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
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 50%
9 * increase in speed on a Power PC using gcc -O3.
16 # ifndef DYNAMIC_CRC_TABLE
17 # define DYNAMIC_CRC_TABLE
18 # endif /* !DYNAMIC_CRC_TABLE */
25 /* Find a four-byte integer type for crc32_little() and crc32_big(). */
27 # ifdef __STDC__ /* need ANSI C limits.h to determine sizes */
30 # if (UINT_MAX == 4294967295)
31 typedef unsigned int u4;
32 # elif (ULONG_MAX == 4294967295)
33 typedef unsigned long u4;
34 # elif (USHRT_MAX == 4294967295)
35 typedef unsigned short u4;
37 # undef BYFOUR /* can't find a four-byte integer type! */
39 # endif /* __STDC__ */
40 #endif /* !NOBYFOUR */
42 /* Definitions for doing the crc four data bytes at a time. */
44 # define REV(w) (((w)>>24)+(((w)>>8)&0xff00)+ \
45 (((w)&0xff00)<<8)+(((w)&0xff)<<24))
46 local unsigned long crc32_little OF((unsigned long,
47 const unsigned char FAR *, unsigned));
48 local unsigned long crc32_big OF((unsigned long,
49 const unsigned char FAR *, unsigned));
55 #ifdef DYNAMIC_CRC_TABLE
57 local int crc_table_empty = 1;
58 local unsigned long FAR crc_table[TBLS][256];
59 local void make_crc_table OF((void));
61 local void write_table OF((FILE *, const unsigned long FAR *));
65 Generate tables for a byte-wise 32-bit CRC calculation on the polynomial:
66 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.
68 Polynomials over GF(2) are represented in binary, one bit per coefficient,
69 with the lowest powers in the most significant bit. Then adding polynomials
70 is just exclusive-or, and multiplying a polynomial by x is a right shift by
71 one. If we call the above polynomial p, and represent a byte as the
72 polynomial q, also with the lowest power in the most significant bit (so the
73 byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,
74 where a mod b means the remainder after dividing a by b.
76 This calculation is done using the shift-register method of multiplying and
77 taking the remainder. The register is initialized to zero, and for each
78 incoming bit, x^32 is added mod p to the register if the bit is a one (where
79 x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
80 x (which is shifting right by one and adding x^32 mod p if the bit shifted
81 out is a one). We start with the highest power (least significant bit) of
82 q and repeat for all eight bits of q.
84 The first table is simply the CRC of all possible eight bit values. This is
85 all the information needed to generate CRCs on data a byte at a time for all
86 combinations of CRC register values and incoming bytes. The remaining tables
87 allow for word-at-a-time CRC calculation for both big-endian and little-
88 endian machines, where a word is four bytes.
90 local void make_crc_table()
94 unsigned long poly; /* polynomial exclusive-or pattern */
95 /* terms of polynomial defining this crc (except x^32): */
96 static const unsigned char p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
98 /* make exclusive-or pattern from polynomial (0xedb88320L) */
100 for (n = 0; n < sizeof(p)/sizeof(unsigned char); n++)
101 poly |= 1UL << (31 - p[n]);
103 /* generate a crc for every 8-bit value */
104 for (n = 0; n < 256; n++) {
105 c = (unsigned long)n;
106 for (k = 0; k < 8; k++)
107 c = c & 1 ? poly ^ (c >> 1) : c >> 1;
112 /* generate crc for each value followed by one, two, and three zeros, and
113 then the byte reversal of those as well as the first table */
114 for (n = 0; n < 256; n++) {
116 crc_table[4][n] = REV(c);
117 for (k = 1; k < 4; k++) {
118 c = crc_table[0][c & 0xff] ^ (c >> 8);
120 crc_table[k + 4][n] = REV(c);
