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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                 :             : uint32_t Extend(uint32_t init_crc, const char* data, size_t n);
      18                 :             : 
      19                 :             : // Return the crc32c of data[0,n-1]
      20         [ +  - ]:      126144 : 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                 :       65862 : inline uint32_t Mask(uint32_t crc) {
      30                 :             :   // Rotate right by 15 bits and add a constant.
      31                 :       65862 :   return ((crc >> 15) | (crc << 17)) + kMaskDelta;
      32                 :             : }
      33                 :             : 
      34                 :             : // Return the crc whose masked representation is masked_crc.
      35                 :       75533 : inline uint32_t Unmask(uint32_t masked_crc) {
      36                 :       75533 :   uint32_t rot = masked_crc - kMaskDelta;
      37         [ +  - ]:       75533 :   return ((rot >> 17) | (rot << 15));
      38                 :             : }
      39                 :             : 
      40                 :             : }  // namespace crc32c
      41                 :             : }  // namespace leveldb
      42                 :             : 
      43                 :             : #endif  // STORAGE_LEVELDB_UTIL_CRC32C_H_