// Standalone teaching demo. Timers simulate I/O; no real API is called.
Future<String> fetchProfile() async {
  await Future<void>.delayed(const Duration(milliseconds: 600));
  return 'Thura';
}

Future<String> fetchAnnouncements() async {
  await Future<void>.delayed(const Duration(milliseconds: 900));
  return '2 announcements';
}

Future<List<String>> loadSequentially() async {
  final profile = await fetchProfile();
  final announcements = await fetchAnnouncements();
  return [profile, announcements];
}

Future<List<String>> loadConcurrently() {
  return Future.wait<String>([
    fetchProfile(),
    fetchAnnouncements(),
  ]);
}

Future<void> main() async {
  for (final entry in {
    'Sequential': loadSequentially,
    'Concurrent': loadConcurrently,
  }.entries) {
    final stopwatch = Stopwatch()..start();
    final result = await entry.value();
    stopwatch.stop();
    if (result.join('|') != 'Thura|2 announcements') {
      throw StateError('Unexpected results or input ordering');
    }
    print('${entry.key}: ${stopwatch.elapsedMilliseconds} ms; $result');
  }

  // Verify default wait behavior when a faster operation fails.
  final events = <String>[];
  try {
    await Future.wait<String>([
      Future<String>.delayed(const Duration(milliseconds: 100), () {
        events.add('slow operation completed');
        return 'ok';
      }),
      Future<String>.delayed(const Duration(milliseconds: 20), () {
        events.add('fast operation failed');
        throw StateError('Simulated failure');
      }),
    ]);
  } on StateError {
    events.add('caller caught error');
  }
  const expected = 'fast operation failed|slow operation completed|caller caught error';
  if (events.join('|') != expected) throw StateError('Unexpected error timing');
  print('Error path: ${events.join(' -> ')}');
}