05Frosted window — fractal ice creeping inward from the frame edge

<main class="stage">
<div class="cd is-demo" id="cdRoot">
  <p class="cd__hint">
    <svg class="cd__ico" viewBox="0 0 256 256" aria-hidden="true"><path fill="none" stroke="currentColor" stroke-width="18" d="M20 20h216v216H20z"/><path fill="none" stroke="currentColor" stroke-width="14" stroke-linecap="round" d="M40 40l60 60M216 40l-60 60M40 216l60-60M216 216l-60-60M128 20v70M128 166v70M20 128h70M166 128h70"/></svg>
    <span>Seed the whole window-frame edge and lower the temperature toward the center, and the crystal spreads inward in a fractal pattern — just like frost on a windowpane</span>
  </p>
  <div class="cd__field" id="cdField" role="application" aria-label="Frosted Window Demo" tabindex="0">
    <canvas class="cd__cv" id="cdCv"></canvas>
  </div>
  <div class="cd__bar">
    <span class="cd__meta">grid cells <b class="cd__val" id="cdGrid"></b> · aggregated <b class="cd__val" id="cdRate"></b>/sec</span>
    <span class="cd__meta cd__meta--dim">Canvas · ImageData · putImageData</span>
  </div>
</div>
</main>

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

// 서리 낀 창(05). @keyframes 를 아예 두지 않는다 — 움직임은 전부 캔버스 rAF 루프
// (창틀 씨앗 DLA 응집 + 온도 필드 확률 + 격자 확대 putImageData)가 만든다.
// 무대는 한겨울 밤잉크. 파스텔 팔레트는 편 319 공통 --flake-* (04 와 동일 값 — 편마다 다시 정하지 않는다).

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

@include stage($stage-ink);

// ── 편 공통 파스텔 팔레트(04 와 동일) ──
$flake-ink:   #20242e;
$flake-dusk:  #4a516e;
$flake-core:  #f4f0fa;
$flake-halo:  #b8cce8;
$flake-mint:  #c8e0d8;
$flake-sky:   #a4bcd8;
$flake-mist:  #dce8f4;

.cd {
  --flake-ink:   #{$flake-ink};
  --flake-dusk:  #{$flake-dusk};
  --flake-core:  #{$flake-core};
  --flake-halo:  #{$flake-halo};
  --flake-mint:  #{$flake-mint};
  --flake-sky:   #{$flake-sky};
  --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__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $flake-halo; }

.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-halo; 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-halo; 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: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.cd {
  --flake-ink: #20242e;
  --flake-dusk: #4a516e;
  --flake-core: #f4f0fa;
  --flake-halo: #b8cce8;
  --flake-mint: #c8e0d8;
  --flake-sky: #a4bcd8;
  --flake-mist: #dce8f4;
  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: #dce8f4;
}

.cd__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #b8cce8;
}

.cd__field {
  position: relative;
  height: 180px;
  border-radius: 16px;
  overflow: hidden;
  background: #20242e;
  box-shadow: inset 0 0 0 1px rgba(220, 232, 244, 0.18);
}

.cd__field:focus-visible {
  outline: 2px solid #b8cce8;
  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(220, 232, 244, 0.72);
}

.cd__val {
  color: #b8cce8;
  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;
  }
}

// 서리 낀 창(05). 창틀 네 변 전체를 씨앗으로 깔고, 온도 필드(가장자리=따뜻해 잘 얼고 →
// 중심=차가워 얼기 어려운 역방향 그라디언트)를 반영한 확률로 DLA 랜덤워크를 돌린다.
// 워커는 아직 얼지 않은 안쪽 어디서나 출발해 사방으로 걷다가 이미 언 이웃에 닿으면
// (이웃 수·온도로 정해지는 확률로) 얼어붙는다 — 서리가 유리창 가장자리에서 안으로
// 기어드는 진짜 모양이다. grid cells(저해상도) 점유를 ImageData 한 장으로 그려 확대해 붙인다.
(function () {
  // ── 변형 상수 블록 — 아래 값 하나만 바꿔 변형 3종을 만든다 ──
  var GW = 72, GH = 45;   // grid cells(480×300 과 같은 8:5 비율) — 커질수록 서리 결이 촘촘해진다
  var SPS = 2600;         // 초당 응집 셀 수 — 변형 "번지는 속도" 축(1400 → 3600)
  var COLD = 0.34;        // 중심 냉기 계수 — 클수록 중심까지 다 얼기 전에 가지가 가늘어진다
  var BIAS = 0.55;        // 온도 필드 드리프트 — 걸음이 가장 가까운 창틀 쪽으로 쏠리는 비율(0.55 = 절반 조금 넘게)
  // ────────────────────────────────────────────────────

