Extending the UserClassifier to actually support classification.

components/ntp_snippets/user_classifier.cc

Index: components/ntp_snippets/user_classifier.cc
diff --git a/components/ntp_snippets/user_classifier.cc b/components/ntp_snippets/user_classifier.cc
index ef179b320a6bcba3e06b1a5530f06a9810dd557e..9e74f3a2e806376ca704f9b42c3282b0c259402c 100644
--- a/components/ntp_snippets/user_classifier.cc
+++ b/components/ntp_snippets/user_classifier.cc
@@ -33,6 +33,15 @@ const double kMaxHours = 7 * 24;
 // do not count again).
 const double kMinHours = 0.5;
 
+// Classification constants.
+const double kFrequentUserScrollsAtLeastOncePerHours = 24;
+const double kOccasionalUserOpensNTPAtMostOncePerHours = 72;
+
+// Default lengths of the intervals for new users.
+const double kNTPFrequencyOfANewUserInHours = 24;
+const double kShowFrequencyOfANewUserInHours = 36;
+const double kUseFrequencyOfANewUserInHours = 48;
+
 const char kHistogramAverageHoursToOpenNTP[] =
     "NewTabPage.UserClassifier.AverageHoursToOpenNTP";
 const char kHistogramAverageHoursToShowSuggestions[] =
@@ -40,26 +49,90 @@ const char kHistogramAverageHoursToShowSuggestions[] =
 const char kHistogramAverageHoursToUseSuggestions[] =
     "NewTabPage.UserClassifier.AverageHoursToUseSuggestions";
 
+// Computes the discount rate.
+double GetDiscountRatePerHour() {
+  static double discount_rate_per_hour = 0.0;
+
+  if (discount_rate_per_hour == 0.0) {
+    // Compute discount_rate_per_hour such that
+    //   kDiscountFactorPerDay = 1 - e^{-discount_rate_per_hour * 24}.
+    discount_rate_per_hour =
+        std::log(1.0 / (1.0 - kDiscountFactorPerDay)) / 24.0;
+  }
+
+  return discount_rate_per_hour;
+}
+
+// Returns the new value of the metric using its |old_value|, assuming
+// |hours_since_last_time| hours have passed since it was last recomputed.
+// If |event_now| is true, the event is assumed to have happened right now,
+// otherwise no event is assumed to happen within the last
+// |hours_since_last_time| hours.
+double RecomputeMetric(double old_value,
+                       double hours_since_last_time,
+                       bool event_now) {
+  // Compute and store the new discounted average according to the formula
+  //   avg_events := 1 + e^{-discount_rate_per_hour * hours_since} * avg_events.
+  return (event_now ? 1 : 0) +
+         std::exp(-GetDiscountRatePerHour() * hours_since_last_time) *
+             old_value;
+}
+
+// Compute the number of hours between two events for the given metric value
+// assuming the events were equally distributed.
+double GetEstimateHoursBetweenEvents(const double metric_value) {
+  // Right after the first update, the metric is equal to 1.
+  if (metric_value <= 1)
+    return kMaxHours;
+
+  // This is the estimate with the assumption that last event happened right
+  // now and the system is in the steady-state. Solve estimate_hours in the
+  // steady-state equation:
+  //   metric_value = 1 + e^{-discount_rate * estimate_hours} * metric_value,
+  // i.e.
+  //   -discount_rate * estimate_hours = log((avg_events - 1) / avg_events),
+  //   discount_rate * estimate_hours = log(avg_events / (avg_events - 1)),
+  //   estimate_hours = log(avg_events / (avg_events - 1)) / discount_rate.
+  double estimate_hours =
+      std::log(metric_value / (metric_value - 1)) / GetDiscountRatePerHour();
+  return std::max(kMinHours, std::min(kMaxHours, estimate_hours));
+}
+
+// The inverse of GetEstimateHoursBetweenEvents().
+double GetMetricValueForEstimateHoursBetweenEvents(double estimate_hours) {
+  // Keep the input value within [kMinHours, kMaxHours].
+  estimate_hours = std::max(kMinHours, std::min(kMaxHours, estimate_hours));
+
+  // Return |metric_value| such that GetEstimateHoursBetweenEvents for
+  // |metric_value| returns |estimate_hours|. Thus, solve |metric_value| in
+  //   metric_value = 1 + e^{-discount_rate * estimate_hours} * metric_value,
+  // i.e.
+  //   metric_value = 1 / (1 - e^{-discount_rate * estimate_hours}).
+  return 1.0 / (1.0 - std::exp(-GetDiscountRatePerHour() * estimate_hours));
+}
+
 }  // namespace
 
 namespace ntp_snippets {
 
 UserClassifier::UserClassifier(PrefService* pref_service)
-    : pref_service_(pref_service),
-      // Compute discount_rate_per_hour such that
-      //   kDiscountFactorPerDay = 1 - e^{-discount_rate_per_hour * 24}.
-      discount_rate_per_hour_(std::log(1 / (1 - kDiscountFactorPerDay)) / 24) {}
+    : pref_service_(pref_service) {}
 
