Scala example source code file (PriorityQueue.scala)

This example Scala source code file (PriorityQueue.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

a, boolean, canbuildfrom, int, ordering, priorityqueue, resizablearrayaccess, serializable, that, unit

The PriorityQueue.scala Scala example source code

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

package scala
package collection
package mutable

import generic._

/** This class implements priority queues using a heap.
 *  To prioritize elements of type A there must be an implicit
 *  Ordering[A] available at creation.
 *
 *  @tparam A    type of the elements in this priority queue.
 *  @param ord   implicit ordering used to compare the elements of type `A`.
 *
 *  @author  Matthias Zenger
 *  @version 1.0, 03/05/2004
 *  @since   1
 *
 *  @define Coll PriorityQueue
 *  @define coll priority queue
 *  @define orderDependent
 *  @define orderDependentFold
 *  @define mayNotTerminateInf
 *  @define willNotTerminateInf
 */
@deprecatedInheritance("PriorityQueue is not intended to be subclassed due to extensive private implementation details.", "2.11.0")
class PriorityQueue[A](implicit val ord: Ordering[A])
   extends AbstractIterable[A]
      with Iterable[A]
      with GenericOrderedTraversableTemplate[A, PriorityQueue]
      with IterableLike[A, PriorityQueue[A]]
      with Growable[A]
      with Builder[A, PriorityQueue[A]]
      with Serializable
      with scala.Cloneable
{
  import ord._

  private class ResizableArrayAccess[A] extends AbstractSeq[A] with ResizableArray[A] with Serializable {
    def p_size0 = size0
    def p_size0_=(s: Int) = size0 = s
    def p_array = array
    def p_ensureSize(n: Int) = super.ensureSize(n)
    def p_swap(a: Int, b: Int) = super.swap(a, b)
  }

  protected[this] override def newBuilder = new PriorityQueue[A]

  private val resarr = new ResizableArrayAccess[A]

  resarr.p_size0 += 1                  // we do not use array(0)
  def length: Int = resarr.length - 1  // adjust length accordingly
  override def size: Int = length
  override def isEmpty: Boolean = resarr.p_size0 < 2
  override def repr = this

  def result = this

  override def orderedCompanion = PriorityQueue

  private def toA(x: AnyRef): A = x.asInstanceOf[A]
  protected def fixUp(as: Array[AnyRef], m: Int): Unit = {
    var k: Int = m
    while (k > 1 && toA(as(k / 2)) < toA(as(k))) {
      resarr.p_swap(k, k / 2)
      k = k / 2
    }
  }

  protected def fixDown(as: Array[AnyRef], m: Int, n: Int): Unit = {
    var k: Int = m
    while (n >= 2 * k) {
      var j = 2 * k
      if (j < n && toA(as(j)) < toA(as(j + 1)))
        j += 1
      if (toA(as(k)) >= toA(as(j)))
        return
      else {
        val h = as(k)
        as(k) = as(j)
        as(j) = h
        k = j
      }
    }
  }

  /** Inserts a single element into the priority queue.
   *
   *  @param  elem        the element to insert.
   *  @return             this $coll.
   */
  def +=(elem: A): this.type = {
    resarr.p_ensureSize(resarr.p_size0 + 1)
    resarr.p_array(resarr.p_size0) = elem.asInstanceOf[AnyRef]
    fixUp(resarr.p_array, resarr.p_size0)
    resarr.p_size0 += 1
    this
  }

  /** Adds all elements provided by a `TraversableOnce` object
   *  into the priority queue.
   *
   *  @param  xs    a traversable object.
   *  @return       a new priority queue containing elements of both `xs` and `this`.
   */
  def ++(xs: GenTraversableOnce[A]): PriorityQueue[A] = { this.clone() ++= xs.seq }

  /** Adds all elements to the queue.
   *
   *  @param  elems       the elements to add.
   */
  def enqueue(elems: A*): Unit = { this ++= elems }

