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

components/ntp_snippets/category.h

 // 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_H_
 #define COMPONENTS_NTP_SNIPPETS_CATEGORY_H_
 
 #include <ostream>
 
 namespace ntp_snippets {
 
-class CategoryFactory;
-
 // These are the categories that the client knows about.
 // The values before LOCAL_CATEGORIES_COUNT are the categories that are provided
 // locally on the device. Categories provided by the server (IDs strictly larger
 // than REMOTE_CATEGORIES_OFFSET) only need to be hard-coded here if they need
 // to be recognized by the client implementation.
 // NOTE: These are persisted, so don't reorder or remove values, and insert new
 // values only in the appropriate places marked below.
 // On Android builds, a Java counterpart will be generated for this enum.
 // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.chrome.browser.ntp.snippets
 enum class KnownCategories {
@@ skipping 28 matching lines @@
 };
 
 // A category groups ContentSuggestions which belong together. Use the
 // CategoryFactory to obtain instances.
 class Category {
  public:
   // An arbitrary but consistent ordering. Can be used to look up categories in
   // a std::map, but should not be used to order categories for other purposes.
   struct CompareByID;
 
+  // Creates a category from a KnownCategory value. The passed |known_category|
+  // must not be one of the special values (LOCAL_CATEGORIES_COUNT or
+  // REMOTE_CATEGORIES_OFFSET).
+  static Category FromKnownCategory(KnownCategories known_category);
+
+  // Creates a category from a category identifier delivered by the server.
+  // |remote_category| must be positive.
+  static Category FromRemoteCategory(int remote_category);
+
+  // Creates a category from an ID as returned by |id()|. |id| must be a
+  // non-negative value.
+  static Category FromIDValue(int id);
+
   // Returns a non-negative identifier that is unique for the category and can
   // be converted back to a Category instance using
   // |CategoryFactory::FromIDValue(id)|.
   int id() const { return id_; }
 
   // Returns whether this category matches the given |known_category|.
   bool IsKnownCategory(KnownCategories known_category) const;
 
  private:
-  friend class CategoryFactory;
-
   explicit Category(int id);
 
   int id_;
 
   // Allow copy and assignment.
 };
 
 bool operator==(const Category& left, const Category& right);
 
 bool operator!=(const Category& left, const Category& right);
 
 struct Category::CompareByID {
   bool operator()(const Category& left, const Category& right) const;
 };
 
 std::ostream& operator<<(std::ostream& os, const Category& obj);
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CATEGORY_H_