/*
 * xmlData class.
 *	Provides a mechanism to retrieve an XML document from the server and then extract its data.
 *
 * As a first it tries to use the XML DOM model for document retrieval.
 * Failing that, we revert to using the XMLHTTP model.
 */

/*
 * Constants for the XML DOM state changes.
 */
var XML_UNINITIALIZED = 0;
var XML_LOADING = 1;
var XML_LOADED = 2;
var XML_INTERACTIVE = 3;
var XML_COMPLETED = 4;

/*
 * xmlDataObject_getXMLElementData()
 *
 * Use the W3C DOM method for finding a tag and
 * returning the data content.
 */
function xmlDataObject_getXMLElementData(tag_name)
{
	/*
	 * Slight difference depending on Mozilla/IE and Opera on how we get the XML elements
	 */
	var xmlDocInstance = this.xmlDoc;
	if (typeof this.xmlDoc.load == "undefined") {
		/*
		 * Opera (and others of this type) have their XML document stored in a responseXML property
		 */
		xmlDocInstance = this.xmlDoc.responseXML;
	} else {
		/*
		 * Mozilla and IE XML DOM have a parser status variable
		 */
		if ((xmlDocInstance.parseError != null) && (xmlDocInstance.parseError.errorCode != 0)) {
			return null;
		}
	}

	var tag = xmlDocInstance.getElementsByTagName(tag_name);
	if (tag == null) {
		return null;
	}
	if (tag.length == 0) {
		return null;
	}
	var node = tag[0].firstChild;
	if ((node == null) || (node.nodeValue == null)) {
		return null;
	}
	return node.nodeValue;
}

/*
 * xmlDataObject_createXMLDocument()
 *	Creates/Recreates the object used to retrieve the XML resource
 */
function xmlDataObject_createXMLDocument()
{
	/*
	 * Destroy any previous document
	 */
	if (this.xmlDoc != null) {
		delete this.xmlDoc;
		this.xmlDoc = null;
		this.operaRepeatedCompletionCallBugFix = false;
	}

	/*
	 * Create a variable that _is_ this object so that the anonymous
	 * functions can access it
	 */
	var xmlDataInstance = this;

	/*
	 * Create an XML DOM for retrieving the data
	 */
	if ((typeof document.implementation == "object") && (typeof document.implementation.createDocument == "function")) {
		/*
		 * This is Mozilla (and others) compatible and is the preferred standard for retrieving an XML document.
		 */
		this.xmlDoc = document.implementation.createDocument("", "", null);

		/*
		 * Opera browsers do not support this method, however, even though they say they can!
		 */
		if (typeof this.xmlDoc.load != "undefined") {
			/*
			 * Supported properly
			 */
			this.xmlDoc.onload = function () {
				/*
				 * Loading can complete in error!
				 * If there is no data then ignore (wait for timeout).
				 */
				if (xmlDataInstance.xmlDoc.firstChild == null) {
					return;
				}

				/*
				 * Clear the timeout and signal success
				 */
				window.clearTimeout(xmlDataInstance.timeoutId);
				xmlDataInstance.dataReadyFunc(xmlDataInstance);
			}
			return;
		}

		/*
		 * Drop through and try Microsoft and Opera
		 */
	}
	
	/*
	 * Try IE and Opera
	 */
	 if (window.ActiveXObject) {
		/*
		 * This is Microsofts way
		 */
		this.xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
		if (this.xmlDoc == null) {
			/*
			 * Try the HTTP model
			 */
			 this.xmlDoc = new ActiveXObject("MSXML2.XMLHTTP");
		}
	} else if (window.XMLHttpRequest) {
		/*
		 * Opera 7.6+ and others
		 */
		this.xmlDoc = new XMLHttpRequest();
	}

	if (this.xmlDoc == null) {
		/*
		 * Old unsupported browser
		 */
		alert("Your browser does not support XML retrieval");
		return;
	}

	this.xmlDoc.onreadystatechange = function () {
		if (xmlDataInstance.xmlDoc.readyState == XML_COMPLETED) {
			if (xmlDataInstance.operaRepeatedCompletionCallBugFix) {
				return;
			}

			/*
			 * Opera browsers have a repeated call bug
			 */
			xmlDataInstance.operaRepeatedCompletionCallBugFix = true;

			/*
			 * Opera browsers have a status field to indicate the HTTP response
			 */
			if (typeof xmlDataInstance.xmlDoc.status != "undefined") {
				if (xmlDataInstance.xmlDoc.status != 200) {
					/*
					 * Rely on timeout
					 */
					return;
				}
			} else {
				/*
				 * IE can still signal 'complete' even in error
				 */
				if (xmlDataInstance.xmlDoc.firstChild == null) {
					/*
					 * Rely on timeout
					 */
					return;
				}
			}

			window.clearTimeout(xmlDataInstance.timeoutId);
			xmlDataInstance.dataReadyFunc(xmlDataInstance);
		}
	}
}

/*
 * xmlDataObject_retrieveData()
 *	Begin retrieval.
 */
function xmlDataObject_retrieveData()
{
	/*
	 * Create the document object that will retrieve the requested doc
	 * NOTE: Any previous one is destroyed.
	 */
	this.createXMLDocument();

	/*
	 * Create a local variable that is an anonumous function to be evaluated when the timeout occurs.
	 * The anonymous function will have access to the xmlDataObject instance!
	 */
	var xmlDataInstance = this;
	var fnx = function() {
		/*
		 * Re-create the document (more importantly: destroy the existing one)
		 */
		xmlDataInstance.createXMLDocument();

		/*
		 * Invoke the timeout callback function
		 */
		xmlDataInstance.dataTimeoutFunc(xmlDataInstance);
	}
	this.timeoutId = window.setTimeout(fnx, this.timeoutMillis);

	/*
	 * Start the loading of the resource
	 */
	if (typeof this.xmlDoc.load != "undefined") {
		/*
		 * Microsoft and Mozilla using XML model
		 */
		this.xmlDoc.async="true";
		this.xmlDoc.load(this.dataURL);
		return;
	}

	/*
	 * Opera and old IE XMLHTTPRequest
	 */
	this.xmlDoc.open('GET', this.dataURL, true);
	this.xmlDoc.send();
}

/*
 * xmlDataObject()
 *	Create an XML data object.
 *
 * Used to retrieve data from the server in XML format.
 * The given function is invoked when the data is retrieved.
 */
function xmlDataObject(dataReadyFunc, dataTimeoutFunc, timeoutMillis, dataURL)
{
	/*
	 * Initialise properties.
	 */
	this.dataReadyFunc = dataReadyFunc;
	this.dataTimeoutFunc = dataTimeoutFunc;
	this.dataURL = dataURL;
	this.timeoutMillis = timeoutMillis;
	this.timeoutId = 0;
	this.xmlDoc = null;

	this.operaRepeatedCompletionCallBugFix = false;		/* To fix opera bug on getting called back more than once */
		
	/*
	 * Initialize the public methods
	 */
	this.retrieveData = xmlDataObject_retrieveData;
	this.getXMLElementData = xmlDataObject_getXMLElementData;

	/*
	 * Initialise private methods
	 */
	this.createXMLDocument = xmlDataObject_createXMLDocument;
}