05Foam particles — grid-cell fragments

<main class="stage">
<div class="fp" id="fp">
  <p class="fp__hint">
    <svg class="fp__ico" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M16 120c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".25"/><path d="M16 168c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".15"/><path d="M228.92 49.09a112 112 0 1 0 14 84.91a8 8 0 0 0-15.5-4a96 96 0 1 1-12-72.91A8 8 0 0 1 216 71.63V40a8 8 0 0 1-8-8h-31.63a8 8 0 0 1-5.65-13.09ZM64 104a12 12 0 1 1 12 12a12 12 0 0 1-12-12m40 64a12 12 0 1 1 12 12a12 12 0 0 1-12-12m72-16a16 16 0 1 1 16 16a16 16 0 0 1-16-16" opacity=".9"/></g></svg>
    <span>Computes foam density per grid cell — no per-particle coordinates, each cell counts its own lifespan from birth; try blowing a stronger wind</span>
  </p>
  <div class="fp__field fp__grid-cell" id="field" role="application" aria-label="Foam particles playground" tabindex="0">
    <canvas class="fp__cv" id="cv"></canvas>
    <b class="fp__badge" aria-hidden="true"><span class="fp__badgeNum" id="foamVal">foam 0</span><span class="fp__badgeUnit" id="windVal">wind 1.0</span></b>
  </div>
</div>
</main>

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

// 물거품 파티클(격자 셀 기반) — 크레스트에서 발생 → advect(흐름 이동) → 지수 감쇠하는 오일러 밀도장.
// 움직임은 전부 캔버스 rAF 안에 있다. @keyframes 를 두지 않는다(077 방식).
// 색은 전부 토큰 또는 mix() 로컬 변수(BD26) — 헥스 리터럴 금지.
// 캔버스가 쓰는 색은 커스텀 프로퍼티로 내려보낸다 — 값은 이 파일 한 곳에만 둔다.

$fp-sea0: mix($subject-blue-deep, $stage-paper, 54%);   // 바탕 바다색 — 진한 파랑
$fp-sea1: mix($subject-sky, $stage-paper, 62%);          // 바탕 바다색 — 밝은 하늘 민트
$fp-foam0: mix($subject-cream, $stage-paper, 70%);       // 거품 밀도 낮음 — 옅은 크림
$fp-foam1: $subject-cream;                               // 거품 밀도 높음 — 진한 크림

$duration: $dur-loop;
$easing: $ease-spring;
$color: $fp-foam1;

@include stage($stage-paper);

.fp {
  --sea0:  #{$fp-sea0};
  --sea1:  #{$fp-sea1};
  --foam0: #{$fp-foam0};
  --foam1: #{$fp-foam1};

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

.fp__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-ink;
}
.fp__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $subject-blue; }

.fp__field {
  position: relative; height: 208px;
  border-radius: $r-card; overflow: hidden;
  background: $fp-sea0;
  box-shadow: inset 0 0 0 1px rgba($subject-ink, .16);
  cursor: crosshair; touch-action: none;
}
.fp__field:focus-visible { outline: 2px solid $subject-blue; outline-offset: 2px; }

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

.fp__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;
}
.fp__badgeNum { font-size: 13px; color: $stage-yellow; }
.fp__badgeUnit { font-weight: 700; }

@media (max-width: 340px) {
  .fp { gap: $sp-1; }
  .fp__field { height: 124px; }
  .fp__badge { font-size: 10px; padding: 0 $sp-1; }
  .fp__badgeNum { font-size: 11px; }
  .fp__hint { font-size: 10px; }
}

// reduce — 루프가 아예 안 돈다. 거품이 이미 자란 정적 한 장이 남으므로
// 끌 수 있다는 신호(커서)만 거둔다.
@media (prefers-reduced-motion: reduce) {
  .fp.is-still .fp__field { cursor: default !important; }
  .fp.is-still .fp__badge { opacity: .92 !important; }
}

