Y2015D19.kt

package aockt.y2015

import io.github.jadarma.aockt.core.Solution

object Y2015D19 : Solution {

    /** Returns all the possible transformations the machine can do. */
    private fun parseInput(input: List<String>): List<Pair<String, String>> =
        input.dropLast(2).map {
            val (first, second) = it.split(" => ")
            first to second
        }

    override fun partOne(input: String): Int {
        val lines = input.lines()
        val molecule = lines.last()

        return parseInput(lines)
            .flatMap { (original, transformed) ->
                Regex(original)
                    .findAll(molecule)
                    .map { molecule.replaceRange(it.range, transformed) }
                    .toList()
            }
            .toSet()
            .size
    }

    override fun partTwo(input: String): Int {
        val lines = input.lines()
        val molecule = lines.last()
        val transformations = parseInput(lines).toMutableList()

        var iterations = 0
        var target = molecule

        while (target != "e") {
            val last = target
            transformations.forEach { (original, transformed) ->
                if (target.contains(transformed)) {
                    target = target.replaceFirst(transformed, original)
                    iterations++
                }
            }
            if (last == target) {
                transformations.shuffle()
                target = molecule
                iterations = 0
            }
        }

        return iterations
    }

    /**
     * This is too cool not to show: you can solve this only by using the medicine molecule if you can figure out that
     * the transformation rules follow an unambiguous grammar. Unfortunately, it won't pass the test on the trivial
     * HOH example since it doesn't use the same grammar, but it works for real input data in linear time! Yay math!
     *
     * Discovered by Reddit user `/u/askalski`.
     * Original explanation: [https://www.reddit.com/r/adventofcode/comments/3xflz8/day_19_solutions/cy4etju].
     */
    @Suppress("unused")
    private fun partTwoByMagic(input: String): Int {
        val molecule = input.lineSequence().last()

        val elements = molecule.count { it in 'A'..'Z' }
        val parens = molecule.windowed(2).count { it == "Rn" || it == "Ar" }
        val commas = molecule.count { it == 'Y' }

        return elements - parens - commas * 2 - 1
    }
}