fix: Make sure it is sorted after removals that make bucket empty (#23)

After discovering an issue with priority sorting in Flame, I dug down until I was able to narrow the issue down to the OrderedSet instance. In this specific situation, it appears the set does not maintain ordering between elements.

It seems that remove failed to remove empty buckets when it was the last element in the bucket, resulting in an invalid ordering between buckets in specific situations. I modified remove to delete empty sets instead.

Aside from making the new test pass, the code actually broke an existing test. On reading it, I believe the expected condition was incorrect. I fixed the test to the actual expected condition, allowing it to pass.

lib/ordered_set.dart

@@ -156,7 +156,8 @@ class OrderedSet<E> extends IterableMixin<E> {
     final result = bucket.remove(e);
     if (result) {
       _length--;
-      _backingSet.remove(<E>[]);
+      // If the removal resulted in an empty bucket, remove the bucket as well.
+      _backingSet.remove(<E>{});
       _validReverseCache = false;
     }
     return result;

test/ordered_set_test.dart

@@ -315,7 +315,18 @@ void main() {
         expect(orderedSet.toList().join(), 'bd');
         orderedSet.addAll([d, b, a, c]);
         expect(orderedSet.removeAll([d, b]).join(), 'db');
-        expect(orderedSet.toList().join(), 'abc');
+        expect(orderedSet.toList().join(), 'ac');
+      });
+
+      test('sorts after remove', () {
+        final orderedSet = OrderedSet<int>();
+        // The initial elements must be in order.
+        orderedSet.addAll([1, 3, 4]);
+        expect(orderedSet.toList().join(), '134');
+        expect(orderedSet.remove(4), true);
+        expect(orderedSet.toList().join(), '13');
+        expect(orderedSet.add(2), true);
+        expect(orderedSet.toList().join(), '123');
       });
     });