gistfile1.scala

import scala.annotation.tailrec

object FirstDup extends App {

  def unfold[A, B](b: B)(f: B => Option[(A, B)]): Stream[A] = f(b) match {
    case None => Stream.empty
    case Some((a, b)) => a #:: unfold(b)(f)
  }

  def prefix[A](as: Traversable[A]): Stream[A] =
    unfold((as, Set[A]())) {
      case (as, acc) =>
        as.headOption match {
          case Some(a) if acc contains a => None
          case Some(a) => Some((a, (as.tail, acc + a)))
          case None => None
        }
    }

  println(prefix(List(1, 2, 3, 2, 3)).toList) // List(1, 2, 3)
  println(prefix(List(1, 2, 3)).toList) // List(1, 2, 3)
  println(prefix(List[Int]()).toList) // List()

}