Discussion on: Advent of Code 2019 Solution Megathread - Day 1: The Tyranny of the Rocket Equation

[Jon Bristow]

My kotlin solution.

import java.nio.file.Files
import java.nio.file.Paths
import kotlin.math.max

private const val FILENAME = "src/main/resources/day01.txt"

fun fuelNeeded(mass: Int) = max((mass / 3) - 2, 0)


fun totalFuelNeeded(mass: Int): Int {
    var fuel = fuelNeeded(mass)
    var totalFuel = fuel
    while (fuel != 0) {
        fuel = fuelNeeded(fuel)
        totalFuel += fuel
    }
    return totalFuel
}

fun part1() = Files.readAllLines(Paths.get(FILENAME))
    .sumBy { fuelNeeded(it.toInt()) }
    .toString()

fun part2() = Files.readAllLines(Paths.get(FILENAME))
    .sumBy { totalFuelNeeded(it.toInt()) }
    .toString()

fun main() {
    println("Part 1: ${part1()}")
    println("Part 2: ${part2()}")
}

I'm going to clean this up to avoid imperative style and post it as a reply to this comment.

github.com/jbristow/adventofcode/t...

[Jon Bristow]

This part 2 solution is much more elegant, and leverages monads.

tailrec fun totalFuelNeeded(mass: Int, fuel: Option<Int> = Option.empty(), totalFuel: Int = 0): Int =
    when (val nextFuel = fuel.fold({ fuelNeeded(mass) }, ::fuelNeeded)) {
        0 -> totalFuel
        else -> totalFuelNeeded(mass, nextFuel.just(), totalFuel + nextFuel)
    }

[Christopher Kruse]

I didn't even think about using a max function - that would have been so much cleaner than making two functions!

[Jon Bristow]

I love cleaning these up into more elegant forms almost as much as I love just getting the solution banged out!