crc.cpp

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#include <septentrio_gnss_driver/crc/crc.hpp>
#include <septentrio_gnss_driver/parsers/parsing_utilities.hpp>
 
namespace crc {
    uint16_t compute16CCITT(const uint8_t* buf,
                            size_t buf_length) // The CRC we choose is 2 bytes,
                                               // remember, hence uint16_t..
    {
        uint16_t crc = 0; // Seed is 0, as suggested by the firmware, will compute
                          // CRC in the forward direction..
 
        for (size_t i = 0; i < buf_length; i++)
        {
            crc = (crc << 8) ^ CRC_LOOK_UP[uint8_t((crc >> 8) ^ buf[i])];
            // The ^ (bitwise XOR) in C or C++ takes two numbers as operands and does
            // XOR on every bit of two numbers. The result of XOR is 1 if the two
            // bits are different. The << (left shift) in C or C++ takes two numbers,
            // left shifts the bits of the first operand, the second operand decides
            // the number of places to shift. The >> (right shift) in C or C++ takes
            // two numbers, right shifts the bits of the first operand, the second
            // operand decides the number of places to shift; you can just loose the
            // smallest values if big-endian. The left shift and right shift
            // operators should not be used for negative numbers. The left-shift and
            // right-shift operators are equivalent to multiplication and division by
            // 2 respectively, hence only rightshift is non-exact (remainder is not
            // retained). CRC_LOOK_UP is constructed from truncated polynomial
            // (divisor). The above implements a kind of CRC 32 algorithm: efficient,
            // fast.
        }
 
        return crc;
    }
 
    bool isValid(const std::vector<uint8_t>& message)
    {
        // We need all of the message except for the first 4 bytes (Sync and CRC),
        // i.e. we start at the address of ID.
        uint16_t length = parsing_utilities::getLength(message);
        if (length > 4)
        {
            uint16_t crc = compute16CCITT(message.data() + 4, length - 4);
            return (crc == parsing_utilities::getCrc(message));
        } else
        {
            return false;
        }
    }
} // namespace crc