1 // Copyright (c) 2012 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_VARIATIONS_ENTROPY_PROVIDER_H_ 6 #define COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_ 7 8 #include <functional> 9 #include <string> 10 #include <vector> 11 12 #include "base/basictypes.h" 13 #include "base/compiler_specific.h" 14 #include "base/metrics/field_trial.h" 15 #include "third_party/mt19937ar/mt19937ar.h" 16 17 namespace metrics { 18 19 // Internals of entropy_provider.cc exposed for testing. 20 namespace internal { 21 22 // A functor that generates random numbers based on a seed, using the Mersenne 23 // Twister algorithm. Suitable for use with std::random_shuffle(). 24 struct SeededRandGenerator : std::unary_function<uint32, uint32> { 25 explicit SeededRandGenerator(uint32 seed); 26 ~SeededRandGenerator(); 27 28 // Returns a random number in range [0, range). 29 uint32 operator()(uint32 range); 30 31 MersenneTwister mersenne_twister_; 32 }; 33 34 // Fills |mapping| to create a bijection of values in the range of 35 // [0, |mapping.size()|), permuted based on |randomization_seed|. 36 void PermuteMappingUsingRandomizationSeed(uint32 randomization_seed, 37 std::vector<uint16>* mapping); 38 39 } // namespace internal 40 41 // SHA1EntropyProvider is an entropy provider suitable for high entropy 42 // sources. It works by taking the first 64 bits of the SHA1 hash of the 43 // entropy source concatenated with the trial name and using that for the 44 // final entropy value. 45 class SHA1EntropyProvider : public base::FieldTrial::EntropyProvider { 46 public: 47 // Creates a SHA1EntropyProvider with the given |entropy_source|, which 48 // should contain a large amount of entropy - for example, a textual 49 // representation of a persistent randomly-generated 128-bit value. 50 explicit SHA1EntropyProvider(const std::string& entropy_source); 51 virtual ~SHA1EntropyProvider(); 52 53 // base::FieldTrial::EntropyProvider implementation: 54 virtual double GetEntropyForTrial(const std::string& trial_name, 55 uint32 randomization_seed) const OVERRIDE; 56 57 private: 58 std::string entropy_source_; 59 60 DISALLOW_COPY_AND_ASSIGN(SHA1EntropyProvider); 61 }; 62 63 // PermutedEntropyProvider is an entropy provider suitable for low entropy 64 // sources (below 16 bits). It uses the field trial name to generate a 65 // permutation of a mapping array from an initial entropy value to a new value. 66 // Note: This provider's performance is O(2^n), where n is the number of bits 67 // in the entropy source. 68 class PermutedEntropyProvider : public base::FieldTrial::EntropyProvider { 69 public: 70 // Creates a PermutedEntropyProvider with the given |low_entropy_source|, 71 // which should have a value in the range of [0, low_entropy_source_max). 72 PermutedEntropyProvider(uint16 low_entropy_source, 73 size_t low_entropy_source_max); 74 virtual ~PermutedEntropyProvider(); 75 76 // base::FieldTrial::EntropyProvider implementation: 77 virtual double GetEntropyForTrial(const std::string& trial_name, 78 uint32 randomization_seed) const OVERRIDE; 79 80 protected: 81 // Performs the permutation algorithm and returns the permuted value that 82 // corresponds to |low_entropy_source_|. 83 virtual uint16 GetPermutedValue(uint32 randomization_seed) const; 84 85 private: 86 uint16 low_entropy_source_; 87 size_t low_entropy_source_max_; 88 89 DISALLOW_COPY_AND_ASSIGN(PermutedEntropyProvider); 90 }; 91 92 } // namespace metrics 93 94 #endif // COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_ 95