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if (window.addEventListener) window.addEventListener('DOMMouseScroll', wheel, false);
window.onmousewheel = document.onmousewheel = wheel;

function wheel(event) {
    var delta = 0;
    if (event.wheelDelta) delta = event.wheelDelta / 120;
    else if (event.detail) delta = -event.detail / 3;

    handle(delta);
    if (event.preventDefault) event.preventDefault();
    event.returnValue = false;
}

var goUp = true;
var end = null;
var interval = null;

function handle(delta) {
	var animationInterval = 10; //lower is faster
  var scrollSpeed = 10; //lower is faster

	if (end == null) {
  	end = jQuery(window).scrollTop();
  }
  end -= 20 * delta;
  goUp = delta > 0;

  if (interval == null) {
    interval = setInterval(function () {
      var scrollTop = jQuery(window).scrollTop();
      var step = Math.round((end - scrollTop) / scrollSpeed);
      if (scrollTop <= 0 ||
          scrollTop >= jQuery(window).prop("scrollHeight") - jQuery(window).height() ||
          goUp && step > -1 ||
          !goUp && step < 1 ) {
        clearInterval(interval);
        interval = null;
        end = null;
      }
      jQuery(window).scrollTop(scrollTop + step );
    }, animationInterval);
  }
}