digitalRoot.kt

fun main() {
    val numbers = listOf(
        12,
        7476589,
        97462994,
        37837,
        38840184,
        2848493,
        1847479
    )

    numbers.map {
        val root = digitalRoot(it)
        println("${root} is the digital root of ${it}") }
}

// keep summing the digits until there is only one digit left
fun digitalRoot(n: Int): Int = when(n) {
    in 0..9 -> n
    else -> digitalRoot(digits(n).reduce { acc, i -> acc + i })
}

// helper function to get a list of digits
fun digits(n: Int): List<Int> {
    return n
        .toString()
        .toList()
        .map { it
            .toString()
            .toInt()
        }
}

output.txt

3 is the digital root of 12
1 is the digital root of 7476589
5 is the digital root of 97462994
1 is the digital root of 37837
9 is the digital root of 38840184
2 is the digital root of 2848493
4 is the digital root of 1847479