cleanup

/**
 * Methods for extending Prototype
 */
var Position = {
	/**
	 * Centers an element against the parent or body
	 * parent should be display: relative
	 */
	center: function(elem, parent){
		
		var elem = $(elem),
		dims = elem.getDimensions(),
		scroll = document.viewport.getScrollOffsets(),
		bgDims, top, left;
		
                // are we using a parent?
		if(parent) bgDims = $(parent).getDimensions();
		else bgDims = document.viewport.getDimensions();

		top  = parseInt( ( ( ( bgDims.height ) - dims.height) / 2 ), 10);
		left = parseInt( ( ( ( bgDims.width ) - dims.width) / 2 ), 10 );

		elem.setStyle({
	            position: 'absolute',
		    top:  top + scroll.top + 'px',
		    left: left + scroll.left + 'px'
		});

		return elem;
	},
	
	/**
	 * Used to keep ensure an element stays within a scrollable div
	 */
	keepWithin: function(elem, parent, padding){
		elem = $(elem);
		parent = $(parent);
		padding = padding || 0;
		
		// return if no scrollbars
		if(parent.scrollHeight < 1) return elem;
		
		var elemViewportBottom = elem.getBoundingClientRect().bottom.ceil();
		var parentViewportBottom = parent.viewportOffset().top + parent.getHeight();

		if(parentViewportBottom < elemViewportBottom) 
			parent.scrollTop = elemViewportBottom - parentViewportBottom + padding;  

		return elem;
	}
};

Element.addMethods(Position);