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

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

.fp {
  --sea0: rgb(48.1176470588%, 62.7294117647%, 95.1294117647%);
  --sea1: rgb(64.7450980392%, 86.6274509804%, 100%);
  --foam0: rgb(100%, 97.8039215686%, 93.137254902%);
  --foam1: #fff7e6;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.fp__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: #17141a;
}

.fp__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #2f6df6;
}

.fp__field {
  position: relative;
  height: 208px;
  border-radius: 16px;
  overflow: hidden;
  background: rgb(48.1176470588%, 62.7294117647%, 95.1294117647%);
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.16);
  cursor: crosshair;
  touch-action: none;
}

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

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

.fp__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;
}

.fp__badgeNum {
  font-size: 13px;
  color: #ffd23f;
}

.fp__badgeUnit {
  font-weight: 700;
}

@media (max-width: 340px) {
  .fp {
    gap: 4px;
  }
  .fp__field {
    height: 124px;
  }
  .fp__badge {
    font-size: 10px;
    padding: 0 4px;
  }
  .fp__badgeNum {
    font-size: 11px;
  }
  .fp__hint {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .fp.is-still .fp__field {
    cursor: default !important;
  }
  .fp.is-still .fp__badge {
    opacity: 0.92 !important;
  }
}

// 물거품 파티클(격자 셀 기반) — 01 과 같은 뿌리(정현파 6개 mulberry32(7))의 2D 파도장에서
// 크레스트(threshold 넘는 칸)를 오일러 관점으로 스캔한다. 개별 파티클 좌표를 매 프레임 새로 뽑지 않고,
// "이 거품이 어느 격자 셀(grid-cell)에서 태어났는가"(cellId)로 밀도(density[cellId])와 수명(life[cellId])을
// 저장해 크레스트에서 발생 → 성장 → 파도 위상을 따라 흐름 이동 → 소멸하는 오일러 필드를 만든다.
// 포인터로 "wind"을 불면 발생량(spawn rate)이 커진다.
var root = document.getElementById('fp');
var field = document.getElementById('field');
var cv = document.getElementById('cv');
var foamVal = document.getElementById('foamVal');
var windVal = document.getElementById('windVal');
var ctx = cv.getContext('2d');

var css = getComputedStyle(root);
function v(name) { return css.getPropertyValue(name).trim(); }
var SEA0 = v('--sea0'), SEA1 = v('--sea1');   // 바탕 바다색 램프
var FOAM0 = v('--foam0'), FOAM1 = v('--foam1'); // foam 밀도 낮음→높음 램프(크림 계열)

var COLS = 90;    // 오일러 격자 가로 칸 수 — 저해상 셀(파티클 좌표 아님)
var STEP = 4;     // 격자 한 칸이 렌더 버퍼에서 차지하는 픽셀
var NW = 6;       // 겹칠 정현파 개수(공통 물리, 01 과 동일 뿌리)
var LIFE_MAX = 26; // foam 한 점의 최대 수명(프레임 단위 근사 — 시간 dt 로 감쇠)

function mulberry32(a) {
  return function () {
    a |= 0; a = (a + 0x6D2B79F5) | 0;
    var t = Math.imul(a ^ (a >>> 15), 1 | a);
    var u = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((u ^ (u >>> 14)) >>> 0) / 4294967296;
  };
}
var rng = mulberry32(7);
var rngSpawn = mulberry32(19);   // 발생 판정 전용 — 파도장 시드와 분리(같은 시드를 재사용하면 상관이 생긴다)

var WAVES = [];
(function () {
  var lam = 34;
  for (var i = 0; i < NW; i++) {
    var ang = rng() * Math.PI * 2;
    WAVES.push({
      lam: lam,
      freq: (Math.PI * 2) / lam,
      dirX: Math.cos(ang), dirY: Math.sin(ang),
      amp: 0.58 / (i * 0.62 + 1),
      spd: 0.55 + rng() * 0.25,
      ph: rng() * Math.PI * 2
    });
    lam *= 0.55;
  }
})();
for (var w = 0; w < NW; w++) {
  WAVES[w].om = WAVES[w].spd * 3.6 * Math.sqrt(1 / WAVES[w].lam) * 6;
}

