06Murmuration — a black cloud that leaves flow lines

<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"><g fill="currentColor"><path d="M24 132c22-6 38 6 56 4s34-12 52-8 36 16 54 12 34-14 46-18l-2-10c-14 4-32 16-46 20s-34-8-52-12-36 8-54 10-34-10-52-4Z" opacity=".25"/><path d="M230.53 28.51a8 8 0 0 0-11.31 0L196.3 51.43A108 108 0 0 0 45.14 196.06l-22.8 22.79a8 8 0 1 0 11.31 11.31l23.24-23.23A108 108 0 0 0 202 60.31l23.17-23.18a8 8 0 0 0 5.36-8.62M38 128a90 90 0 0 1 148-68.66L59.34 186A89.75 89.75 0 0 1 38 128m90 90a89.75 89.75 0 0 1-58-21.06L196.66 70A90 90 0 0 1 128 218"/></g></svg>
    <span>Every frame lightly overlays the background so agent trails linger as flow lines — a pure sine-summed curl-noise wind drives a black wave of 400 agents</span>
  </p>
  <div class="cd__field" id="cdField" role="application" aria-label="Murmuration trail test panel" tabindex="0">
    <canvas class="cd__cv" id="cdCv"></canvas>
  </div>
  <div class="cd__bar">
    <span class="cd__meta"><b class="cd__val" id="cdN">400</b>agents · 0.14 trail overlay</span>
    <span class="cd__meta cd__meta--dim">curl noise · trail accumulation · requestAnimationFrame</span>
  </div>
</div>
</main>

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

// 머뮤레이션(06). CSS 애니메이션·@keyframes 를 아예 두지 않는다 —
// 움직임은 전부 캔버스 rAF 루프(잔상 트레일 + 컬 노이즈 바람 보이드)가 만든다.

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

@include stage($stage-ink);

// ── 편 공통 파스텔 팔레트(315 — 04~06 동일) ──
$flock-ink:   #1b1e26;
$flock-dusk:  #3d4358;
$flock-glow:  #f4cfb2;
$flock-bird:  #23262f;
$flock-lilac: #b9a0ff;
$flock-mist:  #d8dce6;

.cd {
  --flock-ink:   #{$flock-ink};
  --flock-dusk:  #{$flock-dusk};
  --flock-glow:  #{$flock-glow};
  --flock-bird:  #{$flock-bird};
  --flock-lilac: #{$flock-lilac};
  --flock-mist:  #{$flock-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: $flock-mist;
}
.cd__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $flock-mist; }

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

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

.cd__bar { display: flex; align-items: baseline; justify-content: space-between; gap: $sp-2; }
.cd__meta { font-size: 10px; font-weight: 500; color: rgba($flock-mist, .72); }
.cd__val { color: $flock-glow; 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 {
  --flock-ink: #1b1e26;
  --flock-dusk: #3d4358;
  --flock-glow: #f4cfb2;
  --flock-bird: #23262f;
  --flock-lilac: #b9a0ff;
  --flock-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__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  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);
  cursor: default;
  touch-action: none;
}

.cd__field:focus-visible {
  outline: 2px solid #f4cfb2;
  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: #f4cfb2;
  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;
  }
}

// 머뮤레이션(06). 잔상 트레일 — 매 프레임 배경을 rgba(ink, 0.14) 로 덮어 이전 프레임이
// 서서히 지워지며 개체 궤적이 흐름선으로 남는다(첫 프레임은 불투명 full).
// 바람은 2D potential(순수 사인 중첩)을 만들어 그 회전(컬)을 샘플링한다 — 라이브러리 0.
(function () {
  // ── 변형 상수 블록 ──
  var N_BIRD = 400;    // 개체 수
  var TRAIL  = 0.14;   // 잔상 덮개 알파 — 낮을수록 궤적이 길게 남는다
  var WIND   = 1.0;    // 바람 세기 배율 — 변형 "바람 세기" 축(1.0 → 2.0)
  // ────────────────────────────────────────────────────

