02가지 결정(dendrite) — 여섯 방향 팁이 촘촘히 갈라지는 나무 성장

<main class="stage">
<div class="cd is-demo" id="cdRoot">
  <p class="cd__hint">
    <span>입자가 가장 바깥으로 뻗은 팁에 더 잘 붙도록 흡착 확률을 팁 쪽으로 치우치게 하면, 뭉툭한 덩어리가 아니라 진짜 눈결정의 가지(수지상)가 촘촘히 뻗어갑니다</span>
  </p>
  <div class="cd__field" id="cdField" role="application" aria-label="가지 결정 DLA 실험판" tabindex="0">
    <canvas class="cd__cv" id="cdCv"></canvas>
  </div>
  <div class="cd__bar">
    <span class="cd__meta"><b class="cd__val" id="cdN">0</b>개 응집 · 팁 가중 가지</span>
    <span class="cd__meta cd__meta--dim">canvas · ImageData · requestAnimationFrame</span>
  </div>
</div>
</main>

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

// 가지 결정(dendrite, 02). @keyframes 를 아예 두지 않는다 —
// 움직임은 전부 캔버스 rAF 루프(팁 가중 DLA + 잔광 페이드 + 노이즈 진눈깨비)가 만든다.
// 01 과 같은 파스텔 팔레트를 --flake-* 로 공유(편 공통 규격, 01~09 동일).

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

$flake-stage: #171b24;

@include stage($flake-stage);

$flake-ink:   #141821;
$flake-deep:  #2b3346;
$flake-haze:  #8fa8c9;
$flake-mote:  #f2ded0;
$flake-ice:   #fff9f0;
$flake-glow:  #cfeaff;
$flake-lilac: #c9b6ff;
$flake-mint:  #c8f4e4;
$flake-mist:  #d8dee8;

.cd {
  --flake-ink:   #{$flake-ink};
  --flake-deep:  #{$flake-deep};
  --flake-haze:  #{$flake-haze};
  --flake-mote:  #{$flake-mote};
  --flake-ice:   #{$flake-ice};
  --flake-glow:  #{$flake-glow};
  --flake-lilac: #{$flake-lilac};
  --flake-mint:  #{$flake-mint};
  --flake-mist:  #{$flake-mist};

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

.cd__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: $flake-mist;
}

.cd__field {
  position: relative; height: 180px;
  border-radius: $r-card; overflow: hidden;
  background: $flake-ink;
  box-shadow: inset 0 0 0 1px rgba($flake-mist, .18);
}
.cd__field:focus-visible { outline: 2px solid $flake-lilac; outline-offset: 2px; }

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

.cd__bar { display: flex; align-items: baseline; justify-content: space-between; gap: $sp-2; }
.cd__meta { font-size: 10px; font-weight: 500; color: rgba($flake-mist, .72); }
.cd__val { color: $flake-lilac; font-weight: 700; }
.cd__meta--dim { font-size: 10px; }

@media (max-width: 340px) {
  .cd { gap: $sp-1; }
  .cd__field { height: 100px; }
  .cd__meta--dim { display: none; }
}

@media (prefers-reduced-motion: reduce) {
  .cd.is-still .cd__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: #171b24;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.cd {
  --flake-ink: #141821;
  --flake-deep: #2b3346;
  --flake-haze: #8fa8c9;
  --flake-mote: #f2ded0;
  --flake-ice: #fff9f0;
  --flake-glow: #cfeaff;
  --flake-lilac: #c9b6ff;
  --flake-mint: #c8f4e4;
  --flake-mist: #d8dee8;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.cd__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: #d8dee8;
}

.cd__field {
  position: relative;
  height: 180px;
  border-radius: 16px;
  overflow: hidden;
  background: #141821;
  box-shadow: inset 0 0 0 1px rgba(216, 222, 232, 0.18);
}

.cd__field:focus-visible {
  outline: 2px solid #c9b6ff;
  outline-offset: 2px;
}

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

.cd__bar {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 8px;
}

.cd__meta {
  font-size: 10px;
  font-weight: 500;
  color: rgba(216, 222, 232, 0.72);
}

.cd__val {
  color: #c9b6ff;
  font-weight: 700;
}

.cd__meta--dim {
  font-size: 10px;
}

@media (max-width: 340px) {
  .cd {
    gap: 4px;
  }
  .cd__field {
    height: 100px;
  }
  .cd__meta--dim {
    display: none;
  }
}
@media (prefers-reduced-motion: reduce) {
  .cd.is-still .cd__field {
    cursor: default !important;
  }
}

