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Comparing part of a QByteArray with quint16, how to do it best?


  • Moderators

    Hello everyone,

    I'm currently working on a QByteArray based communication between 2 devices. Per definition, the last 2 Bytes should always be the crc value, of everything before.

    I'm using QDatastream to write&read into/from the QByteArray.

    Everything work fine. But I think in the receiving part, I may have choosen a "slow" way to compare the send crc with the calcualted one.

    I know we're talking about ms improvments, if any, but as a matter of principle does anyone know a more elegant or faster way to do this?

    Take a look at my code:

    
    bool Datagram::isCorrectResponse(const QByteArray &data)
    {
         ....
         ....
         //Crc Part
         quint16 crcNew (calculateCrc(data.mid(0,data.length()-2));
         quint18 crcOld;
    
         QByteArray baCrc(data.mid(data.length()-2,2));
         QDataStream out(&baCrc, QIODevice::ReadOnly);
         out >> crcOld;
    
         if(crcNew != crcOld)
             return false;
         else
             return true;
    }
    
    //CRC calculation
    quint16 Datagram::calculateCrc(const QByteArray &data)
    {
        quint16 crc = 0xFFFF;
        for(int pos(0); pos < data.length(); pos++){
            crc ^=(uint16_t)data[pos];
            for(int i(8); i!= 0; i--){
                if((crc & 0x0001) != 0){
                    crc >>= 1;
                    crc ^= 0xA001;
                }else{
                    crc >>=1;
                }
            }
        }
        return qFromBigEndian(crc);
    }
    

  • Qt Champions 2017

    bool Datagram::isCorrectResponse(const QByteArray & data)
    {
         int offset = data.length() - sizeof(quint16);
    
         // Calculate the hash
         quint16 crcNew = calculateCrc(data.constBegin(), offset);
    
         // Read the received hash
         QDataStream out(data);
         if (out.skipRawData(offset) != offset)
             return false;
    
         quint18 crcOld;
         out >> crcOld;
    
         // Compare ...
         return out.status() == QDataStream::Ok && crcNew == crcOld;
    }
    
    quint16 Datagram::calculateCrc(QByteArray::const_iterator iterator, int length)
    {
        // ...
        quint16 crc = 0xFFFF;
        for(QByteArray::const_iterator end = iterator + length; iterator != end; ++iterator)  {
            char byteToHash = *iterator;
            // Use a lookup table here
            // ...
        }
        // ...
    }
    

  • Moderators

    hi @kshegunov ,
    I like your solution a lot, removes a lot of QByteArray-Copying/Constructing that botheres me in the my code. But for example I didn't know that QDataStream had a skipRawData function, I was looking for a backwards rollup ore something instead.

    So I'm defenitly adopting a good part of your example!
    However I'm not sure about the calculateCrc change, I actually tried to avoid a lookup table.


  • Lifetime Qt Champion

    @J.Hilk said in Comparing part of a QByteArray with quint16, how to do it best?:

    However I'm not sure about the calculateCrc change, I actually tried to avoid a lookup table.

    Why? Usually the tables are 256 entries a 16 Bit = 512 Byte, so size should not be a problem.

    If it really brings a performance boost can only be measured with a benchmark. One problem in the "non-table" code could be the if-condition. All other arithmetics should be rather fast.


  • Moderators

    @aha_1980 For me in this case, it's not a performance issue but rather a aesthetics one.
    I dislike huge and bulky definitions in my source code.

    0_1516953636657_embarrassedEmojii.jpg

    However I made the following changes to it:

    //CRC calculation
    quint16 Datagram::calculateCrc(const QByteArray &data, const uchar &offset)
    {
        quint16 crc = 0xFFFF;
        for(int pos(0); pos < data.length() - offset; pos++){
            crc ^=(uint16_t)data[pos];
            for(int i(8); i!= 0; i--){
                if((crc & 0x0001) != 0){
                    crc >>= 1;
                    crc ^= 0xA001;
                }else{
                    crc >>=1;
                }
            }
        }
        return qFromBigEndian(crc);
    }
    

    with a default for offset of 0, so the change doesn't break other parts of my code and calling it from
    isCorrectResponse this way:

    crcNew = calculateCrc(data,2);
    

  • Qt Champions 2017

    @J.Hilk said in Comparing part of a QByteArray with quint16, how to do it best?:

    However I made the following changes to it

    Well, I'd have used the crc16 Qt already provides, unless you're bent on a specific implementation (or Qt version).

    @aha_1980 said in Comparing part of a QByteArray with quint16, how to do it best?:

    If it really brings a performance boost can only be measured with a benchmark.

    I'd bet on the lookup table, almost every time. And the table is only 16 shorts (qbytearray.cpp @ 532 in Qt's source) - small enough to fit a single cache line ...


  • Moderators

    @kshegunov said in Comparing part of a QByteArray with quint16, how to do it best?:

    Well, I'd have used the crc16 Qt already provides, unless you're bent on a specific implementation (or Qt version).

    I'll have to use a crc16, the Datagram is based on Modbus.

    I' had some issues with my function, for some reasons, there were QByteArrays, that resulted in a wrong crc. Far from all, but some ( ca 1 in 20!?)