  var root = document.getElementById('cdRoot');
  var field = document.getElementById('cdField');
  var cv = document.getElementById('cdCv');
  var ctx = cv.getContext('2d');
  document.getElementById('cdGrid').textContent = GW + '×' + GH;
  document.getElementById('cdRate').textContent = String(SPS);

  var SEED = 0x3190405;
  var prngState = SEED >>> 0;
  function rand() {
    prngState = (prngState + 0x6D2B79F5) >>> 0;
    var t = prngState;
    t = Math.imul(t ^ (t >>> 15), t | 1);
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  }

  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_DUSK = tokRGB('--flake-dusk');
  var C_CORE = tokRGB('--flake-core');
  var C_HALO = tokRGB('--flake-halo');
  var C_MIST = tokRGB('--flake-mist');

  // ── grid cells 점유 — 네 변 전체가 씨앗, 중심 냉기 필드에 따라 안쪽으로 자란다 ──
  var occ = new Uint8Array(GW * GH);
  var noise = new Float32Array(GW * GH);     // 셀마다 고정 반짝임 계수(정적 프리컴퓨트) — 얼음 결 질감
  var cx0 = (GW - 1) / 2, cy0 = (GH - 1) / 2;
  var maxDist = Math.sqrt(cx0 * cx0 + cy0 * cy0);
  var walkers = [];
  var WALKERS = 60;
  var filled = 0, TOTAL = GW * GH;
  var phase = 0, shineT = 0;

  function idx(gx, gy) { return gy * GW + gx; }
  function edgeDist(gx, gy) { return Math.min(gx, gy, GW - 1 - gx, GH - 1 - gy); }
  function respawn(w) {
    var tries = 0;
    do {
      w.gx = 1 + ((rand() * (GW - 2)) | 0);
      w.gy = 1 + ((rand() * (GH - 2)) | 0);
      tries++;
    } while (occ[idx(w.gx, w.gy)] && tries < 40);
  }
  var DX = [1, -1, 0, 0], DY = [0, 0, 1, -1];
  // 방향 index: 0=+x(DX[0]=1) · 1=-x(DX[1]=-1) · 2=+y(DY[2]=1) · 3=-y(DY[3]=-1)
  function stepWalker(w) {
    var d;
    if (rand() < BIAS) {
      // 온도 필드 드리프트 — 가장 가까운 창틀로 한 걸음(방향 index: 0=+x,1=-x,2=+y,3=-y)
      var dl = w.gx, dr = GW - 1 - w.gx, dt = w.gy, db = GH - 1 - w.gy;
      var m = Math.min(dl, dr, dt, db);
      d = m === dl ? 1 : m === dr ? 0 : m === dt ? 3 : 2;
    } else {
      d = (rand() * 4) | 0;   // 무작위 걸음 — 이게 없으면 직선으로만 얼어 가지가 안 갈라진다
    }
    var nx = w.gx + DX[d], ny = w.gy + DY[d];
    if (nx < 0 || nx >= GW || ny < 0 || ny >= GH) { respawn(w); return false; }
    w.gx = nx; w.gy = ny;
    var i = idx(nx, ny);
    if (occ[i]) return false;
    var nc = (nx > 0 && occ[idx(nx - 1, ny)] ? 1 : 0) + (nx < GW - 1 && occ[idx(nx + 1, ny)] ? 1 : 0) +
              (ny > 0 && occ[idx(nx, ny - 1)] ? 1 : 0) + (ny < GH - 1 && occ[idx(nx, ny + 1)] ? 1 : 0);
    if (!nc) return false;
    // 온도 필드 — 중심에 가까울수록(dist 작을수록) 차가워 얼기 어렵다(가지가 가늘어진다)
    var dist = Math.min(1, edgeDist(nx, ny) / maxDist);
    var temp = 1 - dist * COLD;
    var p = (nc === 1 ? 1.0 : nc === 2 ? 0.58 : nc === 3 ? 0.24 : 0.12) * temp;
    if (rand() >= p) return false;
    occ[i] = 1;
    filled++;
    respawn(w);
    return true;
  }
  function resetSim() {
    occ.fill(0);
    for (var x = 0; x < GW; x++) { occ[idx(x, 0)] = 1; occ[idx(x, GH - 1)] = 1; }
    for (var y = 0; y < GH; y++) { occ[idx(0, y)] = 1; occ[idx(GW - 1, y)] = 1; }
    filled = 2 * (GW + GH) - 4;
    phase = 0; shineT = 0;
    walkers = [];
    for (var i = 0; i < WALKERS; i++) { var w = { gx: 1, gy: 1 }; respawn(w); walkers.push(w); }
  }
  for (var ni = 0; ni < noise.length; ni++) noise[ni] = rand();
  resetSim();

