/**
 * Set up some IE-detection variables, to handle non-standard behavior.
 *
 * IS_IE: boolean that detects IE, from sarissa.sourceforge.net.
 * NAME_OF_CLASS: name of the 'class' attribute, used in set/get/removeAttribute()
 *
**/

var IS_IE = document.all && window.ActiveXObject && navigator.userAgent.toLowerCase().indexOf("msie") > -1  && navigator.userAgent.toLowerCase().indexOf("opera") == -1;
var NAME_OF_CLASS = IS_IE ? 'className' : 'class';

/**
 * Update an element by retrieving a url
 *
 * This function will create an XmlHttpRequest object, retrieve a url from it, and
 * call a callback function so that the element can be updated with the response.
 *
 * An exception will be thrown if XmlHttpRequest objects are not supported.
 * 
 * The advantage of this function over using XmlHttpRequest directly is that this
 * function also takes care of feedback messages, if you pass in a status_element.
 *
 * The pre_send_callback function is optional. If given, it will be called just 
 * before xmlhttp.send(). It is intended to be used to prepare the status_element,
 * by making it visible, changing its cursor, or something like that.
 *
 * The when_ready_callback function is required, and will be called when 
 * readyState == 4. It will be passed one argument, the xmlhttp object. It is 
 * assumed that the callback function is a closure, and any other necessary variables 
 * (like the element to update) will be available within the function.
 * If the status_element was used and set up with pre_send_callback, then it should
 * be set back the way it was in when_ready_callback.
 * 
 * The status_element argument is optional. If given, then for readyState values
 * 1, 2, and 3, status_element.innerHtml will be set to some feedback text.
 *
 * @argument url The url to GET
 * @argument pre_send_callback A callback function to call just before sending the request
 * @argument when_ready_callback A callback function to call when readyState == 4
 * @argument status_element An element to contain status messages when readyState < 4
 */

function update_element_from_url(url, pre_send_callback, when_ready_callback, status_element) {

    var xmlhttp = HTTP();

    if ( !xmlhttp ) {
        throw new Error("XmlHttpRequest not available");
    }

    xmlhttp.open("GET", url, true);

    xmlhttp.onreadystatechange = function() {
        if (status_element) {
            if (xmlhttp.readyState == 1) {
                status_element.innerHTML = "Loading..";
            }
            else if (xmlhttp.readyState == 2) {
                status_element.innerHTML = "Loading....";
            }
            else if (xmlhttp.readyState == 3) {
                status_element.innerHTML = "Loading......";
            }
        }
        if (xmlhttp.readyState == 4) {
            when_ready_callback(xmlhttp);
        }
    };

    if (pre_send_callback != null) {
        pre_send_callback();
    }

    xmlhttp.send(null);
}



/**
 * Create an XMLHttpRequest object
 *
 * This will return an XmlHttpRequest object for various browsers.
 * If the object isn't supported, an alert will be displayed.
 *
 */
function HTTP() {

    if (window.XMLHttpRequest) {
        return new XMLHttpRequest();
    }

    var xmlhttp = null;
    var objects_to_try = ["Msxml2.XMLHTTP.5.0", "Msxml2.XMLHTTP.4.0", "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLHTTP"];

    for (var i=0; i < objects_to_try.length; i++) {
        try {
            xmlhttp = new ActiveXObject(objects_to_try[i]);
            return xmlhttp;
        }
        catch (e) {
            // Try the next one
        }
    }

    alert("Your browser does not support AJAX methods. Please use IE 6+ or Netscape 7+ (or compatible).");

    return null;
}


/* -------------------------------------------------------------------- */
/*	This is a utility method which finds the source node of an event
	under multiple browsers
*/

function getSrc(evt) {
	evt = (evt) ? evt : ((event) ? event : null);
	if (evt) {
		var elem = (evt.target) ? evt.target
								: ((evt.srcElement) ? evt.srcElement 
													: null);
		return elem;
	}
	return;
}

/* -------------------------------------------------------------------- */
/* This utility method is used to attach event handlers to elements 
   or the window object.
*/

function add_event_handler(object, event_name, handler_function) {

    if (object.addEventListener) {
        object.addEventListener(event_name, handler_function, false);
    }
    else if (object.attachEvent) {
        object.attachEvent('on' + event_name, handler_function);
    }
}