/**
  * main AjaxRequest class
  *
  *  var myRequest = new AjaxRequest(resultHandler, errorHandler);
  *  myRequest.starRequest(operationURL, dataBagMap)
  *
  *  @param resultHandler, String, name of javascript function which handles the result
  *  @param errorHandler, String, name of javascript function which handles possible errors
  *  @param operationURL, String, request url (with optional query string)
  *  @param dataBagMap, Map with information needed by result- or errorHandler later on (can be false)
  *
  *  the resultHandler is called automatically, when ajaxRequest has finished!
  *  (if it is false, ajax will not handle any response, eg. for tracking request purpose...)
  *
*/

function AjaxRequest(resultHandler, errorHandler) {

  // xmlhttprequest object
  var xmlHttp = false;

  // dataBag
  this.dataBag = false;

	this.startRequest = function(strURL, newDataBag) {
	  if (newDataBag) {
	    dataBag = newDataBag;
	  }
		var result = sendRequest(strURL, false, false, false);
		if (!result) {
      // no ajax support
      handleAJAXError(xmlHttp);
    }
	}

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


  /**
   * 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; 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; // NO AJAX!
          }
      }

      // 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 = new Function( "", "processResponse(" + intID + ")" ); ;
      xmlHttp.onreadystatechange = processResponse;

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

      return dataReturn;
  }


  /**
   * process the response data from server
   *
   */
  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
              {
                  processData( xmlHttp );
              }
              // loading not successfull, e.g. page not available
              else
              {
                  if( window.handleAJAXError ) {
                      handleAJAXError( xmlHttp );
                  }
                  else {
                    return false; // something went wrong
                    //alert( "ERROR\n HTTP status = " + xmlHttp.status + "\n" + xmlHttp.statusText ) ;
                  }
              }
      }
  }

  // process result data
  function processData(xmlHttp) {
    if (resultHandler) {
      eval(resultHandler + "(dataBag, xmlHttp.responseText);");
    }
  }

  // handle ajax error
  function handleAJAXError(xmlHttp) {
    if (errorHandler) {
      eval(errorHandler + "(dataBag, xmlHttp);");
    }
  }

} // ajaxRequest