/[pearpc]/src/tools/crc32.cc
This is repository of my old source code which isn't updated any more. Go to git.rot13.org for current projects!
ViewVC logotype

Contents of /src/tools/crc32.cc

Parent Directory Parent Directory | Revision Log Revision Log


Revision 1 - (show annotations)
Wed Sep 5 17:11:21 2007 UTC (11 years, 10 months ago) by dpavlin
File size: 10752 byte(s)
import upstream CVS
1 /*
2 * Function for computing CRC32 for the purpose of adding to Ethernet packets.
3 *
4 */
5
6 #include "crc32.h"
7
8 static const uint32 crc32table[0x100] = {
9 0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL,
10 0x076dc419L, 0x706af48fL, 0xe963a535L, 0x9e6495a3L,
11 0x0edb8832L, 0x79dcb8a4L, 0xe0d5e91eL, 0x97d2d988L,
12 0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L, 0x90bf1d91L,
13 0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL,
14 0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L,
15 0x136c9856L, 0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL,
16 0x14015c4fL, 0x63066cd9L, 0xfa0f3d63L, 0x8d080df5L,
17 0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L, 0xa2677172L,
18 0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL,
19 0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L,
20 0x32d86ce3L, 0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L,
21 0x26d930acL, 0x51de003aL, 0xc8d75180L, 0xbfd06116L,
22 0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L, 0xb8bda50fL,
23 0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L,
24 0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL,
25 0x76dc4190L, 0x01db7106L, 0x98d220bcL, 0xefd5102aL,
26 0x71b18589L, 0x06b6b51fL, 0x9fbfe4a5L, 0xe8b8d433L,
27 0x7807c9a2L, 0x0f00f934L, 0x9609a88eL, 0xe10e9818L,
28 0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L,
29 0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL,
30 0x6c0695edL, 0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L,
31 0x65b0d9c6L, 0x12b7e950L, 0x8bbeb8eaL, 0xfcb9887cL,
32 0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L, 0xfbd44c65L,
33 0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L,
34 0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL,
35 0x4369e96aL, 0x346ed9fcL, 0xad678846L, 0xda60b8d0L,
36 0x44042d73L, 0x33031de5L, 0xaa0a4c5fL, 0xdd0d7cc9L,
37 0x5005713cL, 0x270241aaL, 0xbe0b1010L, 0xc90c2086L,
38 0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL,
39 0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L,
40 0x59b33d17L, 0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL,
41 0xedb88320L, 0x9abfb3b6L, 0x03b6e20cL, 0x74b1d29aL,
42 0xead54739L, 0x9dd277afL, 0x04db2615L, 0x73dc1683L,
43 0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L,
44 0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L,
45 0xf00f9344L, 0x8708a3d2L, 0x1e01f268L, 0x6906c2feL,
46 0xf762575dL, 0x806567cbL, 0x196c3671L, 0x6e6b06e7L,
47 0xfed41b76L, 0x89d32be0L, 0x10da7a5aL, 0x67dd4accL,
48 0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L,
49 0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L,
50 0xd1bb67f1L, 0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL,
51 0xd80d2bdaL, 0xaf0a1b4cL, 0x36034af6L, 0x41047a60L,
52 0xdf60efc3L, 0xa867df55L, 0x316e8eefL, 0x4669be79L,
53 0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L,
54 0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL,
55 0xc5ba3bbeL, 0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L,
56 0xc2d7ffa7L, 0xb5d0cf31L, 0x2cd99e8bL, 0x5bdeae1dL,
57 0x9b64c2b0L, 0xec63f226L, 0x756aa39cL, 0x026d930aL,
58 0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L,
59 0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L,
60 0x92d28e9bL, 0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L,
61 0x86d3d2d4L, 0xf1d4e242L, 0x68ddb3f8L, 0x1fda836eL,
62 0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L, 0x18b74777L,
63 0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL,
