// Copyright (c) 2012 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.

#ifndef COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_BACKEND_H_
#define COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_BACKEND_H_

#include <stddef.h>
#include <stdint.h>

#include <list>
#include <memory>
#include <set>
#include <string>
#include <utility>
#include <vector>

#include "base/callback.h"
#include "base/cancelable_callback.h"
#include "base/containers/flat_set.h"
#include "base/containers/mru_cache.h"
#include "base/files/file_path.h"
#include "base/gtest_prod_util.h"
#include "base/macros.h"
#include "base/memory/memory_pressure_listener.h"
#include "base/observer_list.h"
#include "base/single_thread_task_runner.h"
#include "base/supports_user_data.h"
#include "base/task/cancelable_task_tracker.h"
#include "build/build_config.h"
#include "components/favicon/core/favicon_backend_delegate.h"
#include "components/favicon/core/favicon_database.h"
#include "components/favicon_base/favicon_usage_data.h"
#include "components/history/core/browser/expire_history_backend.h"
#include "components/history/core/browser/history_backend_notifier.h"
#include "components/history/core/browser/history_types.h"
#include "components/history/core/browser/keyword_id.h"
#include "components/history/core/browser/visit_tracker.h"
#include "sql/init_status.h"
#include "third_party/abseil-cpp/absl/types/optional.h"

class SkBitmap;

namespace base {
class SingleThreadTaskRunner;
}

namespace favicon {
class FaviconBackend;
}

namespace syncer {
class ModelTypeControllerDelegate;
}

namespace history {
struct DownloadRow;
class HistoryBackendClient;
class HistoryBackendDBBaseTest;
class HistoryBackendObserver;
class HistoryBackend;
class TestHistoryBackend;
class HistoryBackendTest;
class HistoryDatabase;
struct HistoryDatabaseParams;
class HistoryDBTask;
class InMemoryHistoryBackend;
class HistoryBackendHelper;
class TypedURLSyncBridge;
class URLDatabase;

// Returns a formatted version of `url` with the HTTP/HTTPS scheme, port,
// username/password, and any trivial subdomains (e.g., "www.", "m.") removed.
std::u16string FormatUrlForRedirectComparison(const GURL& url);

// Advances (if `day` >= 0) or backtracks (if `day` < 0) from `time` by
// abs(`day`) calendar days in local timezone and returns the midnight of the
// resulting day.
base::Time MidnightNDaysLater(base::Time time, int days);

// Keeps track of a queued HistoryDBTask. This class lives solely on the
// DB thread.
class QueuedHistoryDBTask {
 public:
  QueuedHistoryDBTask(
      std::unique_ptr<HistoryDBTask> task,
      scoped_refptr<base::SingleThreadTaskRunner> origin_loop,
      const base::CancelableTaskTracker::IsCanceledCallback& is_canceled);
  ~QueuedHistoryDBTask();

  bool is_canceled();
  bool Run(HistoryBackend* backend, HistoryDatabase* db);
  void DoneRun();

 private:
  std::unique_ptr<HistoryDBTask> task_;
  scoped_refptr<base::SingleThreadTaskRunner> origin_loop_;
  base::CancelableTaskTracker::IsCanceledCallback is_canceled_;

  DISALLOW_COPY_AND_ASSIGN(QueuedHistoryDBTask);
};

// *See the .cc file for more information on the design.*
//
// Internal history implementation which does most of the work of the history
// system. This runs on a background thread (to not block the browser when we
// do expensive operations) and is NOT threadsafe, so it must only be called
// from message handlers on the background thread. Invoking on another thread
// requires threadsafe refcounting.
//
// Most functions here are just the implementations of the corresponding
// functions in the history service. These functions are not documented
// here, see the history service for behavior.
class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
                       public HistoryBackendNotifier,
                       public favicon::FaviconBackendDelegate {
 public:
  // Interface implemented by the owner of the HistoryBackend object. Normally,
  // the history service implements this to send stuff back to the main thread.
  // The unit tests can provide a different implementation if they don't have
  // a history service object.
  class Delegate {
   public:
    virtual ~Delegate() = default;

