Boids Flocking

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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
<title>Boids Flocking — sloprun.dev</title>
<style>
/* ── sloprun design tokens (verbatim from demos/tokens.css) ───────────────── */
:root {
  --bg: #F6F6F3;          /* neutral paper, not cream */
  --surface: #FFFFFF;
  --ink: #1A1D21;
  --muted: #5A6068;
  --line: #E3E2DC;
  --accent: #C05600;      /* phosphor amber, darkened for light bg */
  --accent-soft: #FFF3E6;
  --run: #1A7F37;         /* earned green */
  --run-soft: #E7F4EA;
  --danger: #C93C37;
  --radius: 10px;
  --font-sans: system-ui, -apple-system, "Segoe UI", sans-serif;
  --font-mono: ui-monospace, "SF Mono", "Cascadia Code", Menlo, Consolas, monospace;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #14161A; --surface: #1C1F26; --ink: #E8E6E1; --muted: #9AA0A8;
    --line: #2A2E36; --accent: #FFB454; --accent-soft: #2A2118;
    --run: #3FB950; --run-soft: #16281B; --danger: #F47067;
  }
}
:root[data-theme="dark"] {
  --bg: #14161A; --surface: #1C1F26; --ink: #E8E6E1; --muted: #9AA0A8;
  --line: #2A2E36; --accent: #FFB454; --accent-soft: #2A2118;
  --run: #3FB950; --run-soft: #16281B; --danger: #F47067;
}
body { background: var(--bg); color: var(--ink); font-family: var(--font-sans); }
/* ── end tokens ───────────────────────────────────────────────────────────── */

/* demo-local additions (scrims need alpha, tokens are opaque hex) */
:root { --scrim: rgba(246,246,243,.88); --scrim-soft: rgba(246,246,243,.55); --shadow: 0 1px 2px rgba(26,29,33,.05); --lift: 0 6px 24px rgba(26,29,33,.16); }
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) { --scrim: rgba(20,22,26,.88); --scrim-soft: rgba(20,22,26,.55); --shadow: 0 1px 2px rgba(0,0,0,.35); --lift: 0 8px 28px rgba(0,0,0,.55); }
}
:root[data-theme="dark"] { --scrim: rgba(20,22,26,.88); --scrim-soft: rgba(20,22,26,.55); --shadow: 0 1px 2px rgba(0,0,0,.35); --lift: 0 8px 28px rgba(0,0,0,.55); }

* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; }
html { -webkit-text-size-adjust: 100%; }
body { min-height: 100%; overflow-x: hidden; }

.wrap {
  max-width: 1140px; margin: 0 auto;
  padding: 22px 16px 26px;
  display: flex; flex-direction: column; gap: 16px;
}

/* header */
h1 { margin: 0 0 6px; font-size: clamp(21px, 3.6vw, 29px); letter-spacing: -.021em; line-height: 1.15; }
.lede { margin: 0; max-width: 66ch; color: var(--muted); font-size: 14.5px; line-height: 1.55; }
.lede b { color: var(--ink); font-weight: 600; }
.lede code {
  font-family: var(--font-mono); font-size: .87em; color: var(--accent);
  background: var(--accent-soft); border-radius: 4px; padding: 1px 5px;
}

/* layout */
.stage { display: grid; grid-template-columns: minmax(0,1fr) 296px; gap: 16px; align-items: start; }
@media (max-width: 900px) { .stage { grid-template-columns: minmax(0,1fr); } }

.left { display: flex; flex-direction: column; gap: 12px; min-width: 0; }
.toolbar { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; padding: 10px 12px; }
.tbsep { width: 1px; align-self: stretch; background: var(--line); margin: 0 2px; }
@media (max-width: 480px) { .tbsep { display: none; } }

/* the field */
.field {
  position: relative; overflow: hidden;
  border: 1px solid var(--line); border-radius: var(--radius);
  background-color: var(--surface);
  background-image: radial-gradient(var(--line) 1.1px, transparent 1.2px);
  background-size: 26px 26px; background-position: -13px -13px;
  box-shadow: var(--shadow);
  /* fill the viewport height the header and footer leave behind, within sane bounds */
  height: clamp(300px, calc(100vh - 330px), 680px);
}
@media (max-width: 900px) { .field { aspect-ratio: 4 / 3; height: auto; } }
@media (max-width: 560px) { .field { aspect-ratio: 1 / 1; } }
canvas { display: block; width: 100%; height: 100%; touch-action: manipulation; cursor: crosshair; }
canvas:focus { outline: none; }
canvas:focus-visible { outline: 2px solid var(--accent); outline-offset: -3px; }
/* clicking the field focuses it so Enter keeps working, but a mouse user should not
   get a keyboard focus ring for it — the class is dropped the moment they type */
canvas.ptr-focus:focus-visible { outline: none; }

/* hint chip over the field */
.hint {
  position: absolute; left: 10px; bottom: 10px; pointer-events: none;
  font-family: var(--font-mono); font-size: 11px; letter-spacing: .01em;
  color: var(--muted); background: var(--scrim);
  border: 1px solid var(--line); border-radius: 999px; padding: 5px 10px;
  transition: opacity .45s ease; max-width: calc(100% - 20px);
}
.hint[hidden] { display: none; }
.hint.gone { opacity: 0; }

/* paused overlay */
.overlay {
  position: absolute; inset: 0; display: grid; place-items: center;
  background: var(--scrim-soft); padding: 18px; text-align: center;
}
.overlay[hidden] { display: none; }
.overlay .box {
  display: flex; flex-direction: column; align-items: center; gap: 10px; max-width: 34ch;
  background: var(--surface); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 16px 18px; box-shadow: var(--lift);
}
.overlay p { margin: 0; font-size: 13px; line-height: 1.5; color: var(--muted); }

/* panel */
.panel { display: flex; flex-direction: column; gap: 12px; }
.card {
  background: var(--surface); border: 1px solid var(--line);
  border-radius: var(--radius); padding: 12px 13px; box-shadow: var(--shadow);
}
.card h3.sub {
  margin: 15px 0 8px; padding-top: 12px; border-top: 1px solid var(--line);
  font-family: var(--font-mono); font-size: 10.5px; letter-spacing: .1em;
  text-transform: uppercase; color: var(--muted); font-weight: 600;
}
.card h2 {
  margin: 0 0 10px; font-family: var(--font-mono); font-size: 10.5px;
  letter-spacing: .1em; text-transform: uppercase; color: var(--muted); font-weight: 600;
}

