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

components/ntp_snippets/category_rankers/constant_category_ranker.cc

Index: components/ntp_snippets/category_rankers/constant_category_ranker.cc
diff --git a/components/ntp_snippets/category_rankers/constant_category_ranker.cc b/components/ntp_snippets/category_rankers/constant_category_ranker.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c008b7952413534a5d0fcee77a99fc4f8975dfcf
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+++ b/components/ntp_snippets/category_rankers/constant_category_ranker.cc
@@ -0,0 +1,81 @@
+// 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/category_rankers/constant_category_ranker.h"
+
+#include "base/stl_util.h"
+
+namespace ntp_snippets {
+
+ConstantCategoryRanker::ConstantCategoryRanker() {
+  // 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());
+
+  // Known remote categories come after. Other remote categories will be ordered
+  // after these depending on when providers notify us about them using
+  // AppendCategoryIfNecessary.
+  // TODO(treib): Consider not adding ARTICLES here, so that providers can
+  // define the order themselves.
+  AppendKnownCategory(KnownCategories::ARTICLES);
+}
+
+ConstantCategoryRanker::~ConstantCategoryRanker() = default;
+
+bool ConstantCategoryRanker::Compare(Category left, Category right) const {
+  if (!base::ContainsValue(ordered_categories_, left)) {
+    LOG(DFATAL) << "The category with ID " << left.id()
+                << " has not been added using AppendCategoryIfNecessary.";
+  }
+  if (!base::ContainsValue(ordered_categories_, right)) {
+    LOG(DFATAL) << "The category with ID " << right.id()
+                << " has not been added using AppendCategoryIfNecessary.";
+  }
+  if (left == right) {
+    return false;
+  }
+  for (Category category : ordered_categories_) {
+    if (category == left) {
+      return true;
+    }
+    if (category == right) {
+      return false;
+    }
+  }
+  // This fallback is provided only to satisfy "Compare" contract if by mistake
+  // categories are not added using AppendCategoryIfNecessary. One should not
+  // rely on this, instead the order must be defined explicitly using
+  // AppendCategoryIfNecessary.
+  return left.id() < right.id();
+}
+
+void ConstantCategoryRanker::ClearHistory(base::Time begin, base::Time end) {
+  // Ignored, because this implementation doesn't store any history-related
+  // data.
+}
+
+void ConstantCategoryRanker::AppendCategoryIfNecessary(Category category) {
+  if (!base::ContainsValue(ordered_categories_, category)) {
+    ordered_categories_.push_back(category);
+  }
+}
+
+void ConstantCategoryRanker::OnSuggestionOpened(Category category) {
+  // Ignored. The order is constant.
+}
+
+void ConstantCategoryRanker::AppendKnownCategory(
+    KnownCategories known_category) {
+  Category category = Category::FromKnownCategory(known_category);
+  DCHECK(!base::ContainsValue(ordered_categories_, category));
+  ordered_categories_.push_back(category);
+}
+
+}  // namespace ntp_snippets