04Afterglow diffusion — the bolt's halo bleeding through a pixel buffer

<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="currentColor" d="M150 14 66 146h49l-24 96 99-138h-52l28-90z"/></svg>
    <span>The brightness buffer left behind by the lightning is rewritten every frame by averaging it with neighboring pixels — even after the line vanishes, a halo keeps spreading and cooling</span>
  </p>
  <div class="cd__field" id="cdField" role="application" aria-label="Lightning Afterglow Diffusion test panel" tabindex="0">
    <canvas class="cd__cv" id="cdCv"></canvas>
  </div>
  <div class="cd__bar">
    <span class="cd__meta">diffusion <b class="cd__val" id="cdSpread"></b> · cooling <b class="cd__val" id="cdCool"></b></span>
    <span class="cd__meta cd__meta--dim">canvas · ImageData · putImageData</span>
  </div>
</div>
</main>

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

// 잔광 확산(04). @keyframes 를 아예 두지 않는다 — 움직임은 전부 캔버스 rAF 루프
// (밝기 버퍼 확산 + 냉각 + LUT putImageData)가 만든다.
// 무대는 어스름 잉크. 파스텔 팔레트를 --bolt-* 로 편다(편 316 공통 규격).

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

@include stage($stage-ink);

// ── 편 공통 파스텔 팔레트(BD9~11 원칙: 편마다 다시 정하지 않는다) ──
$bolt-ink:   #1b1e26;   // 어스름 무대
$bolt-dusk:  #3d4358;   // 슬레이트
$bolt-core:  #f6e7ff;   // 번개 코어 — 밝은 라벤다 화이트
$bolt-halo:  #b9a0ff;   // 헤일로 라일락
$bolt-mint:  #b9ddd1;   // 민트 보조
$bolt-pink:  #ffb9c8;   // 핑크 보조
$bolt-mist:  #d8dce6;   // 안개 회색빛

.cd {
  // 렌더러가 읽는 색 — JS 는 getComputedStyle 로 --bolt-* 를 받아 쓴다
  --bolt-ink:   #{$bolt-ink};
  --bolt-dusk:  #{$bolt-dusk};
  --bolt-core:  #{$bolt-core};
  --bolt-halo:  #{$bolt-halo};
  --bolt-mint:  #{$bolt-mint};
  --bolt-pink:  #{$bolt-pink};
  --bolt-mist:  #{$bolt-mist};

  // 항목 반경 토큰(01~09 공통 — 실사용은 $r-* SCSS 토큰, 표기 참조용)
  --radius-sm: 4px;
  --radius-md: 10px;
  --radius-lg: 20px;

  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: $bolt-mist;                        // 잉크 무대 위 — 안개 톤 글자
}
.cd__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $bolt-halo; }

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

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

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

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

// reduce — rAF 를 돌리지 않고 잔광이 번진 한 장만 남는다.
@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 {
  --bolt-ink: #1b1e26;
  --bolt-dusk: #3d4358;
  --bolt-core: #f6e7ff;
  --bolt-halo: #b9a0ff;
  --bolt-mint: #b9ddd1;
  --bolt-pink: #ffb9c8;
  --bolt-mist: #d8dce6;
  --radius-sm: 4px;
  --radius-md: 10px;
  --radius-lg: 20px;
  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: #d8dce6;
}

.cd__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #b9a0ff;
}

.cd__field {
  position: relative;
  height: 180px;
  border-radius: 16px;
  overflow: hidden;
  background: #1b1e26;
  box-shadow: inset 0 0 0 1px rgba(216, 220, 230, 0.18);
}

.cd__field:focus-visible {
  outline: 2px solid #b9a0ff;
  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, 220, 230, 0.72);
}

.cd__val {
  color: #b9a0ff;
  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;
  }
}