/* stats */
.stats { display: grid; grid-template-columns: repeat(2, minmax(0,1fr)); gap: 11px 10px; }
.stat { display: flex; flex-direction: column; gap: 2px; min-width: 0; }
.stat .v {
  font-family: var(--font-mono); font-size: 20px; line-height: 1.05;
  font-variant-numeric: tabular-nums; letter-spacing: -.02em;
}
.stat .v .u { font-size: 11px; color: var(--muted); margin-left: 2px; letter-spacing: 0; }
.stat .k {
  font-family: var(--font-mono); font-size: 9.5px; letter-spacing: .09em;
  text-transform: uppercase; color: var(--muted);
}
.stat .v.warn { color: var(--accent); }

.pill {
  display: inline-flex; align-items: center; gap: 6px; margin-top: 11px;
  font-family: var(--font-mono); font-size: 11px; letter-spacing: .04em;
  padding: 3px 9px 3px 7px; border-radius: 999px;
  border: 1px solid var(--line); color: var(--muted); background: var(--bg);
}
.pill .dot { width: 7px; height: 7px; border-radius: 50%; background: var(--muted); flex: none; }
.pill.on { color: var(--run); background: var(--run-soft); border-color: transparent; }
.pill.on .dot { background: var(--run); }

/* controls */
.ctl { display: block; margin-bottom: 12px; }
.ctl:last-child { margin-bottom: 2px; }
.ctl-top { display: flex; justify-content: space-between; align-items: baseline; gap: 8px; margin-bottom: 3px; }
.ctl-top label { font-size: 13px; }
.ctl-top .val {
  font-family: var(--font-mono); font-size: 12px; color: var(--muted);
  font-variant-numeric: tabular-nums;
}
.ctl .desc { display: block; font-size: 11.5px; color: var(--muted); line-height: 1.35; margin-top: 1px; }

input[type=range] {
  -webkit-appearance: none; appearance: none; display: block;
  width: 100%; height: 20px; margin: 0; background: transparent; cursor: pointer;
}
input[type=range]::-webkit-slider-runnable-track { height: 4px; border-radius: 2px; background: var(--line); }
input[type=range]::-webkit-slider-thumb {
  -webkit-appearance: none; width: 15px; height: 15px; margin-top: -5.5px;
  border-radius: 50%; background: var(--accent); border: 2px solid var(--surface);
  box-shadow: 0 0 0 1px var(--line);
}
input[type=range]::-moz-range-track { height: 4px; border-radius: 2px; background: var(--line); }
input[type=range]::-moz-range-thumb {
  width: 13px; height: 13px; border-radius: 50%; background: var(--accent);
  border: 2px solid var(--surface); box-shadow: 0 0 0 1px var(--line);
}
input[type=range]:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; border-radius: 6px; }

/* buttons */
button {
  font: inherit; font-size: 13px; font-family: var(--font-sans);
  color: var(--ink); background: var(--surface);
  border: 1px solid var(--line); border-radius: 8px;
  padding: 7px 11px; cursor: pointer; line-height: 1.2;
  transition: border-color .12s ease, color .12s ease, background .12s ease;
}
button:hover:not(:disabled) { border-color: var(--accent); color: var(--accent); }
button:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }
button:disabled { opacity: .42; cursor: not-allowed; }
button.primary { background: var(--accent); color: var(--surface); border-color: var(--accent); font-weight: 600; }
button.primary:hover:not(:disabled) { color: var(--surface); filter: brightness(1.07); }
button.big { font-size: 14px; padding: 9px 18px; }

.row { display: flex; flex-wrap: wrap; gap: 7px; }
.row > button { flex: 1 1 auto; }
.presets { display: grid; grid-template-columns: repeat(2, minmax(0,1fr)); gap: 7px; }
.presets button[aria-pressed="true"] {
  background: var(--accent-soft); color: var(--accent); border-color: var(--accent);
}

/* trails switch */
.switch { display: flex; align-items: center; gap: 9px; cursor: pointer; font-size: 13px; margin-left: auto; }
@media (max-width: 560px) { .switch { margin-left: 0; } }
.switch input { position: absolute; opacity: 0; width: 0; height: 0; }
.track {
  width: 34px; height: 19px; border-radius: 999px; background: var(--line);
  border: 1px solid var(--line); position: relative; flex: none; transition: background .15s ease;
}
.track::after {
  content: ""; position: absolute; top: 1.5px; left: 2px; width: 14px; height: 14px;
  border-radius: 50%; background: var(--surface); transition: transform .15s ease;
  box-shadow: 0 1px 2px rgba(0,0,0,.25);
}
.switch input:checked + .track { background: var(--accent); border-color: var(--accent); }
.switch input:checked + .track::after { transform: translateX(15px); }
.switch input:focus-visible + .track { outline: 2px solid var(--accent); outline-offset: 2px; }

/* legend */
.legend { display: flex; align-items: center; gap: 8px; margin-top: 10px; }
.legend .bar { flex: 1 1 auto; height: 6px; border-radius: 3px; }
.legend .lbl { font-family: var(--font-mono); font-size: 9.5px; letter-spacing: .07em; text-transform: uppercase; color: var(--muted); }
.legend .num { font-size: 11px; letter-spacing: 0; text-transform: none; color: var(--ink); font-variant-numeric: tabular-nums; min-width: 4ch; text-align: right; }
.cap { margin: 8px 0 0; font-size: 11.5px; line-height: 1.4; color: var(--muted); }
.cap b { color: var(--ink); font-weight: 600; }

footer {
  font-family: var(--font-mono); font-size: 11px; color: var(--muted);
  letter-spacing: .05em; padding-top: 2px;
}

.sr-only {
  position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px;
  overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; border: 0;
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { transition-duration: .001ms !important; animation-duration: .001ms !important; }
}
</style>
</head>
<body>
<div class="wrap">

  <header>
    <h1>Boids Flocking</h1>
    <p class="lede">
      400 birds, no leader, no plan. Each one follows three local rules —
      <b>steer toward</b> neighbours, <b>steer with</b> neighbours, <b>don't crowd</b> neighbours —
      and the murmuration falls out for free. Move the sliders mid-flight;
      <b>click the field</b> to drop a predator and watch the flock split around it.
    </p>
  </header>