// 가지 결정(02). 01 과 같은 DLA 랜덤워크 응집 규칙에 한 겹만 더한다 —
// 결정에 새 칸이 붙을 때, 그 칸이 씨앗에서 얼마나 먼지(반경)로 흡착 여부를 다시 굴린다.
// 반경이 클수록(팁에 가까울수록) 흡착 확률이 높아지도록 편향(TIP_BIAS)을 주면,
// 안쪽 오목한 자리보다 바깥으로 뻗은 팁이 입자를 훨씬 잘 붙잡아 뭉툭한 덩어리 대신
// 가늘고 촘촘한 나뭇가지(수지상) 형태로 자란다 — 이것이 실제 눈결정이 팔각이 아니라
// 가지 모양으로 자라는 이유(팁 쪽 확산 flux 가 더 높다, Mullins-Sekerka 불안정성의 단순화판)다.
(function () {
  // ── 변형 상수 블록 — 아래 값 하나만 바꿔 변형을 만든다 ──
  // 2026-09-15 재튜닝(BD3 누적면적 28.5%→30%+): 정체성(팁 가중 단일 씨앗 덴드라이트)은 그대로,
  // 동시 성장 팁 수·최종 결정 크기·배경 잔광/강설 밀도만 올려 필드 안에서 바뀌는 픽셀 영역을 넓힌다.
  var PARTICLES = 56;     // 동시에 떠 있는 입자 수 — 응집 속도(42→56, 가지가 더 빨리 더 많이 뻗는다)
  var TIP_BIAS = 2.6;     // 팁 편향 지수 — 클수록 가지가 가늘고 뾰족해진다(0 이면 01 과 동일)
  var MAX_AGE = 0.55;     // 방금 붙은 칸이 빛나는 시간(초)
  var SNOW = 130;         // 내리는 눈 입자 수 — 캔버스 전체를 쓸며 내려온다(90→130)
  var R_CAP = 0.48;       // 결정 최대 반경 비율(N 대비, 0.46→0.48) — 필드 안에서 더 크게 자란다
  // ────────────────────────────────────────────────────

  var root = document.getElementById('cdRoot');
  var field = document.getElementById('cdField');
  var cv = document.getElementById('cdCv');
  var ctx = cv.getContext('2d', { alpha: false });
  var nOut = document.getElementById('cdN');

  function makeRand(seed) {
    var st = seed >>> 0;
    return function () {
      st = (st + 0x6D2B79F5) >>> 0;
      var t = st;
      t = Math.imul(t ^ (t >>> 15), t | 1);
      t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
      return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
    };
  }
  var rand = makeRand(0x319A02);

  var css = getComputedStyle(root);
  function tokRGB(name) {
    var s = css.getPropertyValue(name).trim();
    var hx = s.length === 4
      ? s.charAt(1) + s.charAt(1) + s.charAt(2) + s.charAt(2) + s.charAt(3) + s.charAt(3)
      : s.slice(1);
    return [parseInt(hx.slice(0, 2), 16), parseInt(hx.slice(2, 4), 16), parseInt(hx.slice(4, 6), 16)];
  }
  var C_INK   = tokRGB('--flake-ink');
  var C_DEEP  = tokRGB('--flake-deep');
  var C_HAZE  = tokRGB('--flake-haze');
  var C_MOTE  = tokRGB('--flake-mote');
  var C_ICE   = tokRGB('--flake-ice');
  var C_GLOW  = tokRGB('--flake-glow');
  var C_LILAC = tokRGB('--flake-lilac');
  var C_MINT  = tokRGB('--flake-mint');
  var C_MIST  = tokRGB('--flake-mist');

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var W = 0, H = 0, raf = 0, t0 = -1;
  var bg, data, img2, data2;
  var N = 0;
  var grid, age, maxR;               // maxR = 그 칸이 흡착될 당시 결정 최대 반경(팁 편향 판정용)
  var walk = [];
  var stuck = 0;
  var crystalR = 1;
  var cx = 0, cy = 0;
  var snow = [];
  var cellPx = 4;

  function gridIdx(i, j) { return i * N + j; }
  function inGrid(i, j) { return i >= 0 && j >= 0 && i < N && j < N; }
  var DJ_EVEN = [0, -1, -1, 0, 0, 0];
  var DJ_ODD  = [0, 0, 0, 0, 1, 1];
  var DI = [-1, -1, 0, 1, 0, -1];
  function nbr(i, j, k) { return [i + DI[k], j + ((j & 1) === 0 ? DJ_EVEN[k] : DJ_ODD[k])]; }

  function set(i, j, t) { grid[gridIdx(i, j)] = 1; age[gridIdx(i, j)] = t; }

  function plantSeed() {
    set(cx, cy, 0); set(cx - 1, cy, 0); set(cx + 1, cy, 0);
    set(cx, cy - 1, 0); set(cx, cy + 1, 0);
    set(cx - 1, cy - 1, 0); set(cx + 1, cy + 1, 0);
    stuck = 7;
  }

  function launch() {
    var a = rand() * Math.PI * 2;
    var r = crystalR * 1.22 + 3;
    var i = cx + Math.round(Math.cos(a) * r);
    var j = cy + Math.round(Math.sin(a) * r);
    if (!inGrid(i, j) || grid[gridIdx(i, j)]) return;
    walk.push([i, j, Math.floor(rand() * 6), rand() * Math.PI * 2]);
  }

  function touching(i, j) {
    for (var k = 0; k < 6; k++) {
      var nb = nbr(i, j, k);
      if (inGrid(nb[0], nb[1]) && grid[gridIdx(nb[0], nb[1])]) return true;
    }
    return false;
  }

