05Lava flow — heat piling up in grid cells

<main class="stage">
<div class="lv is-demo" id="lvRoot">
  <p class="lv__hint">
    <svg class="lv__ico" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M128 24a16 16 0 0 1 16 16v8a80.06 80.06 0 0 1 63.94 63.94h8a16 16 0 0 1 0 32h-8A80.06 80.06 0 0 1 144 207.94v8a16 16 0 0 1-32 0v-8A80.06 80.06 0 0 1 48.06 144h-8a16 16 0 0 1 0-32h8A80.06 80.06 0 0 1 112 48.06v-8A16 16 0 0 1 128 24m-16 57.35V96a16 16 0 0 0 32 0V81.35A48.09 48.09 0 0 0 81.35 112H96a16 16 0 0 0 0-32h14.65M174.65 144H160a16 16 0 0 0 0 32h14.65A48.09 48.09 0 0 0 144 174.65V160a16 16 0 0 0-32 0v14.65A48.09 48.09 0 0 0 81.35 144" opacity=".2"/><path d="M232 128a104 104 0 1 1-104-104a104 104 0 0 1 104 104m-32 0a8 8 0 0 0-8-8h-9.34A56.1 56.1 0 0 0 136 77.34V68a8 8 0 0 0-16 0v9.34A56.1 56.1 0 0 0 77.34 120H68a8 8 0 0 0 0 16h9.34A56.1 56.1 0 0 0 120 179.66V188a8 8 0 0 0 16 0v-8.34A56.1 56.1 0 0 0 178.66 136H192a8 8 0 0 0 8-8m-40 0a32 32 0 1 1-32-32a32 32 0 0 1 32 32m-16 0a16 16 0 1 0-16 16a16 16 0 0 0 16-16"/></g></svg>
    <span>One diffusion step where neighboring cells trade heat is the whole trick behind lava that hardens and remelts — drag the pointer to move the vent</span>
  </p>
  <div class="lv__field" id="lvField" role="application" aria-label="Lava heat diffusion grid board" tabindex="0">
    <canvas class="lv__cv" id="lvCv"></canvas>
  </div>
  <div class="lv__bar">
    <span class="lv__meta"><b class="lv__val" id="lvRes">96×54</b> heat grid · k 0.34</span>
    <span class="lv__meta lv__meta--dim">ImageData · putImageData</span>
  </div>
</div>
</main>

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

// 용암 흐름(05). CSS 애니메이션·@keyframes 를 아예 두지 않는다 —
// 움직임은 전부 열 확산 시뮬레이션(putImageData)이 만든다.
// 무대는 잉크(#241b1d — 화산 바닥). 파스텔 고정 팔레트를 --fire-* 로 편 편 공통 규격.

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

@include stage($stage-ink);

// ── 편 공통 파스텔 팔레트(BD9~11 원칙) ──
$fire-ink:   #241b1d;
$fire-amber: #f0a868;
$fire-gold:  #f6cf8a;
$fire-rose:  #eca0a8;
$fire-ember: #d97a52;
$fire-smoke: #c9c2c6;

.lv {
  --fire-ink:   #{$fire-ink};
  --fire-amber: #{$fire-amber};
  --fire-gold:  #{$fire-gold};
  --fire-rose:  #{$fire-rose};
  --fire-ember: #{$fire-ember};
  --fire-smoke: #{$fire-smoke};

  // 항목 반경 토큰(01~09 공통)
  --radius-sm: 4px;
  --radius-md: 10px;
  --radius-lg: 20px;

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

.lv__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: $fire-smoke;
}
.lv__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $fire-smoke; }

.lv__field {
  position: relative; height: 180px;
  border-radius: $r-card; overflow: hidden;
  background: $fire-ink;
  box-shadow: inset 0 0 0 1px rgba($fire-smoke, .18);
  cursor: crosshair; touch-action: none;
}
.lv__field:focus-visible { outline: 2px solid $fire-amber; outline-offset: 2px; }

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

.lv__bar { display: flex; align-items: baseline; justify-content: space-between; gap: $sp-2; }
.lv__meta { font-size: 10px; font-weight: 500; color: rgba($fire-smoke, .72); }
.lv__val { color: $fire-amber; font-weight: 700; }
.lv__meta--dim { font-size: 10px; }

@media (max-width: 340px) {
  .lv { gap: $sp-1; }
  .lv__field { height: 100px; }
  .lv__meta--dim { display: none; }
}

