private function buildDigitList($radix)
    {
        if ($radix <= strlen(self::$integerBase)) {
            return str_split(substr(self::$integerBase, 0, $radix));
        } elseif ($radix === 64) {
            return str_split(self::$integerBase64);
        } elseif ($radix <= 256) {
            return range(chr(0), chr($radix - 1));
        }

        return $this->buildArbitraryList($radix);
    }