base/crc32.cc

/*
 * libjingle
 * Copyright 2012, Google, Inc.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *  1. Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *  2. Redistributions in binary form must reproduce the above copyright notice,
 *     this list of conditions and the following disclaimer in the documentation
 *     and/or other materials provided with the distribution.
 *  3. The name of the author may not be used to endorse or promote products
 *     derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
 * EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
 * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
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 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include "talk/base/crc32.h"

#include "talk/base/basicdefs.h"

namespace talk_base {

// This implementation is based on the sample implementation in RFC 1952.

// CRC32 polynomial, in reversed form.
// See RFC 1952, or http://en.wikipedia.org/wiki/Cyclic_redundancy_check
static const uint32 kCrc32Polynomial = 0xEDB88320;
static uint32 kCrc32Table[256] = { 0 };

static void EnsureCrc32TableInited() {
  if (kCrc32Table[ARRAY_SIZE(kCrc32Table) - 1])
    return;  // already inited
  for (uint32 i = 0; i < ARRAY_SIZE(kCrc32Table); ++i) {
    uint32 c = i;
    for (size_t j = 0; j < 8; ++j) {
      if (c & 1) {
        c = kCrc32Polynomial ^ (c >> 1);
      } else {
        c >>= 1;
      }
    }
    kCrc32Table[i] = c;
  }
}

uint32 UpdateCrc32(uint32 start, const void* buf, size_t len) {
  EnsureCrc32TableInited();

  uint32 c = start ^ 0xFFFFFFFF;
  const uint8* u = static_cast<const uint8*>(buf);
  for (size_t i = 0; i < len; ++i) {
    c = kCrc32Table[(c ^ u[i]) & 0xFF] ^ (c >> 8);
  }
  return c ^ 0xFFFFFFFF;
}

}  // namespace talk_base