[LanguageModel] Return top languages only with a reasonable sample set

Before, the LanguageModel returned top languages even if it had only one
data point. Thus the frequencies of top languages could change
drastically in the warm-up phase of the model.

This CL adds a minimum size of the sample set. The model returns empty
list of top languages before this minimal size is reached.

BUG=653058

Review-Url: https://codereview.chromium.org/2391383005
Cr-Commit-Position: refs/heads/master@{#423849}

components/translate/core/browser/language_model.cc

@@ -19,6 +19,7 @@ namespace {
 const char kLanguageModelCounters[] = "language_model_counters";
 
 const int kMaxCountersSum = 1000;
+const int kMinCountersSum = 100;
 const float kCutoffRatio = 0.005f;
 const float kDiscountFactor = 0.75f;
 
@@ -60,9 +61,13 @@ void DiscountAndCleanCounters(base::DictionaryValue* dict) {
 std::vector<LanguageModel::LanguageInfo> GetAllLanguages(
     const base::DictionaryValue& dict) {
 
-  std::vector<LanguageModel::LanguageInfo> top_languages;
   int counters_sum = GetCountersSum(dict);
 
+  // If the sample is not large enough yet, pretend there are no top languages.
+  if (counters_sum < kMinCountersSum)
+    return std::vector<LanguageModel::LanguageInfo>();
+
+  std::vector<LanguageModel::LanguageInfo> top_languages;
   int counter_value = 0;
   for (base::DictionaryValue::Iterator itr(dict); !itr.IsAtEnd();
        itr.Advance()) {
@@ -103,12 +108,15 @@ float LanguageModel::GetLanguageFrequency(
     const std::string& language_code) const {
   const base::DictionaryValue* dict =
       pref_service_->GetDictionary(kLanguageModelCounters);
+  int counters_sum = GetCountersSum(*dict);
+  // If the sample is not large enough yet, pretend there are no top languages.
+  if (counters_sum < kMinCountersSum)
+    return 0;
+
   int counter_value = 0;
   // If the key |language_code| does not exist, |counter_value| stays 0.
   dict->GetInteger(language_code, &counter_value);
 
-  int counters_sum = GetCountersSum(*dict);
-
   return static_cast<float>(counter_value) / counters_sum;
 }
 

components/translate/core/browser/language_model.h

@@ -30,7 +30,7 @@ class LanguageModel : public KeyedService {
     // The current estimated frequency of the language share, a number between 0
     // and 1 (can be understood as the probability that the next page the user
     // opens is in this language). Frequencies over all LanguageInfos from
-    // GetTopLanguages() sum to 1.
+    // GetTopLanguages() sum to 1 (unless there are no top languages, yet).
     float frequency;
 
     bool operator==(const LanguageInfo& m) const {
@@ -45,7 +45,8 @@ class LanguageModel : public KeyedService {
   static void RegisterProfilePrefs(PrefRegistrySimple* registry);
 
   // Returns a list of the languages currently tracked by the model, sorted by
-  // frequency in decreasing order.
+  // frequency in decreasing order. The list is empty, if the model has not
+  // enough data points.
   std::vector<LanguageInfo> GetTopLanguages() const;
 
   // Returns the estimated frequency for the given language or 0 if the language

components/translate/core/browser/language_model_unittest.cc

@@ -27,10 +27,12 @@ TEST(LanguageModelTest, ListSorted) {
   LanguageModel::RegisterProfilePrefs(prefs.registry());
   LanguageModel model(&prefs);
 
-  model.OnPageVisited(kLang1);
-  model.OnPageVisited(kLang1);
-  model.OnPageVisited(kLang1);
-  model.OnPageVisited(kLang2);
+  for (int i = 0; i < 50; i++) {
+    model.OnPageVisited(kLang1);
+    model.OnPageVisited(kLang1);
+    model.OnPageVisited(kLang1);
+    model.OnPageVisited(kLang2);
+  }
 
   EXPECT_THAT(model.GetTopLanguages(),
               ElementsAre(LanguageModel::LanguageInfo{kLang1},
@@ -42,10 +44,12 @@ TEST(LanguageModelTest, ListSortedReversed) {
   LanguageModel::RegisterProfilePrefs(prefs.registry());
   LanguageModel model(&prefs);
 
-  model.OnPageVisited(kLang2);
-  model.OnPageVisited(kLang1);
-  model.OnPageVisited(kLang1);
-  model.OnPageVisited(kLang1);
+  for (int i = 0; i < 50; i++) {
+    model.OnPageVisited(kLang2);
+    model.OnPageVisited(kLang1);
+    model.OnPageVisited(kLang1);
+    model.OnPageVisited(kLang1);
+  }
 
   EXPECT_THAT(model.GetTopLanguages(),
               ElementsAre(LanguageModel::LanguageInfo{kLang1},
@@ -57,10 +61,12 @@ TEST(LanguageModelTest, RightFrequencies) {
   LanguageModel::RegisterProfilePrefs(prefs.registry());
   LanguageModel model(&prefs);
 
-  model.OnPageVisited(kLang1);
-  model.OnPageVisited(kLang1);
-  model.OnPageVisited(kLang1);
-  model.OnPageVisited(kLang2);
+  for (int i = 0; i < 50; i++) {
+    model.OnPageVisited(kLang1);
+    model.OnPageVisited(kLang1);
+    model.OnPageVisited(kLang1);
+    model.OnPageVisited(kLang2);
+  }
 
   // Corresponding frequencies are given by the model.
   EXPECT_THAT(model.GetLanguageFrequency(kLang1), FloatEq(0.75f));