カレンダ整形問題：Kotlin版

MonthCalendar.kt

import java.text.SimpleDateFormat
import java.time.LocalDate
import java.time.Month
import java.time.temporal.ChronoUnit
import java.util.GregorianCalendar
import java.util.Locale

fun main() {
    val now = LocalDate.now()
    println(Main.formatMonthCalendar(now.year, now.month))
}

object Main {
    fun formatMonthCalendar(pYear: Int, pMonth: Month): String {
        val fst = LocalDate.of(pYear, pMonth.value, 1)
        val last = fst.plus(1, ChronoUnit.MONTHS).minusDays(1)
        val offset = if (fst.dayOfWeek.value == 0) 1 else fst.dayOfWeek.value

        val head = SimpleDateFormat("MMMMM Y", Locale.ENGLISH).format(GregorianCalendar(pYear, pMonth.value, 1).time)
        val head2 =  "Su Mo Tu We Th Fr Sa"
        return listOf(centering(head, head2.length))
            .plus(listOf(head2))
            .plus((1..offset).map { null }
            .plus(fst.dayOfMonth..last.dayOfMonth)
            .chunked(7).map { week ->
                week.joinToString(separator = " ", transform = ::toS)
            })
            .joinToString(separator = "\n")
    }

    private fun centering(s: String, lineWidth: Int): String {
        val l = (lineWidth - s.length ) / 2
        return (1..l).map { ' ' }.joinToString(separator = "") + s
    }

    private fun toS(day: Int?): String {
        return when {
            day == null -> "  "
            day < 10 -> " $day"
            else -> day.toString()
        }
    }
}