jQuery get an elements position or offset according to some parent element

$.fn.offsetRelative = function(top){
    var $this = $(this);
    var $parent = $this.offsetParent();
    var offset = $this.position();
    
    // add scroll
    offset.top += $this.scrollTop()+$parent.scrollTop();
    offset.left += $this.scrollLeft()+$parent.scrollLeft();
    if(!top) {
        // Didn't pass a 'top' element
        return offset;
    } else if($parent.get(0).tagName == "BODY") {
        // Reached top of document
        return offset;
    } else if($($(top), $parent).length) {
        // Parent element contains the 'top' element we want the offset to be relative to
        return offset;
    } else if($parent[0] == $(this).closest(top)[0]) {
        // Reached the 'top' element we want the offset to be relative to
        return offset;
    } else {
        // Get parent's relative offset
        var parent_offset = $parent.offsetRelative(top);
        offset.top += parent_offset.top;
        offset.left += parent_offset.left;
        return offset;
    }
};

JSFiddle link