  <main class="stage">

    <div class="left">
    <div class="field" id="field">
      <canvas id="c" tabindex="0" aria-label="Flock field: 400 simulated birds. Press Enter to drop a predator into the flock.">
        A canvas animation of 400 flocking birds. Your browser does not support canvas.
      </canvas>
      <div class="hint" id="hint">click the field to drop a predator</div>
      <div class="overlay" id="overlay" hidden>
        <div class="box">
          <button class="primary big" id="overlayPlay" type="button">▶&nbsp; Play simulation</button>
          <p id="overlayNote">Paused.</p>
        </div>
      </div>
    </div>

      <div class="card toolbar" role="group" aria-label="Simulation controls">
        <button type="button" class="primary" id="playBtn">Pause</button>
        <button type="button" id="resetBtn">Reset flock</button>
        <span class="tbsep" aria-hidden="true"></span>
        <button type="button" id="dropBtn">Drop predator</button>
        <button type="button" id="clearBtn" disabled>Clear</button>
        <label class="switch" for="trails">
          <input type="checkbox" id="trails">
          <span class="track" aria-hidden="true"></span>
          <span>Motion trails</span>
        </label>
      </div>
    </div>

    <div class="panel">

      <section class="card" aria-label="Simulation readout">
        <h2>Readout</h2>
        <div class="stats">
          <div class="stat"><span class="v" id="sFps">—</span><span class="k">fps</span></div>
          <div class="stat"><span class="v" id="sOrder">—</span><span class="k">flock order</span></div>
          <div class="stat"><span class="v" id="sBoids">400</span><span class="k">boids</span></div>
          <div class="stat"><span class="v" id="sPred">0</span><span class="k">predators</span></div>
        </div>
        <span class="pill" id="pill"><span class="dot"></span><span id="pillText">idle</span></span>
        <div class="legend" aria-hidden="true">
          <span class="lbl">alone</span>
          <span class="bar" id="legendBar"></span>
          <span class="lbl">crowded</span>
        </div>
        <p class="cap">Colour shows crowding: <b>grey</b> on the fringe, <b>amber</b> in the thick of it. <b>Flock order</b> is how closely all 400 headings agree &mdash; <b>1.00</b> is one mind, and it sags whenever the flock splits into groups pulling different ways. Drop a predator and watch it fall.</p>
      </section>

      <section class="card" aria-label="Flocking rules">
        <h2>Rules</h2>

        <div class="ctl">
          <div class="ctl-top">
            <label for="coh">Cohesion</label><span class="val" id="cohV" aria-hidden="true">1.30</span>
          </div>
          <input type="range" id="coh" min="0" max="2" step="0.05" value="1.3">
        </div>

        <div class="ctl">
          <div class="ctl-top">
            <label for="ali">Alignment</label><span class="val" id="aliV" aria-hidden="true">1.55</span>
          </div>
          <input type="range" id="ali" min="0" max="2" step="0.05" value="1.55">
        </div>

        <div class="ctl">
          <div class="ctl-top">
            <label for="sep">Separation</label><span class="val" id="sepV" aria-hidden="true">1.05</span>
          </div>
          <input type="range" id="sep" min="0" max="2" step="0.05" value="1.05">
        </div>

        <div class="ctl">
          <div class="ctl-top">
            <label for="spd">Speed</label><span class="val" id="spdV" aria-hidden="true">×1.00</span>
          </div>
          <input type="range" id="spd" min="0.3" max="2.5" step="0.05" value="1">
        </div>

        <h3 class="sub">Presets</h3>
        <div class="presets" id="presets" role="group" aria-label="Rule presets">
          <button type="button" data-p="0" aria-pressed="true">Murmuration</button>
          <button type="button" data-p="1" aria-pressed="false">Tight swarm</button>
          <button type="button" data-p="2" aria-pressed="false">Highway</button>
          <button type="button" data-p="3" aria-pressed="false">Scatter</button>
        </div>
      </section>

    </div>
  </main>

  <footer>demo · sloprun.dev</footer>
</div>

<div class="sr-only" role="status" aria-live="polite" id="live"></div>

<script>
(function () {
  "use strict";

  /* ── theme plumbing ─────────────────────────────────────────────────────── */
  var root = document.documentElement;
  var mqDark = window.matchMedia ? matchMedia("(prefers-color-scheme: dark)") : null;
  var mqRM   = window.matchMedia ? matchMedia("(prefers-reduced-motion: reduce)") : null;

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    if (d && d.type === "sloprun:theme" && (d.theme === "light" || d.theme === "dark")) {
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    if (mqDark.addEventListener) mqDark.addEventListener("change", onScheme);
    else if (mqDark.addListener) mqDark.addListener(onScheme);
  }

  /* ── colour reading ─────────────────────────────────────────────────────── */
  var COL = { surface: [255,255,255], muted: [90,96,104], accent: [192,86,0], danger: [201,60,55], line: [227,226,220] };
  var RAMP_N = 12, ramp = [];

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  function mix(a, b, t) {
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    COL.muted   = tok("--muted",   COL.muted);
    COL.accent  = tok("--accent",  COL.accent);
    COL.danger  = tok("--danger",  COL.danger);
    COL.line    = tok("--line",    COL.line);
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    var lb = document.getElementById("legendBar");
    lb.style.background = "linear-gradient(90deg," + rgba(COL.muted, .7) + "," + rgba(COL.accent, 1) + ")";
  }

  /* ── dom ────────────────────────────────────────────────────────────────── */
  var field   = document.getElementById("field");
  var canvas  = document.getElementById("c");
  var ctx     = canvas.getContext("2d");
  var hint    = document.getElementById("hint");
  var overlay = document.getElementById("overlay");
  var overlayPlay = document.getElementById("overlayPlay");
  var overlayNote = document.getElementById("overlayNote");
  var sFps  = document.getElementById("sFps");
  var sPred = document.getElementById("sPred");
  var sOrder = document.getElementById("sOrder");
  var pill  = document.getElementById("pill");
  var pillText = document.getElementById("pillText");
  var live  = document.getElementById("live");
  var playBtn  = document.getElementById("playBtn");
  var resetBtn = document.getElementById("resetBtn");
  var dropBtn  = document.getElementById("dropBtn");
  var clearBtn = document.getElementById("clearBtn");
  var trailsEl = document.getElementById("trails");
  var presetsEl = document.getElementById("presets");
  var sliders = {
    coh: document.getElementById("coh"),
    ali: document.getElementById("ali"),
    sep: document.getElementById("sep"),
    spd: document.getElementById("spd")
  };
  var vals = {
    coh: document.getElementById("cohV"),
    ali: document.getElementById("aliV"),
    sep: document.getElementById("sepV"),
    spd: document.getElementById("spdV")
  };

