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Scala example source code file (ParIterableLike.scala)

This example Scala source code file (ParIterableLike.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

annotation, collection, combiner, generics, int, iterablesplitter, mutable, option, reflection, repr, s, t, that, this, u

The ParIterableLike.scala Scala example source code

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

package scala
package collection.parallel

import scala.collection.mutable.Builder
import scala.collection.mutable.ArrayBuffer
import scala.collection.IterableLike
import scala.collection.Parallel
import scala.collection.Parallelizable
import scala.collection.CustomParallelizable
import scala.collection.generic._
import scala.collection.GenIterableLike
import scala.collection.GenIterable
import scala.collection.GenTraversableOnce
import scala.collection.GenTraversable
import immutable.HashMapCombiner
import scala.reflect.{ClassTag, classTag}

import java.util.concurrent.atomic.AtomicBoolean

import scala.annotation.unchecked.uncheckedVariance
import scala.annotation.unchecked.uncheckedStable
import scala.language.{ higherKinds, implicitConversions }

import scala.collection.parallel.ParallelCollectionImplicits._


/** A template trait for parallel collections of type `ParIterable[T]`.
 *
 *  $paralleliterableinfo
 *
 *  $sideeffects
 *
 *  @tparam T    the element type of the collection
 *  @tparam Repr the type of the actual collection containing the elements
 *
 *  @define paralleliterableinfo
 *  This is a base trait for Scala parallel collections. It defines behaviour
 *  common to all parallel collections. Concrete parallel collections should
 *  inherit this trait and `ParIterable` if they want to define specific combiner
 *  factories.
 *
 *  Parallel operations are implemented with divide and conquer style algorithms that
 *  parallelize well. The basic idea is to split the collection into smaller parts until
 *  they are small enough to be operated on sequentially.
 *
 *  All of the parallel operations are implemented as tasks within this trait. Tasks rely
 *  on the concept of splitters, which extend iterators. Every parallel collection defines:
 *
 *  {{{
 *     def splitter: IterableSplitter[T]
 *  }}}
 *
 *  which returns an instance of `IterableSplitter[T]`, which is a subtype of `Splitter[T]`.
 *  Splitters have a method `remaining` to check the remaining number of elements,
 *  and method `split` which is defined by splitters. Method `split` divides the splitters
 *  iterate over into disjunct subsets:
 *
 *  {{{
 *     def split: Seq[Splitter]
 *  }}}
 *
 *  which splits the splitter into a sequence of disjunct subsplitters. This is typically a
 *  very fast operation which simply creates wrappers around the receiver collection.
 *  This can be repeated recursively.
 *
 *  Tasks are scheduled for execution through a
 *  [[scala.collection.parallel.TaskSupport]] object, which can be changed
 *  through the `tasksupport` setter of the collection.
 *
 *  Method `newCombiner` produces a new combiner. Combiners are an extension of builders.
 *  They provide a method `combine` which combines two combiners and returns a combiner
 *  containing elements of both combiners.
 *  This method can be implemented by aggressively copying all the elements into the new combiner
 *  or by lazily binding their results. It is recommended to avoid copying all of
 *  the elements for performance reasons, although that cost might be negligible depending on
 *  the use case. Standard parallel collection combiners avoid copying when merging results,
 *  relying either on a two-step lazy construction or specific data-structure properties.
 *
 *  Methods:
 *
 *  {{{
 *     def seq: Sequential
 *     def par: Repr
 *  }}}
 *
 *  produce the sequential or parallel implementation of the collection, respectively.
 *  Method `par` just returns a reference to this parallel collection.
 *  Method `seq` is efficient - it will not copy the elements. Instead,
 *  it will create a sequential version of the collection using the same underlying data structure.
 *  Note that this is not the case for sequential collections in general - they may copy the elements
 *  and produce a different underlying data structure.
 *
 *  The combination of methods `toMap`, `toSeq` or `toSet` along with `par` and `seq` is a flexible
 *  way to change between different collection types.
 *
 *  Since this trait extends the `GenIterable` trait, methods like `size` must also
 *  be implemented in concrete collections, while `iterator` forwards to `splitter` by
 *  default.
 *
 *  Each parallel collection is bound to a specific fork/join pool, on which dormant worker
 *  threads are kept. The fork/join pool contains other information such as the parallelism
 *  level, that is, the number of processors used. When a collection is created, it is assigned the
 *  default fork/join pool found in the `scala.parallel` package object.
 *
 *  Parallel collections are not necessarily ordered in terms of the `foreach`
 *  operation (see `Traversable`). Parallel sequences have a well defined order for iterators - creating
 *  an iterator and traversing the elements linearly will always yield the same order.
 *  However, bulk operations such as `foreach`, `map` or `filter` always occur in undefined orders for all
 *  parallel collections.
 *
 *  Existing parallel collection implementations provide strict parallel iterators. Strict parallel iterators are aware
 *  of the number of elements they have yet to traverse. It's also possible to provide non-strict parallel iterators,
 *  which do not know the number of elements remaining. To do this, the new collection implementation must override
 *  `isStrictSplitterCollection` to `false`. This will make some operations unavailable.
 *
 *  To create a new parallel collection, extend the `ParIterable` trait, and implement `size`, `splitter`,
 *  `newCombiner` and `seq`. Having an implicit combiner factory requires extending this trait in addition, as
 *  well as providing a companion object, as with regular collections.
 *
 *  Method `size` is implemented as a constant time operation for parallel collections, and parallel collection
 *  operations rely on this assumption.
 *
 *  @author Aleksandar Prokopec
 *  @since 2.9
 *
 *  @define sideeffects
 *  The higher-order functions passed to certain operations may contain side-effects. Since implementations
 *  of bulk operations may not be sequential, this means that side-effects may not be predictable and may
 *  produce data-races, deadlocks or invalidation of state if care is not taken. It is up to the programmer
 *  to either avoid using side-effects or to use some form of synchronization when accessing mutable data.
 *
 *  @define pbfinfo
 *  An implicit value of class `CanCombineFrom` which determines the
 *  result class `That` from the current representation type `Repr` and
 *  and the new element type `B`. This builder factory can provide a parallel
 *  builder for the resulting collection.
 *
 *  @define abortsignalling
 *  This method will use `abort` signalling capabilities. This means
 *  that splitters may send and read `abort` signals.
 *
 *  @define indexsignalling
 *  This method will use `indexFlag` signalling capabilities. This means
 *  that splitters may set and read the `indexFlag` state.
 *
 */
trait ParIterableLike[+T, +Repr <: ParIterable[T], +Sequential <: Iterable[T] with IterableLike[T, Sequential]]
extends GenIterableLike[T, Repr]
   with CustomParallelizable[T, Repr]
   with Parallel
   with HasNewCombiner[T, Repr]
{
self: ParIterableLike[T, Repr, Sequential] =>

  @transient
  @volatile
  private var _tasksupport = defaultTaskSupport

  protected def initTaskSupport() {
    _tasksupport = defaultTaskSupport
  }

  /** The task support object which is responsible for scheduling and
   *  load-balancing tasks to processors.
   *
   *  @see [[scala.collection.parallel.TaskSupport]]
   */
  def tasksupport = {
    val ts = _tasksupport
    if (ts eq null) {
      _tasksupport = defaultTaskSupport
      defaultTaskSupport
    } else ts
  }

  /** Changes the task support object which is responsible for scheduling and
   *  load-balancing tasks to processors.
   *
   *  A task support object can be changed in a parallel collection after it
   *  has been created, but only during a quiescent period, i.e. while there
   *  are no concurrent invocations to parallel collection methods.
   *
   *  Here is a way to change the task support of a parallel collection:
   *
   *  {{{
   *  import scala.collection.parallel._
   *  val pc = mutable.ParArray(1, 2, 3)
   *  pc.tasksupport = new ForkJoinTaskSupport(
   *    new scala.concurrent.forkjoin.ForkJoinPool(2))
   *  }}}
   *
   *  @see [[scala.collection.parallel.TaskSupport]]
   */
  def tasksupport_=(ts: TaskSupport) = _tasksupport = ts

  def seq: Sequential

  def repr: Repr = this.asInstanceOf[Repr]

  final def isTraversableAgain = true

  def hasDefiniteSize = true

  def isEmpty = size == 0

  def nonEmpty = size != 0

  def head = iterator.next()

  def headOption = if (nonEmpty) Some(head) else None

  def tail = drop(1)

  def last = {
    var lst = head
    for (x <- this.seq) lst = x
    lst
  }

  def lastOption = if (nonEmpty) Some(last) else None

  def init = take(size - 1)

  /** Creates a new parallel iterator used to traverse the elements of this parallel collection.
   *  This iterator is more specific than the iterator of the returned by `iterator`, and augmented
   *  with additional accessor and transformer methods.
   *
   *  @return          a parallel iterator
   */
  protected[parallel] def splitter: IterableSplitter[T]