  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');
  nOut.textContent = String(N_BIRD);

  var SEED = 0x0b0106;
  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('--flock-ink');
  var C_GLOW = tokRGB('--flock-glow');

  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 firstFrame = true;

  var PERCEP = 26, SEP_R = 11;
  var MAX_SPD = 88, MAX_FRC = 240;
  var W_SEP = 1.5, W_ALI = 1.0, W_COH = 0.8;

  var birds = [];
  for (var i = 0; i < N_BIRD; i++) {
    var an = rand() * TAU;
    birds.push({
      x: rand() * 400, y: rand() * 180,
      vx: Math.cos(an) * MAX_SPD * 0.6, vy: Math.sin(an) * MAX_SPD * 0.6,
      sz: 1.5 + rand() * 1.0
    });
  }

  // ── 컬 노이즈 바람 — potential psi 의 회전(u, v) = (dpsi/dy, -dpsi/dx) ──
  // psi 는 사인 3겹 중첩(방향·주파수 다른 3개) — 라이브러리 없이 계산으로 만드는 흐름장.
  function windAt(x, y, tSec) {
    var s = 0.004;                    // 공간 주파수 기준
    var psi =
      Math.sin(x * s * 1.7 + tSec * 0.31) * Math.cos(y * s * 2.3 - tSec * 0.23) * 1.0 +
      Math.sin(x * s * 0.9 - tSec * 0.17 + 2.1) * Math.cos(y * s * 1.1 + tSec * 0.41 + 0.8) * 0.7 +
      Math.sin((x + y) * s * 1.3 + tSec * 0.53 + 4.2) * 0.45;
    var e = 1.5;                      // 유한차분 간격(px)
    var dpsiDx = (psiAt(x + e, y, tSec) - psiAt(x - e, y, tSec)) / (2 * e);
    var dpsiDy = (psiAt(x, y + e, tSec) - psiAt(x, y - e, tSec)) / (2 * e);
    // 회전장 — 발산 0(공기가 쌓이거나 빠지지 않는다)
    var mag = 46 * WIND;
    return [dpsiDy * mag, -dpsiDx * mag];
  }
  function psiAt(x, y, tSec) {
    var s = 0.004;
    return (
      Math.sin(x * s * 1.7 + tSec * 0.31) * Math.cos(y * s * 2.3 - tSec * 0.23) * 1.0 +
      Math.sin(x * s * 0.9 - tSec * 0.17 + 2.1) * Math.cos(y * s * 1.1 + tSec * 0.41 + 0.8) * 0.7 +
      Math.sin((x + y) * s * 1.3 + tSec * 0.53 + 4.2) * 0.45
    );
  }

  function step(dt, tSec) {
    for (var i = 0; i < birds.length; i++) {
      var b = birds[i];
      var sx = 0, sy = 0, ax = 0, ay = 0, cx0 = 0, cy0 = 0;
      var sepN = 0, aliN = 0, cohN = 0;
      for (var j = 0; j < birds.length; j++) {
        if (i === j) continue;
        var o = birds[j];
        var dx = o.x - b.x, dy = o.y - b.y;
        if (dx > W / 2) dx -= W; else if (dx < -W / 2) dx += W;
        if (dy > H / 2) dy -= H; else if (dy < -H / 2) dy += H;
        var d = Math.hypot(dx, dy);
        if (d < PERCEP && d > 0.0001) {
          if (d < SEP_R) { sx -= dx / d / d * SEP_R; sy -= dy / d / d * SEP_R; sepN++; }
          ax += o.vx; ay += o.vy; aliN++;
          cx0 += dx; cy0 += dy; cohN++;
        }
      }
      var fx = 0, fy = 0;
      if (sepN) { fx += (sx / sepN) * W_SEP; fy += (sy / sepN) * W_SEP; }
      if (aliN) { fx += (ax / aliN - b.vx) * 0.06 * W_ALI; fy += (ay / aliN - b.vy) * 0.06 * W_ALI; }
      if (cohN) { fx += (cx0 / cohN) * 0.045 * W_COH; fy += (cy0 / cohN) * 0.045 * W_COH; }

