crc32.c revision 146081 
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
69local volatile int crc_table_empty = 1;
70local unsigned long FAR crc_table[TBLS][256];
71local 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*/
102local 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
179local 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 */
201const 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/* ========================================================================= */
215unsigned 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/* ========================================================================= */
258local 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/* ========================================================================= */
298local 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 */
334