zircon/third_party/ulib/cksum/crc32.c - fuchsia - Git at Google

 /* crc32.c -- compute the CRC-32 of a data stream
  * Copyright (C) 1995-2006, 2010, 2011, 2012 Mark Adler
  * For conditions of distribution and use, see copyright notice in zlib.h
  *
  * Thanks to Rodney Brown <rbrown64@csc.com.au> 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().

   DYNAMIC_CRC_TABLE and MAKECRCH can be #defined to write out crc32.h.
  */

 #ifdef MAKECRCH
 #  include <stdio.h>
 #  ifndef DYNAMIC_CRC_TABLE
 #    define DYNAMIC_CRC_TABLE
 #  endif /* !DYNAMIC_CRC_TABLE */
 #endif /* MAKECRCH */

 #include "zutil.h"      /* for STDC definitions */

 #include <lib/cksum.h>

 /* Definitions for doing the crc four data bytes at a time. */
 #if !defined(NOBYFOUR) && defined(Z_U4)
 #error
 #  define BYFOUR
 #endif
 #ifdef BYFOUR
    static uint32_t crc32_little OF((uint32_t, const uint8_t FAR *, uint32_t));
    static uint32_t crc32_big OF((uint32_t, const uint8_t FAR *, uint32_t));
 #  define TBLS 8
 #else
 #  define TBLS 1
 #endif /* BYFOUR */

 /* static functions for crc concatenation */
 static uint32_t gf2_matrix_times OF((uint32_t *mat, uint32_t vec));
 static void gf2_matrix_square OF((uint32_t *square, uint32_t *mat));


 #ifdef DYNAMIC_CRC_TABLE

 static volatile int crc_table_empty = 1;
 static uint32_t crc_table[TBLS][256];
 static void make_crc_table OF((void));
 #ifdef MAKECRCH
    static void write_table OF((FILE *, const uint32_t*));
 #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.
 */
 static void make_crc_table()
 {
     uint32_t c;
     int n, k;
     uint32_t 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 uint8_t 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 = 0;
         for (n = 0; n < (int)(sizeof(p)/sizeof(uint8_t)); n++)
             poly |= (uint32_t)1 << (31 - p[n]);

         /* generate a crc for every 8-bit value */
         for (n = 0; n < 256; n++) {
             c = (uint32_t)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] = ZSWAP32(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] = ZSWAP32(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, "static const uint32_t ");
         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
 static void write_table(out, table)
     FILE *out;
     const uint32_t *table;
 {
     int n;

     for (n = 0; n < 256; n++)
         fprintf(out, "%s0x%08lxUL%s", n % 5 ? "" : "    ",
                 (uint32_t)(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 uint32_t* ZEXPORT get_crc_table(void)
 {
 #ifdef DYNAMIC_CRC_TABLE
     if (crc_table_empty)
         make_crc_table();
 #endif /* DYNAMIC_CRC_TABLE */
     return (const uint32_t*)crc_table;
 }

 /* ========================================================================= */
 #define DO1 crc = crc_table[0][(crc ^ (*buf++)) & 0xff] ^ (crc >> 8)
 #define DO8 DO1; DO1; DO1; DO1; DO1; DO1; DO1; DO1

 /* ========================================================================= */
 uint32_t ZEXPORT crc32(uint32_t crc, const uint8_t* buf, size_t len)
 {
     if (buf == Z_NULL) return 0UL;

 #ifdef DYNAMIC_CRC_TABLE
     if (crc_table_empty)
         make_crc_table();
 #endif /* DYNAMIC_CRC_TABLE */

 #ifdef BYFOUR
     if (sizeof(void *) == sizeof(ptrdiff_t)) {
         uint32_t endian;

         endian = 1;
         if (*((uint8_t *)(&endian)))
             return crc32_little(crc, buf, len);
         else
             return crc32_big(crc, buf, len);
     }
 #endif /* BYFOUR */
     crc = crc ^ 0xffffffffUL;
     while (len >= 8) {
         DO8;
         len -= 8;
     }
     if (len) do {
         DO1;
     } while (--len);
     return crc ^ 0xffffffffUL;
 }