    So I looked up, what Qt does in the QModbus - Rtu class:

    /* Table of CRC values for high-order byte */
    static const uint8_t table_crc_hi[] = {
        0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
        0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
        0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
        0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40,
        0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
        0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 0x80, 0x41,
        0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
        0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
        0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
        0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40,
        0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
        0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40,
        0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
        0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40,
        0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
        0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40,
        0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
        0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
        0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
        0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
        0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
        0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40,
        0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
        0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
        0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
        0x80, 0x41, 0x00, 0xC1, 0x81, 0x40
    };
    
    /* Table of CRC values for low-order byte */
    static const uint8_t table_crc_lo[] = {
        0x00, 0xC0, 0xC1, 0x01, 0xC3, 0x03, 0x02, 0xC2, 0xC6, 0x06,
        0x07, 0xC7, 0x05, 0xC5, 0xC4, 0x04, 0xCC, 0x0C, 0x0D, 0xCD,
        0x0F, 0xCF, 0xCE, 0x0E, 0x0A, 0xCA, 0xCB, 0x0B, 0xC9, 0x09,
        0x08, 0xC8, 0xD8, 0x18, 0x19, 0xD9, 0x1B, 0xDB, 0xDA, 0x1A,
        0x1E, 0xDE, 0xDF, 0x1F, 0xDD, 0x1D, 0x1C, 0xDC, 0x14, 0xD4,
        0xD5, 0x15, 0xD7, 0x17, 0x16, 0xD6, 0xD2, 0x12, 0x13, 0xD3,
        0x11, 0xD1, 0xD0, 0x10, 0xF0, 0x30, 0x31, 0xF1, 0x33, 0xF3,
        0xF2, 0x32, 0x36, 0xF6, 0xF7, 0x37, 0xF5, 0x35, 0x34, 0xF4,
        0x3C, 0xFC, 0xFD, 0x3D, 0xFF, 0x3F, 0x3E, 0xFE, 0xFA, 0x3A,
        0x3B, 0xFB, 0x39, 0xF9, 0xF8, 0x38, 0x28, 0xE8, 0xE9, 0x29,
        0xEB, 0x2B, 0x2A, 0xEA, 0xEE, 0x2E, 0x2F, 0xEF, 0x2D, 0xED,
        0xEC, 0x2C, 0xE4, 0x24, 0x25, 0xE5, 0x27, 0xE7, 0xE6, 0x26,
        0x22, 0xE2, 0xE3, 0x23, 0xE1, 0x21, 0x20, 0xE0, 0xA0, 0x60,
        0x61, 0xA1, 0x63, 0xA3, 0xA2, 0x62, 0x66, 0xA6, 0xA7, 0x67,
        0xA5, 0x65, 0x64, 0xA4, 0x6C, 0xAC, 0xAD, 0x6D, 0xAF, 0x6F,
        0x6E, 0xAE, 0xAA, 0x6A, 0x6B, 0xAB, 0x69, 0xA9, 0xA8, 0x68,
        0x78, 0xB8, 0xB9, 0x79, 0xBB, 0x7B, 0x7A, 0xBA, 0xBE, 0x7E,
        0x7F, 0xBF, 0x7D, 0xBD, 0xBC, 0x7C, 0xB4, 0x74, 0x75, 0xB5,
        0x77, 0xB7, 0xB6, 0x76, 0x72, 0xB2, 0xB3, 0x73, 0xB1, 0x71,
        0x70, 0xB0, 0x50, 0x90, 0x91, 0x51, 0x93, 0x53, 0x52, 0x92,
        0x96, 0x56, 0x57, 0x97, 0x55, 0x95, 0x94, 0x54, 0x9C, 0x5C,
        0x5D, 0x9D, 0x5F, 0x9F, 0x9E, 0x5E, 0x5A, 0x9A, 0x9B, 0x5B,
        0x99, 0x59, 0x58, 0x98, 0x88, 0x48, 0x49, 0x89, 0x4B, 0x8B,
        0x8A, 0x4A, 0x4E, 0x8E, 0x8F, 0x4F, 0x8D, 0x4D, 0x4C, 0x8C,
        0x44, 0x84, 0x85, 0x45, 0x87, 0x47, 0x46, 0x86, 0x82, 0x42,
        0x43, 0x83, 0x41, 0x81, 0x80, 0x40
    };
    
    static uint16_t crc16(uint8_t *buffer, uint16_t buffer_length)
    {
        uint8_t crc_hi = 0xFF; /* high CRC byte initialized */
        uint8_t crc_lo = 0xFF; /* low CRC byte initialized */
        unsigned int i; /* will index into CRC lookup */
    
        /* pass through message buffer */
        while (buffer_length--) {
            i = crc_hi ^ *buffer++; /* calculate the CRC  */
            crc_hi = crc_lo ^ table_crc_hi[i];
            crc_lo = table_crc_lo[i];
        }
    
        return (crc_hi << 8 | crc_lo);
    }
    
    //with this call in my class:
    quint16 crcNew = crc16((unsigned char *)(data.data()),offset);
    

    Seems to work fine.


  • Qt Champions 2017

    @J.Hilk said in Comparing part of a QByteArray with quint16, how to do it best?:

    I'll have to use a crc16

    qChecksum is crc16, or more-precisely one implementation out of the many.

    So I looked up, what Qt does in the QModbus - Rtu class

    Perhaps that code should be moved to QtBase ...


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