src/chrome/browser/predictors/autocomplete_action_predictor.cc

   1 // Copyright (c) 2012 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 "chrome/browser/predictors/autocomplete_action_predictor.h"
   6
   7 #include <math.h>
   8
   9 #include <vector>
  10
  11 #include "base/bind.h"
  12 #include "base/guid.h"
  13 #include "base/i18n/case_conversion.h"
  14 #include "base/metrics/histogram.h"
  15 #include "base/strings/string_util.h"
  16 #include "base/strings/stringprintf.h"
  17 #include "base/strings/utf_string_conversions.h"
  18 #include "chrome/browser/autocomplete/autocomplete_match.h"
  19 #include "chrome/browser/autocomplete/autocomplete_result.h"
  20 #include "chrome/browser/chrome_notification_types.h"
  21 #include "chrome/browser/history/history_notifications.h"
  22 #include "chrome/browser/history/history_service.h"
  23 #include "chrome/browser/history/history_service_factory.h"
  24 #include "chrome/browser/history/in_memory_database.h"
  25 #include "chrome/browser/omnibox/omnibox_log.h"
  26 #include "chrome/browser/predictors/autocomplete_action_predictor_factory.h"
  27 #include "chrome/browser/predictors/predictor_database.h"
  28 #include "chrome/browser/predictors/predictor_database_factory.h"
  29 #include "chrome/browser/prerender/prerender_field_trial.h"
  30 #include "chrome/browser/prerender/prerender_handle.h"
  31 #include "chrome/browser/prerender/prerender_manager.h"
  32 #include "chrome/browser/prerender/prerender_manager_factory.h"
  33 #include "chrome/browser/profiles/profile.h"
  34 #include "content/public/browser/browser_thread.h"
  35 #include "content/public/browser/notification_details.h"
  36 #include "content/public/browser/notification_service.h"
  37 #include "content/public/browser/notification_source.h"
  38
  39 namespace {
  40
  41 const float kConfidenceCutoff[] = {
  42   0.8f,
  43   0.5f
  44 };
  45
  46 COMPILE_ASSERT(arraysize(kConfidenceCutoff) ==
  47                predictors::AutocompleteActionPredictor::LAST_PREDICT_ACTION,
  48                ConfidenceCutoff_count_mismatch);
  49
  50 const size_t kMinimumUserTextLength = 1;
  51 const int kMinimumNumberOfHits = 3;
  52
  53 enum DatabaseAction {
  54   DATABASE_ACTION_ADD,
  55   DATABASE_ACTION_UPDATE,
  56   DATABASE_ACTION_DELETE_SOME,
  57   DATABASE_ACTION_DELETE_ALL,
  58   DATABASE_ACTION_COUNT
  59 };
  60
  61 }  // namespace
  62
  63 namespace predictors {
  64
  65 const int AutocompleteActionPredictor::kMaximumDaysToKeepEntry = 14;
  66
  67 AutocompleteActionPredictor::AutocompleteActionPredictor(Profile* profile)
  68     : profile_(profile),
  69       main_profile_predictor_(NULL),
  70       incognito_predictor_(NULL),
  71       initialized_(false) {
  72   if (profile_->IsOffTheRecord()) {
  73     main_profile_predictor_ = AutocompleteActionPredictorFactory::GetForProfile(
  74         profile_->GetOriginalProfile());
  75     DCHECK(main_profile_predictor_);
  76     main_profile_predictor_->incognito_predictor_ = this;
  77     if (main_profile_predictor_->initialized_)
  78       CopyFromMainProfile();
  79   } else {
  80     // Request the in-memory database from the history to force it to load so
  81     // it's available as soon as possible.
  82     HistoryService* history_service = HistoryServiceFactory::GetForProfile(
  83         profile_, Profile::EXPLICIT_ACCESS);
  84     if (history_service)
  85       history_service->InMemoryDatabase();
  86
  87     table_ =
  88         PredictorDatabaseFactory::GetForProfile(profile_)->autocomplete_table();
  89
  90     // Observe all main frame loads so we can wait for the first to complete
  91     // before accessing DB and IO threads to build the local cache.
