03폭풍 하늘 — 무작위 간격으로 겹쳐 치는 여러 낙뢰

<main class="stage">
<div class="cd is-demo" id="cdRoot">
  <p class="cd__hint">
    <span>다음 낙뢰까지의 대기시간을 지수분포로 뽑으면 폭풍이 실제처럼 몰아치다 쉰다 — 평균 간격이 짧을수록 몰아침이 잦아집니다. 먼 번개는 하늘 빛만 바뀌고 가까운 번개는 줄기가 그대로 보입니다</span>
  </p>
  <div class="cd__field" id="cdField" role="application" aria-label="폭풍 하늘 다회 낙뢰 실험판" tabindex="0">
    <canvas class="cd__cv" id="cdCv"></canvas>
  </div>
  <div class="cd__bar">
    <span class="cd__meta">평균 <b class="cd__val" id="cdN">0.5</b>초 간격 · 지수분포 폭풍</span>
    <span class="cd__meta cd__meta--dim">canvas · lineTo · globalCompositeOperation</span>
  </div>
</div>
</main>

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

// 폭풍 하늘 다회 낙뢰(03). @keyframes 를 아예 두지 않는다 —
// 움직임은 전부 캔버스 rAF 루프(지수분포 대기 + 원근 낙뢰 + 구름 실루엣)가 만든다.
// 파스텔 팔레트 --bolt-* 는 편 공통 규격(01~09 동일).

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

@include stage($stage-ink);

// ── 편 공통 파스텔 팔레트(BD9~11 원칙) ──
$bolt-ink:   #1b1e26;
$bolt-dusk:  #3d4358;
$bolt-core:  #f6e7ff;
$bolt-lilac: #b9a0ff;
$bolt-mint:  #b9ddd1;
$bolt-pink:  #ffb9c8;
$bolt-mist:  #d8dce6;

.cd {
  --bolt-ink:   #{$bolt-ink};
  --bolt-dusk:  #{$bolt-dusk};
  --bolt-core:  #{$bolt-core};
  --bolt-lilac: #{$bolt-lilac};
  --bolt-mint:  #{$bolt-mint};
  --bolt-pink:  #{$bolt-pink};
  --bolt-mist:  #{$bolt-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: $bolt-mist;
}

.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-lilac; outline-offset: 2px; }

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

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

// reduce — rAF 없이 낙뢰 한 방의 한 장.
@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-lilac: #b9a0ff;
  --bolt-mint: #b9ddd1;
  --bolt-pink: #ffb9c8;
  --bolt-mist: #d8dce6;
  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__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;
  }
}

// 폭풍 하늘 다회 낙뢰(03). 다음 낙뢰까지의 대기를 지수분포(-ln(u)*MEAN)로 뽑아 몰아침-고요 리듬을 만든다.
// 폭풍 강도(쿨다운)가 사인파로 천천히 오르내려 파도 치는 밤이 되고, 각 낙뢰는 거리(원근)를 가진다 —
// 먼 번개는 줄기 대신 하늘 면적이 확 밝아지는 섬광, 가까운 번개는 중점변위 줄기 + 구름 윤곽 실루엣.
// lighter 합성으로 섬광과 줄기 광이 겹쳐 쌓인다. 다크 모드 히어로 배경용.
(function () {
  // ── 변형 상수 블록 — 아래 값 하나만 바꿔 변형을 만든다 ──
  var MEAN    = 0.5;   // 지수분포 평균 간격(초) — 작을수록 몰아침
  var STORM_A = 0.5;   // 폭풍 강도 진폭 — 몰아침-고요의 대비
  var FARMAX  = 1.0;   // 원근 최대 거리 — 1 은 먼 번개까지, 0.6 은 대부분 가까운 번개
  // ────────────────────────────────────────────────────

  var root = document.getElementById('cdRoot');
  var field = document.getElementById('cdField');
  var cv = document.getElementById('cdCv');
  var ctx = cv.getContext('2d');
  var nOut = document.getElementById('cdN');

  // 시드 고정 PRNG — mulberry32. 미리보기가 항상 같은 폭풍으로 흐른다
  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(0x316C03);

