65 lines
2.4 KiB
C
65 lines
2.4 KiB
C
/* Not copyrighted 1990 Mark Adler */
|
|
|
|
#ifndef lint
|
|
static char rcsid[] = "$Id: makecrc.c,v 1.1 1993/04/10 15:56:11 cgd Exp $";
|
|
#endif
|
|
|
|
#include <stdio.h>
|
|
|
|
main()
|
|
/*
|
|
Generate a table for a byte-wise 32-bit CRC calculation on the polynomial:
|
|
x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1.
|
|
|
|
Polynomials over GF(2) are represented in binary, one bit per coefficient,
|
|
with the lowest powers in the most significant bit. Then adding polynomials
|
|
is just exclusive-or, and multiplying a polynomial by x is a right shift by
|
|
one. If we call the above polynomial p, and represent a byte as the
|
|
polynomial q, also with the lowest power in the most significant bit (so the
|
|
byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,
|
|
where a mod b means the remainder after dividing a by b.
|
|
|
|
This calculation is done using the shift-register method of multiplying and
|
|
taking the remainder. The register is initialized to zero, and for each
|
|
incoming bit, x^32 is added mod p to the register if the bit is a one (where
|
|
x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
|
|
x (which is shifting right by one and adding x^32 mod p if the bit shifted
|
|
out is a one). We start with the highest power (least significant bit) of
|
|
q and repeat for all eight bits of q.
|
|
|
|
The table is simply the CRC of all possible eight bit values. This is all
|
|
the information needed to generate CRC's on data a byte at a time for all
|
|
combinations of CRC register values and incoming bytes. The table is
|
|
written to stdout as 256 long hexadecimal values in C language format.
|
|
*/
|
|
{
|
|
unsigned long c; /* crc shift register */
|
|
unsigned long e; /* polynomial exclusive-or pattern */
|
|
int i; /* counter for all possible eight bit values */
|
|
int k; /* byte being shifted into crc apparatus */
|
|
|
|
/* terms of polynomial defining this crc (except x^32): */
|
|
static int p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
|
|
|
|
/* Make exclusive-or pattern from polynomial */
|
|
e = 0;
|
|
for (i = 0; i < sizeof(p)/sizeof(int); i++)
|
|
e |= 1L << (31 - p[i]);
|
|
|
|
/* Compute and print table of CRC's, five per line */
|
|
printf(" 0x00000000L");
|
|
for (i = 1; i < 256; i++)
|
|
{
|
|
c = 0;
|
|
for (k = i | 256; k != 1; k >>= 1)
|
|
{
|
|
c = c & 1 ? (c >> 1) ^ e : c >> 1;
|
|
if (k & 1)
|
|
c ^= e;
|
|
}
|
|
printf(i % 5 ? ", 0x%08lxL" : ",\n 0x%08lxL", c);
|
|
}
|
|
putchar('\n');
|
|
return 0;
|
|
}
|