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