1// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file. See the AUTHORS file for names of contributors.
4
5#ifndef STORAGE_LEVELDB_UTIL_CRC32C_H_
6#define STORAGE_LEVELDB_UTIL_CRC32C_H_
7
8#include <stddef.h>
9#include <stdint.h>
10
11namespace leveldb {
12namespace crc32c {
13
14// Return the crc32c of concat(A, data[0,n-1]) where init_crc is the
15// crc32c of some string A.  Extend() is often used to maintain the
16// crc32c of a stream of data.
17extern uint32_t Extend(uint32_t init_crc, const char* data, size_t n);
18
19// Return the crc32c of data[0,n-1]
20inline uint32_t Value(const char* data, size_t n) {
21  return Extend(0, data, n);
22}
23
24static const uint32_t kMaskDelta = 0xa282ead8ul;
25
26// Return a masked representation of crc.
27//
28// Motivation: it is problematic to compute the CRC of a string that
29// contains embedded CRCs.  Therefore we recommend that CRCs stored
30// somewhere (e.g., in files) should be masked before being stored.
31inline uint32_t Mask(uint32_t crc) {
32  // Rotate right by 15 bits and add a constant.
33  return ((crc >> 15) | (crc << 17)) + kMaskDelta;
34}
35
36// Return the crc whose masked representation is masked_crc.
37inline uint32_t Unmask(uint32_t masked_crc) {
38  uint32_t rot = masked_crc - kMaskDelta;
39  return ((rot >> 17) | (rot << 15));
40}
41
42}  // namespace crc32c
43}  // namespace leveldb
44
45#endif  // STORAGE_LEVELDB_UTIL_CRC32C_H_
46