1 // Copyright 2022 The Abseil Authors
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 #include "absl/crc/internal/crc_cord_state.h"
16
17 #include <cassert>
18
19 #include "absl/base/config.h"
20 #include "absl/base/no_destructor.h"
21 #include "absl/numeric/bits.h"
22
23 namespace absl {
24 ABSL_NAMESPACE_BEGIN
25 namespace crc_internal {
26
RefSharedEmptyRep()27 CrcCordState::RefcountedRep* CrcCordState::RefSharedEmptyRep() {
28 static absl::NoDestructor<CrcCordState::RefcountedRep> empty;
29
30 assert(empty->count.load(std::memory_order_relaxed) >= 1);
31 assert(empty->rep.removed_prefix.length == 0);
32 assert(empty->rep.prefix_crc.empty());
33
34 Ref(empty.get());
35 return empty.get();
36 }
37
CrcCordState()38 CrcCordState::CrcCordState() : refcounted_rep_(new RefcountedRep) {}
39
CrcCordState(const CrcCordState & other)40 CrcCordState::CrcCordState(const CrcCordState& other)
41 : refcounted_rep_(other.refcounted_rep_) {
42 Ref(refcounted_rep_);
43 }
44
CrcCordState(CrcCordState && other)45 CrcCordState::CrcCordState(CrcCordState&& other)
46 : refcounted_rep_(other.refcounted_rep_) {
47 // Make `other` valid for use after move.
48 other.refcounted_rep_ = RefSharedEmptyRep();
49 }
50
operator =(const CrcCordState & other)51 CrcCordState& CrcCordState::operator=(const CrcCordState& other) {
52 if (this != &other) {
53 Unref(refcounted_rep_);
54 refcounted_rep_ = other.refcounted_rep_;
55 Ref(refcounted_rep_);
56 }
57 return *this;
58 }
59
operator =(CrcCordState && other)60 CrcCordState& CrcCordState::operator=(CrcCordState&& other) {
61 if (this != &other) {
62 Unref(refcounted_rep_);
63 refcounted_rep_ = other.refcounted_rep_;
64 // Make `other` valid for use after move.
65 other.refcounted_rep_ = RefSharedEmptyRep();
66 }
67 return *this;
68 }
69
~CrcCordState()70 CrcCordState::~CrcCordState() {
71 Unref(refcounted_rep_);
72 }
73
Checksum() const74 crc32c_t CrcCordState::Checksum() const {
75 if (rep().prefix_crc.empty()) {
76 return absl::crc32c_t{0};
77 }
78 if (IsNormalized()) {
79 return rep().prefix_crc.back().crc;
80 }
81 return absl::RemoveCrc32cPrefix(
82 rep().removed_prefix.crc, rep().prefix_crc.back().crc,
83 rep().prefix_crc.back().length - rep().removed_prefix.length);
84 }
85
NormalizedPrefixCrcAtNthChunk(size_t n) const86 CrcCordState::PrefixCrc CrcCordState::NormalizedPrefixCrcAtNthChunk(
87 size_t n) const {
88 assert(n < NumChunks());
89 if (IsNormalized()) {
90 return rep().prefix_crc[n];
91 }
92 size_t length = rep().prefix_crc[n].length - rep().removed_prefix.length;
93 return PrefixCrc(length,
94 absl::RemoveCrc32cPrefix(rep().removed_prefix.crc,
95 rep().prefix_crc[n].crc, length));
96 }
97
Normalize()98 void CrcCordState::Normalize() {
99 if (IsNormalized() || rep().prefix_crc.empty()) {
100 return;
101 }
102
103 Rep* r = mutable_rep();
104 for (auto& prefix_crc : r->prefix_crc) {
105 size_t remaining = prefix_crc.length - r->removed_prefix.length;
106 prefix_crc.crc = absl::RemoveCrc32cPrefix(r->removed_prefix.crc,
107 prefix_crc.crc, remaining);
108 prefix_crc.length = remaining;
109 }
110 r->removed_prefix = PrefixCrc();
111 }
112
Poison()113 void CrcCordState::Poison() {
114 Rep* rep = mutable_rep();
115 if (NumChunks() > 0) {
116 for (auto& prefix_crc : rep->prefix_crc) {
117 // This is basically CRC32::Scramble().
118 uint32_t crc = static_cast<uint32_t>(prefix_crc.crc);
119 crc += 0x2e76e41b;
120 crc = absl::rotr(crc, 17);
121 prefix_crc.crc = crc32c_t{crc};
122 }
123 } else {
124 // Add a fake corrupt chunk.
125 rep->prefix_crc.emplace_back(0, crc32c_t{1});
126 }
127 }
128
129 } // namespace crc_internal
130 ABSL_NAMESPACE_END
131 } // namespace absl
132