Day04SecurityThroughObscurity.kt

package adventofcode.year2016

import adventofcode.Puzzle
import adventofcode.PuzzleInput

class Day04SecurityThroughObscurity(
    customInput: PuzzleInput? = null,
) : Puzzle(customInput) {
    private val rooms by lazy {
        input
            .lines()
            .map { room -> ROOM_REGEX.find(room)!!.destructured }
            .map { (name, sectorId, checksum) -> Triple(name, sectorId.toInt(), checksum) }
    }

    override fun partOne() =
        rooms
            .map { (name, sectorId, checksum) -> Triple(name.filterNot { it == '-' }.groupingBy { it }.eachCount(), sectorId, checksum) }
            .filter { (letterCount, _, checksum) ->
                val computedChecksum =
                    letterCount
                        .toList()
                        .sortedBy { (letter, _) -> letter }
                        .sortedByDescending { (_, count) -> count }
                        .take(5)
                        .joinToString("") { (letter, _) -> letter.toString() }

                computedChecksum == checksum
            }.sumOf { (_, sectorId, _) -> sectorId }

    override fun partTwo() =
        rooms
            .map { (name, sectorId, _) ->
                name
                    .map { char ->
                        when (char) {
                            '-' -> ' '
                            else -> ((char.code - 'a'.code + sectorId) % 26 + 'a'.code).toChar()
                        }
                    }.joinToString("") to sectorId
            }.first { (name, _) -> name == NORTHPOLE_OBJECT_STORAGE_ROOM }
            .second

    companion object {
        private val ROOM_REGEX = """([a-z-]+)-(\d+)\[([a-z]{5})]""".toRegex()
        private const val NORTHPOLE_OBJECT_STORAGE_ROOM = "northpole object storage"
    }
}