var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
var W = 0, H = 0, dpr = 1, ROWS = 0;
var hgt = null;     // 파도장 높이(Float32Array)
var dens = null;    // 셀별 foam 밀도(Float32Array, 0..1) — 오일러 필드 본체
var buf = null, bctx = null, out = null;
var RW = 0, RH = 0;
var raf = 0, auto = true, nowSec = 0, lastT = 0;
var wind = 1.0, windT = 1.0;
var windAng = 0.5, windAngT = 0.5;
var lastPX = -1, lastPY = -1, lastPT = 0;
var totalFoam = 0;

function hex2rgb(s) {
  var pm = /^\s*rgb\(\s*([\d.]+)%\s*,\s*([\d.]+)%\s*,\s*([\d.]+)%\s*\)\s*$/.exec(s);
  if (pm) return [Math.round(pm[1] * 2.55), Math.round(pm[2] * 2.55), Math.round(pm[3] * 2.55)];
  s = s.replace('#', '');
  if (s.length === 3) s = s[0] + s[0] + s[1] + s[1] + s[2] + s[2];
  var n = parseInt(s, 16);
  return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
}
var RS0 = hex2rgb(SEA0), RS1 = hex2rgb(SEA1);
var RF0 = hex2rgb(FOAM0), RF1 = hex2rgb(FOAM1);

function fit() {
  var r = field.getBoundingClientRect();
  dpr = window.devicePixelRatio || 1;
  W = r.width; H = r.height;
  cv.width = Math.round(W * dpr);
  cv.height = Math.round(H * dpr);
  ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
  ROWS = Math.max(30, Math.round(COLS * H / W));
  var n = COLS * ROWS;
  hgt = new Float32Array(n);
  dens = new Float32Array(n);
  RW = COLS * STEP; RH = ROWS * STEP;
  buf = document.createElement('canvas');
  buf.width = RW; buf.height = RH;
  bctx = buf.getContext('2d');
  out = bctx.createImageData(RW, RH);
}

// 파도장 높이 합성 — 01 과 같은 뿌리
function fieldcalc(t, wstr, wang) {
  var i = 0;
  for (var y = 0; y < ROWS; y++) {
    for (var x = 0; x < COLS; x++, i++) {
      var h = 0;
      for (var w2 = 0; w2 < NW; w2++) {
        var WV = WAVES[w2];
        var da = wang * 0.5;
        var dx = WV.dirX * Math.cos(da) - WV.dirY * Math.sin(da);
        var dy = WV.dirX * Math.sin(da) + WV.dirY * Math.cos(da);
        var ph = (x * dx + y * dy) * WV.freq * 0.62 + t * WV.om + WV.ph;
        h += WV.amp * wstr * Math.sin(ph);
      }
      hgt[i] = h;
    }
  }
}

