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- // Copyright (c) 2009 The Chromium Authors. All rights reserved.
- // Use of this source code is governed by a BSD-style license that can be
- // found in the LICENSE file.
- // A DifferenceEstimator class provides a means for quickly estimating the
- // difference between two regions of memory.
- #ifndef COURGETTE_DIFFERENCE_ESTIMATOR_H_
- #define COURGETTE_DIFFERENCE_ESTIMATOR_H_
- #include <stddef.h>
- #include <vector>
- #include "courgette/region.h"
- namespace courgette {
- // A DifferenceEstimator simplifies the task of determining which 'Subject' byte
- // strings (stored in regions of memory) are good matches to existing 'Base'
- // regions. The ultimate measure would be to try full differential compression
- // and measure the output size, but an estimate that correlates well with the
- // full compression is more efficient.
- //
- // The measure is asymmetric, if the Subject is a small substring of the Base
- // then it should match very well.
- //
- // The comparison is staged: first make Base and Subject objects for the regions
- // and then call 'Measure' to get the estimate. The staging allows multiple
- // comparisons to be more efficient by precomputing information used in the
- // comparison.
- //
- class DifferenceEstimator {
- public:
- DifferenceEstimator();
- DifferenceEstimator(const DifferenceEstimator&) = delete;
- DifferenceEstimator& operator=(const DifferenceEstimator&) = delete;
- ~DifferenceEstimator();
- class Base;
- class Subject;
- // This DifferenceEstimator owns the objects returned by MakeBase and
- // MakeSubject. Caller continues to own memory at |region| and must not free
- // it until ~DifferenceEstimator has been called.
- Base* MakeBase(const Region& region);
- Subject* MakeSubject(const Region& region);
- // Returns a value correlated with the size of the bsdiff or xdelta difference
- // from |base| to |subject|. Returns zero iff the base and subject regions
- // are bytewise identical.
- size_t Measure(Base* base, Subject* subject);
- private:
- std::vector<Base*> owned_bases_;
- std::vector<Subject*> owned_subjects_;
- };
- } // namespace
- #endif // COURGETTE_DIFFERENCE_ESTIMATOR_H_
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