[NTP::SectionOrder] Replace CategoryFactory with a category ranker.

components/ntp_snippets/category_rankers/category_ranker.h

Index: components/ntp_snippets/category_rankers/category_ranker.h
diff --git a/components/ntp_snippets/category_rankers/category_ranker.h b/components/ntp_snippets/category_rankers/category_ranker.h
new file mode 100644
index 0000000000000000000000000000000000000000..30cf36bf8e2670e0166fa079f0bf66f59d583c09
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+++ b/components/ntp_snippets/category_rankers/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_CATEGORY_RANKERS_CATEGORY_RANKER_H_
+#define COMPONENTS_NTP_SNIPPETS_CATEGORY_RANKERS_CATEGORY_RANKER_H_
+
+#include "base/time/time.h"
+#include "components/ntp_snippets/category.h"
+
+namespace ntp_snippets {
+
+// TODO(vitaliii): Ensure that changes in the order are propagated to the UI.
+// (crbug.com/673743)
+
+// Orders categories.
+// The order may be dynamic and change at any time.
+class CategoryRanker {
+ public:
+  virtual ~CategoryRanker() = default;
+
+  // Compares two categories. True means that |left| precedes |right|, i.e. it
+  // must be located earlier (above) on the NTP. This method must satisfy
+  // "Compare" contract required by sort algorithms.
+  virtual bool Compare(Category left, Category right) const = 0;
+
+  // Deletes all history related data between |begin| and |end|. After this
+  // call, the category order may not depend on this history range anymore.
+  virtual void ClearHistory(base::Time begin, base::Time end) = 0;
+
+  // If |category| has not been added previously, it is added after all already
+  // known categories, otherwise nothing is changed.
+  virtual void AppendCategoryIfNecessary(Category category) = 0;
+
+  // Feedback data from the user to update the ranking.
+
+  // Called whenever a suggestion is opened by the user.
+  virtual void OnSuggestionOpened(Category category) = 0;
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_CATEGORY_RANKERS_CATEGORY_RANKER_H_