// foam 오일러 스텝 — 셀마다: (1) 크레스트면 발생 확률로 밀도를 채운다(spawn),
// (2) 파도 위상 방향으로 밀도를 옆 셀에 조금 옮긴다(advect — 흐름 이동, 개별 좌표 추적 아님),
// (3) 전체를 지수 감쇠시켜 수명이 다한 거품이 자연히 사라지게 한다.
var densNext = null;
function foamStep(dt, wstr, wang) {
  if (!densNext || densNext.length !== dens.length) densNext = new Float32Array(dens.length);
  var eff = 0;
  for (var i = 0; i < NW; i++) eff += WAVES[i].amp;
  eff *= wstr;
  var cut = eff * 0.74;                       // 크레스트 threshold(03 과 같은 비율대)
  var spawnRate = 0.10 + Math.min(0.55, (wstr - 0.6) * 0.28); // 바람이 셀수록 발생량 ↑
  var decay = Math.exp(-dt * (1000 / LIFE_MAX) * 0.06);       // 수명 감쇠(지수)
  var flowX = Math.cos(windAng) * 0.9, flowY = Math.sin(windAng) * 0.9;
  var total = 0;
  for (var y = 0; y < ROWS; y++) {
    var row = y * COLS;
    for (var x = 0; x < COLS; x++) {
      var idx = row + x;
      // advect — 위상 흐름 반대 방향(이전 위치)에서 밀도를 끌어와 이 셀에 놓는다(semi-Lagrangian 근사)
      var sx = x - flowX, sy = y - flowY;
      var ix = Math.round(sx), iy = Math.round(sy);
      if (ix < 0) ix = 0; else if (ix >= COLS) ix = COLS - 1;
      if (iy < 0) iy = 0; else if (iy >= ROWS) iy = ROWS - 1;
      var carried = dens[iy * COLS + ix] * decay;
      // spawn — 크레스트 셀만 결정적 난수로 발생 판정(같은 셀·같은 프레임은 같은 판정 — 시드 고정)
      if (hgt[idx] > cut) {
        var r = rngSpawn();
        if (r < spawnRate) carried = Math.min(1, carried + 0.55 + r * 0.4);
      }
      densNext[idx] = carried;
      total += carried;
    }
  }
  var tmp = dens; dens = densNext; densNext = tmp;
  totalFoam = total;
}

// 렌더 — 바탕 바다색(높이 낮은 대비) 위에 foam 밀도를 크림 램프로 얹는다. 밀도가 진할수록 불투명.
function render() {
  var o = out.data;
  for (var y = 0; y < ROWS; y++) {
    var row = y * COLS;
    for (var x = 0; x < COLS; x++) {
      var idx = row + x;
      var u = hgt[idx] * 0.7 + 0.5;
      if (u < 0) u = 0; else if (u > 1) u = 1;
      var r = RS0[0] + (RS1[0] - RS0[0]) * u;
      var g = RS0[1] + (RS1[1] - RS0[1]) * u;
      var b = RS0[2] + (RS1[2] - RS0[2]) * u;
      var f = dens[idx];
      if (f > 0.02) {
        if (f > 1) f = 1;
        var fr = RF0[0] + (RF1[0] - RF0[0]) * f;
        var fg = RF0[1] + (RF1[1] - RF0[1]) * f;
        var fb = RF0[2] + (RF1[2] - RF0[2]) * f;
        r = r * (1 - f) + fr * f;
        g = g * (1 - f) + fg * f;
        b = b * (1 - f) + fb * f;
      }
      for (var sy = 0; sy < STEP; sy++) {
        var base = ((y * STEP + sy) * RW + x * STEP) * 4;
        for (var sx2 = 0; sx2 < STEP; sx2++, base += 4) {
          o[base] = r;
          o[base + 1] = g;
          o[base + 2] = b;
          o[base + 3] = 255;
        }
      }
    }
  }
  bctx.putImageData(out, 0, 0);
  ctx.drawImage(buf, 0, 0, W, H);
}

function frame(now) {
  nowSec = now / 1000;
  var dt = lastT ? (now - lastT) : 16.7;
  lastT = now;
  if (auto) {
    var th = nowSec * 0.31;
    windT = 1.05 + Math.sin(th * 0.9 + 0.4) * 0.55 + Math.sin(th * 2.3) * 0.12;
    windAngT = 0.5 + Math.sin(th * 0.42) * 0.85;
  }
  wind += (windT - wind) * 0.05;
  windAng += (windAngT - windAng) * 0.05;
  fieldcalc(nowSec, wind, windAng);
  foamStep(dt, wind, windAng);
  render();
  foamVal.textContent = 'foam ' + Math.round(totalFoam);
  windVal.textContent = 'wind ' + wind.toFixed(2);
  raf = requestAnimationFrame(frame);
}

