private data class Pos(val x:Int, val y:Int)

fun main() {
    var input = getInputLines(2020, 24)
    val directions = listOf("e", "se", "sw", "w", "nw", "ne")
    fun parse(s:String): MutableList<String> {
        val out = mutableListOf<String>()
        var i = 0
        while(i<s.length) {
            val n = if (s[i] in listOf('e', 'w')) 1 else 2
            out.add(s.substring(i, i + n))
            i+=n
        }
        return out
    }

    fun move(p:Pos, dir:String) =
        when(dir) {
            "e" -> { Pos(p.x + 1, p.y) }
            "w" -> { Pos(p.x - 1, p.y)}
            "se" -> { Pos(p.x + 1, p.y - 1) }
            "sw" -> { Pos(p.x, p.y - 1) }
            "ne" -> { Pos(p.x, p.y + 1) }
            "nw" -> { Pos(p.x - 1, p.y + 1) }
            else -> throw RuntimeException("Unknown direction $dir")
        }

    var blacks = input.map { parse(it).fold(Pos(0, 0)) { p, d -> move(p, d)}}
        .groupBy { it }.map { it.key to it.value.size }
        .filterNot { it.second % 2  == 0 }.map { it.first }
    println(blacks.size)
    for(i in 1..100) {
        // number of black tiles around
        val ba = blacks.flatMap { b -> directions.map {move(b, it)  } }
            .groupBy { it }.map { it.key to it.value.size }.toMap()
        //  black tiles turning white
        val b2w = blacks.filter { ba[it] == null || ba[it]!!>2 }
        // white tiles turning black
        val w2b = ba.toList().filter { it.second == 2 }.map { it.first }.toSet().minus(blacks)
        // update black tiles
        blacks = blacks.minus(b2w).plus(w2b)
    }
    println(blacks.size)
}