2021 Day 8

   @Test
    fun part1() {
        println("Part 1: ${calculateRisk(data)}")
    }

    @Test
    fun part2() {
        val basins = lowestPoints(data).map { findBasin(data, it).size }.sorted().reversed()
        println("Part 2 ${basins[0] * basins[1] * basins[2]}")
    }

    private fun calculateRisk(cave: List<List<Int>>) =
        lowestPoints(cave).sumBy { cave.heightAt(it) + 1 }

    private fun lowestPoints(cave: List<List<Int>>): List<Int> {
        return (0 until cave.allPoints).filter { position ->
            val pointHeight = cave.heightAt(position)
            pointHeight < cave.heightLeftOf(position) &&
            pointHeight < cave.heightRighttOf(position) &&
            pointHeight < cave.heightAbove(position) &&
            pointHeight < cave.heightBelow(position)
        }
    }

    private fun findBasin(cave: List<List<Int>>, position: Int, basin: Set<Int> = emptySet()): Set<Int> {
        if (basin.contains(position) || cave.heightAt(position) == 9)  return basin
        var basinResult = basin + position
        if (position % cave.width > 0) basinResult = findBasin(cave, position - 1, basinResult)
        if (position % cave.width < cave.width - 1) basinResult = findBasin(cave, position + 1, basinResult)
        if (position / cave.width > 0) basinResult = findBasin(cave, position - cave.width, basinResult)
        if (position / cave.width < cave.size - 1) basinResult = findBasin(cave, position + cave.width, basinResult)
        return basinResult
    }

    private val List<List<Int>>.width get() = this[0].size
    private val List<List<Int>>.allPoints get() = width * size
    private fun List<List<Int>>.heightAt(position: Int) = this[position / width][position % width] - 48
    private fun List<List<Int>>.heightLeftOf(position: Int) = if (position % width == 0) 9 else heightAt(position - 1)
    private fun List<List<Int>>.heightRighttOf(position: Int) = if (position % width == width - 1) 9 else heightAt(position + 1)
    private fun List<List<Int>>.heightAbove(position: Int) = if (position / width == 0) 9 else heightAt(position - width)
    private fun List<List<Int>>.heightBelow(position: Int) = if (position / width == size - 1) 9 else heightAt(position + width)