var xmlHttp;
var i = 0;
var maxNumber = 0;

/*
 * Changes the current shown advert by adding to a counter. 
 * When counter reaches max number of active adverts it resets and starts over.
 */
function changeAd()
{ 	
    xmlHttp=GetXmlHttpObject()
    if (xmlHttp==null)
    {
        alert ("Browser does not support HTTP Request")
        return;
    }
    var url="advert.php"
    url=url+"?ajax=true"
    url=url+"&advert="+i
    url=url+"&sid="+Math.random()

    xmlHttp.onreadystatechange=function() {
        if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete") {
            document.getElementById("advert").innerHTML=xmlHttp.responseText
        }
    } 
    xmlHttp.open("POST",url,true)
    xmlHttp.send(null)
    i++;
    if (i==maxNumber) { i = 0; }
}

/*
 * Keeps track of time and calls the function ChangeAd at the appropriate time.
 */
function updateAds(number, time) {
    maxNumber = number;
    changeAd();
    interval = window.setInterval("changeAd()", time*1000);
}

/*
 * Ajax function
 */
function GetXmlHttpObject(){
    var xmlHttp=null;
    try {
        xmlHttp=new XMLHttpRequest();
    } /* Firefox, IE8+, and other newer and correct web browesers */
    catch(e){
        try {
            xmlHttp=new ActiveXObject("Msxml2.XMLHTTP"); /* some versions IE */
        } catch (e) {
            xmlHttp=new ActiveXObject("Microsoft.XMLHTTP"); /* some versions IE */
        }
    } 
    return xmlHttp;
}