Adding weeks, months, and years to Duration

duration.dart

interface Duration default DurationImplementation {
  // exact
  static final int MONTHS_PER_YEAR = 12;
  static final int DAYS_PER_WEEK = 7;
  static final int MINUTES_PER_HOUR = 60;
  static final int SECONDS_PER_MINUTE = 60;
  static final int SECONDS_PER_HOUR = SECONDS_PER_MINUTE * MINUTES_PER_HOUR;
  static final int MILLISECONDS_PER_HOUR = MILLISECONDS_PER_MINUTE * MINUTES_PER_HOUR;
  static final int MILLISECONDS_PER_MINUTE = MILLISECONDS_PER_SECOND * SECONDS_PER_MINUTE;
  static final int MILLISECONDS_PER_SECOND = 1000;

  // standard
  static final int STANDARD_DAYS_PER_YEAR = 365;
  static final int DAYS_PER_LEAP_YEAR = STANDARD_DAYS_PER_YEAR + 1;
  static final int STANDARD_HOURS_PER_DAY = 24;
  static final int STANDARD_MINUTES_PER_DAY = MINUTES_PER_HOUR * STANDARD_HOURS_PER_DAY;
  static final int STANDARD_SECONDS_PER_DAY = SECONDS_PER_HOUR * STANDARD_HOURS_PER_DAY;
  static final int STANDARD_MILLISECONDS_PER_DAY = MILLISECONDS_PER_HOUR * STANDARD_HOURS_PER_DAY;
  
  // average
  static final num AVERAGE_DAYS_PER_YEAR = STANDARD_DAYS_PER_YEAR + 1/4 - 1/100 + 1/400;
  static final num AVERAGE_DAYS_PER_MONTH = AVERAGE_DAYS_PER_YEAR / MONTHS_PER_YEAR;
  static final num AVERAGE_WEEKS_PER_YEAR = AVERAGE_DAYS_PER_YEAR / DAYS_PER_WEEK;
  static final num AVERAGE_WEEKS_PER_MONTH = AVERAGE_DAYS_PER_MONTH / DAYS_PER_WEEK;

  /// normalizes into milliseconds, days, and months fields
  const Duration([
    int milliseconds,
    int seconds, 
    int minutes, 
    int hours, 
    int days, 
    int weeks, 
    int months, 
    int years]);

  final int milliseconds, days, months;

  // non-exact
  int get inMilliseconds();
  int get inSeconds();
  int get inMinutes();
  int get inHours();
  int get inDays();
  int get inWeeks();
  int get inMonths();
  int get inYears();
  
}

class DurationImplementation {
  const DurationImplementation([
    int years = 0, 
    int months = 0, 
    int weeks = 0, 
    int days = 0, 
    int hours = 0, 
    int minutes = 0, 
    int seconds = 0, 
    int milliseconds = 0]) : 

    months = years * Duration.MONTHS_PER_YEAR + months,

    days = weeks * Duration.DAYS_PER_WEEK + days,

    milliseconds = 
      hours * Duration.MILLISECONDS_PER_HOUR +
      minutes * Duration.MILLISECONDS_PER_MINUTE +
      seconds * Duration.MILLISECONDS_PER_SECOND +
      milliseconds;
  
  final int milliseconds, days, months;

  int get inMilliseconds() => _inMilliseconds.floor();
  int get inSeconds() => _inSeconds.floor();
  int get inMinutes() => _inMinutes.floor();
  int get inHours() => _inHours.floor();
  int get inDays() => _inDays.floor();
  int get inWeeks() => _inWeeks.floor();
  int get inMonths() => _inMonths.floor();
  int get inYears() => _inYears.floor();
  
  num get _inMilliseconds() => milliseconds + (days + _monthsAsDays) * Duration.STANDARD_MILLISECONDS_PER_DAY;
  num get _inSeconds() => _inMilliseconds / Duration.MILLISECONDS_PER_SECOND;
  num get _inMinutes() => _inMilliseconds / Duration.MILLISECONDS_PER_MINUTE;
  num get _inHours() => _inMilliseconds / Duration.MILLISECONDS_PER_HOUR;
  num get _inDays() => days + _monthsAsDays + _millisecondsAsDays;
  num get _inWeeks() => _inDays / Duration.DAYS_PER_WEEK;
  num get _inMonths() => months + (days + _millisecondsAsDays) / Duration.AVERAGE_DAYS_PER_MONTH;
  num get _inYears() => _inMonths / Duration.MONTHS_PER_YEAR;

  num get _monthsAsDays() => months * Duration.AVERAGE_DAYS_PER_MONTH;
  num get _millisecondsAsDays() => milliseconds / Duration.STANDARD_MILLISECONDS_PER_DAY;

}