// reduce — rAF 를 돌리지 않고 시뮬레이션 스텝만 쌓아 한 장을 남긴다.
@media (prefers-reduced-motion: reduce) {
  .lv.is-still .lv__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);
}

.lv {
  --fire-ink: #241b1d;
  --fire-amber: #f0a868;
  --fire-gold: #f6cf8a;
  --fire-rose: #eca0a8;
  --fire-ember: #d97a52;
  --fire-smoke: #c9c2c6;
  --radius-sm: 4px;
  --radius-md: 10px;
  --radius-lg: 20px;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.lv__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: #c9c2c6;
}

.lv__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #c9c2c6;
}

.lv__field {
  position: relative;
  height: 180px;
  border-radius: 16px;
  overflow: hidden;
  background: #241b1d;
  box-shadow: inset 0 0 0 1px rgba(201, 194, 198, 0.18);
  cursor: crosshair;
  touch-action: none;
}

.lv__field:focus-visible {
  outline: 2px solid #f0a868;
  outline-offset: 2px;
}

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

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

.lv__meta {
  font-size: 10px;
  font-weight: 500;
  color: rgba(201, 194, 198, 0.72);
}

.lv__val {
  color: #f0a868;
  font-weight: 700;
}

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

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

// 용암 흐름(05). 화면을 저해상도 격자(GW×GH)로 나눠 각 셀에 열 h 를 저장한다.
// 매 프레임: next[i] = cur[i] + k * (이웃평균 - cur[i]) — 이것이 확산 방정식의
// 가장 단순한 명시적 근사(라플라시안 이완). 화구(vent) 셀에 열을 계속 붓고,
// 밝기가 임곗값 T_MELT 를 넘는 셀만 용암색, 그 아래는 굳은 암석색으로
// ImageData/putImageData 로 픽셀에 직접 찍는다. 열이 퍼지고 식으며 굳는 과정이
// 격자에서 그대로 보인다.
(function () {
  // ── 변형 상수 블록 — 아래 값 하나만 바꿔 변형 3종을 만든다 ──
  var GW = 96;           // 격자 폭(셀) — 변형 "해상도" 축(48 → 144)
  var K_DIFF = 0.34;     // 확산 계수 — 변형 "확산" 축(0.12 → 0.40). 재튜닝(2026-09-15): 0.24→0.34 — 격자 전체가 순환에 들게
  var VENT_N = 5;        // 화구 수 — 변형 "화구" 축(1 → 5). 재튜닝: 3→5 — 하단 폭을 넓게 덮는다
  // ────────────────────────────────────────────────────

  var root = document.getElementById('lvRoot');
  var field = document.getElementById('lvField');
  var cv = document.getElementById('lvCv');
  var ctx = cv.getContext('2d');
  var resOut = document.getElementById('lvRes');

  // 시드 고정 PRNG — mulberry32
  var SEED = 0x5eed05;
  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;
  }

  // 색은 CSS 커스텀 프로퍼티(--fire-*)에서 getComputedStyle 로 읽는다
  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('--fire-ink');
  var C_AMBER = tokRGB('--fire-amber');
  var C_GOLD  = tokRGB('--fire-gold');
  var C_ROSE  = tokRGB('--fire-rose');
  var C_EMBER = tokRGB('--fire-ember');
  var C_SMOKE = tokRGB('--fire-smoke');

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var W = 0, H = 0, GH = 0;
  var img, buf, bctx;
  var heat, heat2;                   // 현재/다음 열장(Float32Array GW×GH)
  var vents = [];                    // 화구 목록 {gx, gy, amp, ph}
  var raf = 0, t0 = -1, tick = 0;
  var curT = 0;                      // 재튜닝: render() 의 잔열 셔머가 쓰는 최근 시각(step 에서 갱신)
  var px01 = 0.5, py01 = 0.7;        // 포인터(화구 후보 위치)
  var manual = false;

  var T_MELT = 0.62;                 // 용암 임곗값 — 이 위면 녹는다
  var T_GLOW = 0.86;                 // 코어 임곗값 — 이 위면 하얗게 달아오른다
  var COOL = 0.006;                  // 냉각(방사) — 매 스텝 열이 조금씩 빠져나간다
  var STILL_TICK = 210;              // reduce 에서 그리는 스텝 — 흐름이 읽히는 한 장

