diff options
Diffstat (limited to 'Build/source/utils/gzip/sample/makecrc.c')
-rw-r--r-- | Build/source/utils/gzip/sample/makecrc.c | 63 |
1 files changed, 63 insertions, 0 deletions
diff --git a/Build/source/utils/gzip/sample/makecrc.c b/Build/source/utils/gzip/sample/makecrc.c new file mode 100644 index 00000000000..2b60f476e6a --- /dev/null +++ b/Build/source/utils/gzip/sample/makecrc.c @@ -0,0 +1,63 @@ +/* Not copyrighted 1990 Mark Adler */ + +#ifndef lint +static char rcsid[] = "$Id: makecrc.c,v 0.6 1993/05/28 07:42:59 jloup 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 (0xedb88320) */ + 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 = i; + /* The idea to initialize the register with the byte instead of + * zero was stolen from Haruhiko Okumura's ar002 + */ + for (k = 8; k; k--) + c = c & 1 ? (c >> 1) ^ e : c >> 1; + printf(i % 5 ? ", 0x%08lxL" : ",\n 0x%08lxL", c); + } + putchar('\n'); + return 0; +} |