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 <cstddef> |
9 | #include <cstdint> |
10 | |
11 | namespace leveldb { |
12 | namespace 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. |
17 | uint32_t Extend(uint32_t init_crc, const char* data, size_t n); |
18 | |
19 | // Return the crc32c of data[0,n-1] |
20 | inline uint32_t Value(const char* data, size_t n) { return Extend(0, data, n); } |
21 | |
22 | static const uint32_t kMaskDelta = 0xa282ead8ul; |
23 | |
24 | // Return a masked representation of crc. |
25 | // |
26 | // Motivation: it is problematic to compute the CRC of a string that |
27 | // contains embedded CRCs. Therefore we recommend that CRCs stored |
28 | // somewhere (e.g., in files) should be masked before being stored. |
29 | inline uint32_t Mask(uint32_t crc) { |
30 | // Rotate right by 15 bits and add a constant. |
31 | return ((crc >> 15) | (crc << 17)) + kMaskDelta; |
32 | } |
33 | |
34 | // Return the crc whose masked representation is masked_crc. |
35 | inline uint32_t Unmask(uint32_t masked_crc) { |
36 | uint32_t rot = masked_crc - kMaskDelta; |
37 | return ((rot >> 17) | (rot << 15)); |
38 | } |
39 | |
40 | } // namespace crc32c |
41 | } // namespace leveldb |
42 | |
43 | #endif // STORAGE_LEVELDB_UTIL_CRC32C_H_ |
44 | |