Day19MonsterMessages.kt

package adventofcode.year2020

import adventofcode.Puzzle
import adventofcode.PuzzleInput

class Day19MonsterMessages(customInput: PuzzleInput? = null) : Puzzle(customInput) {
    private val rules by lazy {
        input
            .split("\n\n")
            .first()
            .lines()
            .map { it.replace("\"", "") }
            .map { it.split(": ") }
            .associate { it.first() to it.last() }
    }

    private val messages by lazy { input.split("\n\n").last().lines() }

    override fun partOne(): Int {
        val rule0 =
            generateSequence(rules["0"]) { previous ->
                previous
                    .split(" ")
                    .joinToString(" ") {
                        when (val ref = NUMBER_REGEX.find(it)) {
                            null -> it
                            else -> it.replace(ref.value, "(${rules[ref.value]})")
                        }
                    }
            }
                .first { regex -> regex.split(" ").none { it.contains(NUMBER_REGEX) } }
                .replace(" ", "")
                .toRegex()

        return messages.count { rule0.matches(it) }
    }

    companion object {
        private val NUMBER_REGEX = """\d+""".toRegex()
    }
}