release/src/router/busybox/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 tarball for details.
  17  */
  18
  19 #include "libbb.h"
  20
  21 uint32_t* FAST_FUNC crc32_filltable(uint32_t *crc_table, int endian)
  22 {
  23         uint32_t polynomial = endian ? 0x04c11db7 : 0xedb88320;
  24         uint32_t c;
  25         int i, j;
  26
  27         if (!crc_table)
  28                 crc_table = xmalloc(256 * sizeof(uint32_t));
  29
  30         for (i = 0; i < 256; i++) {
  31                 c = endian ? (i << 24) : i;
  32                 for (j = 8; j; j--) {
  33                         if (endian)
  34                                 c = (c&0x80000000) ? ((c << 1) ^ polynomial) : (c << 1);
  35                         else
  36                                 c = (c&1) ? ((c >> 1) ^ polynomial) : (c >> 1);
  37                 }
  38                 *crc_table++ = c;
  39         }
  40
  41         return crc_table - 256;
  42 }