  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_DUSK = tokRGB('--bolt-dusk');
  var C_CORE = tokRGB('--bolt-core');
  var C_LILA = tokRGB('--bolt-lilac');
  var C_MINT = tokRGB('--bolt-mint');
  var C_PINK = tokRGB('--bolt-pink');
  var C_MIST = tokRGB('--bolt-mist');

  var TAU = Math.PI * 2;
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var W = 0, H = 0;
  var raf = 0, t0 = -1;

  var DEPTH = 6;
  var ROUGH = 0.4;

  // ── 중점변위 ──
  function displace(ax, ay, bx, by, depth, out) {
    if (depth === 0) { out.push([bx, by]); return; }
    var mx = (ax + bx) / 2, my = (ay + by) / 2;
    var dx = bx - ax, dy = by - ay;
    var len = Math.hypot(dx, dy) || 1;
    var nx = -dy / len, ny = dx / len;
    var off = (rand() * 2 - 1) * len * ROUGH * 0.5;
    mx += nx * off; my += ny * off;
    displace(ax, ay, mx, my, depth - 1, out);
    displace(mx, my, bx, by, depth - 1, out);
  }

  // ── 구름 — 층층이 쌓인 실루엣. 섬광에 따라 명도가 오르내리고, 느린 표류로 계속 움직인다 ──
  var clouds = [];
  function buildClouds() {
    clouds = [];
    var layers = 3;
    for (var L = 0; L < layers; L++) {
      var lobes = [];
      var count = 6 + L * 2;
      for (var i = 0; i < count; i++) {
        lobes.push({
          rx: (i + 0.5) / count + (rand() - 0.5) * 0.06,
          ry: 0.08 + L * 0.05 + (rand() - 0.5) * 0.03,
          r: 0.10 + rand() * 0.08 + (2 - L) * 0.02,
          ph: (i / count) * TAU + L * 1.7
        });
      }
      clouds.push({ lobes: lobes, depth: (L + 1) / layers, spd: 0.05 + L * 0.02 });
    }
  }

  var bolts = [];       // {pts, born, life, dist, ex, flickAt}
  var nextAt = 0.05;
  var cycle = 0;
  var ANCHORS = [0.2, 0.5, 0.82, 0.35, 0.66, 0.08, 0.92, 0.48];

  function spawn(t, storm) {
    // 거리 — 폭풍이 몰아칠 때는 가까운 번개의 비중이 커진다
    var dist = Math.pow(rand(), 1.6 - storm * 0.8) * FARMAX;
    var ax = ANCHORS[cycle % ANCHORS.length] + (rand() - 0.5) * 0.12;
    cycle++;
    var sx = (ax + (dist * 0.5 - 0.25) * 0.2) * W;   // 먼 번개는 중앙 쪽으로 몰림
    var ex = sx + (rand() - 0.5) * W * 0.2 * (1 - dist * 0.5);
    var pts = null;
    if (dist < 0.55) {
      pts = [[sx, -6]];
      displace(sx, -6, ex, H * (0.82 - dist * 0.1), DEPTH - (dist > 0.3 ? 1 : 0), pts);
    }
    bolts.push({ pts: pts, born: t, life: 0.45 + rand() * 0.25, dist: dist, ex: ex, flickAt: t, sx: sx });
  }

  function env(s, t) {
    var u = (t - s.born) / s.life;
    if (u >= 1 || u < 0) return 0;
    var b = Math.pow(1 - u, 1.6);
    if (u > 0.28 && u < 0.36) b = Math.max(b, 0.75);
    return b;
  }

  function rgba(c, a) { return 'rgba(' + c[0] + ',' + c[1] + ',' + c[2] + ',' + a + ')'; }
  function mix(c1, c2, k) {
    return [c1[0] + (c2[0] - c1[0]) * k, c1[1] + (c2[1] - c1[1]) * k, c1[2] + (c2[2] - c1[2]) * k];
  }

  function draw(t, sky) {
    // 하늘 — 어스름 잉크에서 지평선 슬레이트. sky(0..1) 로 전체 명도가 오르내린다
    var top = mix(C_INK, C_DUSK, 0.08 + 0.5 * sky);
    var bot = mix(C_INK, C_DUSK, 0.4 + 0.6 * sky);
    var g = ctx.createLinearGradient(0, 0, 0, H);
    g.addColorStop(0, rgba(top, 1));
    g.addColorStop(1, rgba(bot, 1));
    ctx.fillStyle = g;
    ctx.fillRect(0, 0, W, H);