  /* ── simulation state ───────────────────────────────────────────────────── */
  var N = 400;
  var px = new Float32Array(N), py = new Float32Array(N);
  var vx = new Float32Array(N), vy = new Float32Array(N);
  var ux = new Float32Array(N), uy = new Float32Array(N);   // unit headings, rebuilt each step
  var den = new Float32Array(N);                            // neighbours in view — drives colour

  var W = 0, H = 0, U = 30;           // U = mean spacing, the sim's length unit
  var R = 40, R2 = 1600, SEP = 22, SEP2 = 484;
  var BASE = 150, MAXS = 150, MAXF = 225, FLEE = 120, FLEE2 = 14400;
  var meanOrder = 0, orderInit = false, denAvg = 6;

  var wCoh = 1.3, wAli = 1.55, wSep = 1.05, spdMul = 1;
  var preds = [];
  var MAX_PRED = 6;

  var trails = false;
  var seeded = false;
  var userRunning = true;
  var active = false;
  var rafId = 0, lastT = 0;

  var reduced = !!(mqRM && mqRM.matches);

  /* ── spatial grid (toroidal) ────────────────────────────────────────────── */
  var cols = 1, rows = 1, cellW = 1, cellH = 1, buckets = [], useGrid = false;

  function rebuildGrid() {
    cols = Math.max(1, Math.floor(W / R));
    rows = Math.max(1, Math.floor(H / R));
    useGrid = (cols >= 3 && rows >= 3);
    cellW = W / cols; cellH = H / rows;
    var need = cols * rows;
    buckets = new Array(need);
    for (var i = 0; i < need; i++) buckets[i] = [];
  }

  function fillGrid() {
    for (var i = 0, n = buckets.length; i < n; i++) buckets[i].length = 0;
    for (var b = 0; b < N; b++) {
      var cx = Math.floor(px[b] / cellW); if (cx < 0) cx = 0; else if (cx >= cols) cx = cols - 1;
      var cy = Math.floor(py[b] / cellH); if (cy < 0) cy = 0; else if (cy >= rows) cy = rows - 1;
      buckets[cy * cols + cx].push(b);
    }
  }

  /* ── sizing ─────────────────────────────────────────────────────────────── */
  function resize() {
    var rect = field.getBoundingClientRect();
    var cw = Math.max(120, Math.round(rect.width));
    var ch = Math.max(120, Math.round(rect.height));
    if (cw === W && ch === H && canvas.width) return;

    var dpr = Math.min(2, window.devicePixelRatio || 1);
    canvas.width  = Math.round(cw * dpr);
    canvas.height = Math.round(ch * dpr);
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);

    if (W > 0 && H > 0) {
      var sx = cw / W, sy = ch / H, i;
      for (i = 0; i < N; i++) { px[i] *= sx; py[i] *= sy; }
      for (i = 0; i < preds.length; i++) { preds[i].x *= sx; preds[i].y *= sy; }
    }
    W = cw; H = ch;
    U = Math.sqrt(W * H / N);
    R = 1.35 * U; R2 = R * R;
    SEP = 0.44 * R; SEP2 = SEP * SEP;
    BASE = 5.0 * U;
    FLEE = 4.2 * U; FLEE2 = FLEE * FLEE;
    rebuildGrid();
    if (!active && seeded) draw(0);
  }

  /* ── seeding ────────────────────────────────────────────────────────────── */
  function seed(coherent) {
    // Default: three loose clusters pointing three different ways, so the opening
    // seconds show them finding each other. Coherent: one flock already of one mind —
    // used when the sim starts paused, where the first frame is the ONLY frame.
    var base = Math.random() * Math.PI * 2;
    for (var i = 0; i < N; i++) {
      var gx, gy, spread, hd;
      if (coherent) {
        // two lobes ~57 deg apart: a single blob settles at a degenerate-looking
        // 0.99, two braided subflocks land where a real murmuration sits (~0.90)
        var lobe = i & 1;
        gx = (0.36 + 0.28 * lobe) * W; gy = (0.44 + 0.14 * lobe) * H;
        spread = Math.min(W, H) * 0.30;
        hd = base + (lobe ? 1.0 : 0) + (Math.random() - 0.5) * 0.7;
      } else {
        var g = i % 3;
        gx = (0.25 + 0.25 * g) * W; gy = (0.32 + 0.18 * ((g * 2) % 3)) * H;
        spread = Math.min(W, H) * 0.22;
        hd = (g / 3) * Math.PI * 2 + (Math.random() - 0.5) * 0.9;
      }
      var a = Math.random() * Math.PI * 2;
      var r = Math.pow(Math.random(), coherent ? 0.5 : 0.6) * spread;
      px[i] = ((gx + Math.cos(a) * r) % W + W) % W;
      py[i] = ((gy + Math.sin(a) * r) % H + H) % H;
      var sp = BASE * spdMul * (0.8 + Math.random() * 0.2);
      vx[i] = Math.cos(hd) * sp; vy[i] = Math.sin(hd) * sp;
    }
    orderInit = false;
    seeded = true;
  }

  // Run the sim forward without painting, inside a strict time budget, so a paused
  // demo can still show a settled flock and a truthful order readout.
  function warmUp(steps, budgetMs) {
    var t0 = (window.performance && performance.now) ? performance.now() : 0;
    for (var i = 0; i < steps; i++) {
      step(1 / 60);
      if ((i & 15) === 15 && t0 && performance.now() - t0 > budgetMs) break;
    }
    sOrder.textContent = meanOrder.toFixed(2);
  }

  /* ── predators ──────────────────────────────────────────────────────────── */
  function addPredator(x, y) {
    if (preds.length >= MAX_PRED) preds.shift();
    var a = Math.random() * Math.PI * 2;
    preds.push({ x: x, y: y, vx: Math.cos(a) * BASE * 0.6, vy: Math.sin(a) * BASE * 0.6, wander: a });
    syncPred();
    say(preds.length === 1 ? "Predator dropped. The flock scatters." : preds.length + " predators in the field.");
    dismissHint();
    if (!active) draw(0);
  }
  function clearPredators() {
    if (!preds.length) return;
    preds.length = 0;
    syncPred();
    say("Predators cleared.");
    if (!active) draw(0);
  }
  function syncPred() {
    sPred.textContent = String(preds.length);
    clearBtn.disabled = preds.length === 0;
  }

