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

components/ntp_snippets/section_rankers/default_constant_section_ranker.cc

Index: components/ntp_snippets/section_rankers/default_constant_section_ranker.cc
diff --git a/components/ntp_snippets/section_rankers/default_constant_section_ranker.cc b/components/ntp_snippets/section_rankers/default_constant_section_ranker.cc
new file mode 100644
index 0000000000000000000000000000000000000000..30ba8cc26e71ffda94281192c7f0c61b1517d42a
--- /dev/null
+++ b/components/ntp_snippets/section_rankers/default_constant_section_ranker.cc
@@ -0,0 +1,64 @@
+// 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.
+
+#include "components/ntp_snippets/section_rankers/default_constant_section_ranker.h"
+
+#include "base/stl_util.h"
+
+namespace ntp_snippets {
+
+DefaultConstantSectionRanker::DefaultConstantSectionRanker() {
+  // Add all local categories in a fixed order.
+  AppendKnownCategory(KnownCategories::DOWNLOADS);
+  AppendKnownCategory(KnownCategories::RECENT_TABS);
+  AppendKnownCategory(KnownCategories::FOREIGN_TABS);
+  AppendKnownCategory(KnownCategories::BOOKMARKS);
+  AppendKnownCategory(KnownCategories::PHYSICAL_WEB_PAGES);
+
+  DCHECK_EQ(static_cast<size_t>(KnownCategories::LOCAL_CATEGORIES_COUNT),
+            ordered_categories_.size());
+}
+
+DefaultConstantSectionRanker::~DefaultConstantSectionRanker() = default;
+
+bool DefaultConstantSectionRanker::Compare(Category left,
+                                           Category right) const {
+  for (Category category : ordered_categories_) {
+    if (category == left) {
+      return true;
+    }
+    if (category == right) {
+      return false;
+    }
+  }
+  return left.id() < right.id();
+}
+
+void DefaultConstantSectionRanker::ClearHistory(base::Time begin,
+                                                base::Time end) {
+  // Ignored.
+}
+
+void DefaultConstantSectionRanker::AppendCategoryIfNecessary(
+    Category category) {
+  if (!base::ContainsValue(ordered_categories_, category)) {
+    ordered_categories_.push_back(category);
+  }
+}
+
+void DefaultConstantSectionRanker::OnSuggestionOpened(Category category) {
+  // Ignored. The order is constant.
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Private methods
+
+void DefaultConstantSectionRanker::AppendKnownCategory(
+    KnownCategories known_category) {
+  Category category = Category::FromKnownCategory(known_category);
+  DCHECK(!base::ContainsValue(ordered_categories_, category));
+  ordered_categories_.push_back(category);
+}
+
+}  // namespace ntp_snippets