private def transformToRanges(in: List[Int], start: Int, end: Int, acc: List[(Int, Int)]): List[(Int, Int)] = {
    val N = end + 1
    in match {
      case Nil      => acc :+ (start, end)
      case N :: Nil => acc :+ (start, N)
      case x :: Nil => acc :+ (start, end) :+ (x, x)
      case N :: xs  => transformToRanges(xs, start, N, acc)
      case x :: xs  => transformToRanges(xs, x, x, acc :+ (start, end))
    }
  }

  def transformToRanges(in: List[Int]): List[(Int, Int)] = {
    transformToRanges(in.tail, in.head, in.head, List())
  }