  function sim(dt) {
    if (phase === 1) {
      shineT += dt;
      if (shineT > 650) resetSim();
      return;
    }
    var quota = SPS * dt / 1000;
    var stuck = 0, steps = 0, maxSteps = 400000;   // 순수 배열 연산이라 프레임당 이 정도는 가볍다
    var t0 = performance.now(), budgetMs = 9;      // 시간 예산으로 자른다 — 워커 수 공식보다 실측이 정확하다
    while (stuck < quota && steps < maxSteps) {
      for (var i = 0; i < walkers.length; i++) {
        if (stepWalker(walkers[i])) stuck++;
        steps++;
        if (stuck >= quota) break;
      }
      if ((steps & 2047) === 0 && performance.now() - t0 > budgetMs) break;
    }
    if (filled > TOTAL * 0.62) { phase = 1; shineT = 0; }   // 다 자라면 잠깐 반짝이고 리셋
  }

  var W = 0, H = 0, img = null, gimg = null, gctx = null, gcanvas = null;
  function fit() {
    var r = field.getBoundingClientRect();
    W = Math.max(2, Math.round(r.width));
    H = Math.max(2, Math.round(r.height));
    cv.width = W; cv.height = H;
    img = ctx.createImageData(W, H);
    var d = img.data;
    for (var i = 3; i < d.length; i += 4) d[i] = 255;
    gcanvas = document.createElement('canvas');
    gcanvas.width = GW; gcanvas.height = GH;
    gctx = gcanvas.getContext('2d');
    gimg = gctx.createImageData(GW, GH);
    var gd = gimg.data;
    for (var j = 3; j < gd.length; j += 4) gd[j] = 255;
    prngState = SEED >>> 0;
    resetSim();
  }

  // grid cells 점유 → 작은 캔버스에 ImageData 로 그리고, 큰 캔버스에 확대해 putImageData 결과를 그린다
  function drawGrid() {
    var gd = gimg.data;
    for (var gy = 0; gy < GH; gy++) {
      for (var gx = 0; gx < GW; gx++) {
        var i = gy * GW + gx, j = i << 2;
        if (!occ[i]) { gd[j] = C_INK[0]; gd[j + 1] = C_INK[1]; gd[j + 2] = C_INK[2]; continue; }
        var dist = Math.min(1, edgeDist(gx, gy) / maxDist);
        var wave = Math.sin(fr * 0.11 - dist * 5.2 + noise[i] * 6.283);    // 결정을 타고 도는 결빛 파동(나무결 반짝임) — 여러 파가 겹쳐 자리마다 다른 박자로 밝아진다
        var sparkle = 0.36 + 0.28 * noise[i] + 0.36 * (0.5 + 0.5 * wave);
        var t = Math.min(1, dist * 1.15) * sparkle;
        var r0 = C_HALO[0] + (C_CORE[0] - C_HALO[0]) * t, g0 = C_HALO[1] + (C_CORE[1] - C_HALO[1]) * t, b0 = C_HALO[2] + (C_CORE[2] - C_HALO[2]) * t;
        gd[j] = r0; gd[j + 1] = g0; gd[j + 2] = b0;
      }
    }
    gctx.putImageData(gimg, 0, 0);
  }

