Record default search engine changes.

Add a P3A probe to record when the default search engine is changed,
and whether it's a transition between Brave Search or one of the
other major services. This helps understand what options are seem
better to users.

Closes brave/brave-browser#18224

browser/search_engines/search_engine_tracker.cc

@@ -44,6 +44,40 @@ void RecordSearchEngineP3A(const GURL& search_engine_url,
   UMA_HISTOGRAM_ENUMERATION(kDefaultSearchEngineMetric, answer);
 }
 
+void RecordSwitchP3A(const GURL& to_url, const GURL& from_url) {
+  auto answer = SearchEngineSwitchP3A::kNoSwitch;
+
+  auto to = to_url.host_piece();
+  auto from = from_url.host_piece();
+  auto ncase = base::CompareCase::INSENSITIVE_ASCII;
+  if (base::EndsWith(from, "brave.com", ncase)) {
+    // Switching away from Brave Search.
+    if (base::EndsWith(to, "brave.com", ncase)) {
+      answer = SearchEngineSwitchP3A::kBraveToBrave;
+    } else if (base::EndsWith(to, "google.com", ncase)) {
+      answer = SearchEngineSwitchP3A::kBraveToGoogle;
+    } else if (base::EndsWith(to, "duckduckgo.com", ncase)) {
+      answer = SearchEngineSwitchP3A::kBraveToDDG;
+    } else {
+      answer = SearchEngineSwitchP3A::kBraveToOther;
+    }
+  } else if (base::EndsWith(to, "brave.com", ncase)) {
+    // Switching to Brave Search.
+    if (base::EndsWith(from, "google.com", ncase)) {
+      answer = SearchEngineSwitchP3A::kGoogleToBrave;
+    } else if (base::EndsWith(from, "duckduckgo.com", ncase)) {
+      answer = SearchEngineSwitchP3A::kDDGToBrave;
+    } else {
+      answer = SearchEngineSwitchP3A::kOtherToBrave;
+    }
+  } else {
+    // Any other transition.
+    answer = SearchEngineSwitchP3A::kOtherToOther;
+  }
+
+  UMA_HISTOGRAM_ENUMERATION(kSwitchSearchEngineMetric, answer);
+}
+
 }  // namespace
 
 // static
@@ -105,6 +139,7 @@ void SearchEngineTracker::OnTemplateURLServiceChanged() {
     const GURL& url = template_url->GenerateSearchURL(search_terms);
     if (url != default_search_url_) {
       RecordSearchEngineP3A(url, template_url->GetEngineType(search_terms));
+      RecordSwitchP3A(url, default_search_url_);
     }
   }
 }

browser/search_engines/search_engine_tracker.h

@@ -17,6 +17,7 @@
 
 // Exposed for tests.
 constexpr char kDefaultSearchEngineMetric[] = "Brave.Search.DefaultEngine.4";
+constexpr char kSwitchSearchEngineMetric[] = "Brave.Search.SwitchEngine";
 
 // Note: append-only enumeration! Never remove any existing values, as this enum
 // is used to bucket a UMA histogram, and removing values breaks that.
@@ -33,6 +34,19 @@ enum class SearchEngineP3A {
   kMaxValue = kBrave,
 };
 
+enum class SearchEngineSwitchP3A {
+  kNoSwitch,
+  kBraveToBrave,
+  kBraveToGoogle,
+  kBraveToDDG,
+  kBraveToOther,
+  kGoogleToBrave,
+  kDDGToBrave,
+  kOtherToBrave,
+  kOtherToOther,
+  kMaxValue = kOtherToOther,
+};
+
 class SearchEngineTrackerFactory : public BrowserContextKeyedServiceFactory {
  public:
   static SearchEngineTrackerFactory* GetInstance();

components/p3a/brave_p3a_service.cc

@@ -82,6 +82,7 @@ constexpr const char* kCollectedHistograms[] = {
     "Brave.Rewards.WalletState",
     "Brave.Savings.BandwidthSavingsMB",
     "Brave.Search.DefaultEngine.4",
+    "Brave.Search.SwitchEngine",
     "Brave.Shields.UsageStatus",
     "Brave.SpeedReader.Enabled",
     "Brave.SpeedReader.ToggleCount",