autocomplete [nfc]: Factor out compareNullable

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -261,6 +261,21 @@ class MentionAutocompleteView extends ChangeNotifier {
 
   List<User>? _sortedUsers;
 
+  /// 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.
+  int compareNullable(int? a, int? b) => switch ((a, b)) {
+    (int a, int b) => a.compareTo(b),
+    (int(),     _) => 1,
+    (_,     int()) => -1,
+    _              => 0,
+  };
+
   /// Determines which of the two users is more recent in DM conversations.
   ///
   /// Returns a negative number if [userA] is more recent than [userB],
@@ -272,12 +287,7 @@ 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);
   }
 
   int compareByRelevance({

test/model/autocomplete_test.dart

@@ -465,6 +465,32 @@ void main() {
       view = MentionAutocompleteView.init(store: store, narrow: narrow);
     }
 
+    group('MentionAutocompleteView.compareNullable', () {
+      Future<void> prepareView() => prepare(
+        users: [],
+        dmConversations: [],
+        narrow: const CombinedFeedNarrow(),
+      );
+
+      test('both [a] and [b] are non-null', () async {
+        await prepareView();
+        check(view.compareNullable(2, 5)).isNegative();
+        check(view.compareNullable(5, 2)).isGreaterThan(0);
+        check(view.compareNullable(5, 5)).equals(0);
+      });
+
+      test('one of [a] and [b] is null', () async {
+        await prepareView();
+        check(view.compareNullable(null, 5)).isNegative();
+        check(view.compareNullable(5, null)).isGreaterThan(0);
+      });
+
+      test('both of [a] and [b] are null', () async {
+        await prepareView();
+        check(view.compareNullable(null, null)).equals(0);
+      });
+    });
+
     group('MentionAutocompleteView.compareByDms', () {
       test('has DMs with userA and userB, latest with userA, prioritizes userA', () async {
         await prepare(