addWeekDay method to DateTime

[FMorschel]

Maybe creating an addWorkDay method to DateTime? I suggest something like the following (edited, found a broken case on the first example - when starting on weekends it would add another day-, and added the subtractWorkDays method):

extension AddWorkDays on DateTime {

  bool get isWeekend =>
      (weekday == DateTime.saturday) || (weekday == DateTime.sunday);

  bool get isWorkDay => !isWeekend;

  bool get _notFridayNorSaturday =>
      (weekday != DateTime.friday) && (weekday != DateTime.saturday);

  DateTime addWorkDays(int days) {
    final dayToAdd = (days.isNegative ? -1 : 1).days;
    if (isWorkDay && (days == 0)) return this;
    if (days.isNegative) return subtractWorkDays(-days);
    if (_notFridayNorSaturday) {
      return add(dayToAdd).addWorkDays(days - 1);
    } else {
      return add(dayToAdd).addWorkDays(days);
    }
  }

  bool get _notMondayNorSunday =>
      (weekday != DateTime.monday) && (weekday != DateTime.sunday);

  DateTime subtractWorkDays(int days) {
    if (isWorkDay && (days == 0)) return this;
    if (days.isNegative) return addWorkDays(-days);
    final dayToAdd = (days.isNegative ? -1 : 1).days;
    if (_notMondayNorSunday) {
      return subtract(dayToAdd).subtractWorkDays(days - 1);
    } else {
      return subtract(dayToAdd).subtractWorkDays(days);
    }
  }
}

I'm not sure how useful this would be and about the duration days being added or if it would be better to create another DateTime with the day field increased/decreased by one every time.

[jogboms]

This is nice. I particularly like the isWeekend and isWorkDay.

There is no problem AFAIK with using add and subtract as they would also create a new DateTime every time.

[FMorschel]

I have some doubts:

• About the parameters being int, do you think that's the best approach? Have any suggestions on this?
• Do you think this would be useful?
• Do you think this should be added to the package even though it's not as useful? (For answering this consider my thoughts stated next)

Thoughts:

I was wondering if maybe we could create a new class or an easy-to-use method so that instead of this, we could say which days inside the week the package user wants to skip when adding/subtracting days.

[jogboms]

Well,

• I think keeping ints is good for now and if the need for fractions comes up we could easily use a num without introducing a breaking change.
• isWeekend & isWorkDay I believe I would use. Still, I'm not sure of the value of addWorkDays & subtractWorkDays for the average joe as it is peculiar to a kind of application. I would not like to bloat the API unless requested by maybe a couple of people.

I believe the second bullet should address the third doubt and the final thoughts on bloating the API layer

[FMorschel]

As per your comment on #55, I believe this should be closed. Do you think the isWeekend and isWorkDay should be implemented on a PR?

[jogboms]

I think you can, it sounds good to me @FMorschel