// 잔광 diffusion(04). 밝기 버퍼 lum(픽셀당 실수 1채널)을 매 프레임 이웃 4픽셀 avg 쪽으로 섞고(diffuse)
// 냉각 계수를 곱해 식힌다. 낙뢰는 살아 있는 동안 매 프레임 새 중점변위 경로로 버퍼에 도장찍는다(max 갱신).
// 화면 재구성은 LUT(잉크-진보라-라일락-코어)를 통한 ImageData 한 장 putImageData 로 끝낸다.
(function () {
  // ── 변형 상수 블록 — 아래 값 하나만 바꿔 변형 3종을 만든다 ──
  var SPREAD = 0.44;   // diffusion 계수 — 변형 "번짐" 축(0.38 → 0.62)
  var COOL   = 0.986;  // 냉각 계수 — 변형 "잔광 시간" 축(0.975 → 0.988)
  var GAP_MS = 430;    // avg 낙뢰 간격(ms) — 변형 "하늘 리듬" 축(640 → 1500)
  // ────────────────────────────────────────────────────

  var root = document.getElementById('cdRoot');
  var field = document.getElementById('cdField');
  var cv = document.getElementById('cdCv');
  var ctx = cv.getContext('2d');
  document.getElementById('cdSpread').textContent = String(SPREAD);
  document.getElementById('cdCool').textContent = String(COOL);

  // 시드 고정 PRNG — mulberry32. 미리보기가 항상 같은 흐름으로 번친다
  var SEED = 0x3160404;
  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(--bolt-*)에서 getComputedStyle 로 읽는다 — 파스텔 토큰이 scss 한 곳에
  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('--bolt-ink');
  var C_HALO = tokRGB('--bolt-halo');
  var C_CORE = tokRGB('--bolt-core');

  // LUT — 밝기 t 0(잉크) 부터 1(코어 위 흰끼) 까지 4구간 색 ramp. compose 에서 픽셀당 조회만 한다
  var lutR = new Uint8ClampedArray(256), lutG = new Uint8ClampedArray(256), lutB = new Uint8ClampedArray(256);
  function buildLUT() {
    var m = [0, 0.32, 0.62, 0.88, 1];
    var cs = [
      [C_INK[0], C_INK[1], C_INK[2]],
      [C_INK[0] + (C_HALO[0] - C_INK[0]) * 0.42, C_INK[1] + (C_HALO[1] - C_INK[1]) * 0.42, C_INK[2] + (C_HALO[2] - C_INK[2]) * 0.42],
      [C_HALO[0], C_HALO[1], C_HALO[2]],
      [C_CORE[0], C_CORE[1], C_CORE[2]],
      [255, 252, 255]
    ];
    for (var i = 0; i < 256; i++) {
      var t = i / 255, k = 0;
      while (k < 3 && t > m[k + 1]) k++;
      var f = (t - m[k]) / (m[k + 1] - m[k] || 1);
      lutR[i] = cs[k][0] + (cs[k + 1][0] - cs[k][0]) * f;
      lutG[i] = cs[k][1] + (cs[k + 1][1] - cs[k][1]) * f;
      lutB[i] = cs[k][2] + (cs[k + 1][2] - cs[k][2]) * f;
    }
  }

  var W = 0, H = 0;
  var lum = null, lum2 = null, img = null;   // 밝기 이중 버퍼 + 출력 ImageData
  var strikes = [], timer = 0;
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var raf = 0, prev = 0;

  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;
    lum = new Float32Array(W * H);
    lum2 = new Float32Array(W * H);
    img = ctx.createImageData(W, H);
    var d = img.data;
    for (var i = 3; i < d.length; i += 4) d[i] = 255;   // 알파는 한 번 채운 뒤 건드리지 않는다
    prngState = SEED >>> 0;
    strikes = [];
    timer = 0;   // 첫 프레임에 곧장 낙뢰 — 로드 직후 화면이 이미 산다
  }

  // diffusion — 각 픽셀을 이웃 4개 avg 쪽으로 SPREAD 만큼 끌고 COOL 로 식힌다. 두 버퍼를 교대로 써 편향을 없앤다
  function diffuse() {
    for (var y = 0; y < H; y++) {
      var row = y * W;
      var up = (y > 0 ? y - 1 : y) * W;
      var dn = (y < H - 1 ? y + 1 : y) * W;
      for (var x = 0; x < W; x++) {
        var i = row + x;
        var l = lum[i];
        var av = (lum[row + (x > 0 ? x - 1 : x)] + lum[row + (x < W - 1 ? x + 1 : x)] +
                  lum[up + x] + lum[dn + x]) * 0.25;
        lum2[i] = (l + SPREAD * (av - l)) * COOL;
      }
    }
    var tmp = lum; lum = lum2; lum2 = tmp;
  }

