function addLoadListener(fn)
{
  if (typeof window.addEventListener != 'undefined')
  {
    window.addEventListener('load', fn, false);
  }
  else if (typeof document.addEventListener != 'undefined')
  {
    document.addEventListener('load', fn, false);
  }
  else if (typeof window.attachEvent != 'undefined')
  {
    window.attachEvent('onload', fn);
  }
  else
  {
    var oldfn = window.onload;
    if (typeof window.onload != 'function')
    {
      window.onload = fn;
    }
    else
    {
      window.onload = function()
      {
        oldfn();
        fn();
      };
    }
  }
}

/**
 * Preloads rollover image and swaps/replaces images during button mouseover/mouseout
 */
function preloadImages()
{
	var divList = ['nt1', 'nt2', 'nt3', 'nt4', 'nt5', 'nt6'];
	var links = ['roll-1', 'roll-2', 'roll-3', 'roll-4', 'roll-5', 'roll-6'];
	var imageList = ['mission', 'portfolio', 'publications', 'resources', 'sendnote', 'who_we_are'];
	var divs = [];
	var images = [];
	var folder = 'navtop';
	var count = 0;
	
	for (var i = 0; i < divList.length; i++)
	{
		if (document.getElementById(divList[i]))
		{
			divs[count] = divList[i];
			images[count] = imageList[i];
			count++;
		}
	}
	
	for (var i = 0; i < count; i++)
	{
		loadImage(links[i], divs[i], images[i], folder);
	}

	var divList = ['ml-1', 'ml-2', 'mr-1', 'mr-2'];
	var links = ['roll-7', 'roll-8', 'roll-9', 'roll-10'];
	var imageList = ['brochure', 'journal', 'viewbooks', 'donate'];
	var divs = [];
	var images = [];
	var folder = 'sidebar';
	var count = 0;
	
	for (var i = 0; i < divList.length; i++)
	{
		if (document.getElementById(divList[i]))
		{
			divs[count] = divList[i];
			images[count] = imageList[i];
			count++;
		}
	}
	
	for (var i = 0; i < count; i++)
	{
		loadImage(links[i], divs[i], images[i], folder);
	}
}

function loadImage(linkId, divId, imgName, imgFolder)
{
	var imgObject = new Image();
	imgObject.src = 'images/'+imgFolder+'/'+imgName+'-over.gif';
	var link = document.getElementById(linkId);
	var div = document.getElementById(divId);
	
	link.onmouseover = function()
	{
		div.style.background = 'url('+imgObject.src+')';
	}
	link.onmouseout = function()
	{
		div.style.background = 'url(images/'+imgFolder+'/'+imgName+'.gif)';
	}
}

addLoadListener(preloadImages);