  92     notification_registrar_.Add(this,
  93                                 content::NOTIFICATION_LOAD_COMPLETED_MAIN_FRAME,
  94                                 content::NotificationService::AllSources());
  95   }
  96 }
  97
  98 AutocompleteActionPredictor::~AutocompleteActionPredictor() {
  99   if (main_profile_predictor_)
 100     main_profile_predictor_->incognito_predictor_ = NULL;
 101   else if (incognito_predictor_)
 102     incognito_predictor_->main_profile_predictor_ = NULL;
 103   if (prerender_handle_.get())
 104     prerender_handle_->OnCancel();
 105 }
 106
 107 void AutocompleteActionPredictor::RegisterTransitionalMatches(
 108     const string16& user_text,
 109     const AutocompleteResult& result) {
 110   if (user_text.length() < kMinimumUserTextLength)
 111     return;
 112   const string16 lower_user_text(base::i18n::ToLower(user_text));
 113
 114   // Merge this in to an existing match if we already saw |user_text|
 115   std::vector<TransitionalMatch>::iterator match_it =
 116       std::find(transitional_matches_.begin(), transitional_matches_.end(),
 117                 lower_user_text);
 118
 119   if (match_it == transitional_matches_.end()) {
 120     TransitionalMatch transitional_match;
 121     transitional_match.user_text = lower_user_text;
 122     match_it = transitional_matches_.insert(transitional_matches_.end(),
 123                                             transitional_match);
 124   }
 125
 126   for (AutocompleteResult::const_iterator i(result.begin()); i != result.end();
 127        ++i) {
 128     if (std::find(match_it->urls.begin(), match_it->urls.end(),
 129                   i->destination_url) == match_it->urls.end()) {
 130       match_it->urls.push_back(i->destination_url);
 131     }
 132   }
 133 }
 134
 135 void AutocompleteActionPredictor::ClearTransitionalMatches() {
 136   transitional_matches_.clear();
 137 }
 138
 139 void AutocompleteActionPredictor::StartPrerendering(
 140     const GURL& url,
 141     const content::SessionStorageNamespaceMap& session_storage_namespace_map,
 142     const gfx::Size& size) {
 143   // Only cancel the old prerender after starting the new one, so if the URLs
 144   // are the same, the underlying prerender will be reused.
 145   scoped_ptr<prerender::PrerenderHandle> old_prerender_handle(
 146       prerender_handle_.release());
 147   if (prerender::PrerenderManager* prerender_manager =
 148           prerender::PrerenderManagerFactory::GetForProfile(profile_)) {
 149     content::SessionStorageNamespace* session_storage_namespace = NULL;
 150     content::SessionStorageNamespaceMap::const_iterator it =
 151         session_storage_namespace_map.find(std::string());
 152     if (it != session_storage_namespace_map.end())
 153       session_storage_namespace = it->second.get();
 154     prerender_handle_.reset(prerender_manager->AddPrerenderFromOmnibox(
 155         url, session_storage_namespace, size));
 156   }
 157   if (old_prerender_handle)
 158     old_prerender_handle->OnCancel();
 159 }
 160
 161 // Given a match, return a recommended action.
 162 AutocompleteActionPredictor::Action
 163     AutocompleteActionPredictor::RecommendAction(
 164         const string16& user_text,
 165         const AutocompleteMatch& match) const {
 166   bool is_in_db = false;
 167   const double confidence = CalculateConfidence(user_text, match, &is_in_db);
 168   DCHECK(confidence >= 0.0 && confidence <= 1.0);
 169
 170   UMA_HISTOGRAM_BOOLEAN("AutocompleteActionPredictor.MatchIsInDb", is_in_db);
 171
 172   if (is_in_db) {
 173     // Multiple enties with the same URL are fine as the confidence may be
 174     // different.
 175     tracked_urls_.push_back(std::make_pair(match.destination_url, confidence));
 176     UMA_HISTOGRAM_COUNTS_100("AutocompleteActionPredictor.Confidence",
 177                              confidence * 100);
 178   }
 179
 180   // Map the confidence to an action.
 181   Action action = ACTION_NONE;
 182   for (int i = 0; i < LAST_PREDICT_ACTION; ++i) {
 183     if (confidence >= kConfidenceCutoff[i]) {
 184       action = static_cast<Action>(i);
 185       break;
 186     }
 187   }
 188
 189   // Downgrade prerender to preconnect if this is a search match or if omnibox
 190   // prerendering is disabled. There are cases when Instant will not handle a
 191   // search suggestion and in those cases it would be good to prerender the
 192   // search results, however search engines have not been set up to correctly
 193   // handle being prerendered and until they are we should avoid it.