  /** Creates a new split iterator used to traverse the elements of this collection.
   *
   *  By default, this method is implemented in terms of the protected `splitter` method.
   *
   *  @return         a split iterator
   */
  def iterator: Splitter[T] = splitter

  override def par: Repr = repr

  /** Denotes whether this parallel collection has strict splitters.
   *
   *  This is true in general, and specific collection instances may choose to
   *  override this method. Such collections will fail to execute methods
   *  which rely on splitters being strict, i.e. returning a correct value
   *  in the `remaining` method.
   *
   *  This method helps ensure that such failures occur on method invocations,
   *  rather than later on and in unpredictable ways.
   */
  def isStrictSplitterCollection = true

  /** The `newBuilder` operation returns a parallel builder assigned to this collection's fork/join pool.
   *  This method forwards the call to `newCombiner`.
   */
  //protected[this] def newBuilder: scala.collection.mutable.Builder[T, Repr] = newCombiner

  /** Optionally reuses an existing combiner for better performance. By default it doesn't - subclasses may override this behaviour.
   *  The provided combiner `oldc` that can potentially be reused will be either some combiner from the previous computational task, or `None` if there
   *  was no previous phase (in which case this method must return `newc`).
   *
   *  @param oldc   The combiner that is the result of the previous task, or `None` if there was no previous task.
   *  @param newc   The new, empty combiner that can be used.
   *  @return       Either `newc` or `oldc`.
   */
  protected def reuse[S, That](oldc: Option[Combiner[S, That]], newc: Combiner[S, That]): Combiner[S, That] = newc

  type SSCTask[R, Tp] = StrictSplitterCheckTask[R, Tp]

  /* helper traits - to avoid structural invocations */

  trait TaskOps[R, Tp] {
    def mapResult[R1](mapping: R => R1): ResultMapping[R, Tp, R1]
    // public method with inaccessible types in parameters
    def compose[R3, R2, Tp2](t2: SSCTask[R2, Tp2])(resCombiner: (R, R2) => R3): SeqComposite[R, R2, R3, SSCTask[R, Tp], SSCTask[R2, Tp2]]
    def parallel[R3, R2, Tp2](t2: SSCTask[R2, Tp2])(resCombiner: (R, R2) => R3): ParComposite[R, R2, R3, SSCTask[R, Tp], SSCTask[R2, Tp2]]
  }

  trait BuilderOps[Elem, To] {
    trait Otherwise[Cmb] {
      def otherwise(notbody: => Unit)(implicit t: ClassTag[Cmb]): Unit
    }

    def ifIs[Cmb](isbody: Cmb => Unit): Otherwise[Cmb]
    def isCombiner: Boolean
    def asCombiner: Combiner[Elem, To]
  }

  trait SignallingOps[PI <: DelegatedSignalling] {
    def assign(cntx: Signalling): PI
  }

  /* convenience task operations wrapper */
  protected implicit def task2ops[R, Tp](tsk: SSCTask[R, Tp]) = new TaskOps[R, Tp] {
    def mapResult[R1](mapping: R => R1): ResultMapping[R, Tp, R1] = new ResultMapping[R, Tp, R1](tsk) {
      def map(r: R): R1 = mapping(r)
    }

    def compose[R3, R2, Tp2](t2: SSCTask[R2, Tp2])(resCombiner: (R, R2) => R3) = new SeqComposite[R, R2, R3, SSCTask[R, Tp], SSCTask[R2, Tp2]](tsk, t2) {
      def combineResults(fr: R, sr: R2): R3 = resCombiner(fr, sr)
    }

    def parallel[R3, R2, Tp2](t2: SSCTask[R2, Tp2])(resCombiner: (R, R2) => R3) = new ParComposite[R, R2, R3, SSCTask[R, Tp], SSCTask[R2, Tp2]](tsk, t2) {
      def combineResults(fr: R, sr: R2): R3 = resCombiner(fr, sr)
    }
  }

  protected def wrap[R](body: => R) = new NonDivisible[R] {
    def leaf(prevr: Option[R]) = result = body
    @volatile var result: R = null.asInstanceOf[R]
  }

  /* convenience signalling operations wrapper */
  protected implicit def delegatedSignalling2ops[PI <: DelegatedSignalling](it: PI) = new SignallingOps[PI] {
    def assign(cntx: Signalling): PI = {
      it.signalDelegate = cntx
      it
    }
  }

  protected implicit def builder2ops[Elem, To](cb: Builder[Elem, To]) = new BuilderOps[Elem, To] {
    def ifIs[Cmb](isbody: Cmb => Unit) = new Otherwise[Cmb] {
      def otherwise(notbody: => Unit)(implicit t: ClassTag[Cmb]) {
        if (cb.getClass == t.runtimeClass) isbody(cb.asInstanceOf[Cmb]) else notbody
      }
    }
    def isCombiner = cb.isInstanceOf[Combiner[_, _]]
    def asCombiner = cb.asInstanceOf[Combiner[Elem, To]]
  }

  protected[this] def bf2seq[S, That](bf: CanBuildFrom[Repr, S, That]) = new CanBuildFrom[Sequential, S, That] {
    def apply(from: Sequential) = bf.apply(from.par.asInstanceOf[Repr]) // !!! we only use this on `this.seq`, and know that `this.seq.par.getClass == this.getClass`
    def apply() = bf.apply()
  }

  protected[this] def sequentially[S, That <: Parallel](b: Sequential => Parallelizable[S, That]) = b(seq).par.asInstanceOf[Repr]

  def mkString(start: String, sep: String, end: String): String = seq.mkString(start, sep, end)

  def mkString(sep: String): String = seq.mkString("", sep, "")

  def mkString: String = seq.mkString("")

  override def toString = seq.mkString(stringPrefix + "(", ", ", ")")

  def canEqual(other: Any) = true

  /** Reduces the elements of this sequence using the specified associative binary operator.
   *
   *  $undefinedorder
   *
   *  Note this method has a different signature than the `reduceLeft`
   *  and `reduceRight` methods of the trait `Traversable`.
   *  The result of reducing may only be a supertype of this parallel collection's
   *  type parameter `T`.
   *
   *  @tparam U      A type parameter for the binary operator, a supertype of `T`.
   *  @param op       A binary operator that must be associative.
   *  @return         The result of applying reduce operator `op` between all the elements if the collection is nonempty.
   *  @throws UnsupportedOperationException
   *  if this $coll is empty.
   */
  def reduce[U >: T](op: (U, U) => U): U = {
    tasksupport.executeAndWaitResult(new Reduce(op, splitter) mapResult { _.get })
  }

  /** Optionally reduces the elements of this sequence using the specified associative binary operator.
   *
   *  $undefinedorder
   *
   *  Note this method has a different signature than the `reduceLeftOption`
   *  and `reduceRightOption` methods of the trait `Traversable`.
   *  The result of reducing may only be a supertype of this parallel collection's
   *  type parameter `T`.
   *
   *  @tparam U      A type parameter for the binary operator, a supertype of `T`.
   *  @param op      A binary operator that must be associative.
   *  @return        An option value containing result of applying reduce operator `op` between all
   *                 the elements if the collection is nonempty, and `None` otherwise.
   */
  def reduceOption[U >: T](op: (U, U) => U): Option[U] = if (isEmpty) None else Some(reduce(op))

  /** Folds the elements of this sequence using the specified associative binary operator.
   *  The order in which the elements are reduced is unspecified and may be nondeterministic.
   *
   *  Note this method has a different signature than the `foldLeft`
   *  and `foldRight` methods of the trait `Traversable`.
   *  The result of folding may only be a supertype of this parallel collection's
   *  type parameter `T`.
   *
   *  @tparam U      a type parameter for the binary operator, a supertype of `T`.
   *  @param z       a neutral element for the fold operation, it may be added to the result
   *                 an arbitrary number of times, not changing the result (e.g. `Nil` for list concatenation,
   *                 0 for addition, or 1 for multiplication)
   *  @param op      a binary operator that must be associative
   *  @return        the result of applying fold operator `op` between all the elements and `z`
   */
  def fold[U >: T](z: U)(op: (U, U) => U): U = {
    tasksupport.executeAndWaitResult(new Fold(z, op, splitter))
  }

