<!DOCTYPE html>

<html>
<head>
    <meta name="viewport" content="initial-scale=1.0">
    <style>
        body {
            margin: 0;
        }
        #stage {
            position: relative;
            width: 320px;
            height: 320px;
            border: 1px solid black;
        }
        
        .particle {
            position: absolute;
            height: 20px;
            width: 20px;
            background-color: blue;
            border-radius: 50%;
        }
    </style>
    <script>

        function randomInt(min, max)
        {
            return Math.round(this.random(min, max));
        }

        function random(min, max)
        {
            return (Math.random() * (max - min)) + min;
        }

        function randomColor()
        {
            var min = 32;
            var max = 256 - 32;
            return "#"
                + this.randomInt(min, max).toString(16)
                + this.randomInt(min, max).toString(16)
                + this.randomInt(min, max).toString(16);
        }
        
        function Point(x, y)
        {
            this.x = x;
            this.y = y;
        }

        Point.pointOnCircle = function(angle, radius)
        {
            return new Point(radius * Math.cos(angle), radius * Math.sin(angle));
        }

        Point.prototype =
        {
            add: function(other)
            {
                if(isNaN(other.x))
                    return new Point(this.x + other, this.y + other);
                return new Point(this.x + other.x, this.y + other.y);
            },

            subtract: function(other)
            {
                if(isNaN(other.x))
                    return new Point(this.x - other, this.y - other);
                return new Point(this.x - other.x, this.y - other.y);
            },

            move: function(angle, velocity, timeDelta)
            {
                return this.add(Point.pointOnCircle(angle, velocity * (timeDelta / 1000)));
            },

            multiply: function(other)
            {
                if(isNaN(other.x))
                    return new Point(this.x * other, this.y * other);
                return new Point(this.x * other.x, this.y * other.y);
            },

            length: function() {
                return Math.sqrt( this.x * this.x + this.y * this.y );
            },

            normalize: function() {
                var l = Math.sqrt( this.x * this.x + this.y * this.y );
                this.x /= l;
                this.y /= l;
                return this;
            }
        }

        function Particle(maxPosition)
        {
            this.element = document.createElement('div');
            this.element.className = 'particle';
            this.element.style.backgroundColor = randomColor();
            this.maxPosition = maxPosition;
            this.reset();
            this.move();
        }

        Particle.prototype =
        {
            reset: function()
            {
                this.size = new Point(20, 20);
                this.maxLocation = this.maxPosition.subtract(this.size);
                this.position = new Point(this.maxPosition.x / 2, this.maxPosition.y / 4);

                var angle = Math.random() * Math.PI * 2;
                this._velocity = new Point(Math.sin(angle), Math.cos(angle)).multiply(random(.8, 1.2));
            },

            animate: function(timeDelta)
            {
                this.position = this.position.add(this._velocity.multiply(timeDelta));
                this._velocity.y += 0.03;

                // If particle is going to move off right side
                if (this.position.x > this.maxLocation.x) {
                    if (this._velocity.x > 0)
                        this._velocity.x *= -1;
                    this.position.x = this.maxLocation.x;
                } else if (this.position.x < 0) {
                    // If particle is going to move off left side
                    if (this._velocity.x < 0)
                        this._velocity.x *= -1;
                    this.position.x = 0;
                }

                // If particle is going to move off bottom side
                if (this.position.y > this.maxLocation.y) {
                    // Adjust direction but maintain magnitude
                    var magnitude = this._velocity.length();
                    this._velocity.x *= 1.5 + .005 * this.size.x;
                    this._velocity = this._velocity.normalize().multiply(magnitude);
                    if (Math.abs(this._velocity.y) < 0.7)
                        this.reset();
                    else {
                        if (this._velocity.y > 0)
                            this._velocity.y *= -0.999;
                        this.position.y = this.maxLocation.y;
                    }
                } else if (this.position.y < 0) {
                    // If particle is going to move off top side
                    var magnitude = this._velocity.length();
                    this._velocity.x *= 1.5 + .005 * this.size.x;
                    this._velocity = this._velocity.normalize().multiply(magnitude);
                    if (this._velocity.y < 0)
                        this._velocity.y *= -0.998;
                    this.position.y = 0;
                }
                
                this.move();
            },

            move: function()
            {
                this.element.style.transform = "translate(" + this.position.x + "px," + this.position.y + "px) ";
            }
        }

        var numParticles = 20;
        var particles = [];

        function makeParticles()
        {
            var stage = document.getElementById('stage');
            var maxPosition = new Point(320, 320);
            for (var i = 0; i < numParticles; ++i) {
                particles.push(new Particle(maxPosition));
                stage.appendChild(particles[i].element);
            }
        }
        
        function moveParticles()
        {
            for (var particle of particles)
                particle.animate(1);
        }
        
        function animate()
        {
            moveParticles();
            window.requestAnimationFrame(animate);
        }
        
        function setupAnimation()
        {
            makeParticles();
            animate();
        }
        
        window.addEventListener('load', setupAnimation, false);
    </script>
</head>
<body>

<div id="stage">
</div>

</body>
</html>