  // 색 램프 — 굳은 바위(잉크↔연기) → 잔불 → 주황(amber) → 코어(gold)
  // t: 0(차가움) → 1(가장 뜨거움). 4 구간 선형 보간.
  var ramp = [];
  (function buildRamp() {
    var stops = [
      [0.00, C_INK[0],   C_INK[1],   C_INK[2]],
      [0.30, mixc(C_INK, C_SMOKE, 0.5)[0], mixc(C_INK, C_SMOKE, 0.5)[1], mixc(C_INK, C_SMOKE, 0.5)[2]],
      [0.62, C_EMBER[0], C_EMBER[1], C_EMBER[2]],
      [0.82, C_AMBER[0], C_AMBER[1], C_AMBER[2]],
      [1.00, C_GOLD[0],  C_GOLD[1],  C_GOLD[2]]
    ];
    for (var i = 0; i < 256; i++) {
      var t = i / 255;
      for (var s = 0; s < stops.length - 1; s++) {
        if (t >= stops[s][0] && t <= stops[s + 1][0]) {
          var f = (t - stops[s][0]) / (stops[s + 1][0] - stops[s][0]);
          ramp.push([stops[s][1] + (stops[s + 1][1] - stops[s][1]) * f,
                     stops[s][2] + (stops[s + 1][2] - stops[s][2]) * f,
                     stops[s][3] + (stops[s + 1][3] - stops[s][3]) * f]);
          break;
        }
      }
    }
  })();
  function mixc(a, b, t) { return [a[0] + (b[0] - a[0]) * t, a[1] + (b[1] - a[1]) * t, a[2] + (b[2] - a[2]) * t]; }

  function fit() {
    var r = field.getBoundingClientRect();
    W = r.width; H = r.height;
    cv.width = Math.round(W); cv.height = Math.round(H);
    GH = Math.max(24, Math.round(GW * H / W));
    buf = document.createElement('canvas');
    buf.width = GW; buf.height = GH;
    bctx = buf.getContext('2d');
    img = bctx.createImageData(GW, GH);
    heat = new Float32Array(GW * GH);
    heat2 = new Float32Array(GW * GH);
    // 화구 — 시드 고정 배치. 하단에 두어 위로 번지게(열 상승 근사: 아래 행이 위 행에 열을 더 준다)
    // 재튜닝(2026-09-15): 화구 수(VENT_N)가 늘어도 폭 전체에 고르게 퍼지도록 span 으로 재계산
    vents = [];
    var span = VENT_N > 1 ? 0.78 / (VENT_N - 1) : 0;
    for (var v = 0; v < VENT_N; v++) {
      vents.push({
        gx: Math.floor(GW * (0.11 + span * v + (rand() - 0.5) * 0.06)),
        gy: Math.floor(GH * (0.72 + (rand() - 0.5) * 0.14)),
        amp: 0.9 + rand() * 0.35,
        ph: rand() * Math.PI * 2
      });
    }
    resOut.textContent = GW + '×' + GH;
  }