// 정지판 — 발생·감쇠를 몇 스텝 미리 돌려 거품이 이미 자란 정적 한 장을 만든다(빈 바다가 아니다)
function still() {
  fieldcalc(7.3, 1.35, 0.5);
  windAng = 0.5;
  for (var k = 0; k < 40; k++) foamStep(33, 1.35, 0.5);
  render();
}

function localXY(e) {
  var r = field.getBoundingClientRect();
  return [e.clientX - r.left, e.clientY - r.top];
}

field.addEventListener('pointermove', function (e) {
  var p = localXY(e);
  var t = performance.now();
  if (lastPX >= 0 && t > lastPT) {
    var vx = p[0] - lastPX, vy = p[1] - lastPY;
    var sp2 = Math.sqrt(vx * vx + vy * vy);
    if (sp2 > 0.5) {
      windT = Math.max(0.35, Math.min(2.1, 0.9 + sp2 * 0.02));
      windAngT = Math.atan2(vy, vx) * 0.16;
    }
  }
  lastPX = p[0]; lastPY = p[1]; lastPT = t;
  auto = false;
});
field.addEventListener('pointerleave', function () {
  auto = true;
  lastPX = -1;
});
field.addEventListener('keydown', function (e) {
  var d = { ArrowLeft: [-1, 0], ArrowRight: [1, 0], ArrowUp: [0, -1], ArrowDown: [0, 1] }[e.key];
  if (!d) return;
  e.preventDefault();
  auto = false;
  windT = Math.max(0.35, Math.min(2.1, windT + d[0] * 0.18));
  windAngT = windAngT + d[1] * 0.1;
  if (reduce) { fieldcalc(nowSec || 3.1, windT, windAngT); foamStep(33, windT, windAngT); render(); }
});
window.addEventListener('resize', function () { fit(); if (reduce) still(); });

fit();
if (reduce) {
  root.classList.add('is-still');
  still();
} else {
  raf = requestAnimationFrame(frame);
}

// FoamParticles — vanilla 판(run/313/demo/05-ocean-foam-particles/index.html) 의 React 포트.
// 01 과 같은 뿌리(정현파 6개, 시드 7)의 2D 파도장에서 크레스트를 오일러 관점으로 스캔해
// 셀별 거품 밀도(density)를 저장한다 — 개별 파티클 좌표를 매 프레임 새로 뽑지 않는다.
// 발생(spawn, 결정적 난수) → advect(파도 위상 흐름 이동) → 지수 감쇠(수명)를 한 스텝으로 처리.
// 움직임은 캔버스 rAF 루프가 전부이므로 @keyframes 는 없다.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./FoamParticles.module.scss";

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

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

const COLS = 90;
const STEP = 4;
const NW = 6;
const LIFE_MAX = 26;

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

type Wave = {
  lam: number; freq: number; dirX: number; dirY: number;
  amp: number; spd: number; ph: number; om: number;
};

function buildWaves(): Wave[] {
  const rng = mulberry32(7);
  const out: Wave[] = [];
  let lam = 34;
  for (let i = 0; i < NW; i++) {
    const ang = rng() * Math.PI * 2;
    const wv: Wave = {
      lam,
      freq: (Math.PI * 2) / lam,
      dirX: Math.cos(ang),
      dirY: Math.sin(ang),
      amp: 0.58 / (i * 0.62 + 1),
      spd: 0.55 + rng() * 0.25,
      ph: rng() * Math.PI * 2,
      om: 0,
    };
    wv.om = wv.spd * 3.6 * Math.sqrt(1 / wv.lam) * 6;
    out.push(wv);
    lam *= 0.55;
  }
  return out;
}

function hex2rgb(s: string): [number, number, number] {
  const pm = /^\s*rgb\(\s*([\d.]+)%\s*,\s*([\d.]+)%\s*,\s*([\d.]+)%\s*\)\s*$/.exec(s);
  if (pm) return [Math.round(+pm[1] * 2.55), Math.round(+pm[2] * 2.55), Math.round(+pm[3] * 2.55)];
  let t = s.replace("#", "");
  if (t.length === 3) t = t[0] + t[0] + t[1] + t[1] + t[2] + t[2];
  const n = parseInt(t, 16);
  return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
}

