Samuel Huang | 06f1ae9 | 2018-03-13 18:19:34 +0000 | [diff] [blame] | 1 | // Copyright 2017 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 | #ifndef COMPONENTS_ZUCCHINI_BINARY_DATA_HISTOGRAM_H_ |
| 6 | #define COMPONENTS_ZUCCHINI_BINARY_DATA_HISTOGRAM_H_ |
| 7 | |
| 8 | #include <stddef.h> |
| 9 | #include <stdint.h> |
| 10 | |
| 11 | #include <memory> |
| 12 | #include <string> |
| 13 | |
Samuel Huang | 06f1ae9 | 2018-03-13 18:19:34 +0000 | [diff] [blame] | 14 | #include "components/zucchini/buffer_view.h" |
| 15 | |
| 16 | namespace zucchini { |
| 17 | |
| 18 | // A class to detect outliers in a list of doubles using Chauvenet's criterion: |
| 19 | // Compute mean and standard deviation of observations, then determine whether |
| 20 | // a query value lies beyond a fixed number of standard deviations (sigmas) from |
| 21 | // the mean. The purpose of this test is to reduce the chance of false-positive |
| 22 | // ensemble matches. |
| 23 | class OutlierDetector { |
| 24 | public: |
| 25 | OutlierDetector(); |
Samuel Huang | f137bf4 | 2021-08-13 15:42:26 +0000 | [diff] [blame] | 26 | OutlierDetector(const OutlierDetector&) = delete; |
| 27 | const OutlierDetector& operator=(const OutlierDetector&) = delete; |
Samuel Huang | 06f1ae9 | 2018-03-13 18:19:34 +0000 | [diff] [blame] | 28 | ~OutlierDetector(); |
| 29 | |
| 30 | // Incorporates |sample| into mean and standard deviation. |
| 31 | void Add(double sample); |
| 32 | |
| 33 | // Prepares basic statistics for DecideOutlier() calls. Should be called after |
| 34 | // all samples have been added. |
| 35 | void Prepare(); |
| 36 | |
| 37 | // Renders current statistics as strings for logging. |
| 38 | std::string RenderStats(); |
| 39 | |
| 40 | // Heuristically decides whether |sample| is an outlier. Returns 1 if |sample| |
| 41 | // is "too high", 0 if |sample| is "normal", and -1 if |sample| is "too low". |
| 42 | // Must be called after Prepare(). |
| 43 | int DecideOutlier(double sample); |
| 44 | |
| 45 | private: |
| 46 | size_t n_ = 0; |
| 47 | double sum_ = 0; |
| 48 | double sum_of_squares_ = 0; |
| 49 | double mean_ = 0; |
| 50 | double standard_deviation_ = 0; |
Samuel Huang | 06f1ae9 | 2018-03-13 18:19:34 +0000 | [diff] [blame] | 51 | }; |
| 52 | |
| 53 | // A class to compute similarity score between binary data. The heuristic here |
| 54 | // preprocesses input data to a size-65536 histogram, counting the frequency of |
| 55 | // consecutive 2-byte sequences. Therefore data with lengths < 2 are considered |
| 56 | // invalid -- but this is okay for Zucchini's use case. |
| 57 | class BinaryDataHistogram { |
| 58 | public: |
| 59 | BinaryDataHistogram(); |
Samuel Huang | f137bf4 | 2021-08-13 15:42:26 +0000 | [diff] [blame] | 60 | BinaryDataHistogram(const BinaryDataHistogram&) = delete; |
| 61 | const BinaryDataHistogram& operator=(const BinaryDataHistogram&) = delete; |
Samuel Huang | 06f1ae9 | 2018-03-13 18:19:34 +0000 | [diff] [blame] | 62 | ~BinaryDataHistogram(); |
| 63 | |
| 64 | // Attempts to compute the histogram, returns true iff successful. |
| 65 | bool Compute(ConstBufferView region); |
| 66 | |
| 67 | bool IsValid() const { return static_cast<bool>(histogram_); } |
| 68 | |
| 69 | // Returns distance to another histogram (heuristics). If two binaries are |
| 70 | // identical then their histogram distance is 0. However, the converse is not |
| 71 | // true in general. For example, "aba" and "bab" are different, but their |
| 72 | // histogram distance is 0 (both histograms are {"ab": 1, "ba": 1}). |
| 73 | double Distance(const BinaryDataHistogram& other) const; |
| 74 | |
| 75 | private: |
| 76 | enum { kNumBins = 1 << (sizeof(uint16_t) * 8) }; |
| 77 | static_assert(kNumBins == 65536, "Incorrect constant computation."); |
| 78 | |
| 79 | // Size, in bytes, of the data over which the histogram was computed. |
| 80 | size_t size_ = 0; |
| 81 | |
| 82 | // 2^16 buckets holding counts of all 2-byte sequences in the data. The counts |
| 83 | // are stored as signed values to simplify computing the distance between two |
| 84 | // histograms. |
| 85 | std::unique_ptr<int32_t[]> histogram_; |
Samuel Huang | 06f1ae9 | 2018-03-13 18:19:34 +0000 | [diff] [blame] | 86 | }; |
| 87 | |
| 88 | } // namespace zucchini |
| 89 | |
| 90 | #endif // COMPONENTS_ZUCCHINI_BINARY_DATA_HISTOGRAM_H_ |