01Stable fluid advection

<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="M237.94 107.19l-89.13-89.13a20 20 0 0 0-28.28 0l-89.13 89.13a20 20 0 0 0 0 28.28l89.13 89.13a20 20 0 0 0 28.28 0l89.13-89.13a20 20 0 0 0 0-28.28ZM167 143.46a8 8 0 0 1-11.4 0L128 115.86l-27.6 27.6a8 8 0 1 1-11.4-11.32l33.2-33.46a8 8 0 0 1 11.4 0l33.2 33.46a8 8 0 0 1 .2 11.32Z" opacity=".2"/><path d="M243.54 101.75L154.4 12.61a28 28 0 0 0-39.66 0L25.6 101.75a28 28 0 0 0 0 39.66l89.14 89.14a28 28 0 0 0 39.66 0l89.14-89.14a28 28 0 0 0 0-39.66Zm-11.4 28.26l-89.13 89.14a12 12 0 0 1-17 0L36.9 130a12 12 0 0 1 0-17L126 23.87a12 12 0 0 1 17 0l89.14 89.14a12 12 0 0 1 0 16.98Zm-64.76-83.75a8 8 0 0 0-11.71 0l-33.65 33.87a4 4 0 0 1-5.71 0L82.49 100.1a8 8 0 1 0-11.4 11.24l33.51 33.9a20 20 0 0 0 28.49 0l33.51-33.9a8 8 0 0 0 .18-12.02Z"/></g></svg>
    <span>Each cell traces the flow backward to pull its previous value — hover to bend the current</span>
  </p>
  <div class="fa__field" id="field" role="application" aria-label="Fluid advection 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 *;

// 안정 유체 이류 — 움직임은 전부 캔버스 rAF 루프 안에 있다. CSS 는 무대·판·배지만 맡는다.
// @keyframes 를 하나도 두지 않는다 — 렌더러가 CSS 시계를 고정하지 않고 실시간으로 24장을 찍는다(012·077 선례).
// 염료 팔레트는 파스텔(채도 0.50 이하)로 직접 잡는다 — 이 편은 기능색 편이 아니므로 BD26 기준을 처음부터 지킨다.

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

@include stage($stage-ink);

.fa {
  // 염료 세 층 — 파스텔 하늘색(옅은) → 파스텔 라일락(중간) → 파스텔 산호(진한). 잉크 무대 위 대비 7.6~13.0
  --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;
  }
}

// 안정 유체 이류(semi-Lagrangian) — 다음 프레임을 내다보지 않고, 지금 칸의 값이
// 한 걸음 전 어디에서 왔는지 속도를 거슬러 샘플링한다. 시간 간격이 커져도 폭발하지 않는다.
// 속도장이 자기 자신을 옮기는 자기 이류와 염료 밀도를 옮기는 물질 이류를 한 프레임에 겹쳐 돌린다.
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();

// N — grid 한 변 칸 수. 480px 무대에 32칸이면 한 칸 15px, 60fps 안에 계산이 들어온다
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, seedT = 0;
var stirring = false;

// 화면 픽셀 → grid 좌표 (0..1) 로 바꾸는 비율
function fit() {
  var r = field.getBoundingClientRect();
  W = r.width; H = r.height;
  cv.width = Math.round(W); cv.height = Math.round(H);
}

// 후방 이류 — 칸 (x,y) 의 새 값을, 속도를 dt 만큼 거슬러 간 좌표의 옛 값으로 채운다.
// 좌표는 칸 배열 범위로 가두고(경계 밖 샘플링 방지) 네 이웃의 면적 가중평균(bilinear)로 읽는다.
// 셀 하나가 차지하는 공간은 1/N 이다 — 속도(칸/초) × dt(초) × N 이 되짚는 칸 수가 된다.
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)]);
    }
  }
}

// 경계 — 벽 반사 근사. 유출 속도를 0 으로 두고 밀도는 감쇠시킨다
function bounds() {
  var n2 = N + 2;
  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)] * 0.5; u[IX(i, N + 1)] = u[IX(i, N)] * 0.5;
    v[IX(i, 0)] = v[IX(i, 1)] * 0.5; v[IX(i, N + 1)] = v[IX(i, N)] * 0.5;
  }
  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)] * 0.5; u[IX(N + 1, j)] = u[IX(N, j)] * 0.5;
    v[IX(0, j)] = v[IX(1, j)] * 0.5; v[IX(N + 1, j)] = v[IX(N, j)] * 0.5;
  }
}

