Particle Field + Cursor

:root {
  --bg: #0c0d10;
  --surface: #15161b;
  --surface-2: #1d1f27;
  --ink: #e9eaf0;
  --muted: #9a9cab;
  --accent: #8b5cf6;
  --accent-2: #22d3ee;
  --ok: #34d399;
  --warn: #fbbf24;
  --danger: #f87171;
  --line: rgba(255, 255, 255, 0.1);
  --line-2: rgba(255, 255, 255, 0.18);
  --r-sm: 8px;
  --r-md: 12px;
  --r-lg: 16px;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

html,
body {
  height: 100%;
}

body {
  margin: 0;
  background:
    radial-gradient(1200px 700px at 80% -10%, rgba(139, 92, 246, 0.18), transparent 60%),
    radial-gradient(900px 600px at 0% 110%, rgba(34, 211, 238, 0.12), transparent 55%),
    var(--bg);
  color: var(--ink);
  font-family: "Inter", system-ui, -apple-system, sans-serif;
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
}

.stage {
  position: relative;
  width: 100%;
  height: 100vh;
  height: 100dvh;
  overflow: hidden;
}

.field {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  display: block;
  cursor: crosshair;
  touch-action: none;
}

/* Titlebar */
.titlebar {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  padding: 22px 26px;
  display: flex;
  flex-direction: column;
  gap: 4px;
  pointer-events: none;
  z-index: 2;
}

.titlebar__brand {
  display: flex;
  align-items: center;
  gap: 10px;
}

.titlebar__brand .dot {
  width: 10px;
  height: 10px;
  border-radius: 50%;
  background: var(--accent);
  box-shadow: 0 0 0 4px rgba(139, 92, 246, 0.18), 0 0 14px var(--accent);
}

.titlebar h1 {
  margin: 0;
  font-size: 15px;
  font-weight: 600;
  letter-spacing: 0.2px;
}

.titlebar__hint {
  margin: 0;
  font-size: 12.5px;
  color: var(--muted);
}

/* Panel */
.panel {
  position: absolute;
  right: 20px;
  bottom: 20px;
  width: 268px;
  padding: 16px 16px 14px;
  z-index: 3;
  background: color-mix(in srgb, var(--surface) 82%, transparent);
  border: 1px solid var(--line);
  border-radius: var(--r-lg);
  backdrop-filter: blur(14px) saturate(120%);
  -webkit-backdrop-filter: blur(14px) saturate(120%);
  box-shadow: 0 18px 50px -20px rgba(0, 0, 0, 0.7);
}

.panel__head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: 14px;
}

.panel__head h2 {
  margin: 0;
  font-size: 12px;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.14em;
  color: var(--muted);
}

.stat {
  display: inline-flex;
  align-items: baseline;
  gap: 4px;
  padding: 3px 9px;
  border-radius: 999px;
  background: var(--surface-2);
  border: 1px solid var(--line);
  font-family: "JetBrains Mono", ui-monospace, monospace;
}

.stat__val {
  font-size: 13px;
  font-weight: 500;
  color: var(--accent-2);
  min-width: 20px;
  text-align: right;
}

.stat__unit {
  font-size: 10px;
  color: var(--muted);
}

.control {
  margin-bottom: 14px;
}

.control label,
.control__label {
  display: block;
  font-size: 12px;
  color: var(--ink);
  margin-bottom: 8px;
  font-weight: 500;
}

.control output {
  float: right;
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 12px;
  color: var(--muted);
}

