Week.forDate constructor

Implementation

factory Week.forDate(DateTime date) {
  assert(date.debugCheckIsValidTimetableDate());

  // Algorithm from https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_from_a_month_and_day_of_the_month_or_ordinal_date
  final year = date.year;
  final weekOfYear = (date.dayOfYear + 10 - date.weekday) ~/ 7;

  if (weekOfYear == 0) {
    // If the week number thus obtained equals 0, it means that the given date
    // belongs to the preceding (week-based) year.
    final weekOfYear =
        DateTimeTimetable.date(year - 1, 12, 31).week.weekOfYear;
    return Week(year - 1, weekOfYear);
  }

  if (weekOfYear == 53 &&
      DateTime(year, 12, 31).weekday < DateTime.thursday) {
    // If a week number of 53 is obtained, one must check that the date is not
    // actually in week 1 of the following year.
    return Week(year + 1, 1);
  }

  return Week(year, weekOfYear);
}