  /* ── physics step ───────────────────────────────────────────────────────── */
  var nbrCells = new Int32Array(9);

  function step(dt) {
    MAXS = BASE * spdMul;
    MAXF = 1.5 * MAXS;

    // one sqrt per boid: unit headings feed both the alignment rule and the colour ramp
    for (var u = 0; u < N; u++) {
      var um = Math.sqrt(vx[u] * vx[u] + vy[u] * vy[u]);
      if (um > 1e-6) { ux[u] = vx[u] / um; uy[u] = vy[u] / um; }
      else { ux[u] = 1; uy[u] = 0; }
    }

    if (useGrid) fillGrid();
    var denSum = 0;

    var hw = W * 0.5, hh = H * 0.5;
    var i, j, k;

    for (i = 0; i < N; i++) {
      var x = px[i], y = py[i], bvx = vx[i], bvy = vy[i];
      var cnt = 0, sepCnt = 0;
      var cx = 0, cy = 0, ax = 0, ay = 0, sx = 0, sy = 0;

      if (useGrid) {
        var gcx = Math.floor(x / cellW); if (gcx < 0) gcx = 0; else if (gcx >= cols) gcx = cols - 1;
        var gcy = Math.floor(y / cellH); if (gcy < 0) gcy = 0; else if (gcy >= rows) gcy = rows - 1;
        var m = 0;
        for (var oy = -1; oy <= 1; oy++) {
          var ry = (gcy + oy + rows) % rows;
          for (var ox = -1; ox <= 1; ox++) {
            nbrCells[m++] = ry * cols + ((gcx + ox + cols) % cols);
          }
        }
        for (k = 0; k < 9; k++) {
          var list = buckets[nbrCells[k]];
          for (var q = 0, ql = list.length; q < ql; q++) {
            j = list[q]; if (j === i) continue;
            var dx = px[j] - x; if (dx > hw) dx -= W; else if (dx < -hw) dx += W;
            var dy = py[j] - y; if (dy > hh) dy -= H; else if (dy < -hh) dy += H;
            var d2 = dx * dx + dy * dy;
            if (d2 > 0 && d2 < R2) {
              cnt++; cx += dx; cy += dy; ax += ux[j]; ay += uy[j];
              if (d2 < SEP2) {
                var d = Math.sqrt(d2);
                var f = (SEP - d) / SEP;
                sx -= (dx / d) * f; sy -= (dy / d) * f; sepCnt++;
              }
            }
          }
        }
      } else {
        for (j = 0; j < N; j++) {
          if (j === i) continue;
          var dx2 = px[j] - x; if (dx2 > hw) dx2 -= W; else if (dx2 < -hw) dx2 += W;
          var dy2 = py[j] - y; if (dy2 > hh) dy2 -= H; else if (dy2 < -hh) dy2 += H;
          var dd2 = dx2 * dx2 + dy2 * dy2;
          if (dd2 > 0 && dd2 < R2) {
            cnt++; cx += dx2; cy += dy2; ax += ux[j]; ay += uy[j];
            if (dd2 < SEP2) {
              var dd = Math.sqrt(dd2);
              var ff = (SEP - dd) / SEP;
              sx -= (dx2 / dd) * ff; sy -= (dy2 / dd) * ff; sepCnt++;
            }
          }
        }
      }

      var accx = 0, accy = 0, mag, tx, ty;

      den[i] = cnt; denSum += cnt;

      if (cnt > 0) {
        // alignment: match the average heading of neighbours
        if (wAli > 0) {
          tx = ax; ty = ay;
          mag = Math.sqrt(tx * tx + ty * ty);
          if (mag > 1e-6) {
            var adx = (tx / mag) * MAXS - bvx, ady = (ty / mag) * MAXS - bvy;
            var am = Math.sqrt(adx * adx + ady * ady);
            if (am > MAXF) { adx = adx / am * MAXF; ady = ady / am * MAXF; }
            accx += adx * wAli; accy += ady * wAli;
          }
        }
        // cohesion: steer toward the local centre of mass, but fade it out once you
        // are already at the centre — without this dead zone the flock collapses into
        // a single hard ball instead of a murmuration
        if (wCoh > 0) {
          tx = cx / cnt; ty = cy / cnt;
          mag = Math.sqrt(tx * tx + ty * ty);
          if (mag > 1e-6) {
            var near = mag / (0.30 * R); if (near > 1) near = 1;
            var cdx = (tx / mag) * MAXS - bvx, cdy = (ty / mag) * MAXS - bvy;
            var cm = Math.sqrt(cdx * cdx + cdy * cdy);
            if (cm > MAXF) { cdx = cdx / cm * MAXF; cdy = cdy / cm * MAXF; }
            accx += cdx * wCoh * near; accy += cdy * wCoh * near;
          }
        }
      }
      // separation: push away from anyone too close
      if (sepCnt > 0 && wSep > 0) {
        mag = Math.sqrt(sx * sx + sy * sy);
        if (mag > 1e-6) {
          var sdx = (sx / mag) * MAXS - bvx, sdy = (sy / mag) * MAXS - bvy;
          var sm = Math.sqrt(sdx * sdx + sdy * sdy);
          if (sm > MAXF) { sdx = sdx / sm * MAXF; sdy = sdy / sm * MAXF; }
          accx += sdx * wSep * 1.8; accy += sdy * wSep * 1.8;
        }
      }
      // flee: predators override everything nearby
      for (k = 0; k < preds.length; k++) {
        var p = preds[k];
        var pdx = p.x - x; if (pdx > hw) pdx -= W; else if (pdx < -hw) pdx += W;
        var pdy = p.y - y; if (pdy > hh) pdy -= H; else if (pdy < -hh) pdy += H;
        var pd2 = pdx * pdx + pdy * pdy;
        if (pd2 < FLEE2 && pd2 > 1e-6) {
          var pd = Math.sqrt(pd2);
          var w = 1 - pd / FLEE;
          var fdx = (-pdx / pd) * MAXS - bvx, fdy = (-pdy / pd) * MAXS - bvy;
          var fm = Math.sqrt(fdx * fdx + fdy * fdy);
          var lim = MAXF * 3;
          if (fm > lim) { fdx = fdx / fm * lim; fdy = fdy / fm * lim; }
          accx += fdx * w * 3.2; accy += fdy * w * 3.2;
        }
      }

