02樹枝状結晶(dendrite) — 6方向の先端が密に分岐する木の成長

<main class="stage">
<div class="cd is-demo" id="cdRoot">
  <p class="cd__hint">
    <span>最も外側に伸びた先端ほど粒子が付着しやすいよう吸着確率を偏らせると、丸い塊ではなく本物の雪結晶の枝（樹枝状）が密に伸びていきます</span>
  </p>
  <div class="cd__field" id="cdField" role="application" aria-label="樹枝状結晶DLA実験版" 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>個凝集 · 先端加重の枝</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>
  );
}