  var fr = 0;
  function compose(now) {
    var d = img.data;
    // ① 유리창 배경 — 세로 그라디언트 + 숨쉬는 성에 안개(가장자리 밝게)
    var topR = C_INK[0] + 4, topG = C_INK[1] + 4, topB = C_INK[2] + 8;
    var botR = C_INK[0] + (C_DUSK[0] - C_INK[0]) * 0.30, botG = C_INK[1] + (C_DUSK[1] - C_INK[1]) * 0.30, botB = C_INK[2] + (C_DUSK[2] - C_INK[2]) * 0.30;
    var breathe = 0.5 + 0.5 * Math.sin(fr * 0.05);
    for (var y = 0; y < H; y++) {
      var ty = y / H;
      var br = topR + (botR - topR) * ty, bg = topG + (botG - topG) * ty, bb = topB + (botB - topB) * ty;
      var edgeY = Math.min(y, H - 1 - y) / (H * 0.5);
      var row = y * W;
      for (var x = 0; x < W; x++) {
        var edgeX = Math.min(x, W - 1 - x) / (W * 0.5);
        var edge = 1 - Math.min(edgeX, edgeY);
        var vig = Math.pow(Math.max(0, edge), 2.2) * (0.10 + 0.05 * breathe);
        var j = (row + x) << 2;
        d[j]     = br + (C_MIST[0] - br) * vig;
        d[j + 1] = bg + (C_MIST[1] - bg) * vig;
        d[j + 2] = bb + (C_MIST[2] - bb) * vig;
      }
    }
    // ② 서리 결정 — grid cells 이미지를 확대해 얹는다(가까운 이웃 보간 없이 또렷하게)
    drawGrid();
    var sx = W / GW, sy = H / GH;
    for (var gy2 = 0; gy2 < GH; gy2++) {
      var gy0 = (gy2 * sy) | 0, gy1 = ((gy2 + 1) * sy) | 0;
      for (var gx2 = 0; gx2 < GW; gx2++) {
        var gi = (gy2 * GW + gx2) << 2;
        if (gimg.data[gi] === C_INK[0] && gimg.data[gi + 1] === C_INK[1] && gimg.data[gi + 2] === C_INK[2]) continue;
        var R = gimg.data[gi], G = gimg.data[gi + 1], B = gimg.data[gi + 2];
        var gx0 = (gx2 * sx) | 0, gx1 = ((gx2 + 1) * sx) | 0;
        for (var yy = gy0; yy < gy1 && yy < H; yy++) {
          var rowo = yy * W;
          for (var xx = gx0; xx < gx1 && xx < W; xx++) {
            var jj = (rowo + xx) << 2;
            d[jj] = R; d[jj + 1] = G; d[jj + 2] = B;
          }
        }
      }
    }
    ctx.putImageData(img, 0, 0);

    // ③ 창을 닦고 지나가는 결빛 스윕 — 자란 뒤에도 화면이 계속 산다(성에 위 대각선 하이라이트)
    for (var sw = 0; sw < 2; sw++) {                    // 두 줄기가 반 cycle 어긋나게 — 쉬는 구간 없다
      var ph = ((now * 0.0013) % 1 + 0.5 * sw) % 1;     // 0→1 순환(≈0.77초 cycle)
      var cx = (ph * 1.7 - 0.35) * W, cy = (-0.15 + ph * 0.5) * H;
      var rr = W * 0.34, r2 = rr * rr;
      var x0 = Math.max(0, (cx - rr) | 0), x1 = Math.min(W - 1, (cx + rr) | 0);
      var y0 = Math.max(0, (cy - rr) | 0), y1 = Math.min(H - 1, (cy + rr) | 0);
      if (x1 < x0 || y1 < y0) continue;
      var pd = ctx.getImageData(x0, y0, x1 - x0 + 1, y1 - y0 + 1);
      var pdd = pd.data, pw = x1 - x0 + 1;
      for (var yy = y0; yy <= y1; yy++) {
        var dyy = yy - cy;
        for (var xx = x0; xx <= x1; xx++) {
          var dxx = xx - cx;
          var q = (dxx * dxx + dyy * dyy) / r2;
          if (q > 1) continue;
          var a = Math.pow(1 - q, 2) * 0.16;            // 부드러운 가우시안 느낌의 산
          var pj = ((yy - y0) * pw + (xx - x0)) << 2;
          pdd[pj]     += (C_CORE[0] - pdd[pj]) * a;
          pdd[pj + 1] += (C_CORE[1] - pdd[pj + 1]) * a;
          pdd[pj + 2] += (C_CORE[2] - pdd[pj + 2]) * a;
        }
      }
      ctx.putImageData(pd, x0, y0);
    }
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var raf = 0, prev = 0;
  function frame(now) {
    var dt = Math.min(50, now - (prev || now));
    prev = now;
    sim(dt);
    compose(now);
    fr++;
    raf = requestAnimationFrame(frame);
  }
  function stillDraw() {
    fit();
    for (var i = 0; i < 30; i++) sim(33);
    compose(0);
  }

