public static function decode($str, &$index)
    {
        $result = 0;
        $shift = 0;

        do {
            $c = $str[$index++];
            $digit = Base64::decode($c);
            $continuation = ($digit & self::VLQ_CONTINUATION_BIT) != 0;
            $digit &= self::VLQ_BASE_MASK;
            $result = $result + ($digit << $shift);
            $shift = $shift + self::VLQ_BASE_SHIFT;
        } while ($continuation);

        return self::fromVLQSigned($result);
    }