CRC-CCITT (XModem)

crc_table_gen.c

#include <iostream>
#include <iomanip>

uint16_t  crcTable[256];

#define WIDTH  (8 * sizeof(uint16_t))
#define TOPBIT (1 << (WIDTH - 1))

// Checked with https://www.lammertbies.nl/comm/info/crc-calculation
// It is CRC-CCITT (XModem)
// Use other polynomial to get other results

#define POLYNOMIAL 0x1021

void crcInit(void)
{
    uint16_t remainder;

    for (int dividend = 0; dividend < 256; ++dividend) {
        remainder = dividend << (WIDTH - 8);

        //  modulo-2 division, a bit at a time.
        for (uint8_t bit = 8; bit > 0; --bit) {
            if (remainder & TOPBIT) {
                remainder = (remainder << 1) ^ POLYNOMIAL;
            } else {
                remainder = (remainder << 1);
            }
        }
        crcTable[dividend] = remainder;
    }
}

int main() {
	crcInit();

	// with 0x (use setw(6) in that case: including 0x digits)
	//std::cout << std::showbase << std::internal << std::setfill('0');
	// without
	std::cout << std::setfill('0') << std::setw(4);

	for (int i = 0; i < 256; ++i) {
		std::cout << std::setw(4) << std::hex << crcTable[i] << std::endl;
	}
}