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1 // Copyright (c) 2009 The Chromium Authors. All rights reserved. | |
2 // Use of this source code is governed by a BSD-style license that can be | |
3 // found in the LICENSE file. | |
4 | |
5 // A DifferenceEstimator class provides a means for quickly estimating the | |
6 // difference between two regions of memory. | |
7 | |
8 #ifndef COURGETTE_DIFFERENCE_ESTIMATOR_H_ | |
9 #define COURGETTE_DIFFERENCE_ESTIMATOR_H_ | |
10 | |
11 #include <vector> | |
12 | |
13 #include "third_party/courgette/region.h" | |
14 | |
15 namespace courgette { | |
16 | |
17 // A DifferenceEstimator simplifies the task of determining which 'Subject' byte | |
18 // strings (stored in regions of memory) are good matches to existing 'Base' | |
19 // regions. The ultimate measure would be to try full differential compression | |
20 // and measure the output size, but an estimate that correlates well with the | |
21 // full compression is more efficient. | |
22 // | |
23 // The measure is asymmetric, if the Subject is a small substring of the Base | |
24 // then it should match very well. | |
25 // | |
26 // The comparison is staged: first make Base and Subject objects for the regions | |
27 // and then call 'Measure' to get the estimate. The staging allows multiple | |
28 // comparisons to be more efficient by precomputing information used in the | |
29 // comparison. | |
30 // | |
31 class DifferenceEstimator { | |
32 public: | |
33 DifferenceEstimator(); | |
34 ~DifferenceEstimator(); | |
35 | |
36 class Base; | |
37 class Subject; | |
38 | |
39 // This DifferenceEstimator owns the objects returned by MakeBase and | |
40 // MakeSubject. Caller continues to own memory at |region| and must not free | |
41 // it until ~DifferenceEstimator has been called. | |
42 Base* MakeBase(const Region& region); | |
43 Subject* MakeSubject(const Region& region); | |
44 | |
45 // Returns a value correlated with the size of the bsdiff or xdelta difference | |
46 // from |base| to |subject|. Returns zero iff the base and subject regions | |
47 // are bytewise identical. | |
48 size_t Measure(Base* base, Subject* subject); | |
49 | |
50 private: | |
51 std::vector<Base*> owned_bases_; | |
52 std::vector<Subject*> owned_subjects_; | |
53 DISALLOW_COPY_AND_ASSIGN(DifferenceEstimator); | |
54 }; | |
55 | |
56 } // namespace | |
57 | |
58 #endif // COURGETTE_DIFFERENCE_ESTIMATOR_H_ | |
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