    // 별먼지 — 폭풍 밤바닥의 미세 명도 요동
    for (var i = 0; i < specks.length; i++) {
      var sp = specks[i];
      var a = 0.04 + 0.26 * (0.5 + 0.5 * Math.sin(t * sp.spd + sp.ph));
      ctx.fillStyle = rgba(C_MIST, a);
      ctx.fillRect(sp.rx * W, sp.ry * H, 1.5, 1.5);
    }

    // 구름 — 섬광(sky) 에 반응하는 실루엣. lighter 로 빛이 쌓인다(칩: globalCompositeOperation)
    ctx.globalCompositeOperation = 'lighter';
    for (var c = 0; c < clouds.length; c++) {
      var cl = clouds[c];
      var clCol = mix(mix(C_INK, C_DUSK, 0.3), C_LILA, 0.10 + 0.38 * sky * (1 - cl.depth * 0.3));
      ctx.fillStyle = rgba(clCol, 0.5);
      for (var L = 0; L < cl.lobes.length; L++) {
        var lb = cl.lobes[L];
        var drift = Math.sin(t * cl.spd + lb.ph) * 0.018;   // 느린 표류 — 매 프레임 실루엣이 미세하게 흘러간다
        ctx.beginPath();
        ctx.ellipse((lb.rx + drift) * W, lb.ry * H, lb.r * W * 0.5, lb.r * H * 0.42, 0, 0, TAU);
        ctx.fill();
      }
    }

    // 줄기 — 가까운 번개만. 헤일로·중간광·코어
    for (var j = 0; j < bolts.length; j++) {
      var s = bolts[j];
      var b = env(s, t);
      if (b <= 0 || !s.pts) continue;
      var near = 1 - s.dist / FARMAX;
      var pts = s.pts;
      var k, m;
      ctx.strokeStyle = rgba(C_LILA, 0.16 * b * near);
      ctx.lineWidth = 8;
      ctx.beginPath();
      ctx.moveTo(pts[0][0], pts[0][1]);
      for (k = 1; k < pts.length; k++) ctx.lineTo(pts[k][0], pts[k][1]);
      ctx.stroke();
      ctx.strokeStyle = rgba(C_LILA, 0.5 * b * near);
      ctx.lineWidth = 3;
      ctx.beginPath();
      ctx.moveTo(pts[0][0], pts[0][1]);
      for (k = 1; k < pts.length; k++) ctx.lineTo(pts[k][0], pts[k][1]);
      ctx.stroke();
      ctx.strokeStyle = rgba(C_CORE, b * near);
      ctx.lineWidth = 1.6;
      ctx.beginPath();
      ctx.moveTo(pts[0][0], pts[0][1]);
      for (k = 1; k < pts.length; k++) ctx.lineTo(pts[k][0], pts[k][1]);
      ctx.stroke();
      // 지면 블룸
      var gy = H * (0.82 - s.dist * 0.1);
      var br = 8 + 26 * b * near;
      var g2 = ctx.createRadialGradient(s.ex, gy, 0, s.ex, gy, br);
      g2.addColorStop(0, rgba(C_MINT, 0.45 * b * near));
      g2.addColorStop(1, rgba(C_LILA, 0));
      ctx.fillStyle = g2;
      ctx.fillRect(s.ex - br, gy - br, br * 2, br * 2);
      // 착지 핑크 고리
      ctx.beginPath();
      ctx.arc(s.ex, gy, 4 + 18 * b * near, 0, TAU);
      ctx.strokeStyle = rgba(C_PINK, 0.28 * b * near);
      ctx.lineWidth = 1.3;
      ctx.stroke();
    }
    ctx.globalCompositeOperation = 'source-over';
  }

  var specks = [];
  function buildSpecks() {
    specks = [];
    for (var i = 0; i < 18; i++) {
      specks.push({ rx: rand(), ry: rand() * 0.8, ph: rand() * TAU, spd: 1 + rand() * 1.6 });
    }
  }

  function stormLevel(t) {
    return 0.5 + STORM_A * Math.sin(t * 0.5) * Math.sin(t * 0.23 + 1.7);
  }

  var skyGlow = 0;