  /** Aggregates the results of applying an operator to subsequent elements.
   *
   *  This is a more general form of `fold` and `reduce`. It has similar semantics, but does
   *  not require the result to be a supertype of the element type. It traverses the elements in
   *  different partitions sequentially, using `seqop` to update the result, and then
   *  applies `combop` to results from different partitions. The implementation of this
   *  operation may operate on an arbitrary number of collection partitions, so `combop`
   *  may be invoked arbitrary number of times.
   *
   *  For example, one might want to process some elements and then produce a `Set`. In this
   *  case, `seqop` would process an element and append it to the set, while `combop`
   *  would concatenate two sets from different partitions together. The initial value
   *  `z` would be an empty set.
   *
   *  {{{
   *    pc.aggregate(Set[Int]())(_ += process(_), _ ++ _)
   *  }}}
   *
   *  Another example is calculating geometric mean from a collection of doubles
   *  (one would typically require big doubles for this).
   *
   *  @tparam S        the type of accumulated results
   *  @param z         the initial value for the accumulated result of the partition - this
   *                   will typically be the neutral element for the `seqop` operator (e.g.
   *                   `Nil` for list concatenation or `0` for summation) and may be evaluated
   *                   more than once
   *  @param seqop     an operator used to accumulate results within a partition
   *  @param combop    an associative operator used to combine results from different partitions
   */
  def aggregate[S](z: =>S)(seqop: (S, T) => S, combop: (S, S) => S): S = {
    tasksupport.executeAndWaitResult(new Aggregate(() => z, seqop, combop, splitter))
  }

  def foldLeft[S](z: S)(op: (S, T) => S): S = seq.foldLeft(z)(op)

  def foldRight[S](z: S)(op: (T, S) => S): S = seq.foldRight(z)(op)

  def reduceLeft[U >: T](op: (U, T) => U): U = seq.reduceLeft(op)

  def reduceRight[U >: T](op: (T, U) => U): U = seq.reduceRight(op)

  def reduceLeftOption[U >: T](op: (U, T) => U): Option[U] = seq.reduceLeftOption(op)

  def reduceRightOption[U >: T](op: (T, U) => U): Option[U] = seq.reduceRightOption(op)

  /** Applies a function `f` to all the elements of $coll in an undefined order.
   *
   *  @tparam U    the result type of the function applied to each element, which is always discarded
   *  @param f     function applied to each element
   */
  def foreach[U](f: T => U) = {
    tasksupport.executeAndWaitResult(new Foreach(f, splitter))
  }

  def count(p: T => Boolean): Int = {
    tasksupport.executeAndWaitResult(new Count(p, splitter))
  }

  def sum[U >: T](implicit num: Numeric[U]): U = {
    tasksupport.executeAndWaitResult(new Sum[U](num, splitter))
  }

  def product[U >: T](implicit num: Numeric[U]): U = {
    tasksupport.executeAndWaitResult(new Product[U](num, splitter))
  }

  def min[U >: T](implicit ord: Ordering[U]): T = {
    tasksupport.executeAndWaitResult(new Min(ord, splitter) mapResult { _.get }).asInstanceOf[T]
  }

  def max[U >: T](implicit ord: Ordering[U]): T = {
    tasksupport.executeAndWaitResult(new Max(ord, splitter) mapResult { _.get }).asInstanceOf[T]
  }

  def maxBy[S](f: T => S)(implicit cmp: Ordering[S]): T = {
    if (isEmpty) throw new UnsupportedOperationException("empty.maxBy")

    reduce((x, y) => if (cmp.gteq(f(x), f(y))) x else y)
  }

  def minBy[S](f: T => S)(implicit cmp: Ordering[S]): T = {
    if (isEmpty) throw new UnsupportedOperationException("empty.minBy")

    reduce((x, y) => if (cmp.lteq(f(x), f(y))) x else y)
  }

  def map[S, That](f: T => S)(implicit bf: CanBuildFrom[Repr, S, That]): That = if (bf(repr).isCombiner) {
    tasksupport.executeAndWaitResult(new Map[S, That](f, combinerFactory(() => bf(repr).asCombiner), splitter) mapResult { _.resultWithTaskSupport })
  } else setTaskSupport(seq.map(f)(bf2seq(bf)), tasksupport)
  /*bf ifParallel { pbf =>
    tasksupport.executeAndWaitResult(new Map[S, That](f, pbf, splitter) mapResult { _.result })
  } otherwise seq.map(f)(bf2seq(bf))*/

  def collect[S, That](pf: PartialFunction[T, S])(implicit bf: CanBuildFrom[Repr, S, That]): That = if (bf(repr).isCombiner) {
    tasksupport.executeAndWaitResult(new Collect[S, That](pf, combinerFactory(() => bf(repr).asCombiner), splitter) mapResult { _.resultWithTaskSupport })
  } else setTaskSupport(seq.collect(pf)(bf2seq(bf)), tasksupport)
  /*bf ifParallel { pbf =>
    tasksupport.executeAndWaitResult(new Collect[S, That](pf, pbf, splitter) mapResult { _.result })
  } otherwise seq.collect(pf)(bf2seq(bf))*/

  def flatMap[S, That](f: T => GenTraversableOnce[S])(implicit bf: CanBuildFrom[Repr, S, That]): That = if (bf(repr).isCombiner) {
    tasksupport.executeAndWaitResult(new FlatMap[S, That](f, combinerFactory(() => bf(repr).asCombiner), splitter) mapResult { _.resultWithTaskSupport })
  } else setTaskSupport(seq.flatMap(f)(bf2seq(bf)), tasksupport)
  /*bf ifParallel { pbf =>
    tasksupport.executeAndWaitResult(new FlatMap[S, That](f, pbf, splitter) mapResult { _.result })
  } otherwise seq.flatMap(f)(bf2seq(bf))*/

  /** Tests whether a predicate holds for all elements of this $coll.
   *
   *  $abortsignalling
   *
   *  @param pred    a predicate used to test elements
   *  @return        true if `p` holds for all elements, false otherwise
   */
  def forall(pred: T => Boolean): Boolean = {
    tasksupport.executeAndWaitResult(new Forall(pred, splitter assign new DefaultSignalling with VolatileAbort))
  }

  /** Tests whether a predicate holds for some element of this $coll.
   *
   *  $abortsignalling
   *
   *  @param pred    a predicate used to test elements
   *  @return        true if `p` holds for some element, false otherwise
   */
  def exists(pred: T => Boolean): Boolean = {
    tasksupport.executeAndWaitResult(new Exists(pred, splitter assign new DefaultSignalling with VolatileAbort))
  }

  /** Finds some element in the collection for which the predicate holds, if such
   *  an element exists. The element may not necessarily be the first such element
   *  in the iteration order.
   *
   *  If there are multiple elements obeying the predicate, the choice is nondeterministic.
   *
   *  $abortsignalling
   *
   *  @param pred     predicate used to test the elements
   *  @return         an option value with the element if such an element exists, or `None` otherwise
   */
  def find(pred: T => Boolean): Option[T] = {
    tasksupport.executeAndWaitResult(new Find(pred, splitter assign new DefaultSignalling with VolatileAbort))
  }

  /** Creates a combiner factory. Each combiner factory instance is used
   *  once per invocation of a parallel transformer method for a single
   *  collection.
   *
   *  The default combiner factory creates a new combiner every time it
   *  is requested, unless the combiner is thread-safe as indicated by its
   *  `canBeShared` method. In this case, the method returns a factory which
   *  returns the same combiner each time. This is typically done for
   *  concurrent parallel collections, the combiners of which allow
   *  thread safe access.
   */
  protected[this] def combinerFactory = {
    val combiner = newCombiner
    combiner.combinerTaskSupport = tasksupport
    if (combiner.canBeShared) new CombinerFactory[T, Repr] {
      val shared = combiner
      def apply() = shared
      def doesShareCombiners = true
    } else new CombinerFactory[T, Repr] {
      def apply() = newCombiner
      def doesShareCombiners = false
    }
  }

  protected[this] def combinerFactory[S, That](cbf: () => Combiner[S, That]) = {
    val combiner = cbf()
    combiner.combinerTaskSupport = tasksupport
    if (combiner.canBeShared) new CombinerFactory[S, That] {
      val shared = combiner
      def apply() = shared
      def doesShareCombiners = true
    } else new CombinerFactory[S, That] {
      def apply() = cbf()
      def doesShareCombiners = false
    }
  }

  def withFilter(pred: T => Boolean): Repr = filter(pred)

  def filter(pred: T => Boolean): Repr = {
    tasksupport.executeAndWaitResult(new Filter(pred, combinerFactory, splitter) mapResult { _.resultWithTaskSupport })
  }

  def filterNot(pred: T => Boolean): Repr = {
    tasksupport.executeAndWaitResult(new FilterNot(pred, combinerFactory, splitter) mapResult { _.resultWithTaskSupport })
  }

