06Trail-age coloring — the freshest paths glow neon-bright

<main class="stage">
<div class="sm is-demo" id="smRoot">
  <p class="sm__hint">
    <span>Timestamp each cell's last visit and fade its color with age — actively used trunks stay neon-bright while abandoned branches quietly die out</span>
  </p>
  <div class="sm__field" id="smField" role="application" aria-label="Trail-Age Coloring Slime Demo" tabindex="0">
    <canvas class="sm__cv" id="smCv"></canvas>
  </div>
  <div class="sm__bar">
    <span class="sm__meta"><b class="sm__val" id="smN">0</b><span id="smUnit"> steps · trail age</span></span>
    <span class="sm__meta sm__meta--dim">canvas · ImageData · requestAnimationFrame</span>
  </div>
</div>
</main>

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

// 트레일 나이 채색(06). @keyframes 를 두지 않는다 —
// 움직임은 전부 캔버스 rAF 루프(3센서 감지 → 회전 → 마지막 방문 시각 기록 → 나이 페이드)가 만든다.
// 무대는 다시 밤(ink) 바탕 — 네온 잔상 컨셉에 맞춘 여섯 번째 순환(01밤·02종이·03밀짚·04노을·05먹).

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

// ── 이 항목만의 팔레트(어두운 무대 L0.1 미만 + 네온 잔상 파스텔 L0.85 이상 — BD26 안전) ──
$sm-stage: #171426;   // 새벽 남색 먹 무대

@include stage($sm-stage);

$sm-ink:   #171426;   // 필드 바닥 — 무대와 같은 톤
$sm-old:   #4a4470;   // 오래된 트레일 — 죽어가는 흐린 남보라(어두운 배경 위 낮은 채도)
$sm-fresh: #d9f7ff;   // 방금 밟힌 트레일 — 파스텔 네온 시안(밝은 계열)
$sm-text:  #f0eaf5;   // 남색 먹 위 — 크림 라벤더 글자(대비)

.sm {
  --sm-ink:   #{$sm-ink};
  --sm-old:   #{$sm-old};
  --sm-fresh: #{$sm-fresh};

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

.sm__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: $sm-text;
}

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

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

.sm__bar { display: flex; align-items: center; justify-content: space-between; gap: $sp-2; }

.sm__val { color: $sm-fresh; }
.sm__meta { font-size: 10px; font-weight: 700; color: rgba($sm-text, .72); }
.sm__meta--dim { font-size: 10px; font-weight: 500; color: rgba($sm-text, .72); }

@media (max-width: 340px) {
  .sm { gap: $sp-1; }
  .sm__field { height: 100px; }
  .sm__meta--dim { display: none; }
}

// reduce — rAF 를 돌리지 않고 나이가 섞인 한 장만 남는다.
@media (prefers-reduced-motion: reduce) {
  .sm.is-still { opacity: 1; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #171426;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.sm {
  --sm-ink: #171426;
  --sm-old: #4a4470;
  --sm-fresh: #d9f7ff;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.sm__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: #f0eaf5;
}

.sm__field {
  position: relative;
  height: 180px;
  border-radius: 16px;
  overflow: hidden;
  background: #171426;
  box-shadow: inset 0 0 0 1px rgba(240, 234, 245, 0.18);
}

.sm__field:focus-visible {
  outline: 2px solid #f0eaf5;
  outline-offset: 2px;
}

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

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

.sm__val {
  color: #d9f7ff;
}

.sm__meta {
  font-size: 10px;
  font-weight: 700;
  color: rgba(240, 234, 245, 0.72);
}

.sm__meta--dim {
  font-size: 10px;
  font-weight: 500;
  color: rgba(240, 234, 245, 0.72);
}

@media (max-width: 340px) {
  .sm {
    gap: 4px;
  }
  .sm__field {
    height: 100px;
  }
  .sm__meta--dim {
    display: none;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sm.is-still {
    opacity: 1;
  }
}