    // Called when the database cannot be read correctly for some reason.
    // `diagnostics` contains information about the underlying database
    // which can help in identifying the cause of the profile error.
    virtual void NotifyProfileError(sql::InitStatus init_status,
                                    const std::string& diagnostics) = 0;

    // Sets the in-memory history backend. The in-memory backend is created by
    // the main backend. For non-unit tests, this happens on the background
    // thread. It is to be used on the main thread, so this would transfer
    // it to the history service. Unit tests can override this behavior.
    //
    // This function is NOT guaranteed to be called. If there is an error,
    // there may be no in-memory database.
    virtual void SetInMemoryBackend(
        std::unique_ptr<InMemoryHistoryBackend> backend) = 0;

    // Notify HistoryService that the favicons for the given page URLs (e.g.
    // http://www.google.com) and the given icon URL (e.g.
    // http://www.google.com/favicon.ico) have changed. HistoryService notifies
    // any registered callbacks. It is valid to call NotifyFaviconsChanged()
    // with non-empty `page_urls` and an empty `icon_url` and vice versa.
    virtual void NotifyFaviconsChanged(const std::set<GURL>& page_urls,
                                       const GURL& icon_url) = 0;

    // Notify HistoryService that the user is visiting an URL. The event will
    // be forwarded to the HistoryServiceObservers in the correct thread.
    virtual void NotifyURLVisited(ui::PageTransition transition,
                                  const URLRow& row,
                                  const RedirectList& redirects,
                                  base::Time visit_time) = 0;

    // Notify HistoryService that some URLs have been modified. The event will
    // be forwarded to the HistoryServiceObservers in the correct thread.
    virtual void NotifyURLsModified(const URLRows& changed_urls) = 0;

    // TODO(https://crbug.com/1141501): this is for an experiment, and will be
    // removed once data is collected from experiment.
    virtual void NotifyURLsModified(const URLRows& changed_urls,
                                    UrlsModifiedReason reason);

    // Notify HistoryService that some or all of the URLs have been deleted.
    // The event will be forwarded to the HistoryServiceObservers in the correct
    // thread.
    virtual void NotifyURLsDeleted(DeletionInfo deletion_info) = 0;

    // Notify HistoryService that some keyword has been searched using omnibox.
    // The event will be forwarded to the HistoryServiceObservers in the correct
    // thread.
    virtual void NotifyKeywordSearchTermUpdated(const URLRow& row,
                                                KeywordID keyword_id,
                                                const std::u16string& term) = 0;

    // Notify HistoryService that keyword search term has been deleted.
    // The event will be forwarded to the HistoryServiceObservers in the correct
    // thread.
    virtual void NotifyKeywordSearchTermDeleted(URLID url_id) = 0;

    // Invoked when the backend has finished loading the db.
    virtual void DBLoaded() = 0;
  };

  // Check if the transition should increment the typed_count of a visit.
  static bool IsTypedIncrement(ui::PageTransition transition);

  // Init must be called to complete object creation. This object can be
  // constructed on any thread, but all other functions including Init() must
  // be called on the history thread.
  //
  // `history_dir` is the directory where the history files will be placed.
  // See the definition of BroadcastNotificationsCallback above. This function
  // takes ownership of the callback pointer.
  //
  // `history_client` is used to determine bookmarked URLs when deleting and
  // may be null.
  //
  // This constructor is fast and does no I/O, so can be called at any time.
  HistoryBackend(std::unique_ptr<Delegate> delegate,
                 std::unique_ptr<HistoryBackendClient> backend_client,
                 scoped_refptr<base::SequencedTaskRunner> task_runner);

  // Must be called after creation but before any objects are created. If this
  // fails, all other functions will fail as well. (Since this runs on another
  // thread, we don't bother returning failure.)
  //
  // `force_fail` can be set during unittests to unconditionally fail to init.
  void Init(bool force_fail,
            const HistoryDatabaseParams& history_database_params);

  // Notification that the history system is shutting down. This will break
  // the refs owned by the delegate and any pending transaction so it will
  // actually be deleted.
  void Closing();

#if defined(OS_IOS)
  // Persists any in-flight state, without actually shutting down the history
  // system. This is intended for use when the application is backgrounded.
  void PersistState();
#endif

  void ClearCachedDataForContextID(ContextID context_id);