 #ifdef BYFOUR

 /* ========================================================================= */
 #define DOLIT4 c ^= *buf4++; \
         c = crc_table[3][c & 0xff] ^ crc_table[2][(c >> 8) & 0xff] ^ \
             crc_table[1][(c >> 16) & 0xff] ^ crc_table[0][c >> 24]
 #define DOLIT32 DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4

 /* ========================================================================= */
 static uint32_t crc32_little(uint32_t crc, const uint8_t* buf, size_t len)
 {
     register uint32_t c;
     register const uint32_t *buf4;

     c = (uint32_t)crc;
     c = ~c;
     while (len && ((ptrdiff_t)buf & 3)) {
         c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
         len--;
     }

     buf4 = (const uint32_t*)(const void*)buf;
     while (len >= 32) {
         DOLIT32;
         len -= 32;
     }
     while (len >= 4) {
         DOLIT4;
         len -= 4;
     }
     buf = (const uint8_t*)buf4;

     if (len) do {
         c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
     } while (--len);
     c = ~c;
     return (uint32_t)c;
 }

 /* ========================================================================= */
 #define DOBIG4 c ^= *++buf4; \
         c = crc_table[4][c & 0xff] ^ crc_table[5][(c >> 8) & 0xff] ^ \
             crc_table[6][(c >> 16) & 0xff] ^ crc_table[7][c >> 24]
 #define DOBIG32 DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4

 /* ========================================================================= */
 static uint32_t crc32_big(uint32_t crc, const uint8_t* buf, size_t len)
 {
     register uint32_t c;
     register const uint32_t* buf4;

     c = ZSWAP32((uint32_t)crc);
     c = ~c;
     while (len && ((ptrdiff_t)buf & 3)) {
         c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
         len--;
     }

     buf4 = (const uint32_t*)(const void*)buf;
     buf4--;
     while (len >= 32) {
         DOBIG32;
         len -= 32;
     }
     while (len >= 4) {
         DOBIG4;
         len -= 4;
     }
     buf4++;
     buf = (const uint8_t*)buf4;

     if (len) do {
         c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
     } while (--len);
     c = ~c;
     return (uint32_t)(ZSWAP32(c));
 }

 #endif /* BYFOUR */

 #define GF2_DIM 32      /* dimension of GF(2) vectors (length of CRC) */

 /* ========================================================================= */
 static uint32_t gf2_matrix_times(uint32_t* mat, uint32_t vec)
 {
     uint32_t sum;

     sum = 0;
     while (vec) {
         if (vec & 1)
             sum ^= *mat;
         vec >>= 1;
         mat++;
     }
     return sum;
 }

 /* ========================================================================= */
 static void gf2_matrix_square(uint32_t* square, uint32_t* mat)
 {
     int n;

     for (n = 0; n < GF2_DIM; n++)
         square[n] = gf2_matrix_times(mat, mat[n]);
 }

 /* ========================================================================= */
 uint32_t ZEXPORT crc32_combine(uint32_t crc1, uint32_t crc2, size_t len2)
 {
     int n;
     uint32_t row;
     uint32_t even[GF2_DIM];    /* even-power-of-two zeros operator */
     uint32_t odd[GF2_DIM];     /* odd-power-of-two zeros operator */

     /* degenerate case (also disallow negative lengths) */
     if (len2 <= 0)
         return crc1;

     /* put operator for one zero bit in odd */
     odd[0] = 0xedb88320UL;          /* CRC-32 polynomial */
     row = 1;
     for (n = 1; n < GF2_DIM; n++) {
         odd[n] = row;
         row <<= 1;
     }

     /* put operator for two zero bits in even */
     gf2_matrix_square(even, odd);

     /* put operator for four zero bits in odd */
     gf2_matrix_square(odd, even);

     /* apply len2 zeros to crc1 (first square will put the operator for one
        zero byte, eight zero bits, in even) */
     do {
         /* apply zeros operator for this bit of len2 */
         gf2_matrix_square(even, odd);
         if (len2 & 1)
             crc1 = gf2_matrix_times(even, crc1);
         len2 >>= 1;

         /* if no more bits set, then done */
         if (len2 == 0)
             break;

         /* another iteration of the loop with odd and even swapped */
         gf2_matrix_square(odd, even);
         if (len2 & 1)
             crc1 = gf2_matrix_times(odd, crc1);
         len2 >>= 1;

         /* if no more bits set, then done */
     } while (len2 != 0);

     /* return combined crc */
     crc1 ^= crc2;
     return crc1;
 }
	/* crc32.c -- compute the CRC-32 of a data stream
	* Copyright (C) 1995-2006, 2010, 2011, 2012 Mark Adler
	* For conditions of distribution and use, see copyright notice in zlib.h
	*
	* Thanks to Rodney Brown <rbrown64@csc.com.au> 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().