// 트레일 나이 채색(06). Physarum 스티그머지 + 셀별 마지막 방문 시각 —
// trail 대신 lastVisit Float32Array 에 "이 셀을 마지막으로 밟은 스텝 번호"를 기록한다.
// 렌더는 나이(age = 현재스텝 - 마지막방문)로 색을 정한다: 방금 밟힌 셀은 네온처럼 밝고,
// 나이가 차면 어둡게 죽는다. 여러 먹이 앵커를 두고 트레일이 쓰이는 간선만 살아남는 모습.
//
// 화면이 사는 장치 — 077 캔버스 기준의 "매 프레임 계산":
//  · 캔버스는 절대 clear 하지 않는다. 나이 필드를 매 프레임 새로 계산해 얹는다.
//  · 앰비언트 글로우 레이어(두 옥타브 값노이즈)가 전면을 확산 구조로 흔들어 누적면적을 채운다.
(function () {
  // ── 변형 상수 블록 ──
  var AGENTS = 1400;        // 동시에 걷는 에이전트 수
  var DECAY_STEPS = 46;     // 트레일이 완전히 죽는 데 걸리는 스텝 수(나이 수명)
  // ────────────────────────────────────
  var SENSOR_ANGLE = 0.5;
  var SENSOR_DIST = 5.0;
  var TURN = 0.5;
  var STEP = 1.05;
  var DIFFUSE = 0.5;
  var SUBSTEPS = 3;
  var FOOD_W = 2.0;

  var root = document.getElementById('smRoot');
  var field = document.getElementById('smField');
  var cv = document.getElementById('smCv');
  var ctx = cv.getContext('2d', { alpha: false });
  var nOut = document.getElementById('smN');

  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(0x320A06);

  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('--sm-ink');
  var C_OLD    = tokRGB('--sm-old');
  var C_FRESH  = tokRGB('--sm-fresh');

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var W = 0, H = 0, fw = 0, fh = 0, raf = 0, t0 = -1;
  var lastVisit, trail, trailB, food, img, data;
  var ax, ay, ah;
  var stepCount = 0;
  var foods = [];

  // ── 값노이즈(앰비언트 글로우용) ──
  var perm = new Uint8Array(512);
  (function () {
    var p = [];
    for (var i = 0; i < 256; i++) p.push(i);
    for (var i2 = 255; i2 > 0; i2--) {
      var k2 = Math.floor(rand() * (i2 + 1));
      var tmp = p[i2]; p[i2] = p[k2]; p[k2] = tmp;
    }
    for (var i3 = 0; i3 < 512; i3++) perm[i3] = p[i3 & 255];
  })();
  function fade(k) { return k * k * (3 - 2 * k); }
  function vnoise(x, y) {
    var xi = Math.floor(x), yi = Math.floor(y);
    var xf = x - xi, yf = y - yi;
    var a2 = perm[(perm[xi & 255] + yi) & 255];
    var b2 = perm[(perm[(xi + 1) & 255] + yi) & 255];
    var c2 = perm[(perm[xi & 255] + yi + 1) & 255];
    var d2 = perm[(perm[(xi + 1) & 255] + yi + 1) & 255];
    var u = fade(xf), v = fade(yf);
    return (a2 + (b2 - a2) * u + (c2 - a2) * v + (a2 - b2 - c2 + d2) * u * v) / 255;
  }

  function idx(x, y) {
    x = x % fw; if (x < 0) x += fw;
    y = y % fh; if (y < 0) y += fh;
    return (y * fw + x) | 0;
  }
  function clampIdx(x, y) {
    x = Math.max(0, Math.min(fw - 1, Math.round(x)));
    y = Math.max(0, Math.min(fh - 1, Math.round(y)));
    return y * fw + x;
  }

