crc.cpp

/*
* The width of the CRC calculation and result.
* Modify the typedef for a 16 or 32-bit CRC standard.
*/
typedef uint8_t crc;

#define WIDTH  (8 * sizeof(crc))
#define TOPBIT (1 << (WIDTH - 1))

crc crcSlow(uint8_t const message[], int nBytes) {

    crc  remainder = 0;	

    // Perform modulo-2 division, a byte at a time.
    for (int byte = 0; byte < nBytes; ++byte) {

    // Bring the next byte into the remainder.
    remainder ^= (message[byte] << (WIDTH - 8));
    // Perform modulo-2 division, a bit at a time.

    for (uint8_t bit = 8; bit > 0; --bit) {
    // Try to divide the current data bit.
    if (remainder & TOPBIT)
    remainder = (remainder << 1) ^ POLYNOMIAL;
    else
    remainder = (remainder << 1);
    }
    }

    // The final remainder is the CRC result.
    return (remainder);

}