Measure usage metrics to prepare for adaptive fetching rates in M55

components/ntp_snippets/user_classifier.h

Index: components/ntp_snippets/user_classifier.h
diff --git a/components/ntp_snippets/user_classifier.h b/components/ntp_snippets/user_classifier.h
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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 "base/macros.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.
+// TODO(jkrcal): Add classification of users based on the metrics and getters
+// for the classification as well as for the metrics.
+class UserClassifier {
+ public:
+  // 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);
+
+  // When the user opens a new NTP - this indicates potential use of content
+  // suggestions.
+  void OnNTPOpened();
+
+  // When the content suggestions are shown to the user - in the current
+  // implementation when the user scrolls below the fold.
+  void OnSuggestionsShown();
+
+  // When the user clicks on some suggestions or on some "More" button.
+  void OnSuggestionsUsed();
+
+ private:
+  // The event has happened, recompute and store the metric accordingly.
+  void UpdateMetricOnEvent(const char* metric_pref_name,
+                           const char* last_time_pref_name);
+
+  // Compute the number of hours between two events for the given metric value
+  // assuming the events were equally distributed.
+  double GetEstimateHoursBetweenEvents(const char* metric_pref_name);
+
+  // Returns the number of hours since the last event of the same type or
+  // DBL_MAX if there is no last event of that type.
+  double GetHoursSinceLastTime(const char* last_time_pref_name);
+  void SetLastTimeToNow(const char* last_time_pref_name);
+
+  PrefService* pref_service_;
+  double const discount_rate_per_hour_;
+
+  DISALLOW_COPY_AND_ASSIGN(UserClassifier);
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_USER_CLASSIFIER_H_