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 // -----------------------------------------------------------------------------
16 // File: crc32c.h
17 // -----------------------------------------------------------------------------
18 //
19 // This header file defines the API for computing CRC32C values as checksums
20 // for arbitrary sequences of bytes provided as a string buffer.
21 //
22 // The API includes the basic functions for computing such CRC32C values and
23 // some utility functions for performing more efficient mathematical
24 // computations using an existing checksum.
25 #ifndef ABSL_CRC_CRC32C_H_
26 #define ABSL_CRC_CRC32C_H_
27
28 #include <cstdint>
29 #include <ostream>
30
31 #include "absl/crc/internal/crc32c_inline.h"
32 #include "absl/strings/string_view.h"
33
34 namespace absl {
35 ABSL_NAMESPACE_BEGIN
36
37 //-----------------------------------------------------------------------------
38 // crc32c_t
39 //-----------------------------------------------------------------------------
40
41 // `crc32c_t` defines a strongly-typed integer for holding a CRC32C value.
42 //
43 // Some operators are intentionally omitted. Only equality operators are defined
44 // so that `crc32c_t` can be directly compared. Methods for putting `crc32c_t`
45 // directly into a set are omitted because this is bug-prone due to checksum
46 // collisions. Use an explicit conversion to the `uint32_t` space for operations
47 // that treat `crc32c_t` as an integer.
48 class crc32c_t final {
49 public:
50 crc32c_t() = default;
crc32c_t(uint32_t crc)51 constexpr explicit crc32c_t(uint32_t crc) : crc_(crc) {}
52
53 crc32c_t(const crc32c_t&) = default;
54 crc32c_t& operator=(const crc32c_t&) = default;
55
uint32_t()56 explicit operator uint32_t() const { return crc_; }
57
58 friend bool operator==(crc32c_t lhs, crc32c_t rhs) {
59 return static_cast<uint32_t>(lhs) == static_cast<uint32_t>(rhs);
60 }
61
62 friend bool operator!=(crc32c_t lhs, crc32c_t rhs) { return !(lhs == rhs); }
63
64 private:
65 uint32_t crc_;
66 };
67
68 namespace crc_internal {
69 // Non-inline code path for `absl::ExtendCrc32c()`. Do not call directly.
70 // Call `absl::ExtendCrc32c()` (defined below) instead.
71 crc32c_t ExtendCrc32cInternal(crc32c_t initial_crc,
72 absl::string_view buf_to_add);
73 } // namespace crc_internal
74
75 // -----------------------------------------------------------------------------
76 // CRC32C Computation Functions
77 // -----------------------------------------------------------------------------
78
79 // ComputeCrc32c()
80 //
81 // Returns the CRC32C value of the provided string.
82 crc32c_t ComputeCrc32c(absl::string_view buf);
83
84 // ExtendCrc32c()
85 //
86 // Computes a CRC32C value from an `initial_crc` CRC32C value including the
87 // `buf_to_add` bytes of an additional buffer. Using this function is more
88 // efficient than computing a CRC32C value for the combined buffer from
89 // scratch.
90 //
91 // Note: `ExtendCrc32c` with an initial_crc of 0 is equivalent to
92 // `ComputeCrc32c`.
93 //
94 // This operation has a runtime cost of O(`buf_to_add.size()`)
ExtendCrc32c(crc32c_t initial_crc,absl::string_view buf_to_add)95 inline crc32c_t ExtendCrc32c(crc32c_t initial_crc,
96 absl::string_view buf_to_add) {
97 // Approximately 75% of calls have size <= 64.
98 if (buf_to_add.size() <= 64) {
99 uint32_t crc = static_cast<uint32_t>(initial_crc);
100 if (crc_internal::ExtendCrc32cInline(&crc, buf_to_add.data(),
101 buf_to_add.size())) {
102 return crc32c_t{crc};
103 }
104 }
105 return crc_internal::ExtendCrc32cInternal(initial_crc, buf_to_add);
106 }
107
108 // ExtendCrc32cByZeroes()
109 //
110 // Computes a CRC32C value for a buffer with an `initial_crc` CRC32C value,
111 // where `length` bytes with a value of 0 are appended to the buffer. Using this
112 // function is more efficient than computing a CRC32C value for the combined
113 // buffer from scratch.
114 //
115 // This operation has a runtime cost of O(log(`length`))
116 crc32c_t ExtendCrc32cByZeroes(crc32c_t initial_crc, size_t length);
117
118 // MemcpyCrc32c()
119 //
120 // Copies `src` to `dest` using `memcpy()` semantics, returning the CRC32C
121 // value of the copied buffer.
122 //
123 // Using `MemcpyCrc32c()` is potentially faster than performing the `memcpy()`
124 // and `ComputeCrc32c()` operations separately.
125 crc32c_t MemcpyCrc32c(void* dest, const void* src, size_t count,
126 crc32c_t initial_crc = crc32c_t{0});
127
128 // -----------------------------------------------------------------------------
129 // CRC32C Arithmetic Functions
130 // -----------------------------------------------------------------------------
131
132 // The following functions perform arithmetic on CRC32C values, which are
133 // generally more efficient than recalculating any given result's CRC32C value.
134
135 // ConcatCrc32c()
136 //
137 // Calculates the CRC32C value of two buffers with known CRC32C values
138 // concatenated together.
139 //
140 // Given a buffer with CRC32C value `crc1` and a buffer with
141 // CRC32C value `crc2` and length, `crc2_length`, returns the CRC32C value of
142 // the concatenation of these two buffers.
143 //
144 // This operation has a runtime cost of O(log(`crc2_length`)).
145 crc32c_t ConcatCrc32c(crc32c_t crc1, crc32c_t crc2, size_t crc2_length);
146
147 // RemoveCrc32cPrefix()
148 //
149 // Calculates the CRC32C value of an existing buffer with a series of bytes
150 // (the prefix) removed from the beginning of that buffer.
151 //
152 // Given the CRC32C value of an existing buffer, `full_string_crc`; The CRC32C
153 // value of a prefix of that buffer, `prefix_crc`; and the length of the buffer
154 // with the prefix removed, `remaining_string_length` , return the CRC32C
155 // value of the buffer with the prefix removed.
156 //
157 // This operation has a runtime cost of O(log(`remaining_string_length`)).
158 crc32c_t RemoveCrc32cPrefix(crc32c_t prefix_crc, crc32c_t full_string_crc,
159 size_t remaining_string_length);
160 // RemoveCrc32cSuffix()
161 //
162 // Calculates the CRC32C value of an existing buffer with a series of bytes
163 // (the suffix) removed from the end of that buffer.
164 //
165 // Given a CRC32C value of an existing buffer `full_string_crc`, the CRC32C
166 // value of the suffix to remove `suffix_crc`, and the length of that suffix
167 // `suffix_len`, returns the CRC32C value of the buffer with suffix removed.
168 //
169 // This operation has a runtime cost of O(log(`suffix_len`))
170 crc32c_t RemoveCrc32cSuffix(crc32c_t full_string_crc, crc32c_t suffix_crc,
171 size_t suffix_length);
172
173 // operator<<
174 //
175 // Streams the CRC32C value `crc` to the stream `os`.
176 inline std::ostream& operator<<(std::ostream& os, crc32c_t crc) {
177 return os << static_cast<uint32_t>(crc);
178 }
179
180 ABSL_NAMESPACE_END
181 } // namespace absl
182
183 #endif // ABSL_CRC_CRC32C_H_
184