  // Clears all on-demand favicons.
  void ClearAllOnDemandFavicons();

  // Gets the counts and last last time of URLs that belong to `origins` in the
  // history database. Origins that are not in the history database will be in
  // the map with a count and time of 0.
  // Returns an empty map if db_ is not initialized.
  OriginCountAndLastVisitMap GetCountsAndLastVisitForOrigins(
      const std::set<GURL>& origins) const;

  // Navigation ----------------------------------------------------------------

  // `request.time` must be unique with high probability.
  void AddPage(const HistoryAddPageArgs& request);
  virtual void SetPageTitle(const GURL& url, const std::u16string& title);
  void AddPageNoVisitForBookmark(const GURL& url, const std::u16string& title);
  void UpdateWithPageEndTime(ContextID context_id,
                             int nav_entry_id,
                             const GURL& url,
                             base::Time end_ts);
  void SetFlocAllowed(ContextID context_id, int nav_entry_id, const GURL& url);
  void AddContentModelAnnotationsForVisit(
      VisitID visit_id,
      const VisitContentModelAnnotations& model_annotations);

  // Querying ------------------------------------------------------------------

  // Run the `callback` on the History thread.
  // `callback` should handle the null database case.
  void ScheduleAutocomplete(
      base::OnceCallback<void(HistoryBackend*, URLDatabase*)> callback);

  QueryURLResult QueryURL(const GURL& url, bool want_visits);
  QueryResults QueryHistory(const std::u16string& text_query,
                            const QueryOptions& options);

  // Computes the most recent URL(s) that the given canonical URL has
  // redirected to. There may be more than one redirect in a row, so this
  // function will fill the given array with the entire chain. If there are
  // no redirects for the most recent visit of the URL, or the URL is not
  // in history, the array will be empty.
  RedirectList QueryRedirectsFrom(const GURL& url);

  // Similar to above function except computes a chain of redirects to the
  // given URL. Stores the most recent list of redirects ending at `url` in the
  // given RedirectList. For example, if we have the redirect list A -> B -> C,
  // then calling this function with url=C would fill redirects with {B, A}.
  RedirectList QueryRedirectsTo(const GURL& url);

  VisibleVisitCountToHostResult GetVisibleVisitCountToHost(const GURL& url);

  // Request the `result_count` most visited URLs and the chain of
  // redirects leading to each of these URLs. `days_back` is the
  // number of days of history to use. Used by TopSites.
  MostVisitedURLList QueryMostVisitedURLs(int result_count, int days_back);

  // Statistics ----------------------------------------------------------------

  // Gets the number of URLs as seen in chrome://history within the time range
  // [`begin_time`, `end_time`). Each URL is counted only once per day. For
  // determination of the date, timestamps are converted to dates using local
  // time.
  HistoryCountResult GetHistoryCount(const base::Time& begin_time,
                                     const base::Time& end_time);

  // Returns the number of hosts visited in the last month.
  HistoryCountResult CountUniqueHostsVisitedLastMonth();

  // Returns a collection of domain diversity metrics. Each metric is an
  // unsigned integer representing the number of unique domains (effective
  // top-level domain (eTLD) + 1, e.g. "foo.com", "bar.co.uk") visited within
  // the 1-day, 7-day or 28-day span that ends at a midnight in local timezone.
  //
  // For each of the most recent `number_of_days_to_report` midnights before
  // `report_time`(inclusive), this function computes a subset of
  // {1-day, 7-day, 28-day} metrics whose spanning periods all end on that
  // midnight. This subset of metrics to compute is specified by a bitmask
  // `metric_type_bitmask`, which takes a bitwise combination of
  // kEnableLast1DayMetric, kEnableLast7DayMetric and kEnableLast28DayMetric.
  //
  // All computed metrics are stored in DomainDiversityResults, which represents
  // a collection of DomainMetricSet's. Each DomainMetricSet contains up to 3
  // metrics ending at one unique midnight in the time range of
  // `number_of_days_to_report` days before `report_time`. The collection of
  // DomainMetricSet is sorted reverse chronologically by the ending midnight.
  //
  // For example, when `report_time` = 2019/11/01 00:01am, `number_of_days` = 3,
  // `metric_type_bitmask` = kEnableLast28DayMetric | kEnableLast1DayMetric,
  // DomainDiversityResults will hold 3 DomainMetricSets, each containing 2
  // metrics measuring domain visit counts spanning the following date ranges
  // (all dates are inclusive):
  // {{10/30, 10/3~10/30}, {10/29, 10/2~10/29}, {10/28, 10/1~10/28}}
  DomainDiversityResults GetDomainDiversity(
      base::Time report_time,
      int number_of_days_to_report,
      DomainMetricBitmaskType metric_type_bitmask);

