Advent of Code 2020 Kotlin

aoc2020-day1.kt

import java.io.File
import java.util.*

// https://rosettacode.org/wiki/Combinations#Kotlin
inline fun <reified T> combinations(arr: List<T>, m: Int) = sequence {
    val n = arr.size
    val result = Array(m) { arr[0] }
    val stack = LinkedList<Int>()
    stack.push(0)
    while (stack.isNotEmpty()) {
        var resIndex = stack.size - 1;
        var arrIndex = stack.pop()

        while (arrIndex < n) {
            result[resIndex++] = arr[arrIndex++]
            stack.push(arrIndex)

            if (resIndex == m) {
                yield(result.toList())
                break
            }
        }
    }
}

fun findExpense(expenses: List<Int>, n: Int): Int {
    val combinations = combinations(expenses, n);
    val tuple = combinations.find() { it.fold(0) { acc, e -> acc + e } == 2020 }!!
    tuple.fold(1) { fac, e -> fac * e }
}

fun main(args: Array<String>) {
    val expenses = File("1.txt")
        .useLines { it.toList() }
        .map { Integer.parseInt(it) }

    var expense1 = findExpense(expenses, 2)
    println(expense1)

    var expense2 = findExpense(expenses, 3)
    println(expense2)
}