	DYNAMIC_CRC_TABLE and MAKECRCH can be #defined to write out crc32.h.
	*/

	#ifdef MAKECRCH
	# include <stdio.h>
	# ifndef DYNAMIC_CRC_TABLE
	# define DYNAMIC_CRC_TABLE
	# endif /* !DYNAMIC_CRC_TABLE */
	#endif /* MAKECRCH */

	#include "zutil.h" /* for STDC definitions */

	#include <lib/cksum.h>

	/* Definitions for doing the crc four data bytes at a time. */
	#if !defined(NOBYFOUR) && defined(Z_U4)
	#error
	# define BYFOUR
	#endif
	#ifdef BYFOUR
	static uint32_t crc32_little OF((uint32_t, const uint8_t FAR *, uint32_t));
	static uint32_t crc32_big OF((uint32_t, const uint8_t FAR *, uint32_t));
	# define TBLS 8
	#else
	# define TBLS 1
	#endif /* BYFOUR */

	/* static functions for crc concatenation */
	static uint32_t gf2_matrix_times OF((uint32_t *mat, uint32_t vec));
	static void gf2_matrix_square OF((uint32_t square, uint32_t mat));


	#ifdef DYNAMIC_CRC_TABLE

	static volatile int crc_table_empty = 1;
	static uint32_t crc_table[TBLS][256];
	static void make_crc_table OF((void));
	#ifdef MAKECRCH
	static void write_table OF((FILE , const uint32_t));
	#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.
	*/
	static void make_crc_table()
	{
	uint32_t c;
	int n, k;
	uint32_t 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 uint8_t 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 = 0;
	for (n = 0; n < (int)(sizeof(p)/sizeof(uint8_t)); n++)
	poly \|= (uint32_t)1 << (31 - p[n]);

	/* generate a crc for every 8-bit value */
	for (n = 0; n < 256; n++) {
	c = (uint32_t)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] = ZSWAP32(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] = ZSWAP32(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, "static const uint32_t ");
	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
	static void write_table(out, table)
	FILE *out;
	const uint32_t *table;
	{
	int n;

	for (n = 0; n < 256; n++)
	fprintf(out, "%s0x%08lxUL%s", n % 5 ? "" : " ",
	(uint32_t)(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 uint32_t* ZEXPORT get_crc_table(void)
	{
	#ifdef DYNAMIC_CRC_TABLE
	if (crc_table_empty)
	make_crc_table();
	#endif /* DYNAMIC_CRC_TABLE */
	return (const uint32_t*)crc_table;
	}

	/* ========================================================================= */
	#define DO1 crc = crc_table[0][(crc ^ (*buf++)) & 0xff] ^ (crc >> 8)
	#define DO8 DO1; DO1; DO1; DO1; DO1; DO1; DO1; DO1

	/* ========================================================================= */
	uint32_t ZEXPORT crc32(uint32_t crc, const uint8_t* buf, size_t len)
	{
	if (buf == Z_NULL) return 0UL;

	#ifdef DYNAMIC_CRC_TABLE
	if (crc_table_empty)
	make_crc_table();
	#endif /* DYNAMIC_CRC_TABLE */

	#ifdef BYFOUR
	if (sizeof(void *) == sizeof(ptrdiff_t)) {
	uint32_t endian;

	endian = 1;
	if (((uint8_t )(&endian)))
	return crc32_little(crc, buf, len);
	else
	return crc32_big(crc, buf, len);
	}
	#endif /* BYFOUR */
	crc = crc ^ 0xffffffffUL;
	while (len >= 8) {
	DO8;
	len -= 8;
	}
	if (len) do {
	DO1;
	} while (--len);
	return crc ^ 0xffffffffUL;
	}

	#ifdef BYFOUR

	/* ========================================================================= */
	#define DOLIT4 c ^= *buf4++; \
	c = crc_table[3][c & 0xff] ^ crc_table[2][(c >> 8) & 0xff] ^ \
	crc_table[1][(c >> 16) & 0xff] ^ crc_table[0][c >> 24]
	#define DOLIT32 DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4

