components/variations/caching_permuted_entropy_provider.cc

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "components/variations/caching_permuted_entropy_provider.h"
   6
   7 #include <string>
   8
   9 #include "base/base64.h"
  10 #include "base/logging.h"
  11 #include "base/prefs/pref_registry_simple.h"
  12 #include "base/prefs/pref_service.h"
  13 #include "components/variations/pref_names.h"
  14
  15 namespace metrics {
  16
  17 CachingPermutedEntropyProvider::CachingPermutedEntropyProvider(
  18     PrefService* local_state,
  19     uint16 low_entropy_source,
  20     size_t low_entropy_source_max)
  21     : PermutedEntropyProvider(low_entropy_source, low_entropy_source_max),
  22       local_state_(local_state) {
  23   ReadFromLocalState();
  24 }
  25
  26 CachingPermutedEntropyProvider::~CachingPermutedEntropyProvider() {
  27 }
  28
  29 // static
  30 void CachingPermutedEntropyProvider::RegisterPrefs(
  31     PrefRegistrySimple* registry) {
  32   registry->RegisterStringPref(
  33       variations::prefs::kVariationsPermutedEntropyCache, std::string());
  34 }
  35
  36 // static
  37 void CachingPermutedEntropyProvider::ClearCache(PrefService* local_state) {
  38   local_state->ClearPref(variations::prefs::kVariationsPermutedEntropyCache);
  39 }
  40
  41 uint16 CachingPermutedEntropyProvider::GetPermutedValue(
  42     uint32 randomization_seed) const {
  43   DCHECK(thread_checker_.CalledOnValidThread());
  44
  45   uint16 value = 0;
  46   if (!FindValue(randomization_seed, &value)) {
  47     value = PermutedEntropyProvider::GetPermutedValue(randomization_seed);
  48     AddToCache(randomization_seed, value);
  49   }
  50   return value;
  51 }
  52
  53 void CachingPermutedEntropyProvider::ReadFromLocalState() const {
  54   const std::string base64_cache_data = local_state_->GetString(
  55       variations::prefs::kVariationsPermutedEntropyCache);
  56   std::string cache_data;
  57   if (!base::Base64Decode(base64_cache_data, &cache_data) ||
  58       !cache_.ParseFromString(cache_data)) {
  59     local_state_->ClearPref(variations::prefs::kVariationsPermutedEntropyCache);
  60     NOTREACHED();
  61   }
  62 }
  63
  64 void CachingPermutedEntropyProvider::UpdateLocalState() const {
  65   std::string serialized;
  66   cache_.SerializeToString(&serialized);
  67
  68   std::string base64_encoded;
  69   base::Base64Encode(serialized, &base64_encoded);
  70   local_state_->SetString(variations::prefs::kVariationsPermutedEntropyCache,
  71                           base64_encoded);
  72 }
  73
  74 void CachingPermutedEntropyProvider::AddToCache(uint32 randomization_seed,
  75                                                 uint16 value) const {
  76   PermutedEntropyCache::Entry* entry;
  77   const int kMaxSize = 25;
  78   if (cache_.entry_size() >= kMaxSize) {
  79     // If the cache is full, evict the first entry, swapping later entries in
  80     // to take its place. This effectively creates a FIFO cache, which is good
  81     // enough here because the expectation is that there shouldn't be more than
  82     // |kMaxSize| field trials at any given time, so eviction should happen very
  83     // rarely, only as new trials are introduced, evicting old expired trials.
  84     for (int i = 1; i < kMaxSize; ++i)
  85       cache_.mutable_entry()->SwapElements(i - 1, i);
  86     entry = cache_.mutable_entry(kMaxSize - 1);
  87   } else {
  88     entry = cache_.add_entry();
  89   }
  90
  91   entry->set_randomization_seed(randomization_seed);
  92   entry->set_value(value);
  93
  94   UpdateLocalState();
  95 }
  96
  97 bool CachingPermutedEntropyProvider::FindValue(uint32 randomization_seed,
  98                                                uint16* value) const {
  99   for (int i = 0; i < cache_.entry_size(); ++i) {
 100     if (cache_.entry(i).randomization_seed() == randomization_seed) {
 101       *value = cache_.entry(i).value();
 102       return true;
 103     }
 104   }
 105   return false;
 106 }
 107
 108 }  // namespace metrics