components/variations/entropy_provider.cc

   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 #include "components/variations/entropy_provider.h"
   6
   7 #include <algorithm>
   8 #include <limits>
   9 #include <vector>
  10
  11 #include "base/logging.h"
  12 #include "base/rand_util.h"
  13 #include "base/sha1.h"
  14 #include "base/sys_byteorder.h"
  15 #include "components/variations/metrics_util.h"
  16
  17 namespace metrics {
  18
  19 namespace internal {
  20
  21 SeededRandGenerator::SeededRandGenerator(uint32 seed) {
  22   mersenne_twister_.init_genrand(seed);
  23 }
  24
  25 SeededRandGenerator::~SeededRandGenerator() {
  26 }
  27
  28 uint32 SeededRandGenerator::operator()(uint32 range) {
  29   // Based on base::RandGenerator().
  30   DCHECK_GT(range, 0u);
  31
  32   // We must discard random results above this number, as they would
  33   // make the random generator non-uniform (consider e.g. if
  34   // MAX_UINT64 was 7 and |range| was 5, then a result of 1 would be twice
  35   // as likely as a result of 3 or 4).
  36   uint32 max_acceptable_value =
  37       (std::numeric_limits<uint32>::max() / range) * range - 1;
  38
  39   uint32 value;
  40   do {
  41     value = mersenne_twister_.genrand_int32();
  42   } while (value > max_acceptable_value);
  43
  44   return value % range;
  45 }
  46
  47 void PermuteMappingUsingRandomizationSeed(uint32 randomization_seed,
  48                                           std::vector<uint16>* mapping) {
  49   for (size_t i = 0; i < mapping->size(); ++i)
  50     (*mapping)[i] = static_cast<uint16>(i);
  51
  52   SeededRandGenerator generator(randomization_seed);
  53
  54   // Do a deterministic random shuffle of the mapping using |generator|.
  55   //
  56   // Note: This logic is identical to the following call with libstdc++ and VS:
  57   //
  58   //   std::random_shuffle(mapping->begin(), mapping->end(), generator);
  59   //
  60   // However, this is not guaranteed by the spec and some implementations (e.g.
  61   // libc++) use a different algorithm. To ensure results are consistent
  62   // regardless of the compiler toolchain used, use our own version.
  63   for (size_t i = 1; i < mapping->size(); ++i) {
  64     // Pick an element in mapping[:i+1] with which to exchange mapping[i].
  65     size_t j = generator(i + 1);
  66     std::swap((*mapping)[i], (*mapping)[j]);
  67   }
  68 }
  69
  70 }  // namespace internal
  71
  72 SHA1EntropyProvider::SHA1EntropyProvider(const std::string& entropy_source)
  73     : entropy_source_(entropy_source) {
  74 }
  75
  76 SHA1EntropyProvider::~SHA1EntropyProvider() {
  77 }
  78
  79 double SHA1EntropyProvider::GetEntropyForTrial(
  80     const std::string& trial_name,
  81     uint32 randomization_seed) const {
  82   // Given enough input entropy, SHA-1 will produce a uniformly random spread
  83   // in its output space. In this case, the input entropy that is used is the
  84   // combination of the original |entropy_source_| and the |trial_name|.
  85   //
  86   // Note: If |entropy_source_| has very low entropy, such as 13 bits or less,
  87   // it has been observed that this method does not result in a uniform
  88   // distribution given the same |trial_name|. When using such a low entropy
  89   // source, PermutedEntropyProvider should be used instead.
  90   std::string input(entropy_source_ + trial_name);
  91   unsigned char sha1_hash[base::kSHA1Length];
  92   base::SHA1HashBytes(reinterpret_cast<const unsigned char*>(input.c_str()),
  93                       input.size(),
  94                       sha1_hash);
  95
  96   uint64 bits;
  97   static_assert(sizeof(bits) < sizeof(sha1_hash), "more data required");
  98   memcpy(&bits, sha1_hash, sizeof(bits));
  99   bits = base::ByteSwapToLE64(bits);
 100
 101   return base::BitsToOpenEndedUnitInterval(bits);
 102 }
 103
 104 PermutedEntropyProvider::PermutedEntropyProvider(
 105     uint16 low_entropy_source,
 106     size_t low_entropy_source_max)
 107     : low_entropy_source_(low_entropy_source),
 108       low_entropy_source_max_(low_entropy_source_max) {
 109   DCHECK_LT(low_entropy_source, low_entropy_source_max);
 110   DCHECK_LE(low_entropy_source_max, std::numeric_limits<uint16>::max());
 111 }
 112
 113 PermutedEntropyProvider::~PermutedEntropyProvider() {
 114 }
 115
 116 double PermutedEntropyProvider::GetEntropyForTrial(
 117     const std::string& trial_name,
 118     uint32 randomization_seed) const {
 119   if (randomization_seed == 0)
 120     randomization_seed = HashName(trial_name);
 121
 122   return GetPermutedValue(randomization_seed) /
 123          static_cast<double>(low_entropy_source_max_);
 124 }
 125
 126 uint16 PermutedEntropyProvider::GetPermutedValue(
 127     uint32 randomization_seed) const {
 128   std::vector<uint16> mapping(low_entropy_source_max_);
 129   internal::PermuteMappingUsingRandomizationSeed(randomization_seed, &mapping);
 130   return mapping[low_entropy_source_];
 131 }
 132
 133 }  // namespace metrics