 UserClassifier::~UserClassifier() {}
 
 // static
 void UserClassifier::RegisterProfilePrefs(PrefRegistrySimple* registry) {
+  registry->RegisterDoublePref(prefs::kUserClassifierAverageNTPOpenedPerHour,
+                               GetMetricValueForEstimateHoursBetweenEvents(
+                                   kNTPFrequencyOfANewUserInHours));
   registry->RegisterDoublePref(
-      prefs::kUserClassifierAverageNTPOpenedPerHour, 1);
+      prefs::kUserClassifierAverageSuggestionsShownPerHour,
+      GetMetricValueForEstimateHoursBetweenEvents(
+          kShowFrequencyOfANewUserInHours));
   registry->RegisterDoublePref(
-      prefs::kUserClassifierAverageSuggestionsShownPerHour, 1);
-  registry->RegisterDoublePref(
-      prefs::kUserClassifierAverageSuggestionsUsedPerHour, 1);
+      prefs::kUserClassifierAverageSuggestionsUsedPerHour,
+      GetMetricValueForEstimateHoursBetweenEvents(
+          kUseFrequencyOfANewUserInHours));
 
   registry->RegisterInt64Pref(prefs::kUserClassifierLastTimeToOpenNTP, 0);
   registry->RegisterInt64Pref(prefs::kUserClassifierLastTimeToShowSuggestions,
@@ -69,78 +142,136 @@ void UserClassifier::RegisterProfilePrefs(PrefRegistrySimple* registry) {
 }
 
 void UserClassifier::OnNTPOpened() {
-  UpdateMetricOnEvent(prefs::kUserClassifierAverageNTPOpenedPerHour,
-                      prefs::kUserClassifierLastTimeToOpenNTP);
+  double metric =
+      UpdateMetricOnEvent(prefs::kUserClassifierAverageNTPOpenedPerHour,
+                          prefs::kUserClassifierLastTimeToOpenNTP);
 
-  double avg = GetEstimateHoursBetweenEvents(
-      prefs::kUserClassifierAverageNTPOpenedPerHour);
+  double avg = GetEstimateHoursBetweenEvents(metric);
   UMA_HISTOGRAM_CUSTOM_COUNTS(kHistogramAverageHoursToOpenNTP, avg, 1,
                               kMaxHours, 50);
 }
 
 void UserClassifier::OnSuggestionsShown() {
-  UpdateMetricOnEvent(prefs::kUserClassifierAverageSuggestionsShownPerHour,
-                      prefs::kUserClassifierLastTimeToShowSuggestions);
+  double metric =
+      UpdateMetricOnEvent(prefs::kUserClassifierAverageSuggestionsShownPerHour,
+                          prefs::kUserClassifierLastTimeToShowSuggestions);
 
-  double avg = GetEstimateHoursBetweenEvents(
-      prefs::kUserClassifierAverageSuggestionsShownPerHour);
+  double avg = GetEstimateHoursBetweenEvents(metric);
   UMA_HISTOGRAM_CUSTOM_COUNTS(kHistogramAverageHoursToShowSuggestions, avg, 1,
                               kMaxHours, 50);
 }
 
 void UserClassifier::OnSuggestionsUsed() {
-  UpdateMetricOnEvent(prefs::kUserClassifierAverageSuggestionsUsedPerHour,
-                      prefs::kUserClassifierLastTimeToUseSuggestions);
+  double metric =
+      UpdateMetricOnEvent(prefs::kUserClassifierAverageSuggestionsUsedPerHour,
+                          prefs::kUserClassifierLastTimeToUseSuggestions);
 
-  double avg = GetEstimateHoursBetweenEvents(
-      prefs::kUserClassifierAverageSuggestionsUsedPerHour);
+  double avg = GetEstimateHoursBetweenEvents(metric);
   UMA_HISTOGRAM_CUSTOM_COUNTS(kHistogramAverageHoursToUseSuggestions, avg, 1,
                               kMaxHours, 50);
 }
 