  // 중점변위 — 선분을 depth 번 반분하며 중점을 법선 방향으로 무작위 오프셋. 호출마다 새 경로가 나온다
  function genBolt(ax, ay, bx, by, mag, depth) {
    var pts = [{ x: ax, y: ay }, { x: bx, y: by }];
    var dx = bx - ax, dy = by - ay;
    var len = Math.hypot(dx, dy) || 1;
    var nx = -dy / len, ny = dx / len;   // 법선 — 변위가 놓이는 방향
    var off = mag;
    for (var d = 0; d < depth; d++) {
      var next = [pts[0]];
      for (var i = 0; i < pts.length - 1; i++) {
        var p = pts[i], q = pts[i + 1];
        var r = (rand() * 2 - 1) * off;
        next.push({ x: (p.x + q.x) * 0.5 + nx * r, y: (p.y + q.y) * 0.5 + ny * r }, q);
      }
      pts = next;
      off *= 0.55;
    }
    return pts;
  }

  // 점 하나를 밝기 버퍼에 도장 — 코어는 과노출(1.25), 둘레는 거리 비례 감쇠. 기존 값과 비교해 max 만 취한다
  function stampPoint(px, py, radius) {
    var cx = Math.round(px), cy = Math.round(py);
    for (var yy = cy - radius; yy <= cy + radius; yy++) {
      if (yy < 0 || yy >= H) continue;
      for (var xx = cx - radius; xx <= cx + radius; xx++) {
        if (xx < 0 || xx >= W) continue;
        var dxx = xx - cx, dyy = yy - cy;
        var d = Math.sqrt(dxx * dxx + dyy * dyy);
        if (d > radius) continue;
        var v = d <= 1 ? 1.25 : (1 - d / radius) * 0.62;
        var idx = yy * W + xx;
        if (v > lum[idx]) lum[idx] = v;
      }
    }
  }

  // 경로 도장 — 선분을 2px 간격으로 보간해 찍어 끊김 없는 줄기를 만든다
  function stampPath(pts, radius) {
    for (var i = 0; i < pts.length - 1; i++) {
      var p = pts[i], q = pts[i + 1];
      var seg = Math.max(1, Math.ceil(Math.hypot(q.x - p.x, q.y - p.y) / 2));
      for (var s = 0; s < seg; s++) {
        var f = s / seg;
        stampPoint(p.x + (q.x - p.x) * f, p.y + (q.y - p.y) * f, radius);
      }
    }
    stampPoint(pts[pts.length - 1].x, pts[pts.length - 1].y, radius);
  }

  // 낙뢰 정의 — 메인 줄기(상단→하단)와 본 줄기 중간에서 갈라지는 곁가지 2~3개
  function newStrike() {
    var ax = W * (0.12 + rand() * 0.76);
    var bx = W * (0.12 + rand() * 0.76);
    var s = { ax: ax, bx: bx, life: 4 + (rand() * 3 | 0), branches: [] };
    var nb = 2 + (rand() * 2 | 0);
    for (var k = 0; k < nb; k++) {
      var t = 0.3 + rand() * 0.45;
      var dir = rand() < 0.5 ? -1 : 1;
      s.branches.push({
        sx: ax + (bx - ax) * t,
        sy: H * t,
        ex: ax + (bx - ax) * t + dir * (W * 0.1 + rand() * W * 0.2),
        ey: H * (0.7 + rand() * 0.35),
        mag: H * 0.1
      });
    }
    return s;
  }

  // 살아 있는 낙뢰는 매 프레임 새 경로로 다시 도장 — 줄기가 몇 프레임 떨리다 지워진다
  function stepStrikes(dtMs) {
    timer -= dtMs;
    if (timer <= 0) {
      strikes.push(newStrike());
      timer = GAP_MS * (0.5 + rand());   // 몰아치다 쉬는 불규칙 간격
    }
    for (var i = strikes.length - 1; i >= 0; i--) {
      var s = strikes[i];
      stampPath(genBolt(s.ax, -8, s.bx, H + 8, H * 0.14, 5), 6);
      for (var k = 0; k < s.branches.length; k++) {
        var b = s.branches[k];
        stampPath(genBolt(b.sx, b.sy, b.ex, b.ey, b.mag, 4), 5);
      }
      s.life--;
      if (s.life <= 0) strikes.splice(i, 1);
    }
  }