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

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

// FrostWindowPane — vanilla 판(run/319/demo/05-snowflake-frost-window-pane/index.html) 의 React 포트.
// 창틀 네 변 전체를 씨앗으로 깔고 온도 필드(가장자리=잘 얼고 → 중심=얼기 어려운 역방향 그라디언트)를
// 확률에 반영한 DLA 랜덤워크 — 서리가 유리창 가장자리에서 안쪽으로 프랙탈로 기어드는 진짜 모양.
// 격자(저해상도) 점유를 ImageData 한 장에 그려 확대해 붙이고 결빛 스윕이 지나간다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./FrostWindowPane.module.scss";

export type FrostWindowPaneProps = {
  duration?: string;
  easing?: string;
  color?: string;
  gw?: number;   // 격자 폭(셀 수) — 변형 "결의 촘촘함" 축(72 ↔ 96)
  sps?: number;  // 초당 응집 셀 수 — 변형 "번지는 속도" 축(1400 ↔ 3600)
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type RGB = [number, number, number];
type Walker = { gx: number; gy: number };
type Colors = { ink: RGB; dusk: RGB; core: RGB; halo: RGB; mist: RGB };

const FIELD_LABEL = "서리 낀 창 실험판";

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 FrostWindowPane({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#b8cce8",
  gw = 72,
  sps = 2600,
}: FrostWindowPaneProps) {
  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 imgRef = useRef<ImageData | null>(null) as Box<ImageData | null>;
  const gimgRef = useRef<ImageData | null>(null) as Box<ImageData | null>;
  const gctxRef = useRef<CanvasRenderingContext2D | null>(null);
  const randRef = useRef<() => number>(makePRNG(0x3190405)) as Box<() => number>;
  const rafRef = useRef(0) as Box<number>;
  const [still, setStill] = useState(false);
  const [gridShown] = useState(`${gw}×${Math.round(gw * 0.625)}`);
  const [spsShown] = useState(sps);
  const vars: Vars = { "--duration": duration, "--easing": easing, "--color": color };

  // ── 서리 상태(ref 로 보관 — rAF 루프에서만 갱신, 리렌더 유발 안 함) ──
  const GH = Math.round(gw * 0.625);
  const COLD = 0.34;
  const BIAS = 0.55;
  const occRef = useRef<Uint8Array>(new Uint8Array(gw * GH)) as Box<Uint8Array>;
  const noiseRef = useRef<Float32Array>(new Float32Array(gw * GH)) as Box<Float32Array>;
  const filledRef = useRef(0) as Box<number>;
  const phaseRef = useRef(0) as Box<number>;
  const shineTRef = useRef(0) as Box<number>;
  const walkersRef = useRef<Walker[]>([]) as Box<Walker[]>;
  const frRef = useRef(0) as Box<number>;
  const maxDistRef = useRef(1) as Box<number>;
  const WALKERS = 60;

  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 idx(gx: number, gy: number): number {
    return gy * gw + gx;
  }
  function edgeDist(gx: number, gy: number): number {
    return Math.min(gx, gy, gw - 1 - gx, GH - 1 - gy);
  }

  function respawn(w: Walker): void {
    const rand = randRef.current;
    let tries = 0;
    do {
      w.gx = 1 + ((rand() * (gw - 2)) | 0);
      w.gy = 1 + ((rand() * (GH - 2)) | 0);
      tries++;
    } while (occRef.current[idx(w.gx, w.gy)] && tries < 40);
  }