.control output .u {
  opacity: 0.6;
  margin-left: 1px;
}

input[type="range"] {
  -webkit-appearance: none;
  appearance: none;
  width: 100%;
  height: 4px;
  border-radius: 999px;
  background: linear-gradient(90deg, var(--accent), var(--accent-2));
  background-repeat: no-repeat;
  outline: none;
  cursor: pointer;
}

input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none;
  appearance: none;
  width: 16px;
  height: 16px;
  border-radius: 50%;
  background: var(--ink);
  border: 3px solid var(--surface);
  box-shadow: 0 0 0 1px var(--line-2), 0 2px 6px rgba(0, 0, 0, 0.5);
  transition: transform 0.12s ease;
}

input[type="range"]::-webkit-slider-thumb:hover {
  transform: scale(1.12);
}

input[type="range"]::-moz-range-thumb {
  width: 16px;
  height: 16px;
  border-radius: 50%;
  background: var(--ink);
  border: 3px solid var(--surface);
  box-shadow: 0 0 0 1px var(--line-2);
}

input[type="range"]:focus-visible {
  box-shadow: 0 0 0 3px rgba(139, 92, 246, 0.4);
}

/* Segmented control */
.control--seg {
  margin-bottom: 16px;
}

.seg {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 4px;
  padding: 4px;
  background: var(--surface-2);
  border: 1px solid var(--line);
  border-radius: var(--r-md);
}

.seg__btn {
  appearance: none;
  border: 0;
  padding: 7px 0;
  border-radius: var(--r-sm);
  background: transparent;
  color: var(--muted);
  font: inherit;
  font-size: 12.5px;
  font-weight: 500;
  cursor: pointer;
  transition: background 0.15s ease, color 0.15s ease;
}

.seg__btn:hover {
  color: var(--ink);
}

.seg__btn.is-active {
  background: linear-gradient(180deg, var(--accent), color-mix(in srgb, var(--accent) 80%, black));
  color: #fff;
  box-shadow: 0 2px 10px -2px rgba(139, 92, 246, 0.6);
}

.seg__btn:focus-visible {
  outline: 2px solid var(--accent-2);
  outline-offset: 2px;
}

/* Footer */
.panel__foot {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding-top: 12px;
  border-top: 1px solid var(--line);
}

.btn {
  appearance: none;
  border: 1px solid var(--line-2);
  background: var(--surface-2);
  color: var(--ink);
  font: inherit;
  font-size: 12.5px;
  font-weight: 500;
  padding: 7px 16px;
  border-radius: var(--r-sm);
  cursor: pointer;
  transition: background 0.15s ease, border-color 0.15s ease, transform 0.06s ease;
}

.btn:hover {
  background: #262833;
  border-color: var(--accent);
}

.btn:active {
  transform: translateY(1px);
}

.btn[aria-pressed="true"] {
  color: var(--warn);
  border-color: color-mix(in srgb, var(--warn) 50%, var(--line-2));
}

.btn:focus-visible {
  outline: 2px solid var(--accent-2);
  outline-offset: 2px;
}

.panel__meta {
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 11.5px;
  color: var(--muted);
}

.panel__meta #linkCount {
  color: var(--accent-2);
}

@media (max-width: 520px) {
  .titlebar {
    padding: 16px 18px;
  }
  .titlebar__hint {
    display: none;
  }
  .panel {
    left: 14px;
    right: 14px;
    bottom: 14px;
    width: auto;
  }
}

(function () {
  "use strict";

  var canvas = document.getElementById("field");
  var ctx = canvas && canvas.getContext ? canvas.getContext("2d") : null;

  // Graceful fallback if Canvas 2D is unavailable.
  if (!ctx) {
    if (canvas) {
      canvas.setAttribute("aria-hidden", "false");
      canvas.outerHTML =
        '<p style="position:absolute;inset:0;display:grid;place-items:center;color:#9a9cab">' +
        "Canvas 2D is not supported in this browser.</p>";
    }
    return;
  }

  // --- Controls -------------------------------------------------------------
  var countEl = document.getElementById("count");
  var countOut = document.getElementById("countOut");
  var distEl = document.getElementById("dist");
  var distOut = document.getElementById("distOut");
  var toggleBtn = document.getElementById("toggle");
  var fpsEl = document.getElementById("fps");
  var linkCountEl = document.getElementById("linkCount");
  var segBtns = Array.prototype.slice.call(document.querySelectorAll(".seg__btn"));

  var reduceMotion =
    window.matchMedia && window.matchMedia("(prefers-reduced-motion: reduce)").matches;