  def ++[U >: T, That](that: GenTraversableOnce[U])(implicit bf: CanBuildFrom[Repr, U, That]): That = {
    if (that.isParallel && bf.isParallel) {
      // println("case both are parallel")
      val other = that.asParIterable
      val pbf = bf.asParallel
      val cfactory = combinerFactory(() => pbf(repr))
      val copythis = new Copy(cfactory, splitter)
      val copythat = wrap {
        val othtask = new other.Copy(cfactory, other.splitter)
        tasksupport.executeAndWaitResult(othtask)
      }
      val task = (copythis parallel copythat) { _ combine _ } mapResult {
        _.resultWithTaskSupport
      }
      tasksupport.executeAndWaitResult(task)
    } else if (bf(repr).isCombiner) {
      // println("case parallel builder, `that` not parallel")
      val copythis = new Copy(combinerFactory(() => bf(repr).asCombiner), splitter)
      val copythat = wrap {
        val cb = bf(repr).asCombiner
        for (elem <- that.seq) cb += elem
        cb
      }
      tasksupport.executeAndWaitResult((copythis parallel copythat) { _ combine _ } mapResult { _.resultWithTaskSupport })
    } else {
      // println("case not a parallel builder")
      val b = bf(repr)
      this.splitter.copy2builder[U, That, Builder[U, That]](b)
      for (elem <- that.seq) b += elem
      setTaskSupport(b.result(), tasksupport)
    }
  }

  def partition(pred: T => Boolean): (Repr, Repr) = {
    tasksupport.executeAndWaitResult(
      new Partition(pred, combinerFactory, combinerFactory, splitter) mapResult {
        p => (p._1.resultWithTaskSupport, p._2.resultWithTaskSupport)
      }
    )
  }

  def groupBy[K](f: T => K): immutable.ParMap[K, Repr] = {
    val r = tasksupport.executeAndWaitResult(new GroupBy(f, () => HashMapCombiner[K, T], splitter) mapResult {
      rcb => rcb.groupByKey(() => combinerFactory())
    })
    setTaskSupport(r, tasksupport)
  }

  def take(n: Int): Repr = {
    val actualn = if (size > n) n else size
    if (actualn < MIN_FOR_COPY) take_sequential(actualn)
    else tasksupport.executeAndWaitResult(new Take(actualn, combinerFactory, splitter) mapResult {
      _.resultWithTaskSupport
    })
  }

  private def take_sequential(n: Int) = {
    val cb = newCombiner
    cb.sizeHint(n)
    val it = splitter
    var left = n
    while (left > 0) {
      cb += it.next
      left -= 1
    }
    cb.resultWithTaskSupport
  }

  def drop(n: Int): Repr = {
    val actualn = if (size > n) n else size
    if ((size - actualn) < MIN_FOR_COPY) drop_sequential(actualn)
    else tasksupport.executeAndWaitResult(new Drop(actualn, combinerFactory, splitter) mapResult { _.resultWithTaskSupport })
  }

  private def drop_sequential(n: Int) = {
    val it = splitter drop n
    val cb = newCombiner
    cb.sizeHint(size - n)
    while (it.hasNext) cb += it.next
    cb.resultWithTaskSupport
  }

  override def slice(unc_from: Int, unc_until: Int): Repr = {
    val from = unc_from min size max 0
    val until = unc_until min size max from
    if ((until - from) <= MIN_FOR_COPY) slice_sequential(from, until)
    else tasksupport.executeAndWaitResult(new Slice(from, until, combinerFactory, splitter) mapResult { _.resultWithTaskSupport })
  }

  private def slice_sequential(from: Int, until: Int): Repr = {
    val cb = newCombiner
    var left = until - from
    val it = splitter drop from
    while (left > 0) {
      cb += it.next
      left -= 1
    }
    cb.resultWithTaskSupport
  }

  def splitAt(n: Int): (Repr, Repr) = {
    tasksupport.executeAndWaitResult(
      new SplitAt(n, combinerFactory, combinerFactory, splitter) mapResult {
        p => (p._1.resultWithTaskSupport, p._2.resultWithTaskSupport)
      }
    )
  }

  /** Computes a prefix scan of the elements of the collection.
   *
   *  Note: The neutral element `z` may be applied more than once.
   *
   *  @tparam U         element type of the resulting collection
   *  @tparam That      type of the resulting collection
   *  @param z          neutral element for the operator `op`
   *  @param op         the associative operator for the scan
   *  @param bf         $bfinfo
   *  @return           a collection containing the prefix scan of the elements in the original collection
   *
   *  @usecase def scan(z: T)(op: (T, T) => T): $Coll[T]
   *    @inheritdoc
   *
   *    @return           a new $coll containing the prefix scan of the elements in this $coll
   */
  def scan[U >: T, That](z: U)(op: (U, U) => U)(implicit bf: CanBuildFrom[Repr, U, That]): That = if (bf(repr).isCombiner) {
    if (tasksupport.parallelismLevel > 1) {
      if (size > 0) tasksupport.executeAndWaitResult(new CreateScanTree(0, size, z, op, splitter) mapResult {
        tree => tasksupport.executeAndWaitResult(new FromScanTree(tree, z, op, combinerFactory(() => bf(repr).asCombiner)) mapResult {
          cb => cb.resultWithTaskSupport
        })
      }) else setTaskSupport((bf(repr) += z).result(), tasksupport)
    } else setTaskSupport(seq.scan(z)(op)(bf2seq(bf)), tasksupport)
  } else setTaskSupport(seq.scan(z)(op)(bf2seq(bf)), tasksupport)

  def scanLeft[S, That](z: S)(op: (S, T) => S)(implicit bf: CanBuildFrom[Repr, S, That]) = setTaskSupport(seq.scanLeft(z)(op)(bf2seq(bf)), tasksupport)

  def scanRight[S, That](z: S)(op: (T, S) => S)(implicit bf: CanBuildFrom[Repr, S, That]) = setTaskSupport(seq.scanRight(z)(op)(bf2seq(bf)), tasksupport)

  /** Takes the longest prefix of elements that satisfy the predicate.
   *
   *  $indexsignalling
   *  The index flag is initially set to maximum integer value.
   *
   *  @param pred   the predicate used to test the elements
   *  @return       the longest prefix of this $coll of elements that satisy the predicate `pred`
   */
  def takeWhile(pred: T => Boolean): Repr = {
    val cbf = combinerFactory
    if (cbf.doesShareCombiners) {
      val parseqspan = toSeq.takeWhile(pred)
      tasksupport.executeAndWaitResult(new Copy(combinerFactory, parseqspan.splitter) mapResult {
        _.resultWithTaskSupport
      })
    } else {
      val cntx = new DefaultSignalling with AtomicIndexFlag
      cntx.setIndexFlag(Int.MaxValue)
      tasksupport.executeAndWaitResult(new TakeWhile(0, pred, combinerFactory, splitter assign cntx) mapResult {
        _._1.resultWithTaskSupport
      })
    }
  }

  /** Splits this $coll into a prefix/suffix pair according to a predicate.
   *
   *  $indexsignalling
   *  The index flag is initially set to maximum integer value.
   *
   *  @param pred   the predicate used to test the elements
   *  @return       a pair consisting of the longest prefix of the collection for which all
   *                the elements satisfy `pred`, and the rest of the collection
   */
  def span(pred: T => Boolean): (Repr, Repr) = {
    val cbf = combinerFactory
    if (cbf.doesShareCombiners) {
      val (xs, ys) = toSeq.span(pred)
      val copyxs = new Copy(combinerFactory, xs.splitter) mapResult { _.resultWithTaskSupport }
      val copyys = new Copy(combinerFactory, ys.splitter) mapResult { _.resultWithTaskSupport }
      val copyall = (copyxs parallel copyys) {
        (xr, yr) => (xr, yr)
      }
      tasksupport.executeAndWaitResult(copyall)
    } else {
      val cntx = new DefaultSignalling with AtomicIndexFlag
      cntx.setIndexFlag(Int.MaxValue)
      tasksupport.executeAndWaitResult(new Span(0, pred, combinerFactory, combinerFactory, splitter assign cntx) mapResult {
        p => (p._1.resultWithTaskSupport, p._2.resultWithTaskSupport)
      })
    }
  }

