AoC day 18

import scala.annotation.tailrec

object day18 extends App {

  def nextRow(prev: String): String = {
    ('.' +: prev :+ '.').sliding(3).map(c => if (c.head != c.last) '^' else '.').mkString
  }

  def safeTiles(n: Int): Int = {
    @tailrec
    def countUp(row: String, acc: (Int, Int)): Int = acc match {
      case (x, y) if x == n => y
      case (x, y) => countUp(nextRow(row), (x + 1, y + row.count(_ == '.')))
    }

    val row1 = "......^.^^.....^^^^^^^^^...^.^..^^.^^^..^.^..^.^^^.^^^^..^^.^.^.....^^^^^..^..^^^..^^.^.^..^^..^^^.."
    countUp(row1, (0, 0))
  }

  safeTiles(40)
  safeTiles(400000)

}