64 0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L,
65 0xa00ae278L, 0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L,
66 0xa7672661L, 0xd06016f7L, 0x4969474dL, 0x3e6e77dbL,
67 0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L, 0x37d83bf0L,
68 0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L,
69 0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L,
70 0xbad03605L, 0xcdd70693L, 0x54de5729L, 0x23d967bfL,
71 0xb3667a2eL, 0xc4614ab8L, 0x5d681b02L, 0x2a6f2b94L,
72 0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL, 0x2d02ef8dL
73 };
74
75
76 // The previous table could have been built using the following function :
77
78 /*
79
80 #define CRC32_POLY 0xedb88320; // this is a 0x04c11db7 reflection
81
82 void init_crc32()
83 {
84 int i, j, b;
85 uint32 c;
86
87 for (i = 0; i < 0x100; i++) {
88 for (c = i, j = 0; j < 8; j++) {
89 b = c & 1;
90 c >>= 1;
91 if (b)
92 c ^= CRC32_POLY;
93 }
94 crc32table[i] = c;
95 }
96 }
97 */
98
99 // With this macro defined, the function runs about 35% faster, but the code is about 3 times bigger :
100 #define RUN_FASTER
101
102 #define DO_CRC(b) crc = (crc >> 8) ^ crc32table[(crc & 0xff) ^ (b)]
103
104 uint32 ether_crc(size_t len, const byte *p)
105 {
106 uint32 crc = 0xffffffff; // preload shift register, per CRC-32 spec
107
108 for (; len>0; len--) {
109 DO_CRC(*p++);
110 }
111 return ~crc; // transmit complement, per CRC-32 spec
112 }
113
114 /*
115 * A brief CRC tutorial.
116 *
117 * A CRC is a long-division remainder. You add the CRC to the message,
118 * and the whole thing (message+CRC) is a multiple of the given
119 * CRC polynomial. To check the CRC, you can either check that the
120 * CRC matches the recomputed value, *or* you can check that the
121 * remainder computed on the message+CRC is 0. This latter approach
122 * is used by a lot of hardware implementations, and is why so many
123 * protocols put the end-of-frame flag after the CRC.
124 *
125 * It's actually the same long division you learned in school, except that
126 * - We're working in binary, so the digits are only 0 and 1, and
127 * - When dividing polynomials, there are no carries. Rather than add and
128 * subtract, we just xor. Thus, we tend to get a bit sloppy about
129 * the difference between adding and subtracting.
130 *
131 * A 32-bit CRC polynomial is actually 33 bits long. But since it's
132 * 33 bits long, bit 32 is always going to be set, so usually the CRC
133 * is written in hex with the most significant bit omitted. (If you're
134 * familiar with the IEEE 754 floating-point format, it's the same idea.)
135 *
136 * Note that a CRC is computed over a string of *bits*, so you have
137 * to decide on the endianness of the bits within each byte. To get
138 * the best error-detecting properties, this should correspond to the
139 * order they're actually sent. For example, standard RS-232 serial is
140 * little-endian; the most significant bit (sometimes used for parity)
141 * is sent last. And when appending a CRC word to a message, you should
142 * do it in the right order, matching the endianness.
143 *
144 * Just like with ordinary division, the remainder is always smaller than
145 * the divisor (the CRC polynomial) you're dividing by. Each step of the
146 * division, you take one more digit (bit) of the dividend and append it
147 * to the current remainder. Then you figure out the appropriate multiple
148 * of the divisor to subtract to being the remainder back into range.
149 * In binary, it's easy - it has to be either 0 or 1, and to make the
150 * XOR cancel, it's just a copy of bit 32 of the remainder.
151 *
152 * When computing a CRC, we don't care about the quotient, so we can
153 * throw the quotient bit away, but subtract the appropriate multiple of
154 * the polynomial from the remainder and we're back to where we started,
155 * ready to process the next bit.
