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// 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
//
//
// 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.
#ifndef ABSL_CRC_INTERNAL_CRC_H_
#define ABSL_CRC_INTERNAL_CRC_H_
#include <cstdint>
#include "absl/base/config.h"
// 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
#endif // ABSL_CRC_INTERNAL_CRC_H_