  function buildFoods() {
    foods = [];
    var n = 5;
    var cx = fw / 2, cy = fh / 2, r = Math.min(fw, fh) * 0.4;
    for (var i = 0; i < n; i++) {
      var a = (i / n) * Math.PI * 2 + rand() * 0.6;
      var rr = r * (0.72 + rand() * 0.28);
      foods.push([cx + Math.cos(a) * rr, cy + Math.sin(a) * rr, rand() * Math.PI * 2]);
    }
    food = new Float32Array(fw * fh);
    for (var y = 0; y < fh; y++) {
      for (var x = 0; x < fw; x++) {
        var best = 0;
        for (var p = 0; p < foods.length; p++) {
          var dx = x - foods[p][0], dy = y - foods[p][1];
          var v = Math.exp(-(dx * dx + dy * dy) / (fw * 0.9));
          if (v > best) best = v;
        }
        food[y * fw + x] = best;
      }
    }
  }

  function sense(x, y) {
    var i = clampIdx(x, y);
    // 나이 필드를 감지에도 그대로 쓴다: 최근 밟힌 셀 = 진한 트레일
    var lv = lastVisit[i];
    var ageK = lv < 0 ? 0 : Math.max(0, 1 - (stepCount - lv) / DECAY_STEPS);
    return ageK * 4 + food[i] * FOOD_W;
  }

  function initAgents() {
    ax = new Float32Array(AGENTS); ay = new Float32Array(AGENTS); ah = new Float32Array(AGENTS);
    var cx = fw / 2, cy = fh / 2, r = Math.min(fw, fh) * 0.14;
    for (var i = 0; i < AGENTS; i++) {
      var a = rand() * Math.PI * 2, rr = rand() * r;
      ax[i] = cx + Math.cos(a) * rr;
      ay[i] = cy + Math.sin(a) * rr;
      ah[i] = rand() * Math.PI * 2;
    }
  }

  function stepAgents() {
    for (var i = 0; i < AGENTS; i++) {
      var x = ax[i], y = ay[i], h = ah[i];
      var sf = sense(x + Math.cos(h) * SENSOR_DIST, y + Math.sin(h) * SENSOR_DIST);
      var sl = sense(x + Math.cos(h + SENSOR_ANGLE) * SENSOR_DIST, y + Math.sin(h + SENSOR_ANGLE) * SENSOR_DIST);
      var sr = sense(x + Math.cos(h - SENSOR_ANGLE) * SENSOR_DIST, y + Math.sin(h - SENSOR_ANGLE) * SENSOR_DIST);
      if (sf >= sl && sf >= sr) {
        // 직진
      } else if (sl > sr) {
        h += TURN;
      } else if (sr > sl) {
        h -= TURN;
      } else {
        h += (rand() < 0.5 ? 1 : -1) * TURN;
      }
      h += (rand() - 0.5) * 0.14;
      var nx = x + Math.cos(h) * STEP, ny = y + Math.sin(h) * STEP;
      nx = ((nx % fw) + fw) % fw; ny = ((ny % fh) + fh) % fh;
      ax[i] = nx; ay[i] = ny; ah[i] = h;
      var ii = idx(Math.round(nx), Math.round(ny));
      lastVisit[ii] = stepCount;
    }
    stepCount++;
  }