export default function FoamParticles({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#fff7e6",
}: FoamParticlesProps) {
  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 foamValRef = useRef<HTMLSpanElement | null>(null);
  const windValRef = useRef<HTMLSpanElement | null>(null);
  const wavesRef = useRef<Wave[] | null>(null);
  const sizeRef = useRef({ w: 0, h: 0 }) as Box<{ w: number; h: number }>;
  const rafRef = useRef(0) as Box<number>;
  const lastTRef = useRef(0) as Box<number>;
  const stillRef = useRef(false) as Box<boolean>;
  const rngSpawnRef = useRef<(() => number) | null>(null);
  type St = {
    rows: number; rw: number; rh: number;
    hgt: Float32Array | null;
    dens: Float32Array | null;
    densNext: Float32Array | null;
    out: ImageData | null;
    bctx: CanvasRenderingContext2D | null;
    buf: HTMLCanvasElement | null;
    auto: boolean;
    wind: number; windT: number;
    windAng: number; windAngT: number;
    lastPX: number; lastPY: number; lastPT: number;
    totalFoam: number;
    sea0: [number, number, number] | null;
    sea1: [number, number, number] | null;
    foam0: [number, number, number] | null;
    foam1: [number, number, number] | null;
  };
  const stRef = useRef({
    rows: 0, rw: 0, rh: 0,
    hgt: null, dens: null, densNext: null, out: null, bctx: null, buf: null,
    auto: true,
    wind: 1.0, windT: 1.0,
    windAng: 0.5, windAngT: 0.5,
    lastPX: -1, lastPY: -1, lastPT: 0,
    totalFoam: 0,
    sea0: null, sea1: null, foam0: null, foam1: null,
  } as St) as Box<St>;
  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, cv = cvRef.current;
    if (!field || !cv) return;
    const r = field.getBoundingClientRect();
    const dpr = window.devicePixelRatio || 1;
    sizeRef.current.w = r.width;
    sizeRef.current.h = r.height;
    cv.width = Math.round(r.width * dpr);
    cv.height = Math.round(r.height * dpr);
    const ctx = ctxRef.current;
    if (ctx) ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    const st = stRef.current;
    st.rows = Math.max(30, Math.round(COLS * r.height / r.width));
    const n = COLS * st.rows;
    st.hgt = new Float32Array(n);
    st.dens = new Float32Array(n);
    st.densNext = new Float32Array(n);
    st.rw = COLS * STEP; st.rh = st.rows * STEP;
    const buf = document.createElement("canvas");
    buf.width = st.rw; buf.height = st.rh;
    st.buf = buf;
    st.bctx = buf.getContext("2d");
    if (st.bctx) st.out = st.bctx.createImageData(st.rw, st.rh);
  }

  function fieldcalc(t: number, wstr: number, wang: number): void {
    const st = stRef.current;
    const WV = wavesRef.current;
    if (!st.hgt || !WV) return;
    let i = 0;
    for (let y = 0; y < st.rows; y++) {
      for (let x = 0; x < COLS; x++, i++) {
        let h = 0;
        for (let k = 0; k < NW; k++) {
          const wv = WV[k];
          const da = wang * 0.5;
          const dx = wv.dirX * Math.cos(da) - wv.dirY * Math.sin(da);
          const dy = wv.dirX * Math.sin(da) + wv.dirY * Math.cos(da);
          const ph = (x * dx + y * dy) * wv.freq * 0.62 + t * wv.om + wv.ph;
          h += wv.amp * wstr * Math.sin(ph);
        }
        st.hgt[i] = h;
      }
    }
  }

