Prevent site engagement scores from decaying when Chrome isn't in use.

chrome/browser/engagement/site_engagement_score.h

 // Copyright 2016 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 CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SCORE_H_
 #define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SCORE_H_
 
 #include <memory>
 
 #include "base/gtest_prod_util.h"
@@ skipping 44 matching lines @@
 
     // The number of points that the engagement service must accumulate to be
     // considered 'useful'.
     BOOTSTRAP_POINTS,
 
     // The boundaries between low/medium and medium/high engagement as returned
     // by GetEngagementLevel().
     MEDIUM_ENGAGEMENT_BOUNDARY,
     HIGH_ENGAGEMENT_BOUNDARY,
 
+    // The maximum number of decays that a SiteEngagementScore can incur before
+    // entering a grace period. MAX_DECAYS_PER_SCORE * DECAY_PERIOD_IN_DAYS is
+    // the max decay period, i.e. the maximum duration permitted for
+    // (clock_->Now() - score.last_engagement_time()).
+    MAX_DECAYS_PER_SCORE,
+
+    // If a SiteEngagamentScore has not been accessed or updated for a period
+    // longer than the max decay period + LAST_ENGAGEMENT_GRACE_PERIOD_IN_HOURS
+    // (see above), its last engagement time will be reset to be max decay
+    // period prior to clock_->Now().
+    LAST_ENGAGEMENT_GRACE_PERIOD_IN_HOURS,
+
     MAX_VARIATION
   };
 
   // The maximum number of points that are allowed.
   static const double kMaxPoints;
 
   static double GetMaxPointsPerDay();
   static double GetDecayPeriodInDays();
   static double GetDecayPoints();
   static double GetNavigationPoints();
   static double GetUserInputPoints();
   static double GetVisibleMediaPoints();
   static double GetHiddenMediaPoints();
   static double GetWebAppInstalledPoints();
   static double GetFirstDailyEngagementPoints();
   static double GetBootstrapPoints();
   static double GetMediumEngagementBoundary();
   static double GetHighEngagementBoundary();
+  static double GetMaxDecaysPerScore();
+  static double GetLastEngagementGracePeriodInHours();
 
   // Update the default engagement settings via variations.
   static void UpdateFromVariations(const char* param_name);
 
   // The SiteEngagementScore does not take ownership of |clock|. It is the
   // responsibility of the caller to make sure |clock| outlives this
   // SiteEngagementScore.
   SiteEngagementScore(base::Clock* clock,
                       const GURL& origin,
                       HostContentSettingsMap* settings_map);
   SiteEngagementScore(SiteEngagementScore&& other);
   ~SiteEngagementScore();
 
   SiteEngagementScore& operator=(SiteEngagementScore&& other);
 
   // Adds |points| to this score, respecting daily limits and the maximum
   // possible score. Decays the score if it has not been updated recently
   // enough.
   void AddPoints(double points);
   double GetScore() const;
 
   // Writes the values in this score into |settings_map_|.
   void Commit();
 
   // Returns true if the maximum number of points today has been added.
   bool MaxPointsPerDayAdded() const;
 
   // Resets the score to |points| and resets the daily point limit. If
-  // |updated_time| is non-null, sets the last engagement time and last
-  // shortcut launch time (if it is non-null) to |updated_time|. Otherwise, last
-  // engagement time is set to the current time and last shortcut launch time is
-  // left unchanged.
+  // |updated_time| is non-null, sets the last engagement time to that value.
   void Reset(double points, const base::Time updated_time);
 
   // Get/set the last time this origin was launched from an installed shortcut.
   base::Time last_shortcut_launch_time() const {
     return last_shortcut_launch_time_;
   }
   void set_last_shortcut_launch_time(const base::Time& time) {
     last_shortcut_launch_time_ = time;
   }
 
+  // Get/set the last time this origin recorded an engagement change.
+  base::Time last_engagement_time() const {
+    return last_engagement_time_;
+  }
+  void set_last_engagement_time(const base::Time& time) {
+    last_engagement_time_ = time;
+  }
+
  private:
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementScoreTest, PartiallyEmptyDictionary);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementScoreTest, PopulatedDictionary);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementScoreTest, Reset);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementScoreTest, FirstDailyEngagementBonus);
   friend class ImportantSitesUtilTest;
   friend class SiteEngagementHelperTest;
   friend class SiteEngagementScoreTest;
   friend class SiteEngagementServiceTest;
 
@@ skipping 50 matching lines @@
   // The origin this score represents.
   GURL origin_;
 
   // The settings map to write this score to when Commit() is called.
   HostContentSettingsMap* settings_map_;
 
   DISALLOW_COPY_AND_ASSIGN(SiteEngagementScore);
 };
 
 #endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SCORE_H_