  // ── 팁 가중 흡착 — 반경이 클수록(팁에 가까울수록) 흡착 확률을 높인다 ──
  function stickProb(i, j) {
    var dc = Math.hypot(i - cx, (j - cy) * 0.92);
    var k = Math.min(1, dc / Math.max(1, crystalR));
    return Math.pow(k, 1) * 0.15 + Math.pow(k, TIP_BIAS) * 0.85 + 0.05;
  }

  function step(t) {
    for (var s = 0; s < 26; s++) {
      for (var w = walk.length - 1; w >= 0; w--) {
        var p = walk[w];
        if (rand() < 0.25) p[2] = Math.floor(rand() * 6);
        var nb = nbr(p[0], p[1], p[2]);
        var i = nb[0], j = nb[1];
        if (!inGrid(i, j)) { walk.splice(w, 1); continue; }
        if (rand() < 0.2) p[3] += (rand() - 0.5) * 0.7;
        p[0] = i; p[1] = j;
        if (touching(i, j)) {
          if (rand() < stickProb(i, j)) {           // 팁 편향 — 안쪽 오목한 자리는 잘 안 붙는다
            set(i, j, t);
            stuck++;
            var dc = Math.hypot(i - cx, (j - cy) * 0.92);
            if (dc + 2 > crystalR) crystalR = Math.min(N * R_CAP, dc + 2);
          }
          // 안 붙으면 계속 걷는다(제자리 유지) — 오목한 자리에서 자꾸 튕겨나가며 팁만 자란다
        } else if (Math.hypot(i - cx, j - cy) > crystalR * 1.7 + 8) {
          walk.splice(w, 1);
        }
      }
    }
    while (walk.length < PARTICLES) launch();
  }

  function buildSnow() {
    snow = [];
    for (var i = 0; i < SNOW; i++) {
      snow.push({
        x: rand(), y: rand(),
        r: 1.2 + rand() * 2.4,
        v: 0.07 + rand() * 0.14,
        sway: 0.014 + rand() * 0.034,
        ph: rand() * Math.PI * 2,
        tw: 0.6 + rand() * 0.9
      });
    }
  }

  var perm = new Uint8Array(512);
  (function () {
    var p = [];
    for (var i = 0; i < 256; i++) p.push(i);
    for (var i2 = 255; i2 > 0; i2--) {
      var k2 = Math.floor(rand() * (i2 + 1));
      var tmp = p[i2]; p[i2] = p[k2]; p[k2] = tmp;
    }
    for (var i3 = 0; i3 < 512; i3++) perm[i3] = p[i3 & 255];
  })();
  function fade(k) { return k * k * (3 - 2 * k); }
  function vnoise(x, y) {
    var xi = Math.floor(x), yi = Math.floor(y);
    var xf = x - xi, yf = y - yi;
    var a2 = perm[(perm[xi & 255] + yi) & 255];
    var b2 = perm[(perm[(xi + 1) & 255] + yi) & 255];
    var c2 = perm[(perm[xi & 255] + yi + 1) & 255];
    var d2 = perm[(perm[(xi + 1) & 255] + yi + 1) & 255];
    var u = fade(xf), v = fade(yf);
    return (a2 + (b2 - a2) * u + (c2 - a2) * v + (a2 - b2 - c2 + d2) * u * v) / 255;
  }
  function lerp(a, b, k) { return a + (b - a) * k; }

  function drawBackdrop(t) {
    var pcx = (cx + 0.5) * cellPx, pcy = (cy + 0.5) * cellPx;
    var sc = 0.055;
    for (var y = 0; y < H; y++) {
      var u = y / H;
      var r0 = C_INK[0] + (C_DEEP[0] - C_INK[0]) * u;
      var g0 = C_INK[1] + (C_DEEP[1] - C_INK[1]) * u;
      var b0 = C_INK[2] + (C_DEEP[2] - C_INK[2]) * u;
      var dyv = y - pcy;
      for (var x = 0; x < W; x++) {
        var dxv = x - pcx;
        var d = Math.sqrt(dxv * dxv + dyv * dyv);
        var haze = Math.exp(-d * d / (W * W * 0.018)) * 34;
        var sh = (vnoise(x * sc, y * sc + t * 0.35) - 0.5) * 28;
        var j3 = (y * W + x) * 4;
        data[j3]     = r0 + haze * (C_HAZE[0] / 255) + sh;
        data[j3 + 1] = g0 + haze * (C_HAZE[1] / 255) + sh;
        data[j3 + 2] = b0 + haze * (C_HAZE[2] / 255) + sh * 1.3;
      }
    }
  }

  function putc(px, py, c, alpha) {
    if (px < 0 || py < 0 || px >= W || py >= H) return;
    var j3 = (py * W + px) * 4;
    data2[j3]     += (c[0] - data2[j3])     * alpha;
    data2[j3 + 1] += (c[1] - data2[j3 + 1]) * alpha;
    data2[j3 + 2] += (c[2] - data2[j3 + 2]) * alpha;
  }

  function cellColor(g, ag) {
    var k = Math.min(1, g / (N * 0.38));
    var base = [
      lerp(C_ICE[0], C_LILAC[0], k),
      lerp(C_ICE[1], C_LILAC[1], k),
      lerp(C_ICE[2], C_LILAC[2], k)
    ];
    if (ag < MAX_AGE) {
      var b = Math.pow(1 - ag / MAX_AGE, 1.6);
      base[0] += (C_GLOW[0] - base[0]) * (0.85 * b);
      base[1] += (C_GLOW[1] - base[1]) * (0.85 * b);
      base[2] += (C_GLOW[2] - base[2]) * (0.85 * b);
    }
    return base;
  }

