Add ModbusRequest.toRawCRC() function

[dlugaz]

Add convenience function to generate a complete RTU frame. It's also logical to have a complementary function to ::fromRawCRC()

[david-drinn]

Are the CRC bytes appended in the right order?

// ie. unsigned short
const auto crc = MB::utils::calculateCRC(result);
result.push_back(reinterpret_cast<const uint8_t *>(&crc)[0]);
result.push_back(reinterpret_cast<const uint8_t *>(&crc)[1]);

It looks correct to me after testing and reviewing the spec and this code.

The Modbus implementation guide says the least-significant byte gets transmitted first.

The calculateCRC() function in this library doesn't preemptively try to reorder it for you, instead just giving you the 16-bit CRC value as-is, leaving it to you to serialize it as needed.

// modbusUtils.hpp
inline uint16_t calculateCRC(const std::vector<uint8_t> &buffer) {
    return MB::CRC::calculateCRC(buffer);
}

// crc.hpp
//! Calculate CRC based on the input vector of bytes
inline uint16_t calculateCRC(const std::vector<uint8_t> &buffer,
                             std::optional<std::size_t> len = std::nullopt) {
    std::size_t bufferLength = buffer.size();
    if (len.has_value() && bufferLength >= *len) {
        bufferLength = *len;
    }

    return calculateCRC(static_cast<const uint8_t *>(&(*buffer.begin())), bufferLength);
}

// crc.cpp
uint16_t MB::mbCRC::calculateCRC(const uint8_t *buff, std::size_t len) {
    constexpr uint16_t wCRCTable[] = {
        // snipped table
        0X4380, 0X8341, 0X4100, 0X81C1, 0X8081, 0X4040};

    uint8_t nTemp;
    uint16_t wCRCWord = 0xFFFF;

    while (len--) {
        nTemp = *buff++ ^ wCRCWord;
        wCRCWord >>= 8;
        wCRCWord ^= wCRCTable[nTemp];
    }
    return wCRCWord;
}

The above result.push_back() calls indeed put the least-significant byte first -- (&crc)[0] first, (&crc)[1] second -- in the serialized message.