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 11namespace leveldb { 12namespace 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. 17extern uint32_t Extend(uint32_t init_crc, const char* data, size_t n); 18 19// Return the crc32c of data[0,n-1] 20inline uint32_t Value(const char* data, size_t n) { 21 return Extend(0, data, n); 22} 23 24static 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. 31inline 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. 37inline 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