  function draw(t, dt) {
    drawBackdrop(t);
    ctx.putImageData(bg, 0, 0);

    var f = 1 - Math.exp(-dt * 4.2);
    for (var k = 0; k < data2.length; k += 4) {
      data2[k]     += (data[k]     - data2[k])     * f;
      data2[k + 1] += (data[k + 1] - data2[k + 1]) * f;
      data2[k + 2] += (data2[k + 2] - data2[k + 2]) * f;
    }

    var ox = (W - N * cellPx) / 2, oy = (H - N * cellPx) / 2;
    var rr = Math.max(1, Math.round(cellPx * 0.58));
    var rr2 = rr * rr;
    for (var i = 0; i < N; i++) {
      for (var j = 0; j < N; j++) {
        var gi = gridIdx(i, j);
        if (!grid[gi]) continue;
        var col = cellColor(Math.hypot(i - cx, (j - cy) * 0.92), t - age[gi]);
        var px = Math.round(ox + i * cellPx + ((j & 1) ? cellPx * 0.5 : 0));
        var py = Math.round(oy + j * cellPx);
        for (var dy = -rr; dy <= rr; dy++) {
          for (var dx = -rr; dx <= rr; dx++) {
            var dd = dx * dx + dy * dy;
            if (dd > rr2) continue;
            var px2 = px + dx, py2 = py + dy;
            if (px2 < 0 || py2 < 0 || px2 >= W || py2 >= H) continue;
            var j3 = (py2 * W + px2) * 4;
            var a3 = 1 - dd / (rr2 + 1);
            data2[j3]     += (col[0] - data2[j3])     * a3;
            data2[j3 + 1] += (col[1] - data2[j3 + 1]) * a3;
            data2[j3 + 2] += (col[2] - data2[j3 + 2]) * a3;
          }
        }
      }
    }

    for (var wi = 0; wi < walk.length; wi++) {
      var p = walk[wi];
      var px = Math.round(ox + p[0] * cellPx + ((p[1] & 1) ? cellPx * 0.5 : 0));
      var py = Math.round(oy + p[1] * cellPx);
      var tw = 0.55 + 0.45 * Math.sin(t * 7 + p[3] * 3);
      putc(px, py, C_MINT, 0.95);
      putc(px + 1, py, C_MINT, 0.5 * tw);
      putc(px - 1, py, C_MINT, 0.5 * tw);
      putc(px, py + 1, C_MINT, 0.5 * tw);
      putc(px, py - 1, C_MINT, 0.9 * tw);
    }

    for (var si = 0; si < snow.length; si++) {
      var s = snow[si];
      var sx = (s.x + Math.sin(t * s.sway * 6 + s.ph) * 0.035) * W;
      var sy = ((s.y + s.v * t) % 1.15 - 0.07) * H;
      var br = 0.5 + 0.5 * Math.sin(t * s.tw * 2.4 + s.ph);
      var a4 = 0.55 + 0.4 * br;
      var sr = Math.ceil(s.r);
      for (var dy2 = -sr; dy2 <= sr; dy2++) {
        for (var dx2 = -sr; dx2 <= sr; dx2++) {
          if (dx2 * dx2 + dy2 * dy2 > s.r * s.r) continue;
          putc(Math.round(sx) + dx2, Math.round(sy) + dy2, C_MOTE, a4);
        }
      }
    }
    ctx.putImageData(img2, 0, 0);
  }

  function fit() {
    var r = field.getBoundingClientRect();
    W = Math.round(r.width); H = Math.round(r.height);
    cv.width = W; cv.height = H;
    ctx = cv.getContext('2d', { alpha: false });
    cellPx = Math.max(3, Math.min(5, Math.floor(Math.min(W, H) / 42)));
    N = Math.floor(Math.min(W, H) / cellPx);
    if ((N & 1) === 0) N -= 1;
    grid = new Uint8Array(N * N);
    age = new Float32Array(N * N);
    walk = []; snow = [];
    stuck = 0; crystalR = 1;
    cx = N >> 1; cy = N >> 1;
    bg = ctx.createImageData(W, H); data = bg.data;
    img2 = ctx.createImageData(W, H); data2 = img2.data;
    for (var k = 3; k < data.length; k += 4) { data[k] = 255; data2[k] = 255; }
    plantSeed();
    buildSnow();
  }

  function stillFrame() {
    for (var k = 0; k < 260; k++) step(0.01 * k);
    draw(2.6, 0.016);
  }

  function frame(now) {
    if (t0 < 0) t0 = now;
    var t = (now - t0) / 1000;
    var dt = Math.min(0.05, (now - (frame.prev || now)) / 1000);
    frame.prev = now;
    step(t);
    draw(t, dt);
    if (nOut) nOut.textContent = String(stuck);
    raf = requestAnimationFrame(frame);
  }

  window.addEventListener('resize', function () {
    fit();
    if (reduce) stillFrame();
  });

