[NTP::SectionOrder] Add a flag to choose category ranker.

components/ntp_snippets/features.cc

Index: components/ntp_snippets/features.cc
diff --git a/components/ntp_snippets/features.cc b/components/ntp_snippets/features.cc
index 03cfbc3bf45f0831f3d6c9836aa39d7f2e8254df..c3c7fc87baf2a2ac811d3e24bfabae0c1ee721ae 100644
--- a/components/ntp_snippets/features.cc
+++ b/components/ntp_snippets/features.cc
@@ -4,6 +4,8 @@
 
 #include "components/ntp_snippets/features.h"
 
+#include "components/ntp_snippets/category_rankers/click_based_category_ranker.h"
+#include "components/ntp_snippets/category_rankers/constant_category_ranker.h"
 #include "components/variations/variations_associated_data.h"
 
 namespace ntp_snippets {
@@ -44,4 +46,47 @@ const base::Feature kFetchMoreFeature{"NTPSuggestionsFetchMore",
 const base::Feature kPreferAmpUrlsFeature{"NTPPreferAmpUrls",
                                           base::FEATURE_ENABLED_BY_DEFAULT};
 
+const base::Feature kCategoryRanker{"ContentSuggestionsCategoryRanker",
+                                    base::FEATURE_ENABLED_BY_DEFAULT};
+
+const char kCategoryRankerParameter[] = "category_ranker";
+const char kCategoryRankerConstantRanker[] = "constant";
+const char kCategoryRankerClickBasedRanker[] = "click_based";
+
+CategoryRankerChoice GetSelectedCategoryRanker() {
+  std::string category_ranker_value =
+      variations::GetVariationParamValueByFeature(kCategoryRanker,
+                                                  kCategoryRankerParameter);
+
+  if (category_ranker_value.empty()) {
+    // Default, Enabled or Disabled.
+    return CategoryRankerChoice::CONSTANT;
+  }
+  if (category_ranker_value == kCategoryRankerConstantRanker) {
+    return CategoryRankerChoice::CONSTANT;
+  }
+  if (category_ranker_value == kCategoryRankerClickBasedRanker) {
+    return CategoryRankerChoice::CLICK_BASED;
+  }
+
+  NOTREACHED() << "The " << kCategoryRankerParameter << " parameter value is '"
+               << category_ranker_value << "'";
+  return CategoryRankerChoice::CONSTANT;
+}
+
+std::unique_ptr<CategoryRanker> BuildSelectedCategoryRanker(
+    PrefService* pref_service) {
+  CategoryRankerChoice choice = ntp_snippets::GetSelectedCategoryRanker();
+  switch (choice) {
+    case CategoryRankerChoice::CONSTANT:
+      return base::MakeUnique<ConstantCategoryRanker>();
+    case CategoryRankerChoice::CLICK_BASED:
+      return base::MakeUnique<ClickBasedCategoryRanker>(pref_service);
+    default:
+      NOTREACHED() << "The category ranker choice value is "
+                   << static_cast<int>(choice);
+  }
+  return nullptr;
+}
+
 }  // namespace ntp_snippets