public function groupLinksByDepth(){
        $final_items = [];
        $this->each(function(Link $linkObj, $uri) use(&$final_items){
            $final_items[$linkObj->getCrawlDepth()][$uri] = [
               'link'  => $linkObj->getAbsoluteUrl(),
               'source_page' => $linkObj->getParentUrl(),
            ];          
        });
        
        ksort($final_items);
        return $this->make($final_items);
    }