private function groupByScheme(SearchResult $urls)
    {
        $grouped = [];

        foreach ($urls as $url) {
            $scheme = parse_url($url->url, PHP_URL_SCHEME);
            if (!$scheme) {
                continue;
            }

            if (!isset($grouped[$scheme])) {
                $grouped[$scheme] = [];
            }

            $grouped[$scheme][] = $url;
        }

        return $grouped;
    }