AoC 2020 Day 1

main.kt

import java.io.File


fun main(args: Array<String>) {
    val result = DayOneTask.run()
    println(result)
}


object DayOneTask {

    fun run(): Int {
        val inputData = File("input")
            .readLines()
            .map(String::toInt)
        return (0 until inputData.size - 2)
            .mapNotNull { firstPosition ->
                val first = inputData[firstPosition]
                val lookup = 2020 - first
                (firstPosition until inputData.size - 3)
                    .mapNotNull { secondPosition ->
                        inputData
                            .drop(secondPosition)
                            .headWithLeftover(lookup)
                            { left, right ->
                                left * right
                            }
                            ?.let { it * first }
                    }.firstOrNull()
            }
            .first()
    }


    private fun List<Int>.headWithLeftover(
        target: Int,
        f: (Int, Int) -> Int
    ):
            Int? {

        if (size < 2) {
            return null
        }

        val head = first()
        val lookup = target - head
        return if (
            lookup > 0 &&
            drop(1)
                .contains(lookup)
        ) {
            f(head, lookup)
        } else {
            null
        }

    }
}