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

components/ntp_snippets/section_rankers/section_ranker.h

Index: components/ntp_snippets/section_rankers/section_ranker.h
diff --git a/components/ntp_snippets/section_rankers/section_ranker.h b/components/ntp_snippets/section_rankers/section_ranker.h
new file mode 100644
index 0000000000000000000000000000000000000000..6fe55babe76700aee0d2b187f4f7b4b5d9b6f173
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+++ b/components/ntp_snippets/section_rankers/section_ranker.h
@@ -0,0 +1,42 @@
+// 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_SECTION_RANKER_H_
+#define COMPONENTS_NTP_SNIPPETS_SECTION_RANKERS_SECTION_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.

[Marc Treib, 2016/12/13 12:22:42]

This should have a bug assigned.

[vitaliii, 2016/12/14 08:59:38]

Done.

+
+// Orders sections (represented as categories).
+// The order can be changed.

[Marc Treib, 2016/12/13 12:22:42]

What does this mean?

[vitaliii, 2016/12/14 08:59:38]

Done.

+class SectionRanker {

[Marc Treib, 2016/12/13 12:22:42]

CategoryRanker.
"Section" is the UI term, which we're not using around here.

[vitaliii, 2016/12/14 08:59:38]

Done.

+ public:
+  // Compare two categories. True means that |left| precedes |right|, i.e. it

[Marc Treib, 2016/12/13 12:22:42]

"Compares two categories", similar below. The style is:
// Does the thing.
void DoTheThing();

[vitaliii, 2016/12/14 08:59:38]

Done.

+  // 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;
+
+  // Delete all history related data between |begin| and |end|. The order
+  // after this call should not reflect the history in any way.

[Marc Treib, 2016/12/13 12:22:42]

After this call, the category order may not depend on this history range
anymore.
?

[vitaliii, 2016/12/14 08:59:38]

Done.

+  virtual void ClearHistory(base::Time begin, base::Time end) = 0;
+
+  // If there is no previous data for |category|, it is added to the end with
+  // the score 0, otherwise does nothing. Before the data for new categories

[Marc Treib, 2016/12/13 12:22:42]

"score 0" is an implementation detail of some possible implementations of the
interface, so it shouldn't be here.
The same more or less applies to the rest of the comment.

[vitaliii, 2016/12/14 08:59:38]

Done.

+  // is obtained, the order of the categories must be the same as the order
+  // they have been added in.
+  virtual void AppendCategoryIfNecessary(Category category) = 0;
+
+  // Feedback data from the user to update the ranking.
+
+  // Suggestion click data.

[Marc Treib, 2016/12/13 12:22:42]

This comment isn't useful.
"Called whenever a suggestion is opened by the user." ?

[vitaliii, 2016/12/14 08:59:38]

Done.

+  virtual void OnSuggestionOpened(Category category) = 0;
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_SECTION_RANKERS_SECTION_RANKER_H_