LXR 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
  21uint32_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}
  43