// holds an instance of XMLHttpRequest
var xmlHttp = createXmlHttpRequestObject();
var glb_resp_cb;

// creates an XMLHttpRequest instance
function createXmlHttpRequestObject()
{
    // will store the reference to the XMLHttpRequest object
    var xmlHttp;
    // this should work for all browsers except IE6 and older
    try
    {
        // try to create XMLHttpRequest object
        xmlHttp = new XMLHttpRequest();
    }
    catch(e)
    {
        // assume IE6 or older
        var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
                                        "MSXML2.XMLHTTP.5.0",
                                        "MSXML2.XMLHTTP.4.0",
                                        "MSXML2.XMLHTTP.3.0",
                                        "MSXML2.XMLHTTP",
                                        "Microsoft.XMLHTTP");
        // try every prog id until one works
        for (var i=0; i<XmlHttpVersions.length && !xmlHttp; i++)
        {
            try
            {
                // try to create XMLHttpRequest object
                xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
            }
            catch (e) {}
        }
    }

    // return the created object or display an error message
    if (!xmlHttp)
        alert("Error creating the XMLHttpRequest object.");
    else
        return xmlHttp;
}


// read a file from the server
function postUrl(strurl, strparam, progress_cb, response_cb)
{
    // only continue if xmlHttp isn't void
    if (xmlHttp)
    {
        // try to connect to the server
        try
        {
            if (xmlHttp.readyState == 0
                || xmlHttp.readyState == 4) {
                progress_cb();
                glb_resp_cb = response_cb;

                // initiate the asynchronous HTTP request
                xmlHttp.open('POST', strurl, true);
                xmlHttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
                xmlHttp.onreadystatechange = handleRequestStateChange;
                xmlHttp.send(strparam);
            }
        }
        // display the error in case of failure
        catch (e)
        {
            alert("Can't connect to server:\n" + e.toString());
        }
    }
}


// function called when the state of the HTTP request changes
function handleRequestStateChange()
{
    // when readyState is 4, we are ready to read the server response
    if (xmlHttp.readyState == 4)
    {
        // continue only if HTTP status is "OK"
        if (xmlHttp.status == 200)
        {
            try
            {
                // do something with the response from the server
                handleServerResponse();
            }
            catch(e)
            {
                // display error message
                alert("Error reading the response: " + e.toString());
            }
        }
        else
        {
            // display status message
            alert("There was a problem retrieving the data:\n" + xmlHttp.statusText);
        }
    }
}


// handles the response received from the server
function handleServerResponse()
{
    // retrieve the server's response packaged as an XML DOM object
    var xmlResponse = xmlHttp.responseXML;

    // catching potential errors with IE and Opera
    if (!xmlResponse || !xmlResponse.documentElement)
        throw("Invalid XML structure (ID):\n" + xmlHttp.responseText);

    // catching potential errors with Firefox
    var rootNodeName = xmlResponse.documentElement.nodeName;
    if (rootNodeName == "parsererror")
        throw("Invalid XML structure1 (Ff):\n" + xmlHttp.responseText);

    // getting the root element (the document element)
    xmlRoot = xmlResponse.documentElement;
    // testing that we received the XML document we expect
    if (rootNodeName != "response" || !xmlRoot.firstChild)
        throw("Invalid XML structure (notag):\n" + xmlHttp.responseText);

    // the value we need to display is the child of the root <response> element
    // display the user message
    glb_resp_cb(xmlRoot.firstChild.data);
}