 194   // http://crbug.com/117495
 195   if (action == ACTION_PRERENDER &&
 196       (AutocompleteMatch::IsSearchType(match.type) ||
 197        !prerender::IsOmniboxEnabled(profile_))) {
 198     action = ACTION_PRECONNECT;
 199   }
 200
 201   return action;
 202 }
 203
 204 // Return true if the suggestion type warrants a TCP/IP preconnection.
 205 // i.e., it is now quite likely that the user will select the related domain.
 206 // static
 207 bool AutocompleteActionPredictor::IsPreconnectable(
 208     const AutocompleteMatch& match) {
 209   return AutocompleteMatch::IsSearchType(match.type);
 210 }
 211
 212 void AutocompleteActionPredictor::Observe(
 213     int type,
 214     const content::NotificationSource& source,
 215     const content::NotificationDetails& details) {
 216   switch (type) {
 217     case content::NOTIFICATION_LOAD_COMPLETED_MAIN_FRAME:
 218       CreateLocalCachesFromDatabase();
 219       notification_registrar_.Remove(
 220           this,
 221           content::NOTIFICATION_LOAD_COMPLETED_MAIN_FRAME,
 222           content::NotificationService::AllSources());
 223       break;
 224
 225     case chrome::NOTIFICATION_HISTORY_URLS_DELETED: {
 226       DCHECK(initialized_);
 227       const content::Details<const history::URLsDeletedDetails>
 228           urls_deleted_details =
 229               content::Details<const history::URLsDeletedDetails>(details);
 230       if (urls_deleted_details->all_history)
 231         DeleteAllRows();
 232       else
 233         DeleteRowsWithURLs(urls_deleted_details->rows);
 234       break;
 235     }
 236
 237     // This notification does not catch all instances of the user navigating
 238     // from the Omnibox, but it does catch the cases where the dropdown is open
 239     // and those are the events we're most interested in.
 240     case chrome::NOTIFICATION_OMNIBOX_OPENED_URL: {
 241       DCHECK(initialized_);
 242
 243       // TODO(dominich): This doesn't need to be synchronous. Investigate
 244       // posting it as a task to be run later.
 245       OnOmniboxOpenedUrl(*content::Details<OmniboxLog>(details).ptr());
 246       break;
 247     }
 248
 249     case chrome::NOTIFICATION_HISTORY_LOADED: {
 250       TryDeleteOldEntries(content::Details<HistoryService>(details).ptr());
 251
 252       notification_registrar_.Remove(this,
 253                                      chrome::NOTIFICATION_HISTORY_LOADED,
 254                                      content::Source<Profile>(profile_));
 255       break;
 256     }
 257
 258     default:
 259       NOTREACHED() << "Unexpected notification observed.";
 260       break;
 261   }
 262 }
 263
 264 void AutocompleteActionPredictor::CreateLocalCachesFromDatabase() {
 265   // Create local caches using the database as loaded. We will garbage collect
 266   // rows from the caches and the database once the history service is
 267   // available.
 268   std::vector<AutocompleteActionPredictorTable::Row>* rows =
 269       new std::vector<AutocompleteActionPredictorTable::Row>();
 270   content::BrowserThread::PostTaskAndReply(content::BrowserThread::DB,
 271       FROM_HERE,
 272       base::Bind(&AutocompleteActionPredictorTable::GetAllRows, table_, rows),
 273       base::Bind(&AutocompleteActionPredictor::CreateCaches, AsWeakPtr(),
 274                  base::Owned(rows)));
 275 }
 276
 277 void AutocompleteActionPredictor::DeleteAllRows() {
 278   if (!initialized_)
 279     return;
 280
 281   db_cache_.clear();
 282   db_id_cache_.clear();
 283
 284   if (table_.get()) {
 285     content::BrowserThread::PostTask(content::BrowserThread::DB, FROM_HERE,
 286         base::Bind(&AutocompleteActionPredictorTable::DeleteAllRows,
 287                    table_));
 288   }
 289
 290   UMA_HISTOGRAM_ENUMERATION("AutocompleteActionPredictor.DatabaseAction",