  // LUT 로 픽셀 색을 매기고 한 장에 뿌린다
  function compose() {
    var d = img.data, n = W * H;
    for (var i = 0; i < n; i++) {
      var v = lum[i];
      var li = v >= 1 ? 255 : (v <= 0 ? 0 : (v * 255) | 0);
      var j = i << 2;
      d[j] = lutR[li]; d[j + 1] = lutG[li]; d[j + 2] = lutB[li];
    }
    ctx.putImageData(img, 0, 0);
  }

  // reduce 용 한 장 — rAF 없이 낙뢰 직후 잔광이 번진 상태를 시뮬레이션으로 만들어 찍는다
  function stillDraw() {
    fit();
    diffuse();
    stepStrikes(1000);
    for (var w = 0; w < 6; w++) { diffuse(); stepStrikes(16); }
    compose();
  }

  function frame(now) {
    var dt = Math.min(50, now - (prev || now));
    prev = now;
    diffuse();
    stepStrikes(dt);
    compose();
    raf = requestAnimationFrame(frame);
  }

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

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

// HaloDiffusion — vanilla 판(run/316/demo/04-lightning-glow-halo-diffusion/index.html) 의 React 포트.
// 밝기 버퍼(Float32Array) 를 매 프레임 이웃 4픽셀 평균 쪽으로 확산 + 냉각하고, 살아 있는 낙뢰를
// 중점변위 경로로 도장찍는다. 화면 재구성은 LUT 조회 + putImageData 한 번으로 끝낸다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./HaloDiffusion.module.scss";

export type HaloDiffusionProps = {
  duration?: string;
  easing?: string;
  color?: string;
  spread?: number;   // 확산 계수 — 변형 "번짐" 축(0.44 → 0.62)
  cool?: number;      // 냉각 계수 — 변형 "잔광 시간" 축(0.986 → 0.988)
  gapMs?: number;     // 평균 낙뢰 간격(ms) — 변형 "하늘 리듬" 축(430 → 1500)
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type RGB = [number, number, number];
type Pt = { x: number; y: number };
type Branch = { sx: number; sy: number; ex: number; ey: number; mag: number };
type Strike = { ax: number; bx: number; life: number; branches: Branch[] };
type Colors = { ink: RGB; halo: RGB; core: RGB };

const FIELD_LABEL = "번개 잔광 확산 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 HaloDiffusion({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#b9a0ff",
  spread = 0.44,
  cool = 0.986,
  gapMs = 430,
}: HaloDiffusionProps) {
  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 spreadRef = useRef(spread) as Box<number>;
  const coolRef = useRef(cool) as Box<number>;
  const gapRef = useRef(gapMs) as Box<number>;
  const lumRef = useRef<Float32Array | null>(null) as Box<Float32Array | null>;
  const lum2Ref = useRef<Float32Array | null>(null) as Box<Float32Array | null>;
  const imgRef = useRef<ImageData | null>(null) as Box<ImageData | null>;
  const lutRef = useRef<{ r: Uint8ClampedArray; g: Uint8ClampedArray; b: Uint8ClampedArray } | null>(null);
  const strikesRef = useRef<Strike[]>([]) as Box<Strike[]>;
  const timerRef = useRef(0) as Box<number>;
  const randRef = useRef<() => number>(makePRNG(0x3160404)) as Box<() => number>;
  const rafRef = useRef(0) as Box<number>;
  const [still, setStill] = useState(false);
  const [spreadShown] = useState(spread);
  const [coolShown] = useState(cool);
  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 buildLUT(col: Colors): void {
    const r = new Uint8ClampedArray(256), g = new Uint8ClampedArray(256), b = new Uint8ClampedArray(256);
    const m = [0, 0.32, 0.62, 0.88, 1];
    const cs: RGB[] = [
      col.ink,
      [col.ink[0] + (col.halo[0] - col.ink[0]) * 0.42, col.ink[1] + (col.halo[1] - col.ink[1]) * 0.42, col.ink[2] + (col.halo[2] - col.ink[2]) * 0.42],
      col.halo,
      col.core,
      [255, 252, 255],
    ];
    for (let i = 0; i < 256; i++) {
      const t = i / 255;
      let k = 0;
      while (k < 3 && t > m[k + 1]) k++;
      const f = (t - m[k]) / (m[k + 1] - m[k] || 1);
      r[i] = cs[k][0] + (cs[k + 1][0] - cs[k][0]) * f;
      g[i] = cs[k][1] + (cs[k + 1][1] - cs[k][1]) * f;
      b[i] = cs[k][2] + (cs[k + 1][2] - cs[k][2]) * f;
    }
    lutRef.current = { r, g, b };
  }