  /** Returns the element with the highest priority in the queue,
   *  and removes this element from the queue.
   *
   *  @throws Predef.NoSuchElementException
   *  @return   the element with the highest priority.
   */
  def dequeue(): A =
    if (resarr.p_size0 > 1) {
      resarr.p_size0 = resarr.p_size0 - 1
      resarr.p_swap(1, resarr.p_size0)
      fixDown(resarr.p_array, 1, resarr.p_size0 - 1)
      toA(resarr.p_array(resarr.p_size0))
    } else
      throw new NoSuchElementException("no element to remove from heap")

  def dequeueAll[A1 >: A, That](implicit bf: CanBuildFrom[_, A1, That]): That = {
    val b = bf.apply()
    while (nonEmpty) {
      b += dequeue()
    }
    b.result()
  }

  /** Returns the element with the highest priority in the queue,
   *  or throws an error if there is no element contained in the queue.
   *
   *  @return   the element with the highest priority.
   */
  override def head: A = if (resarr.p_size0 > 1) toA(resarr.p_array(1)) else throw new NoSuchElementException("queue is empty")

  /** Removes all elements from the queue. After this operation is completed,
   *  the queue will be empty.
   */
  def clear(): Unit = { resarr.p_size0 = 1 }

  /** Returns an iterator which yields all the elements.
   *
   *  Note: The order of elements returned is undefined.
   *  If you want to traverse the elements in priority queue
   *  order, use `clone().dequeueAll.iterator`.
   *
   *  @return  an iterator over all the elements.
   */
  override def iterator: Iterator[A] = new AbstractIterator[A] {
    private var i = 1
    def hasNext: Boolean = i < resarr.p_size0
    def next(): A = {
      val n = resarr.p_array(i)
      i += 1
      toA(n)
    }
  }

  /** Returns the reverse of this queue. The priority queue that gets
   *  returned will have an inversed ordering - if for some elements
   *  `x` and `y` the original queue's ordering
   *  had `compare` returning an integer ''w'', the new one will return ''-w'',
   *  assuming the original ordering abides its contract.
   *
   *  Note that the order of the elements will be reversed unless the
   *  `compare` method returns 0. In this case, such elements
   *  will be subsequent, but their corresponding subinterval may be inappropriately
   *  reversed. However, due to the compare-equals contract, they will also be equal.
   *
   *  @return   A reversed priority queue.
   */
  def reverse = {
    val revq = new PriorityQueue[A]()(new scala.math.Ordering[A] {
      def compare(x: A, y: A) = ord.compare(y, x)
    })
    for (i <- 1 until resarr.length) revq += resarr(i)
    revq
  }

  /** Returns an iterator which yields all the elements in the reverse order
   *  than that returned by the method `iterator`.
   *
   *  Note: The order of elements returned is undefined.
   *
   *  @return  an iterator over all elements sorted in descending order.
   */
  def reverseIterator: Iterator[A] = new AbstractIterator[A] {
    private var i = resarr.p_size0 - 1
    def hasNext: Boolean = i >= 1
    def next(): A = {
      val n = resarr.p_array(i)
      i -= 1
      toA(n)
    }
  }

  /** The hashCode method always yields an error, since it is not
   *  safe to use mutable queues as keys in hash tables.
   *
   *  @return never.
   */
  override def hashCode(): Int =
    throw new UnsupportedOperationException("unsuitable as hash key")

  /** Returns a regular queue containing the same elements.
   *
   *  Note: the order of elements is undefined.
   */
  def toQueue: Queue[A] = new Queue[A] ++= this.iterator

  /** Returns a textual representation of a queue as a string.
   *
   *  @return the string representation of this queue.
   */
  override def toString() = toList.mkString("PriorityQueue(", ", ", ")")

  /** Converts this $coll to a list.
   *
   *  Note: the order of elements is undefined.
   *
   *  @return a list containing all elements of this $coll.
   */
  override def toList = this.iterator.toList

  /** This method clones the priority queue.
   *
   *  @return  a priority queue with the same elements.
   */
  override def clone(): PriorityQueue[A] = new PriorityQueue[A] ++= this.iterator

  // def printstate() {
  //   println("-----------------------")
  //   println("Size: " + resarr.p_size0)
  //   println("Internal array: " + resarr.p_array.toList)
  //   println(toString)
  // }
}


object PriorityQueue extends OrderedTraversableFactory[PriorityQueue] {
  def newBuilder[A](implicit ord: Ordering[A]) = new PriorityQueue[A]
  implicit def canBuildFrom[A](implicit ord: Ordering[A]): CanBuildFrom[Coll, A, PriorityQueue[A]] = new GenericCanBuildFrom[A]
}