  function render(t) {
    for (var y = 0; y < fh; y++) {
      for (var x = 0; x < fw; x++) {
        var i = y * fw + x;
        var j = i * 4;
        var lv = lastVisit[i];
        var k = 0;
        if (lv >= 0) {
          var age = stepCount - lv;
          if (age < DECAY_STEPS) k = 1 - age / DECAY_STEPS;
        }
        k = k * k * (3 - 2 * k);   // 나이 페이드를 부드럽게
        var rr = C_INK[0] + (C_OLD[0] - C_INK[0]) * Math.min(1, k * 2.4) + (C_FRESH[0] - C_OLD[0]) * Math.max(0, k - 0.35) * 1.54;
        var gg = C_INK[1] + (C_OLD[1] - C_INK[1]) * Math.min(1, k * 2.4) + (C_FRESH[1] - C_OLD[1]) * Math.max(0, k - 0.35) * 1.54;
        var bb = C_INK[2] + (C_OLD[2] - C_INK[2]) * Math.min(1, k * 2.4) + (C_FRESH[2] - C_OLD[2]) * Math.max(0, k - 0.35) * 1.54;
        for (var p = 0; p < foods.length; p++) {
          var dx = x - foods[p][0], dy = y - foods[p][1];
          var pulse = 0.55 + 0.45 * Math.sin(t * 2.2 + foods[p][2]);
          var glow = Math.exp(-(dx * dx + dy * dy) / 18) * pulse * 0.5;
          rr += (C_FRESH[0] - rr) * glow; gg += (C_FRESH[1] - gg) * glow; bb += (C_FRESH[2] - bb) * glow;
        }
        var n1 = vnoise(x * 0.09, y * 0.09 + t * 0.9);
        var n2 = vnoise(x * 0.03 + 40, y * 0.03 + t * 0.6);
        var sh = (n1 - 0.5) * 30 + (n2 - 0.5) * 20;
        rr += sh; gg += sh; bb += sh;
        data[j] = rr; data[j + 1] = gg; data[j + 2] = bb; data[j + 3] = 255;
      }
    }
    ctx.putImageData(img, 0, 0);
  }

  function fit() {
    var r = field.getBoundingClientRect();
    W = Math.round(r.width); H = Math.round(r.height);
    fw = Math.max(96, Math.min(200, Math.round(W / 3)));
    fh = Math.max(52, Math.min(120, Math.round(H / 3)));
    cv.width = fw; cv.height = fh;
    cv.style.width = '100%'; cv.style.height = '100%';
    ctx = cv.getContext('2d', { alpha: false });
    img = ctx.createImageData(fw, fh); data = img.data;
    lastVisit = new Float32Array(fw * fh);
    for (var i = 0; i < lastVisit.length; i++) lastVisit[i] = -9999;
    stepCount = 0;
    buildFoods();
    initAgents();
  }

  function stillFrame() {
    for (var k = 0; k < 300; k++) stepAgents();
    render(2.6);
  }

  function frame(now) {
    if (t0 < 0) t0 = now;
    var t = (now - t0) / 1000;
    for (var s = 0; s < SUBSTEPS; s++) stepAgents();
    render(t);
    if (nOut) nOut.textContent = String(stepCount);
    raf = requestAnimationFrame(frame);
  }

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

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

// TrailDecayColorAge — vanilla 판(run/320/demo/06-slime-trail-decay-color-age/index.html) 의 React 포트.
// 트레일 나이 채색. trail 대신 lastVisit(마지막 방문 스텝)을 기록해 나이(age = 현재스텝 - 마지막방문)로
// 색을 정한다 — 방금 밟힌 셀은 네온처럼 밝고, 나이가 차면 어둡게 죽는다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./TrailDecayColorAge.module.scss";

export type TrailDecayColorAgeProps = {
  duration?: string;
  easing?: string;
  color?: string;
  agents?: number;
  decaySteps?: number;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type RGB = [number, number, number];
type Food = [number, number, number];

const FIELD_LABEL = "트레일 나이 채색 점균 실험판";
const SENSOR_ANGLE = 0.5;
const SENSOR_DIST = 5.0;
const TURN = 0.5;
const STEP = 1.05;
const SUBSTEPS = 3;
const FOOD_W = 2.0;

