#!/usr/bin/env dub
/+ dub.sdl:
    name "main"
    dependency "mir-algorithm" version="~>3.10.12"
+/

import std;
import mir.combinatorics;

void main()
{
    File("input", "r").byLine.map!(to!int).array.multiplyNEntriesThatSumTo(3, 2020).writeln;
}

alias product = partial!(reverseArgs!(fold!((a, b) => a * b)), 1);

int multiplyNEntriesThatSumTo(int[] input, int n, int requiredSum)
{
    return input.combinations(n).filter!(combination => combination.sum == requiredSum)
        .map!(combination => combination.product)
        .front;
}

unittest
{
    auto input = [1721, 979, 366, 299, 675, 1456];

    assert(input.multiplyNEntriesThatSumTo(3, 2020) == 241_861_950);
}