/* crc32.c -- compute the CRC-32 of a data stream
* Copyright (C) 1995-2005 Mark Adler
* For conditions of distribution and use, see copyright notice in zlib.h
*
* Thanks to Rodney Brown <[email protected]> for his contribution of faster
* CRC methods: exclusive-oring 32 bits of data at a time, and pre-computing
* tables for updating the shift register in one step with three exclusive-ors
* instead of four steps with four exclusive-ors. This results in about a
* factor of two increase in speed on a Power PC G4 (PPC7455) using gcc -O3.
*/
/* @(#) $Id$ */
/*
Note on the use of DYNAMIC_CRC_TABLE: there is no mutex or semaphore
protection on the static variables used to control the first-use generation
of the crc tables. Therefore, if you #define DYNAMIC_CRC_TABLE, you should
first call get_crc_table() to initialize the tables before allowing more than
one thread to use crc32().
*/
#ifdef MAKECRCH
# include <stdio.h>
# ifndef DYNAMIC_CRC_TABLE
# define DYNAMIC_CRC_TABLE
# endif /* !DYNAMIC_CRC_TABLE */
#endif /* MAKECRCH */
#include "zutil.h" /* for STDC and FAR definitions */
#define local static
/* Find a four-byte integer type for crc32_little() and crc32_big(). */
#ifndef NOBYFOUR
# ifdef STDC /* need ANSI C limits.h to determine sizes */
# include <limits.h>
# define BYFOUR
# if (UINT_MAX == 0xffffffffUL)
typedef unsigned int u4;
# else
# if (ULONG_MAX == 0xffffffffUL)
typedef unsigned long u4;
# else
# if (USHRT_MAX == 0xffffffffUL)
typedef unsigned short u4;
# else
# undef BYFOUR /* can't find a four-byte integer type! */
# endif
# endif
# endif
# endif /* STDC */
#endif /* !NOBYFOUR */
/* Definitions for doing the crc four data bytes at a time. */
#ifdef BYFOUR
# define REV(w) (((w)>>24)+(((w)>>8)&0xff00)+ \
(((w)&0xff00)<<8)+(((w)&0xff)<<24))
local unsigned long crc32_little OF((unsigned long,
const unsigned char FAR *, unsigned));
local unsigned long crc32_big OF((unsigned long,
const unsigned char FAR *, unsigned));
# define TBLS 8
#else
# define TBLS 1
#endif /* BYFOUR */
/* Local functions for crc concatenation */
local unsigned long gf2_matrix_times OF((unsigned long *mat,
unsigned long vec));
local void gf2_matrix_square OF((unsigned long *square, unsigned long *mat));
#ifdef DYNAMIC_CRC_TABLE
local volatile int crc_table_empty = 1;
local unsigned long FAR crc_table[TBLS][256];
local void make_crc_table OF((void));
#ifdef MAKECRCH
local void write_table OF((FILE *, const unsigned long FAR *));
#endif /* MAKECRCH */
/*
Generate tables 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 first table is simply the CRC of all possible eight bit values. This is
all the information needed to generate CRCs on data a byte at a time for all
combinations of CRC register values and incoming bytes. The remaining tables
allow for word-at-a-time CRC calculation for both big-endian and little-
endian machines, where a word is four bytes.
*/
local void make_crc_table()
{
unsigned long c;
int n, k;
unsigned long poly; /* polynomial exclusive-or pattern */
/* terms of polynomial defining this crc (except x^32): */
static volatile int first = 1; /* flag to limit concurrent making */
static const unsigned char p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
/* See if another task is already doing this (not thread-safe, but better
than nothing -- significantly reduces duration of vulnerability in
case the advice about DYNAMIC_CRC_TABLE is ignored) */
if (first) {
first = 0;
/* make exclusive-or pattern from polynomial (0xedb88320UL) */
poly = 0UL;
for (n = 0; n < sizeof(p)/sizeof(unsigned char); n++)
poly |= 1UL << (31 - p[n]);
/* generate a crc for every 8-bit value */
for (n = 0; n < 256; n++) {
c = (unsigned long)n;
for (k = 0; k < 8; k++)
c = c & 1 ? poly ^ (c >> 1) : c >> 1;
crc_table[0][n] = c;
}
#ifdef BYFOUR
/* generate crc for each value followed by one, two, and three zeros,
and then the byte reversal of those as well as the first table */
for (n = 0; n < 256; n++) {
c = crc_table[0][n];
crc_table[4][n] = REV(c);
for (k = 1; k < 4; k++) {
c = crc_table[0][c & 0xff] ^ (c >> 8);
crc_table[k][n] = c;
crc_table[k + 4][n] = REV(c);
}
}
#endif /* BYFOUR */
crc_table_empty = 0;
}
else { /* not first */
/* wait for the other guy to finish (not efficient, but rare) */
while (crc_table_empty)
;
}
#ifdef MAKECRCH
/* write out CRC tables to crc32.h */
{
FILE *out;
out = fopen("crc32.h", "w");
if (out == NULL) return;
fprintf(out, "/* crc32.h -- tables for rapid CRC calculation\n");
fprintf(out, " * Generated automatically by crc32.c\n */\n\n");
fprintf(out, "local const unsigned long FAR ");
fprintf(out, "crc_table[TBLS][256] =\n{\n {\n");
write_table(out, crc_table[0]);
# ifdef BYFOUR
fprintf(out, "#ifdef BYFOUR\n");
for (k = 1; k < 8; k++) {
fprintf(out, " },\n {\n");
write_table(out, crc_table[k]);
}
fprintf(out, "#endif\n");
# endif /* BYFOUR */
fprintf(out, " }\n};\n");
fclose(out);
}
#endif /* MAKECRCH */
}
#ifdef MAKECRCH
local void write_table(out, table)
FILE *out;
const unsigned long FAR *table;
{
int n;
for (n = 0; n < 256; n++)
fprintf(out, "%s0x%08lxUL%s", n % 5 ? "" : " ", table[n],
n == 255 ? "\n" : (n % 5 == 4 ? ",\n" : ", "));
}
#endif /* MAKECRCH */
#else /* !DYNAMIC_CRC_TABLE */
/* ========================================================================
* Tables of CRC-32s of all single-byte values, made by make_crc_table().
*/
#include "crc32.h"
#endif /* DYNAMIC_CRC_TABLE */
/* =========================================================================
* This function can be used by asm versions of crc32()
*/
const unsigned long FAR * ZEXPORT get_crc_table()
{
#ifdef DYNAMIC_CRC_TABLE
if (crc_table_empty)
make_crc_table();
#endif /* DYNAMIC_CRC_TABLE */
return (const unsigned long FAR *)crc_table;
}
/* ========================================================================= */
#define DO1 crc
