crc16t.c

/***********************************************************
    crc16t.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 crctable[UCHAR_MAX + 1];

void makecrctable1(void)
{
    unsigned int i, j, r;

    for (i = 0; i <= UCHAR_MAX; i++) {
        r = i << (16 - CHAR_BIT);
        for (j = 0; j < CHAR_BIT; j++)
            if (r & 0x8000U) r = (r << 1) ^ CRCPOLY1;
            else             r <<= 1;
        crctable[i] = r & 0xFFFFU;
    }
}

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

    r = 0xFFFFU;
    while (--n >= 0)
        r = (r << CHAR_BIT) ^ crctable[(byte)(r >> (16 - CHAR_BIT)) ^ *c++];
    return ~r & 0xFFFFU;
}

void makecrctable2(void)
{
    unsigned int i, j, r;

    for (i = 0; i <= UCHAR_MAX; i++) {
        r = i;
        for (j = 0; j < CHAR_BIT; j++)
            if (r & 1) r = (r >> 1) ^ CRCPOLY2;
            else       r >>= 1;
        crctable[i] = r;
    }
}

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

    r = 0xFFFFU;
    while (--n >= 0)
        r = (r >> CHAR_BIT) ^ crctable[(byte)r ^ *c++];
    return r ^ 0xFFFFU;
}

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

    makecrctable1();
    printf("crc1(%s) = %04X\n", s, crc1(13, s));
    makecrctable2();
    printf("crc2(%s) = %04X\n", s, crc2(13, s));
    return 0;
}