duration_plus.dart

enum Timespan {
  year,
  month,
  week,
  day,
  hour,
  minute,
  moment;

  String get plural => '${name}s';
  String get singleItem => '1 $name';
  String nItems(int n) =>
      this == moment ? 'just a moment' : (n == 1 ? singleItem : '$n $plural');
}

typedef TimeAmount = ({Timespan span, int count});

extension on Duration {
  int get inYears => (inDays / 365).floor();
  int get inMonths => (inDays / 30).floor();
  int get inWeeks => (inDays / 7).floor();

  int convert(Timespan t) {
    return switch (t) {
      Timespan.year => inYears,
      Timespan.month => inMonths,
      Timespan.week => inWeeks,
      Timespan.day => inDays,
      Timespan.hour => inHours,
      Timespan.minute => inMinutes,
      Timespan.moment => 1,
    };
  }

  String toCourseWordsString() {
    assert(!isNegative, "Doesn't work with negative durations");
    final pair = Timespan.values
        .map<TimeAmount>((span) => (span: span, count: convert(span)))
        .firstWhere((pair) => pair.count > 0);
    return pair.span.nItems(pair.count);
  }
}