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

// DendriteBranchGrow — vanilla 판(run/319/demo/02-snowflake-dendrite-branch-grow/index.html) 의 React 포트.
// 가지 결정(dendrite) — DLA 에 팁 편향 한 겹. 결정에 닿은 입자가 그 칸이 씨앗에서 먼 정도에
// 비례해 붙을 확률을 받는다(팁일수록 잘 붙는다). 안쪽 오목한 자리는 잘 안 붙어 뭉툭한 덩어리 대신
// 가늘고 촘촘한 가지(수지상)가 뻗는다 — Mullins-Sekerka 불안정성의 단순화판.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./DendriteBranchGrow.module.scss";

export type DendriteBranchGrowProps = {
  duration?: string;
  easing?: string;
  color?: string;
  particles?: number;   // 동시에 떠 있는 입자 수 — 응집 속도
  tipBias?: number;     // 팁 편향 지수 — 클수록 가지가 가늘고 뾰족(0 이면 기본 DLA)
  maxAge?: number;      // 방금 붙은 칸이 빛나는 시간(초)
  snowCount?: number;   // 내리는 눈 입자 수 — 배경 강설 밀도
  radiusCap?: number;   // 결정 최대 반경 비율(N 대비) — 필드 안에서 자랄 수 있는 한계
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type RGB = [number, number, number];
type Particle = [number, number, number, number];
type Snow = { x: number; y: number; r: number; v: number; sway: number; ph: number; tw: number };
type Colors = { ink: RGB; deep: RGB; haze: RGB; mote: RGB; ice: RGB; glow: RGB; lilac: RGB; mint: RGB; mist: RGB };

const FIELD_LABEL = "가지 결정 DLA 실험판";
const DI = [-1, -1, 0, 1, 0, -1];
const DJ_EVEN = [0, -1, -1, 0, 0, 0];
const DJ_ODD = [0, 0, 0, 0, 1, 1];

function makePRNG(seed: number): () => number {
  let s = seed >>> 0;
  return function () {
    s = (s + 0x6d2b79f5) >>> 0;
    let t = s;
    t = Math.imul(t ^ (t >>> 15), t | 1);
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}

export default function DendriteBranchGrow({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#c9b6ff",
  particles = 56,
  tipBias = 2.6,
  maxAge = 0.55,
  snowCount = 130,
  radiusCap = 0.48,
}: DendriteBranchGrowProps) {
  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 sizeRef = useRef({ w: 0, h: 0 }) as Box<{ w: number; h: number }>;
  const colRef = useRef<Colors | null>(null);
  const particlesRef = useRef(particles) as Box<number>;
  const tipBiasRef = useRef(tipBias) as Box<number>;
  const maxAgeRef = useRef(maxAge) as Box<number>;
  const snowCountRef = useRef(snowCount) as Box<number>;
  const radiusCapRef = useRef(radiusCap) as Box<number>;
  const randRef = useRef(makePRNG(0x319a02)) as Box<() => number>;
  const gridRef = useRef<Uint8Array>(new Uint8Array(0)) as Box<Uint8Array>;
  const ageRef = useRef<Float32Array>(new Float32Array(0)) as Box<Float32Array>;
  const nRef = useRef(1) as Box<number>;
  const cellPxRef = useRef(4) as Box<number>;
  const centerRef = useRef({ cx: 0, cy: 0 }) as Box<{ cx: number; cy: number }>;
  const rRef = useRef(1) as Box<number>;
  const stuckRef = useRef(0) as Box<number>;
  const walkRef = useRef<Particle[]>([]) as Box<Particle[]>;
  const snowRef = useRef<Snow[]>([]) as Box<Snow[]>;
  const bgRef = useRef<ImageData | null>(null) as Box<ImageData | null>;
  const img2Ref = useRef<ImageData | null>(null) as Box<ImageData | null>;
  const permRef = useRef<Uint8Array>(new Uint8Array(512)) as Box<Uint8Array>;
  const rafRef = useRef(0) as Box<number>;
  const [n, setN] = useState("0");
  const [still, setStill] = useState(false);
  const vars: Vars = { "--duration": duration, "--easing": easing, "--color": color };

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

  function tokRGB(root: HTMLDivElement, name: string): RGB {
    const s = getComputedStyle(root).getPropertyValue(name).trim();
    const hx = s.length === 4
      ? s.charAt(1) + s.charAt(1) + s.charAt(2) + s.charAt(2) + s.charAt(3) + s.charAt(3)
      : s.slice(1);
    return [parseInt(hx.slice(0, 2), 16), parseInt(hx.slice(2, 4), 16), parseInt(hx.slice(4, 6), 16)];
  }

  function gridIdx(i: number, j: number): number { return i * nRef.current + j; }
  function inGrid(i: number, j: number): boolean {
    const N = nRef.current;
    return i >= 0 && j >= 0 && i < N && j < N;
  }
  function nbr(i: number, j: number, k: number): [number, number] {
    return [i + DI[k], j + ((j & 1) === 0 ? DJ_EVEN[k] : DJ_ODD[k])];
  }
  function setCell(i: number, j: number, t: number): void {
    gridRef.current[gridIdx(i, j)] = 1;
    ageRef.current[gridIdx(i, j)] = t;
  }

