Allow getting the week number of a LocalDate and a LocalDateTime

Hello,

I think it’d be great to have natively the ability to know which week number a particular date is from. It’s very easy to do with GregorianCalendar in the JVM or Android (calendar.get(Calendar.WEEK_OF_YEAR)), but I see no such facility for Kotlin common code yet.

Have a great day!

[dkhalanskyjb]

Let’s go back to the use case of checking if two dates belong to the same week. How would one implement that, if we did provide the week numbering?

// wrong attempt 1
fun LocalDate.sameWeekAs(other: LocalDate) =
    weekNumber(weekStart = DayOfWeek.MONDAY) ==
    other.weekNumber(weekStart = DayOfWeek.MONDAY)

Obviously, this won’t work if the dates are from different years. Jan 1st, 2007 is not the same week as Jan 1st, 2023.

// wrong attempt 2
fun LocalDate.sameWeekAs(other: LocalDate) =
    year == other.year &&
    weekNumber(weekStart = DayOfWeek.MONDAY) ==
    other.weekNumber(weekStart = DayOfWeek.MONDAY)

This is much less obviously wrong, but still wrong since some weeks are shared by two years.

// wrong attempt 3
fun LocalDate.sameWeekAs(other: LocalDate) =
    monthsUntil(other).absoluteValue < 12 &&
    weekNumber(weekStart = DayOfWeek.MONDAY) ==
    other.weekNumber(weekStart = DayOfWeek.MONDAY)

Now, this will almost always work, but not always! For example, Jan 1st, 2007, is week 1, but Dec 31st, 2007, is also week 1. The reason is that, by the ISO-8601 numbering, if a week is split between two years, it is assigned a number in terms of the year that has the bigger part of the week.

I don’t see a simple way to check if two dates are in the same week using a week numbering. Here’s a working approach though:

fun LocalDate.sameWeekAs(other: LocalDate, firstDayOfWeek: DayOfWeek): Boolean {
    fun LocalDate.firstDayInWeek(): LocalDate =
        minus((dayOfWeek.isoDayNumber - firstDayOfWeek.isoDayNumber).mod(7), DateTimeUnit.DAY)
    return firstDayInWeek() == other.firstDayInWeek()
}

Maybe this is the approach we should be expanding, especially if operations like “find next Monday” or “find last day before this one that was in January” have some other uses.

[4shutosh]

One use case that I can think of is check if two dates belong to same week, to show a header or something like “This week”. Although I guess this can be achieved by floorDiv of dayOfYear by 7.