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// Copyright 2013 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 "chrome/common/metrics/caching_permuted_entropy_provider.h"
#include <string>
#include "base/base64.h"
#include "base/logging.h"
#include "base/prefs/pref_registry_simple.h"
#include "base/prefs/pref_service.h"
#include "chrome/common/pref_names.h"
namespace metrics {
CachingPermutedEntropyProvider::CachingPermutedEntropyProvider(
PrefService* local_state,
uint16 low_entropy_source,
size_t low_entropy_source_max)
: PermutedEntropyProvider(low_entropy_source, low_entropy_source_max),
local_state_(local_state) {
ReadFromLocalState();
}
CachingPermutedEntropyProvider::~CachingPermutedEntropyProvider() {
}
// static
void CachingPermutedEntropyProvider::RegisterPrefs(
PrefRegistrySimple* registry) {
registry->RegisterStringPref(prefs::kMetricsPermutedEntropyCache,
std::string());
}
// static
void CachingPermutedEntropyProvider::ClearCache(PrefService* local_state) {
local_state->ClearPref(prefs::kMetricsPermutedEntropyCache);
}
uint16 CachingPermutedEntropyProvider::GetPermutedValue(
uint32 randomization_seed) const {
DCHECK(thread_checker_.CalledOnValidThread());
uint16 value = 0;
if (!FindValue(randomization_seed, &value)) {
value = PermutedEntropyProvider::GetPermutedValue(randomization_seed);
AddToCache(randomization_seed, value);
}
return value;
}
void CachingPermutedEntropyProvider::ReadFromLocalState() const {
const std::string base64_cache_data =
local_state_->GetString(prefs::kMetricsPermutedEntropyCache);
std::string cache_data;
if (!base::Base64Decode(base64_cache_data, &cache_data) ||
!cache_.ParseFromString(cache_data)) {
local_state_->ClearPref(prefs::kMetricsPermutedEntropyCache);
NOTREACHED();
}
}
void CachingPermutedEntropyProvider::UpdateLocalState() const {
std::string serialized;
cache_.SerializeToString(&serialized);
std::string base64_encoded;
if (!base::Base64Encode(serialized, &base64_encoded)) {
NOTREACHED();
return;
}
local_state_->SetString(prefs::kMetricsPermutedEntropyCache, base64_encoded);
}
void CachingPermutedEntropyProvider::AddToCache(uint32 randomization_seed,
uint16 value) const {
PermutedEntropyCache::Entry* entry;
const int kMaxSize = 25;
if (cache_.entry_size() >= kMaxSize) {
// If the cache is full, evict the first entry, swapping later entries in
// to take its place. This effectively creates a FIFO cache, which is good
// enough here because the expectation is that there shouldn't be more than
// |kMaxSize| field trials at any given time, so eviction should happen very
// rarely, only as new trials are introduced, evicting old expired trials.
for (int i = 1; i < kMaxSize; ++i)
cache_.mutable_entry()->SwapElements(i - 1, i);
entry = cache_.mutable_entry(kMaxSize - 1);
} else {
entry = cache_.add_entry();
}
entry->set_randomization_seed(randomization_seed);
entry->set_value(value);
UpdateLocalState();
}
bool CachingPermutedEntropyProvider::FindValue(uint32 randomization_seed,
uint16* value) const {
for (int i = 0; i < cache_.entry_size(); ++i) {
if (cache_.entry(i).randomization_seed() == randomization_seed) {
*value = cache_.entry(i).value();
return true;
}
}
return false;
}
} // namespace metrics
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