libbb/crc32.c

   1 /* vi: set sw=4 ts=4: */
   2 /*
   3  * CRC32 table fill function
   4  * Copyright (C) 2006 by Rob Sullivan <cogito.ergo.cogito@gmail.com>
   5  * (I can't really claim much credit however, as the algorithm is
   6  * very well-known)
   7  *
   8  * The following function creates a CRC32 table depending on whether
   9  * a big-endian (0x04c11db7) or little-endian (0xedb88320) CRC32 is
  10  * required. Admittedly, there are other CRC32 polynomials floating
  11  * around, but Busybox doesn't use them.
  12  *
  13  * endian = 1: big-endian
  14  * endian = 0: little-endian
  15  *
  16  * Licensed under GPLv2, see file LICENSE in this source tree.
  17  */
  18 #include "libbb.h"
  19
  20 uint32_t *global_crc32_table;
  21
  22 uint32_t* FAST_FUNC crc32_filltable(uint32_t *crc_table, int endian)
  23 {
  24         uint32_t polynomial = endian ? 0x04c11db7 : 0xedb88320;
  25         uint32_t c;
  26         unsigned i, j;
  27
  28         if (!crc_table)
  29                 crc_table = xmalloc(256 * sizeof(uint32_t));
  30
  31         for (i = 0; i < 256; i++) {
  32                 c = endian ? (i << 24) : i;
  33                 for (j = 8; j; j--) {
  34                         if (endian)
  35                                 c = (c&0x80000000) ? ((c << 1) ^ polynomial) : (c << 1);
  36                         else
  37                                 c = (c&1) ? ((c >> 1) ^ polynomial) : (c >> 1);
  38                 }
  39                 *crc_table++ = c;
  40         }
  41
  42         return crc_table - 256;
  43 }
  44 /* Common uses: */
  45 uint32_t* FAST_FUNC crc32_new_table_le(void)
  46 {
  47         return crc32_filltable(NULL, 0);
  48 }
  49 uint32_t* FAST_FUNC global_crc32_new_table_le(void)
  50 {
  51         global_crc32_table = crc32_new_table_le();
  52         return global_crc32_table;
  53 }
  54
  55 uint32_t FAST_FUNC crc32_block_endian1(uint32_t val, const void *buf, unsigned len, uint32_t *crc_table)
  56 {
  57         const void *end = (uint8_t*)buf + len;
  58
  59         while (buf != end) {
  60                 val = (val << 8) ^ crc_table[(val >> 24) ^ *(uint8_t*)buf];
  61                 buf = (uint8_t*)buf + 1;
  62         }
  63         return val;
  64 }
  65
  66 uint32_t FAST_FUNC crc32_block_endian0(uint32_t val, const void *buf, unsigned len, uint32_t *crc_table)
  67 {
  68         const void *end = (uint8_t*)buf + len;
  69
  70         while (buf != end) {
  71                 val = crc_table[(uint8_t)val ^ *(uint8_t*)buf] ^ (val >> 8);
  72                 buf = (uint8_t*)buf + 1;
  73         }
  74         return val;
  75 }