AoC2021 Day 9 part 2

Day9B.kt

package day9

import java.nio.file.Files
import java.nio.file.Path


class Day9b(val inputs: Array<IntArray>) {
    val inner = inputs[0].size - 1
    val outer = inputs.size - 1
    val used = mutableSetOf<Pair<Int, Int>>()
    fun solve(): Int {
        val basins = sequence {
            (0..outer).forEach { iO ->
                (0..inner).forEach { iI ->
                    if(isMinimum(Pair(iO, iI))) {
                        yield(basin(Pair(iO, iI)))
                    }
                }
            }
        }
        return basins.sortedDescending()
            .take(3)
            .reduce(Int::times)
    }

    private fun basin(p:Pair<Int, Int>): Int {
        val res = basinPoints(p).toSet()
        return res.size + 1
    }

    private fun basinPoints(p: Pair<Int,Int>): List<Pair<Int, Int>> {
        if(used.contains(p)) return listOf()
        used.add(p)
        val neighbors = neighbours(p)
            .filter { inBasin(it) }
            .toList() - used
        return neighbors + neighbors.flatMap { basinPoints(it) }
    }

    private fun inBasin(v:Pair<Int, Int>) = inputs[v.first][v.second] < 9


    private fun isMinimum(p: Pair<Int,Int>): Boolean {
        return neighbours(p).map { inputs[it.first][it.second] }
            .all { it > inputs[p.first][p.second] }
    }

    private fun neighbours(p: Pair<Int,Int>): Sequence<Pair<Int,Int>> {
        val (o, i) = p
        return sequence {
            if(i > 0) yield(Pair(o, i - 1))
            if(i < inner) yield(Pair(o, i + 1))
            if(o > 0) yield(Pair(o - 1, i))
            if(o < outer) yield(Pair(o + 1, i))
        }
    }
}


fun main(args: Array<String>) {
    val path = Path.of(args[0])
    val inputs = Files.readAllLines(path)
        .map{ Day9.parse(it) }.toTypedArray()
    println(Day9b(inputs).solve())
}