var anzBackgrounds = 13;
var expireMinutes = 30;
var bgNum = 0;

function checkCookie() {
	var cookie = document.cookie;
	
	if(cookie.indexOf("bgNum=") == 0) {
		bgNum = cookie.substr(6);
		//security check
		if(!((bgNum >= 0) && (bgNum < anzBackgrounds))) {
			bgNum = getNewBgNum();
		} 
		//refresh cookie
		setCookie(bgNum);
	}
	else {
		bgNum = getNewBgNum();
		setCookie(bgNum);
	}
}

function setCookie(valToSet) {
	var expDate = new Date();
	expDate.setMinutes(expDate.getMinutes() + expireMinutes);
	document.cookie = "bgNum=" + valToSet + "; expires=" + expDate.toGMTString();
}

function getNewBgNum() {
	return Math.floor(Math.random()*anzBackgrounds);
}