p0, p1, p2];
    }
    fit();
    window.addEventListener("resize", onResize);
    seedVortex(N * 0.5, N * 0.5, 1);
    if (reduced()) {
      if (root) root.classList.add("is-still");
      for (let k = 0; k < 40; k++) step(1 / 30);
      render();
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.root} ref={rootRef} style={vars}>
        <p className={styles.hint}>
          <svg className={styles.ico} viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M128 216a88 88 0 1 1 88-88 8 8 0 0 1-16 0 72 72 0 1 0-72 72 8 8 0 0 1 0 16Z" opacity=".2"/><path d="M128 24a104.2 104.2 0 1 0 73.5 30.5 8 8 0 0 0-11.3 11.3A88 88 0 1 1 40 128a8 8 0 0 0-16 0A104 104 0 0 0 128 24Zm0 40a64 64 0 1 1-64 64 8 8 0 0 0 16 0 48 48 0 1 0 48-48 8 8 0 0 1 0-16Z"/></g></svg>
          <span>중심은 강체처럼 함께 돌고 바깥은 멀어질수록 느려지며, 점성으로 시간이 갈수록 흐려져 사라집니다</span>
        </p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label="소용돌이 감쇠 실험판"
          tabIndex={0}
          onPointerDown={onDown}
          onKeyDown={onKey}
        >
          <canvas className={styles.cv} ref={cvRef}></canvas>
          <b className={styles.badge} aria-hidden="true">32<span className={styles.badgeUnit}>격자</span></b>
        </div>
      </div>
    </main>
  );
}