      // 컬 노이즈 바람 — 무리 전체에 검은 물결을 만드는 주 동력
      var wv = windAt(b.x, b.y, tSec);
      fx += wv[0]; fy += wv[1];

      var fl = Math.hypot(fx, fy);
      if (fl > MAX_FRC) { fx = fx / fl * MAX_FRC; fy = fy / fl * MAX_FRC; }
      b.vx += fx * dt; b.vy += fy * dt;
      var sp = Math.hypot(b.vx, b.vy);
      if (sp > MAX_SPD) { b.vx = b.vx / sp * MAX_SPD; b.vy = b.vy / sp * MAX_SPD; }
      else if (sp < MAX_SPD * 0.3) { b.vx = b.vx / (sp || 1) * MAX_SPD * 0.3; b.vy = b.vy / (sp || 1) * MAX_SPD * 0.3; }
      b.x += b.vx * dt; b.y += b.vy * dt;
      if (b.x < 0) b.x += W; else if (b.x > W) b.x -= W;
      if (b.y < 0) b.y += H; else if (b.y > H) b.y -= H;
    }
  }

  function fit() {
    var r = field.getBoundingClientRect();
    W = r.width; H = r.height;
    cv.width = Math.round(W); cv.height = Math.round(H);
    firstFrame = true;   // 리사이즈마다 잔상 캔버스를 다시 깔아야 한다
  }

  function draw(dt, tSec) {
    // 1) 잔상 덮개 — 첫 프레임은 불투명 full, 이후 반투명 0.14 로 궤적을 남긴다
    ctx.globalCompositeOperation = 'source-over';
    ctx.fillStyle = 'rgba(' + C_INK[0] + ',' + C_INK[1] + ',' + C_INK[2] + ',' + (firstFrame ? 1 : TRAIL).toFixed(3) + ')';
    ctx.fillRect(0, 0, W, H);
    firstFrame = false;

    // 2) 지평선 따뜻한 띠 — 매 프레임 얇게 다시 그려 잔상 위에 산다
    var hy = H * 0.82;
    var hb = ctx.createLinearGradient(0, hy - 10, 0, hy + 8);
    hb.addColorStop(0, 'rgba(' + C_GLOW[0] + ',' + C_GLOW[1] + ',' + C_GLOW[2] + ',0)');
    hb.addColorStop(0.5, 'rgba(' + C_GLOW[0] + ',' + C_GLOW[1] + ',' + C_GLOW[2] + ',0.5)');
    hb.addColorStop(1, 'rgba(' + C_GLOW[0] + ',' + C_GLOW[1] + ',' + C_GLOW[2] + ',0)');
    ctx.fillStyle = hb;
    ctx.fillRect(0, hy - 10, W, 18);

    // 3) 개체 — 실루엣 잉크 점. 배경보다 살짝 밝은 먹(잔상 잉크와 대비 유지)
    for (var i = 0; i < birds.length; i++) {
      var b = birds[i];
      ctx.fillStyle = 'rgba(' + C_GLOW[0] + ',' + C_GLOW[1] + ',' + C_GLOW[2] + ',0.30)';
      ctx.fillRect(b.x - b.sz, b.y - b.sz, b.sz * 2, b.sz * 2);
    }
  }

  function frame(now) {
    if (t0 < 0) t0 = now;
    var t = (now - t0) / 1000;
    var dt = Math.min(0.05, (now - (frame.prev || now)) / 1000);
    frame.prev = now;
    step(dt, t);
    draw(dt, t);
    raf = requestAnimationFrame(frame);
  }

  window.addEventListener('resize', function () { fit(); if (reduce) draw(0, 1.2); });

  fit();
  if (reduce) {
    // reduce — 바람이 궤적을 그린 뒤의 한 장만 고정해 그린다
    for (var s = 0; s < 60; s++) step(0.033, 1.0 + s * 0.033);
    firstFrame = true;
    draw(0, 3.0);
    root.classList.add('is-still');
  } else {
    raf = requestAnimationFrame(frame);
  }
})();