  function update(t, dt) {
    var storm = stormLevel(t);
    if (t >= nextAt) {
      spawn(t, storm);
      // 지수분포 대기 — 다음 낙뢰까지. 몰아칠 때는 평균이 짧아진다
      var mean = MEAN * (1.65 - storm * 1.1);
      nextAt = t + -Math.log(1 - rand()) * mean;
    }
    var target = 0;
    for (var i = 0; i < bolts.length; i++) {
      var s = bolts[i];
      var b = env(s, t);
      if (b > 0) target = Math.max(target, b * (0.25 + 0.75 * (1 - s.dist / FARMAX)));
      if (t - s.born < s.life * 0.8 && t - s.flickAt > 0.06) {
        s.flickAt = t;
        if (s.pts && s.dist < 0.4) {
          var pts = [[s.sx, -6]];
          displace(s.sx, -6, s.ex, H * (0.82 - s.dist * 0.1), DEPTH - (s.dist > 0.3 ? 1 : 0), pts);
          s.pts = pts;
        }
      }
    }
    skyGlow += (target - skyGlow) * Math.min(1, dt * 14);
    while (bolts.length && t - bolts[0].born > 1.2) bolts.shift();
    return storm;
  }

  function fit() {
    var r = field.getBoundingClientRect();
    W = r.width; H = r.height;
    cv.width = Math.round(W); cv.height = Math.round(H);
    ctx.lineCap = 'round';
    ctx.lineJoin = 'round';
  }

  function stillFrame() {
    bolts = [];
    var pts = [[W * 0.5, -6]];
    displace(W * 0.5, -6, W * 0.56, H * 0.8, DEPTH, pts);
    bolts.push({ pts: pts, born: 0.04, life: 0.55, dist: 0.12, ex: W * 0.56, flickAt: 0, sx: W * 0.5 });
    draw(0.04, 0.8);
  }

  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;
    update(t, dt);
    draw(t, Math.max(0, skyGlow + 0.04));
    raf = requestAnimationFrame(frame);
  }

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

  fit();
  buildClouds();
  buildSpecks();
  nOut.textContent = MEAN.toFixed(1);
  if (reduce) {
    stillFrame();
    root.classList.add('is-still');
  } else {
    raf = requestAnimationFrame(frame);
  }
})();

// StormSky — vanilla 판(run/316/demo/03-lightning-storm-sky-multi-strike/index.html) 의 React 포트.
// 폭풍 하늘 다회 낙뢰 — 다음 낙뢰까지의 대기를 지수분포(-ln(u)*MEAN)로 뽑아 몰아침-고요 리듬을 만든다.
// 폭풍 강도가 사인파로 오르내리고, 각 낙뢰는 거리(원근)를 가진다 — 먼 번개는 하늘 섬광, 가까운 번개는
// 중점변위 줄기 + 구름 실루엣. lighter 합성으로 섬광과 줄기 광이 겹쳐 쌓인다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./StormSky.module.scss";

export type StormSkyProps = {
  duration?: string;
  easing?: string;
  color?: string;
  mean?: number;      // 지수분포 평균 간격(초) — 변형 "평균 간격" 축(작을수록 몰아침)
  stormAmp?: number;  // 폭풍 강도 진폭 — 몰아침-고요의 대비
  farMax?: number;    // 원근 최대 거리 — 1 은 먼 번개까지, 0.6 은 대부분 가까운 번개
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type RGB = [number, number, number];
type Pt = [number, number];
type Bolt = { pts: Pt[] | null; born: number; life: number; dist: number; ex: number; flickAt: number; sx: number };
type Speck = { rx: number; ry: number; ph: number; spd: number };
type Lobe = { rx: number; ry: number; r: number; ph: number };
type Cloud = { lobes: Lobe[]; depth: number; spd: number };
type Colors = { ink: RGB; dusk: RGB; core: RGB; lilac: RGB; mint: RGB; pink: RGB; mist: RGB };

const TAU = Math.PI * 2;
const FIELD_LABEL = "폭풍 하늘 다회 낙뢰 실험판";
const DEPTH = 6;
const ROUGH = 0.4;
const ANCHORS = [0.2, 0.5, 0.82, 0.35, 0.66, 0.08, 0.92, 0.48];

