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
17namespace metrics {
18
19namespace internal {
20
21SeededRandGenerator::SeededRandGenerator(uint32 seed) {
22  mersenne_twister_.init_genrand(seed);
23}
24
25SeededRandGenerator::~SeededRandGenerator() {
26}
27
28uint32 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
47void 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  std::random_shuffle(mapping->begin(), mapping->end(), generator);
54}
55
56}  // namespace internal
57
58SHA1EntropyProvider::SHA1EntropyProvider(const std::string& entropy_source)
59    : entropy_source_(entropy_source) {
60}
61
62SHA1EntropyProvider::~SHA1EntropyProvider() {
63}
64
65double SHA1EntropyProvider::GetEntropyForTrial(
66    const std::string& trial_name,
67    uint32 randomization_seed) const {
68  // Given enough input entropy, SHA-1 will produce a uniformly random spread
69  // in its output space. In this case, the input entropy that is used is the
70  // combination of the original |entropy_source_| and the |trial_name|.
71  //
72  // Note: If |entropy_source_| has very low entropy, such as 13 bits or less,
73  // it has been observed that this method does not result in a uniform
74  // distribution given the same |trial_name|. When using such a low entropy
75  // source, PermutedEntropyProvider should be used instead.
76  std::string input(entropy_source_ + trial_name);
77  unsigned char sha1_hash[base::kSHA1Length];
78  base::SHA1HashBytes(reinterpret_cast<const unsigned char*>(input.c_str()),
79                      input.size(),
80                      sha1_hash);
81
82  uint64 bits;
83  COMPILE_ASSERT(sizeof(bits) < sizeof(sha1_hash), need_more_data);
84  memcpy(&bits, sha1_hash, sizeof(bits));
85  bits = base::ByteSwapToLE64(bits);
86
87  return base::BitsToOpenEndedUnitInterval(bits);
88}
89
90PermutedEntropyProvider::PermutedEntropyProvider(
91    uint16 low_entropy_source,
92    size_t low_entropy_source_max)
93    : low_entropy_source_(low_entropy_source),
94      low_entropy_source_max_(low_entropy_source_max) {
95  DCHECK_LT(low_entropy_source, low_entropy_source_max);
96  DCHECK_LE(low_entropy_source_max, std::numeric_limits<uint16>::max());
97}
98
99PermutedEntropyProvider::~PermutedEntropyProvider() {
100}
101
102double PermutedEntropyProvider::GetEntropyForTrial(
103    const std::string& trial_name,
104    uint32 randomization_seed) const {
105  if (randomization_seed == 0)
106    randomization_seed = HashName(trial_name);
107
108  return GetPermutedValue(randomization_seed) /
109         static_cast<double>(low_entropy_source_max_);
110}
111
112uint16 PermutedEntropyProvider::GetPermutedValue(
113    uint32 randomization_seed) const {
114  std::vector<uint16> mapping(low_entropy_source_max_);
115  internal::PermuteMappingUsingRandomizationSeed(randomization_seed, &mapping);
116  return mapping[low_entropy_source_];
117}
118
119}  // namespace metrics
120