// easing functions http://goo.gl/5HLl8
Math.easeInOutQuad = function (t, b, c, d) {
  t /= d/2;
  if (t < 1) {
    return c/2*t*t + b
  }
  t--;
  return -c/2 * (t*(t-2) - 1) + b;
};

Math.easeInCubic = function(t, b, c, d) {
  var tc = (t/=d)*t*t;
  return b+c*(tc);
};

Math.inOutQuintic = function(t, b, c, d) {
  var ts = (t/=d)*t,
  tc = ts*t;
  return b+c*(6*tc*ts + -15*ts*ts + 10*tc);
};

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
var requestAnimFrame = (function(){
  return  window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || function( callback ){ window.setTimeout(callback, 1000 / 60); };
})();

function scrollTo(to, callback, duration) {
  // figure out if this is moz || IE because they use documentElement
  var doc = (document.documentElement.classList.contains('mozilla-firefox') || document.documentElement.classList.contains('internet-explorer')) ? document.documentElement : document.body,
  start = doc.scrollTop,
  change = to - start,
  currentTime = 0,
  increment = 20;
  duration = (typeof(duration) === 'undefined') ? 500: duration;
  var animateScroll = function(){
    // increment the time
    currentTime += increment;
    // find the value with the quadratic in-out easing function
    var val = Math.easeInOutQuad(currentTime, start, change, duration);
    // move the document.body
    doc.scrollTop = val;
    // do the animation unless its over
    if(currentTime < duration) {
      requestAnimFrame(animateScroll);
    } else {
      if (callback && typeof(callback) === 'function') {
        // the animation is done so lets callback
        callback();
      }
    }
  };
  animateScroll();
}