public function checkMap(string $url, string $sha1)
    {
        $this->map = $this->getMap();

        if (isset($this->map[$url]['sri'])) {
            return $sri = (string) $this->map[$url]['sri'];
        }
        if (mb_strpos($url, '//') !== 0) { // Should check `//`?
            ${'//'} = preg_replace('/^https?\:/ui', '', $url);

            if (isset($this->map[${'//'}]['sri'])) {
                return $sri = (string) $this->map[${'//'}]['sri'];
            } // i.e., If a map entry covers all schemes.
        }
        return $sri = null; // Not in the map.
    }