[NTP::SectionOrder] First version of click based category ranker.

components/ntp_snippets/category_rankers/click_based_category_ranker.h

Index: components/ntp_snippets/category_rankers/click_based_category_ranker.h
diff --git a/components/ntp_snippets/category_rankers/click_based_category_ranker.h b/components/ntp_snippets/category_rankers/click_based_category_ranker.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_SECTION_RANKERS_CLICK_BASED_SECTION_RANKER_H_
+#define COMPONENTS_NTP_SNIPPETS_SECTION_RANKERS_CLICK_BASED_SECTION_RANKER_H_
+
+#include <vector>
+
+#include "base/time/time.h"
+#include "components/ntp_snippets/category.h"
+#include "components/ntp_snippets/category_rankers/category_ranker.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace ntp_snippets {
+
+// An implementation of a CategoryRanker based on a number of clicks per
+// category. Initial order is hardcoded, but sections with more clicks are moved
+// to the top. The new remote categories must be registered using
+// AppendCategoryIfNecessary. All other categories must be hardcoded in the
+// initial order. The order and category usage data are persisted in prefs and
+// reloaded on startup. TODO(crbug.com/675929): Remove unused categories from
+// prefs.
+class ClickBasedCategoryRanker : public CategoryRanker {
+ public:
+  explicit ClickBasedCategoryRanker(PrefService* pref_service);
+  ~ClickBasedCategoryRanker() override;
+
+  // CategoryRanker implementation.
+  bool Compare(Category left, Category right) const override;
+  void ClearHistory(base::Time begin, base::Time end) override;
+  void AppendCategoryIfNecessary(Category category) override;
+  void OnSuggestionOpened(Category category) override;
+
+  static void RegisterProfilePrefs(PrefRegistrySimple* registry);
+
+  // Returns passing margin, i.e. a number of extra clicks required to move a
+  // category upwards. For testing only.
+  static int GetPassingMargin();
+
+  // Returns number of top categories with extra margin (i.e. with increased
+  // passing margin). For testing only.
+  static int GetNumTopCategoriesWithExtraMargin();
+
+ private:
+  struct RankedCategory {
+    Category category;
+    int clicks;
+
+    RankedCategory(Category category, int clicks);
+  };
+
+  int GetPositionPassingMargin(
+      std::vector<RankedCategory>::const_iterator category_position) const;
+  void RestoreDefaultOrder();
+  void AppendKnownCategory(KnownCategories known_category);
+  bool ReadOrderFromPrefs(std::vector<RankedCategory>* result_categories);
+  void StoreOrderToPrefs(const std::vector<RankedCategory>& ordered_categories);
+  std::vector<RankedCategory>::iterator FindCategory(Category category);
+  bool ContainsCategory(Category category) const;
+
+  std::vector<RankedCategory> ordered_categories_;
+  PrefService* pref_service_;
+
+  DISALLOW_COPY_AND_ASSIGN(ClickBasedCategoryRanker);
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_SECTION_RANKERS_CLICK_BASED_SECTION_RANKER_H_