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 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