src/zlib/crc32.c

   1 /* crc32.c -- compute the CRC-32 of a data stream
   2  * Copyright (C) 1995-2003 Mark Adler
   3  * For conditions of distribution and use, see copyright notice in zlib.h
   4  *
   5  * Thanks to Rodney Brown <rbrown64@csc.com.au> for his contribution of faster
   6  * CRC methods: exclusive-oring 32 bits of data at a time, and pre-computing
   7  * tables for updating the shift register in one step with three exclusive-ors
   8  * instead of four steps with four exclusive-ors.  This results about a factor
   9  * of two increase in speed on a Power PC G4 (PPC7455) using gcc -O3.
  10  */
  11
  12 /* @(#) $Id$ */
  13
  14 /*
  15   Note on the use of DYNAMIC_CRC_TABLE: there is no mutex or semaphore
  16   protection on the static variables used to control the first-use generation
  17   of the crc tables.  Therefore, if you #define DYNAMIC_CRC_TABLE, you should
  18   first call get_crc_table() to initialize the tables before allowing more than
  19   one thread to use crc32().
  20  */
  21
  22 #ifdef MAKECRCH
  23 #  include <stdio.h>
  24 #  ifndef DYNAMIC_CRC_TABLE
  25 #    define DYNAMIC_CRC_TABLE
  26 #  endif /* !DYNAMIC_CRC_TABLE */
  27 #endif /* MAKECRCH */
  28
  29 #include "zutil.h"      /* for STDC and FAR definitions */
  30
  31 #define local static
  32
  33 /* Find a four-byte integer type for crc32_little() and crc32_big(). */
  34 #ifndef NOBYFOUR
  35 #  ifdef STDC           /* need ANSI C limits.h to determine sizes */
  36 #    include <limits.h>
  37 #    define BYFOUR
  38 #    if (UINT_MAX == 0xffffffffUL)
  39        typedef unsigned int u4;
  40 #    else
  41 #      if (ULONG_MAX == 0xffffffffUL)
  42          typedef unsigned long u4;
  43 #      else
  44 #        if (USHRT_MAX == 0xffffffffUL)
  45            typedef unsigned short u4;
  46 #        else
  47 #          undef BYFOUR     /* can't find a four-byte integer type! */
  48 #        endif
  49 #      endif
  50 #    endif
  51 #  endif /* STDC */
  52 #endif /* !NOBYFOUR */
  53
  54 /* Definitions for doing the crc four data bytes at a time. */
  55 #ifdef BYFOUR
  56 #  define REV(w) (((w)>>24)+(((w)>>8)&0xff00)+ \
  57                 (((w)&0xff00)<<8)+(((w)&0xff)<<24))
  58    local unsigned long crc32_little OF((unsigned long,
  59                         const unsigned char FAR *, unsigned));
  60    local unsigned long crc32_big OF((unsigned long,
  61                         const unsigned char FAR *, unsigned));
  62 #  define TBLS 8
  63 #else
  64 #  define TBLS 1
  65 #endif /* BYFOUR */
  66
  67 #ifdef DYNAMIC_CRC_TABLE
  68
  69 local volatile int crc_table_empty = 1;
  70 local unsigned long FAR crc_table[TBLS][256];
  71 local void make_crc_table OF((void));
  72 #ifdef MAKECRCH
  73    local void write_table OF((FILE *, const unsigned long FAR *));
  74 #endif /* MAKECRCH */
  75
  76 /*
  77   Generate tables for a byte-wise 32-bit CRC calculation on the polynomial:
  78   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.
  79
  80   Polynomials over GF(2) are represented in binary, one bit per coefficient,
  81   with the lowest powers in the most significant bit.  Then adding polynomials
  82   is just exclusive-or, and multiplying a polynomial by x is a right shift by
  83   one.  If we call the above polynomial p, and represent a byte as the
  84   polynomial q, also with the lowest power in the most significant bit (so the
  85   byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,
  86   where a mod b means the remainder after dividing a by b.
  87
  88   This calculation is done using the shift-register method of multiplying and
  89   taking the remainder.  The register is initialized to zero, and for each
  90   incoming bit, x^32 is added mod p to the register if the bit is a one (where
  91   x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
  92   x (which is shifting right by one and adding x^32 mod p if the bit shifted
  93   out is a one).  We start with the highest power (least significant bit) of
  94   q and repeat for all eight bits of q.