  const DX = [1, -1, 0, 0], DY = [0, 0, 1, -1];
  // 방향 index: 0=+x · 1=-x · 2=+y · 3=-y
  function stepWalker(w: Walker): boolean {
    const rand = randRef.current;
    const occ = occRef.current;
    let d: number;
    if (rand() < BIAS) {
      // 온도 필드 드리프트 — 가장 가까운 창틀로 한 걸음
      const dl = w.gx, dr = gw - 1 - w.gx, dt = w.gy, db = GH - 1 - w.gy;
      const m = Math.min(dl, dr, dt, db);
      d = m === dl ? 1 : m === dr ? 0 : m === dt ? 3 : 2;
    } else {
      d = (rand() * 4) | 0;   // 무작위 걸음 — 이게 없으면 가지가 안 갈라진다
    }
    const nx = w.gx + DX[d], ny = w.gy + DY[d];
    if (nx < 0 || nx >= gw || ny < 0 || ny >= GH) { respawn(w); return false; }
    w.gx = nx; w.gy = ny;
    const i = idx(nx, ny);
    if (occ[i]) return false;
    const nc = (nx > 0 && occ[idx(nx - 1, ny)] ? 1 : 0) + (nx < gw - 1 && occ[idx(nx + 1, ny)] ? 1 : 0) +
              (ny > 0 && occ[idx(nx, ny - 1)] ? 1 : 0) + (ny < GH - 1 && occ[idx(nx, ny + 1)] ? 1 : 0);
    if (!nc) return false;
    // 온도 필드 — 중심에 가까울수록 차가워 얼기 어렵다
    const dist = Math.min(1, edgeDist(nx, ny) / maxDistRef.current);
    const temp = 1 - dist * COLD;
    const p = (nc === 1 ? 1.0 : nc === 2 ? 0.58 : nc === 3 ? 0.24 : 0.12) * temp;
    if (rand() >= p) return false;
    occ[i] = 1;
    filledRef.current++;
    respawn(w);
    return true;
  }

  function resetSim(): void {
    const occ = occRef.current;
    occ.fill(0);
    for (let x = 0; x < gw; x++) { occ[idx(x, 0)] = 1; occ[idx(x, GH - 1)] = 1; }
    for (let y = 0; y < GH; y++) { occ[idx(0, y)] = 1; occ[idx(gw - 1, y)] = 1; }
    filledRef.current = 2 * (gw + GH) - 4;
    phaseRef.current = 0;
    shineTRef.current = 0;
    const ws: Walker[] = [];
    for (let i = 0; i < WALKERS; i++) { const w = { gx: 1, gy: 1 }; respawn(w); ws.push(w); }
    walkersRef.current = ws;
  }

  function sim(dt: number): void {
    if (phaseRef.current === 1) {
      shineTRef.current += dt;
      if (shineTRef.current > 650) resetSim();
      return;
    }
    const quota = sps * dt / 1000;
    let stuck = 0, steps = 0;
    const maxSteps = 400000;
    const t0 = performance.now(), budgetMs = 9;
    while (stuck < quota && steps < maxSteps) {
      const ws = walkersRef.current;
      for (let i = 0; i < ws.length; i++) {
        if (stepWalker(ws[i])) stuck++;
        steps++;
        if (stuck >= quota) break;
      }
      if ((steps & 2047) === 0 && performance.now() - t0 > budgetMs) break;
    }
    if (filledRef.current > gw * GH * 0.62) { phaseRef.current = 1; shineTRef.current = 0; }
  }

  function fit(): void {
    const field = fieldRef.current, cv = cvRef.current;
    if (!field || !cv) return;
    const r = field.getBoundingClientRect();
    const W = Math.max(2, Math.round(r.width));
    const H = Math.max(2, Math.round(r.height));
    sizeRef.current = { w: W, h: H };
    cv.width = W; cv.height = H;
    const ctx = cv.getContext("2d");
    if (!ctx) return;
    ctxRef.current = ctx;
    const img = ctx.createImageData(W, H);
    for (let i = 3; i < img.data.length; i += 4) img.data[i] = 255;
    imgRef.current = img;
    const gcanvas = document.createElement("canvas");
    gcanvas.width = gw; gcanvas.height = GH;
    const gctx = gcanvas.getContext("2d");
    if (!gctx) return;
    gctxRef.current = gctx;
    const gimg = gctx.createImageData(gw, GH);
    for (let j = 3; j < gimg.data.length; j += 4) gimg.data[j] = 255;
    gimgRef.current = gimg;
    const cx0 = (gw - 1) / 2, cy0 = (GH - 1) / 2;
    maxDistRef.current = Math.sqrt(cx0 * cx0 + cy0 * cy0);
    randRef.current = makePRNG(0x3190405);
    const noise = noiseRef.current;
    for (let ni = 0; ni < noise.length; ni++) noise[ni] = randRef.current();
    resetSim();
  }

