crc16.c

/***********************************************************
    crc16.c -- CRC
***********************************************************/
#include <limits.h>
#define CRCPOLY1  0x1021U  /* x^{16}+x^{12}+x^5+1 */
#define CRCPOLY2  0x8408U  /* 左右逆転 */
typedef unsigned char byte;

unsigned int crc1(int n, byte c[])
{
    unsigned int i, j, r;

    r = 0xFFFFU;
    for (i = 0; i < n; i++) {
        r ^= (unsigned int)c[i] << (16 - CHAR_BIT);
        for (j = 0; j < CHAR_BIT; j++)
            if (r & 0x8000U) r = (r << 1) ^ CRCPOLY1;
            else             r <<= 1;
    }
    return ~r & 0xFFFFU;
}

unsigned int crc2(int n, byte c[])
{
    unsigned int i, j, r;

    r = 0xFFFFU;
    for (i = 0; i < n; i++) {
        r ^= c[i];
        for (j = 0; j < CHAR_BIT; j++)
            if (r & 1) r = (r >> 1) ^ CRCPOLY2;
            else       r >>= 1;
    }
    return r ^ 0xFFFFU;
}

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
    static byte s[] = "Hello, world!";

    printf("crc1(%s) = %04X\n", s, crc1(13, s));
    printf("crc2(%s) = %04X\n", s, crc2(13, s));
    return 0;
}