  /** Drops all elements in the longest prefix of elements that satisfy the predicate,
   *  and returns a collection composed of the remaining elements.
   *
   *  $indexsignalling
   *  The index flag is initially set to maximum integer value.
   *
   *  @param pred   the predicate used to test the elements
   *  @return       a collection composed of all the elements after the longest prefix of elements
   *                in this $coll that satisfy the predicate `pred`
   */
  def dropWhile(pred: T => Boolean): Repr = {
    val cntx = new DefaultSignalling with AtomicIndexFlag
    cntx.setIndexFlag(Int.MaxValue)
    tasksupport.executeAndWaitResult(
      new Span(0, pred, combinerFactory, combinerFactory, splitter assign cntx) mapResult {
        _._2.resultWithTaskSupport
      }
    )
  }

  def copyToArray[U >: T](xs: Array[U]) = copyToArray(xs, 0)

  def copyToArray[U >: T](xs: Array[U], start: Int) = copyToArray(xs, start, xs.length - start)

  def copyToArray[U >: T](xs: Array[U], start: Int, len: Int) = if (len > 0) {
    tasksupport.executeAndWaitResult(new CopyToArray(start, len, xs, splitter))
  }

  def sameElements[U >: T](that: GenIterable[U]) = seq.sameElements(that)

  def zip[U >: T, S, That](that: GenIterable[S])(implicit bf: CanBuildFrom[Repr, (U, S), That]): That = if (bf(repr).isCombiner && that.isParSeq) {
    val thatseq = that.asParSeq
    tasksupport.executeAndWaitResult(new Zip(combinerFactory(() => bf(repr).asCombiner), splitter, thatseq.splitter) mapResult { _.resultWithTaskSupport })
  } else setTaskSupport(seq.zip(that)(bf2seq(bf)), tasksupport)

  def zipWithIndex[U >: T, That](implicit bf: CanBuildFrom[Repr, (U, Int), That]): That = this zip immutable.ParRange(0, size, 1, inclusive = false)

  def zipAll[S, U >: T, That](that: GenIterable[S], thisElem: U, thatElem: S)(implicit bf: CanBuildFrom[Repr, (U, S), That]): That = if (bf(repr).isCombiner && that.isParSeq) {
    val thatseq = that.asParSeq
    tasksupport.executeAndWaitResult(
      new ZipAll(size max thatseq.length, thisElem, thatElem, combinerFactory(() => bf(repr).asCombiner), splitter, thatseq.splitter) mapResult {
        _.resultWithTaskSupport
      }
    )
  } else setTaskSupport(seq.zipAll(that, thisElem, thatElem)(bf2seq(bf)), tasksupport)

  protected def toParCollection[U >: T, That](cbf: () => Combiner[U, That]): That = {
    tasksupport.executeAndWaitResult(new ToParCollection(combinerFactory(cbf), splitter) mapResult { _.resultWithTaskSupport })
  }

  protected def toParMap[K, V, That](cbf: () => Combiner[(K, V), That])(implicit ev: T <:< (K, V)): That = {
    tasksupport.executeAndWaitResult(new ToParMap(combinerFactory(cbf), splitter)(ev) mapResult { _.resultWithTaskSupport })
  }

  @deprecated("Use .seq.view instead", "2.11.0")
  def view = seq.view

  override def toArray[U >: T: ClassTag]: Array[U] = {
    val arr = new Array[U](size)
    copyToArray(arr)
    arr
  }

  override def toList: List[T] = seq.toList

  override def toIndexedSeq: scala.collection.immutable.IndexedSeq[T] = seq.toIndexedSeq

  override def toStream: Stream[T] = seq.toStream

  override def toIterator: Iterator[T] = splitter

  // the methods below are overridden

  override def toBuffer[U >: T]: scala.collection.mutable.Buffer[U] = seq.toBuffer // have additional, parallel buffers?

  override def toTraversable: GenTraversable[T] = this.asInstanceOf[GenTraversable[T]]

  override def toIterable: ParIterable[T] = this.asInstanceOf[ParIterable[T]]

  override def toSeq: ParSeq[T] = toParCollection[T, ParSeq[T]](() => ParSeq.newCombiner[T])

  override def toSet[U >: T]: immutable.ParSet[U] = toParCollection[U, immutable.ParSet[U]](() => immutable.ParSet.newCombiner[U])

  override def toMap[K, V](implicit ev: T <:< (K, V)): immutable.ParMap[K, V] = toParMap[K, V, immutable.ParMap[K, V]](() => immutable.ParMap.newCombiner[K, V])

  override def toVector: Vector[T] = to[Vector]

  override def to[Col[_]](implicit cbf: CanBuildFrom[Nothing, T, Col[T @uncheckedVariance]]): Col[T @uncheckedVariance] = if (cbf().isCombiner) {
    toParCollection[T, Col[T]](() => cbf().asCombiner)
  } else seq.to(cbf)

  /* tasks */

  protected trait StrictSplitterCheckTask[R, Tp] extends Task[R, Tp] {
    def requiresStrictSplitters = false
    if (requiresStrictSplitters && !isStrictSplitterCollection)
      throw new UnsupportedOperationException("This collection does not provide strict splitters.")
  }

  /** Standard accessor task that iterates over the elements of the collection.
   *
   *  @tparam R    type of the result of this method (`R` for result).
   *  @tparam Tp   the representation type of the task at hand.
   */
  protected trait Accessor[R, Tp]
  extends StrictSplitterCheckTask[R, Tp] {
    protected[this] val pit: IterableSplitter[T]
    protected[this] def newSubtask(p: IterableSplitter[T]): Accessor[R, Tp]
    def shouldSplitFurther = pit.shouldSplitFurther(self.repr, tasksupport.parallelismLevel)
    def split = pit.splitWithSignalling.map(newSubtask(_)) // default split procedure
    private[parallel] override def signalAbort = pit.abort()
    override def toString = this.getClass.getSimpleName + "(" + pit.toString + ")(" + result + ")(supername: " + super.toString + ")"
  }

  protected[this] trait NonDivisibleTask[R, Tp] extends StrictSplitterCheckTask[R, Tp] {
    def shouldSplitFurther = false
    def split = throw new UnsupportedOperationException("Does not split.")
  }

  protected[this] trait NonDivisible[R] extends NonDivisibleTask[R, NonDivisible[R]]

  protected[this] abstract class Composite[FR, SR, R, First <: StrictSplitterCheckTask[FR, _], Second <: StrictSplitterCheckTask[SR, _]]
    (val ft: First, val st: Second)
  extends NonDivisibleTask[R, Composite[FR, SR, R, First, Second]] {
    def combineResults(fr: FR, sr: SR): R
    @volatile var result: R = null.asInstanceOf[R]
    private[parallel] override def signalAbort() {
      ft.signalAbort()
      st.signalAbort()
    }
    protected def mergeSubtasks() {
      ft mergeThrowables st
      if (throwable eq null) result = combineResults(ft.result, st.result)
    }
    override def requiresStrictSplitters = ft.requiresStrictSplitters || st.requiresStrictSplitters
  }

  /** Sequentially performs one task after another. */
  protected[this] abstract class SeqComposite[FR, SR, R, First <: StrictSplitterCheckTask[FR, _], Second <: StrictSplitterCheckTask[SR, _]]
  (f: First, s: Second)
  extends Composite[FR, SR, R, First, Second](f, s) {
    def leaf(prevr: Option[R]) = {
      tasksupport.executeAndWaitResult(ft) : Any
      tasksupport.executeAndWaitResult(st) : Any
      mergeSubtasks()
    }
  }

  /** Performs two tasks in parallel, and waits for both to finish. */
  protected[this] abstract class ParComposite[FR, SR, R, First <: StrictSplitterCheckTask[FR, _], Second <: StrictSplitterCheckTask[SR, _]]
  (f: First, s: Second)
  extends Composite[FR, SR, R, First, Second](f, s) {
    def leaf(prevr: Option[R]) = {
      val ftfuture: () => Any = tasksupport.execute(ft)
      tasksupport.executeAndWaitResult(st) : Any
      ftfuture()
      mergeSubtasks()
    }
  }

  protected[this] abstract class ResultMapping[R, Tp, R1](val inner: StrictSplitterCheckTask[R, Tp])
  extends NonDivisibleTask[R1, ResultMapping[R, Tp, R1]] {
    @volatile var result: R1 = null.asInstanceOf[R1]
    def map(r: R): R1
    def leaf(prevr: Option[R1]) = {
      val initialResult = tasksupport.executeAndWaitResult(inner)
      result = map(initialResult)
    }
    private[parallel] override def signalAbort() {
      inner.signalAbort()
    }
    override def requiresStrictSplitters = inner.requiresStrictSplitters
  }

  protected trait Transformer[R, Tp] extends Accessor[R, Tp]

  protected[this] class Foreach[S](op: T => S, protected[this] val pit: IterableSplitter[T])
  extends Accessor[Unit, Foreach[S]] {
    @volatile var result: Unit = ()
    def leaf(prevr: Option[Unit]) = pit.foreach(op)
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Foreach[S](op, p)
  }

  protected[this] class Count(pred: T => Boolean, protected[this] val pit: IterableSplitter[T])
  extends Accessor[Int, Count] {
    // val pittxt = pit.toString
    @volatile var result: Int = 0
    def leaf(prevr: Option[Int]) = result = pit.count(pred)
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Count(pred, p)
    override def merge(that: Count) = result = result + that.result
    // override def toString = "CountTask(" + pittxt + ")"
  }