  // 거품 오일러 스텝 — 발생(크레스트 문턱, 결정적 난수) → advect(위상 흐름 이동) → 지수 감쇠
  function foamStep(dt: number, wstr: number, wang: number): void {
    const st = stRef.current;
    const WV = wavesRef.current;
    const rngSpawn = rngSpawnRef.current;
    if (!st.hgt || !st.dens || !st.densNext || !WV || !rngSpawn) return;
    let eff = 0;
    for (let i = 0; i < NW; i++) eff += WV[i].amp;
    eff *= wstr;
    const cut = eff * 0.74;
    const spawnRate = 0.10 + Math.min(0.55, (wstr - 0.6) * 0.28);
    const decay = Math.exp(-dt * (1000 / LIFE_MAX) * 0.06);
    const flowX = Math.cos(wang) * 0.9, flowY = Math.sin(wang) * 0.9;
    let total = 0;
    for (let y = 0; y < st.rows; y++) {
      const row = y * COLS;
      for (let x = 0; x < COLS; x++) {
        const idx = row + x;
        const sx = x - flowX, sy = y - flowY;
        let ix = Math.round(sx), iy = Math.round(sy);
        if (ix < 0) ix = 0; else if (ix >= COLS) ix = COLS - 1;
        if (iy < 0) iy = 0; else if (iy >= st.rows) iy = st.rows - 1;
        let carried = st.dens[iy * COLS + ix] * decay;
        if (st.hgt[idx] > cut) {
          const r = rngSpawn();
          if (r < spawnRate) carried = Math.min(1, carried + 0.55 + r * 0.4);
        }
        st.densNext[idx] = carried;
        total += carried;
      }
    }
    const tmp = st.dens; st.dens = st.densNext; st.densNext = tmp;
    st.totalFoam = total;
  }

  function render(): void {
    const st = stRef.current;
    const ctx = ctxRef.current;
    if (!st.out || !st.bctx || !st.buf || !ctx || !st.hgt || !st.dens) return;
    if (!st.sea0 || !st.sea1 || !st.foam0 || !st.foam1) return;
    const o = st.out.data;
    const RS0 = st.sea0, RS1 = st.sea1, RF0 = st.foam0, RF1 = st.foam1;
    for (let y = 0; y < st.rows; y++) {
      const row = y * COLS;
      for (let x = 0; x < COLS; x++) {
        const idx = row + x;
        let u = st.hgt[idx] * 0.7 + 0.5;
        if (u < 0) u = 0; else if (u > 1) u = 1;
        let r = RS0[0] + (RS1[0] - RS0[0]) * u;
        let g = RS0[1] + (RS1[1] - RS0[1]) * u;
        let b = RS0[2] + (RS1[2] - RS0[2]) * u;
        let f = st.dens[idx];
        if (f > 0.02) {
          if (f > 1) f = 1;
          const fr = RF0[0] + (RF1[0] - RF0[0]) * f;
          const fg = RF0[1] + (RF1[1] - RF0[1]) * f;
          const fb = RF0[2] + (RF1[2] - RF0[2]) * f;
          r = r * (1 - f) + fr * f;
          g = g * (1 - f) + fg * f;
          b = b * (1 - f) + fb * f;
        }
        for (let sy = 0; sy < STEP; sy++) {
          let base = ((y * STEP + sy) * st.rw + x * STEP) * 4;
          for (let sx2 = 0; sx2 < STEP; sx2++, base += 4) {
            o[base] = r;
            o[base + 1] = g;
            o[base + 2] = b;
            o[base + 3] = 255;
          }
        }
      }
    }
    st.bctx.putImageData(st.out, 0, 0);
    ctx.drawImage(st.buf, 0, 0, sizeRef.current.w, sizeRef.current.h);
  }

  function frame(now: number): void {
    const st = stRef.current;
    const sec = now / 1000;
    const dt = lastTRef.current ? now - lastTRef.current : 16.7;
    lastTRef.current = now;
    if (st.auto) {
      const th = sec * 0.31;
      st.windT = 1.05 + Math.sin(th * 0.9 + 0.4) * 0.55 + Math.sin(th * 2.3) * 0.12;
      st.windAngT = 0.5 + Math.sin(th * 0.42) * 0.85;
    }
    st.wind += (st.windT - st.wind) * 0.05;
    st.windAng += (st.windAngT - st.windAng) * 0.05;
    fieldcalc(sec, st.wind, st.windAng);
    foamStep(dt, st.wind, st.windAng);
    render();
    if (foamValRef.current) foamValRef.current.textContent = "거품 " + Math.round(st.totalFoam);
    if (windValRef.current) windValRef.current.textContent = "바람 " + st.wind.toFixed(2);
    rafRef.current = requestAnimationFrame(frame);
  }