  // Gets the last time any webpage on the given host was visited within the
  // time range [`begin_time`, `end_time`). If the given host has not been
  // visited in the given time range, the result will have a null base::Time,
  // but still report success.
  HistoryLastVisitResult GetLastVisitToHost(const GURL& host,
                                            base::Time begin_time,
                                            base::Time end_time);

  // Gets the last time `url` was visited before `end_time`. If the given URL
  // has not been visited in the past, the result will have a null base::Time,
  // but still report success.
  HistoryLastVisitResult GetLastVisitToURL(const GURL& url,
                                           base::Time end_time);

  // Gets counts for total visits and days visited for pages matching `host`'s
  // scheme, port, and host. Counts only user-visible visits.
  DailyVisitsResult GetDailyVisitsToHost(const GURL& host,
                                         base::Time begin_time,
                                         base::Time end_time);

  // Favicon -------------------------------------------------------------------

  std::vector<favicon_base::FaviconRawBitmapResult> GetFavicon(
      const GURL& icon_url,
      favicon_base::IconType icon_type,
      const std::vector<int>& desired_sizes);

  favicon_base::FaviconRawBitmapResult GetLargestFaviconForURL(
      const GURL& page_url,
      const std::vector<favicon_base::IconTypeSet>& icon_types_list,
      int minimum_size_in_pixels);

  std::vector<favicon_base::FaviconRawBitmapResult> GetFaviconsForURL(
      const GURL& page_url,
      const favicon_base::IconTypeSet& icon_types,
      const std::vector<int>& desired_sizes,
      bool fallback_to_host);

  std::vector<favicon_base::FaviconRawBitmapResult> GetFaviconForID(
      favicon_base::FaviconID favicon_id,
      int desired_size);

  std::vector<favicon_base::FaviconRawBitmapResult>
  UpdateFaviconMappingsAndFetch(const base::flat_set<GURL>& page_urls,
                                const GURL& icon_url,
                                favicon_base::IconType icon_type,
                                const std::vector<int>& desired_sizes);

  void DeleteFaviconMappings(const base::flat_set<GURL>& page_urls,
                             favicon_base::IconType icon_type);

  void MergeFavicon(const GURL& page_url,
                    const GURL& icon_url,
                    favicon_base::IconType icon_type,
                    scoped_refptr<base::RefCountedMemory> bitmap_data,
                    const gfx::Size& pixel_size);

  // `page_urls` must not be empty.
  void SetFavicons(const base::flat_set<GURL>& page_urls,
                   favicon_base::IconType icon_type,
                   const GURL& icon_url,
                   const std::vector<SkBitmap>& bitmaps);

  void CloneFaviconMappingsForPages(
      const GURL& page_url_to_read,
      const favicon_base::IconTypeSet& icon_types,
      const base::flat_set<GURL>& page_urls_to_write);

  bool SetOnDemandFavicons(const GURL& page_url,
                           favicon_base::IconType icon_type,
                           const GURL& icon_url,
                           const std::vector<SkBitmap>& bitmaps);

  bool CanSetOnDemandFavicons(const GURL& page_url,
                              favicon_base::IconType icon_type);

  void SetFaviconsOutOfDateForPage(const GURL& page_url);

  void SetFaviconsOutOfDateBetween(base::Time begin, base::Time end);

  void TouchOnDemandFavicon(const GURL& icon_url);

  void SetImportedFavicons(
      const favicon_base::FaviconUsageDataList& favicon_usage);

  // Downloads -----------------------------------------------------------------

  uint32_t GetNextDownloadId();
  std::vector<DownloadRow> QueryDownloads();
  void UpdateDownload(const DownloadRow& data, bool should_commit_immediately);
  bool CreateDownload(const DownloadRow& history_info);
  void RemoveDownloads(const std::set<uint32_t>& ids);

