private function getTopologyGroup(string $char): string
    {
        $groups = $this->chars;

        if (\strpos($groups[0], $char) !== false) {
            return 'u';
        }

        if (\strpos($groups[1], $char) !== false) {
            return 'l';
        }

        if (\strpos($groups[2], $char) !== false) {
            return 'd';
        }

        if (\strpos($groups[3], $char) !== false) {
            return 's';
        }

        throw new InvalidArgumentException('Out of group character provided.');
    }