  // --- State ----------------------------------------------------------------
  var dpr = Math.min(window.devicePixelRatio || 1, 2);
  var W = 0; // CSS pixels
  var H = 0;
  var particles = [];
  var targetCount = parseInt(countEl.value, 10);
  var linkDist = parseInt(distEl.value, 10);
  var mode = "attract"; // 'attract' | 'repel'
  var running = !reduceMotion;

  var pointer = { x: -9999, y: -9999, active: false };
  var CURSOR_RADIUS = 150; // influence radius in CSS px
  var CURSOR_FORCE = 0.55;

  // Accent colors sampled from the design tokens.
  var PARTICLE_RGB = [186, 176, 255]; // soft violet-white
  var LINE_RGB = [139, 92, 246]; // --accent

  function rand(a, b) {
    return a + Math.random() * (b - a);
  }

  function makeParticle() {
    return {
      x: Math.random() * W,
      y: Math.random() * H,
      vx: rand(-0.35, 0.35),
      vy: rand(-0.35, 0.35),
      r: rand(0.8, 2.2),
    };
  }

  function syncCount() {
    while (particles.length < targetCount) particles.push(makeParticle());
    if (particles.length > targetCount) particles.length = targetCount;
  }

  // --- Sizing (DPR-aware) ---------------------------------------------------
  function resize() {
    var rect = canvas.getBoundingClientRect();
    W = Math.max(1, rect.width);
    H = Math.max(1, rect.height);
    dpr = Math.min(window.devicePixelRatio || 1, 2);
    canvas.width = Math.round(W * dpr);
    canvas.height = Math.round(H * dpr);
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
  }

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

  // --- Physics --------------------------------------------------------------
  function step() {
    var i, p;
    for (i = 0; i < particles.length; i++) {
      p = particles[i];

      // Cursor force
      if (pointer.active) {
        var dx = p.x - pointer.x;
        var dy = p.y - pointer.y;
        var d2 = dx * dx + dy * dy;
        var R = CURSOR_RADIUS;
        if (d2 < R * R && d2 > 0.01) {
          var d = Math.sqrt(d2);
          var falloff = (1 - d / R) * CURSOR_FORCE;
          var nx = dx / d;
          var ny = dy / d;
          var sign = mode === "repel" ? 1 : -1;
          p.vx += nx * falloff * sign;
          p.vy += ny * falloff * sign;
        }
      }

      // Integrate + gentle damping so the field settles back to a drift.
      p.vx *= 0.96;
      p.vy *= 0.96;
      // Keep a minimum ambient drift alive.
      var speed = Math.hypot(p.vx, p.vy);
      if (speed < 0.12) {
        p.vx += rand(-0.06, 0.06);
        p.vy += rand(-0.06, 0.06);
      }

      p.x += p.vx;
      p.y += p.vy;

      // Wrap around edges.
      if (p.x < -5) p.x = W + 5;
      else if (p.x > W + 5) p.x = -5;
      if (p.y < -5) p.y = H + 5;
      else if (p.y > H + 5) p.y = -5;
    }
  }

  // --- Rendering with a spatial grid for the connection pass -----------------
  var links = 0;

  function draw() {
    ctx.clearRect(0, 0, W, H);
    links = 0;

    var cell = linkDist;
    var cols = Math.max(1, Math.ceil(W / cell));
    var rows = Math.max(1, Math.ceil(H / cell));
    var grid = new Array(cols * rows);
    var i, p;

    for (i = 0; i < particles.length; i++) {
      p = particles[i];
      var cx = Math.min(cols - 1, Math.max(0, (p.x / cell) | 0));
      var cy = Math.min(rows - 1, Math.max(0, (p.y / cell) | 0));
      var idx = cy * cols + cx;
      p._cx = cx;
      p._cy = cy;
      (grid[idx] || (grid[idx] = [])).push(p);
    }

