feat: 5095 - matomo anonymous visitor id now starts with a persistent…

… letter (openfoodfacts#5107)

Impacted files:
* `analytics_helper.dart`: new getter `_anonymousVisitorId`; refactoring
* `main.dart`: minor refactoring
* `user_preferences.dart`: new method `getUniqueRandom()` that is used to compute and store the first letter of the anonymous visitor ID.

packages/smooth_app/lib/data_models/preferences/user_preferences.dart

@@ -75,6 +75,7 @@ class UserPreferences extends ChangeNotifier {
   static const String _TAG_CRASH_REPORTS = 'crash_reports';
   static const String _TAG_EXCLUDED_ATTRIBUTE_IDS = 'excluded_attributes';
   static const String _TAG_USER_GROUP = '_user_group';
+  static const String _TAG_UNIQUE_RANDOM = '_unique_random';
 
   /// Camera preferences
 
@@ -171,6 +172,18 @@ class UserPreferences extends ChangeNotifier {
   /// A random int between 0 and 10 (a naive implementation to allow A/B testing)
   int get userGroup => _sharedPreferences.getInt(_TAG_USER_GROUP)!;
 
+  /// Returns a huge random value that will be computed just once.
+  Future<int> getUniqueRandom() async {
+    const String tag = _TAG_UNIQUE_RANDOM;
+    int? result = _sharedPreferences.getInt(tag);
+    if (result != null) {
+      return result;
+    }
+    result = Random().nextInt(1 << 32);
+    await _sharedPreferences.setInt(tag, result);
+    return result;
+  }
+
   Future<void> setCrashReports(final bool state) async {
     await _sharedPreferences.setBool(_TAG_CRASH_REPORTS, state);
     onCrashReportingChanged.value = state;

packages/smooth_app/lib/helpers/analytics_helper.dart

@@ -193,7 +193,9 @@ class AnalyticsHelper {
 
   static String latestSearch = '';
 
-  static void linkPreferences(UserPreferences userPreferences) {
+  static late int _uniqueRandom;
+
+  static Future<void> linkPreferences(UserPreferences userPreferences) async {
     // Init the value
     _setAnalyticsReports(userPreferences.onAnalyticsChanged.value);
     _setCrashReports(userPreferences.onCrashReportingChanged.value);
@@ -206,6 +208,8 @@ class AnalyticsHelper {
     userPreferences.onCrashReportingChanged.addListener(() {
       _setCrashReports(userPreferences.onCrashReportingChanged.value);
     });
+
+    _uniqueRandom = await userPreferences.getUniqueRandom();
   }
 
   static Future<void> initSentry({
@@ -253,7 +257,7 @@ class AnalyticsHelper {
     }
 
     if (MatomoTracker.instance.initialized) {
-      MatomoTracker.instance.setVisitorUserId(_uuid);
+      MatomoTracker.instance.setVisitorUserId(_visitorId);
     }
   }
 
@@ -281,30 +285,37 @@ class AnalyticsHelper {
       await MatomoTracker.instance.initialize(
         url: 'https://analytics.openfoodfacts.org/matomo.php',
         siteId: 2,
-        visitorId: _uuid,
+        visitorId: _visitorId,
       );
     } catch (err) {
       // With Hot Reload, this may trigger a late field already initialized
     }
   }
 
-  /// A UUID must be at least one 16 characters
-  static String? get _uuid {
+  /// A visitor id should have a length of 16 characters.
+  static String? get _visitorId {
     // if user opts out then track anonymously with userId containing zeros
     if (kDebugMode) {
       return 'smoothie_debug--';
     }
 
     switch (_analyticsReporting) {
       case _AnalyticsTrackingMode.anonymous:
-        return '0' * 16;
+        return _anonymousVisitorId;
       case _AnalyticsTrackingMode.disabled:
         return '';
       case _AnalyticsTrackingMode.enabled:
         return OpenFoodAPIConfiguration.uuid;
     }
   }
 
+  /// Returns a unique visitor id that starts with a letter between A and Z.
+  static String get _anonymousVisitorId => _uniqueLetter + ('0' * 15);
+
+  /// Returns a letter between A and Z, depending on [_uniqueRandom].
+  static String get _uniqueLetter =>
+      String.fromCharCode('A'.codeUnitAt(0) + _uniqueRandom % 26);
+
   static void trackEvent(
     AnalyticsEvent msg, {
     int? eventValue,

packages/smooth_app/lib/main.dart

@@ -138,7 +138,7 @@ Future<bool> _init1() async {
   );
   UserManagementProvider().checkUserLoginValidity();
 
-  AnalyticsHelper.linkPreferences(_userPreferences);
+  await AnalyticsHelper.linkPreferences(_userPreferences);
 
   await ProductQuery.initCountry(_userPreferences);
   _themeProvider = ThemeProvider(_userPreferences);