var startPosition= 0;
var numberOfVisibleItems= 0;
var totalNumberOfItems= 0;


function scrollup()
{
	if (startPosition-1 < 0)
		return;
		
	startPosition--;
	var firstVisibleItem= startPosition;
	var lastVisibleItem= startPosition+numberOfVisibleItems-1;
	
	for (var i= 0; i<totalNumberOfItems; i++) {
		elem= document.getElementById('scroller_'+i); 
		if (i >= firstVisibleItem && i <= lastVisibleItem) {
			elem.style.display= 'block';
		} else {
			elem.style.display= 'none';
		}
	}
}


function scrolldown()
{
	if (startPosition+numberOfVisibleItems >= totalNumberOfItems)
		return;
		
	startPosition++;
	var firstVisibleItem= startPosition;
	var lastVisibleItem= startPosition+numberOfVisibleItems-1;
	
	for (var i= 0; i<totalNumberOfItems; i++) {
		elem= document.getElementById('scroller_'+i); 
		if (i >= firstVisibleItem && i <= lastVisibleItem) {
			elem.style.display= 'block';
		} else {
			elem.style.display= 'none';
		}
	}
}


function setStartPosition(n) { startPosition= n; }
function setNumberOfVisibleItems(n) { numberOfVisibleItems= n; }
function setTotalNumberOfItems(n) { totalNumberOfItems= n; }