  // ── 시뮬레이션 한 스텝 ──
  // ① 화구에 열 주입(시간에 따라 맥동 — 분출이 쉬었다 불다)
  // ② 확산: next[i] = cur[i] + k*(이웃평균 - cur[i])
  //    이웃평균은 상하좌우 4셀 평균(경계는 클램프 — 벽은 단열)
  // ③ 위쪽으로 편향 전달(부유 근사): 아래→위 열 이동을 살짝 비대칭 계수로
  // ④ 냉각: h *= (1 - COOL) — 방사로 식는다. 임곗값 아래로 떨어지면 굳는다
  function step(tSec) {
    curT = tSec;
    // ① 화구 — 맥동 진폭(0.35..1). 분출이 쉬는 구간에 흐름이 멈춘 모양이 나온다
    for (var v = 0; v < vents.length; v++) {
      var vt = vents[v];
      var pulse = 0.35 + 0.65 * (0.5 + 0.5 * Math.sin(tSec * 1.1 + vt.ph));
      for (var dy = -3; dy <= 3; dy++) {
        for (var dx = -3; dx <= 3; dx++) {
          var x = vt.gx + dx, y = vt.gy + dy;
          if (x < 1 || x >= GW - 1 || y < 1 || y >= GH - 1) continue;
          var falloff = 1 - Math.sqrt(dx * dx + dy * dy) / 4.4;
          if (falloff <= 0) continue;
          var idx = y * GW + x;
          heat[idx] += vt.amp * pulse * 0.10 * falloff;
        }
      }
    }
    // 포인터가 살린 화구(사용자가 대는 손) — manual 일 때만
    if (manual) {
      var pgx = Math.floor(px01 * (GW - 2)) + 1;
      var pgy = Math.floor(py01 * (GH - 2)) + 1;
      if (pgx >= 1 && pgx < GW - 1 && pgy >= 1 && pgy < GH - 1) {
        heat[pgy * GW + pgx] += 0.12;
      }
    }
    // ②③ 확산 + 부유 편향
    for (var yy = 1; yy < GH - 1; yy++) {
      for (var xx = 1; xx < GW - 1; xx++) {
        var i2 = yy * GW + xx;
        var c = heat[i2];
        var nU = heat[i2 - GW], nD = heat[i2 + GW], nL = heat[i2 - 1], nR = heat[i2 + 1];
        // 이웃평균 — 아래 셀에 가중치를 더 준다(위로 자라는 흐름)
        var avg = (nU * 0.9 + nD * 1.1 + nL + nR) / 4;
        var nh = c + K_DIFF * (avg - c);
        // ④ 냉각 — 매 스텝 소량. 높은 열일수록 더 식는다(방사 ∝ h)
        nh = nh - nh * COOL * (0.6 + nh);
        if (nh < 0) nh = 0;
        if (nh > 1.4) nh = 1.4;
        heat2[i2] = nh;
      }
    }
    // 경계는 이전 값 유지(단열 벽)
    var tmp = heat; heat = heat2; heat2 = tmp;
  }

  // ── 렌더 — 열값 → 색 램프 → ImageData 픽셀 ──
  // 임곗값 이하(굳은 바위)는 어둡게, T_MELT 위는 용암색.
  // 용암 경계에 로즈 톤 스트로크를 1셀 얹어 "갓 굳은 껍질"을 만든다.
  function render() {
    var data = img.data;
    var p = 0;
    for (var yy = 0; yy < GH; yy++) {
      for (var xx = 0; xx < GW; xx++) {
        var i2 = yy * GW + xx;
        var h = heat[i2];
        var t01 = h >= T_GLOW ? 1 : (h / T_GLOW);
        var ci = (t01 * 255) | 0;
        var col = ramp[ci > 255 ? 255 : ci];
        var r = col[0], g = col[1], b = col[2];
        if (h < T_MELT * 0.5) {
          // 차가운 암석 — 잉크에 가깝게 눌러 바위와 용암의 대비를 만든다
          // 재튜닝(2026-09-15): 잔열색(ember) 셔머를 미묘하게 얹어 정적 검은 영역을 없앤다(위치+시간 위상)
          var k = h / (T_MELT * 0.5);
          var shimmer = 0.05 + 0.05 * Math.sin(curT * 1.3 + xx * 0.13 + yy * 0.19);
          r = C_INK[0] + (r - C_INK[0]) * k + (C_EMBER[0] - C_INK[0]) * shimmer;
          g = C_INK[1] + (g - C_INK[1]) * k + (C_EMBER[1] - C_INK[1]) * shimmer;
          b = C_INK[2] + (b - C_INK[2]) * k + (C_EMBER[2] - C_INK[2]) * shimmer;
        } else if (h >= T_MELT && h < T_MELT + 0.06) {
          // 갓 굳은 껍질 — 로즈 톤 혼합
          var k2 = (h - T_MELT) / 0.06;
          r = r + (C_ROSE[0] - r) * 0.35 * (1 - k2);
          g = g + (C_ROSE[1] - g) * 0.35 * (1 - k2);
          b = b + (C_ROSE[2] - b) * 0.35 * (1 - k2);
        }
        data[p] = r | 0; data[p + 1] = g | 0; data[p + 2] = b | 0; data[p + 3] = 255;
        p += 4;
      }
    }
    bctx.putImageData(img, 0, 0);
    ctx.imageSmoothingEnabled = true;
    ctx.imageSmoothingQuality = 'high';
    ctx.clearRect(0, 0, W, H);
    ctx.drawImage(buf, 0, 0, W, H);
  }

  function frame(now) {
    if (t0 < 0) t0 = now;
    var t = (now - t0) / 1000;
    step(t);
    render();
    raf = requestAnimationFrame(frame);
  }