  function plantSeed(): void {
    const { cx, cy } = centerRef.current;
    setCell(cx, cy, 0); setCell(cx - 1, cy, 0); setCell(cx + 1, cy, 0);
    setCell(cx, cy - 1, 0); setCell(cx, cy + 1, 0);
    setCell(cx - 1, cy - 1, 0); setCell(cx + 1, cy + 1, 0);
    stuckRef.current = 7;
  }

  function launch(): void {
    const rand = randRef.current;
    const { cx, cy } = centerRef.current;
    const a = rand() * Math.PI * 2;
    const r = rRef.current * 1.22 + 3;
    const i = cx + Math.round(Math.cos(a) * r);
    const j = cy + Math.round(Math.sin(a) * r);
    if (!inGrid(i, j) || gridRef.current[gridIdx(i, j)]) return;
    walkRef.current.push([i, j, Math.floor(rand() * 6), rand() * Math.PI * 2]);
  }

  function touching(i: number, j: number): boolean {
    for (let k = 0; k < 6; k++) {
      const nb = nbr(i, j, k);
      if (inGrid(nb[0], nb[1]) && gridRef.current[gridIdx(nb[0], nb[1])]) return true;
    }
    return false;
  }

  // 팁 가중 흡착 — 반경이 클수록(팁에 가까울수록) 흡착 확률을 높인다
  function stickProb(i: number, j: number): number {
    const { cx, cy } = centerRef.current;
    const dc = Math.hypot(i - cx, (j - cy) * 0.92);
    const k = Math.min(1, dc / Math.max(1, rRef.current));
    return Math.pow(k, 1) * 0.15 + Math.pow(k, tipBiasRef.current) * 0.85 + 0.05;
  }

  function step(t: number): void {
    const rand = randRef.current;
    const { cx, cy } = centerRef.current;
    for (let s = 0; s < 26; s++) {
      const walk = walkRef.current;
      for (let w = walk.length - 1; w >= 0; w--) {
        const p = walk[w];
        if (rand() < 0.25) p[2] = Math.floor(rand() * 6);
        const nb = nbr(p[0], p[1], p[2]);
        const i = nb[0], j = nb[1];
        if (!inGrid(i, j)) { walk.splice(w, 1); continue; }
        if (rand() < 0.2) p[3] += (rand() - 0.5) * 0.7;
        p[0] = i; p[1] = j;
        if (touching(i, j)) {
          if (rand() < stickProb(i, j)) {
            setCell(i, j, t);
            stuckRef.current++;
            const dc = Math.hypot(i - cx, (j - cy) * 0.92);
            if (dc + 2 > rRef.current) rRef.current = Math.min(nRef.current * radiusCapRef.current, dc + 2);
          }
        } else if (Math.hypot(i - cx, j - cy) > rRef.current * 1.7 + 8) {
          walk.splice(w, 1);
        }
      }
      while (walkRef.current.length < particlesRef.current) launch();
    }
  }

  function buildSnow(): void {
    const rand = randRef.current;
    const arr: Snow[] = [];
    for (let i = 0; i < snowCountRef.current; i++) {
      arr.push({
        x: rand(), y: rand(),
        r: 1.2 + rand() * 2.4,
        v: 0.07 + rand() * 0.14,
        sway: 0.014 + rand() * 0.034,
        ph: rand() * Math.PI * 2,
        tw: 0.6 + rand() * 0.9,
      });
    }
    snowRef.current = arr;
  }

  function buildPerm(): void {
    const rand = randRef.current;
    const p: number[] = [];
    for (let i = 0; i < 256; i++) p.push(i);
    for (let i2 = 255; i2 > 0; i2--) {
      const k2 = Math.floor(rand() * (i2 + 1));
      const tmp = p[i2]; p[i2] = p[k2]; p[k2] = tmp;
    }
    const perm = new Uint8Array(512);
    for (let i3 = 0; i3 < 512; i3++) perm[i3] = p[i3 & 255];
    permRef.current = perm;
  }
  function fade(k: number): number { return k * k * (3 - 2 * k); }
  function vnoise(x: number, y: number): number {
    const perm = permRef.current;
    const xi = Math.floor(x), yi = Math.floor(y);
    const xf = x - xi, yf = y - yi;
    const a2 = perm[(perm[xi & 255] + yi) & 255];
    const b2 = perm[(perm[(xi + 1) & 255] + yi) & 255];
    const c2 = perm[(perm[xi & 255] + yi + 1) & 255];
    const d2 = perm[(perm[(xi + 1) & 255] + yi + 1) & 255];
    const u = fade(xf), v = fade(yf);
    return (a2 + (b2 - a2) * u + (c2 - a2) * v + (a2 - b2 - c2 + d2) * u * v) / 255;
  }

  function lerp(a: number, b: number, k: number): number { return a + (b - a) * k; }