  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();
    const W = Math.max(2, Math.round(r.width));
    const H = Math.max(2, Math.round(r.height));
    sizeRef.current.w = W;
    sizeRef.current.h = H;
    cv.width = W; cv.height = H;
    ctxRef.current = ctx;
    lumRef.current = new Float32Array(W * H);
    lum2Ref.current = new Float32Array(W * H);
    const img = ctx.createImageData(W, H);
    const d = img.data;
    for (let i = 3; i < d.length; i += 4) d[i] = 255;
    imgRef.current = img;
    randRef.current = makePRNG(0x3160404);
    strikesRef.current = [];
    timerRef.current = 0;
  }

  function diffuse(): void {
    const { w: W, h: H } = sizeRef.current;
    let lum = lumRef.current, lum2 = lum2Ref.current;
    if (!lum || !lum2) return;
    const SP = spreadRef.current, CL = coolRef.current;
    for (let y = 0; y < H; y++) {
      const row = y * W;
      const up = (y > 0 ? y - 1 : y) * W;
      const dn = (y < H - 1 ? y + 1 : y) * W;
      for (let x = 0; x < W; x++) {
        const i = row + x;
        const l = lum[i];
        const av = (lum[row + (x > 0 ? x - 1 : x)] + lum[row + (x < W - 1 ? x + 1 : x)] + lum[up + x] + lum[dn + x]) * 0.25;
        lum2[i] = (l + SP * (av - l)) * CL;
      }
    }
    lumRef.current = lum2;
    lum2Ref.current = lum;
  }

  function genBolt(ax: number, ay: number, bx: number, by: number, mag: number, depth: number): Pt[] {
    const rand = randRef.current;
    let pts: Pt[] = [{ x: ax, y: ay }, { x: bx, y: by }];
    const dx = bx - ax, dy = by - ay;
    const len = Math.hypot(dx, dy) || 1;
    const nx = -dy / len, ny = dx / len;
    let off = mag;
    for (let d = 0; d < depth; d++) {
      const next: Pt[] = [pts[0]];
      for (let i = 0; i < pts.length - 1; i++) {
        const p = pts[i], q = pts[i + 1];
        const r = (rand() * 2 - 1) * off;
        next.push({ x: (p.x + q.x) * 0.5 + nx * r, y: (p.y + q.y) * 0.5 + ny * r }, q);
      }
      pts = next;
      off *= 0.55;
    }
    return pts;
  }

  function stampPoint(px: number, py: number, radius: number): void {
    const lum = lumRef.current;
    const { w: W, h: H } = sizeRef.current;
    if (!lum) return;
    const cx = Math.round(px), cy = Math.round(py);
    for (let yy = cy - radius; yy <= cy + radius; yy++) {
      if (yy < 0 || yy >= H) continue;
      for (let xx = cx - radius; xx <= cx + radius; xx++) {
        if (xx < 0 || xx >= W) continue;
        const dxx = xx - cx, dyy = yy - cy;
        const d = Math.sqrt(dxx * dxx + dyy * dyy);
        if (d > radius) continue;
        const v = d <= 1 ? 1.25 : (1 - d / radius) * 0.62;
        const idx = yy * W + xx;
        if (v > lum[idx]) lum[idx] = v;
      }
    }
  }

  function stampPath(pts: Pt[], radius: number): void {
    for (let i = 0; i < pts.length - 1; i++) {
      const p = pts[i], q = pts[i + 1];
      const seg = Math.max(1, Math.ceil(Math.hypot(q.x - p.x, q.y - p.y) / 2));
      for (let s = 0; s < seg; s++) {
        const f = s / seg;
        stampPoint(p.x + (q.x - p.x) * f, p.y + (q.y - p.y) * f, radius);
      }
    }
    stampPoint(pts[pts.length - 1].x, pts[pts.length - 1].y, radius);
  }

