unfold.scala

object Unfold extends App {
  def read(pos: Int, num: Int): Seq[Int] = {
    val data = (0 to 100).toVector
    data.drop(pos).take(num)
  }

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

    loop(seed)(Stream.empty)
  }

  def all: Stream[Seq[Int]] = {
    val BatchSize = 10
    unfold(0) { pos =>
      read(pos, BatchSize) match {
        case Nil => None
        case xs => Some(pos + BatchSize, xs)
      }
    }.reverse
  }
}