    // Connection lines — only compare against neighbouring cells.
    var maxD2 = linkDist * linkDist;
    for (i = 0; i < particles.length; i++) {
      p = particles[i];
      for (var oy = -1; oy <= 1; oy++) {
        for (var ox = -1; ox <= 1; ox++) {
          var gx = p._cx + ox;
          var gy = p._cy + oy;
          if (gx < 0 || gy < 0 || gx >= cols || gy >= rows) continue;
          var bucket = grid[gy * cols + gx];
          if (!bucket) continue;
          for (var j = 0; j < bucket.length; j++) {
            var q = bucket[j];
            if (q === p) continue;
            // Avoid drawing each pair twice.
            if (q.x < p.x || (q.x === p.x && q.y <= p.y)) continue;
            var ddx = p.x - q.x;
            var ddy = p.y - q.y;
            var dd2 = ddx * ddx + ddy * ddy;
            if (dd2 > maxD2) continue;
            var alpha = (1 - Math.sqrt(dd2) / linkDist) * 0.55;
            ctx.strokeStyle =
              "rgba(" + LINE_RGB[0] + "," + LINE_RGB[1] + "," + LINE_RGB[2] + "," + alpha + ")";
            ctx.lineWidth = 1;
            ctx.beginPath();
            ctx.moveTo(p.x, p.y);
            ctx.lineTo(q.x, q.y);
            ctx.stroke();
            links++;
          }
        }
      }
    }

    // Particles on top.
    var pr = PARTICLE_RGB;
    for (i = 0; i < particles.length; i++) {
      p = particles[i];
      var glow = 0;
      if (pointer.active) {
        var mdx = p.x - pointer.x;
        var mdy = p.y - pointer.y;
        var md2 = mdx * mdx + mdy * mdy;
        if (md2 < CURSOR_RADIUS * CURSOR_RADIUS) {
          glow = 1 - Math.sqrt(md2) / CURSOR_RADIUS;
        }
      }
      var rr = p.r + glow * 1.6;
      ctx.beginPath();
      ctx.arc(p.x, p.y, rr, 0, Math.PI * 2);
      ctx.fillStyle =
        "rgba(" + pr[0] + "," + pr[1] + "," + pr[2] + "," + (0.55 + glow * 0.45) + ")";
      ctx.fill();
    }

    // Cursor influence ring.
    if (pointer.active) {
      ctx.beginPath();
      ctx.arc(pointer.x, pointer.y, CURSOR_RADIUS, 0, Math.PI * 2);
      ctx.strokeStyle = "rgba(34,211,238,0.14)";
      ctx.lineWidth = 1;
      ctx.stroke();
    }
  }

  // --- Loop with FPS metering ----------------------------------------------
  var lastFrame = performance.now();
  var fpsAcc = 0;
  var fpsFrames = 0;
  var lastFpsUpdate = lastFrame;
  var rafId = null;

  function frame(now) {
    var dt = now - lastFrame;
    lastFrame = now;

    if (running) step();
    draw();

    // FPS (smoothed, updated ~4x/sec).
    fpsAcc += dt;
    fpsFrames++;
    if (now - lastFpsUpdate > 250) {
      var fps = Math.round(1000 / (fpsAcc / fpsFrames));
      fpsEl.textContent = String(Math.max(0, Math.min(fps, 120)));
      linkCountEl.textContent = String(links);
      fpsAcc = 0;
      fpsFrames = 0;
      lastFpsUpdate = now;
    }

    rafId = requestAnimationFrame(frame);
  }

  // --- Pointer --------------------------------------------------------------
  function setPointer(e) {
    var rect = canvas.getBoundingClientRect();
    var pt = e.touches ? e.touches[0] : e;
    pointer.x = pt.clientX - rect.left;
    pointer.y = pt.clientY - rect.top;
    pointer.active = true;
  }
  canvas.addEventListener("mousemove", setPointer);
  canvas.addEventListener(
    "touchmove",
    function (e) {
      setPointer(e);
      e.preventDefault();
    },
    { passive: false }
  );
  canvas.addEventListener("mouseleave", function () {
    pointer.active = false;
    pointer.x = pointer.y = -9999;
  });
  canvas.addEventListener("touchend", function () {
    pointer.active = false;
  });