  // Keyword search terms ------------------------------------------------------

  void SetKeywordSearchTermsForURL(const GURL& url,
                                   KeywordID keyword_id,
                                   const std::u16string& term);

  void DeleteAllSearchTermsForKeyword(KeywordID keyword_id);

  void DeleteKeywordSearchTermForURL(const GURL& url);

  void DeleteMatchingURLsForKeyword(KeywordID keyword_id,
                                    const std::u16string& term);

  // Clusters ------------------------------------------------------------------

  void AddContextAnnotationsForVisit(
      VisitID visit_id,
      const VisitContextAnnotations& visit_context_annotations);

  std::vector<AnnotatedVisit> GetAnnotatedVisits(int max_results);

  // Observers -----------------------------------------------------------------

  void AddObserver(HistoryBackendObserver* observer);
  void RemoveObserver(HistoryBackendObserver* observer);

  // Generic operations --------------------------------------------------------

  void ProcessDBTask(
      std::unique_ptr<HistoryDBTask> task,
      scoped_refptr<base::SingleThreadTaskRunner> origin_loop,
      const base::CancelableTaskTracker::IsCanceledCallback& is_canceled);

  virtual bool GetAllTypedURLs(URLRows* urls);

  virtual bool GetVisitsForURL(URLID id, VisitVector* visits);

  // Fetches up to `max_visits` most recent visits for the passed URL.
  virtual bool GetMostRecentVisitsForURL(URLID id,
                                         int max_visits,
                                         VisitVector* visits);

  // For each element in `urls`, updates the pre-existing URLRow in the database
  // with the same ID; or ignores the element if no such row exists. Returns the
  // number of records successfully updated.
  virtual size_t UpdateURLs(const URLRows& urls);

  // While adding visits in batch, the source needs to be provided.
  virtual bool AddVisits(const GURL& url,
                         const std::vector<VisitInfo>& visits,
                         VisitSource visit_source);

  virtual bool RemoveVisits(const VisitVector& visits);

  // Returns the VisitSource associated with each one of the passed visits.
  // If there is no entry in the map for a given visit, that means the visit
  // was SOURCE_BROWSED. Returns false if there is no HistoryDatabase..
  bool GetVisitsSource(const VisitVector& visits, VisitSourceMap* sources);

  virtual bool GetURL(const GURL& url, URLRow* url_row);

  bool GetURLByID(URLID url_id, URLRow* url_row);

  // Returns the sync controller delegate for syncing typed urls. The returned
  // delegate is owned by `this` object.
  base::WeakPtr<syncer::ModelTypeControllerDelegate>
  GetTypedURLSyncControllerDelegate();

  // Deleting ------------------------------------------------------------------

  void DeleteURLs(const std::vector<GURL>& urls);

  void DeleteURL(const GURL& url);

  // Deletes all visits to urls until the corresponding timestamp.
  void DeleteURLsUntil(
      const std::vector<std::pair<GURL, base::Time>>& urls_and_timestamps);

  // Calls ExpireHistoryBackend::ExpireHistoryBetween and commits the change.
  void ExpireHistoryBetween(const std::set<GURL>& restrict_urls,
                            base::Time begin_time,
                            base::Time end_time,
                            bool user_initiated);

  // Finds the URLs visited at `times` and expires all their visits within
  // [`begin_time`, `end_time`). All times in `times` should be in
  // [`begin_time`, `end_time`). This is used when expiration request is from
  // server side, i.e. web history deletes, where only visit times (possibly
  // incomplete) are transmitted to protect user's privacy.
  void ExpireHistoryForTimes(const std::set<base::Time>& times,
                             base::Time begin_time,
                             base::Time end_time);

  // Calls ExpireHistoryBetween() once for each element in the vector.
  // The fields of `ExpireHistoryArgs` map directly to the arguments of
  // of ExpireHistoryBetween().
  void ExpireHistory(const std::vector<ExpireHistoryArgs>& expire_list);

  // Expires all visits before and including the given time, updating the URLs
  // accordingly.
  void ExpireHistoryBeforeForTesting(base::Time end_time);