// MurmurationFlowline — vanilla 판(run/315/demo/06-flock-murmuration-flowline/index.html) 의 React 포트.
// 매 프레임 배경을 반투명(0.14)으로 덮어 개체 궤적이 흐름선으로 남고,
// 순수 사인 중첩 컬 노이즈 바람이 400개체의 검은 물결을 몰아간다(라이브러리 0).
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./MurmurationFlowline.module.scss";

export type MurmurationFlowlineProps = {
  duration?: string;
  easing?: string;
  color?: string;
  birds?: number;   // 개체 수
  trail?: number;   // 잔상 덮개 알파 — 낮을수록 궤적이 길게 남는다
  wind?: number;    // 바람 세기 배율 — 변형 "바람 세기" 축(1.0 → 2.0)
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
// useRef 는 T | null 을 돌려준다 — 처음부터 값이 있고 절대 비지 않는 상자는 이렇게 좁혀 쓴다
type Box<T> = { current: T };
type RGB = [number, number, number];
type Bird = { x: number; y: number; vx: number; vy: number; sz: number };
type Colors = { ink: RGB; glow: RGB };

const TAU = Math.PI * 2;
const FIELD_LABEL = "머뮤레이션 잔상 실험판";
const STILL_T = 3.0;
const PERCEP = 26;
const SEP_R = 11;
const MAX_SPD = 88;
const MAX_FRC = 240;
const W_SEP = 1.5;
const W_ALI = 1.0;
const W_COH = 0.8;

// 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;
  };
}

// 2D potential — 사인 3겹 중첩(방향·주파수 다른 3개). 라이브러리 없이 계산으로 만드는 흐름장
function psiAt(x: number, y: number, tSec: number): number {
  const s = 0.004;
  return (
    Math.sin(x * s * 1.7 + tSec * 0.31) * Math.cos(y * s * 2.3 - tSec * 0.23) * 1.0 +
    Math.sin(x * s * 0.9 - tSec * 0.17 + 2.1) * Math.cos(y * s * 1.1 + tSec * 0.41 + 0.8) * 0.7 +
    Math.sin((x + y) * s * 1.3 + tSec * 0.53 + 4.2) * 0.45
  );
}

export default function MurmurationFlowline({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#f4cfb2",
  birds = 400,
  trail = 0.14,
  wind = 1.0,
}: MurmurationFlowlineProps) {
  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 trailRef = useRef(trail) as Box<number>;
  const windRef = useRef(wind) as Box<number>;
  const birdsArrRef = useRef<Bird[]>([]) as Box<Bird[]>;
  const firstRef = useRef(true) as Box<boolean>;
  const rafRef = useRef(0) as Box<number>;
  const t0Ref = useRef(-1) as Box<number>;
  const prevRef = useRef(0) as Box<number>;
  const [n, setN] = useState(String(birds));
  const [still, setStill] = useState(false);
  const vars: Vars = { "--duration": duration, "--easing": easing, "--color": color };

  // 색은 CSS 커스텀 프로퍼티(--flock-*)에서 getComputedStyle 로 읽는다 — 파스텔 토큰이 scss 한 곳에
  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)];
  }

  // 개체 풀 리빌드 — birds prop 이 바뀌거나 최초 마운트 때
  function buildBirds(): void {
    const rand = makePRNG(0x0b0106);
    const arr: Bird[] = [];
    for (let i = 0; i < birds; i++) {
      const an = rand() * TAU;
      arr.push({
        x: rand() * 400,
        y: rand() * 180,
        vx: Math.cos(an) * MAX_SPD * 0.6,
        vy: Math.sin(an) * MAX_SPD * 0.6,
        sz: 1.5 + rand() * 1.0,
      });
    }
    birdsArrRef.current = arr;
    setN(String(birds));
  }

