| // Copyright (c) 2012 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #include "components/variations/entropy_provider.h" |
| |
| #include <algorithm> |
| #include <limits> |
| #include <vector> |
| |
| #include "base/hash/sha1.h" |
| #include "base/logging.h" |
| #include "base/rand_util.h" |
| #include "base/strings/string_number_conversions.h" |
| #include "base/sys_byteorder.h" |
| #include "components/variations/hashing.h" |
| #include "components/variations/variations_murmur_hash.h" |
| |
| namespace variations { |
| |
| SHA1EntropyProvider::SHA1EntropyProvider(const std::string& entropy_source) |
| : entropy_source_(entropy_source) { |
| } |
| |
| SHA1EntropyProvider::~SHA1EntropyProvider() { |
| } |
| |
| double SHA1EntropyProvider::GetEntropyForTrial( |
| const std::string& trial_name, |
| uint32_t randomization_seed) const { |
| // Given enough input entropy, SHA-1 will produce a uniformly random spread |
| // in its output space. In this case, the input entropy that is used is the |
| // combination of the original |entropy_source_| and the |trial_name|. |
| // |
| // Note: If |entropy_source_| has very low entropy, such as 13 bits or less, |
| // it has been observed that this method does not result in a uniform |
| // distribution given the same |trial_name|. When using such a low entropy |
| // source, NormalizedMurmurHashEntropyProvider should be used instead. |
| std::string input(entropy_source_); |
| input.append(randomization_seed == 0 |
| ? trial_name |
| : base::NumberToString(randomization_seed)); |
| |
| unsigned char sha1_hash[base::kSHA1Length]; |
| base::SHA1HashBytes(reinterpret_cast<const unsigned char*>(input.c_str()), |
| input.size(), |
| sha1_hash); |
| |
| uint64_t bits; |
| static_assert(sizeof(bits) < sizeof(sha1_hash), "more data required"); |
| memcpy(&bits, sha1_hash, sizeof(bits)); |
| bits = base::ByteSwapToLE64(bits); |
| |
| return base::BitsToOpenEndedUnitInterval(bits); |
| } |
| |
| NormalizedMurmurHashEntropyProvider::NormalizedMurmurHashEntropyProvider( |
| uint16_t low_entropy_source, |
| size_t low_entropy_source_max) |
| : low_entropy_source_(low_entropy_source), |
| low_entropy_source_max_(low_entropy_source_max) { |
| DCHECK_LT(low_entropy_source, low_entropy_source_max); |
| DCHECK_LE(low_entropy_source_max, std::numeric_limits<uint16_t>::max()); |
| } |
| |
| NormalizedMurmurHashEntropyProvider::~NormalizedMurmurHashEntropyProvider() {} |
| |
| double NormalizedMurmurHashEntropyProvider::GetEntropyForTrial( |
| const std::string& trial_name, |
| uint32_t randomization_seed) const { |
| if (randomization_seed == 0) { |
| randomization_seed = internal::VariationsMurmurHash::Hash( |
| internal::VariationsMurmurHash::StringToLE32(trial_name), |
| trial_name.length()); |
| } |
| |
| uint32_t x = internal::VariationsMurmurHash::Hash16(randomization_seed, |
| low_entropy_source_); |
| int x_ordinal = 0; |
| for (uint32_t i = 0; i < low_entropy_source_max_; i++) { |
| uint32_t y = internal::VariationsMurmurHash::Hash16(randomization_seed, i); |
| x_ordinal += (y < x); |
| } |
| |
| DCHECK_GE(x_ordinal, 0); |
| // There must have been at least one iteration where |x| == |y|, because |
| // |i| == |low_entropy_source_|, and |x_ordinal| was not incremented in that |
| // iteration, so |x_ordinal| < |low_entropy_source_max_|. |
| DCHECK_LT(static_cast<size_t>(x_ordinal), low_entropy_source_max_); |
| |
| return static_cast<double>(x_ordinal) / low_entropy_source_max_; |
| } |
| |
| } // namespace variations |