  95
  96   The first table is simply the CRC of all possible eight bit values.  This is
  97   all the information needed to generate CRCs on data a byte at a time for all
  98   combinations of CRC register values and incoming bytes.  The remaining tables
  99   allow for word-at-a-time CRC calculation for both big-endian and little-
 100   endian machines, where a word is four bytes.
 101 */
 102 local void make_crc_table()
 103 {
 104     unsigned long c;
 105     int n, k;
 106     unsigned long poly;                 /* polynomial exclusive-or pattern */
 107     /* terms of polynomial defining this crc (except x^32): */
 108     static volatile int first = 1;      /* flag to limit concurrent making */
 109     static const unsigned char p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
 110
 111     /* See if another task is already doing this (not thread-safe, but better
 112        than nothing -- significantly reduces duration of vulnerability in
 113        case the advice about DYNAMIC_CRC_TABLE is ignored) */
 114     if (first) {
 115         first = 0;
 116
 117         /* make exclusive-or pattern from polynomial (0xedb88320UL) */
 118         poly = 0UL;
 119         for (n = 0; n < sizeof(p)/sizeof(unsigned char); n++)
 120             poly |= 1UL << (31 - p[n]);
 121
 122         /* generate a crc for every 8-bit value */
 123         for (n = 0; n < 256; n++) {
 124             c = (unsigned long)n;
 125             for (k = 0; k < 8; k++)
 126                 c = c & 1 ? poly ^ (c >> 1) : c >> 1;
 127             crc_table[0][n] = c;
 128         }
 129
 130 #ifdef BYFOUR
 131         /* generate crc for each value followed by one, two, and three zeros,
 132            and then the byte reversal of those as well as the first table */
 133         for (n = 0; n < 256; n++) {
 134             c = crc_table[0][n];
 135             crc_table[4][n] = REV(c);
 136             for (k = 1; k < 4; k++) {
 137                 c = crc_table[0][c & 0xff] ^ (c >> 8);
 138                 crc_table[k][n] = c;
 139                 crc_table[k + 4][n] = REV(c);
 140             }
 141         }
 142 #endif /* BYFOUR */
 143
 144         crc_table_empty = 0;
 145     }
 146     else {      /* not first */
 147         /* wait for the other guy to finish (not efficient, but rare) */
 148         while (crc_table_empty)
 149             ;
 150     }
 151
 152 #ifdef MAKECRCH
 153     /* write out CRC tables to crc32.h */
 154     {
 155         FILE *out;
 156
 157         out = fopen("crc32.h", "w");
 158         if (out == NULL) return;
 159         fprintf(out, "/* crc32.h -- tables for rapid CRC calculation\n");
 160         fprintf(out, " * Generated automatically by crc32.c\n */\n\n");
 161         fprintf(out, "local const unsigned long FAR ");
 162         fprintf(out, "crc_table[TBLS][256] =\n{\n  {\n");
 163         write_table(out, crc_table[0]);
 164 #  ifdef BYFOUR
 165         fprintf(out, "#ifdef BYFOUR\n");
 166         for (k = 1; k < 8; k++) {
 167             fprintf(out, "  },\n  {\n");
 168             write_table(out, crc_table[k]);
 169         }
 170         fprintf(out, "#endif\n");
 171 #  endif /* BYFOUR */
 172         fprintf(out, "  }\n};\n");
 173         fclose(out);
 174     }
 175 #endif /* MAKECRCH */
 176 }
 177
 178 #ifdef MAKECRCH
 179 local void write_table(out, table)
 180     FILE *out;
 181     const unsigned long FAR *table;
 182 {
 183     int n;
 184
 185     for (n = 0; n < 256; n++)
 186         fprintf(out, "%s0x%08lxUL%s", n % 5 ? "" : "    ", table[n],
 187                 n == 255 ? "\n" : (n % 5 == 4 ? ",\n" : ", "));
 188 }
 189 #endif /* MAKECRCH */
 190
 191 #else /* !DYNAMIC_CRC_TABLE */
 192 /* ========================================================================
 193  * Tables of CRC-32s of all single-byte values, made by make_crc_table().