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 TrailDecayColorAge({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#f0eaf5",
  agents = 1400,
  decaySteps = 46,
}: TrailDecayColorAgeProps) {
  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 colRef = useRef<{ ink: RGB; old: RGB; fresh: RGB } | null>(null);
  const agentsRef = useRef(agents) as Box<number>;
  const decayStepsRef = useRef(decaySteps) as Box<number>;
  const randRef = useRef(makePRNG(0x320a06)) as Box<() => number>;
  const fwRef = useRef(0) as Box<number>;
  const fhRef = useRef(0) as Box<number>;
  const lastVisitRef = useRef<Float32Array>(new Float32Array(0)) as Box<Float32Array>;
  const foodRef = useRef<Float32Array>(new Float32Array(0)) as Box<Float32Array>;
  const foodsRef = useRef<Food[]>([]) as Box<Food[]>;
  const axRef = useRef<Float32Array>(new Float32Array(0)) as Box<Float32Array>;
  const ayRef = useRef<Float32Array>(new Float32Array(0)) as Box<Float32Array>;
  const ahRef = useRef<Float32Array>(new Float32Array(0)) as Box<Float32Array>;
  const imgRef = useRef<ImageData | null>(null) as Box<ImageData | null>;
  const permRef = useRef<Uint8Array>(new Uint8Array(512)) as Box<Uint8Array>;
  const stepCountRef = useRef(0) as Box<number>;
  const rafRef = useRef(0) as Box<number>;
  const [n, setN] = useState("0");
  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 buildPerm(): void {
    const rand = randRef.current;
    const p: number[] = [];
    for (let i = 0; i < 256; i++) p.push(i);
    for (let i2 = 255; i2 > 0; i2--) {
      const k2 = Math.floor(rand() * (i2 + 1));
      const tmp = p[i2]; p[i2] = p[k2]; p[k2] = tmp;
    }
    const perm = new Uint8Array(512);
    for (let i3 = 0; i3 < 512; i3++) perm[i3] = p[i3 & 255];
    permRef.current = perm;
  }
  function fade(k: number): number { return k * k * (3 - 2 * k); }
  function vnoise(x: number, y: number): number {
    const perm = permRef.current;
    const xi = Math.floor(x), yi = Math.floor(y);
    const xf = x - xi, yf = y - yi;
    const a2 = perm[(perm[xi & 255] + yi) & 255];
    const b2 = perm[(perm[(xi + 1) & 255] + yi) & 255];
    const c2 = perm[(perm[xi & 255] + yi + 1) & 255];
    const d2 = perm[(perm[(xi + 1) & 255] + yi + 1) & 255];
    const u = fade(xf), v = fade(yf);
    return (a2 + (b2 - a2) * u + (c2 - a2) * v + (a2 - b2 - c2 + d2) * u * v) / 255;
  }

  function idx(x: number, y: number): number {
    const fw = fwRef.current, fh = fhRef.current;
    let xx = x % fw; if (xx < 0) xx += fw;
    let yy = y % fh; if (yy < 0) yy += fh;
    return (yy * fw + xx) | 0;
  }
  function clampIdx(x: number, y: number): number {
    const fw = fwRef.current, fh = fhRef.current;
    const xx = Math.max(0, Math.min(fw - 1, Math.round(x)));
    const yy = Math.max(0, Math.min(fh - 1, Math.round(y)));
    return yy * fw + xx;
  }

  function buildFoods(): void {
    const rand = randRef.current;
    const fw = fwRef.current, fh = fhRef.current;
    const foods: Food[] = [];
    const n2 = 5;
    const cx = fw / 2, cy = fh / 2, r = Math.min(fw, fh) * 0.4;
    for (let i = 0; i < n2; i++) {
      const a = (i / n2) * Math.PI * 2 + rand() * 0.6;
      const rr = r * (0.72 + rand() * 0.28);
      foods.push([cx + Math.cos(a) * rr, cy + Math.sin(a) * rr, rand() * Math.PI * 2]);
    }
    foodsRef.current = foods;
    const food = new Float32Array(fw * fh);
    for (let y = 0; y < fh; y++) {
      for (let x = 0; x < fw; x++) {
        let best = 0;
        for (let p = 0; p < foods.length; p++) {
          const dx = x - foods[p][0], dy = y - foods[p][1];
          const v = Math.exp(-(dx * dx + dy * dy) / (fw * 0.9));
          if (v > best) best = v;
        }
        food[y * fw + x] = best;
      }
    }
    foodRef.current = food;
  }

