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+// 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 COMPONENTS_NTP_SNIPPETS_USER_CLASSIFIER_H_
+#define COMPONENTS_NTP_SNIPPETS_USER_CLASSIFIER_H_
+
+#include <string>
+
+#include "base/macros.h"
+#include "base/time/time.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace ntp_snippets {
+
+// Collects data about user usage patterns of content suggestions, computes
+// long-term user metrics locally using pref, and reports the metrics to UMA.
+// Based on these long-term user metrics, it classifies the user in a UserClass.
+class UserClassifier {
+ public:
+  // Enumeration listing user classes
+  enum class UserClass {
+    RARE_NTP_USER,
+    ACTIVE_NTP_USER,
+    ACTIVE_SUGGESTIONS_CONSUMER,
+  };
+
+  // For estimating the average length of the intervals between two successive
+  // events, we keep a simple frequency model, a single value that we call
+  // "metric" below.
+  // We track exponentially-discounted rate of the given event per hour where
+  // the continuous utility function between two successive events (e.g. opening
+  // a NTP) at times t1 < t2 is 1 / (t2-t1), i.e. intuitively the rate of this
+  // event in this time interval.
+  // See https://en.wikipedia.org/wiki/Exponential_discounting for more details.
+  // We keep track of the following events.
+  // NOTE: if you add any element, add it also in the static arrays in .cc and
+  // create another histogram.
+  enum class Metric {
+    NTP_OPENED,  // When the user opens a new NTP - this indicates potential
+                 // use of content suggestions.
+    SUGGESTIONS_SHOWN,  // When the content suggestions are shown to the user -
+                        // in the current implementation when the user scrolls
+                        // below the fold.
+    SUGGESTIONS_USED,   // When the user clicks on some suggestions or on some
+                        // "More" button.
+    COUNT               // Keep this as the last element.
+  };
+
+  // The provided |pref_service| may be nullptr in unit-tests.
+  explicit UserClassifier(PrefService* pref_service);
+  ~UserClassifier();
+
+  // Registers profile prefs for all metrics. Called from browser_prefs.cc.
+  static void RegisterProfilePrefs(PrefRegistrySimple* registry);
+
+  // Informs the UserClassifier about a new event for |metric|. The
+  // classification is based on these calls.
+  void OnEvent(Metric metric);
+
+  // Get the estimate average length of the interval between two successive
+  // events of the given type.
+  double GetEstimatedAvgTime(Metric metric) const;
+
+  // Return the classification of the current user.
+  UserClass GetUserClass() const;
+  std::string GetUserClassDescriptionForDebugging() const;
+
+  // Resets the classification (emulates a fresh upgrade / install).
+  void ClearClassificationForDebugging();
+
+ private:
+  // The event has happened, recompute the metric accordingly. Then store and
+  // return the new value.
+  double UpdateMetricOnEvent(Metric metric);
+  // No event has happened but we need to get up-to-date metric, recompute and
+  // return the new value. This function does not store the recomputed metric.
+  double GetUpToDateMetricValue(Metric metric) const;
+
+  // Returns the number of hours since the last event of the same type.
+  // If there is no last event of that type, assume it happened just now and
+  // return 0.
+  double GetHoursSinceLastTime(Metric metric) const;
+  bool HasLastTime(Metric metric) const;
+  void SetLastTimeToNow(Metric metric);
+
+  double GetMetricValue(Metric metric) const;
+  void SetMetricValue(Metric metric, double metric_value);
+  void ClearMetricValue(Metric metric);
+
+  PrefService* pref_service_;
+
+  // Params of the metric.
+  const double discount_rate_per_hour_;
+  const double min_hours_;
+  const double max_hours_;
+
+  // Params of the classification.
+  const double active_consumer_scrolls_at_least_once_per_hours_;
+  const double rare_user_opens_ntp_at_most_once_per_hours_;
+
+  DISALLOW_COPY_AND_ASSIGN(UserClassifier);
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_USER_CLASSIFIER_H_