  protected[this] class Reduce[U >: T](op: (U, U) => U, protected[this] val pit: IterableSplitter[T])
  extends Accessor[Option[U], Reduce[U]] {
    @volatile var result: Option[U] = None
    def leaf(prevr: Option[Option[U]]) = if (pit.remaining > 0) result = Some(pit.reduce(op))
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Reduce(op, p)
    override def merge(that: Reduce[U]) =
      if (this.result == None) result = that.result
      else if (that.result != None) result = Some(op(result.get, that.result.get))
    override def requiresStrictSplitters = true
  }

  protected[this] class Fold[U >: T](z: U, op: (U, U) => U, protected[this] val pit: IterableSplitter[T])
  extends Accessor[U, Fold[U]] {
    @volatile var result: U = null.asInstanceOf[U]
    def leaf(prevr: Option[U]) = result = pit.fold(z)(op)
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Fold(z, op, p)
    override def merge(that: Fold[U]) = result = op(result, that.result)
  }

  protected[this] class Aggregate[S](z: () => S, seqop: (S, T) => S, combop: (S, S) => S, protected[this] val pit: IterableSplitter[T])
  extends Accessor[S, Aggregate[S]] {
    @volatile var result: S = null.asInstanceOf[S]
    def leaf(prevr: Option[S]) = result = pit.foldLeft(z())(seqop)
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Aggregate(z, seqop, combop, p)
    override def merge(that: Aggregate[S]) = result = combop(result, that.result)
  }

  protected[this] class Sum[U >: T](num: Numeric[U], protected[this] val pit: IterableSplitter[T])
  extends Accessor[U, Sum[U]] {
    @volatile var result: U = null.asInstanceOf[U]
    def leaf(prevr: Option[U]) = result = pit.sum(num)
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Sum(num, p)
    override def merge(that: Sum[U]) = result = num.plus(result, that.result)
  }

  protected[this] class Product[U >: T](num: Numeric[U], protected[this] val pit: IterableSplitter[T])
  extends Accessor[U, Product[U]] {
    @volatile var result: U = null.asInstanceOf[U]
    def leaf(prevr: Option[U]) = result = pit.product(num)
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Product(num, p)
    override def merge(that: Product[U]) = result = num.times(result, that.result)
  }

  protected[this] class Min[U >: T](ord: Ordering[U], protected[this] val pit: IterableSplitter[T])
  extends Accessor[Option[U], Min[U]] {
    @volatile var result: Option[U] = None
    def leaf(prevr: Option[Option[U]]) = if (pit.remaining > 0) result = Some(pit.min(ord))
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Min(ord, p)
    override def merge(that: Min[U]) =
      if (this.result == None) result = that.result
      else if (that.result != None) result = if (ord.lteq(result.get, that.result.get)) result else that.result
    override def requiresStrictSplitters = true
  }

  protected[this] class Max[U >: T](ord: Ordering[U], protected[this] val pit: IterableSplitter[T])
  extends Accessor[Option[U], Max[U]] {
    @volatile var result: Option[U] = None
    def leaf(prevr: Option[Option[U]]) = if (pit.remaining > 0) result = Some(pit.max(ord))
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Max(ord, p)
    override def merge(that: Max[U]) =
      if (this.result == None) result = that.result
      else if (that.result != None) result = if (ord.gteq(result.get, that.result.get)) result else that.result
    override def requiresStrictSplitters = true
  }

  protected[this] class Map[S, That](f: T => S, cbf: CombinerFactory[S, That], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[S, That], Map[S, That]] {
    @volatile var result: Combiner[S, That] = null
    def leaf(prev: Option[Combiner[S, That]]) = result = pit.map2combiner(f, reuse(prev, cbf()))
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Map(f, cbf, p)
    override def merge(that: Map[S, That]) = result = result combine that.result
  }

  protected[this] class Collect[S, That]
  (pf: PartialFunction[T, S], pbf: CombinerFactory[S, That], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[S, That], Collect[S, That]] {
    @volatile var result: Combiner[S, That] = null
    def leaf(prev: Option[Combiner[S, That]]) = result = pit.collect2combiner[S, That](pf, pbf())
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Collect(pf, pbf, p)
    override def merge(that: Collect[S, That]) = result = result combine that.result
  }

  protected[this] class FlatMap[S, That]
  (f: T => GenTraversableOnce[S], pbf: CombinerFactory[S, That], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[S, That], FlatMap[S, That]] {
    @volatile var result: Combiner[S, That] = null
    def leaf(prev: Option[Combiner[S, That]]) = result = pit.flatmap2combiner(f, pbf())
    protected[this] def newSubtask(p: IterableSplitter[T]) = new FlatMap(f, pbf, p)
    override def merge(that: FlatMap[S, That]) = {
      //debuglog("merging " + result + " and " + that.result)
      result = result combine that.result
      //debuglog("merged into " + result)
    }
  }

  protected[this] class Forall(pred: T => Boolean, protected[this] val pit: IterableSplitter[T])
  extends Accessor[Boolean, Forall] {
    @volatile var result: Boolean = true
    def leaf(prev: Option[Boolean]) = { if (!pit.isAborted) result = pit.forall(pred); if (result == false) pit.abort() }
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Forall(pred, p)
    override def merge(that: Forall) = result = result && that.result
  }

  protected[this] class Exists(pred: T => Boolean, protected[this] val pit: IterableSplitter[T])
  extends Accessor[Boolean, Exists] {
    @volatile var result: Boolean = false
    def leaf(prev: Option[Boolean]) = { if (!pit.isAborted) result = pit.exists(pred); if (result == true) pit.abort() }
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Exists(pred, p)
    override def merge(that: Exists) = result = result || that.result
  }

  protected[this] class Find[U >: T](pred: T => Boolean, protected[this] val pit: IterableSplitter[T])
  extends Accessor[Option[U], Find[U]] {
    @volatile var result: Option[U] = None
    def leaf(prev: Option[Option[U]]) = { if (!pit.isAborted) result = pit.find(pred); if (result != None) pit.abort() }
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Find(pred, p)
    override def merge(that: Find[U]) = if (this.result == None) result = that.result
  }

  protected[this] class Filter[U >: T, This >: Repr](pred: T => Boolean, cbf: CombinerFactory[U, This], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[U, This], Filter[U, This]] {
    @volatile var result: Combiner[U, This] = null
    def leaf(prev: Option[Combiner[U, This]]) = {
      result = pit.filter2combiner(pred, reuse(prev, cbf()))
    }
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Filter(pred, cbf, p)
    override def merge(that: Filter[U, This]) = result = result combine that.result
  }

  protected[this] class FilterNot[U >: T, This >: Repr](pred: T => Boolean, cbf: CombinerFactory[U, This], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[U, This], FilterNot[U, This]] {
    @volatile var result: Combiner[U, This] = null
    def leaf(prev: Option[Combiner[U, This]]) = {
      result = pit.filterNot2combiner(pred, reuse(prev, cbf()))
    }
    protected[this] def newSubtask(p: IterableSplitter[T]) = new FilterNot(pred, cbf, p)
    override def merge(that: FilterNot[U, This]) = result = result combine that.result
  }

  protected class Copy[U >: T, That](cfactory: CombinerFactory[U, That], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[U, That], Copy[U, That]] {
    @volatile var result: Combiner[U, That] = null
    def leaf(prev: Option[Combiner[U, That]]) = result = pit.copy2builder[U, That, Combiner[U, That]](reuse(prev, cfactory()))
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Copy[U, That](cfactory, p)
    override def merge(that: Copy[U, That]) = result = result combine that.result
  }

  protected[this] class Partition[U >: T, This >: Repr]
  (pred: T => Boolean, cbfTrue: CombinerFactory[U, This], cbfFalse: CombinerFactory[U, This], protected[this] val pit: IterableSplitter[T])
  extends Transformer[(Combiner[U, This], Combiner[U, This]), Partition[U, This]] {
    @volatile var result: (Combiner[U, This], Combiner[U, This]) = null
    def leaf(prev: Option[(Combiner[U, This], Combiner[U, This])]) = result = pit.partition2combiners(pred, reuse(prev.map(_._1), cbfTrue()), reuse(prev.map(_._2), cbfFalse()))
    protected[this] def newSubtask(p: IterableSplitter[T]) = new Partition(pred, cbfTrue, cbfFalse, p)
    override def merge(that: Partition[U, This]) = result = (result._1 combine that.result._1, result._2 combine that.result._2)
  }

