package java2scala.homeworks.funcs

trait ChurchList[A] {
  def fold[B](z: B)(f: A => B => B): B

  def ::(x: A): ChurchList[A]                          = new ChurchList.Cons[A](this, x)

  def map[B](f: A => B): ChurchList[B]                 = fold(ChurchList.empty[B])(e => acc => f(e) :: acc)
  def filter(f: A => Boolean): ChurchList[A]           = fold(ChurchList.empty[A])(e => acc => if (f(e)) e :: acc else acc)
  def flatMap[B](f: A => ChurchList[B]): ChurchList[B] = fold(ChurchList.empty[B])(e => acc => f(e) ++ acc)
  def headOption: Option[A]                            = fold(Option.empty[A])(e => _ => Some(e))
  def drop(count: Int): ChurchList[A]                  =
    fold((ChurchList.empty[A], 0))(e => acc => if (acc._2 < count) (acc._1, acc._2 + 1) else (e :: acc._1, acc._2 + 1))._1
  def toList: List[A]                                  = fold(List[A]())(e => acc => e :: acc)
  def ++(that: ChurchList[A]): ChurchList[A]           = fold(that)(e => acc => e :: acc)
}

object ChurchList {
  def apply[A](xs: A*): ChurchList[A] = {
    def go(seq: Seq[A], acc: ChurchList[A]): ChurchList[A] = seq match {
      case Seq()     => acc
      case h +: tail => go(tail, h :: acc)
    }

    go(xs.reverse, empty[A])
  }

  def empty[A]: ChurchList[A] = new ChurchList[A] {
    override def fold[B](z: B)(f: A => B => B): B = z
  }

  class Cons[A] private[ChurchList] (list: ChurchList[A], x: A) extends ChurchList[A] {
    override def fold[B](z: B)(f: A => B => B): B = f(x)(list.fold(z)(f))
  }
}