object HailstoneSequence extends App {
  def hailstone(n: Int): Stream[Int] =
    n #:: (if (n == 1) Stream.empty else hailstone(if (n % 2 == 0) n / 2 else n * 3 + 1))

  val nr = args.headOption.map(_.toInt).getOrElse(27)
  val collatz = hailstone(nr)
  println(s"Use the routine to show that the hailstone sequence for the number: $nr.")
  println(collatz.toList)
  println(s"It has ${collatz.length} elements.")
  println
  println(
    "Compute the number < 100,000, which has the longest hailstone sequence with that sequence's length.")
  val (n, len) = (1 until 100000).map(n => (n, hailstone(n).length)).maxBy(_._2)
  println(s"Longest hailstone sequence length= $len occurring with number $n.")
}