      bvx += accx * dt; bvy += accy * dt;
      // birds cruise: steering turns them, it does not stall them. Hard turns bleed a
      // little speed, and they spend it back over the next fraction of a second.
      var s = Math.sqrt(bvx * bvx + bvy * bvy);
      if (s < 1e-4) {
        var ra = Math.random() * 6.2831853;
        bvx = Math.cos(ra) * MAXS; bvy = Math.sin(ra) * MAXS;
      } else {
        var want = s + (MAXS - s) * Math.min(1, dt * 6);
        if (want > MAXS) want = MAXS;
        else if (want < MAXS * 0.72) want = MAXS * 0.72;
        bvx = bvx / s * want; bvy = bvy / s * want;
      }
      vx[i] = bvx; vy[i] = bvy;
      var nx = x + bvx * dt, ny = y + bvy * dt;
      if (nx < 0) nx += W; else if (nx >= W) nx -= W;
      if (ny < 0) ny += H; else if (ny >= H) ny -= H;
      px[i] = nx; py[i] = ny;
    }

    // Flock order = the Vicsek order parameter, |Σ v̂| / N over ALL N birds: 1.00 when
    // every bird points the same way, ~0 when the headings cancel out. This is a global
    // measure on purpose — averaging *local* neighbour agreement instead reads ~0.95 even
    // with all three rules switched off, because neighbours are trivially alike.
    var gsx = 0, gsy = 0;
    for (i = 0; i < N; i++) {
      var gm = Math.sqrt(vx[i] * vx[i] + vy[i] * vy[i]);
      if (gm > 1e-6) { gsx += vx[i] / gm; gsy += vy[i] / gm; }
    }
    var orderNow = Math.sqrt(gsx * gsx + gsy * gsy) / N;
    // light easing (~0.2 s) so the digits read as an instrument, not a strobe
    if (!orderInit) { meanOrder = orderNow; orderInit = true; }
    else meanOrder += (orderNow - meanOrder) * Math.min(1, dt * 5);
    // the flock condenses far past uniform density, so normalise colour against the
    // flock's own current crowding, eased so the ramp never flickers
    denAvg += (denSum / N - denAvg) * Math.min(1, dt * 2);
    if (denAvg < 1) denAvg = 1;
    stepPredators(dt, hw, hh);
  }

  function stepPredators(dt, hw, hh) {
    var pspeed = MAXS * 0.9, hunt = 7 * U, hunt2 = hunt * hunt;
    for (var k = 0; k < preds.length; k++) {
      var p = preds[k], tx = 0, ty = 0, cnt = 0;
      for (var i = 0; i < N; i++) {
        var dx = px[i] - p.x; if (dx > hw) dx -= W; else if (dx < -hw) dx += W;
        var dy = py[i] - p.y; if (dy > hh) dy -= H; else if (dy < -hh) dy += H;
        if (dx * dx + dy * dy < hunt2) { tx += dx; ty += dy; cnt++; }
      }
      var dvx, dvy;
      if (cnt > 0) {
        var m = Math.sqrt(tx * tx + ty * ty);
        if (m > 1e-6) { dvx = (tx / m) * pspeed; dvy = (ty / m) * pspeed; }
        else { dvx = p.vx; dvy = p.vy; }
      } else {
        p.wander += (Math.random() - 0.5) * 1.6 * dt;
        dvx = Math.cos(p.wander) * pspeed; dvy = Math.sin(p.wander) * pspeed;
      }
      // ease toward the desired velocity so the chase reads as a curve, not a snap
      var e = Math.min(1, dt * 1.9);
      p.vx += (dvx - p.vx) * e; p.vy += (dvy - p.vy) * e;
      var sp = Math.sqrt(p.vx * p.vx + p.vy * p.vy);
      if (sp > pspeed) { p.vx = p.vx / sp * pspeed; p.vy = p.vy / sp * pspeed; }
      p.x += p.vx * dt; p.y += p.vy * dt;
      if (p.x < 0) p.x += W; else if (p.x >= W) p.x -= W;
      if (p.y < 0) p.y += H; else if (p.y >= H) p.y -= H;
      if (cnt === 0) p.wander = Math.atan2(p.vy, p.vx);
    }
  }

  /* ── rendering ──────────────────────────────────────────────────────────── */
  var groups = [];
  for (var gi = 0; gi < RAMP_N; gi++) groups.push([]);

  // triangle for one bird, appended to whatever path is already open
  function tri(x, y, hx, hy, L, Wd) {
    var bx = x - hx * L * 0.45, by = y - hy * L * 0.45;
    ctx.moveTo(x + hx * L * 0.55, y + hy * L * 0.55);
    ctx.lineTo(bx - hy * Wd, by + hx * Wd);
    ctx.lineTo(bx + hy * Wd, by - hx * Wd);
    ctx.closePath();
  }

  var offX = [0, 0], offY = [0, 0];

  // dt > 0 means "another animation frame"; dt <= 0 is a static repaint (paused, theme
  // swap, slider nudge). Static repaints must be idempotent: fading and re-stamping on
  // every `input` event would scrub the trails away and pile the predator ring up to
  // full opacity, so they always start from a clean canvas instead.
  function draw(dt) {
    if (!W || !H) return;

    var live = dt > 0;
    var fade = 1;
    if (trails && live) {
      fade = 1 - Math.pow(0.86, dt * 60);
      ctx.globalCompositeOperation = "destination-out";
      ctx.fillStyle = "rgba(0,0,0," + fade.toFixed(4) + ")";
      ctx.fillRect(0, 0, W, H);
      ctx.globalCompositeOperation = "source-over";
    } else {
      ctx.clearRect(0, 0, W, H);
    }

    var i, k, ox, oy, nox, noy;

