JS1k: The JavaScript code golfing competition

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c.width = width = window.innerWidth;
c.height = height = window.innerHeight;
centerX = width / 2;
centerY = height / 2;

var circles = [];

function drawCircle(x, y, r) {
  var circle = new Path2D();
  circle.arc(x, y, r, 0, 2 * Math.PI);
  c.stroke(circle);
}

function update() {
  c.fillStyle = 'black';
  c.fillRect(0,0, width, height);

  startCircles();
  updateCircles();
  drawCircles();

  window.requestAnimationFrame(update);
}

function startCircles() {
  if (circles.length < 120) {
    createCircle();
  }
}

function updateCircles() {
  for(var i = 0; i < circles.length; i++) {
    circles[i].x += circles[i].velX;
    circles[i].y += circles[i].velY;
    circles[i].r += circles[i].speed;
    circles[i].hue++;
    if (circles[i].r > circles[i].target) {
      circles[i].opacity -= 0.05;
      if (circles[i].opacity <= 0) {
        circles.splice(i, 1);
      }
    }
  }
}

function drawCircles() {
  for(var i = 0; i < circles.length; i++) {
    var circle = circles[i];
    c.lineWidth = 4;
    c.strokeStyle = "hsla(" + circle.hue + ", 100%, 50%, " + circle.opacity + ")";
    drawCircle(circle.x, circle.y, circle.r);
  }
}


function createCircle() {
  var x = Math.random() * width;
  var y = Math.random() * height;
  var speed = 2 + Math.random();
  var hue_start = Math.random() * 360;
  var opacity_step = Math.PI / 20;
  var target_size = width / Math.floor((Math.random() * 16) + 6);
  var velX = (Math.random() * 8) - 4;
  var velY = (Math.random() * 8) - 4;
  for (var i = 0; i < 20; i++) {
    circles.push({
      x: x,
      y: y,
      r: i*2,
      velX: velX,
      velY: velY,
      target: target_size,
      speed: speed,
      hue: hue_start + i * 10,
      opacity: 1 - Math.abs(Math.cos(opacity_step * i))
    });
  }
}

update();