10225501448 李度 10225101546 陈胤遒 10215501422 高宇菲

45 行
1.4 KiB

  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. #ifndef STORAGE_LEVELDB_UTIL_CRC32C_H_
  5. #define STORAGE_LEVELDB_UTIL_CRC32C_H_
  6. #include <stddef.h>
  7. #include <stdint.h>
  8. namespace leveldb {
  9. namespace crc32c {
  10. // Return the crc32c of concat(A, data[0,n-1]) where init_crc is the
  11. // crc32c of some string A. Extend() is often used to maintain the
  12. // crc32c of a stream of data.
  13. uint32_t Extend(uint32_t init_crc, const char* data, size_t n);
  14. // Return the crc32c of data[0,n-1]
  15. inline uint32_t Value(const char* data, size_t n) {
  16. return Extend(0, data, n);
  17. }
  18. static const uint32_t kMaskDelta = 0xa282ead8ul;
  19. // Return a masked representation of crc.
  20. //
  21. // Motivation: it is problematic to compute the CRC of a string that
  22. // contains embedded CRCs. Therefore we recommend that CRCs stored
  23. // somewhere (e.g., in files) should be masked before being stored.
  24. inline uint32_t Mask(uint32_t crc) {
  25. // Rotate right by 15 bits and add a constant.
  26. return ((crc >> 15) | (crc << 17)) + kMaskDelta;
  27. }
  28. // Return the crc whose masked representation is masked_crc.
  29. inline uint32_t Unmask(uint32_t masked_crc) {
  30. uint32_t rot = masked_crc - kMaskDelta;
  31. return ((rot >> 17) | (rot << 15));
  32. }
  33. } // namespace crc32c
  34. } // namespace leveldb
  35. #endif // STORAGE_LEVELDB_UTIL_CRC32C_H_