  field.addEventListener('pointermove', function (e) {
    var r = field.getBoundingClientRect();
    px01 = Math.min(1, Math.max(0, (e.clientX - r.left) / r.width));
    py01 = Math.min(1, Math.max(0, (e.clientY - r.top) / r.height));
    if (!manual) { manual = true; root.classList.remove('is-demo'); }
    if (reduce) { step(0.9); render(); }
  });
  field.addEventListener('pointerleave', function () {
    if (manual) { px01 = 0.5; py01 = 0.7; }
  });
  field.addEventListener('keydown', function (e) {
    var d = { ArrowLeft: [-0.05, 0], ArrowRight: [0.05, 0], ArrowUp: [0, -0.05], ArrowDown: [0, 0.05] }[e.key];
    if (!d) return;
    e.preventDefault();
    if (!manual) { manual = true; root.classList.remove('is-demo'); }
    px01 = Math.max(0, Math.min(1, px01 + d[0]));
    py01 = Math.max(0, Math.min(1, py01 + d[1]));
    if (reduce) { step(0.9); render(); }
  });
  window.addEventListener('resize', function () { fit(); if (reduce) { for (var s = 0; s < STILL_TICK; s++) step(0.9); render(); } });

  fit();
  if (reduce) {
    // reduce — rAF 없이 STILL_TICK 스텝만 시뮬레이션해 흐름이 자리 잡은 한 장을 그린다
    for (var s = 0; s < STILL_TICK; s++) step(0.9);
    render();
    root.classList.add('is-still');
  } else {
    raf = requestAnimationFrame(frame);
  }
})();

// FireLavaHeatCells — vanilla 판(run/314/demo/05-fire-lava-heat-cells/index.html) 의 React 포트.
// 저해상도 격자의 각 셀이 열을 저장하고, 매 프레임
// next[i] = cur[i] + k*(이웃평균 - cur[i]) 확산과 냉각을 반복한다.
// 밝기가 임곗값을 넘는 셀만 용암색 램프로 ImageData/putImageData 로 찍는다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./FireLavaHeatCells.module.scss";

export type FireLavaHeatCellsProps = {
  duration?: string;
  easing?: string;
  color?: string;
  gridWidth?: number;   // 격자 폭(셀) — 변형 "해상도" 축
  kDiff?: number;       // 확산 계수 — 변형 "확산" 축
  ventCount?: number;   // 화구 수 — 변형 "화구" 축
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type PointerEvt = { clientX: number; clientY: number };
type KeyEvt = { key: string; preventDefault(): void };
type RGB = [number, number, number];
type Vent = { gx: number; gy: number; amp: number; ph: number };
type Colors = { ink: RGB; amber: RGB; gold: RGB; rose: RGB; ember: RGB; smoke: RGB };

const TAU = Math.PI * 2;
const FIELD_LABEL = "용암 열 확산 격자 실험판";
const STILL_TICK = 210;  // reduce 에서 쌓는 시뮬레이션 스텝 수
const T_GLOW = 0.86;     // 코어 임곗값 — 램프 정점
const COOL = 0.006;      // 냉각(방사) 계수

// 시드 고정 PRNG — mulberry32
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;
  };
}

function mixc(a: RGB, b: RGB, t: number): RGB {
  return [a[0] + (b[0] - a[0]) * t, a[1] + (b[1] - a[1]) * t, a[2] + (b[2] - a[2]) * t];
}

// 색 램프(256단계) — 굳은 바위 → 잔불 → 주황 → 코어. vanilla 판과 같은 정점표.
function buildRamp(c: Colors): RGB[] {
  const s0: RGB = mixc(c.ink, c.smoke, 0.5);
  const stops: Array<[number, RGB]> = [
    [0.0, c.ink],
    [0.3, s0],
    [0.62, c.ember],
    [0.82, c.amber],
    [1.0, c.gold],
  ];
  const ramp: RGB[] = [];
  for (let i = 0; i < 256; i++) {
    const t = i / 255;
    for (let s = 0; s < stops.length - 1; s++) {
      if (t >= stops[s][0] && t <= stops[s + 1][0]) {
        const f = (t - stops[s][0]) / (stops[s + 1][0] - stops[s][0]);
        ramp.push(mixc(stops[s][1], stops[s + 1][1], f));
        break;
      }
    }
  }
  return ramp;
}