  // --- Control wiring --------------------------------------------------------
  countEl.addEventListener("input", function () {
    targetCount = parseInt(countEl.value, 10);
    countOut.textContent = String(targetCount);
    syncCount();
  });

  distEl.addEventListener("input", function () {
    linkDist = parseInt(distEl.value, 10);
    distOut.innerHTML = linkDist + '<span class="u">px</span>';
  });

  segBtns.forEach(function (btn) {
    btn.addEventListener("click", function () {
      segBtns.forEach(function (b) {
        b.classList.remove("is-active");
        b.setAttribute("aria-checked", "false");
      });
      btn.classList.add("is-active");
      btn.setAttribute("aria-checked", "true");
      mode = btn.getAttribute("data-mode");
    });
  });

  toggleBtn.addEventListener("click", function () {
    running = !running;
    toggleBtn.textContent = running ? "Pause" : "Resume";
    toggleBtn.setAttribute("aria-pressed", String(!running));
  });

  // --- Boot -----------------------------------------------------------------
  resize();
  syncCount();

  if (reduceMotion) {
    // Respect reduced-motion: render a single static frame, let user resume.
    running = false;
    toggleBtn.textContent = "Resume";
    toggleBtn.setAttribute("aria-pressed", "true");
    draw();
  }

  rafId = requestAnimationFrame(frame);
})();

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <title>Particle Field + Cursor</title>
  <link rel="preconnect" href="https://fonts.googleapis.com" />
  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
  <link
    href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500&display=swap"
    rel="stylesheet"
  />
  <link rel="stylesheet" href="style.css" />
</head>
<body>
  <main class="stage" aria-label="Interactive particle field">
    <canvas id="field" class="field" aria-hidden="true"></canvas>

    <header class="titlebar">
      <div class="titlebar__brand">
        <span class="dot" aria-hidden="true"></span>
        <h1>Particle Field</h1>
      </div>
      <p class="titlebar__hint">Move your cursor across the field — the swarm reacts.</p>
    </header>

    <section class="panel" aria-label="Field controls">
      <div class="panel__head">
        <h2>Controls</h2>
        <div class="stat" role="status" aria-live="off">
          <span class="stat__val" id="fps">60</span>
          <span class="stat__unit">fps</span>
        </div>
      </div>

      <div class="control">
        <label for="count">Particles</label>
        <input id="count" type="range" min="40" max="600" step="10" value="220" />
        <output id="countOut" for="count">220</output>
      </div>

      <div class="control">
        <label for="dist">Connection distance</label>
        <input id="dist" type="range" min="40" max="200" step="5" value="120" />
        <output id="distOut" for="dist">120<span class="u">px</span></output>
      </div>

      <div class="control control--seg">
        <span class="control__label" id="modeLabel">Cursor force</span>
        <div class="seg" role="radiogroup" aria-labelledby="modeLabel">
          <button type="button" class="seg__btn is-active" data-mode="attract" role="radio" aria-checked="true">Attract</button>
          <button type="button" class="seg__btn" data-mode="repel" role="radio" aria-checked="false">Repel</button>
        </div>
      </div>

      <div class="panel__foot">
        <button type="button" id="toggle" class="btn" aria-pressed="false">Pause</button>
        <span class="panel__meta"><span id="linkCount">0</span> links</span>
      </div>
    </section>
  </main>

  <script src="script.js"></script>
</body>
</html>