// 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;
  };
}

export default function StormSky({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#b9a0ff",
  mean = 0.5,
  stormAmp = 0.5,
  farMax = 1.0,
}: StormSkyProps) {
  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 meanRef = useRef(mean) as Box<number>;
  const ampRef = useRef(stormAmp) as Box<number>;
  const farRef = useRef(farMax) as Box<number>;
  const boltsRef = useRef<Bolt[]>([]) as Box<Bolt[]>;
  const specksRef = useRef<Speck[]>([]) as Box<Speck[]>;
  const cloudsRef = useRef<Cloud[]>([]) as Box<Cloud[]>;
  const schedRef = useRef({ nextAt: 0.05, cycle: 0 }) as Box<{ nextAt: number; cycle: number }>;
  const skyRef = useRef(0) as Box<number>;
  const rafRef = useRef(0) as Box<number>;
  const randRef = useRef(makePRNG(0x316c03)) as Box<() => number>;
  const [label, setLabel] = useState(mean.toFixed(1));
  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)];
  }

  // ── 중점변위 ──
  function displace(ax: number, ay: number, bx: number, by: number, d: number, out: Pt[]): void {
    if (d === 0) { out.push([bx, by]); return; }
    let mx = (ax + bx) / 2, my = (ay + by) / 2;
    const dx = bx - ax, dy = by - ay;
    const len = Math.hypot(dx, dy) || 1;
    const nx = -dy / len, ny = dx / len;
    const off = (randRef.current() * 2 - 1) * len * ROUGH * 0.5;
    mx += nx * off; my += ny * off;
    displace(ax, ay, mx, my, d - 1, out);
    displace(mx, my, bx, by, d - 1, out);
  }

  // ── 구름 — 층층이 쌓인 실루엣. 섬광에 따라 명도가 오르내리고, 느린 표류로 계속 움직인다 ──
  function buildClouds(): void {
    const rand = randRef.current;
    const arr: Cloud[] = [];
    for (let L = 0; L < 3; L++) {
      const lobes: Lobe[] = [];
      const count = 6 + L * 2;
      for (let i = 0; i < count; i++) {
        lobes.push({
          rx: (i + 0.5) / count + (rand() - 0.5) * 0.06,
          ry: 0.08 + L * 0.05 + (rand() - 0.5) * 0.03,
          r: 0.1 + rand() * 0.08 + (2 - L) * 0.02,
          ph: (i / count) * TAU + L * 1.7,
        });
      }
      arr.push({ lobes, depth: (L + 1) / 3, spd: 0.05 + L * 0.02 });
    }
    cloudsRef.current = arr;
  }

  function buildSpecks(): void {
    const rand = randRef.current;
    const arr: Speck[] = [];
    for (let i = 0; i < 18; i++) {
      arr.push({ rx: rand(), ry: rand() * 0.8, ph: rand() * TAU, spd: 1 + rand() * 1.6 });
    }
    specksRef.current = arr;
  }

  function spawn(t: number, storm: number): void {
    const rand = randRef.current;
    const { w: W, h: H } = sizeRef.current;
    const sched = schedRef.current;
    // 거리 — 폭풍이 몰아칠 때는 가까운 번개의 비중이 커진다
    const dist = Math.pow(rand(), 1.6 - storm * 0.8) * farRef.current;
    const ax = ANCHORS[sched.cycle % ANCHORS.length] + (rand() - 0.5) * 0.12;
    sched.cycle++;
    const sx = (ax + (dist * 0.5 - 0.25) * 0.2) * W;   // 먼 번개는 중앙 쪽으로 몰림
    const ex = sx + (rand() - 0.5) * W * 0.2 * (1 - dist * 0.5);
    let pts: Pt[] | null = null;
    if (dist < 0.55) {
      pts = [[sx, -6]];
      displace(sx, -6, ex, H * (0.82 - dist * 0.1), DEPTH - (dist > 0.3 ? 1 : 0), pts);
    }
    boltsRef.current.push({ pts, born: t, life: 0.45 + rand() * 0.25, dist, ex, flickAt: t, sx });
  }

  function env(s: Bolt, t: number): number {
    const u = (t - s.born) / s.life;
    if (u >= 1 || u < 0) return 0;
    let b = Math.pow(1 - u, 1.6);
    if (u > 0.28 && u < 0.36) b = Math.max(b, 0.75);
    return b;
  }

  function rgba(c: RGB, a: number): string {
    return `rgba(${c[0]},${c[1]},${c[2]},${a})`;
  }
  function mix(c1: RGB, c2: RGB, k: number): RGB {
    return [c1[0] + (c2[0] - c1[0]) * k, c1[1] + (c2[1] - c1[1]) * k, c1[2] + (c2[2] - c1[2]) * k];
  }