 291                             DATABASE_ACTION_DELETE_ALL, DATABASE_ACTION_COUNT);
 292 }
 293
 294 void AutocompleteActionPredictor::DeleteRowsWithURLs(
 295     const history::URLRows& rows) {
 296   if (!initialized_)
 297     return;
 298
 299   std::vector<AutocompleteActionPredictorTable::Row::Id> id_list;
 300
 301   for (DBCacheMap::iterator it = db_cache_.begin(); it != db_cache_.end();) {
 302     if (std::find_if(rows.begin(), rows.end(),
 303         history::URLRow::URLRowHasURL(it->first.url)) != rows.end()) {
 304       const DBIdCacheMap::iterator id_it = db_id_cache_.find(it->first);
 305       DCHECK(id_it != db_id_cache_.end());
 306       id_list.push_back(id_it->second);
 307       db_id_cache_.erase(id_it);
 308       db_cache_.erase(it++);
 309     } else {
 310       ++it;
 311     }
 312   }
 313
 314   if (table_.get()) {
 315     content::BrowserThread::PostTask(content::BrowserThread::DB, FROM_HERE,
 316         base::Bind(&AutocompleteActionPredictorTable::DeleteRows, table_,
 317                    id_list));
 318   }
 319
 320   UMA_HISTOGRAM_ENUMERATION("AutocompleteActionPredictor.DatabaseAction",
 321                             DATABASE_ACTION_DELETE_SOME, DATABASE_ACTION_COUNT);
 322 }
 323
 324 void AutocompleteActionPredictor::OnOmniboxOpenedUrl(const OmniboxLog& log) {
 325   if (log.text.length() < kMinimumUserTextLength)
 326     return;
 327
 328   const AutocompleteMatch& match = log.result.match_at(log.selected_index);
 329
 330   UMA_HISTOGRAM_BOOLEAN(
 331       base::StringPrintf("Prerender.OmniboxNavigationsCouldPrerender%s",
 332                          prerender::PrerenderManager::GetModeString()).c_str(),
 333       prerender::IsOmniboxEnabled(profile_));
 334
 335   const GURL& opened_url = match.destination_url;
 336   const string16 lower_user_text(base::i18n::ToLower(log.text));
 337
 338   // Traverse transitional matches for those that have a user_text that is a
 339   // prefix of |lower_user_text|.
 340   std::vector<AutocompleteActionPredictorTable::Row> rows_to_add;
 341   std::vector<AutocompleteActionPredictorTable::Row> rows_to_update;
 342
 343   for (std::vector<TransitionalMatch>::const_iterator it =
 344         transitional_matches_.begin(); it != transitional_matches_.end();
 345         ++it) {
 346     if (!StartsWith(lower_user_text, it->user_text, true))
 347       continue;
 348
 349     // Add entries to the database for those matches.
 350     for (std::vector<GURL>::const_iterator url_it = it->urls.begin();
 351           url_it != it->urls.end(); ++url_it) {
 352       DCHECK(it->user_text.length() >= kMinimumUserTextLength);
 353       const DBCacheKey key = { it->user_text, *url_it };
 354       const bool is_hit = (*url_it == opened_url);
 355
 356       AutocompleteActionPredictorTable::Row row;
 357       row.user_text = key.user_text;
 358       row.url = key.url;
 359
 360       DBCacheMap::iterator it = db_cache_.find(key);
 361       if (it == db_cache_.end()) {
 362         row.id = base::GenerateGUID();
 363         row.number_of_hits = is_hit ? 1 : 0;
 364         row.number_of_misses = is_hit ? 0 : 1;
 365
 366         rows_to_add.push_back(row);
 367       } else {
 368         DCHECK(db_id_cache_.find(key) != db_id_cache_.end());
 369         row.id = db_id_cache_.find(key)->second;
 370         row.number_of_hits = it->second.number_of_hits + (is_hit ? 1 : 0);
 371         row.number_of_misses = it->second.number_of_misses + (is_hit ? 0 : 1);
 372
 373         rows_to_update.push_back(row);
 374       }
 375     }
 376   }
 377   if (rows_to_add.size() > 0 || rows_to_update.size() > 0)
 378     AddAndUpdateRows(rows_to_add, rows_to_update);
 379
 380   ClearTransitionalMatches();
 381
 382   // Check against tracked urls and log accuracy for the confidence we
 383   // predicted.
