Day02.kt

fun checksum(spreadsheet: String, lineSummer: (List<Int>) -> Int = ::minAndMax): Int {
    val lines = spreadsheet.split("\n")
    val rows: List<List<Int>> = lines.map { line -> line.split(Regex("""\s""")).map { num -> Integer.valueOf(num) } }

    return rows.fold(0, { sum: Int, list ->  sum + lineSummer(list)})
}

fun minAndMax(list: List<Int>): Int {
    return abs((list.max() ?: 0)  - (list.min() ?: 0))
}

fun evenlyDivides(list: List<Int>): Int {
    val pair : Pair<Int,Int>? = allPairs(list).find { it -> it.first % it.second == 0 || it.second % it.first == 0 }
    return when {
        pair == null -> 0
        pair.first > pair.second -> pair.first / pair.second
        else -> pair.second / pair.first
    }
}

fun <T> allPairs(list: List<T>): List<Pair<T, T>> {
    val pairs = arrayListOf<Pair<T, T>>()
    for ((index, left) in list.withIndex()) {
        if (index != list.size - 1) {
            list.subList(index + 1, list.size).mapTo(pairs) { left to it }
        }
    }
    return pairs
}

Day02Spec.kt

object Day02Spec : Spek({

    given("Day Two : Part One") {
        val spreadsheet = """5 1 9 5
                            .7 5 3
                            .2 4 6 8""".trimMargin(".")
        on("checksum") {
            assertEquals(18,checksum(spreadsheet))
        }
    }

    given("Day Two : Part Two") {
        val spreadsheet = """5 9 2 8
                            .9 4 7 3
                            .3 8 6 5""".trimMargin(".")
        on("checksum with passed in function") {
            assertEquals(9,checksum(spreadsheet, ::evenlyDivides))
        }
    }
})