  // 격자 점유 → 작은 캔버스 ImageData → 큰 캔버스에 확대해 얹는다
  function drawGrid(): void {
    const gimg = gimgRef.current, occ = occRef.current, noise = noiseRef.current, col = colRef.current;
    if (!gimg || !col) return;
    const gd = gimg.data;
    const fr = frRef.current;
    for (let gy = 0; gy < GH; gy++) {
      for (let gx = 0; gx < gw; gx++) {
        const i = gy * gw + gx, j = i << 2;
        if (!occ[i]) { gd[j] = col.ink[0]; gd[j + 1] = col.ink[1]; gd[j + 2] = col.ink[2]; continue; }
        const dist = Math.min(1, edgeDist(gx, gy) / maxDistRef.current);
        const wave = Math.sin(fr * 0.11 - dist * 5.2 + noise[i] * 6.283);   // 결빛 파동 — 자리마다 다른 박자
        const sparkle = 0.36 + 0.28 * noise[i] + 0.36 * (0.5 + 0.5 * wave);
        const t = Math.min(1, dist * 1.15) * sparkle;
        gd[j] = col.halo[0] + (col.core[0] - col.halo[0]) * t;
        gd[j + 1] = col.halo[1] + (col.core[1] - col.halo[1]) * t;
        gd[j + 2] = col.halo[2] + (col.core[2] - col.halo[2]) * t;
      }
    }
    if (gctxRef.current) gctxRef.current.putImageData(gimg, 0, 0);
  }

  function compose(now: number): void {
    const ctx = ctxRef.current, img = imgRef.current, col = colRef.current;
    if (!ctx || !img || !col) return;
    const W = sizeRef.current.w, H = sizeRef.current.h;
    const d = img.data;
    // ① 유리창 배경 — 세로 그라디언트 + 숨쉬는 성에 안개
    const topR = col.ink[0] + 4, topG = col.ink[1] + 4, topB = col.ink[2] + 8;
    const botR = col.ink[0] + (col.dusk[0] - col.ink[0]) * 0.30, botG = col.ink[1] + (col.dusk[1] - col.ink[1]) * 0.30, botB = col.ink[2] + (col.dusk[2] - col.ink[2]) * 0.30;
    const breathe = 0.5 + 0.5 * Math.sin(frRef.current * 0.05);
    for (let y = 0; y < H; y++) {
      const ty = y / H;
      const br = topR + (botR - topR) * ty, bg = topG + (botG - topG) * ty, bb = topB + (botB - topB) * ty;
      const edgeY = Math.min(y, H - 1 - y) / (H * 0.5);
      const row = y * W;
      for (let x = 0; x < W; x++) {
        const edgeX = Math.min(x, W - 1 - x) / (W * 0.5);
        const edge = 1 - Math.min(edgeX, edgeY);
        const vig = Math.pow(Math.max(0, edge), 2.2) * (0.10 + 0.05 * breathe);
        const j = (row + x) << 2;
        d[j] = br + (col.mist[0] - br) * vig;
        d[j + 1] = bg + (col.mist[1] - bg) * vig;
        d[j + 2] = bb + (col.mist[2] - bb) * vig;
      }
    }
    // ② 서리 결정 — 격자 이미지를 확대해 얹는다
    drawGrid();
    const gimg = gimgRef.current;
    if (gimg) {
      const sx = W / gw, sy = H / GH;
      for (let gy2 = 0; gy2 < GH; gy2++) {
        const gy0 = (gy2 * sy) | 0, gy1 = ((gy2 + 1) * sy) | 0;
        for (let gx2 = 0; gx2 < gw; gx2++) {
          const gi = (gy2 * gw + gx2) << 2;
          if (gimg.data[gi] === col.ink[0] && gimg.data[gi + 1] === col.ink[1] && gimg.data[gi + 2] === col.ink[2]) continue;
          const R = gimg.data[gi], G = gimg.data[gi + 1], B = gimg.data[gi + 2];
          const gx0 = (gx2 * sx) | 0, gx1 = ((gx2 + 1) * sx) | 0;
          for (let yy = gy0; yy < gy1 && yy < H; yy++) {
            const rowo = yy * W;
            for (let xx = gx0; xx < gx1 && xx < W; xx++) {
              const jj = (rowo + xx) << 2;
              d[jj] = R; d[jj + 1] = G; d[jj + 2] = B;
            }
          }
        }
      }
    }
    ctx.putImageData(img, 0, 0);