 384   for (std::vector<std::pair<GURL, double> >::const_iterator it =
 385        tracked_urls_.begin(); it != tracked_urls_.end();
 386        ++it) {
 387     if (opened_url == it->first) {
 388       UMA_HISTOGRAM_COUNTS_100("AutocompleteActionPredictor.AccurateCount",
 389                                it->second * 100);
 390     }
 391   }
 392   tracked_urls_.clear();
 393 }
 394
 395 void AutocompleteActionPredictor::AddAndUpdateRows(
 396     const AutocompleteActionPredictorTable::Rows& rows_to_add,
 397     const AutocompleteActionPredictorTable::Rows& rows_to_update) {
 398   if (!initialized_)
 399     return;
 400
 401   for (AutocompleteActionPredictorTable::Rows::const_iterator it =
 402        rows_to_add.begin(); it != rows_to_add.end(); ++it) {
 403     const DBCacheKey key = { it->user_text, it->url };
 404     DBCacheValue value = { it->number_of_hits, it->number_of_misses };
 405
 406     DCHECK(db_cache_.find(key) == db_cache_.end());
 407
 408     db_cache_[key] = value;
 409     db_id_cache_[key] = it->id;
 410     UMA_HISTOGRAM_ENUMERATION("AutocompleteActionPredictor.DatabaseAction",
 411                               DATABASE_ACTION_ADD, DATABASE_ACTION_COUNT);
 412   }
 413   for (AutocompleteActionPredictorTable::Rows::const_iterator it =
 414        rows_to_update.begin(); it != rows_to_update.end(); ++it) {
 415     const DBCacheKey key = { it->user_text, it->url };
 416
 417     DBCacheMap::iterator db_it = db_cache_.find(key);
 418     DCHECK(db_it != db_cache_.end());
 419     DCHECK(db_id_cache_.find(key) != db_id_cache_.end());
 420
 421     db_it->second.number_of_hits = it->number_of_hits;
 422     db_it->second.number_of_misses = it->number_of_misses;
 423     UMA_HISTOGRAM_ENUMERATION("AutocompleteActionPredictor.DatabaseAction",
 424                               DATABASE_ACTION_UPDATE, DATABASE_ACTION_COUNT);
 425   }
 426
 427   if (table_.get()) {
 428     content::BrowserThread::PostTask(content::BrowserThread::DB, FROM_HERE,
 429         base::Bind(&AutocompleteActionPredictorTable::AddAndUpdateRows,
 430                    table_, rows_to_add, rows_to_update));
 431   }
 432 }
 433
 434 void AutocompleteActionPredictor::CreateCaches(
 435     std::vector<AutocompleteActionPredictorTable::Row>* rows) {
 436   CHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 437   DCHECK(!profile_->IsOffTheRecord());
 438   DCHECK(!initialized_);
 439   DCHECK(db_cache_.empty());
 440   DCHECK(db_id_cache_.empty());
 441
 442   for (std::vector<AutocompleteActionPredictorTable::Row>::const_iterator it =
 443        rows->begin(); it != rows->end(); ++it) {
 444     const DBCacheKey key = { it->user_text, it->url };
 445     const DBCacheValue value = { it->number_of_hits, it->number_of_misses };
 446     db_cache_[key] = value;
 447     db_id_cache_[key] = it->id;
 448   }
 449
 450   // If the history service is ready, delete any old or invalid entries.
 451   HistoryService* history_service =
 452       HistoryServiceFactory::GetForProfile(profile_, Profile::EXPLICIT_ACCESS);
 453   if (!TryDeleteOldEntries(history_service)) {
 454     // Wait for the notification that the history service is ready and the URL
 455     // DB is loaded.