  function sense(x: number, y: number): number {
    const i = clampIdx(x, y);
    const lv = lastVisitRef.current[i];
    const ageK = lv < 0 ? 0 : Math.max(0, 1 - (stepCountRef.current - lv) / decayStepsRef.current);
    return ageK * 4 + foodRef.current[i] * FOOD_W;
  }

  function initAgents(): void {
    const rand = randRef.current;
    const fw = fwRef.current, fh = fhRef.current;
    const A = agentsRef.current;
    const ax = new Float32Array(A), ay = new Float32Array(A), ah = new Float32Array(A);
    const cx = fw / 2, cy = fh / 2, r = Math.min(fw, fh) * 0.14;
    for (let i = 0; i < A; i++) {
      const a = rand() * Math.PI * 2, rr = rand() * r;
      ax[i] = cx + Math.cos(a) * rr;
      ay[i] = cy + Math.sin(a) * rr;
      ah[i] = rand() * Math.PI * 2;
    }
    axRef.current = ax; ayRef.current = ay; ahRef.current = ah;
  }

  function stepAgents(): void {
    const rand = randRef.current;
    const A = agentsRef.current;
    const fw = fwRef.current, fh = fhRef.current;
    const ax = axRef.current, ay = ayRef.current, ah = ahRef.current;
    const lastVisit = lastVisitRef.current;
    for (let i = 0; i < A; i++) {
      const x = ax[i], y = ay[i];
      let h = ah[i];
      const sf = sense(x + Math.cos(h) * SENSOR_DIST, y + Math.sin(h) * SENSOR_DIST);
      const sl = sense(x + Math.cos(h + SENSOR_ANGLE) * SENSOR_DIST, y + Math.sin(h + SENSOR_ANGLE) * SENSOR_DIST);
      const sr = sense(x + Math.cos(h - SENSOR_ANGLE) * SENSOR_DIST, y + Math.sin(h - SENSOR_ANGLE) * SENSOR_DIST);
      if (sf >= sl && sf >= sr) {
        // 직진
      } else if (sl > sr) {
        h += TURN;
      } else if (sr > sl) {
        h -= TURN;
      } else {
        h += (rand() < 0.5 ? 1 : -1) * TURN;
      }
      h += (rand() - 0.5) * 0.14;
      let nx = x + Math.cos(h) * STEP, ny = y + Math.sin(h) * STEP;
      nx = ((nx % fw) + fw) % fw; ny = ((ny % fh) + fh) % fh;
      ax[i] = nx; ay[i] = ny; ah[i] = h;
      const ii = idx(Math.round(nx), Math.round(ny));
      lastVisit[ii] = stepCountRef.current;
    }
    stepCountRef.current++;
  }

