var xmlHttp = getXMLRequester();
var callback;

/**
 * instantiates a new xmlhttprequest object
 *
 * @return xmlhttprequest object or false
 */
function getXMLRequester( )
{
    var xmlHttp = false;
            
       // Mozilla, Opera und Safari
       if (window.XMLHttpRequest)
        {
            xmlHttp = new XMLHttpRequest();
        }
        // Internet Explorer
        else if (window.ActiveXObject)
        {
			xmlHttp = new ActiveXObject( "Microsoft.XMLHTTP" );    
        }
    

    return xmlHttp ;
}


// constants
var REQUEST_GET        = 0;
var REQUEST_POST        = 2;
var REQUEST_HEAD    = 1;
var REQUEST_XML        = 3;

/**
 * sends a http request to server
 *
 * @param strSource, String, datasource on server, e.g. data.php
 *
 * @param strData, String, data to send to server, optionally
 *
 * @param intType, Integer,request type, possible values: REQUEST_GET, REQUEST_POST, REQUEST_XML, REQUEST_HEAD default REQUEST_GET
 *
 * @param intID, Integer, ID of this request, will be given to registered event handler onreadystatechange', optionally
 *
 * @return String, request data or data source
 */
function sendRequest( strSource, strData, callb, intType )
{
    if( !strData )
        strData = '';

    // default type (0 = GET, 1 = xml, 2 = POST )
    if( isNaN( intType ) )
        intType = 0; // GET

	callback = callb;

    // previous request not finished yet, abort it before sending a new request
    if( xmlHttp && xmlHttp.readyState )
    {
        xmlHttp.abort( );
        xmlHttp = false;
    }


    // create a new instance of xmlhttprequest object
    // if it fails, return
    if( !xmlHttp )
    {
        xmlHttp = getXMLRequester();
        if( !xmlHttp )
            return;
    }
    
   
    // parse query string
    if( intType != 1 && ( strData && strData.substr( 0, 1 ) == '&' || strData.substr( 0, 1 ) == '?' ) )
        strData = strData.substring( 1, strData.length );

    // data to send using POST
    var dataReturn = strData ? strData : strSource;
    
    switch( intType )
    {
        case 1:    // xml
            strData = "xml=" + strData;
        case 2: // POST
            // open the connection 
            xmlHttp.open( "POST", strSource, true );
            xmlHttp.setRequestHeader( 'Content-Type', 'application/x-www-form-urlencoded' );
            xmlHttp.setRequestHeader( 'Content-length', strData.length );
            break;
        case 3: // HEAD
            // open the connection 
            xmlHttp.open( "HEAD", strSource, true );
            strData = null;
            break;
        default: // GET
            // open the connection 
            var strDataFile = strSource + (strData ? '?' + strData : '' );
            xmlHttp.open( "GET", strDataFile, true );
            strData = null;
    }
    
    // set onload data event-handler
    xmlHttp.onreadystatechange = processResponse;

    // send request to server
    xmlHttp.send( strData );    // param = POST data
    
    return dataReturn;
}

/**
 * process the response data from server
 *
 * @param intID, Integer, ID of this response
 */
function processResponse()
{
    // status 0 UNINITIALIZED open() has not been called yet.
    // status 1 LOADING send() has not been called yet.
    // status 2 LOADED send() has been called, headers and status are available.
    // status 3 INTERACTIVE Downloading, responseText holds the partial data.
    // status 4 COMPLETED Finished with all operations.
    switch( xmlHttp.readyState )
    {
        // uninitialized
        case 0:
        // loading
        case 1:
        // loaded
        case 2:
        // interactive
        case 3:
            break;
        // complete
        case 4:    
            // check http status
            if( xmlHttp.status == 200 )    // success
            {
                callback(xmlHttp.responseXML);
            }
            // loading not successfull, e.g. page not available
            else
            {
                if( window.handleAJAXError )
                    handleAJAXError( xmlHttp );
                else
                    alert( "ERROR\n HTTP status = " + xmlHttp.status + "\n" + xmlHttp.statusText ) ;
            }
    }
}


// process data from server
function processData( xmlHttp )
{
    // process text data
}


// handle response errors
function handleAJAXError( xmlHttp )
{
}