[NTP Snippets] Metrics: switch over known categories

This ensures that there'll be a compile error if we add a new category but don't update the metrics code.
Also, add a histogram suffix for PHYSICAL_WEB_PAGES which was missing.

BUG=none

Review-Url: https://codereview.chromium.org/2337103003
Cr-Commit-Position: refs/heads/master@{#419420}

components/ntp_snippets/category.cc

@@ -4,11 +4,15 @@
 
 #include "components/ntp_snippets/category.h"
 
+#include "base/logging.h"
+
 namespace ntp_snippets {
 
 Category::Category(int id) : id_(id) {}
 
 bool Category::IsKnownCategory(KnownCategories known_category) const {
+  DCHECK_NE(known_category, KnownCategories::LOCAL_CATEGORIES_COUNT);
+  DCHECK_NE(known_category, KnownCategories::REMOTE_CATEGORIES_OFFSET);
   return id_ == static_cast<int>(known_category);
 }
 

components/ntp_snippets/category.h

@@ -34,6 +34,7 @@ enum class KnownCategories {
   // Follows the last local category.
   LOCAL_CATEGORIES_COUNT,
 
+  // Remote categories come after this.
   REMOTE_CATEGORIES_OFFSET = 10000,
 
   // Articles for you.
@@ -55,6 +56,7 @@ class Category {
   // the application, so they should not be persisted.
   int id() const { return id_; }
 
+  // Returns whether this category matches the given |known_category|.
   bool IsKnownCategory(KnownCategories known_category) const;
 
  private:

components/ntp_snippets/category_factory.cc

@@ -16,6 +16,9 @@ CategoryFactory::CategoryFactory() {
   AddKnownCategory(KnownCategories::RECENT_TABS);
   AddKnownCategory(KnownCategories::BOOKMARKS);
   AddKnownCategory(KnownCategories::PHYSICAL_WEB_PAGES);
+
+  DCHECK_EQ(static_cast<size_t>(KnownCategories::LOCAL_CATEGORIES_COUNT),
+            ordered_categories_.size());
 }
 
 CategoryFactory::~CategoryFactory() {}

components/ntp_snippets/content_suggestions_metrics.cc

@@ -5,10 +5,12 @@
 #include "components/ntp_snippets/content_suggestions_metrics.h"
 
 #include <string>
+#include <type_traits>
 
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/strings/stringprintf.h"
+#include "base/template_util.h"
 
 namespace ntp_snippets {
 namespace metrics {
@@ -44,17 +46,32 @@ const char kHistogramMoreButtonClicked[] =
 const char kPerCategoryHistogramFormat[] = "%s.%s";
 
 std::string GetCategorySuffix(Category category) {
-  // TODO(treib): Find a way to produce a compile error if a known category
-  // isn't listed here.
-  if (category.IsKnownCategory(KnownCategories::RECENT_TABS))
+  static_assert(
+      std::is_same<decltype(category.id()), typename base::underlying_type<
+                                                KnownCategories>::type>::value,
+      "KnownCategories must have the same underlying type as category.id()");
+  // Note: Since the underlying type of KnownCategories is int, it's legal to
+  // cast from int to KnownCategories, even if the given value isn't listed in
+  // the enumeration. The switch still makes sure that all known values are
+  // listed here.
+  KnownCategories known_category = static_cast<KnownCategories>(category.id());
+  switch (known_category) {
+    case KnownCategories::RECENT_TABS:
       return "RecentTabs";
-  if (category.IsKnownCategory(KnownCategories::DOWNLOADS))
+    case KnownCategories::DOWNLOADS:
       return "Downloads";
-  if (category.IsKnownCategory(KnownCategories::BOOKMARKS))
+    case KnownCategories::BOOKMARKS:
       return "Bookmarks";
-  if (category.IsKnownCategory(KnownCategories::ARTICLES))
+    case KnownCategories::PHYSICAL_WEB_PAGES:
+      return "PhysicalWeb";
+    case KnownCategories::ARTICLES:
       return "Articles";
-  // All other categories go into a single "Experimental" bucket.
+    case KnownCategories::LOCAL_CATEGORIES_COUNT:
+    case KnownCategories::REMOTE_CATEGORIES_OFFSET:
+      NOTREACHED();
+      break;
+  }
+  // All other (unknown) categories go into a single "Experimental" bucket.
   return "Experimental";
 }
 

tools/metrics/histograms/histograms.xml

@@ -100595,6 +100595,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <suffix name="RecentTabs" label="Recent (offline) tabs"/>
   <suffix name="Downloads" label="Downloads"/>
   <suffix name="Bookmarks" label="Bookmarks"/>
+  <suffix name="PhysicalWeb" label="Physical Web pages"/>
   <suffix name="Articles" label="Articles for you"/>
   <suffix name="Experimental" label="Experimental"/>
   <affected-histogram name="NewTabPage.ContentSuggestions.CountOnNtpOpened"/>