  // Bookmarks -----------------------------------------------------------------

  // Notification that a URL is no longer bookmarked. If there are no visits
  // for the specified url, it is deleted.
  void URLsNoLongerBookmarked(const std::set<GURL>& urls);

  // Callbacks To Kill Database When It Gets Corrupted -------------------------

  // Called by the database to report errors.  Schedules one call to
  // KillHistoryDatabase() in case of corruption.
  void DatabaseErrorCallback(int error, sql::Statement* stmt);

  // Raze the history database. It will be recreated in a future run. Hopefully
  // things go better then. Continue running but without reading or storing any
  // state into the HistoryBackend databases. Close all of the databases managed
  // HistoryBackend as there are no provisions for accessing the other databases
  // managed by HistoryBackend when the history database cannot be accessed.
  void KillHistoryDatabase();

  // SupportsUserData ----------------------------------------------------------

  // The user data allows the clients to associate data with this object.
  // Multiple user data values can be stored under different keys.
  base::SupportsUserData::Data* GetUserData(const void* key) const;
  void SetUserData(const void* key,
                   std::unique_ptr<base::SupportsUserData::Data> data);

  // Testing -------------------------------------------------------------------

  // Sets the task to run and the message loop to run it on when this object
  // is destroyed. See HistoryService::SetOnBackendDestroyTask for a more
  // complete description.
  void SetOnBackendDestroyTask(
      scoped_refptr<base::SingleThreadTaskRunner> task_runner,
      base::OnceClosure task);

  // Adds the given rows to the database if it doesn't exist. A visit will be
  // added for each given URL at the last visit time in the URLRow if the
  // passed visit type != SOURCE_SYNCED (the sync code manages visits itself).
  // Each visit will have the visit_source type set.
  void AddPagesWithDetails(const URLRows& info, VisitSource visit_source);

#if defined(UNIT_TEST)
  HistoryDatabase* db() const { return db_.get(); }

  ExpireHistoryBackend* expire_backend() { return &expirer_; }
#endif

  void SetTypedURLSyncBridgeForTest(std::unique_ptr<TypedURLSyncBridge> bridge);

  // Returns true if the passed visit time is already expired (used by the sync
  // code to avoid syncing visits that would immediately be expired).
  virtual bool IsExpiredVisitTime(const base::Time& time);

  base::Time GetFirstRecordedTimeForTest() { return first_recorded_time_; }

 protected:
  ~HistoryBackend() override;

 private:
  friend class base::RefCountedThreadSafe<HistoryBackend>;
  friend class TestHistoryBackend;
  friend class HistoryBackendTest;
  friend class HistoryBackendDBBaseTest;  // So the unit tests can poke our
                                          // innards.

  // Returns the name of the Favicons database.
  base::FilePath GetFaviconsFileName() const;

  class URLQuerier;
  friend class URLQuerier;

  // Does the work of Init.
  void InitImpl(const HistoryDatabaseParams& history_database_params);

  // Called when the system is under memory pressure.
  void OnMemoryPressure(
      base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level);

  // Closes all databases managed by HistoryBackend. Commits any pending
  // transactions.
  void CloseAllDatabases();

  // Adds a single visit to the database, updating the URL information such
  // as visit and typed count. The visit ID of the added visit and the URL ID
  // of the associated URL (whether added or not) is returned. Both values will
  // be 0 on failure.
  //
  // This does not schedule database commits, it is intended to be used as a
  // subroutine for AddPage only. It also assumes the database is valid.
  std::pair<URLID, VisitID> AddPageVisit(
      const GURL& url,
      base::Time time,
      VisitID referring_visit,
      ui::PageTransition transition,
      bool hidden,
      VisitSource visit_source,
      bool should_increment_typed_count,
      bool floc_allowed,
      absl::optional<std::u16string> title = absl::nullopt);

  // Returns a redirect chain in `redirects` for the VisitID
  // `cur_visit`. `cur_visit` is assumed to be valid. Assumes that
  // this HistoryBackend object has been Init()ed successfully.
  void GetRedirectsFromSpecificVisit(VisitID cur_visit,
                                     RedirectList* redirects);

