Curl noise flow field

// Cheap smooth noise: a few layered sines, good enough for a flow field.
export function noise(x, y, t) {
  return (
    Math.sin(x * 1.7 + t) * Math.cos(y * 1.3 - t * 0.7) +
    Math.sin((x + y) * 0.9 + t * 0.5) * 0.5
  );
}

// Curl of the noise: a direction field that swirls without sinks or sources.
export function curl(x, y, t) {
  const e = 0.01;
  const dx = (noise(x + e, y, t) - noise(x - e, y, t)) / (2 * e);
  const dy = (noise(x, y + e, t) - noise(x, y - e, t)) / (2 * e);
  return [dy, -dx];
}

// Move every walker one step along the field; respawn the ones that leave.
export function advance(walkers, w, h, t, scale) {
  for (const p of walkers) {
    const [vx, vy] = curl(p.x / scale, p.y / scale, t);
    p.px = p.x;
    p.py = p.y;
    p.x += vx * 1.6;
    p.y += vy * 1.6;
    if (p.x < 0 || p.x > w || p.y < 0 || p.y > h || --p.life < 0) {
      p.x = p.px = Math.random() * w;
      p.y = p.py = Math.random() * h;
      p.life = 80 + Math.random() * 160;
    }
  }
}