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      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