export default function FireLavaHeatCells({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#d97a52",
  gridWidth = 96,
  kDiff = 0.34,
  ventCount = 5,
}: FireLavaHeatCellsProps) {
  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 bufRef = useRef<HTMLCanvasElement | null>(null);
  const bctxRef = useRef<CanvasRenderingContext2D | null>(null);
  const imgRef = useRef<ImageData | null>(null);
  const sizeRef = useRef({ w: 0, h: 0 }) as Box<{ w: number; h: number }>;
  const colRef = useRef<Colors | null>(null);
  const rampRef = useRef<RGB[]>([]) as Box<RGB[]>;
  const gwRef = useRef(gridWidth) as Box<number>;
  const ghRef = useRef(0) as Box<number>;
  const kRef = useRef(kDiff) as Box<number>;
  const ventNRef = useRef(ventCount) as Box<number>;
  const ventsRef = useRef<Vent[]>([]) as Box<Vent[]>;
  const heatRef = useRef(new Float32Array(0)) as Box<Float32Array>;
  const heat2Ref = useRef(new Float32Array(0)) as Box<Float32Array>;
  const pxRef = useRef(0.5) as Box<number>;
  const pyRef = useRef(0.7) as Box<number>;
  const manualRef = useRef(false) as Box<boolean>;
  const rafRef = useRef(0) as Box<number>;
  const t0Ref = useRef(-1) as Box<number>;
  const curTRef = useRef(0) as Box<number>;   // 재튜닝: render() 잔열 셔머가 쓰는 최근 시각(step 에서 갱신)
  const [res, setRes] = useState("96×54");
  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 fit(): void {
    const field = fieldRef.current;
    const cv = cvRef.current;
    if (!field || !cv) return;
    const ctx = cv.getContext("2d");
    if (!ctx) return;
    const r = field.getBoundingClientRect();
    sizeRef.current.w = r.width;
    sizeRef.current.h = r.height;
    cv.width = Math.round(r.width);
    cv.height = Math.round(r.height);
    ctxRef.current = ctx;
    const GW = gwRef.current;
    const GH = Math.max(24, Math.round(GW * r.height / r.width));
    ghRef.current = GH;
    const buf = document.createElement("canvas");
    buf.width = GW; buf.height = GH;
    const bctx = buf.getContext("2d");
    if (!bctx) return;
    bufRef.current = buf;
    bctxRef.current = bctx;
    imgRef.current = bctx.createImageData(GW, GH);
    heatRef.current = new Float32Array(GW * GH);
    heat2Ref.current = new Float32Array(GW * GH);
    // 화구 — 시드 고정 배치(하단). 재튜닝(2026-09-15): span 으로 화구 수에 맞춰 폭 전체에 고르게 배치
    const rand = makePRNG(0x5eed05);
    const vents: Vent[] = [];
    const span = ventNRef.current > 1 ? 0.78 / (ventNRef.current - 1) : 0;
    for (let v = 0; v < ventNRef.current; v++) {
      vents.push({
        gx: Math.floor(GW * (0.11 + span * v + (rand() - 0.5) * 0.06)),
        gy: Math.floor(GH * (0.72 + (rand() - 0.5) * 0.14)),
        amp: 0.9 + rand() * 0.35,
        ph: rand() * TAU,
      });
    }
    ventsRef.current = vents;
    setRes(GW + "×" + GH);
  }