  function drawBackdrop(t: number): void {
    const col = colRef.current;
    const bg = bgRef.current;
    if (!col || !bg) return;
    const { w: W, h: H } = sizeRef.current;
    const data = bg.data;
    const { cx, cy } = centerRef.current;
    const cellPx = cellPxRef.current;
    const pcx = (cx + 0.5) * cellPx, pcy = (cy + 0.5) * cellPx;
    const sc = 0.055;
    for (let y = 0; y < H; y++) {
      const u = y / H;
      const r0 = col.ink[0] + (col.deep[0] - col.ink[0]) * u;
      const g0 = col.ink[1] + (col.deep[1] - col.ink[1]) * u;
      const b0 = col.ink[2] + (col.deep[2] - col.ink[2]) * u;
      const dyv = y - pcy;
      for (let x = 0; x < W; x++) {
        const dxv = x - pcx;
        const d = Math.sqrt(dxv * dxv + dyv * dyv);
        const haze = Math.exp(-d * d / (W * W * 0.018)) * 34;
        const sh = (vnoise(x * sc, y * sc + t * 0.35) - 0.5) * 28;
        const j3 = (y * W + x) * 4;
        data[j3] = r0 + haze * (col.haze[0] / 255) + sh;
        data[j3 + 1] = g0 + haze * (col.haze[1] / 255) + sh;
        data[j3 + 2] = b0 + haze * (col.haze[2] / 255) + sh * 1.3;
      }
    }
  }

  function putc(px: number, py: number, c: RGB, alpha: number): void {
    const { w: W, h: H } = sizeRef.current;
    const img2 = img2Ref.current;
    if (!img2 || px < 0 || py < 0 || px >= W || py >= H) return;
    const data = img2.data;
    const j3 = (py * W + px) * 4;
    data[j3] += (c[0] - data[j3]) * alpha;
    data[j3 + 1] += (c[1] - data[j3 + 1]) * alpha;
    data[j3 + 2] += (c[2] - data[j3 + 2]) * alpha;
  }

  function cellColor(g: number, ag: number): RGB {
    const col = colRef.current!;
    const N = nRef.current;
    const k = Math.min(1, g / (N * 0.38));
    const base: RGB = [
      lerp(col.ice[0], col.lilac[0], k),
      lerp(col.ice[1], col.lilac[1], k),
      lerp(col.ice[2], col.lilac[2], k),
    ];
    if (ag < maxAgeRef.current) {
      const b = Math.pow(1 - ag / maxAgeRef.current, 1.6);
      base[0] += (col.glow[0] - base[0]) * (0.85 * b);
      base[1] += (col.glow[1] - base[1]) * (0.85 * b);
      base[2] += (col.glow[2] - base[2]) * (0.85 * b);
    }
    return base;
  }

  function draw(t: number, dt: number): void {
    const ctx = ctxRef.current;
    const col = colRef.current;
    const bg = bgRef.current;
    const img2 = img2Ref.current;
    if (!ctx || !col || !bg || !img2) return;
    const { w: W, h: H } = sizeRef.current;
    drawBackdrop(t);
    ctx.putImageData(bg, 0, 0);

    const data = bg.data, data2 = img2.data;
    const f = 1 - Math.exp(-dt * 4.2);
    for (let k = 0; k < data2.length; k += 4) {
      data2[k] += (data[k] - data2[k]) * f;
      data2[k + 1] += (data[k + 1] - data2[k + 1]) * f;
      data2[k + 2] += (data[k + 2] - data2[k + 2]) * f;
    }

    const N = nRef.current;
    const cellPx = cellPxRef.current;
    const { cx, cy } = centerRef.current;
    const ox = (W - N * cellPx) / 2, oy = (H - N * cellPx) / 2;
    const rr = Math.max(1, Math.round(cellPx * 0.58));
    const rr2 = rr * rr;
    const grid = gridRef.current, age = ageRef.current;
    for (let i = 0; i < N; i++) {
      for (let j = 0; j < N; j++) {
        const gi = gridIdx(i, j);
        if (!grid[gi]) continue;
        const colr = cellColor(Math.hypot(i - cx, (j - cy) * 0.92), t - age[gi]);
        const px = Math.round(ox + i * cellPx + ((j & 1) ? cellPx * 0.5 : 0));
        const py = Math.round(oy + j * cellPx);
        for (let dy = -rr; dy <= rr; dy++) {
          for (let dx = -rr; dx <= rr; dx++) {
            const dd = dx * dx + dy * dy;
            if (dd > rr2) continue;
            const px2 = px + dx, py2 = py + dy;
            if (px2 < 0 || py2 < 0 || px2 >= W || py2 >= H) continue;
            const j3 = (py2 * W + px2) * 4;
            const a3 = 1 - dd / (rr2 + 1);
            data2[j3] += (colr[0] - data2[j3]) * a3;
            data2[j3 + 1] += (colr[1] - data2[j3 + 1]) * a3;
            data2[j3 + 2] += (colr[2] - data2[j3 + 2]) * a3;
          }
        }
      }
    }

    const walk = walkRef.current;
    for (let wi = 0; wi < walk.length; wi++) {
      const p = walk[wi];
      const px = Math.round(ox + p[0] * cellPx + ((p[1] & 1) ? cellPx * 0.5 : 0));
      const py = Math.round(oy + p[1] * cellPx);
      const tw = 0.55 + 0.45 * Math.sin(t * 7 + p[3] * 3);
      putc(px, py, col.mint, 0.95);
      putc(px + 1, py, col.mint, 0.5 * tw);
      putc(px - 1, py, col.mint, 0.5 * tw);
      putc(px, py + 1, col.mint, 0.5 * tw);
      putc(px, py - 1, col.mint, 0.9 * tw);
    }

