sample/makecrc.c

   1 /* Not copyrighted 1990 Mark Adler */
   2
   3 #include <stdio.h>
   4
   5 main()
   6 /*
   7   Generate a table for a byte-wise 32-bit CRC calculation on the polynomial:
   8   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.
   9
  10   Polynomials over GF(2) are represented in binary, one bit per coefficient,
  11   with the lowest powers in the most significant bit.  Then adding polynomials
  12   is just exclusive-or, and multiplying a polynomial by x is a right shift by
  13   one.  If we call the above polynomial p, and represent a byte as the
  14   polynomial q, also with the lowest power in the most significant bit (so the
  15   byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,
  16   where a mod b means the remainder after dividing a by b.
  17
  18   This calculation is done using the shift-register method of multiplying and
  19   taking the remainder.  The register is initialized to zero, and for each
  20   incoming bit, x^32 is added mod p to the register if the bit is a one (where
  21   x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
  22   x (which is shifting right by one and adding x^32 mod p if the bit shifted
  23   out is a one).  We start with the highest power (least significant bit) of
  24   q and repeat for all eight bits of q.
  25
  26   The table is simply the CRC of all possible eight bit values.  This is all
  27   the information needed to generate CRC's on data a byte at a time for all
  28   combinations of CRC register values and incoming bytes.  The table is
  29   written to stdout as 256 long hexadecimal values in C language format.
  30 */
  31 {
  32   unsigned long c;      /* crc shift register */
  33   unsigned long e;      /* polynomial exclusive-or pattern */
  34   int i;                /* counter for all possible eight bit values */
  35   int k;                /* byte being shifted into crc apparatus */
  36
  37   /* terms of polynomial defining this crc (except x^32): */
  38   static int p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
  39
  40   /* Make exclusive-or pattern from polynomial (0xedb88320) */
  41   e = 0;
  42   for (i = 0; i < sizeof(p)/sizeof(int); i++)
  43     e |= 1L << (31 - p[i]);
  44
  45   /* Compute and print table of CRC's, five per line */
  46   printf("  0x00000000L");
  47   for (i = 1; i < 256; i++)
  48   {
  49     c = i;
  50     /* The idea to initialize the register with the byte instead of
  51      * zero was stolen from Haruhiko Okumura's ar002
  52      */
  53     for (k = 8; k; k--)
  54       c = c & 1 ? (c >> 1) ^ e : c >> 1;
  55     printf(i % 5 ? ", 0x%08lxL" : ",\n  0x%08lxL", c);
  56   }
  57   putchar('\n');
  58   return 0;
  59 }