private static function checksum(string $number) : int
    {
        $number = strrev($number);
        $length = strlen($number);

        $sum = 0;

        for ($i = 0; $i < $length; $i++) {
            $value = $number[$i] * ($i % 2 + 1);
            $sum += ($value >= 10 ? $value - 9 : $value);
        }

        return $sum % 10;
    }