  // Similar to the above function except returns a redirect list ending
  // at `cur_visit`.
  void GetRedirectsToSpecificVisit(VisitID cur_visit, RedirectList* redirects);

  // Updates the visit_duration information in visits table.
  void UpdateVisitDuration(VisitID visit_id, const base::Time end_ts);

  // Returns whether `url` is on an untyped intranet host.
  bool IsUntypedIntranetHost(const GURL& url);

  // Querying ------------------------------------------------------------------

  // Backends for QueryHistory. *Basic() handles queries that are not
  // text search queries and can just be given directly to the history DB.
  // The *Text() version performs a brute force query of the history DB to
  // search for results which match the given text query.
  // Both functions assume QueryHistory already checked the DB for validity.
  void QueryHistoryBasic(const QueryOptions& options, QueryResults* result);
  void QueryHistoryText(const std::u16string& text_query,
                        const QueryOptions& options,
                        QueryResults* result);

  // Committing ----------------------------------------------------------------

  // We always keep a transaction open on the history database so that multiple
  // transactions can be batched. Periodically, these are flushed (use
  // ScheduleCommit). This function does the commit to write any new changes to
  // disk and opens a new transaction. This will be called automatically by
  // ScheduleCommit, or it can be called explicitly if a caller really wants
  // to write something to disk.
  void Commit();

  // Schedules a commit to happen in the future. We do this so that many
  // operations over a period of time will be batched together. If there is
  // already a commit scheduled for the future, this will do nothing.
  void ScheduleCommit();

  // Cancels the scheduled commit, if any. If there is no scheduled commit,
  // does nothing.
  void CancelScheduledCommit();

  // Segments ------------------------------------------------------------------

  // Walks back a segment chain to find the last visit with a non null segment
  // id and returns it. If there is none found, returns 0.
  SegmentID GetLastSegmentID(VisitID from_visit);

  // Update the segment information. This is called internally when a page is
  // added. Return the segment id of the segment that has been updated.
  SegmentID UpdateSegments(const GURL& url,
                           VisitID from_visit,
                           VisitID visit_id,
                           ui::PageTransition transition_type,
                           const base::Time ts);

  // Favicons ------------------------------------------------------------------

  // Returns all the page URLs in the redirect chain for `page_url`. If there
  // are no known redirects for `page_url`, returns a vector with `page_url`.
  RedirectList GetCachedRecentRedirects(const GURL& page_url);

  // Send notification that the favicon has changed for `page_url` and all its
  // redirects. This should be called if the mapping between the page URL
  // (e.g. http://www.google.com) and the icon URL (e.g.
  // http://www.google.com/favicon.ico) has changed.
  void SendFaviconChangedNotificationForPageAndRedirects(const GURL& page_url);

  // Send notification that the bitmap data for the favicon at `icon_url` has
  // changed. Sending this notification is important because the favicon at
  // `icon_url` may be mapped to hundreds of page URLs.
  void SendFaviconChangedNotificationForIconURL(const GURL& icon_url);

  // Generic stuff -------------------------------------------------------------

  // Processes the next scheduled HistoryDBTask, scheduling this method
  // to be invoked again if there are more tasks that need to run.
  void ProcessDBTaskImpl();

  // HistoryBackendNotifier:
  void NotifyFaviconsChanged(const std::set<GURL>& page_urls,
                             const GURL& icon_url) override;
  void NotifyURLVisited(ui::PageTransition transition,
                        const URLRow& row,
                        const RedirectList& redirects,
                        base::Time visit_time) override;
  void NotifyURLsModified(const URLRows& changed_urls,
                          UrlsModifiedReason reason) override;
  void NotifyURLsDeleted(DeletionInfo deletion_info) override;

  // Deleting all history ------------------------------------------------------

  // Deletes all history. This is a special case of deleting that is separated
  // from our normal dependency-following method for performance reasons. The
  // logic lives here instead of ExpireHistoryBackend since it will cause
  // re-initialization of some databases (e.g. favicons) that could fail.
  // When these databases are not valid, our pointers must be null, so we need
  // to handle this type of operation to keep the pointers in sync.
  void DeleteAllHistory();

