$(function(){
	var items = $('.item'),
		itemsCount = items.length,
		j = 0,
		tabsLinks = $('#tabs > h2'),
		prevLink = $('h2#prev'),
		nextLink = $('h2#next'),
		groups = $('.group');
				
	var slides = Math.ceil(itemsCount/12);
	for (var i=0; i<slides; i++){
		items.slice((i)*12, (i+1)*12).wrapAll('<div class="group"></div>');
	}
	
	var groups = $('.group');
	
	// initialization
	groups.eq(0).show();
	prevLink.addClass('off');
	
	prevLink.click(function(){
		if($(this).is('.off') || groups.is(':animated')){return false;}
		j--;
		groups.eq(j+1).slideUp(300, function(){
			groups.eq(j).slideDown(300);
		});
		checkEnd(j, 'min');
		return false;
	});
	
	nextLink.click(function(){
		if($(this).is('.off') || groups.is(':animated')){return false;}
		j++;
		groups.eq(j-1).slideUp(300, function(){
			groups.eq(j).slideDown(300);
		});
		checkEnd(j, 'max');
		return false;
	});
	
	function checkEnd(curr, limit){
		if(limit == 'max'){
			if (curr == slides-1) {
				nextLink.addClass('off');
				return true;
			} else {
				tabsLinks.filter('.off').removeClass('off');
				return false;
			}
		} else if (limit == 'min'){
			if (curr == 0){
				prevLink.addClass('off');
				return true;
			} else {
				tabsLinks.filter('.off').removeClass('off');	
				return false;
			}
		}
	}

});