/**
   * More at: http://hakim.se/experiments/
   * Twitter: @hakimel
   */
  
  var SCREEN_WIDTH = window.innerWidth;
  var SCREEN_HEIGHT = window.innerHeight;
  
  var RADIUS = 100;
  var QUANTITY = 25;

  var canvas;
  var context;
  var particles;
  
  var mouseX = SCREEN_WIDTH * 0.5;
  var mouseY = SCREEN_HEIGHT * 0.5;
  var mouseIsDown = false;

  init();

  function init() {

    canvas = document.getElementById( 'world' );
    
    if (canvas && canvas.getContext) {
      context = canvas.getContext('2d');
      
      document.addEventListener('mousemove', documentMouseMoveHandler, false);
      document.addEventListener('mousedown', documentMouseDownHandler, false);
      document.addEventListener('mouseup', documentMouseUpHandler, false);
      document.addEventListener('touchstart', documentTouchStartHandler, false);
      document.addEventListener('touchmove', documentTouchMoveHandler, false);
      window.addEventListener('resize', windowResizeHandler, false);
      
      createParticles();
      
      windowResizeHandler();
      
      setInterval( loop, 1000 / 60 );
    }
  }

  function createParticles() {
    particles = [];
    
    for (var i = 0; i < QUANTITY; i++) {
      var particle = {
        size: 2,
        position: { x: mouseX, y: mouseY },
        offset: { x: 0, y: 0 },
        shift: { x: mouseX, y: mouseY },
        speed: 0.01+Math.random()*0.04,
        targetSize: 2,
        fillColor: '#' + (Math.random() * 0x404040 + 0xaaaaaa | 0).toString(16),
        orbit: RADIUS*.5 + (RADIUS * .5 * Math.random())
      };
      
      particles.push( particle );
    }
  }
  
  function documentMouseMoveHandler(event) {
    mouseX = event.clientX;
    mouseY = event.clientY;
  }
  
  function documentMouseDownHandler(event) {
    mouseIsDown = true;
  }
  
  function documentMouseUpHandler(event) {
    mouseIsDown = false;
  }

  function documentTouchStartHandler(event) {
    if(event.touches.length == 1) {
      event.preventDefault();

      mouseX = event.touches[0].pageX - (window.innerWidth - SCREEN_WIDTH) * .5;;
      mouseY = event.touches[0].pageY - (window.innerHeight - SCREEN_HEIGHT) * .5;
    }
  }
  
  function documentTouchMoveHandler(event) {
    if(event.touches.length == 1) {
      event.preventDefault();

      mouseX = event.touches[0].pageX - (window.innerWidth - SCREEN_WIDTH) * .5;;
      mouseY = event.touches[0].pageY - (window.innerHeight - SCREEN_HEIGHT) * .5;
    }
  }
  
  function windowResizeHandler() {
    SCREEN_WIDTH = window.innerWidth;
    SCREEN_HEIGHT = window.innerHeight;
    
    canvas.width = SCREEN_WIDTH;
    canvas.height = SCREEN_HEIGHT;
  }
  
  function loop() {
    
    context.shadowBlur = 3;
    
    for (i = 0, len = particles.length; i < len; i++) {
      var particle = particles[i];
      
      // Rotation
      particle.offset.x += particle.speed;
      particle.offset.y += particle.speed;
      
      // Follow mouse with some lag
      particle.shift.x += ( mouseX - particle.shift.x) * (particle.speed);
      particle.shift.y += ( mouseY - particle.shift.y) * (particle.speed);
      
      // Apply position
      particle.position.x = particle.shift.x + Math.cos(i + particle.offset.x) * particle.orbit;
      particle.position.y = particle.shift.y + Math.sin(i + particle.offset.y) * particle.orbit;
      
      // Limit to screen bounds
      particle.position.x = Math.max( Math.min( particle.position.x, SCREEN_WIDTH ), 0 );
      particle.position.y = Math.max( Math.min( particle.position.y, SCREEN_HEIGHT ), 0 );
      
      particle.size += ( particle.targetSize - particle.size ) * 0.05;
      
      if( Math.round( particle.size ) == Math.round( particle.targetSize ) ) {
        particle.targetSize = 1 + Math.random() * 7;
      }
      
      context.beginPath();
      context.fillStyle = particle.fillColor;;
      context.moveTo(particle.position.x, particle.position.y);
      context.arc(particle.position.x, particle.position.y, particle.size, 0, Math.PI*2, true);
      context.fill();
    }
  }