    // predator influence fields, drawn under the flock. The field wraps like the sim
    // does, so a predator near an edge still shows the ring the birds across the seam
    // are actually fleeing from.
    for (k = 0; k < preds.length; k++) {
      var p = preds[k];
      // with trails on this ring is re-stamped every frame, so pre-scale it by the fade
      // rate to land on the same steady-state opacity as the no-trails path
      ctx.fillStyle = rgba(COL.danger, 0.055 * fade);
      ctx.strokeStyle = rgba(COL.danger, 0.22 * fade);
      ctx.lineWidth = 1;
      nox = 1; noy = 1; offX[0] = 0; offY[0] = 0;
      if (p.x < FLEE) { offX[1] = W; nox = 2; } else if (p.x > W - FLEE) { offX[1] = -W; nox = 2; }
      if (p.y < FLEE) { offY[1] = H; noy = 2; } else if (p.y > H - FLEE) { offY[1] = -H; noy = 2; }
      for (ox = 0; ox < nox; ox++) {
        for (oy = 0; oy < noy; oy++) {
          ctx.beginPath();
          ctx.arc(p.x + offX[ox], p.y + offY[oy], FLEE, 0, 6.2831853);
          ctx.fill(); ctx.stroke();
        }
      }
    }

    // bucket boids by crowding so we issue ~12 fills instead of 400 strokes
    for (k = 0; k < RAMP_N; k++) groups[k].length = 0;
    var dLo = 0.25 * denAvg, dSpan = 1.6 * denAvg;
    for (i = 0; i < N; i++) {
      var t = (den[i] - dLo) / dSpan;
      var b = (t * RAMP_N) | 0;
      if (b < 0) b = 0; else if (b >= RAMP_N) b = RAMP_N - 1;
      groups[b].push(i);
    }

    var L = Math.max(5, U * 0.30), Wd = L * 0.44;
    for (k = 0; k < RAMP_N; k++) {
      var g = groups[k];
      if (!g.length) continue;
      ctx.beginPath();
      for (var q = 0; q < g.length; q++) {
        var id = g[q];
        var hx = vx[id], hy = vy[id];
        var m = Math.sqrt(hx * hx + hy * hy) || 1;
        hx /= m; hy /= m;
        var x = px[id], y = py[id];
        tri(x, y, hx, hy, L, Wd);
        // the space is a torus, so a bird straddling an edge is drawn on both sides —
        // otherwise it winks out at the border instead of gliding through it
        var wrapX = x < L ? W : (x > W - L ? -W : 0);
        var wrapY = y < L ? H : (y > H - L ? -H : 0);
        if (wrapX) tri(x + wrapX, y, hx, hy, L, Wd);
        if (wrapY) tri(x, y + wrapY, hx, hy, L, Wd);
        if (wrapX && wrapY) tri(x + wrapX, y + wrapY, hx, hy, L, Wd);
      }
      ctx.fillStyle = ramp[k];
      ctx.fill();
    }

    // predators on top
    var PL = Math.max(11, U * 0.8), PW = PL * 0.46;
    for (k = 0; k < preds.length; k++) {
      var pr = preds[k];
      var pm = Math.sqrt(pr.vx * pr.vx + pr.vy * pr.vy) || 1;
      var ax = pr.vx / pm, ay = pr.vy / pm;
      ctx.fillStyle = rgba(COL.danger, 1);
      ctx.lineWidth = 1.5;
      ctx.strokeStyle = rgba(COL.surface, 0.85);
      nox = 1; noy = 1; offX[0] = 0; offY[0] = 0;
      if (pr.x < PL) { offX[1] = W; nox = 2; } else if (pr.x > W - PL) { offX[1] = -W; nox = 2; }
      if (pr.y < PL) { offY[1] = H; noy = 2; } else if (pr.y > H - PL) { offY[1] = -H; noy = 2; }
      for (ox = 0; ox < nox; ox++) {
        for (oy = 0; oy < noy; oy++) {
          var prx = pr.x + offX[ox], pry = pr.y + offY[oy];
          var pbx = prx - ax * PL * 0.45, pby = pry - ay * PL * 0.45;
          ctx.beginPath();
          ctx.moveTo(prx + ax * PL * 0.6, pry + ay * PL * 0.6);
          ctx.lineTo(pbx - ay * PW, pby + ax * PW);
          ctx.lineTo(pbx + ax * PL * 0.18, pby + ay * PL * 0.18);
          ctx.lineTo(pbx + ay * PW, pby - ax * PW);
          ctx.closePath();
          ctx.fill();
          ctx.stroke();
        }
      }
    }
  }

  /* ── loop ───────────────────────────────────────────────────────────────── */
  var fCnt = 0, fShown = 0;

  function frame(now) {
    rafId = requestAnimationFrame(frame);
    var dt = (now - lastT) / 1000;
    lastT = now;
    if (!(dt > 0)) return;
    if (dt > 0.05) dt = 0.05;

    step(dt);
    draw(dt);

    fCnt++;
    // wall clock, not the clamped sim dt — otherwise a 10fps device reports 20
    var span = now - fShown;
    if (span > 260) {
      var fps = fCnt * 1000 / span;
      fShown = now; fCnt = 0;
      sFps.textContent = String(Math.round(fps));
      sFps.className = fps < 40 ? "v warn" : "v";
      sOrder.textContent = meanOrder.toFixed(2);
    }
  }

  function start() {
    if (active) return;
    active = true;
    lastT = performance.now();
    fCnt = 0; fShown = lastT;
    rafId = requestAnimationFrame(frame);
    setStatus();
  }
  function stop() {
    if (!active) return;
    active = false;
    cancelAnimationFrame(rafId);
    sFps.textContent = "—";
    sFps.className = "v";
    // land on the crisp still frame the static repaint path will keep redrawing, so a
    // paused canvas does not sit on a half-faded trail smear
    if (seeded) draw(0);
    setStatus();
  }

  function setStatus() {
    var on = active;
    pill.className = on ? "pill on" : "pill";
    pillText.textContent = on ? "running" : "paused";
    playBtn.textContent = userRunning ? "Pause" : "Play";
    playBtn.setAttribute("aria-label", userRunning ? "Pause simulation" : "Play simulation");
    overlay.hidden = userRunning;
  }

  function setRunning(on) {
    userRunning = on;
    if (on && !document.hidden) start(); else stop();
    setStatus();
    if (on) revealHint();
    say(on ? "Simulation running." : "Simulation paused.");
  }

