(function($) {
    var horizontalPositions = /left|center|right/,
    	horizontalDefault = "center",
    	verticalPositions = /top|center|bottom/,
    	verticalDefault = "center",
    	_position = $.fn.position,
    	_offset = $.fn.offset;

    $.fn.position = function( options, callback ) {
    	if ( !options || !options.of ) {
    		return _position.apply( this, arguments );
    	}

    	// make a copy, we don't want to modify arguments
    	options = $.extend( {}, options );

    	var target = $( options.of ),
    		collision = ( options.collision || "flip" ).split( " " ),
    		offset = options.offset ? options.offset.split( " " ) : [ 0, 0 ],
    		targetWidth,
    		targetHeight,
    		basePosition;

    	if ( options.of.nodeType === 9 ) {
    		targetWidth = target.width();
    		targetHeight = target.height();
    		basePosition = { top: 0, left: 0 };
    	} else if ( options.of.scrollTo && options.of.document ) {
    		targetWidth = target.width();
    		targetHeight = target.height();
    		basePosition = { top: target.scrollTop(), left: target.scrollLeft() };
    	} else if ( options.of.preventDefault ) {
    		// force left top to allow flipping
    		options.at = "left top";
    		targetWidth = targetHeight = 0;
    		basePosition = { top: options.of.pageY, left: options.of.pageX };
    	} else {
    		targetWidth = target.outerWidth();
    		targetHeight = target.outerHeight();
    		basePosition = target.offset();
    	}

    	// force my and at to have valid horizontal and veritcal positions
    	// if a value is missing or invalid, it will be converted to center 
    	$.each( [ "my", "at" ], function() {
    		var pos = ( options[this] || "" ).split( " " );
    		if ( pos.length === 1) {
    			pos = horizontalPositions.test( pos[0] ) ?
    				pos.concat( [verticalDefault] ) :
    				verticalPositions.test( pos[0] ) ?
    					[ horizontalDefault ].concat( pos ) :
    					[ horizontalDefault, verticalDefault ];
    		}
    		pos[ 0 ] = horizontalPositions.test( pos[0] ) ? pos[ 0 ] : horizontalDefault;
    		pos[ 1 ] = verticalPositions.test( pos[1] ) ? pos[ 1 ] : verticalDefault;
    		options[ this ] = pos;
    	});

    	// normalize collision option
    	if ( collision.length === 1 ) {
    		collision[ 1 ] = collision[ 0 ];
    	}

    	// normalize offset option
    	offset[ 0 ] = parseInt( offset[0], 10 ) || 0;
    	if ( offset.length === 1 ) {
    		offset[ 1 ] = offset[ 0 ];
    	}
    	offset[ 1 ] = parseInt( offset[1], 10 ) || 0;

    	if ( options.at[0] === "right" ) {
    		basePosition.left += targetWidth;
    	} else if (options.at[0] === horizontalDefault ) {
    		basePosition.left += targetWidth / 2;
    	}

    	if ( options.at[1] === "bottom" ) {
    		basePosition.top += targetHeight;
    	} else if ( options.at[1] === verticalDefault ) {
    		basePosition.top += targetHeight / 2;
    	}

    	basePosition.left += offset[ 0 ];
    	basePosition.top += offset[ 1 ];

    	return this.each(function() {
    		var elem = $( this ),
    			elemWidth = elem.outerWidth(),
    			elemHeight = elem.outerHeight(),
    			position = $.extend( {}, basePosition );

    		if ( options.my[0] === "right" ) {
    			position.left -= elemWidth;
    		} else if ( options.my[0] === horizontalDefault ) {
    			position.left -= elemWidth / 2;
    		}

    		if ( options.my[1] === "bottom" ) {
    			position.top -= elemHeight;
    		} else if ( options.my[1] === verticalDefault ) {
    			position.top -= elemHeight / 2;
    		}

    		// prevent fractions (see #5280)
    		position.left = parseInt( position.left );
    		position.top = parseInt( position.top );

    		$.each( [ "left", "top" ], function( i, dir ) {
    			if ( $.ui.position[ collision[i] ] ) {
    				$.ui.position[ collision[i] ][ dir ]( position, {
    					targetWidth: targetWidth,
    					targetHeight: targetHeight,
    					elemWidth: elemWidth,
    					elemHeight: elemHeight,
    					offset: offset,
    					my: options.my,
    					at: options.at
    				});
    			}
    		});

    		if ( $.fn.bgiframe ) {
    			elem.bgiframe();
    		}
    		elem.offset( $.extend( position, { using: options.using } ) );
    		if (callback) { callback.call(this, elem, options); }
    	});
    };
    
    $.extend($.ui.position, {
        flip2: {
            left: function( position, data ) {
                if ( data.at[0] === "center" ) {
                    return;
                }
                var win = $( window ),
                over = position.left + data.elemWidth - win.width() - win.scrollLeft(),
                myOffset = data.my[ 0 ] === "left" ?
                -data.elemWidth :
                data.my[ 0 ] === "right" ?
                data.elemWidth :
                0,
                offset = -2 * data.offset[ 0 ];
                position.left += position.left < 0 ?
                myOffset + data.targetWidth + offset :
                over > 0 ?
                myOffset - data.targetWidth + offset :
                0;
            },
            top: function(position, data) {
                if (data.at[1] === 'center') { return; }
                var win = $(window);
                var notPassed = (position.top + data.elemHeight) > (win.height() + win.scrollTop());
                var scrolledPassed = win.scrollTop() > position.top;
                var offset = -2 * data.offset[1];
                var atOffset = data.at[1] === 'top' ? data.targetHeight : -data.targetHeight;
                var myOffset;
                var flip = false;

                if (data.my[1] === "top") {
                    myOffset = -data.elemHeight;
                } else if (data.my[ 1 ] === "bottom") {
                    myOffset = data.elemHeight;
                } else {
                    myOffset = 0;
                }

                if (position.top < 0) {
                    position.top += myOffset + data.targetHeight + offset;
                    flip = true;
                } else if ((data.at[1] == 'bottom' && notPassed) || (data.at[1] == 'top' && scrolledPassed)) {
                    position.top += myOffset + atOffset + offset;
                    flip = true;
                }

                if (flip) { data.at[1] = (data.at[1] == 'bottom') ? 'top' : 'bottom'; }
            }
        }
    });
})(jQuery);