 194  */
 195 #include "crc32.h"
 196 #endif /* DYNAMIC_CRC_TABLE */
 197
 198 /* =========================================================================
 199  * This function can be used by asm versions of crc32()
 200  */
 201 const unsigned long FAR * ZEXPORT get_crc_table()
 202 {
 203 #ifdef DYNAMIC_CRC_TABLE
 204     if (crc_table_empty)
 205         make_crc_table();
 206 #endif /* DYNAMIC_CRC_TABLE */
 207     return (const unsigned long FAR *)crc_table;
 208 }
 209
 210 /* ========================================================================= */
 211 #define DO1 crc = crc_table[0][((int)crc ^ (*buf++)) & 0xff] ^ (crc >> 8)
 212 #define DO8 DO1; DO1; DO1; DO1; DO1; DO1; DO1; DO1
 213
 214 /* ========================================================================= */
 215 unsigned long ZEXPORT crc32(crc, buf, len)
 216     unsigned long crc;
 217     const unsigned char FAR *buf;
 218     unsigned len;
 219 {
 220     if (buf == Z_NULL) return 0UL;
 221
 222 #ifdef DYNAMIC_CRC_TABLE
 223     if (crc_table_empty)
 224         make_crc_table();
 225 #endif /* DYNAMIC_CRC_TABLE */
 226
 227 #ifdef BYFOUR
 228     if (sizeof(void *) == sizeof(ptrdiff_t)) {
 229         u4 endian;
 230
 231         endian = 1;
 232         if (*((unsigned char *)(&endian)))
 233             return crc32_little(crc, buf, len);
 234         else
 235             return crc32_big(crc, buf, len);
 236     }
 237 #endif /* BYFOUR */
 238     crc = crc ^ 0xffffffffUL;
 239     while (len >= 8) {
 240         DO8;
 241         len -= 8;
 242     }
 243     if (len) do {
 244         DO1;
 245     } while (--len);
 246     return crc ^ 0xffffffffUL;
 247 }
 248
 249 #ifdef BYFOUR
 250
 251 /* ========================================================================= */
 252 #define DOLIT4 c ^= *buf4++; \
 253         c = crc_table[3][c & 0xff] ^ crc_table[2][(c >> 8) & 0xff] ^ \
 254             crc_table[1][(c >> 16) & 0xff] ^ crc_table[0][c >> 24]
 255 #define DOLIT32 DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4
 256
 257 /* ========================================================================= */
 258 local unsigned long crc32_little(crc, buf, len)
 259     unsigned long crc;
 260     const unsigned char FAR *buf;
 261     unsigned len;
 262 {
 263     register u4 c;
 264     register const u4 FAR *buf4;
 265
 266     c = (u4)crc;
 267     c = ~c;
 268     while (len && ((ptrdiff_t)buf & 3)) {
 269         c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
 270         len--;
 271     }
 272
 273     buf4 = (const u4 FAR *)buf;
 274     while (len >= 32) {
 275         DOLIT32;
 276         len -= 32;
 277     }
 278     while (len >= 4) {
 279         DOLIT4;
 280         len -= 4;
 281     }
 282     buf = (const unsigned char FAR *)buf4;
 283
 284     if (len) do {
 285         c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
 286     } while (--len);
 287     c = ~c;
 288     return (unsigned long)c;
 289 }
 290
 291 /* ========================================================================= */
 292 #define DOBIG4 c ^= *++buf4; \
 293         c = crc_table[4][c & 0xff] ^ crc_table[5][(c >> 8) & 0xff] ^ \
 294             crc_table[6][(c >> 16) & 0xff] ^ crc_table[7][c >> 24]
 295 #define DOBIG32 DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4
 296
 297 /* ========================================================================= */
 298 local unsigned long crc32_big(crc, buf, len)
 299     unsigned long crc;
 300     const unsigned char FAR *buf;
 301     unsigned len;
 302 {
 303     register u4 c;
 304     register const u4 FAR *buf4;
 305
 306     c = REV((u4)crc);
 307     c = ~c;
 308     while (len && ((ptrdiff_t)buf & 3)) {
 309         c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
 310         len--;
 311     }
 312
 313     buf4 = (const u4 FAR *)buf;
 314     buf4--;
 315     while (len >= 32) {
 316         DOBIG32;
 317         len -= 32;
 318     }
 319     while (len >= 4) {
 320         DOBIG4;
 321         len -= 4;
 322     }
 323     buf4++;
 324     buf = (const unsigned char FAR *)buf4;
 325
 326     if (len) do {
 327         c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
 328     } while (--len);
 329     c = ~c;
 330     return (unsigned long)(REV(c));
 331 }
 332
 333 #endif /* BYFOUR */