  function newStrike(): Strike {
    const rand = randRef.current;
    const { w: W, h: H } = sizeRef.current;
    const ax = W * (0.12 + rand() * 0.76);
    const bx = W * (0.12 + rand() * 0.76);
    const s: Strike = { ax, bx, life: 4 + ((rand() * 3) | 0), branches: [] };
    const nb = 2 + ((rand() * 2) | 0);
    for (let k = 0; k < nb; k++) {
      const t = 0.3 + rand() * 0.45;
      const dir = rand() < 0.5 ? -1 : 1;
      s.branches.push({
        sx: ax + (bx - ax) * t,
        sy: H * t,
        ex: ax + (bx - ax) * t + dir * (W * 0.1 + rand() * W * 0.2),
        ey: H * (0.7 + rand() * 0.35),
        mag: H * 0.1,
      });
    }
    return s;
  }

  function stepStrikes(dtMs: number): void {
    const rand = randRef.current;
    const { h: H } = sizeRef.current;
    timerRef.current -= dtMs;
    if (timerRef.current <= 0) {
      strikesRef.current.push(newStrike());
      timerRef.current = gapRef.current * (0.5 + rand());
    }
    const strikes = strikesRef.current;
    for (let i = strikes.length - 1; i >= 0; i--) {
      const s = strikes[i];
      stampPath(genBolt(s.ax, -8, s.bx, H + 8, H * 0.14, 5), 6);
      for (let k = 0; k < s.branches.length; k++) {
        const b = s.branches[k];
        stampPath(genBolt(b.sx, b.sy, b.ex, b.ey, b.mag, 4), 5);
      }
      s.life--;
      if (s.life <= 0) strikes.splice(i, 1);
    }
  }

  function compose(): void {
    const ctx = ctxRef.current;
    const img = imgRef.current;
    const lum = lumRef.current;
    const lut = lutRef.current;
    const { w: W, h: H } = sizeRef.current;
    if (!ctx || !img || !lum || !lut) return;
    const d = img.data, n = W * H;
    for (let i = 0; i < n; i++) {
      const v = lum[i];
      const li = v >= 1 ? 255 : v <= 0 ? 0 : (v * 255) | 0;
      const j = i << 2;
      d[j] = lut.r[li]; d[j + 1] = lut.g[li]; d[j + 2] = lut.b[li];
    }
    ctx.putImageData(img, 0, 0);
  }

  let t0 = -1;
  let prev = 0;
  function frame(now: number): void {
    if (t0 < 0) t0 = now;
    const dt = Math.min(50, now - (prev || now));
    prev = now;
    diffuse();
    stepStrikes(dt);
    compose();
    rafRef.current = requestAnimationFrame(frame);
  }

  function stillDraw(): void {
    fit();
    diffuse();
    stepStrikes(1000);
    for (let w = 0; w < 6; w++) { diffuse(); stepStrikes(16); }
    compose();
  }

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    spreadRef.current = spread;
    coolRef.current = cool;
    gapRef.current = gapMs;
    colRef.current = {
      ink: tokRGB(root, "--bolt-ink"),
      halo: tokRGB(root, "--bolt-halo"),
      core: tokRGB(root, "--bolt-core"),
    };
    fit();
    buildLUT(colRef.current);
    function onResize(): void {
      fit();
      if (colRef.current) buildLUT(colRef.current);
      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
  }, [spread, cool, gapMs]);

  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="currentColor" d="M150 14 66 146h49l-24 96 99-138h-52l28-90z"/></svg>
          <span>번개가 지나간 자리의 밝기 버퍼를 매 프레임 이웃 픽셀 평균으로 섞어 다시 씁니다 — 선이 사라진 뒤에도 헤일로가 번지다 식습니다</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}>확산 <b className={styles.val}>{spreadShown}</b> · 냉각 <b className={styles.val}>{coolShown}</b></span>
          <span className={styles.metaDim}>캔버스 · ImageData · putImageData</span>
        </div>
      </div>
    </main>
  );
}