// 미리보기 주입원 — 시드 고정. 잎새 모양 두 바퀴를 그리는 속도 소스와 궤적을 따라 얹히는 염료.
// seedT 로 결정적: 같은 편의 gif·poster 는 언제나 같은 흐름이 담긴다.
function inject(t) {
  var cx = N * 0.5 + Math.sin(t * 0.9) * N * 0.22;
  var cy = N * 0.55 + Math.sin(t * 1.7) * N * 0.16;
  var dx = Math.cos(t * 0.9) * 0.9 * N * 0.22, dy = Math.cos(t * 1.7) * 1.7 * N * 0.16 * 0.35;
  var amp = 42;
  var r = 2.6;
  for (var j = -r; j <= r; j++) {
    for (var i = -r; i <= r; i++) {
      var q = (i * i + j * j) / (r * r);
      if (q > 1) continue;
      var f = Math.exp(-q * 3.2) * amp;
      var idx = IX(Math.round(cx + i), Math.round(cy + j));
      u0[idx] += dx * f * 0.06;
      v0[idx] += dy * f * 0.06;
      var hue = (t * 0.22) % 1;
      var amt = Math.exp(-q * 2.2) * 0.9;
      d0[idx] += amt * (hue < 0.5 ? 1 : 0.55);
    }
  }
}

function step(dt) {
  u0.set(u); v0.set(v); d0.set(d);
  inject(seedT);
  advect('u', dt); u0.set(u);
  advect('v', dt); v0.set(v);
  advect('d', dt);
  bounds();
  for (var k = 0; k < SIZE; k++) d[k] *= 0.995;   // 밀도는 천천히 죽는다 — 화면이 꽉 찬 채 굳지 않게
  seedT += dt;
}

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);
}

// 사람이 휘젓기 — 포인터 이동량을 그 칸 주변에 지수 반경으로 얹는다
function splat(gx, gy, fx, fy, amt) {
  var r = 3;
  for (var j = -r; j <= r; j++) {
    for (var i = -r; i <= r; i++) {
      var q = (i * i + j * j) / (r * r);
      if (q > 1) continue;
      var f = Math.exp(-q * 3) * amt;
      var idx = IX(gx + i, gy + j);
      if (gx + i < 1 || gx + i > N || gy + j < 1 || gy + j > N) continue;
      u[idx] += fx * f; v[idx] += fy * f;
      d[idx] = Math.min(2.2, d[idx] + f * 0.7);
    }
  }
}
var last = null;
field.addEventListener('pointermove', function (e) {
  var r = field.getBoundingClientRect();
  var gx = 1 + Math.round((e.clientX - r.left) / r.width * (N - 1));
  var gy = 1 + Math.round((e.clientY - r.top) / r.height * (N - 1));
  if (last) {
    var fx = (e.clientX - last.x) * 0.14, fy = (e.clientY - last.y) * 0.14;
    splat(gx, gy, fx, fy, Math.min(1.6, Math.hypot(fx, fy)));
  } else {
    splat(gx, gy, 0, 0, 0.8);
  }
  last = { x: e.clientX, y: e.clientY };
  if (reduce) render();
});
field.addEventListener('pointerleave', function () { last = null; stirring = false; });
field.addEventListener('keydown', function (e) {
  var dir = { ArrowLeft: [-2, 0], ArrowRight: [2, 0], ArrowUp: [0, -2], ArrowDown: [0, 2] }[e.key];
  if (!dir) return;
  e.preventDefault();
  var gx = N >> 1, gy = N >> 1;
  splat(gx, gy, dir[0], dir[1], 1.2);
  if (reduce) render();
});
window.addEventListener('resize', function () { fit(); if (reduce) render(); });

fit();
if (reduce) {
  // reduce — 시뮬레이션을 40 걸음만 앞으로 감아 흐름이 자리잡은 한 장을 그린다(빈 화면이 아니다)
  for (var k = 0; k < 40; k++) step(1 / 30);
  render();
} else {
  raf = requestAnimationFrame(frame);
}

// StableAdvect — vanilla 판(run/308/demo/01-fluid-stable-advect/index.html) 의 React 포트.
// 안정 유체(Stable Fluids) semi-Lagrangian 후방 이류. 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 "./StableAdvect.module.scss";

export type StableAdvectProps = {
  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; }

export default function StableAdvect({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#2f6df6",
}: StableAdvectProps) {
  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 sizeRef = useRef({ w: 0, h: 0 }) as Box<{ w: number; h: number }>;
  const seedTRef = useRef(0) as Box<number>;
  const lastRef = useRef<{ x: number; y: number } | null>(null) as Box<{ x: number; y: number } | null>;
  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();
    sizeRef.current.w = r.width;
    sizeRef.current.h = r.height;
    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 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)] * 0.5; uu[IX(i, N + 1)] = uu[IX(i, N)] * 0.5;
      vv[IX(i, 0)] = vv[IX(i, 1)] * 0.5; vv[IX(i, N + 1)] = vv[IX(i, N)] * 0.5;
    }
    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)] * 0.5; uu[IX(N + 1, j)] = uu[IX(N, j)] * 0.5;
      vv[IX(0, j)] = vv[IX(1, j)] * 0.5; vv[IX(N + 1, j)] = vv[IX(N, j)] * 0.5;
    }
  }