  /* ── controls ───────────────────────────────────────────────────────────── */
  var PRESETS = [
    { name: "Murmuration", coh: 1.30, ali: 1.55, sep: 1.05, spd: 1.00 },
    { name: "Tight swarm", coh: 2.00, ali: 0.35, sep: 1.55, spd: 0.75 },
    { name: "Highway",     coh: 0.55, ali: 2.00, sep: 1.15, spd: 1.70 },
    { name: "Scatter",     coh: 0.10, ali: 0.20, sep: 1.90, spd: 1.35 }
  ];

  function fmt(key, v) { return key === "spd" ? "×" + v.toFixed(2) : v.toFixed(2); }

  function syncSlider(key) {
    var v = parseFloat(sliders[key].value);
    vals[key].textContent = fmt(key, v);
    if (key === "coh") wCoh = v;
    else if (key === "ali") wAli = v;
    else if (key === "sep") wSep = v;
    else spdMul = v;
  }

  Object.keys(sliders).forEach(function (key) {
    sliders[key].addEventListener("input", function () {
      syncSlider(key);
      markPreset(-1);
      if (!active) draw(0);
    });
  });

  function markPreset(idx) {
    var btns = presetsEl.querySelectorAll("button");
    for (var i = 0; i < btns.length; i++) {
      btns[i].setAttribute("aria-pressed", String(i === idx));
    }
  }

  presetsEl.addEventListener("click", function (e) {
    var btn = e.target.closest ? e.target.closest("button[data-p]") : null;
    if (!btn) return;
    var idx = parseInt(btn.getAttribute("data-p"), 10);
    var p = PRESETS[idx];
    sliders.coh.value = p.coh; sliders.ali.value = p.ali;
    sliders.sep.value = p.sep; sliders.spd.value = p.spd;
    ["coh", "ali", "sep", "spd"].forEach(syncSlider);
    markPreset(idx);
    say(p.name + " preset applied.");
    dismissHint();
    if (!active) draw(0);
  });

  trailsEl.addEventListener("change", function () {
    trails = trailsEl.checked;
    ctx.clearRect(0, 0, W, H);
    if (!active) draw(0);
  });

  playBtn.addEventListener("click", function () { setRunning(!userRunning); });
  overlayPlay.addEventListener("click", function () { setRunning(true); canvas.focus(); });
  // the scrim covers the whole field, so a click on it should mean "play", not nothing
  overlay.addEventListener("click", function (e) { if (e.target === overlay) setRunning(true); });

  resetBtn.addEventListener("click", function () {
    // reset means "back to the opening state", and the opening state differs: running
    // starts scattered so you can watch it organise, paused must land on a settled
    // still frame because no frame after this one is coming
    var still = !active;
    seed(still);
    preds.length = 0; syncPred();
    ctx.clearRect(0, 0, W, H);
    if (still) { warmUp(150, 110); draw(0); }
    say(still ? "Flock reset." : "Flock reset. 400 boids re-scattered.");
  });

  dropBtn.addEventListener("click", function () {
    var i = (Math.random() * N) | 0;      // drop it right on top of a bird — instant drama
    addPredator(px[i], py[i]);
  });
  clearBtn.addEventListener("click", clearPredators);

  function canvasPoint(e) {
    var r = canvas.getBoundingClientRect();
    return { x: (e.clientX - r.left) / r.width * W, y: (e.clientY - r.top) / r.height * H };
  }
  canvas.addEventListener("pointerdown", function (e) {
    if (e.button !== undefined && e.button !== 0) return;
    var p = canvasPoint(e);
    addPredator(p.x, p.y);
    canvas.classList.add("ptr-focus");
    canvas.focus();
  });
  canvas.addEventListener("blur", function () { canvas.classList.remove("ptr-focus"); });
  canvas.addEventListener("keydown", function (e) {
    canvas.classList.remove("ptr-focus");
    if (e.key === "Enter" || e.key === " " || e.key === "Spacebar") {
      e.preventDefault();
      var i = (Math.random() * N) | 0;
      addPredator(px[i], py[i]);
    }
  });

  var hintTimer = 0, hintShown = false;
  // a reduced-motion visitor opens behind the play overlay, where "click the field"
  // would mean "resume", so the hint waits until they have actually pressed play
  function revealHint() {
    if (hintShown) return;
    hintShown = true;
    hint.hidden = false;
    hintTimer = setTimeout(dismissHint, 7000);
  }
  function dismissHint() {
    hintShown = true;
    if (hint.hidden || hint.classList.contains("gone")) return;
    hint.classList.add("gone");
    clearTimeout(hintTimer);
    setTimeout(function () { hint.hidden = true; }, 500);
  }

  var sayTimer = 0;
  function say(msg) {
    clearTimeout(sayTimer);
    sayTimer = setTimeout(function () { live.textContent = msg; }, 120);
  }

  /* ── lifecycle ──────────────────────────────────────────────────────────── */
  document.addEventListener("visibilitychange", function () {
    if (document.hidden) stop();
    else if (userRunning) start();
  });

  if (window.ResizeObserver) {
    new ResizeObserver(function () { resize(); }).observe(field);
  } else {
    window.addEventListener("resize", resize);
  }
  window.addEventListener("resize", resize);

  /* ── boot ───────────────────────────────────────────────────────────────── */
  readTheme();
  resize();
  // pin the rules to the Murmuration preset before seeding: a browser that restores
  // form values across a reload must not leave the sliders, the highlighted preset
  // and the seed speed disagreeing with each other
  sliders.coh.value = PRESETS[0].coh; sliders.ali.value = PRESETS[0].ali;
  sliders.sep.value = PRESETS[0].sep; sliders.spd.value = PRESETS[0].spd;
  ["coh", "ali", "sep", "spd"].forEach(syncSlider);
  markPreset(0);
  syncPred();

  if (reduced) {
    userRunning = false;
    trailsEl.checked = false; trails = false;
    overlayNote.textContent = "Your system asks for reduced motion, so this simulation starts paused. It animates continuously once you press play.";
    // The still frame is the only frame a reduced-motion visitor sees before they opt
    // in, so seed a flock that is already of one mind and settle it just long enough
    // for real local structure — the raw three-cluster seed reads as random confetti,
    // and integrating it to a murmuration takes ~15s of sim, far past any load budget.
    seed(true);
    warmUp(150, 110);
    draw(0);
    setStatus();
    hint.hidden = true;
  } else {
    seed();
    trails = trailsEl.checked;   // honour a checkbox the browser restored on reload
    setStatus();
    start();
    revealHint();
  }
})();
</script>
</body>
</html>