Day10SyntaxScoring.kt

package adventofcode.year2021

import adventofcode.Puzzle
import adventofcode.PuzzleInput

class Day10SyntaxScoring(customInput: PuzzleInput? = null) : Puzzle(customInput) {
    private val lines by lazy { input.lines() }

    private val bracketMap =
        mapOf(
            '(' to ')',
            '[' to ']',
            '{' to '}',
            '<' to '>',
        )

    private val scoreMapPartOne =
        mapOf(
            ')' to 3,
            ']' to 57,
            '}' to 1197,
            '>' to 25137,
        )

    private val scoreMapPartTwo =
        mapOf(
            ')' to 1,
            ']' to 2,
            '}' to 3,
            '>' to 4,
        )

    private fun List<Any>.middle() = this[size / 2]

    override fun partOne() =
        lines
            .mapNotNull { line ->
                val stack = ArrayDeque<Char>()

                line.forEach { char ->
                    when (char) {
                        in setOf('(', '[', '{', '<') -> stack.add(char)
                        else -> if (char != bracketMap[stack.removeLast()]) return@mapNotNull char
                    }
                }

                null
            }
            .sumOf { char -> scoreMapPartOne[char]!! }

    override fun partTwo() =
        lines
            .mapNotNull { line ->
                val stack = ArrayDeque<Char>()

                line.forEach { char ->
                    when (char) {
                        in setOf('(', '[', '{', '<') -> stack.add(char)
                        else -> if (char != bracketMap[stack.removeLast()]) return@mapNotNull null
                    }
                }

                stack.reversed().map { openingBracket -> bracketMap[openingBracket]!! }
            }
            .map { completion -> completion.fold(0L) { acc, char -> acc * 5 + scoreMapPartTwo[char]!! } }
            .sorted()
            .middle()
}