/*
--- Day 1: Report Repair ---

After saving Christmas five years in a row, you've decided to take a vacation at a nice resort on a
tropical island. Surely, Christmas will go on without you.

The tropical island has its own currency and is entirely cash-only. The gold coins used there have a
little picture of a starfish; the locals just call them stars. None of the currency exchanges seem
to have heard of them, but somehow, you'll need to find fifty of these coins by the time you arrive
so you can pay the deposit on your room.

To save your vacation, you need to get all fifty stars by December 25th.

Collect stars by solving puzzles. Two puzzles will be made available on each day in the Advent
calendar; the second puzzle is unlocked when you complete the first. Each puzzle grants one star.
Good luck!

Before you leave, the Elves in accounting just need you to fix your expense report (your puzzle
input); apparently, something isn't quite adding up.

Specifically, they need you to find the two entries that sum to 2020 and then multiply those two
numbers together.

For example, suppose your expense report contained the following:

1721
979
366
299
675
1456

In this list, the two entries that sum to 2020 are 1721 and 299. Multiplying them together produces
1721 * 299 = 514579, so the correct answer is 514579.

Of course, your expense report is much larger. Find the two entries that sum to 2020; what do you
get if you multiply them together?
*/

package com.curtislb.adventofcode.year2020.day01.part1

import com.curtislb.adventofcode.common.search.findPairSum
import java.nio.file.Path
import java.nio.file.Paths

/**
 * Returns the solution to the puzzle for 2020, day 1, part 1.
 *
 * @param inputPath The path to the input file for this puzzle.
 */
fun solve(inputPath: Path = Paths.get("..", "input", "input.txt")): Int? {
    val file = inputPath.toFile()
    val entries = file.readLines().map { it.toInt() }
    return entries.findPairSum(2020)?.let { it.first * it.second }
}

fun main() = when (val solution = solve()) {
    null -> println("No solution found.")
    else -> println(solution)
}
