List Sorting Invariants

This quick example shows how to test a pure function with property-based techniques. We verify that our stableSort helper keeps the length of the list, preserves the elements, and is idempotent.

import 'package:collection/collection.dart' show IterableEquality;
import 'package:property_testing/property_testing.dart';
import 'package:test/test.dart';

List<int> stableSort(List<int> values) =>
    [...values]..sort((a, b) => a.compareTo(b));

void main() {
  test('stableSort preserves list invariants', () async {
    final listGen = Gen.integer(min: -100, max: 100).list(maxLength: 30);

    final runner = PropertyTestRunner(
      listGen,
      (original) {
        final sorted = stableSort(original);

        expect(sorted.length, equals(original.length),
            reason: 'length mismatch for $original');

        // Ensure both lists contain the same multiset of elements.
        final equality = const IterableEquality<int>();
        final sortedCopy = [...sorted]..sort();
        final originalCopy = [...original]..sort();
        expect(equality.equals(sortedCopy, originalCopy), isTrue,
            reason: 'element mismatch for $original');

        // Sorting twice should produce the same list (idempotence).
        expect(stableSort(sorted), equals(sorted));
      },
      PropertyConfig(numTests: 300),
    );

    final result = await runner.run();
    expect(result.success, isTrue, reason: result.report);
  });
}

Because the generator exercises thousands of permutations of list sizes and values, subtle regressions (duplicates lost, idempotence broken, etc.) will surface immediately with a shrunk counterexample.