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 17namespace metrics { 18 19// Internals of entropy_provider.cc exposed for testing. 20namespace 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(). 24struct 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|. 36void 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. 45class 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. 68class 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