  // Given a vector of all URLs that we will keep, removes all favicons that
  // aren't used by those URLs.
  bool ClearAllFaviconHistory(const std::vector<GURL>& kept_urls);

  // Deletes all information in the history database, except for the supplied
  // set of URLs in the URL table (these should correspond to the bookmarked
  // URLs).
  //
  // The IDs of the URLs may change.
  bool ClearAllMainHistory(const URLRows& kept_urls);

  // Deletes the FTS index database files, which are no longer used.
  void DeleteFTSIndexDatabases();

  // favicon::FaviconBackendDelegate
  std::vector<GURL> GetCachedRecentRedirectsForPage(
      const GURL& page_url) override;

  bool ProcessSetFaviconsResult(const favicon::SetFaviconsResult& result,
                                const GURL& icon_url);
  // Data ----------------------------------------------------------------------

  // Delegate. See the class definition above for more information. This will
  // be null before Init is called and after Cleanup, but is guaranteed
  // non-null in between.
  std::unique_ptr<Delegate> delegate_;

  // Directory where database files will be stored, empty until Init is called.
  base::FilePath history_dir_;

  // The history/favicon databases. Either may be null if the database could
  // not be opened, all users must first check for null and return immediately
  // if it is. The favicon DB may be null when the history one isn't, but not
  // vice-versa.
  std::unique_ptr<HistoryDatabase> db_;
  bool scheduled_kill_db_;  // Database is being killed due to error.
  std::unique_ptr<favicon::FaviconBackend> favicon_backend_;

  // Manages expiration between the various databases.
  ExpireHistoryBackend expirer_;

  // A commit has been scheduled to occur sometime in the future. We can check
  // !IsCancelled() to see if there is a commit scheduled in the future (note
  // that CancelableOnceClosure starts cancelled with the default constructor),
  // and we can use Cancel() to cancel the scheduled commit. There can be only
  // one scheduled commit at a time (see ScheduleCommit).
  base::CancelableOnceClosure scheduled_commit_;

  // Maps recent redirect destination pages to the chain of redirects that
  // brought us to there. Pages that did not have redirects or were not the
  // final redirect in a chain will not be in this list, as well as pages that
  // redirected "too long" ago (as determined by ExpireOldRedirects above).
  // It is used to set titles & favicons for redirects to that of the
  // destination.
  //
  // As with AddPage, the last item in the redirect chain will be the
  // destination of the redirect (i.e., the key into recent_redirects_);
  typedef base::MRUCache<GURL, RedirectList> RedirectCache;
  RedirectCache recent_redirects_;

  // Timestamp of the first entry in our database.
  base::Time first_recorded_time_;

  // When set, this is the task that should be invoked on destruction.
  scoped_refptr<base::SingleThreadTaskRunner> backend_destroy_task_runner_;
  base::OnceClosure backend_destroy_task_;

  // Tracks page transition types.
  VisitTracker tracker_;

  // List of QueuedHistoryDBTasks to run;
  std::list<std::unique_ptr<QueuedHistoryDBTask>> queued_history_db_tasks_;

  // Used to determine if a URL is bookmarked; may be null.
  std::unique_ptr<HistoryBackendClient> backend_client_;

  scoped_refptr<base::SequencedTaskRunner> task_runner_;

  // Used to allow embedder code to stash random data by key. Those object will
  // be deleted before closing the databases (hence the member variable instead
  // of inheritance from base::SupportsUserData).
  std::unique_ptr<HistoryBackendHelper> supports_user_data_helper_;

  // Listens for the system being under memory pressure.
  std::unique_ptr<base::MemoryPressureListener> memory_pressure_listener_;

  // Contains diagnostic information about the sql database that is non-empty
  // when a catastrophic error occurs.
  std::string db_diagnostics_;

  // List of observers
  base::ObserverList<HistoryBackendObserver>::Unchecked observers_;

  // Used to manage syncing of the typed urls datatype. It will be null before
  // HistoryBackend::Init is called. Defined after observers_ because
  // it unregisters itself as observer during destruction.
  std::unique_ptr<TypedURLSyncBridge> typed_url_sync_bridge_;

  DISALLOW_COPY_AND_ASSIGN(HistoryBackend);
};

}  // namespace history

#endif  // COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_BACKEND_H_