  protected[this] class GroupBy[K, U >: T](
    f: U => K,
    mcf: () => HashMapCombiner[K, U],
    protected[this] val pit: IterableSplitter[T]
  ) extends Transformer[HashMapCombiner[K, U], GroupBy[K, U]] {
    @volatile var result: Result = null
    final def leaf(prev: Option[Result]) = {
      // note: HashMapCombiner doesn't merge same keys until evaluation
      val cb = mcf()
      while (pit.hasNext) {
        val elem = pit.next()
        cb += f(elem) -> elem
      }
      result = cb
    }
    protected[this] def newSubtask(p: IterableSplitter[T]) = new GroupBy(f, mcf, p)
    override def merge(that: GroupBy[K, U]) = {
      // note: this works because we know that a HashMapCombiner doesn't merge same keys until evaluation
      // --> we know we're not dropping any mappings
      result = (result combine that.result).asInstanceOf[HashMapCombiner[K, U]]
    }
  }

  protected[this] class Take[U >: T, This >: Repr]
  (n: Int, cbf: CombinerFactory[U, This], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[U, This], Take[U, This]] {
    @volatile var result: Combiner[U, This] = null
    def leaf(prev: Option[Combiner[U, This]]) = {
      result = pit.take2combiner(n, reuse(prev, cbf()))
    }
    protected[this] def newSubtask(p: IterableSplitter[T]) = throw new UnsupportedOperationException
    override def split = {
      val pits = pit.splitWithSignalling
      val sizes = pits.scanLeft(0)(_ + _.remaining)
      for ((p, untilp) <- pits zip sizes; if untilp <= n) yield {
        if (untilp + p.remaining < n) new Take(p.remaining, cbf, p)
        else new Take(n - untilp, cbf, p)
      }
    }
    override def merge(that: Take[U, This]) = result = result combine that.result
    override def requiresStrictSplitters = true
  }

  protected[this] class Drop[U >: T, This >: Repr]
  (n: Int, cbf: CombinerFactory[U, This], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[U, This], Drop[U, This]] {
    @volatile var result: Combiner[U, This] = null
    def leaf(prev: Option[Combiner[U, This]]) = result = pit.drop2combiner(n, reuse(prev, cbf()))
    protected[this] def newSubtask(p: IterableSplitter[T]) = throw new UnsupportedOperationException
    override def split = {
      val pits = pit.splitWithSignalling
      val sizes = pits.scanLeft(0)(_ + _.remaining)
      for ((p, withp) <- pits zip sizes.tail; if withp >= n) yield {
        if (withp - p.remaining > n) new Drop(0, cbf, p)
        else new Drop(n - withp + p.remaining, cbf, p)
      }
    }
    override def merge(that: Drop[U, This]) = result = result combine that.result
    override def requiresStrictSplitters = true
  }

  protected[this] class Slice[U >: T, This >: Repr]
  (from: Int, until: Int, cbf: CombinerFactory[U, This], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[U, This], Slice[U, This]] {
    @volatile var result: Combiner[U, This] = null
    def leaf(prev: Option[Combiner[U, This]]) = result = pit.slice2combiner(from, until, reuse(prev, cbf()))
    protected[this] def newSubtask(p: IterableSplitter[T]) = throw new UnsupportedOperationException
    override def split = {
      val pits = pit.splitWithSignalling
      val sizes = pits.scanLeft(0)(_ + _.remaining)
      for ((p, untilp) <- pits zip sizes; if untilp + p.remaining >= from || untilp <= until) yield {
        val f = (from max untilp) - untilp
        val u = (until min (untilp + p.remaining)) - untilp
        new Slice(f, u, cbf, p)
      }
    }
    override def merge(that: Slice[U, This]) = result = result combine that.result
    override def requiresStrictSplitters = true
  }

  protected[this] class SplitAt[U >: T, This >: Repr]
  (at: Int, cbfBefore: CombinerFactory[U, This], cbfAfter: CombinerFactory[U, This], protected[this] val pit: IterableSplitter[T])
  extends Transformer[(Combiner[U, This], Combiner[U, This]), SplitAt[U, This]] {
    @volatile var result: (Combiner[U, This], Combiner[U, This]) = null
    def leaf(prev: Option[(Combiner[U, This], Combiner[U, This])]) = result = pit.splitAt2combiners(at, reuse(prev.map(_._1), cbfBefore()), reuse(prev.map(_._2), cbfAfter()))
    protected[this] def newSubtask(p: IterableSplitter[T]) = throw new UnsupportedOperationException
    override def split = {
      val pits = pit.splitWithSignalling
      val sizes = pits.scanLeft(0)(_ + _.remaining)
      for ((p, untilp) <- pits zip sizes) yield new SplitAt((at max untilp min (untilp + p.remaining)) - untilp, cbfBefore, cbfAfter, p)
    }
    override def merge(that: SplitAt[U, This]) = result = (result._1 combine that.result._1, result._2 combine that.result._2)
    override def requiresStrictSplitters = true
  }

  protected[this] class TakeWhile[U >: T, This >: Repr]
  (pos: Int, pred: T => Boolean, cbf: CombinerFactory[U, This], protected[this] val pit: IterableSplitter[T])
  extends Transformer[(Combiner[U, This], Boolean), TakeWhile[U, This]] {
    @volatile var result: (Combiner[U, This], Boolean) = null
    def leaf(prev: Option[(Combiner[U, This], Boolean)]) = if (pos < pit.indexFlag) {
      result = pit.takeWhile2combiner(pred, reuse(prev.map(_._1), cbf()))
      if (!result._2) pit.setIndexFlagIfLesser(pos)
    } else result = (reuse(prev.map(_._1), cbf()), false)
    protected[this] def newSubtask(p: IterableSplitter[T]) = throw new UnsupportedOperationException
    override def split = {
      val pits = pit.splitWithSignalling
      for ((p, untilp) <- pits zip pits.scanLeft(0)(_ + _.remaining)) yield new TakeWhile(pos + untilp, pred, cbf, p)
    }
    override def merge(that: TakeWhile[U, This]) = if (result._2) {
      result = (result._1 combine that.result._1, that.result._2)
    }
    override def requiresStrictSplitters = true
  }

  protected[this] class Span[U >: T, This >: Repr]
  (pos: Int, pred: T => Boolean, cbfBefore: CombinerFactory[U, This], cbfAfter: CombinerFactory[U, This], protected[this] val pit: IterableSplitter[T])
  extends Transformer[(Combiner[U, This], Combiner[U, This]), Span[U, This]] {
    @volatile var result: (Combiner[U, This], Combiner[U, This]) = null
    def leaf(prev: Option[(Combiner[U, This], Combiner[U, This])]) = if (pos < pit.indexFlag) {
      // val lst = pit.toList
      // val pa = mutable.ParArray(lst: _*)
      // val str = "At leaf we will iterate: " + pa.splitter.toList
      result = pit.span2combiners(pred, cbfBefore(), cbfAfter()) // do NOT reuse old combiners here, lest ye be surprised
      // println("\nAt leaf result is: " + result)
      if (result._2.size > 0) pit.setIndexFlagIfLesser(pos)
    } else {
      result = (reuse(prev.map(_._2), cbfBefore()), pit.copy2builder[U, This, Combiner[U, This]](reuse(prev.map(_._2), cbfAfter())))
    }
    protected[this] def newSubtask(p: IterableSplitter[T]) = throw new UnsupportedOperationException
    override def split = {
      val pits = pit.splitWithSignalling
      for ((p, untilp) <- pits zip pits.scanLeft(0)(_ + _.remaining)) yield new Span(pos + untilp, pred, cbfBefore, cbfAfter, p)
    }
    override def merge(that: Span[U, This]) = result = if (result._2.size == 0) {
      (result._1 combine that.result._1, that.result._2)
    } else {
      (result._1, result._2 combine that.result._1 combine that.result._2)
    }
    override def requiresStrictSplitters = true
  }

  protected[this] class Zip[U >: T, S, That](pbf: CombinerFactory[(U, S), That], protected[this] val pit: IterableSplitter[T], val othpit: SeqSplitter[S])
  extends Transformer[Combiner[(U, S), That], Zip[U, S, That]] {
    @volatile var result: Result = null
    def leaf(prev: Option[Result]) = result = pit.zip2combiner[U, S, That](othpit, pbf())
    protected[this] def newSubtask(p: IterableSplitter[T]) = unsupported
    override def split = {
      val pits = pit.splitWithSignalling
      val sizes = pits.map(_.remaining)
      val opits = othpit.psplitWithSignalling(sizes: _*)
      (pits zip opits) map { p => new Zip(pbf, p._1, p._2) }
    }
    override def merge(that: Zip[U, S, That]) = result = result combine that.result
    override def requiresStrictSplitters = true
  }

