ceph/src/rocksdb/util/crc32c.h

   1 //  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
   2 //  This source code is licensed under both the GPLv2 (found in the
   3 //  COPYING file in the root directory) and Apache 2.0 License
   4 //  (found in the LICENSE.Apache file in the root directory).
   5 //
   6 // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
   7 // Use of this source code is governed by a BSD-style license that can be
   8 // found in the LICENSE file. See the AUTHORS file for names of contributors.
   9
  10 #pragma once
  11 #include <stddef.h>
  12 #include <stdint.h>
  13
  14 #include <string>
  15
  16 #include "rocksdb/rocksdb_namespace.h"
  17
  18 namespace ROCKSDB_NAMESPACE {
  19 namespace crc32c {
  20
  21 extern std::string IsFastCrc32Supported();
  22
  23 // Return the crc32c of concat(A, data[0,n-1]) where init_crc is the
  24 // crc32c of some string A.  Extend() is often used to maintain the
  25 // crc32c of a stream of data.
  26 extern uint32_t Extend(uint32_t init_crc, const char* data, size_t n);
  27
  28 // Takes two unmasked crc32c values, and the length of the string from
  29 // which `crc2` was computed, and computes a crc32c value for the
  30 // concatenation of the original two input strings. Running time is
  31 // ~ log(crc2len).
  32 extern uint32_t Crc32cCombine(uint32_t crc1, uint32_t crc2, size_t crc2len);
  33
  34 // Return the crc32c of data[0,n-1]
  35 inline uint32_t Value(const char* data, size_t n) { return Extend(0, data, n); }
  36
  37 static const uint32_t kMaskDelta = 0xa282ead8ul;
  38
  39 // Return a masked representation of crc.
  40 //
  41 // Motivation: it is problematic to compute the CRC of a string that
  42 // contains embedded CRCs.  Therefore we recommend that CRCs stored
  43 // somewhere (e.g., in files) should be masked before being stored.
  44 inline uint32_t Mask(uint32_t crc) {
  45   // Rotate right by 15 bits and add a constant.
  46   return ((crc >> 15) | (crc << 17)) + kMaskDelta;
  47 }
  48
  49 // Return the crc whose masked representation is masked_crc.
  50 inline uint32_t Unmask(uint32_t masked_crc) {
  51   uint32_t rot = masked_crc - kMaskDelta;
  52   return ((rot >> 17) | (rot << 15));
  53 }
  54
  55 }  // namespace crc32c
  56 }  // namespace ROCKSDB_NAMESPACE