  function inject(t: number): void {
    const cx = N * 0.5 + Math.sin(t * 0.9) * N * 0.22;
    const cy = N * 0.55 + Math.sin(t * 1.7) * N * 0.16;
    const dx = Math.cos(t * 0.9) * 0.9 * N * 0.22, dy = Math.cos(t * 1.7) * 1.7 * N * 0.16 * 0.35;
    const amp = 42, r = 2.6;
    const u0v = u0.current, v0v = v0.current, d0v = d0.current;
    for (let j = -r; j <= r; j++) {
      for (let i = -r; i <= r; i++) {
        const q = (i * i + j * j) / (r * r);
        if (q > 1) continue;
        const f = Math.exp(-q * 3.2) * amp;
        const idx = IX(Math.round(cx + i), Math.round(cy + j));
        u0v[idx] += dx * f * 0.06;
        v0v[idx] += dy * f * 0.06;
        const hue = (t * 0.22) % 1;
        const amt = Math.exp(-q * 2.2) * 0.9;
        d0v[idx] += amt * (hue < 0.5 ? 1 : 0.55);
      }
    }
  }

  function step(dt: number): void {
    u0.current.set(u.current); v0.current.set(v.current); d0.current.set(d.current);
    inject(seedTRef.current);
    advect("u", dt); u0.current.set(u.current);
    advect("v", dt); v0.current.set(v.current);
    advect("d", dt);
    bounds();
    const dd = d.current;
    for (let k = 0; k < SIZE; k++) dd[k] *= 0.995;
    seedTRef.current += dt;
  }

  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 splat(gx: number, gy: number, fx: number, fy: number, amt: number): void {
    const r = 3;
    const uu = u.current, vv = v.current, dd = d.current;
    for (let j = -r; j <= r; j++) {
      for (let i = -r; i <= r; i++) {
        const q = (i * i + j * j) / (r * r);
        if (q > 1) continue;
        if (gx + i < 1 || gx + i > N || gy + j < 1 || gy + j > N) continue;
        const f = Math.exp(-q * 3) * amt;
        const idx = IX(gx + i, gy + j);
        uu[idx] += fx * f; vv[idx] += fy * f;
        dd[idx] = Math.min(2.2, dd[idx] + f * 0.7);
      }
    }
  }

  function onMove(e: PtrEvt): void {
    const field = fieldRef.current;
    if (!field) return;
    const r = field.getBoundingClientRect();
    const gx = 1 + Math.round((e.clientX - r.left) / r.width * (N - 1));
    const gy = 1 + Math.round((e.clientY - r.top) / r.height * (N - 1));
    const last = lastRef.current;
    if (last) {
      const fx = (e.clientX - last.x) * 0.14, fy = (e.clientY - last.y) * 0.14;
      splat(gx, gy, fx, fy, Math.min(1.6, Math.hypot(fx, fy)));
    } else {
      splat(gx, gy, 0, 0, 0.8);
    }
    lastRef.current = { x: e.clientX, y: e.clientY };
    if (reduced()) render();
  }

  function onLeave(): void { lastRef.current = null; }

  function onKey(e: KeyEvt): void {
    const dir: Record<string, number[]> = { ArrowLeft: [-2, 0], ArrowRight: [2, 0], ArrowUp: [0, -2], ArrowDown: [0, 2] };
    const dd2 = dir[e.key];
    if (!dd2) return;
    e.preventDefault();
    splat(N >> 1, N >> 1, dd2[0], dd2[1], 1.2);
    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);
    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="M237.94 107.19l-89.13-89.13a20 20 0 0 0-28.28 0l-89.13 89.13a20 20 0 0 0 0 28.28l89.13 89.13a20 20 0 0 0 28.28 0l89.13-89.13a20 20 0 0 0 0-28.28ZM167 143.46a8 8 0 0 1-11.4 0L128 115.86l-27.6 27.6a8 8 0 1 1-11.4-11.32l33.2-33.46a8 8 0 0 1 11.4 0l33.2 33.46a8 8 0 0 1 .2 11.32Z" opacity=".2"/><path d="M243.54 101.75L154.4 12.61a28 28 0 0 0-39.66 0L25.6 101.75a28 28 0 0 0 0 39.66l89.14 89.14a28 28 0 0 0 39.66 0l89.14-89.14a28 28 0 0 0 0-39.66Zm-11.4 28.26l-89.13 89.14a12 12 0 0 1-17 0L36.9 130a12 12 0 0 1 0-17L126 23.87a12 12 0 0 1 17 0l89.14 89.14a12 12 0 0 1 0 16.98Zm-64.76-83.75a8 8 0 0 0-11.71 0l-33.65 33.87a4 4 0 0 1-5.71 0L82.49 100.1a8 8 0 1 0-11.4 11.24l33.51 33.9a20 20 0 0 0 28.49 0l33.51-33.9a8 8 0 0 0 .18-12.02Z"/></g></svg>
          <span>각 칸은 물결을 되짚어 이전 값을 가져옵니다 — 커서를 얹으면 흐름이 구부러집니다</span>
        </p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label="유체 이류 실험판"
          tabIndex={0}
          onPointerMove={onMove}
          onPointerLeave={onLeave}
          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>
  );
}