  function render(t: number): void {
    const ctx = ctxRef.current;
    const col = colRef.current;
    const img = imgRef.current;
    if (!ctx || !col || !img) return;
    const fw = fwRef.current, fh = fhRef.current;
    const lastVisit = lastVisitRef.current;
    const decaySteps0 = decayStepsRef.current;
    const stepCount = stepCountRef.current;
    const foods = foodsRef.current;
    const data = img.data;
    for (let y = 0; y < fh; y++) {
      for (let x = 0; x < fw; x++) {
        const i = y * fw + x;
        const j = i * 4;
        const lv = lastVisit[i];
        let k = 0;
        if (lv >= 0) {
          const age = stepCount - lv;
          if (age < decaySteps0) k = 1 - age / decaySteps0;
        }
        k = k * k * (3 - 2 * k);
        let rr = col.ink[0] + (col.old[0] - col.ink[0]) * Math.min(1, k * 2.4) + (col.fresh[0] - col.old[0]) * Math.max(0, k - 0.35) * 1.54;
        let gg = col.ink[1] + (col.old[1] - col.ink[1]) * Math.min(1, k * 2.4) + (col.fresh[1] - col.old[1]) * Math.max(0, k - 0.35) * 1.54;
        let bb = col.ink[2] + (col.old[2] - col.ink[2]) * Math.min(1, k * 2.4) + (col.fresh[2] - col.old[2]) * Math.max(0, k - 0.35) * 1.54;
        for (let p = 0; p < foods.length; p++) {
          const dx = x - foods[p][0], dy = y - foods[p][1];
          const pulse = 0.55 + 0.45 * Math.sin(t * 2.2 + foods[p][2]);
          const glow = Math.exp(-(dx * dx + dy * dy) / 18) * pulse * 0.5;
          rr += (col.fresh[0] - rr) * glow; gg += (col.fresh[1] - gg) * glow; bb += (col.fresh[2] - bb) * glow;
        }
        const n1 = vnoise(x * 0.09, y * 0.09 + t * 0.9);
        const n2 = vnoise(x * 0.03 + 40, y * 0.03 + t * 0.6);
        const sh = (n1 - 0.5) * 30 + (n2 - 0.5) * 20;
        rr += sh; gg += sh; bb += sh;
        data[j] = rr; data[j + 1] = gg; data[j + 2] = bb; data[j + 3] = 255;
      }
    }
    ctx.putImageData(img, 0, 0);
  }

  function fit(): void {
    const field = fieldRef.current;
    const cv = cvRef.current;
    if (!field || !cv) return;
    const r = field.getBoundingClientRect();
    const W = Math.round(r.width), H = Math.round(r.height);
    const fw = Math.max(96, Math.min(200, Math.round(W / 3)));
    const fh = Math.max(52, Math.min(120, Math.round(H / 3)));
    fwRef.current = fw; fhRef.current = fh;
    cv.width = fw; cv.height = fh;
    cv.style.width = "100%"; cv.style.height = "100%";
    const ctx = cv.getContext("2d", { alpha: false });
    if (!ctx) return;
    ctxRef.current = ctx;
    const img = ctx.createImageData(fw, fh);
    imgRef.current = img;
    const lastVisit = new Float32Array(fw * fh);
    for (let i = 0; i < lastVisit.length; i++) lastVisit[i] = -9999;
    lastVisitRef.current = lastVisit;
    stepCountRef.current = 0;
    buildFoods();
    initAgents();
  }

  function stillFrame(): void {
    for (let k = 0; k < 300; k++) stepAgents();
    render(2.6);
  }

  let t0 = -1;
  function frame(now: number): void {
    if (t0 < 0) t0 = now;
    const t = (now - t0) / 1000;
    for (let s = 0; s < SUBSTEPS; s++) stepAgents();
    render(t);
    setN(String(stepCountRef.current));
    rafRef.current = requestAnimationFrame(frame);
  }

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    agentsRef.current = agents;
    decayStepsRef.current = decaySteps;
    randRef.current = makePRNG(0x320a06);
    colRef.current = {
      ink: tokRGB(root, "--sm-ink"),
      old: tokRGB(root, "--sm-old"),
      fresh: tokRGB(root, "--sm-fresh"),
    };
    buildPerm();
    fit();
    function onResize(): void {
      fit();
      if (reduced()) stillFrame();
    }
    window.addEventListener("resize", onResize);
    if (reduced()) {
      stillFrame();
      setStill(true);
    } else {
      t0 = -1;
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [agents, decaySteps]);

  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>각 셀에 마지막으로 밟힌 시각을 기록해 오래될수록 색을 어둡게 죽이면, 활발히 쓰이는 간선만 네온처럼 밝게 남고 버려진 지선은 자연히 사라집니다</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>
            <span> 스텝 · 트레일 나이</span>
          </span>
          <span className={styles.metaDim}>canvas · ImageData · requestAnimationFrame</span>
        </div>
      </div>
    </main>
  );
}