  function draw(t: number, sky: number): void {
    const ctx = ctxRef.current;
    const col = colRef.current;
    if (!ctx || !col) return;
    const { w: W, h: H } = sizeRef.current;
    // 하늘 — 어스름 잉크에서 지평선 슬레이트. sky(0..1) 로 전체 명도가 오르내린다
    const top = mix(col.ink, col.dusk, 0.08 + 0.5 * sky);
    const bot = mix(col.ink, col.dusk, 0.4 + 0.6 * sky);
    const g = ctx.createLinearGradient(0, 0, 0, H);
    g.addColorStop(0, rgba(top, 1));
    g.addColorStop(1, rgba(bot, 1));
    ctx.fillStyle = g;
    ctx.fillRect(0, 0, W, H);

    // 별먼지 — 폭풍 밤바닥의 미세 명도 요동
    const specks = specksRef.current;
    for (let i = 0; i < specks.length; i++) {
      const sp = specks[i];
      const a = 0.04 + 0.26 * (0.5 + 0.5 * Math.sin(t * sp.spd + sp.ph));
      ctx.fillStyle = rgba(col.mist, a);
      ctx.fillRect(sp.rx * W, sp.ry * H, 1.5, 1.5);
    }

    // 구름 — 섬광(sky) 에 반응하는 실루엣. lighter 로 빛이 쌓인다
    ctx.globalCompositeOperation = "lighter";
    const clouds = cloudsRef.current;
    for (let c = 0; c < clouds.length; c++) {
      const cl = clouds[c];
      const clCol = mix(mix(col.ink, col.dusk, 0.3), col.lilac, 0.1 + 0.38 * sky * (1 - cl.depth * 0.3));
      ctx.fillStyle = rgba(clCol, 0.5);
      for (let L = 0; L < cl.lobes.length; L++) {
        const lb = cl.lobes[L];
        const drift = Math.sin(t * cl.spd + lb.ph) * 0.018;   // 느린 표류 — 매 프레임 실루엣이 미세하게 흘러간다
        ctx.beginPath();
        ctx.ellipse((lb.rx + drift) * W, lb.ry * H, lb.r * W * 0.5, lb.r * H * 0.42, 0, 0, TAU);
        ctx.fill();
      }
    }

    // 줄기 — 가까운 번개만. 헤일로·중간광·코어
    const bolts = boltsRef.current;
    for (let j = 0; j < bolts.length; j++) {
      const s = bolts[j];
      const b = env(s, t);
      if (b <= 0 || !s.pts) continue;
      const near = 1 - s.dist / farRef.current;
      const pts = s.pts;
      ctx.strokeStyle = rgba(col.lilac, 0.16 * b * near);
      ctx.lineWidth = 8;
      ctx.beginPath();
      ctx.moveTo(pts[0][0], pts[0][1]);
      for (let k = 1; k < pts.length; k++) ctx.lineTo(pts[k][0], pts[k][1]);
      ctx.stroke();
      ctx.strokeStyle = rgba(col.lilac, 0.5 * b * near);
      ctx.lineWidth = 3;
      ctx.beginPath();
      ctx.moveTo(pts[0][0], pts[0][1]);
      for (let k = 1; k < pts.length; k++) ctx.lineTo(pts[k][0], pts[k][1]);
      ctx.stroke();
      ctx.strokeStyle = rgba(col.core, b * near);
      ctx.lineWidth = 1.6;
      ctx.beginPath();
      ctx.moveTo(pts[0][0], pts[0][1]);
      for (let k = 1; k < pts.length; k++) ctx.lineTo(pts[k][0], pts[k][1]);
      ctx.stroke();
      // 지면 블룸
      const gy = H * (0.82 - s.dist * 0.1);
      const br = 8 + 26 * b * near;
      const g2 = ctx.createRadialGradient(s.ex, gy, 0, s.ex, gy, br);
      g2.addColorStop(0, rgba(col.mint, 0.45 * b * near));
      g2.addColorStop(1, rgba(col.lilac, 0));
      ctx.fillStyle = g2;
      ctx.fillRect(s.ex - br, gy - br, br * 2, br * 2);
      // 착지 핑크 고리
      ctx.beginPath();
      ctx.arc(s.ex, gy, 4 + 18 * b * near, 0, TAU);
      ctx.strokeStyle = rgba(col.pink, 0.28 * b * near);
      ctx.lineWidth = 1.3;
      ctx.stroke();
    }
    ctx.globalCompositeOperation = "source-over";
  }