 456     notification_registrar_.Add(this, chrome::NOTIFICATION_HISTORY_LOADED,
 457                                 content::Source<Profile>(profile_));
 458   }
 459 }
 460
 461 bool AutocompleteActionPredictor::TryDeleteOldEntries(HistoryService* service) {
 462   CHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 463   DCHECK(!profile_->IsOffTheRecord());
 464   DCHECK(!initialized_);
 465
 466   if (!service)
 467     return false;
 468
 469   history::URLDatabase* url_db = service->InMemoryDatabase();
 470   if (!url_db)
 471     return false;
 472
 473   DeleteOldEntries(url_db);
 474   return true;
 475 }
 476
 477 void AutocompleteActionPredictor::DeleteOldEntries(
 478     history::URLDatabase* url_db) {
 479   CHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 480   DCHECK(!profile_->IsOffTheRecord());
 481   DCHECK(!initialized_);
 482   DCHECK(table_.get());
 483
 484   std::vector<AutocompleteActionPredictorTable::Row::Id> ids_to_delete;
 485   DeleteOldIdsFromCaches(url_db, &ids_to_delete);
 486
 487   content::BrowserThread::PostTask(content::BrowserThread::DB, FROM_HERE,
 488       base::Bind(&AutocompleteActionPredictorTable::DeleteRows, table_,
 489                  ids_to_delete));
 490
 491   FinishInitialization();
 492   if (incognito_predictor_)
 493     incognito_predictor_->CopyFromMainProfile();
 494 }
 495
 496 void AutocompleteActionPredictor::DeleteOldIdsFromCaches(
 497     history::URLDatabase* url_db,
 498     std::vector<AutocompleteActionPredictorTable::Row::Id>* id_list) {
 499   CHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 500   DCHECK(!profile_->IsOffTheRecord());
 501   DCHECK(!initialized_);
 502   DCHECK(url_db);
 503   DCHECK(id_list);
 504
 505   id_list->clear();
 506   for (DBCacheMap::iterator it = db_cache_.begin(); it != db_cache_.end();) {
 507     history::URLRow url_row;
 508
 509     if ((url_db->GetRowForURL(it->first.url, &url_row) == 0) ||
 510         ((base::Time::Now() - url_row.last_visit()).InDays() >
 511          kMaximumDaysToKeepEntry)) {
 512       const DBIdCacheMap::iterator id_it = db_id_cache_.find(it->first);
 513       DCHECK(id_it != db_id_cache_.end());
 514       id_list->push_back(id_it->second);
 515       db_id_cache_.erase(id_it);
 516       db_cache_.erase(it++);
 517     } else {
 518       ++it;
 519     }
 520   }
 521 }
 522
 523 void AutocompleteActionPredictor::CopyFromMainProfile() {
 524   CHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 525   DCHECK(profile_->IsOffTheRecord());
 526   DCHECK(!initialized_);
 527   DCHECK(main_profile_predictor_);
 528   DCHECK(main_profile_predictor_->initialized_);
 529
 530   db_cache_ = main_profile_predictor_->db_cache_;
 531   db_id_cache_ = main_profile_predictor_->db_id_cache_;
 532   FinishInitialization();
 533 }
 534
 535 void AutocompleteActionPredictor::FinishInitialization() {
 536   CHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 537   DCHECK(!initialized_);
 538
 539   // Incognito and normal profiles should listen only to omnibox notifications
 540   // from their own profile, but both should listen to history deletions from
 541   // the main profile, since opening the history page in either case actually
 542   // opens the non-incognito history (and lets users delete from there).
 543   notification_registrar_.Add(this, chrome::NOTIFICATION_OMNIBOX_OPENED_URL,
 544                               content::Source<Profile>(profile_));
 545   notification_registrar_.Add(this, chrome::NOTIFICATION_HISTORY_URLS_DELETED,
 546       content::Source<Profile>(profile_->GetOriginalProfile()));
 547   initialized_ = true;
 548 }
 549
 550 double AutocompleteActionPredictor::CalculateConfidence(
 551     const string16& user_text,
 552     const AutocompleteMatch& match,
 553     bool* is_in_db) const {
 554   const DBCacheKey key = { user_text, match.destination_url };
 555
 556   *is_in_db = false;
 557   if (user_text.length() < kMinimumUserTextLength)
 558     return 0.0;
 559
 560   const DBCacheMap::const_iterator iter = db_cache_.find(key);
 561   if (iter == db_cache_.end())
 562     return 0.0;
 563
 564   *is_in_db = true;
 565   return CalculateConfidenceForDbEntry(iter);
 566 }
 567
 568 double AutocompleteActionPredictor::CalculateConfidenceForDbEntry(
 569     DBCacheMap::const_iterator iter) const {
 570   const DBCacheValue& value = iter->second;
 571   if (value.number_of_hits < kMinimumNumberOfHits)
 572     return 0.0;
 573
 574   const double number_of_hits = static_cast<double>(value.number_of_hits);
 575   return number_of_hits / (number_of_hits + value.number_of_misses);
 576 }
 577
 578 AutocompleteActionPredictor::TransitionalMatch::TransitionalMatch() {
 579 }
 580
 581 AutocompleteActionPredictor::TransitionalMatch::~TransitionalMatch() {
 582 }
 583
 584 }  // namespace predictors