//rollover function

window.onload = function() {
	var conf = {
		className : 'btn',
		postfix : '_ov'
	};
	
	var imgNodeList = getElementsByClassName(conf.className);
	var node;
	
	for( var i=0, len=imgNodeList.length; i<len; i++ ) {
		node = imgNodeList[i];
		node.originalSrc = node.src;
		node.rolloverSrc = node.originalSrc.replace( /(\.gif|\.jpg|\.png)/,conf.postfix+"$1" );
		
		preloadImage( node.rolloverSrc );
		
		node.onmouseover = function() {
			this.src = this.rolloverSrc;
		};
		node.onmouseout = function() {
			this.src = this.originalSrc;
		};
	};
};

function getElementsByClassName( name ) {
	var elements = [];
	var allElements = document.getElementsByTagName('*');
	for( var i=0, len=allElements.length; i<len; i++ ) {
		if( allElements[i].className == name ) {
			elements.push( allElements[i] );
		};
	};
	return elements;
};

preloadImages = [];

function preloadImage( url ) {
	var p = preloadImages;
	var l = p.length;
	p[l] = new Image();
	p[l].src = url;
};