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crc32c.h
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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]
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.
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.
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_
uint32_t Unmask(uint32_t masked_crc)
Definition: crc32c.h:37
uint32_t Mask(uint32_t crc)
Definition: crc32c.h:31
unsigned int uint32_t
Definition: stdint.h:21
uint32_t Value(const char *data, size_t n)
Definition: crc32c.h:20
uint32_t Extend(uint32_t crc, const char *buf, size_t size)
Definition: crc32c.cc:286