Day06.kt

package name.mido.aoc.y2024

import name.mido.aoc.y2023.Direction
import name.mido.aoc.y2023.Point
import name.mido.aoc.y2023.StringScreen
import java.io.File
import kotlin.time.measureTime

fun main() {
    val screen = StringScreen(File("src/inputs/2024/day06.txt").readLines())

    val start = screen.collectPoints('^').first()

    val guard = Guard(Direction.NORTH, start, screen)

    val solA = guard.walk().second.size
    println("SolutionA is $solA")

    val bTime = measureTime {
        val solB = guard.findLoops()
        println("SolutionB is $solB")
    }
    println("SolutionB took $bTime")

}

class Guard(
    val baseDir: Direction,
    val basePos: Point,
    var screen: StringScreen,
) {
    var dir = baseDir
    var pos = basePos

    fun reset() {
        dir = baseDir
        pos = basePos
    }

    fun step(obstacles: Set<Point>): Boolean {
        val newPos = pos.move(dir)
        return if(screen.contains(newPos)) {
            if(screen[newPos] == '#' || obstacles.contains(newPos)) {
                dir = dir.right()
            } else {
                pos = newPos
            }
            true
        } else {
            false
        }
    }

    fun walk(obstacles: Set<Point> = emptySet()): Pair<Boolean, Set<Point>> {
        reset()
        val moveHistory = mutableSetOf(Pair(dir, pos))

        while(step(obstacles)) {
            val state = Pair(dir, pos)
            if(moveHistory.contains(state)) {
                return Pair(true, emptySet())
            }
            moveHistory.add(state)
        }

        return Pair(false, moveHistory.map { it.second }.toSet())
    }

    fun findLoops(): Int {
        val candidates = walk().second

        return candidates.filter { walk(setOf(it)).first }.size
    }
}