BerlinClock.kt

import java.time.LocalTime
import java.time.temporal.ChronoUnit

fun main() {
    val time = LocalTime.now().truncatedTo(ChronoUnit.SECONDS)
    val clock = time.toBerlinClock()
    println("$time => $clock")
}

const val Off = "O"
const val Red = "R"
const val Yellow = "Y"

fun LocalTime.toBerlinClock(): String {
    val seconds = if (second % 2 == 0) Yellow else Off
    val hours = (hour / 5 to hour % 5).let { (first, second) ->
        Array(4) { if (it < first) Red else Off } + Array(4) { if (it < second) Red else Off }
    }
    val minutes = (minute / 5 to minute % 5).let { (first, second) ->
        Array(11) { if (it < first) if (it % 4 == 3) Red else Yellow else Off } + Array(4) { if (it < second) Yellow else Off }
    }
    return (listOf(seconds) + hours + minutes).joinToString(separator = "")
}