Scala example source code file (MQueue.scala)

This example Scala source code file (MQueue.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

actor, any, b, boolean, int, mqueue, mqueueelement, msg, none, option, outputchannel

The MQueue.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2005-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala.actors

private[actors] class MQueueElement[Msg >: Null](val msg: Msg, val session: OutputChannel[Any], var next: MQueueElement[Msg]) {
  def this() = this(null, null, null)
  def this(msg: Msg, session: OutputChannel[Any]) = this(msg, session, null)
}

private[actors] class MQueue[Msg >: Null](protected val label: String) {
  protected var first: MQueueElement[Msg] = null
  protected var last: MQueueElement[Msg] = null  // last eq null iff list is empty
  private var _size = 0

  def size = _size
  final def isEmpty = last eq null

  protected def changeSize(diff: Int) {
    _size += diff
  }

  def prepend(other: MQueue[Msg]) {
    if (!other.isEmpty) {
      other.last.next = first
      first = other.first
    }
  }

  def clear() {
    first = null
    last = null
    _size = 0
  }


  def append(msg: Msg, session: OutputChannel[Any]) {
    changeSize(1) // size always increases by 1
    val el = new MQueueElement(msg, session)

    if (isEmpty) first = el
    else last.next = el

    last = el
  }

  def append(el: MQueueElement[Msg]) {
    changeSize(1) // size always increases by 1

    if (isEmpty) first = el
    else last.next = el

    last = el
  }

  def foreach(f: (Msg, OutputChannel[Any]) => Unit) {
    var curr = first
    while (curr != null) {
      f(curr.msg, curr.session)
      curr = curr.next
    }
  }

  def foreachAppend(target: MQueue[Msg]) {
    var curr = first
    while (curr != null) {
      target.append(curr)
      curr = curr.next
    }
  }

  def foreachDequeue(target: MQueue[Msg]) {
    var curr = first
    while (curr != null) {
      target.append(curr)
      curr = curr.next
    }
    first = null
    last = null
    _size = 0
  }

  def foldLeft[B](z: B)(f: (B, Msg) => B): B = {
    var acc = z
    var curr = first
    while (curr != null) {
      acc = f(acc, curr.msg)
      curr = curr.next
    }
    acc
  }

  /** Returns the n-th message that satisfies the predicate `p`
   *  without removing it.
   */
  def get(n: Int)(p: Msg => Boolean): Option[Msg] = {
    var pos = 0

    def test(msg: Msg): Boolean =
      p(msg) && (pos == n || { pos += 1; false })

    var curr = first
    while (curr != null)
      if (test(curr.msg)) return Some(curr.msg) // early return
      else curr = curr.next

    None
  }

  /** Removes the n-th message that satisfies the predicate <code>p</code>.
   */
  def remove(n: Int)(p: (Msg, OutputChannel[Any]) => Boolean): Option[(Msg, OutputChannel[Any])] =
    removeInternal(n)(p) map (x => (x.msg, x.session))

  /** Extracts the first message that satisfies the predicate `p`
   *  or `'''null'''` if `p` fails for all of them.
   */
  def extractFirst(p: (Msg, OutputChannel[Any]) => Boolean): MQueueElement[Msg] =
    removeInternal(0)(p).orNull

  def extractFirst(pf: PartialFunction[Msg, Any]): MQueueElement[Msg] = {
    if (isEmpty)    // early return
      return null

    // special handling if returning the head
    if (pf.isDefinedAt(first.msg)) {
      val res = first
      first = first.next
      if (res eq last)
        last = null

      changeSize(-1)
      res
    }
    else {
      var curr = first.next   // init to element #2
      var prev = first

      while (curr != null) {
        if (pf.isDefinedAt(curr.msg)) {
          prev.next = curr.next
          if (curr eq last)
            last = prev

          changeSize(-1)
          return curr // early return
        }
        else {
          prev = curr
          curr = curr.next
        }
      }
      // not found
      null
    }
  }

  private def removeInternal(n: Int)(p: (Msg, OutputChannel[Any]) => Boolean): Option[MQueueElement[Msg]] = {
    var pos = 0

    def foundMsg(x: MQueueElement[Msg]) = {
      changeSize(-1)
      Some(x)
    }
    def test(msg: Msg, session: OutputChannel[Any]): Boolean =
      p(msg, session) && (pos == n || { pos += 1 ; false })

    if (isEmpty)    // early return
      return None

    // special handling if returning the head
    if (test(first.msg, first.session)) {
      val res = first
      first = first.next
      if (res eq last)
        last = null

      foundMsg(res)
    }
    else {
      var curr = first.next   // init to element #2
      var prev = first

      while (curr != null) {
        if (test(curr.msg, curr.session)) {
          prev.next = curr.next
          if (curr eq last)
            last = prev

          return foundMsg(curr) // early return
        }
        else {
          prev = curr
          curr = curr.next
        }
      }
      // not found
      None
    }
  }
}

/** Debugging trait.
 */
private[actors] trait MessageQueueTracer extends MQueue[Any]
{
  private val queueNumber = MessageQueueTracer.getQueueNumber

  override def append(msg: Any, session: OutputChannel[Any]) {
    super.append(msg, session)
    printQueue("APPEND %s" format msg)
  }
  override def get(n: Int)(p: Any => Boolean): Option[Any] = {
    val res = super.get(n)(p)
    printQueue("GET %s" format res)
    res
  }
  override def remove(n: Int)(p: (Any, OutputChannel[Any]) => Boolean): Option[(Any, OutputChannel[Any])] = {
    val res = super.remove(n)(p)
    printQueue("REMOVE %s" format res)
    res
  }
  override def extractFirst(p: (Any, OutputChannel[Any]) => Boolean): MQueueElement[Any] = {
    val res = super.extractFirst(p)
    printQueue("EXTRACT_FIRST %s" format res)
    res
  }

  private def printQueue(msg: String) = {
    def firstMsg = if (first eq null) "null" else first.msg
    def lastMsg = if (last eq null) "null" else last.msg

    println("[%s size=%d] [%s] first = %s, last = %s".format(this, size, msg, firstMsg, lastMsg))
  }
  override def toString() = "%s:%d".format(label, queueNumber)
}

private[actors] object MessageQueueTracer {
  // for tracing purposes
  private var queueNumberAssigner = 0
  private def getQueueNumber = synchronized {
    queueNumberAssigner += 1
    queueNumberAssigner
  }
}