  // 시뮬레이션 한 스텝 — 화구 주입 → 확산(부유 편향) → 냉각
  function step(tSec: number): void {
    curTRef.current = tSec;
    const GW = gwRef.current, GH = ghRef.current;
    const heat = heatRef.current, heat2 = heat2Ref.current;
    if (!heat || !heat2 || GW < 3 || GH < 3) return;
    const vents = ventsRef.current;
    for (let v = 0; v < vents.length; v++) {
      const vt = vents[v];
      const pulse = 0.35 + 0.65 * (0.5 + 0.5 * Math.sin(tSec * 1.1 + vt.ph));
      for (let dy = -3; dy <= 3; dy++) {
        for (let dx = -3; dx <= 3; dx++) {
          const x = vt.gx + dx, y = vt.gy + dy;
          if (x < 1 || x >= GW - 1 || y < 1 || y >= GH - 1) continue;
          const falloff = 1 - Math.sqrt(dx * dx + dy * dy) / 4.4;
          if (falloff <= 0) continue;
          heat[y * GW + x] += vt.amp * pulse * 0.1 * falloff;
        }
      }
    }
    if (manualRef.current) {
      const pgx = Math.floor(pxRef.current * (GW - 2)) + 1;
      const pgy = Math.floor(pyRef.current * (GH - 2)) + 1;
      if (pgx >= 1 && pgx < GW - 1 && pgy >= 1 && pgy < GH - 1) {
        heat[pgy * GW + pgx] += 0.12;
      }
    }
    const K = kRef.current;
    for (let yy = 1; yy < GH - 1; yy++) {
      for (let xx = 1; xx < GW - 1; xx++) {
        const i = yy * GW + xx;
        const c = heat[i];
        const nU = heat[i - GW], nD = heat[i + GW], nL = heat[i - 1], nR = heat[i + 1];
        const avg = (nU * 0.9 + nD * 1.1 + nL + nR) / 4;   // 아래 셀 가중 — 위로 자라는 흐름
        let nh = c + K * (avg - c);
        nh = nh - nh * COOL * (0.6 + nh);                  // 방사 냉각 ∝ h
        if (nh < 0) nh = 0;
        if (nh > 1.4) nh = 1.4;
        heat2[i] = nh;
      }
    }
    const tmp = heatRef.current;
    heatRef.current = heat2Ref.current;
    heat2Ref.current = tmp;
  }

  // 렌더 — 열값 → 램프 → ImageData. T_MELT 경계에 로즈 껍질.
  function render(): void {
    const img = imgRef.current, bctx = bctxRef.current, buf = bufRef.current, ctx = ctxRef.current;
    const col = colRef.current, ramp = rampRef.current;
    if (!img || !bctx || !buf || !ctx || !col || ramp.length === 0) return;
    const data = img.data;
    const GW = gwRef.current, GH = ghRef.current;
    const heat = heatRef.current;
    const { w: W, h: H } = sizeRef.current;
    const T_MELT = 0.62;
    let p = 0;
    for (let yy = 0; yy < GH; yy++) {
      for (let xx = 0; xx < GW; xx++) {
        const h = heat[yy * GW + xx];
        const t01 = h >= T_GLOW ? 1 : h / T_GLOW;
        let ci = (t01 * 255) | 0;
        if (ci > 255) ci = 255;
        let r = ramp[ci][0], g = ramp[ci][1], b = ramp[ci][2];
        if (h < T_MELT * 0.5) {
          const k = h / (T_MELT * 0.5);
          // 재튜닝(2026-09-15): 잔열색(ember) 셔머 — 정적 검은 영역을 없앤다(위치+시간 위상)
          const shimmer = 0.05 + 0.05 * Math.sin(curTRef.current * 1.3 + xx * 0.13 + yy * 0.19);
          r = col.ink[0] + (r - col.ink[0]) * k + (col.ember[0] - col.ink[0]) * shimmer;
          g = col.ink[1] + (g - col.ink[1]) * k + (col.ember[1] - col.ink[1]) * shimmer;
          b = col.ink[2] + (b - col.ink[2]) * k + (col.ember[2] - col.ink[2]) * shimmer;
        } else if (h >= T_MELT && h < T_MELT + 0.06) {
          const k2 = (h - T_MELT) / 0.06;
          r = r + (col.rose[0] - r) * 0.35 * (1 - k2);
          g = g + (col.rose[1] - g) * 0.35 * (1 - k2);
          b = b + (col.rose[2] - b) * 0.35 * (1 - k2);
        }
        data[p] = r | 0; data[p + 1] = g | 0; data[p + 2] = b | 0; data[p + 3] = 255;
        p += 4;
      }
    }
    bctx.putImageData(img, 0, 0);
    ctx.imageSmoothingEnabled = true;
    ctx.imageSmoothingQuality = "high";
    ctx.clearRect(0, 0, W, H);
    ctx.drawImage(buf, 0, 0, W, H);
  }

  function frame(now: number): void {
    if (t0Ref.current < 0) t0Ref.current = now;
    const t = (now - t0Ref.current) / 1000;
    step(t);
    render();
    rafRef.current = requestAnimationFrame(frame);
  }