156 *
157 * A big-endian CRC written this way would be coded like:
158 * for (i = 0; i < input_bits; i++) {
159 * multiple = remainder & 0x80000000 ? CRCPOLY : 0;
160 * remainder = (remainder << 1 | next_input_bit()) ^ multiple;
161 * }
162 * Notice how, to get at bit 32 of the shifted remainder, we look
163 * at bit 31 of the remainder *before* shifting it.
164 *
165 * But also notice how the next_input_bit() bits we're shifting into
166 * the remainder don't actually affect any decision-making until
167 * 32 bits later. Thus, the first 32 cycles of this are pretty boring.
168 * Also, to add the CRC to a message, we need a 32-bit-long hole for it at
169 * the end, so we have to add 32 extra cycles shifting in zeros at the
170 * end of every message,
171 *
172 * So the standard trick is to rearrage merging in the next_input_bit()
173 * until the moment it's needed. Then the first 32 cycles can be precomputed,
174 * and merging in the final 32 zero bits to make room for the CRC can be
175 * skipped entirely.
176 * This changes the code to:
177 * for (i = 0; i < input_bits; i++) {
178 * remainder ^= next_input_bit() << 31;
179 * multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
180 * remainder = (remainder << 1) ^ multiple;
181 * }
182 * With this optimization, the little-endian code is simpler:
183 * for (i = 0; i < input_bits; i++) {
184 * remainder ^= next_input_bit();
185 * multiple = (remainder & 1) ? CRCPOLY : 0;
186 * remainder = (remainder >> 1) ^ multiple;
187 * }
188 *
189 * Note that the other details of endianness have been hidden in CRCPOLY
190 * (which must be bit-reversed) and next_input_bit().
191 *
192 * However, as long as next_input_bit is returning the bits in a sensible
193 * order, we can actually do the merging 8 or more bits at a time rather
194 * than one bit at a time:
195 * for (i = 0; i < input_bytes; i++) {
196 * remainder ^= next_input_byte() << 24;
197 * for (j = 0; j < 8; j++) {
198 * multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
199 * remainder = (remainder << 1) ^ multiple;
200 * }
201 * }
202 * Or in little-endian:
203 * for (i = 0; i < input_bytes; i++) {
204 * remainder ^= next_input_byte();
205 * for (j = 0; j < 8; j++) {
206 * multiple = (remainder & 1) ? CRCPOLY : 0;
207 * remainder = (remainder >> 1) ^ multiple;
208 * }
209 * }
210 * If the input is a multiple of 32 bits, you can even XOR in a 32-bit
211 * word at a time and increase the inner loop count to 32.
212 *
213 * You can also mix and match the two loop styles, for example doing the
214 * bulk of a message byte-at-a-time and adding bit-at-a-time processing
215 * for any fractional bytes at the end.
216 *
217 * The only remaining optimization is to the byte-at-a-time table method.
218 * Here, rather than just shifting one bit of the remainder to decide
219 * in the correct multiple to subtract, we can shift a byte at a time.
220 * This produces a 40-bit (rather than a 33-bit) intermediate remainder,
221 * but again the multiple of the polynomial to subtract depends only on
222 * the high bits, the high 8 bits in this case.
223 *
224 * The multile we need in that case is the low 32 bits of a 40-bit
225 * value whose high 8 bits are given, and which is a multiple of the
226 * generator polynomial. This is simply the CRC-32 of the given
227 * one-byte message.
228 *
229 * Two more details: normally, appending zero bits to a message which
230 * is already a multiple of a polynomial produces a larger multiple of that
231 * polynomial. To enable a CRC to detect this condition, it's common to
232 * invert the CRC before appending it. This makes the remainder of the
233 * message+crc come out not as zero, but some fixed non-zero value.
234 *
235 * The same problem applies to zero bits prepended to the message, and
236 * a similar solution is used. Instead of starting with a remainder of
237 * 0, an initial remainder of all ones is used. As long as you start
238 * the same way on decoding, it doesn't make a difference.
239 */

  ViewVC Help
Powered by ViewVC 1.1.26