Question

Kotlin Language : How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

Answer 1

Here is your solution:

fun sundaysInTwentCentury(): Int {
    fun daysInMon(month: Int, year: Int) = when(month) {
        1,3,5,7,8,10,12 -> 31
        4,6,9,11 -> 30
        2 -> when {
            year % 400 == 0 -> 29
            year % 100 == 0 -> 28
            year % 4 == 0 -> 29
            else -> 28
        }
        else -> throw Exception("Invalid Month ${month}")
    }
    fun daysInYears(year: Int): Int {
        var days = 0
        for(month in 1..12) {
            days += daysInMon(month, year)
        }
        return days
    }
    fun dayOfWeekAfter(currentDow: Int, numberOfDays: Int) = ((currentDow-1) + numberOfDays) % 7 + 1
    var weekDayOnFirstOfMonth = dayOfWeekAfter(2, daysInYears(1900))
    var totalSundays = 0
    for (year in 1901..2000) {
        for(month in 1..12) {
            if (weekDayOnFirstOfMonth == 1) totalSundays++
            weekDayOnFirstOfMonth = dayOfWeekAfter(weekDayOnFirstOfMonth, daysInMon(month,year))
        }
    }
    return totalSundays
}