aoc_2024/day06.kts

#!/usr/bin/env kotlin
import java.io.File
import java.lang.RuntimeException
import java.util.Scanner

data class LocationState(val location: Pair<Int, Int>, val direction: Direction)
enum class Direction(val offset: Pair<Int, Int>) {
    NORTH(Pair(0, -1)),
    EAST(Pair(1, 0)),
    SOUTH(Pair(0, 1)),
    WEST(Pair(-1, 0));

    fun move(currentLocation: Pair<Int, Int>): Pair<Int, Int> {
        return Pair(
            currentLocation.first + offset.first,
            currentLocation.second + offset.second
        )
    }
    fun turnLeft(): Direction {
        return when (this) {
            NORTH -> WEST
            WEST -> SOUTH
            SOUTH -> EAST
            EAST -> NORTH
        }
    }
    fun turnRight(): Direction {
        return when (this) {
            NORTH -> EAST
            EAST -> SOUTH
            SOUTH -> WEST
            WEST -> NORTH
        }
    }
    fun moveByPartOneRules(
        currentLocation: Pair<Int, Int>,
        blocks: Set<Pair<Int, Int>>
    ): LocationState {
        var currentDirection = this
        do {
            val potentialMove = currentDirection.move(currentLocation)
            if (potentialMove !in blocks) {
                return LocationState(potentialMove, currentDirection)
            }
            currentDirection = currentDirection.turnRight()
        } while (currentDirection != this)
        throw RuntimeException("Nowhere to go! $currentLocation $this")
    }
}
fun isPositionInTheRoom(player: Pair<Int, Int>, xMax: Int, yMax: Int): Boolean {
    return player.first in 0..xMax && player.second in 0..yMax
}

fun isLoop(
    playerLocation: Pair<Int, Int>,
    playerDirection: Direction,
    history: List<LocationState>,
    blocks: MutableSet<Pair<Int, Int>>,
    potentialBlockLocation: Pair<Int, Int>,
    xMax: Int,
    yMax: Int
): Boolean {
    var localLocation = playerLocation
    var localDirection = playerDirection
    val localHistory = history.toMutableList()
    val localBlocks = blocks.toMutableSet()
    localBlocks.add(potentialBlockLocation)
    while (LocationState(localLocation, localDirection) !in localHistory) {
        localHistory.add(LocationState(localLocation, localDirection))
        val (newLocation, newDirection) = localDirection.moveByPartOneRules(localLocation, localBlocks)
        localLocation = newLocation
        localDirection = newDirection
        if (!isPositionInTheRoom(localLocation, xMax, yMax)) {
            return false
        }
    }
    return true
}


val scanner = Scanner(System.`in`)
val visited = mutableSetOf<Pair<Int, Int>>()
val history = mutableListOf<LocationState>()
var playerLocation = Pair(0, 0)
var playerDirection: Direction = Direction.NORTH
val blocks = mutableSetOf<Pair<Int, Int>>()
val potentialBlocks = mutableSetOf<Pair<Int, Int>>()
var yMax = 0
var xMax = 0
while (scanner.hasNextLine()) {
    val line = scanner.nextLine()
    for (i in line.indices) {
        if (line[i] == '.') {
            continue
        }
        if (line[i] == '>') {
            playerDirection = Direction.EAST
            playerLocation = Pair(i, yMax)
        } else if (line[i] == '<') {
            playerDirection = Direction.WEST
            playerLocation = Pair(i, yMax)
        } else if (line[i] == '^') {
            playerDirection = Direction.NORTH
            playerLocation = Pair(i, yMax)
        } else if (line[i] == 'v') {
            playerDirection = Direction.SOUTH
            playerLocation = Pair(i, yMax)
        } else if (line[i] == '#') {
            blocks.add(Pair(i, yMax))
        }
    }
    xMax = xMax.coerceAtLeast(line.lastIndex)
    yMax++
}
yMax-- // Should represent the max index.
val initialPosition = playerLocation.copy()
while (isPositionInTheRoom(playerLocation, xMax, yMax)) {
    visited.add(playerLocation)
    if (checkForLoopIfObstacleInFront(playerLocation, playerDirection, history)) {
        potentialBlocks.add(playerDirection.move(playerLocation))
    }
    history.add(LocationState(playerLocation, playerDirection))
    val (newLocation, newDirection) = playerDirection.moveByPartOneRules(playerLocation, blocks)
    playerLocation = newLocation
    playerDirection = newDirection
}

println("Player location: ${visited.size}")
println("Potential blocks: ${potentialBlocks.size}")

fun checkForLoopIfObstacleInFront(
    playerLocation: Pair<Int, Int>,
    playerDirection: Direction,
    history: MutableList<LocationState>
): Boolean {
    val potentialBlockLocation = playerDirection.moveByPartOneRules(playerLocation, blocks).location
    val alreadyVisited = potentialBlockLocation in visited
    val positionNotInTheRoom = !isPositionInTheRoom(potentialBlockLocation, xMax, yMax)
    if (alreadyVisited || positionNotInTheRoom) {
        return false
    }
    return isLoop(playerLocation, playerDirection, history, blocks, potentialBlockLocation, xMax, yMax)
}