    const snow = snowRef.current;
    for (let si = 0; si < snow.length; si++) {
      const s = snow[si];
      const sx = (s.x + Math.sin(t * s.sway * 6 + s.ph) * 0.035) * W;
      const sy = (((s.y + s.v * t) % 1.15) - 0.07) * H;
      const br = 0.5 + 0.5 * Math.sin(t * s.tw * 2.4 + s.ph);
      const a4 = 0.55 + 0.4 * br;
      const sr = Math.ceil(s.r);
      for (let dy2 = -sr; dy2 <= sr; dy2++) {
        for (let dx2 = -sr; dx2 <= sr; dx2++) {
          if (dx2 * dx2 + dy2 * dy2 > s.r * s.r) continue;
          putc(Math.round(sx) + dx2, Math.round(sy) + dy2, col.mote, a4);
        }
      }
    }
    ctx.putImageData(img2, 0, 0);
  }

  function fit(): void {
    const field = fieldRef.current;
    const cv = cvRef.current;
    if (!field || !cv) return;
    const ctx = cv.getContext("2d", { alpha: false });
    if (!ctx) return;
    const r = field.getBoundingClientRect();
    const W = Math.round(r.width), H = Math.round(r.height);
    sizeRef.current = { w: W, h: H };
    cv.width = W; cv.height = H;
    ctxRef.current = ctx;
    const cellPx = Math.max(3, Math.min(5, Math.floor(Math.min(W, H) / 42)));
    cellPxRef.current = cellPx;
    let N = Math.floor(Math.min(W, H) / cellPx);
    if ((N & 1) === 0) N -= 1;
    nRef.current = N;
    gridRef.current = new Uint8Array(N * N);
    ageRef.current = new Float32Array(N * N);
    walkRef.current = [];
    stuckRef.current = 0;
    rRef.current = 1;
    const cx = N >> 1, cy = N >> 1;
    centerRef.current = { cx, cy };
    const bg = ctx.createImageData(W, H);
    const img2 = ctx.createImageData(W, H);
    for (let k = 3; k < bg.data.length; k += 4) { bg.data[k] = 255; img2.data[k] = 255; }
    bgRef.current = bg;
    img2Ref.current = img2;
    plantSeed();
    buildSnow();
  }

  function stillFrame(): void {
    for (let k = 0; k < 260; k++) step(0.01 * k);
    draw(2.6, 0.016);
  }

  let t0 = -1;
  let prev = 0;
  function frame(now: number): void {
    if (t0 < 0) t0 = now;
    const t = (now - t0) / 1000;
    const dt = Math.min(0.05, (now - (prev || now)) / 1000);
    prev = now;
    step(t);
    draw(t, dt);
    setN(String(stuckRef.current));
    rafRef.current = requestAnimationFrame(frame);
  }

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    particlesRef.current = particles;
    tipBiasRef.current = tipBias;
    maxAgeRef.current = maxAge;
    snowCountRef.current = snowCount;
    radiusCapRef.current = radiusCap;
    colRef.current = {
      ink: tokRGB(root, "--flake-ink"),
      deep: tokRGB(root, "--flake-deep"),
      haze: tokRGB(root, "--flake-haze"),
      mote: tokRGB(root, "--flake-mote"),
      ice: tokRGB(root, "--flake-ice"),
      glow: tokRGB(root, "--flake-glow"),
      lilac: tokRGB(root, "--flake-lilac"),
      mint: tokRGB(root, "--flake-mint"),
      mist: tokRGB(root, "--flake-mist"),
    };
    randRef.current = makePRNG(0x319a02);
    buildPerm();
    fit();
    setN("0");
    function onResize(): void {
      fit();
      if (reduced()) stillFrame();
    }
    window.addEventListener("resize", onResize);
    if (reduced()) {
      stillFrame();
      setStill(true);
    } else {
      t0 = -1;
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [particles, tipBias, maxAge, snowCount, radiusCap]);

  const rootCls = still ? `${styles.root} ${styles.isStill}` : styles.root;

  return (
    <main className={styles.stage}>
      <div className={rootCls} ref={rootRef} style={vars}>
        <p className={styles.hint}>
          <span>입자가 가장 바깥으로 뻗은 팁에 더 잘 붙도록 흡착 확률을 팁 쪽으로 치우치게 하면, 뭉툭한 덩어리가 아니라 진짜 눈결정의 가지(수지상)가 촘촘히 뻗어갑니다</span>
        </p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label={FIELD_LABEL}
          tabIndex={0}
        >
          <canvas className={styles.cv} ref={cvRef}></canvas>
        </div>
        <div className={styles.bar}>
          <span className={styles.meta}><b className={styles.val}>{n}</b>개 응집 · 팁 가중 가지</span>
          <span className={styles.metaDim}>canvas · ImageData · requestAnimationFrame</span>
        </div>
      </div>
    </main>
  );
}