autocomplete [nfc]: Factor out compareNullable

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -204,14 +204,24 @@ class MentionAutocompleteView extends ChangeNotifier {
     final aLatestMessageId = recentDms.latestMessagesByRecipient[userA.userId];
     final bLatestMessageId = recentDms.latestMessagesByRecipient[userB.userId];
 
-    return switch((aLatestMessageId, bLatestMessageId)) {
-      (int a, int b) => -a.compareTo(b),
-      (int(),     _) => -1,
-      (_,     int()) => 1,
-      _              => 0,
-    };
+    return -compareNullable(aLatestMessageId, bLatestMessageId);
   }
 
+  /// Compares [a] to [b].
+  ///
+  /// If both are non-null, returns [a.compareTo(b)].
+  ///
+  /// If one of them is null, returns a negative number if [a] is null and [b] is
+  /// non-null and returns a positive number if [b] is null and [a] is non-null.
+  ///
+  /// If both are null, returns zero.
+  static int compareNullable(int? a, int? b) => switch ((a, b)) {
+    (int a, int b) => a.compareTo(b),
+    (int(),     _) => 1,
+    (_,     int()) => -1,
+    _              => 0,
+  };
+
   @override
   void dispose() {
     store.autocompleteViewManager.unregisterMentionAutocomplete(this);

test/model/autocomplete_test.dart

@@ -364,6 +364,23 @@ void main() {
       await store.addUsers(users);
     }
 
+    group('MentionAutocompleteView.compareNullable', () {
+      test('both [a] and [b] are non-null', () async {
+        check(MentionAutocompleteView.compareNullable(2, 5)).isLessThan(0);
+        check(MentionAutocompleteView.compareNullable(5, 2)).isGreaterThan(0);
+        check(MentionAutocompleteView.compareNullable(5, 5)).equals(0);
+      });
+
+      test('one of [a] and [b] is null', () async {
+        check(MentionAutocompleteView.compareNullable(null, 5)).isLessThan(0);
+        check(MentionAutocompleteView.compareNullable(5, null)).isGreaterThan(0);
+      });
+
+      test('both of [a] and [b] are null', () async {
+        check(MentionAutocompleteView.compareNullable(null, null)).equals(0);
+      });
+    });
+
     group('MentionAutocompleteView.compareByDms', () {
       const idA = 1;
       const idB = 2;