    // ③ 창을 닦고 지나가는 결빛 스윕 — 자란 뒤에도 화면이 계속 산다
    for (let sw = 0; sw < 2; sw++) {
      const ph = ((now * 0.0013) % 1 + 0.5 * sw) % 1;
      const cx = (ph * 1.7 - 0.35) * W, cy = (-0.15 + ph * 0.5) * H;
      const rr = W * 0.34, r2 = rr * rr;
      const x0 = Math.max(0, (cx - rr) | 0), x1 = Math.min(W - 1, (cx + rr) | 0);
      const y0 = Math.max(0, (cy - rr) | 0), y1 = Math.min(H - 1, (cy + rr) | 0);
      if (x1 < x0 || y1 < y0) continue;
      const pd = ctx.getImageData(x0, y0, x1 - x0 + 1, y1 - y0 + 1);
      const pdd = pd.data, pw = x1 - x0 + 1;
      for (let yy = y0; yy <= y1; yy++) {
        const dyy = yy - cy;
        for (let xx = x0; xx <= x1; xx++) {
          const dxx = xx - cx;
          const q = (dxx * dxx + dyy * dyy) / r2;
          if (q > 1) continue;
          const a = Math.pow(1 - q, 2) * 0.16;
          const pj = ((yy - y0) * pw + (xx - x0)) << 2;
          pdd[pj] += (col.core[0] - pdd[pj]) * a;
          pdd[pj + 1] += (col.core[1] - pdd[pj + 1]) * a;
          pdd[pj + 2] += (col.core[2] - pdd[pj + 2]) * a;
        }
      }
      ctx.putImageData(pd, x0, y0);
    }
  }

  const prevRef = useRef(0) as Box<number>;
  function frame(now: number): void {
    const dt = Math.min(50, now - (prevRef.current || now));
    prevRef.current = now;
    sim(dt);
    compose(now);
    frRef.current++;
    rafRef.current = requestAnimationFrame(frame);
  }
  function stillDraw(): void {
    fit();
    for (let i = 0; i < 30; i++) sim(33);
    compose(0);
  }

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    colRef.current = {
      ink: tokRGB(root, "--flake-ink"),
      dusk: tokRGB(root, "--flake-dusk"),
      core: tokRGB(root, "--flake-core"),
      halo: tokRGB(root, "--flake-halo"),
      mist: tokRGB(root, "--flake-mist"),
    };
    fit();
    function onResize(): void {
      fit();
      if (reduced()) stillDraw();
    }
    window.addEventListener("resize", onResize);
    if (reduced()) {
      stillDraw();
      setStill(true);
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [gw, sps]);

  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}>
          <svg className={styles.ico} viewBox="0 0 256 256" aria-hidden="true"><path fill="none" stroke="currentColor" strokeWidth={18} d="M20 20h216v216H20z"/><path fill="none" stroke="currentColor" strokeWidth={14} strokeLinecap="round" d="M40 40l60 60M216 40l-60 60M40 216l60-60M216 216l-60-60M128 20v70M128 166v70M20 128h70M166 128h70"/></svg>
          <span>씨앗을 창틀 가장자리 전체에 깔고 온도 필드를 안쪽으로 낮추면, 응집이 창가 서리처럼 가장자리에서 안쪽으로 프랙탈 무늬로 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}>격자 <b className={styles.val}>{gridShown}</b> · 응집 <b className={styles.val}>{spsShown}</b>/초</span>
          <span className={styles.metaDim}>캔버스 · ImageData · putImageData</span>
        </div>
      </div>
    </main>
  );
}