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// Copyright 2020 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_METRICS_ENTROPY_STATE_H_
#define COMPONENTS_METRICS_ENTROPY_STATE_H_
#include <string>
#include "base/gtest_prod_util.h"
#include "base/memory/raw_ptr.h"
#include "components/prefs/pref_registry_simple.h"
class PrefService;
namespace metrics {
// A class to get entropy source values from the PrefService.
class EntropyState final {
public:
// Creates the EntropyState with the given |local_state| to get
// the entropy source value from this helper class.
explicit EntropyState(PrefService* local_state);
EntropyState(const EntropyState&) = delete;
EntropyState& operator=(const EntropyState&) = delete;
// Clears low_entropy_source and old_low_entropy_source in the prefs.
static void ClearPrefs(PrefService* local_state);
// Registers low_entropy_source and old_low_entropy_source in the prefs.
static void RegisterPrefs(PrefRegistrySimple* registry);
// Returns the high entropy source for this client, which is composed of a
// client ID and the low entropy source. This is intended to be unique for
// each install. |initial_client_id| is the client_id that was used to
// randomize field trials and must not be empty.
std::string GetHighEntropySource(const std::string& initial_client_id);
// Returns the low entropy source that is used to randomize field trials on
// startup for this client. Generates a new value if there is none. See the
// |low_entropy_source_| comment for more info.
int GetLowEntropySource();
// Returns the pseudo low entropy source for this client. Generates a new
// value if there is none. See the |pseudo_low_entropy_source_| comment
// for more info.
int GetPseudoLowEntropySource();
// Returns the old low entropy source for this client. Does not generate a new
// value, but instead returns |kLowEntropySourceNotSet|, if there is none. See
// the |old_low_entropy_source_| comment for more info.
int GetOldLowEntropySource();
// Returns the limited entropy randomization source that is used to randomize
// field trials in a limited entropy layer. Generates a new value if there is
// none. See the |limited_entropy_randomization_source_| comment for more
// info.
std::string_view GetLimitedEntropyRandomizationSource();
// The argument used to generate a non-identifying entropy source. We want no
// more than 13 bits of entropy, so use this max to return a number in the
// range [0, 7999] as the entropy source (12.97 bits of entropy).
//
// The value should be kept consistent with
// LowEntropySource.MAX_LOW_ENTROPY_SIZE in Java.
static constexpr int kMaxLowEntropySize = 8000;
private:
FRIEND_TEST_ALL_PREFIXES(EntropyStateTest, LowEntropySourceNotReset);
FRIEND_TEST_ALL_PREFIXES(EntropyStateTest, PseudoLowEntropySourceNotReset);
FRIEND_TEST_ALL_PREFIXES(EntropyStateTest, HaveNoLowEntropySource);
FRIEND_TEST_ALL_PREFIXES(EntropyStateTest, HaveOnlyNewLowEntropySource);
FRIEND_TEST_ALL_PREFIXES(EntropyStateTest, HaveOnlyOldLowEntropySource);
FRIEND_TEST_ALL_PREFIXES(EntropyStateTest, CorruptNewLowEntropySources);
FRIEND_TEST_ALL_PREFIXES(EntropyStateTest, CorruptOldLowEntropySources);
FRIEND_TEST_ALL_PREFIXES(EntropyStateTest,
ValidLimitedEntropyRandomizationSource);
FRIEND_TEST_ALL_PREFIXES(EntropyStateTest,
InvalidLimitedEntropyRandomizationSource);
// Default value for prefs::kMetricsLowEntropySource.
static constexpr int kLowEntropySourceNotSet = -1;
// Loads the low entropy source values from prefs. Creates the new source
// value if it doesn't exist, but doesn't create the old source value. After
// this function finishes, |low_entropy_source_| will be set, but
// |old_low_entropy_source_| may still be |kLowEntropySourceNotSet|.
void UpdateLowEntropySources();
// Returns the limited entropy randomization source value if one is already
// set. Otherwise, attempts to generate a new one either when there isn't a
// previously set one stored in prefs, or when the command line flag
// --reset-variation-state is set. In both cases, the newly generated value
// will be set and stored stored in prefs before returning.
void UpdateLimitedEntropyRandomizationSource();
// Checks whether a value is on the range of allowed low entropy source
// values.
static bool IsValidLowEntropySource(int value);
// Checks whether the given value is a valid limited entropy randomization
// source.
static bool IsValidLimitedEntropyRandomizationSource(std::string_view value);
// Generates a new limited entropy randomization source.
std::string GenerateLimitedEntropyRandomizationSource();
// The local state prefs store.
const raw_ptr<PrefService> local_state_;
// The non-identifying low entropy source values. These values seed the
// pseudorandom generators which pick experimental groups. The "old" value is
// thought to be biased in the wild, and is no longer used for experiments
// requiring low entropy. Clients which already have an "old" value continue
// incorporating it into the high entropy source, to avoid changing those
// group assignments. New clients only have the new source.
//
// The pseudo-low entropy source is not used for experiment diversion, but
// only for statistical validation. (Since it's not used for experiment
// diversion, it won't be subject to drift over time as experiment effects
// accumulate in actual low entropy source buckets.)
//
// During startup these are set to the values used for randomizing field
// trials and won't be changed within the session even after calling
// |ClearPrefs|.
int low_entropy_source_ = kLowEntropySourceNotSet;
int old_low_entropy_source_ = kLowEntropySourceNotSet;
int pseudo_low_entropy_source_ = kLowEntropySourceNotSet;
// This value is used to seed the randomization for field trials in the
// limited entropy layer. During startup this value is set to one that's
// previously set. If a previously set value is not available, it will
// generate a new one. See more in the comments of
// |UpdateLimitedEntropyRandomizationSource|.
//
// Similar to the |low_entropy_source_| above, this value won't be changed
// within the session, even after calling |ClearPrefs|.
std::string limited_entropy_randomization_source_;
};
} // namespace metrics
#endif // COMPONENTS_METRICS_ENTROPY_STATE_H_
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