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- // 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 "base/check_op.h"
- #include "base/hash/sha1.h"
- #include "base/rand_util.h"
- #include "base/strings/strcat.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(
- base::StringPiece 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 = base::StrCat(
- {entropy_source_, 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(
- base::StringPiece 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
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