  function stormLevel(t: number): number {
    return 0.5 + ampRef.current * Math.sin(t * 0.5) * Math.sin(t * 0.23 + 1.7);
  }

  function update(t: number, dt: number): number {
    const sched = schedRef.current;
    const storm = stormLevel(t);
    if (t >= sched.nextAt) {
      spawn(t, storm);
      // 지수분포 대기 — 다음 낙뢰까지. 몰아칠 때는 평균이 짧아진다
      const m = meanRef.current * (1.65 - storm * 1.1);
      sched.nextAt = t + -Math.log(1 - randRef.current()) * m;
    }
    let target = 0;
    const bolts = boltsRef.current;
    for (let i = 0; i < bolts.length; i++) {
      const s = bolts[i];
      const b = env(s, t);
      if (b > 0) target = Math.max(target, b * (0.25 + 0.75 * (1 - s.dist / farRef.current)));
      if (t - s.born < s.life * 0.8 && t - s.flickAt > 0.06) {
        s.flickAt = t;
        if (s.pts && s.dist < 0.4) {
          const { h: H } = sizeRef.current;
          const pts: Pt[] = [[s.sx, -6]];
          displace(s.sx, -6, s.ex, H * (0.82 - s.dist * 0.1), DEPTH - (s.dist > 0.3 ? 1 : 0), pts);
          s.pts = pts;
        }
      }
    }
    skyRef.current += (target - skyRef.current) * Math.min(1, dt * 14);
    while (bolts.length && t - bolts[0].born > 1.2) bolts.shift();
    return storm;
  }

  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);
    ctx.lineCap = "round";
    ctx.lineJoin = "round";
    ctxRef.current = ctx;
  }

  function stillFrame(): void {
    const { w: W, h: H } = sizeRef.current;
    boltsRef.current = [];
    const pts: Pt[] = [[W * 0.5, -6]];
    displace(W * 0.5, -6, W * 0.56, H * 0.8, DEPTH, pts);
    boltsRef.current.push({ pts, born: 0.04, life: 0.55, dist: 0.12, ex: W * 0.56, flickAt: 0, sx: W * 0.5 });
    draw(0.04, 0.8);
  }

  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;
    update(t, dt);
    draw(t, Math.max(0, skyRef.current + 0.04));
    rafRef.current = requestAnimationFrame(frame);
  }

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    meanRef.current = mean;
    ampRef.current = stormAmp;
    farRef.current = farMax;
    colRef.current = {
      ink: tokRGB(root, "--bolt-ink"),
      dusk: tokRGB(root, "--bolt-dusk"),
      core: tokRGB(root, "--bolt-core"),
      lilac: tokRGB(root, "--bolt-lilac"),
      mint: tokRGB(root, "--bolt-mint"),
      pink: tokRGB(root, "--bolt-pink"),
      mist: tokRGB(root, "--bolt-mist"),
    };
    randRef.current = makePRNG(0x316c03);
    schedRef.current = { nextAt: 0.05, cycle: 0 };
    boltsRef.current = [];
    skyRef.current = 0;
    fit();
    buildClouds();
    buildSpecks();
    setLabel(mean.toFixed(1));
    function onResize(): void {
      fit();
      if (reduced()) stillFrame();
    }
    window.addEventListener("resize", onResize);
    if (reduced()) {
      stillFrame();
      setStill(true);
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [mean, stormAmp, farMax]);

  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>다음 낙뢰까지의 대기시간을 지수분포로 뽑으면 폭풍이 실제처럼 몰아치다 쉰다 — 평균 간격이 짧을수록 몰아침이 잦아집니다. 먼 번개는 하늘 빛만 바뀌고 가까운 번개는 줄기가 그대로 보입니다</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}>평균 <b className={styles.val}>{label}</b>초 간격 · 지수분포 폭풍</span>
          <span className={styles.metaDim}>canvas · lineTo · globalCompositeOperation</span>
        </div>
      </div>
    </main>
  );
}