private function getNumericRepresentation($letterRepresentation)
    {
        $numericRepresentation = '';

        foreach (str_split($letterRepresentation) as $char) {
            $ord = ord($char);
            if ($ord >= 65 && $ord <= 90) {
                $numericRepresentation .= (string) ($ord - 55);
            } elseif ($ord >= 48 && $ord <= 57) {
                $numericRepresentation .= (string) ($ord - 48);
            }
        }

        return $numericRepresentation;
    }