-void UserClassifier::UpdateMetricOnEvent(const char* metric_pref_name,
-                                         const char* last_time_pref_name) {
+double UserClassifier::GetEstimatedAvgTimeToOpenNTP() const {
+  double metric =
+      GetUpToDateMetricValue(prefs::kUserClassifierAverageNTPOpenedPerHour,
+                             prefs::kUserClassifierLastTimeToOpenNTP);
+  return GetEstimateHoursBetweenEvents(metric);
+}
+
+double UserClassifier::GetEstimatedAvgTimeToShowSuggestions() const {
+  double metric = GetUpToDateMetricValue(
+      prefs::kUserClassifierAverageSuggestionsShownPerHour,
+      prefs::kUserClassifierLastTimeToShowSuggestions);
+  return GetEstimateHoursBetweenEvents(metric);
+}
+
+double UserClassifier::GetEstimatedAvgTimeToUseSuggestions() const {
+  double metric = GetUpToDateMetricValue(
+      prefs::kUserClassifierAverageSuggestionsUsedPerHour,
+      prefs::kUserClassifierLastTimeToUseSuggestions);
+  return GetEstimateHoursBetweenEvents(metric);
+}
+
+UserClassifier::UserClass UserClassifier::GetUserClass() const {
+  if (GetEstimatedAvgTimeToOpenNTP() >=
+      kOccasionalUserOpensNTPAtMostOncePerHours) {
+    return UserClass::OCCASIONAL_NTP_USER;
+  }
+
+  if (GetEstimatedAvgTimeToUseSuggestions() <=
+      kFrequentUserScrollsAtLeastOncePerHours) {
+    return UserClass::FREQUENT_NTP_USER;
+  }
+
+  return UserClass::NORMAL_NTP_USER;
+}
+
+std::string UserClassifier::GetUserClassDescriptionForDebugging() const {
+  switch (GetUserClass()) {
+    case UserClass::OCCASIONAL_NTP_USER:
+      return "Occasional user of the NTP";
+    case UserClass::NORMAL_NTP_USER:
+      return "Normal user of the NTP";
+    case UserClass::FREQUENT_NTP_USER:
+      return "Frequent user of the NTP";
+  }
+  NOTREACHED();
+  return "Unknown user class";
+}
+
+void UserClassifier::ClearClassificationForDebugging() {
+  pref_service_->ClearPref(prefs::kUserClassifierAverageNTPOpenedPerHour);
+  pref_service_->ClearPref(
+      prefs::kUserClassifierAverageSuggestionsShownPerHour);
+  pref_service_->ClearPref(prefs::kUserClassifierAverageSuggestionsUsedPerHour);
+
+  pref_service_->ClearPref(prefs::kUserClassifierLastTimeToOpenNTP);
+  pref_service_->ClearPref(prefs::kUserClassifierLastTimeToShowSuggestions);
+  pref_service_->ClearPref(prefs::kUserClassifierLastTimeToUseSuggestions);
+}
+
+double UserClassifier::UpdateMetricOnEvent(const char* metric_pref_name,
+                                           const char* last_time_pref_name) {
+  // The pref_service_ can be null in tests.
   if (!pref_service_)
-    return;
+    return 0;
 
   double hours_since_last_time =
       std::min(kMaxHours, GetHoursSinceLastTime(last_time_pref_name));
+  // If the "last time" is not defined, set it.
+  if (!hours_since_last_time)

[Marc Treib, 2016/09/20 10:27:02]

This will check for zero - is that what you want? Doesn't seem very safe...

[jkrcal, 2016/09/20 13:10:13]

I agree, this a bit obscure, to say the least :) Not in the code anymore.

+    SetLastTimeToNow(last_time_pref_name);
   // Ignore events within the same "browsing session".
   if (hours_since_last_time < kMinHours)
-    return;
+    return GetUpToDateMetricValue(metric_pref_name, last_time_pref_name);
+
   SetLastTimeToNow(last_time_pref_name);
 
   double avg_events_per_hour = pref_service_->GetDouble(metric_pref_name);
-  // Compute and store the new discounted average according to the formula
-  //   avg_events := 1 + e^{-discount_rate_per_hour * hours_since} * avg_events.
   double new_avg_events_per_hour =
-      1 +
-      std::exp(discount_rate_per_hour_ * hours_since_last_time) *
-          avg_events_per_hour;
+      RecomputeMetric(avg_events_per_hour, hours_since_last_time, true);
   pref_service_->SetDouble(metric_pref_name, new_avg_events_per_hour);
+  return new_avg_events_per_hour;
 }
 
-double UserClassifier::GetEstimateHoursBetweenEvents(
-    const char* metric_pref_name) {
-  double avg_events_per_hour = pref_service_->GetDouble(metric_pref_name);
+double UserClassifier::GetUpToDateMetricValue(
+    const char* metric_pref_name,
+    const char* last_time_pref_name) const {
+  // The pref_service_ can be null in tests.
+  if (!pref_service_)
+    return 0;
 
-  // Right after the first update, the metric is equal to 1.
-  if (avg_events_per_hour <= 1)
-    return kMaxHours;
+  double hours_since_last_time =
+      std::min(kMaxHours, GetHoursSinceLastTime(last_time_pref_name));
 
-  // This is the estimate with the assumption that last event happened right
-  // now and the system is in the steady-state. Solve estimate_hours in the
-  // steady-state equation:
-  //   avg_events = 1 + e^{-discount_rate * estimate_hours} * avg_events.
-  return std::min(kMaxHours,
-                  std::log(avg_events_per_hour / (avg_events_per_hour - 1)) /
-                      discount_rate_per_hour_);
+  double avg_events_per_hour = pref_service_->GetDouble(metric_pref_name);
+  return RecomputeMetric(avg_events_per_hour, hours_since_last_time, true);
 }
 
 double UserClassifier::GetHoursSinceLastTime(
-    const char* last_time_pref_name) {
+    const char* last_time_pref_name) const {
   if (!pref_service_->HasPrefPath(last_time_pref_name))
-    return DBL_MAX;
+    return 0;
 
   base::TimeDelta since_last_time =
       base::Time::Now() - base::Time::FromInternalValue(