DayOne.kt

fun captcha(input: String, stepping: Int = 1): Int {
    val ints = input.map { Character.getNumericValue(it) }

    fun calcValueAtStepping(currentIndex: Int) : Int {
        val wrappedIndex = currentIndex + stepping
        return if (wrappedIndex < ints.size - 1) ints[wrappedIndex] else ints[wrappedIndex % ints.size]
    }

    var sum = 0
    for ((index, current) in ints.withIndex()) {
        sum += if (current == calcValueAtStepping(index)) current else 0
    }
    return sum
}