Example implementation of Swiss Bank Holidays in Kotlin

BankHolidays.kt

import BankHoliday.Companion.isBankHoliday
import java.time.LocalDate
import java.time.Year

fun main(args: Array<String>) {

    // some tests (should be done using JUnit of course ;)
    mapOf(
        LocalDate.of(2023, 1, 1) to true,
        LocalDate.of(2023, 8, 1) to true,
        LocalDate.of(2023, 12, 25) to true,
        LocalDate.of(2023, 12, 26) to true,
        LocalDate.of(2023, 4, 9) to true,
        LocalDate.of(2023, 4, 7) to true,
        LocalDate.of(2023, 9, 5) to false,
        LocalDate.of(2023, 1, 2) to false
    ).forEach { (date, assumption) ->
        val res = if (date.isBankHoliday() == assumption) "✓" else "×"
        println("Asserting that $date isBankHoliday should be $assumption: $res")
    }
}

// Sealed supertype which can only be implemented from the same file / package
sealed class BankHoliday(
    val date: LocalDate
) {

    companion object {

        private fun holidays(year: Year) = listOf(
            Easter(year),
            AscensionDay(year),
            EasterMonday(year),
            GoodFriday(year),
            WhitMonday(year),
            BoxingDay(year),
            ChristmasDay(year),
            FirstOfJanuary(year),
            NationalDay(year)
        )

        fun LocalDate.isBankHoliday(): Boolean =
            holidays(Year.of(this.year)).any { h ->
                this.compareTo(h.date) == 0
            }
    }
}

class Easter(year: Year) : BankHoliday(calculate(year)) {

    companion object {

        private fun calculate(year: Year): LocalDate {
            val y = year.value
            val n = y % 19
            val c = y / 100
            val u = y % 100
            val s = c / 4
            val t = c % 4
            val p = (c + 8) / 25
            val q = (c - p + 1) / 3
            val e = (19 * n + c - s - q + 15) % 30
            val b = u / 4
            val d = u % 4
            val L = (2 * t + 2 * b - e - d + 32) % 7
            val h = (n + 11 * e + 22 * L) / 451
            val calculatedMonth = (e + L - 7 * h + 114) / 31
            val calculatedDay = (e + L - 7 * h + 114) % 31 + 1

            return LocalDate.of(y, calculatedMonth, calculatedDay)
        }

    }
}

// Complex dates based on easter
class AscensionDay(year: Year) : BankHoliday(
    Easter(year)
        .date
        .plusDays(39)
)

class EasterMonday(year: Year) : BankHoliday(
    Easter(year)
        .date
        .plusDays(1)
)

class GoodFriday(year: Year) : BankHoliday(
    Easter(year)
        .date
        .minusDays(2)
)

class WhitMonday(year: Year) : BankHoliday(
    Easter(year)
        .date
        .plusDays(50)
)

// Simple dates
class BoxingDay(year: Year) : BankHoliday(
    year.atMonth(12).atDay(26)
)

class ChristmasDay(year: Year) : BankHoliday(
    year.atMonth(12).atDay(25)
)

class FirstOfJanuary(year: Year) : BankHoliday(
    year.atMonth(1).atDay(1)
)

class NationalDay(year: Year) : BankHoliday(
    year.atMonth(8).atDay(1)
)