  function onPoint(e: PointerEvt): void {
    const field = fieldRef.current;
    if (!field) return;
    const r = field.getBoundingClientRect();
    pxRef.current = Math.min(1, Math.max(0, (e.clientX - r.left) / r.width));
    pyRef.current = Math.min(1, Math.max(0, (e.clientY - r.top) / r.height));
    if (!manualRef.current) manualRef.current = true;
    if (reduced()) { step(0.9); render(); }
  }

  function onLeave(): void {
    if (manualRef.current) { pxRef.current = 0.5; pyRef.current = 0.7; }
  }

  function onKeyDown(e: KeyEvt): void {
    const deltas: Record<string, [number, number]> = {
      ArrowLeft: [-0.05, 0], ArrowRight: [0.05, 0], ArrowUp: [0, -0.05], ArrowDown: [0, 0.05],
    };
    const d = deltas[e.key];
    if (!d) return;
    e.preventDefault();
    if (!manualRef.current) manualRef.current = true;
    pxRef.current = Math.max(0, Math.min(1, pxRef.current + d[0]));
    pyRef.current = Math.max(0, Math.min(1, pyRef.current + d[1]));
    if (reduced()) { step(0.9); render(); }
  }

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    gwRef.current = gridWidth;
    kRef.current = kDiff;
    ventNRef.current = ventCount;
    colRef.current = {
      ink: tokRGB(root, "--fire-ink"),
      amber: tokRGB(root, "--fire-amber"),
      gold: tokRGB(root, "--fire-gold"),
      rose: tokRGB(root, "--fire-rose"),
      ember: tokRGB(root, "--fire-ember"),
      smoke: tokRGB(root, "--fire-smoke"),
    };
    rampRef.current = buildRamp(colRef.current);
    fit();
    function onResize(): void {
      fit();
      if (reduced()) { for (let s = 0; s < STILL_TICK; s++) step(0.9); render(); }
    }
    window.addEventListener("resize", onResize);
    if (reduced()) {
      for (let s = 0; s < STILL_TICK; s++) step(0.9);
      render();
      setStill(true);
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [gridWidth, kDiff, ventCount]);

  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"><g fill="currentColor"><path d="M128 24a16 16 0 0 1 16 16v8a80.06 80.06 0 0 1 63.94 63.94h8a16 16 0 0 1 0 32h-8A80.06 80.06 0 0 1 144 207.94v8a16 16 0 0 1-32 0v-8A80.06 80.06 0 0 1 48.06 144h-8a16 16 0 0 1 0-32h8A80.06 80.06 0 0 1 112 48.06v-8A16 16 0 0 1 128 24m-16 57.35V96a16 16 0 0 0 32 0V81.35A48.09 48.09 0 0 0 81.35 112H96a16 16 0 0 0 0-32h14.65M174.65 144H160a16 16 0 0 0 0 32h14.65A48.09 48.09 0 0 0 144 174.65V160a16 16 0 0 0-32 0v14.65A48.09 48.09 0 0 0 81.35 144" opacity=".2"/><path d="M232 128a104 104 0 1 1-104-104a104 104 0 0 1 104 104m-32 0a8 8 0 0 0-8-8h-9.34A56.1 56.1 0 0 0 136 77.34V68a8 8 0 0 0-16 0v9.34A56.1 56.1 0 0 0 77.34 120H68a8 8 0 0 0 0 16h9.34A56.1 56.1 0 0 0 120 179.66V188a8 8 0 0 0 16 0v-8.34A56.1 56.1 0 0 0 178.66 136H192a8 8 0 0 0 8-8m-40 0a32 32 0 1 1-32-32a32 32 0 0 1 32 32m-16 0a16 16 0 1 0-16 16a16 16 0 0 0 16-16"/></g></svg>
          <span>이웃 셀끼리 열을 주고받는 확산 한 단계가 용암이 굳었다 녹는 전부입니다 — 포인터로 화구를 옮깁니다</span>
        </p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label={FIELD_LABEL}
          tabIndex={0}
          onPointerMove={onPoint}
          onPointerLeave={onLeave}
          onKeyDown={onKeyDown}
        >
          <canvas className={styles.cv} ref={cvRef}></canvas>
        </div>
        <div className={styles.bar}>
          <span className={styles.meta}><b className={styles.val}>{res}</b> 열 격자 · k {kDiff}</span>
          <span className={styles.metaDim}>ImageData · putImageData</span>
        </div>
      </div>
    </main>
  );
}