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Standard library for the Scala Programming Language
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/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala
package collection
package immutable

import generic._
import scala.annotation.tailrec

/**
  * $factoryInfo
  *
  * Note that each element insertion takes O(n) time, which means that creating a list set with
  * n elements will take O(n^2^) time. This makes the builder suitable only for a small number of
  * elements.
  *
  * @since 1
  * @define Coll ListSet
  * @define coll list set
  */
object ListSet extends ImmutableSetFactory[ListSet] {

  /**
    * $setCanBuildFromInfo
    */
  implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, ListSet[A]] =
    setCanBuildFrom[A]

  @SerialVersionUID(5010379588739277132L)
  private object EmptyListSet extends ListSet[Any]
  private[collection] def emptyInstance: ListSet[Any] = EmptyListSet
}

/**
  * This class implements immutable sets using a list-based data structure. List set iterators and
  * traversal methods visit elements in the order whey were first inserted.
  *
  * Elements are stored internally in reversed insertion order, which means the newest element is at
  * the head of the list. As such, methods such as `head` and `tail` are O(n), while `last` and
  * `init` are O(1). Other operations, such as inserting or removing entries, are also O(n), which
  * makes this collection suitable only for a small number of elements.
  *
  * Instances of `ListSet` represent empty sets; they can be either created by calling the
  * constructor directly, or by applying the function `ListSet.empty`.
  *
  * @tparam A the type of the elements contained in this list set
  *
  * @author Matthias Zenger
  * @version 1.0, 09/07/2003
  * @since 1
  * @define Coll ListSet
  * @define coll list set
  * @define mayNotTerminateInf
  * @define willNotTerminateInf
  */
@SerialVersionUID(-8417059026623606218L)
sealed class ListSet[A] extends AbstractSet[A]
  with Set[A]
  with GenericSetTemplate[A, ListSet]
  with SetLike[A, ListSet[A]]
  with Serializable {

  override def companion: GenericCompanion[ListSet] = ListSet

  override def size: Int = 0
  override def isEmpty: Boolean = true

  def contains(elem: A): Boolean = false

  def +(elem: A): ListSet[A] = new Node(elem)
  def -(elem: A): ListSet[A] = this

  override def ++(xs: GenTraversableOnce[A]): ListSet[A] =
    if (xs.isEmpty) this
    else (repr /: xs) (_ + _)

  def iterator: Iterator[A] = {
    def reverseList = {
      var curr: ListSet[A] = this
      var res: List[A] = Nil
      while (!curr.isEmpty) {
        res = curr.elem :: res
        curr = curr.next
      }
      res
    }
    reverseList.iterator
  }

  protected def elem: A = throw new NoSuchElementException("elem of empty set")
  protected def next: ListSet[A] = throw new NoSuchElementException("next of empty set")

  override def toSet[B >: A]: Set[B] = this.asInstanceOf[ListSet[B]]

  override def stringPrefix = "ListSet"

  /**
    * Represents an entry in the `ListSet`.
    */
  @SerialVersionUID(-787710309854855049L)
  protected class Node(override protected val elem: A) extends ListSet[A] with Serializable {

    override def size = sizeInternal(this, 0)

    @tailrec private[this] def sizeInternal(n: ListSet[A], acc: Int): Int =
      if (n.isEmpty) acc
      else sizeInternal(n.next, acc + 1)

    override def isEmpty: Boolean = false

    override def contains(e: A) = containsInternal(this, e)

    @tailrec private[this] def containsInternal(n: ListSet[A], e: A): Boolean =
      !n.isEmpty && (n.elem == e || containsInternal(n.next, e))

    override def +(e: A): ListSet[A] = if (contains(e)) this else new Node(e)

    override def -(e: A): ListSet[A] = removeInternal(e, this, Nil)

    @tailrec private[this] def removeInternal(k: A, cur: ListSet[A], acc: List[ListSet[A]]): ListSet[A] =
      if (cur.isEmpty) acc.last
      else if (k == cur.elem) (cur.next /: acc) { case (t, h) => new t.Node(h.elem) }
      else removeInternal(k, cur.next, cur :: acc)

    override protected def next: ListSet[A] = ListSet.this

    override def last: A = elem
    override def init: ListSet[A] = next
  }
}