AOC - 2024 day 10

package Y2024.D10

class TrailHead(lines: List<String>) {
    private val startPositions = mutableListOf<Pair<Int, Int>>()
    private val trailMap = mutableMapOf<Pair<Int, Int>, Int>()

    init {
        lines.forEachIndexed { y, line ->
            line.forEachIndexed { x, tile ->
                if (tile == '0') startPositions.add(y to x)
                trailMap[y to x] = tile.digitToInt()
            }
        }
    }

    fun sumOfTrails(): Int {
        val alreadyWalked = mapOf<Pair<Int, Int>, Int>()
        return startPositions.sumOf { pos ->
            walkTheTrail(pos, 0).size
        }
    }

    fun walkTheTrail(pos: Pair<Int, Int>, height: Int): List<Pair<Int,Int>> {
        if (height == 9) return listOf(pos)
        val possibleSteps = trailMap.findNeighbours(pos).filter { it.second == height + 1 }
        if (possibleSteps.isEmpty()) return emptyList()
        return possibleSteps.map { walkTheTrail(it.first, height + 1) }.flatten()
    }

    private fun Map<Pair<Int, Int>, Int>.findNeighbours(pos: Pair<Int, Int>): List<Pair<Pair<Int, Int>, Int>> {
        return listOf<Pair<Pair<Int, Int>, Int>>() +
                trailMap.neighbourAt(pos + (-1 to 0)) +
                trailMap.neighbourAt(pos + (1 to 0)) +
                trailMap.neighbourAt(pos + (0 to -1)) +
                trailMap.neighbourAt(pos + (0 to 1))

    }
}

private fun MutableMap<Pair<Int, Int>, Int>.neighbourAt(pos: Pair<Int, Int>): Pair<Pair<Int, Int>, Int> {
    return pos to (this[pos] ?: -1)
}

private operator fun Pair<Int, Int>.plus(other: Pair<Int, Int>): Pair<Int, Int> {
    return first + other.first to second + other.second
}