src/leveldb/util/crc32c.h

   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_