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// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_
#define COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_
#include <stddef.h>
#include <stdint.h>
#include <string>
#include "base/component_export.h"
#include "base/metrics/field_trial.h"
#include "third_party/abseil-cpp/absl/types/optional.h"
namespace variations {
// SHA1EntropyProvider is an entropy provider suitable for high entropy sources.
// It works by taking the first 64 bits of the SHA1 hash of the entropy source
// concatenated with the trial name, or randomization seed and using that for
// the final entropy value.
class COMPONENT_EXPORT(VARIATIONS) SHA1EntropyProvider
: public base::FieldTrial::EntropyProvider {
public:
// Creates a SHA1EntropyProvider with the given |entropy_source|, which
// should contain a large amount of entropy - for example, a textual
// representation of a persistent randomly-generated 128-bit value.
explicit SHA1EntropyProvider(const std::string& entropy_source);
SHA1EntropyProvider(const SHA1EntropyProvider&) = delete;
SHA1EntropyProvider& operator=(const SHA1EntropyProvider&) = delete;
~SHA1EntropyProvider() override;
// base::FieldTrial::EntropyProvider implementation:
double GetEntropyForTrial(base::StringPiece trial_name,
uint32_t randomization_seed) const override;
private:
const std::string entropy_source_;
};
// NormalizedMurmurHashEntropyProvider is an entropy provider suitable for low
// entropy sources (below 16 bits). It uses MurmurHash3_32 to hash the study
// name along with all possible low entropy sources. It finds the index where
// the actual low entropy source's hash would fall in the sorted list of all
// those hashes, and uses that as the final value. For more info, see:
// https://docs.google.com/document/d/1cPF5PruriWNP2Z5gSkq4MBTm0wSZqLyIJkUO9ekibeo
class COMPONENT_EXPORT(VARIATIONS) NormalizedMurmurHashEntropyProvider final
: public base::FieldTrial::EntropyProvider {
public:
// Creates a provider with |entropy_value| in the domain [0, entropy_domain).
NormalizedMurmurHashEntropyProvider(uint16_t entropy_value,
size_t entropy_domain);
NormalizedMurmurHashEntropyProvider(
const NormalizedMurmurHashEntropyProvider&) = default;
~NormalizedMurmurHashEntropyProvider() override;
// base::FieldTrial::EntropyProvider:
double GetEntropyForTrial(base::StringPiece trial_name,
uint32_t randomization_seed) const override;
size_t entropy_domain() const { return entropy_domain_; }
private:
const uint16_t entropy_value_;
const size_t entropy_domain_;
};
class COMPONENT_EXPORT(VARIATIONS) EntropyProviders {
public:
// Construct providers from the given entropy sources.
// If |high_entropy_source| is empty, no high entropy provider is created.
EntropyProviders(const std::string& high_entropy_value,
uint16_t low_entropy_value,
size_t low_entropy_domain);
EntropyProviders(const EntropyProviders&) = delete;
EntropyProviders& operator=(const EntropyProviders&) = delete;
virtual ~EntropyProviders();
// Accessors are virtual for testing purposes.
// Gets the high entropy source, if available, otherwise returns low entropy.
virtual const base::FieldTrial::EntropyProvider& default_entropy() const;
// Gets the low entropy source.
virtual const base::FieldTrial::EntropyProvider& low_entropy() const;
bool default_entropy_is_high_entropy() const {
return high_entropy_.has_value();
}
size_t low_entropy_domain() const { return low_entropy_.entropy_domain(); }
private:
absl::optional<SHA1EntropyProvider> high_entropy_;
NormalizedMurmurHashEntropyProvider low_entropy_;
};
} // namespace variations
#endif // COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_
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