  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;
    firstRef.current = true;   // 리사이즈마다 잔상 캔버스를 다시 깔아야 한다
  }

  // ── 컬 노이즈 바람 — potential psi 의 회전(u, v) = (dpsi/dy, -dpsi/dx). 발산 0 ──
  function windAt(x: number, y: number, tSec: number): [number, number] {
    const e = 1.5;
    const dpsiDx = (psiAt(x + e, y, tSec) - psiAt(x - e, y, tSec)) / (2 * e);
    const dpsiDy = (psiAt(x, y + e, tSec) - psiAt(x, y - e, tSec)) / (2 * e);
    const mag = 46 * windRef.current;
    return [dpsiDy * mag, -dpsiDx * mag];
  }

  function step(dt: number, tSec: number): void {
    const birdsArr = birdsArrRef.current;
    const { w: W, h: H } = sizeRef.current;
    for (let i = 0; i < birdsArr.length; i++) {
      const b = birdsArr[i];
      let sx = 0, sy = 0, ax = 0, ay = 0, cx0 = 0, cy0 = 0;
      let sepN = 0, aliN = 0, cohN = 0;
      for (let j = 0; j < birdsArr.length; j++) {
        if (i === j) continue;
        const o = birdsArr[j];
        let dx = o.x - b.x, dy = o.y - b.y;
        if (dx > W / 2) dx -= W; else if (dx < -W / 2) dx += W;
        if (dy > H / 2) dy -= H; else if (dy < -H / 2) dy += H;
        const d = Math.hypot(dx, dy);
        if (d < PERCEP && d > 0.0001) {
          if (d < SEP_R) { sx -= (dx / d / d) * SEP_R; sy -= (dy / d / d) * SEP_R; sepN++; }
          ax += o.vx; ay += o.vy; aliN++;
          cx0 += dx; cy0 += dy; cohN++;
        }
      }
      let fx = 0, fy = 0;
      if (sepN) { fx += (sx / sepN) * W_SEP; fy += (sy / sepN) * W_SEP; }
      if (aliN) { fx += (ax / aliN - b.vx) * 0.06 * W_ALI; fy += (ay / aliN - b.vy) * 0.06 * W_ALI; }
      if (cohN) { fx += (cx0 / cohN) * 0.045 * W_COH; fy += (cy0 / cohN) * 0.045 * W_COH; }

      const wv = windAt(b.x, b.y, tSec);
      fx += wv[0];
      fy += wv[1];

      const fl = Math.hypot(fx, fy);
      if (fl > MAX_FRC) { fx = (fx / fl) * MAX_FRC; fy = (fy / fl) * MAX_FRC; }
      b.vx += fx * dt;
      b.vy += fy * dt;
      const sp = Math.hypot(b.vx, b.vy);
      if (sp > MAX_SPD) { b.vx = (b.vx / sp) * MAX_SPD; b.vy = (b.vy / sp) * MAX_SPD; }
      else if (sp < MAX_SPD * 0.3) { b.vx = (b.vx / (sp || 1)) * MAX_SPD * 0.3; b.vy = (b.vy / (sp || 1)) * MAX_SPD * 0.3; }
      b.x += b.vx * dt;
      b.y += b.vy * dt;
      if (b.x < 0) b.x += W; else if (b.x > W) b.x -= W;
      if (b.y < 0) b.y += H; else if (b.y > H) b.y -= H;
    }
  }

  function draw(): void {
    const ctx = ctxRef.current;
    const col = colRef.current;
    const birdsArr = birdsArrRef.current;
    if (!ctx || !col || birdsArr.length === 0) return;
    const { w: W, h: H } = sizeRef.current;

    // 1) 잔상 덮개 — 첫 프레임은 불투명 full, 이후 반투명 0.14 로 궤적을 남긴다
    ctx.globalCompositeOperation = "source-over";
    ctx.fillStyle = `rgba(${col.ink[0]},${col.ink[1]},${col.ink[2]},${firstRef.current ? 1 : trailRef.current})`;
    ctx.fillRect(0, 0, W, H);
    firstRef.current = false;

