var xhrCallbacks, xhrSupported,
	xhrId = 0,
	// #5280: Internet Explorer will keep connections alive if we don't abort on unload
	xhrOnUnloadAbort = window.ActiveXObject && function() {
		// Abort all pending requests
		var key;
		for ( key in xhrCallbacks ) {
			xhrCallbacks[ key ]( undefined, true );
		}
	};

// Functions to create xhrs
function createStandardXHR() {
	try {
		return new window.XMLHttpRequest();
	} catch( e ) {}
}

function createActiveXHR() {
	try {
		return new window.ActiveXObject("Microsoft.XMLHTTP");
	} catch( e ) {}
}

// Create the request object
// (This is still attached to ajaxSettings for backward compatibility)
jQuery.ajaxSettings.xhr = window.ActiveXObject ?
	/* Microsoft failed to properly
	 * implement the XMLHttpRequest in IE7 (can't request local files),
	 * so we use the ActiveXObject when it is available
	 * Additionally XMLHttpRequest can be disabled in IE7/IE8 so
	 * we need a fallback.
	 */
	function() {
		return !this.isLocal && createStandardXHR() || createActiveXHR();
	} :
	// For all other browsers, use the standard XMLHttpRequest object
	createStandardXHR;

// Determine support properties
xhrSupported = jQuery.ajaxSettings.xhr();
jQuery.support.cors = !!xhrSupported && ( "withCredentials" in xhrSupported );
xhrSupported = jQuery.support.ajax = !!xhrSupported;

// Create transport if the browser can provide an xhr
if ( xhrSupported ) {

	jQuery.ajaxTransport(function( s ) {
		// Cross domain only allowed if supported through XMLHttpRequest
		if ( !s.crossDomain || jQuery.support.cors ) {

			var callback;

			return {
				send: function( headers, complete ) {

					// Get a new xhr
					var handle, i,
						xhr = s.xhr();

					// Open the socket
					// Passing null username, generates a login popup on Opera (#2865)
					if ( s.username ) {
						xhr.open( s.type, s.url, s.async, s.username, s.password );
					} else {
						xhr.open( s.type, s.url, s.async );
					}

					// Apply custom fields if provided
					if ( s.xhrFields ) {
						for ( i in s.xhrFields ) {
							xhr[ i ] = s.xhrFields[ i ];
						}
					}

					// Override mime type if needed
					if ( s.mimeType && xhr.overrideMimeType ) {
						xhr.overrideMimeType( s.mimeType );
					}

					// X-Requested-With header
					// For cross-domain requests, seeing as conditions for a preflight are
					// akin to a jigsaw puzzle, we simply never set it to be sure.
					// (it can always be set on a per-request basis or even using ajaxSetup)
					// For same-domain requests, won't change header if already provided.
					if ( !s.crossDomain && !headers["X-Requested-With"] ) {
						headers["X-Requested-With"] = "XMLHttpRequest";
					}

					// Need an extra try/catch for cross domain requests in Firefox 3
					try {
						for ( i in headers ) {
							xhr.setRequestHeader( i, headers[ i ] );
						}
					} catch( err ) {}

					// Do send the request
					// This may raise an exception which is actually
					// handled in jQuery.ajax (so no try/catch here)
					xhr.send( ( s.hasContent && s.data ) || null );

					// Listener
					callback = function( _, isAbort ) {
						var status, responseHeaders, statusText, responses;

						// Firefox throws exceptions when accessing properties
						// of an xhr when a network error occurred
						// http://helpful.knobs-dials.com/index.php/Component_returned_failure_code:_0x80040111_(NS_ERROR_NOT_AVAILABLE)
						try {

							// Was never called and is aborted or complete
							if ( callback && ( isAbort || xhr.readyState === 4 ) ) {

								// Only called once
								callback = undefined;

								// Do not keep as active anymore
								if ( handle ) {
									xhr.onreadystatechange = jQuery.noop;
									if ( xhrOnUnloadAbort ) {
										delete xhrCallbacks[ handle ];
									}
								}

								// If it's an abort
								if ( isAbort ) {
									// Abort it manually if needed
									if ( xhr.readyState !== 4 ) {
										xhr.abort();
									}
								} else {
									responses = {};
									status = xhr.status;
									responseHeaders = xhr.getAllResponseHeaders();

									// When requesting binary data, IE6-9 will throw an exception
									// on any attempt to access responseText (#11426)
									if ( typeof xhr.responseText === "string" ) {
										responses.text = xhr.responseText;
									}

									// Firefox throws an exception when accessing
									// statusText for faulty cross-domain requests
									try {
										statusText = xhr.statusText;
									} catch( e ) {
										// We normalize with Webkit giving an empty statusText
										statusText = "";
									}

									// Filter status for non standard behaviors

									// If the request is local and we have data: assume a success
									// (success with no data won't get notified, that's the best we
									// can do given current implementations)
									if ( !status && s.isLocal && !s.crossDomain ) {
										status = responses.text ? 200 : 404;
									// IE - #1450: sometimes returns 1223 when it should be 204
									} else if ( status === 1223 ) {
										status = 204;
									}
								}
							}
						} catch( firefoxAccessException ) {
							if ( !isAbort ) {
								complete( -1, firefoxAccessException );
							}
						}

						// Call complete if needed
						if ( responses ) {
							complete( status, statusText, responses, responseHeaders );
						}
					};

					if ( !s.async ) {
						// if we're in sync mode we fire the callback
						callback();
					} else if ( xhr.readyState === 4 ) {
						// (IE6 & IE7) if it's in cache and has been
						// retrieved directly we need to fire the callback
						setTimeout( callback );
					} else {
						handle = ++xhrId;
						if ( xhrOnUnloadAbort ) {
							// Create the active xhrs callbacks list if needed
							// and attach the unload handler
							if ( !xhrCallbacks ) {
								xhrCallbacks = {};
								jQuery( window ).unload( xhrOnUnloadAbort );
							}
							// Add to list of active xhrs callbacks
							xhrCallbacks[ handle ] = callback;
						}
						xhr.onreadystatechange = callback;
					}
				},

				abort: function() {
					if ( callback ) {
						callback( undefined, true );
					}
				}
			};
		}
	});
}