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

// gets the xmlHttp Object
var xmlHttp = null;
window.setTimeout( "getXMLRequester( )", 0 );

/**
 * instantiates a new xmlhttprequest object
 *
 * @return xmlhttprequest object or false
 */
function getXMLRequester( )
{
   //var xmlHttp = false;
            
    // try to create a new instance of the xmlhttprequest object        
    try
    {
        // Internet Explorer
        if( window.ActiveXObject )
        {
            for( var i = 5; i > 0; i-- )
            {
                try
                {
                    // loading of a newer version of msxml dll (msxml3 - msxml5) failed
                    // use fallback solution
                    // old style msxml version independent, deprecated
                    if( i == 2 )
                    {
                        xmlHttp = new ActiveXObject( "Microsoft.XMLHTTP" );    
                    }
                    // try to use the latest msxml dll
                    else
                    {
                        
                        xmlHttp = new ActiveXObject( "Msxml2.XMLHTTP." + i + ".0" );
                    }
                    break;
                }
                catch( excNotLoadable )
                {                        
                    xmlHttp = false;
                }
            }
        }
        // Mozilla, Opera und Safari
        else if( window.XMLHttpRequest )
        {
            xmlHttp = new XMLHttpRequest( );
        }
    }
    // loading of xmlhttp object failed
    catch( excNotLoadable )
    {
        xmlHttp = false;
    }
    return xmlHttp ;
}

/**
 * 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, intType, intID )
{
	if( !strData )
	{
        strData = '';
	}

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

    // 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 false;
		}
    }
            
    // 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 = "";
	if( strData )
	{
		dataReturn = strData;
	}
	else
	{
		dataReturn = 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 = new Function( "", "processResponse(" + intID + ")" ); ;

    // 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( intID )
{
    // 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:
			break;
        // loading
        case 1:
			break;
        // loaded
        case 2:
			break;
        // interactive
        case 3:
            break;
        // complete
        case 4:    
            // check http status
            if( xmlHttp.status == 200 )    // success
            {
                processData( xmlHttp, intID );
            }
            // loading not successfull, e.g. page not available
            else
            {
                if( window.handleAJAXError )
				{
                    handleAJAXError( xmlHttp, intID );
				}
                else
				{
                    //alert( "ERROR\n HTTP status = " + xmlHttp.status + "\n" + xmlHttp.statusText );
				}
            }
			break;
    }
}

// process data from server
function processData( xmlHttp, intID )
{
	// process text data
	// updateMenu( xmlHttp.responseText );
	switch( intID )
	{
		case GET_MESSAGES:
			fctHandleResponseMessages( xmlHttp );
			break;
		case INSERT_MESSAGE:
			fctHandleResponseInsertMessage( xmlHttp );
			break;
		case CHECK_MESSAGES:
			fctHandelCheckOnNewMessages( xmlHttp );
			break;
		default:
			alert( "[ ID not found ]" );
			break;
	}
}


// handle response errors
function handleAJAXError( xmlHttp, intID )
{
    //...
	//alert( "ERROR\n HTTP status = " + xmlHttp.status + "\n" + xmlHttp.statusText );
}