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
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 extern uint32_t Extend(uint32_t init_crc, const char* data, size_t n);
18
19 // Return the crc32c of data[0,n-1]
Value(const char * data,size_t n)20 inline uint32_t Value(const char* data, size_t n) {
21 return Extend(0, data, n);
22 }
23
24 static 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.
Mask(uint32_t crc)31 inline 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.
Unmask(uint32_t masked_crc)37 inline 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