  function onPointerMove(e: PointerMotionEvent): void {
    const field = fieldRef.current;
    if (!field) return;
    const r = field.getBoundingClientRect();
    const px = e.clientX - r.left, py = e.clientY - r.top;
    const t = performance.now();
    const st = stRef.current;
    if (st.lastPX >= 0 && t > st.lastPT) {
      const vx = px - st.lastPX, vy = py - st.lastPY;
      const sp = Math.sqrt(vx * vx + vy * vy);
      if (sp > 0.5) {
        st.windT = Math.max(0.35, Math.min(2.1, 0.9 + sp * 0.02));
        st.windAngT = Math.atan2(vy, vx) * 0.16;
      }
    }
    st.lastPX = px; st.lastPY = py; st.lastPT = t;
    st.auto = false;
  }

  function onPointerLeave(): void {
    stRef.current.auto = true;
    stRef.current.lastPX = -1;
  }

  function onResize(): void {
    fit();
    if (reduced()) {
      const st = stRef.current;
      fieldcalc(7.3, 1.35, 0.5);
      st.windAng = 0.5;
      for (let k = 0; k < 40; k++) foamStep(33, 1.35, 0.5);
      render();
    }
  }

  useEffect(() => {
    const cv = cvRef.current;
    const root = rootRef.current;
    if (!cv || !root) return;
    ctxRef.current = cv.getContext("2d");
    if (!ctxRef.current) return;
    wavesRef.current = buildWaves();
    rngSpawnRef.current = mulberry32(19);
    const cs = getComputedStyle(root);
    const tok = (name: string) => cs.getPropertyValue(name).trim();
    const st = stRef.current;
    st.sea0 = hex2rgb(tok("--sea0"));
    st.sea1 = hex2rgb(tok("--sea1"));
    st.foam0 = hex2rgb(tok("--foam0"));
    st.foam1 = hex2rgb(tok("--foam1"));
    fit();
    window.addEventListener("resize", onResize);
    if (reduced()) {
      stillRef.current = true;
      fieldcalc(7.3, 1.35, 0.5);
      st.windAng = 0.5;
      for (let k = 0; k < 40; k++) foamStep(33, 1.35, 0.5);
      render();
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  const cls = [styles.root, stillRef.current ? styles.isStill : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls} 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="M16 120c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".25"/><path d="M16 168c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".15"/><path d="M228.92 49.09a112 112 0 1 0 14 84.91a8 8 0 0 0-15.5-4a96 96 0 1 1-12-72.91A8 8 0 0 1 216 71.63V40a8 8 0 0 1-8-8h-31.63a8 8 0 0 1-5.65-13.09ZM64 104a12 12 0 1 1 12 12a12 12 0 0 1-12-12m40 64a12 12 0 1 1 12 12a12 12 0 0 1-12-12m72-16a16 16 0 1 1 16 16a16 16 0 0 1-16-16" opacity=".9"/></g></svg>
          <span>격자 셀마다 거품 밀도를 계산합니다 — 좌표를 다시 뽑지 않고 태어난 칸에서 수명을 셉니다. 바람을 세게 불어보세요</span>
        </p>
        <div className={styles.field} ref={fieldRef} onPointerMove={onPointerMove} onPointerLeave={onPointerLeave}>
          <canvas className={styles.cv} ref={cvRef}></canvas>
          <b className={styles.badge} aria-hidden="true"><span className={styles.badgeNum} ref={foamValRef}>거품 0</span><span className={styles.badgeUnit} ref={windValRef}>바람 1.0</span></b>
        </div>
      </div>
    </main>
  );
}