/**
* mark@sullivans.id.au
*/
 
function calculateQuantityChange(form)
{

	if (isNaN(form.input_qty.value)) 
	{
		alert("Invalid quantity entered");
		return false;
	}
	
	var input_qty    = parseInt(form.input_qty.value);
	var original_qty = parseInt(form.original_qty.value);

	if (input_qty > original_qty)
	{
 		form.qty.value = input_qty - original_qty;
 		form.op.value = "up"
 	} else {
 		form.qty.value = original_qty - input_qty;
 		form.op.value = "down"
 	}
 	
 	return true;
}