	/* ========================================================================= */
	static uint32_t crc32_little(uint32_t crc, const uint8_t* buf, size_t len)
	{
	register uint32_t c;
	register const uint32_t *buf4;

	c = (uint32_t)crc;
	c = ~c;
	while (len && ((ptrdiff_t)buf & 3)) {
	c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
	len--;
	}

	buf4 = (const uint32_t)(const void)buf;
	while (len >= 32) {
	DOLIT32;
	len -= 32;
	}
	while (len >= 4) {
	DOLIT4;
	len -= 4;
	}
	buf = (const uint8_t*)buf4;

	if (len) do {
	c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
	} while (--len);
	c = ~c;
	return (uint32_t)c;
	}

	/* ========================================================================= */
	#define DOBIG4 c ^= *++buf4; \
	c = crc_table[4][c & 0xff] ^ crc_table[5][(c >> 8) & 0xff] ^ \
	crc_table[6][(c >> 16) & 0xff] ^ crc_table[7][c >> 24]
	#define DOBIG32 DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4

	/* ========================================================================= */
	static uint32_t crc32_big(uint32_t crc, const uint8_t* buf, size_t len)
	{
	register uint32_t c;
	register const uint32_t* buf4;

	c = ZSWAP32((uint32_t)crc);
	c = ~c;
	while (len && ((ptrdiff_t)buf & 3)) {
	c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
	len--;
	}

	buf4 = (const uint32_t)(const void)buf;
	buf4--;
	while (len >= 32) {
	DOBIG32;
	len -= 32;
	}
	while (len >= 4) {
	DOBIG4;
	len -= 4;
	}
	buf4++;
	buf = (const uint8_t*)buf4;

	if (len) do {
	c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
	} while (--len);
	c = ~c;
	return (uint32_t)(ZSWAP32(c));
	}

	#endif /* BYFOUR */

	#define GF2_DIM 32 /* dimension of GF(2) vectors (length of CRC) */

	/* ========================================================================= */
	static uint32_t gf2_matrix_times(uint32_t* mat, uint32_t vec)
	{
	uint32_t sum;

	sum = 0;
	while (vec) {
	if (vec & 1)
	sum ^= *mat;
	vec >>= 1;
	mat++;
	}
	return sum;
	}

	/* ========================================================================= */
	static void gf2_matrix_square(uint32_t* square, uint32_t* mat)
	{
	int n;

	for (n = 0; n < GF2_DIM; n++)
	square[n] = gf2_matrix_times(mat, mat[n]);
	}

	/* ========================================================================= */
	uint32_t ZEXPORT crc32_combine(uint32_t crc1, uint32_t crc2, size_t len2)
	{
	int n;
	uint32_t row;
	uint32_t even[GF2_DIM]; /* even-power-of-two zeros operator */
	uint32_t odd[GF2_DIM]; /* odd-power-of-two zeros operator */

	/* degenerate case (also disallow negative lengths) */
	if (len2 <= 0)
	return crc1;

	/* put operator for one zero bit in odd */
	odd[0] = 0xedb88320UL; /* CRC-32 polynomial */
	row = 1;
	for (n = 1; n < GF2_DIM; n++) {
	odd[n] = row;
	row <<= 1;
	}

	/* put operator for two zero bits in even */
	gf2_matrix_square(even, odd);

	/* put operator for four zero bits in odd */
	gf2_matrix_square(odd, even);

	/* apply len2 zeros to crc1 (first square will put the operator for one
	zero byte, eight zero bits, in even) */
	do {
	/* apply zeros operator for this bit of len2 */
	gf2_matrix_square(even, odd);
	if (len2 & 1)
	crc1 = gf2_matrix_times(even, crc1);
	len2 >>= 1;

	/* if no more bits set, then done */
	if (len2 == 0)
	break;

	/* another iteration of the loop with odd and even swapped */
	gf2_matrix_square(odd, even);
	if (len2 & 1)
	crc1 = gf2_matrix_times(odd, crc1);
	len2 >>= 1;

	/* if no more bits set, then done */
	} while (len2 != 0);

	/* return combined crc */
	crc1 ^= crc2;
	return crc1;
	}