128 /* write out CRC tables to crc32.h */
132 out = fopen("crc32.h", "w");
133 if (out == NULL) return;
134 fprintf(out, "/* crc32.h -- tables for rapid CRC calculation\n");
135 fprintf(out, " * Generated automatically by crc32.c\n */\n\n");
136 fprintf(out, "local const unsigned long FAR ");
137 fprintf(out, "crc_table[TBLS][256] =\n{\n {\n");
138 write_table(out, crc_table[0]);
140 fprintf(out, "#ifdef BYFOUR\n");
141 for (k = 1; k < 8; k++) {
142 fprintf(out, " },\n {\n");
143 write_table(out, crc_table[k]);
145 fprintf(out, "#endif\n");
147 fprintf(out, " }\n};\n");
150 #endif /* MAKECRCH */
154 local void write_table(out, table)
156 const unsigned long FAR *table;
160 for (n = 0; n < 256; n++)
161 fprintf(out, "%s0x%08lxUL%s", n % 5 ? "" : " ", table[n],
162 n == 255 ? "\n" : (n % 5 == 4 ? ",\n" : ", "));
164 #endif /* MAKECRCH */
166 #else /* !DYNAMIC_CRC_TABLE */
167 /* ========================================================================
168 * Tables of CRC-32s of all single-byte values, made by make_crc_table().
171 #endif /* DYNAMIC_CRC_TABLE */
173 /* =========================================================================
174 * This function can be used by asm versions of crc32()
176 const unsigned long FAR * ZEXPORT get_crc_table()
178 #ifdef DYNAMIC_CRC_TABLE
179 if (crc_table_empty) make_crc_table();
180 #endif /* DYNAMIC_CRC_TABLE */
181 return (const unsigned long FAR *)crc_table;
184 /* ========================================================================= */
185 #define DO1 crc = crc_table[0][((int)crc ^ (*buf++)) & 0xff] ^ (crc >> 8)
186 #define DO8 DO1; DO1; DO1; DO1; DO1; DO1; DO1; DO1
188 /* ========================================================================= */
189 unsigned long ZEXPORT crc32(crc, buf, len)
191 const unsigned char FAR *buf;
194 if (buf == Z_NULL) return 0UL;
196 #ifdef DYNAMIC_CRC_TABLE
199 #endif /* DYNAMIC_CRC_TABLE */
206 if (*((unsigned char *)(&endian)))
207 return crc32_little(crc, buf, len);
209 return crc32_big(crc, buf, len);
212 crc = crc ^ 0xffffffffUL;
220 return crc ^ 0xffffffffUL;
226 /* ========================================================================= */
227 #define DOLIT4 c ^= *buf4++; \
228 c = crc_table[3][c & 0xff] ^ crc_table[2][(c >> 8) & 0xff] ^ \
229 crc_table[1][(c >> 16) & 0xff] ^ crc_table[0][c >> 24]
230 #define DOLIT32 DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4
232 /* ========================================================================= */
233 local unsigned long crc32_little(crc, buf, len)
235 const unsigned char FAR *buf;
239 register const u4 FAR *buf4;
243 while (len && ((int)buf & 3)) {
244 c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
248 buf4 = (const u4 FAR *)buf;
257 buf = (const unsigned char FAR *)buf4;
260 c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
263 return (unsigned long)c;
266 /* ========================================================================= */
267 #define DOBIG4 c ^= *++buf4; \
268 c = crc_table[4][c & 0xff] ^ crc_table[5][(c >> 8) & 0xff] ^ \
269 crc_table[6][(c >> 16) & 0xff] ^ crc_table[7][c >> 24]
270 #define DOBIG32 DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4
272 /* ========================================================================= */
273 local unsigned long crc32_big(crc, buf, len)
275 const unsigned char FAR *buf;
279 register const u4 FAR *buf4;
283 while (len && ((int)buf & 3)) {
284 c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
288 buf4 = (const u4 FAR *)buf;
299 buf = (const unsigned char FAR *)buf4;
302 c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
305 return (unsigned long)(REV(c));