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_ 46
