RS_code 编码和解码

1. RS_Code是什么
        RS_Code是纠删码的一种。 
       Erasure Code（EC），即纠删码，是一种前向错误纠正技术（Forward Error Correction，FEC），主要应用在网络传输中避免包的丢失， 存储系统利用它来提高 存储 可靠性。相比多副本复制而言， 纠删码能够以更小的数据冗余度获得更高数据可靠性， 但编码方式较复杂，需要大量计算 。纠删码只能容忍数据丢失，无法容忍数据篡改，纠删码正是得名与此。
      Erasure Code是一种编码技术，它可以将n份原始数据，增加m份数据，并能通过n+m份中的任意n份数据，还原为原始数据。即如果有任意小于等于m份的数据失效，仍然能通过剩下的数据还原出来。
      目前，纠删码技术在分布式存储 系统中的应用主要有三类，阵列纠删码（Array Code: RAID5、RAID6等）、RS(Reed-Solomon)里德-所罗门类纠删码和LDPC(LowDensity Parity Check Code)低密度奇偶校验纠删码。
2. RS_Code库

INCLUDEPATH += $$PWD/../depends/rscode/include
Debug:{
   LIBS += -L$$PWD/../depends/rscode/lib/x64/Debug -lrscode
}
Release:{
   LIBS += -L$$PWD/../depends/rscode/lib/x64/Release -lrscode
}

3. RS_Code的使用

QByteArray origMsg = QByteArray::fromHex( "09 0c 09 0a 0c 0a 0b 0f 0c 0c 10 0c 0d 0d 10 11 0f 15 08 15 11 0b 0d 00 0c 00");

    qDebug() << "mac = " << mac << "before, origMsg = " << origMsg.toHex();
    QByteArray inbandMsg = ConvertOrigMsgToInbandMsg(origMsg);
    qDebug() << "mac = " << mac << "before, inbandMsg = " << inbandMsg.toHex();
    QList<int> lostFlag;
    lostFlag.clear();
    for (int i = 0; i< 26-3;i++)
    {
        lostFlag << (0);
    }
    lostFlag << (1);
    lostFlag << (0);
    lostFlag << (1);
    QVector<qint32> erasure = GetRSCodeErasure(lostFlag);
    RSCode::getInstance().Decode(inbandMsg, erasure);
    qDebug() << "mac = " << mac << "after,  inbandMsg = " << inbandMsg.toHex();
    origMsg = InbandMsgToConvertOrigMsg(inbandMsg);
    qDebug() << "mac = " << mac << "after,  origMsg = " << origMsg.toHex();

QByteArray InbandMsg::ConvertOrigMsgToInbandMsg(QByteArray &origMsg)
{
    QByteArray inbandMsg;

    //未拼接的原始消息长度必须是2的倍数（原始消息每次传4bit）
    if ((origMsg.size() % 2) != 0)
    {
        return inbandMsg;
    }

    int inbandMsgSize = origMsg.size() / 2;
    int high4Bit;
    int low4Bit;
    int byteValue;
    for (int i = 0; i < inbandMsgSize; i++)
    {
        high4Bit = (origMsg[2*i] - ORIG_MSG_CONTENT_BEGINE) & 0x0f;
        low4Bit = (origMsg[2*i + 1] - ORIG_MSG_CONTENT_BEGINE) & 0x0f;
        byteValue = (high4Bit << 4) + low4Bit;
        inbandMsg += static_cast<char>(byteValue);
    }
    return inbandMsg;
}


QByteArray InbandMsg::InbandMsgToConvertOrigMsg(QByteArray &inbandMsg)
{
    QByteArray origMsg;
    int high4Bit;
    int low4Bit;
    for (int i = 0; i < inbandMsg.size(); i++)
    {
        high4Bit = (inbandMsg.at(i) & 0xf0) >> 4;
        origMsg+= static_cast<char>(high4Bit);
        low4Bit = inbandMsg.at(i) & 0x0f;
        origMsg+= static_cast<char>(low4Bit);
    }

    return origMsg;
}

4. 参考文献
Erasure Code - EC纠删码原理     https://blog.csdn.net/shelldon/article/details/54144730