  protected[this] class ZipAll[U >: T, S, That]
  (len: Int, thiselem: U, thatelem: S, pbf: CombinerFactory[(U, S), That], protected[this] val pit: IterableSplitter[T], val othpit: SeqSplitter[S])
  extends Transformer[Combiner[(U, S), That], ZipAll[U, S, That]] {
    @volatile var result: Result = null
    def leaf(prev: Option[Result]) = result = pit.zipAll2combiner[U, S, That](othpit, thiselem, thatelem, pbf())
    protected[this] def newSubtask(p: IterableSplitter[T]) = unsupported
    override def split = if (pit.remaining <= len) {
      val pits = pit.splitWithSignalling
      val sizes = pits.map(_.remaining)
      val opits = othpit.psplitWithSignalling(sizes: _*)
      ((pits zip opits) zip sizes) map { t => new ZipAll(t._2, thiselem, thatelem, pbf, t._1._1, t._1._2) }
    } else {
      val opits = othpit.psplitWithSignalling(pit.remaining)
      val diff = len - pit.remaining
      Seq(
        new ZipAll(pit.remaining, thiselem, thatelem, pbf, pit, opits(0)), // nothing wrong will happen with the cast below - elem T is never accessed
        new ZipAll(diff, thiselem, thatelem, pbf, immutable.repetition(thiselem, diff).splitter.asInstanceOf[IterableSplitter[T]], opits(1))
      )
    }
    override def merge(that: ZipAll[U, S, That]) = result = result combine that.result
    override def requiresStrictSplitters = true
  }

  protected[this] class CopyToArray[U >: T, This >: Repr](from: Int, len: Int, array: Array[U], protected[this] val pit: IterableSplitter[T])
  extends Accessor[Unit, CopyToArray[U, This]] {
    @volatile var result: Unit = ()
    def leaf(prev: Option[Unit]) = pit.copyToArray(array, from, len)
    protected[this] def newSubtask(p: IterableSplitter[T]) = unsupported
    override def split = {
      val pits = pit.splitWithSignalling
      for ((p, untilp) <- pits zip pits.scanLeft(0)(_ + _.remaining); if untilp < len) yield {
        val plen = p.remaining min (len - untilp)
        new CopyToArray[U, This](from + untilp, plen, array, p)
      }
    }
    override def requiresStrictSplitters = true
  }

  protected[this] class ToParCollection[U >: T, That](cbf: CombinerFactory[U, That], protected[this] val pit: IterableSplitter[T])
  extends Transformer[Combiner[U, That], ToParCollection[U, That]] {
    @volatile var result: Result = null
    def leaf(prev: Option[Combiner[U, That]]) {
      result = cbf()
      while (pit.hasNext) result += pit.next
    }
    protected[this] def newSubtask(p: IterableSplitter[T]) = new ToParCollection[U, That](cbf, p)
    override def merge(that: ToParCollection[U, That]) = result = result combine that.result
  }

  protected[this] class ToParMap[K, V, That](cbf: CombinerFactory[(K, V), That], protected[this] val pit: IterableSplitter[T])(implicit ev: T <:< (K, V))
  extends Transformer[Combiner[(K, V), That], ToParMap[K, V, That]] {
    @volatile var result: Result = null
    def leaf(prev: Option[Combiner[(K, V), That]]) {
      result = cbf()
      while (pit.hasNext) result += pit.next
    }
    protected[this] def newSubtask(p: IterableSplitter[T]) = new ToParMap[K, V, That](cbf, p)(ev)
    override def merge(that: ToParMap[K, V, That]) = result = result combine that.result
  }

  protected[this] class CreateScanTree[U >: T](from: Int, len: Int, z: U, op: (U, U) => U, protected[this] val pit: IterableSplitter[T])
  extends Transformer[ScanTree[U], CreateScanTree[U]] {
    @volatile var result: ScanTree[U] = null
    def leaf(prev: Option[ScanTree[U]]) = if (pit.remaining > 0) {
      val trees = ArrayBuffer[ScanTree[U]]()
      var i = from
      val until = from + len
      val blocksize = scanBlockSize
      while (i < until) {
        trees += scanBlock(i, scala.math.min(blocksize, pit.remaining))
        i += blocksize
      }

      // merge trees
      result = mergeTrees(trees, 0, trees.length)
    } else result = null // no elements to scan (merge will take care of `null`s)
    private def scanBlock(from: Int, len: Int): ScanTree[U] = {
      val pitdup = pit.dup
      new ScanLeaf(pitdup, op, from, len, None, pit.reduceLeft(len, op))
    }
    private def mergeTrees(trees: ArrayBuffer[ScanTree[U]], from: Int, howmany: Int): ScanTree[U] = if (howmany > 1) {
      val half = howmany / 2
      ScanNode(mergeTrees(trees, from, half), mergeTrees(trees, from + half, howmany - half))
    } else trees(from)
    protected[this] def newSubtask(pit: IterableSplitter[T]) = unsupported
    override def split = {
      val pits = pit.splitWithSignalling
      for ((p, untilp) <- pits zip pits.scanLeft(from)(_ + _.remaining)) yield {
        new CreateScanTree(untilp, p.remaining, z, op, p)
      }
    }
    override def merge(that: CreateScanTree[U]) = if (this.result != null) {
      if (that.result != null) result = ScanNode(result, that.result)
    } else result = that.result
    override def requiresStrictSplitters = true
  }

  protected[this] class FromScanTree[U >: T, That]
  (tree: ScanTree[U], z: U, op: (U, U) => U, cbf: CombinerFactory[U, That])
  extends StrictSplitterCheckTask[Combiner[U, That], FromScanTree[U, That]] {
    @volatile var result: Combiner[U, That] = null
    def leaf(prev: Option[Combiner[U, That]]) {
      val cb = reuse(prev, cbf())
      iterate(tree, cb)
      result = cb
    }
    private def iterate(tree: ScanTree[U], cb: Combiner[U, That]): Unit = tree match {
      case ScanNode(left, right) =>
        iterate(left, cb)
        iterate(right, cb)
      case ScanLeaf(p, _, _, len, Some(prev), _) =>
        p.scanToCombiner(len, prev.acc, op, cb)
      case ScanLeaf(p, _, _, len, None, _) =>
        cb += z
        p.scanToCombiner(len, z, op, cb)
    }
    def split = tree match {
      case ScanNode(left, right) => Seq(
        new FromScanTree(left, z, op, cbf),
        new FromScanTree(right, z, op, cbf)
      )
      case _ => unsupportedop("Cannot be split further")
    }
    def shouldSplitFurther = tree match {
      case ScanNode(_, _) => true
      case ScanLeaf(_, _, _, _, _, _) => false
    }
    override def merge(that: FromScanTree[U, That]) = result = result combine that.result
  }

  /* scan tree */

  protected[this] def scanBlockSize = (thresholdFromSize(size, tasksupport.parallelismLevel) / 2) max 1

  protected[this] trait ScanTree[U >: T] {
    def beginsAt: Int
    def pushdown(v: U): Unit
    def leftmost: ScanLeaf[U]
    def rightmost: ScanLeaf[U]
    def print(depth: Int = 0): Unit
  }

  protected[this] case class ScanNode[U >: T](left: ScanTree[U], right: ScanTree[U]) extends ScanTree[U] {
    right.pushdown(left.rightmost.acc)
    right.leftmost.prev = Some(left.rightmost)

    val leftmost = left.leftmost
    val rightmost = right.rightmost

    def beginsAt = left.beginsAt
    def pushdown(v: U) {
      left.pushdown(v)
      right.pushdown(v)
    }
    def print(depth: Int) {
      println((" " * depth) + "ScanNode, begins at " + beginsAt)
      left.print(depth + 1)
      right.print(depth + 1)
    }
  }

  protected[this] case class ScanLeaf[U >: T]
  (pit: IterableSplitter[U], op: (U, U) => U, from: Int, len: Int, var prev: Option[ScanLeaf[U]], var acc: U)
  extends ScanTree[U] {
    def beginsAt = from
    def pushdown(v: U) = {
      acc = op(v, acc)
    }
    def leftmost = this
    def rightmost = this
    def print(depth: Int) = println((" " * depth) + this)
  }

  /* alias methods */

  def /:[S](z: S)(op: (S, T) => S): S = foldLeft(z)(op)

  def :\[S](z: S)(op: (T, S) => S): S = foldRight(z)(op)

  /* debug information */

  private[parallel] def debugInformation = "Parallel collection: " + this.getClass

  private[parallel] def brokenInvariants = Seq[String]()

  // private val dbbuff = ArrayBuffer[String]()
  // def debugBuffer: ArrayBuffer[String] = dbbuff
  def debugBuffer: ArrayBuffer[String] = null

  private[parallel] def debugclear() = synchronized {
    debugBuffer.clear()
  }

  private[parallel] def debuglog(s: String) = synchronized {
    debugBuffer += s
  }

  import scala.collection.DebugUtils._
  private[parallel] def printDebugBuffer() = println(buildString {
    append =>
    for (s <- debugBuffer) {
      append(s)
    }
  })

}

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