| // Copyright 2022 The Abseil Authors. | |
| // | |
| // Licensed under the Apache License, Version 2.0 (the "License"); | |
| // you may not use this file except in compliance with the License. | |
| // You may obtain a copy of the License at | |
| // | |
| // https://www.apache.org/licenses/LICENSE-2.0 | |
| // | |
| // Unless required by applicable law or agreed to in writing, software | |
| // distributed under the License is distributed on an "AS IS" BASIS, | |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | |
| // See the License for the specific language governing permissions and | |
| // limitations under the License. | |
| // This class implements CRCs (aka Rabin Fingerprints). | |
| // Treats the input as a polynomial with coefficients in Z(2), | |
| // and finds the remainder when divided by an primitive polynomial | |
| // of the appropriate length. | |
| // A polynomial is represented by the bit pattern formed by its coefficients, | |
| // but with the highest order bit not stored. | |
| // The highest degree coefficient is stored in the lowest numbered bit | |
| // in the lowest addressed byte. Thus, in what follows, the highest degree | |
| // coefficient that is stored is in the low order bit of "lo" or "*lo". | |
| // Hardware acceleration is used when available. | |
| namespace absl { | |
| ABSL_NAMESPACE_BEGIN | |
| namespace crc_internal { | |
| class CRC { | |
| public: | |
| virtual ~CRC(); | |
| // If "*crc" is the CRC of bytestring A, place the CRC of | |
| // the bytestring formed from the concatenation of A and the "length" | |
| // bytes at "bytes" into "*crc". | |
| virtual void Extend(uint32_t* crc, const void* bytes, | |
| size_t length) const = 0; | |
| // Equivalent to Extend(crc, bytes, length) where "bytes" | |
| // points to an array of "length" zero bytes. | |
| virtual void ExtendByZeroes(uint32_t* crc, size_t length) const = 0; | |
| // Inverse operation of ExtendByZeroes. If `crc` is the CRC value of a string | |
| // ending in `length` zero bytes, this returns a CRC value of that string | |
| // with those zero bytes removed. | |
| virtual void UnextendByZeroes(uint32_t* crc, size_t length) const = 0; | |
| // Apply a non-linear transformation to "*crc" so that | |
| // it is safe to CRC the result with the same polynomial without | |
| // any reduction of error-detection ability in the outer CRC. | |
| // Unscramble() performs the inverse transformation. | |
| // It is strongly recommended that CRCs be scrambled before storage or | |
| // transmission, and unscrambled at the other end before further manipulation. | |
| virtual void Scramble(uint32_t* crc) const = 0; | |
| virtual void Unscramble(uint32_t* crc) const = 0; | |
| // Crc32c() returns the singleton implementation of CRC for the CRC32C | |
| // polynomial. Returns a handle that MUST NOT be destroyed with delete. | |
| static CRC* Crc32c(); | |
| protected: | |
| CRC(); // Clients may not call constructor; use Crc32c() instead. | |
| private: | |
| CRC(const CRC&) = delete; | |
| CRC& operator=(const CRC&) = delete; | |
| }; | |
| } // namespace crc_internal | |
| ABSL_NAMESPACE_END | |
| } // namespace absl | |