Get specific weekday in month (null-safety version)

main.dart

void main() {
  print(getSpecificWeekdayInMonth(
    year: 2020,
    month: 8,
    whichWeek: 5,
    whichWeekday: 7,
  ));
}

DateTime? getSpecificWeekdayInMonth({
  required int year,
  required int month,
  required int whichWeek,
  required int whichWeekday,
}) {
  if (year < 1 || month < 1 || whichWeek < 1 || whichWeekday < 1) {
    throw ArgumentError('Everthing must more than 1');
  }

  final DateTime baseDate = DateTime(year, month, 1);

  /// Use first monday for the date offset.
  DateTime firstMonday;
  if (baseDate.weekday == 1) {
    firstMonday = baseDate;
  } else {
    firstMonday = baseDate.add(Duration(days: 8 - baseDate.weekday));
  }

  /// Return if requested for first monday.
  if (whichWeek == 1 && whichWeekday == 1) {
    return firstMonday;
  }

  /// Reduce 1 day for validation.
  final Duration offsetDuration = Duration(
    days: (whichWeek - 1) * 7 + whichWeekday - 1,
  );
  final DateTime calcDate = firstMonday.add(offsetDuration);

  /// If the result is out of the constraint, return nothing.
  if (calcDate.year != year || calcDate.month != month) {
    return null;
  }
  return calcDate;
}