crc32.cc revision 0c4e06b4c6107a1b94f764e279e4fb4161e905b0 
1/*
2 *  Copyright 2012 The WebRTC Project Authors. All rights reserved.
3 *
4 *  Use of this source code is governed by a BSD-style license
5 *  that can be found in the LICENSE file in the root of the source
6 *  tree. An additional intellectual property rights grant can be found
7 *  in the file PATENTS.  All contributing project authors may
8 *  be found in the AUTHORS file in the root of the source tree.
9 */
10
11#include "webrtc/base/crc32.h"
12
13#include "webrtc/base/basicdefs.h"
14
15namespace rtc {
16
17// This implementation is based on the sample implementation in RFC 1952.
18
19// CRC32 polynomial, in reversed form.
20// See RFC 1952, or http://en.wikipedia.org/wiki/Cyclic_redundancy_check
21static const uint32_t kCrc32Polynomial = 0xEDB88320;
22static uint32_t kCrc32Table[256] = {0};
23
24static void EnsureCrc32TableInited() {
25  if (kCrc32Table[ARRAY_SIZE(kCrc32Table) - 1])
26    return;  // already inited
27  for (uint32_t i = 0; i < ARRAY_SIZE(kCrc32Table); ++i) {
28    uint32_t c = i;
29    for (size_t j = 0; j < 8; ++j) {
30      if (c & 1) {
31        c = kCrc32Polynomial ^ (c >> 1);
32      } else {
33        c >>= 1;
34      }
35    }
36    kCrc32Table[i] = c;
37  }
38}
39
40uint32_t UpdateCrc32(uint32_t start, const void* buf, size_t len) {
41  EnsureCrc32TableInited();
42
43  uint32_t c = start ^ 0xFFFFFFFF;
44  const uint8_t* u = static_cast<const uint8_t*>(buf);
45  for (size_t i = 0; i < len; ++i) {
46    c = kCrc32Table[(c ^ u[i]) & 0xFF] ^ (c >> 8);
47  }
48  return c ^ 0xFFFFFFFF;
49}
50
51}  // namespace rtc
52
53