    // 2) 지평선 따뜻한 띠 — 매 프레임 얇게 다시 그려 잔상 위에 산다
    const hy = H * 0.82;
    const hb = ctx.createLinearGradient(0, hy - 10, 0, hy + 8);
    hb.addColorStop(0, `rgba(${col.glow[0]},${col.glow[1]},${col.glow[2]},0)`);
    hb.addColorStop(0.5, `rgba(${col.glow[0]},${col.glow[1]},${col.glow[2]},0.5)`);
    hb.addColorStop(1, `rgba(${col.glow[0]},${col.glow[1]},${col.glow[2]},0)`);
    ctx.fillStyle = hb;
    ctx.fillRect(0, hy - 10, W, 18);

    // 3) 개체 — 지평선 톤 점(잔상 잉크 위 대비 유지)
    ctx.fillStyle = `rgba(${col.glow[0]},${col.glow[1]},${col.glow[2]},0.30)`;
    for (let i = 0; i < birdsArr.length; i++) {
      const b = birdsArr[i];
      ctx.fillRect(b.x - b.sz, b.y - b.sz, b.sz * 2, b.sz * 2);
    }
  }

  function frame(now: number): void {
    if (t0Ref.current < 0) t0Ref.current = now;
    const t = (now - t0Ref.current) / 1000;
    const dt = Math.min(0.05, (now - (prevRef.current || now)) / 1000);
    prevRef.current = now;
    step(dt, t);
    draw();
    rafRef.current = requestAnimationFrame(frame);
  }

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    trailRef.current = trail;
    windRef.current = wind;
    colRef.current = {
      ink: tokRGB(root, "--flock-ink"),
      glow: tokRGB(root, "--flock-glow"),
    };
    buildBirds();
    fit();
    function onResize(): void {
      fit();
      if (window.matchMedia("(prefers-reduced-motion: reduce)").matches) draw();
    }
    window.addEventListener("resize", onResize);
    if (window.matchMedia("(prefers-reduced-motion: reduce)").matches) {
      // reduce — 바람이 궤적을 그린 뒤의 한 장만 고정해 그린다
      for (let s = 0; s < 60; s++) step(0.033, 1.0 + s * 0.033);
      firstRef.current = true;
      draw();
      setStill(true);
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [birds, trail, wind]);

  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="M24 132c22-6 38 6 56 4s34-12 52-8 36 16 54 12 34-14 46-18l-2-10c-14 4-32 16-46 20s-34-8-52-12-36 8-54 10-34-10-52-4Z" opacity=".25"/><path d="M230.53 28.51a8 8 0 0 0-11.31 0L196.3 51.43A108 108 0 0 0 45.14 196.06l-22.8 22.79a8 8 0 1 0 11.31 11.31l23.24-23.23A108 108 0 0 0 202 60.31l23.17-23.18a8 8 0 0 0 5.36-8.62M38 128a90 90 0 0 1 148-68.66L59.34 186A89.75 89.75 0 0 1 38 128m90 90a89.75 89.75 0 0 1-58-21.06L196.66 70A90 90 0 0 1 128 218"/></g></svg>
          <span>매 프레임 배경을 반투명하게 덮어 개체 궤적이 흐름선으로 남습니다 — 순수 사인 중첩 컬 노이즈 바람이 400개체의 검은 물결을 몰아갑니다</span>
        </p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label={FIELD_LABEL}
          tabIndex={0}
        >
          <canvas className={styles.cv} ref={cvRef}></canvas>
        </div>
        <div className={styles.bar}>
          <span className={styles.meta}><b className={styles.val}>{n}</b>개체 · 잔상 0.14 덮개</span>
          <span className={styles.metaDim}>curl noise · 트레일 누적 · requestAnimationFrame</span>
        </div>
      </div>
    </main>
  );
}