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	/*
	* Copyright (c) 2012, 2017, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/
	package java.util.stream;

	import java.util.Spliterator;
	import java.util.concurrent.CountedCompleter;
	import java.util.concurrent.ForkJoinPool;
	import java.util.concurrent.ForkJoinWorkerThread;

	/**
	* Abstract base class for most fork-join tasks used to implement stream ops.
	* Manages splitting logic, tracking of child tasks, and intermediate results.
	* Each task is associated with a {@link Spliterator} that describes the portion
	* of the input associated with the subtree rooted at this task.
	* Tasks may be leaf nodes (which will traverse the elements of
	* the {@code Spliterator}) or internal nodes (which split the
	* {@code Spliterator} into multiple child tasks).
	*
	* @implNote
	* <p>This class is based on {@link CountedCompleter}, a form of fork-join task
	* where each task has a semaphore-like count of uncompleted children, and the
	* task is implicitly completed and notified when its last child completes.
	* Internal node tasks will likely override the {@code onCompletion} method from
	* {@code CountedCompleter} to merge the results from child tasks into the
	* current task's result.
	*
	* <p>Splitting and setting up the child task links is done by {@code compute()}
	* for internal nodes. At {@code compute()} time for leaf nodes, it is
	* guaranteed that the parent's child-related fields (including sibling links
	* for the parent's children) will be set up for all children.
	*
	* <p>For example, a task that performs a reduce would override {@code doLeaf()}
	* to perform a reduction on that leaf node's chunk using the
	* {@code Spliterator}, and override {@code onCompletion()} to merge the results
	* of the child tasks for internal nodes:
	*
	* <pre>{@code
	* protected S doLeaf() {
	* spliterator.forEach(...);
	* return localReductionResult;
	* }
	*
	* public void onCompletion(CountedCompleter caller) {
	* if (!isLeaf()) {
	* ReduceTask<P_IN, P_OUT, T, R> child = children;
	* R result = child.getLocalResult();
	* child = child.nextSibling;
	* for (; child != null; child = child.nextSibling)
	* result = combine(result, child.getLocalResult());
	* setLocalResult(result);
	* }
	* }
	* }</pre>
	*
	* <p>Serialization is not supported as there is no intention to serialize
	* tasks managed by stream ops.
	*
	* @param <P_IN> Type of elements input to the pipeline
	* @param <P_OUT> Type of elements output from the pipeline
	* @param <R> Type of intermediate result, which may be different from operation
	* result type
	* @param <K> Type of parent, child and sibling tasks
	* @since 1.8
	*/
	@SuppressWarnings("serial")
	abstract class AbstractTask<P_IN, P_OUT, R,
	K extends AbstractTask<P_IN, P_OUT, R, K>>
	extends CountedCompleter<R> {

	private static final int LEAF_TARGET = ForkJoinPool.getCommonPoolParallelism() << 2;

	/** The pipeline helper, common to all tasks in a computation */
	protected final PipelineHelper<P_OUT> helper;

	/**
	* The spliterator for the portion of the input associated with the subtree
	* rooted at this task
	*/
	protected Spliterator<P_IN> spliterator;

	/** Target leaf size, common to all tasks in a computation */
	protected long targetSize; // may be lazily initialized

	/**
	* The left child.
	* null if no children
	* if non-null rightChild is non-null
	*/
	protected K leftChild;

	/**
	* The right child.
	* null if no children
	* if non-null leftChild is non-null
	*/
	protected K rightChild;

	/** The result of this node, if completed */
	private R localResult;

	/**
	* Constructor for root nodes.
	*
	* @param helper The {@code PipelineHelper} describing the stream pipeline
	* up to this operation
	* @param spliterator The {@code Spliterator} describing the source for this
	* pipeline
	*/
	protected AbstractTask(PipelineHelper<P_OUT> helper,
	Spliterator<P_IN> spliterator) {
	super(null);
	this.helper = helper;
	this.spliterator = spliterator;
	this.targetSize = 0L;
	}

	/**
	* Constructor for non-root nodes.
	*
	* @param parent this node's parent task
	* @param spliterator {@code Spliterator} describing the subtree rooted at
	* this node, obtained by splitting the parent {@code Spliterator}
	*/
	protected AbstractTask(K parent,
	Spliterator<P_IN> spliterator) {
	super(parent);
	this.spliterator = spliterator;
	this.helper = parent.helper;
	this.targetSize = parent.targetSize;
	}

	/**
	* Default target of leaf tasks for parallel decomposition.
	* To allow load balancing, we over-partition, currently to approximately
	* four tasks per processor, which enables others to help out
	* if leaf tasks are uneven or some processors are otherwise busy.
	*/
	public static int getLeafTarget() {
	Thread t = Thread.currentThread();
	if (t instanceof ForkJoinWorkerThread) {
	return ((ForkJoinWorkerThread) t).getPool().getParallelism() << 2;
	}
	else {
	return LEAF_TARGET;
	}
	}

	/**
	* Constructs a new node of type T whose parent is the receiver; must call
	* the AbstractTask(T, Spliterator) constructor with the receiver and the
	* provided Spliterator.
	*
	* @param spliterator {@code Spliterator} describing the subtree rooted at
	* this node, obtained by splitting the parent {@code Spliterator}
	* @return newly constructed child node
	*/
	protected abstract K makeChild(Spliterator<P_IN> spliterator);

	/**
	* Computes the result associated with a leaf node. Will be called by
	* {@code compute()} and the result passed to @{code setLocalResult()}
	*
	* @return the computed result of a leaf node
	*/
	protected abstract R doLeaf();

	/**
	* Returns a suggested target leaf size based on the initial size estimate.
	*
	* @return suggested target leaf size
	*/
	public static long suggestTargetSize(long sizeEstimate) {
	long est = sizeEstimate / getLeafTarget();
	return est > 0L ? est : 1L;
	}

	/**
	* Returns the targetSize, initializing it via the supplied
	* size estimate if not already initialized.
	*/
	protected final long getTargetSize(long sizeEstimate) {
	long s;
	return ((s = targetSize) != 0 ? s :
	(targetSize = suggestTargetSize(sizeEstimate)));
	}

	/**
	* Returns the local result, if any. Subclasses should use
	* {@link #setLocalResult(Object)} and {@link #getLocalResult()} to manage
	* results. This returns the local result so that calls from within the
	* fork-join framework will return the correct result.
	*
	* @return local result for this node previously stored with
	* {@link #setLocalResult}
	*/
	@Override
	public R getRawResult() {
	return localResult;
	}

	/**
	* Does nothing; instead, subclasses should use
	* {@link #setLocalResult(Object)}} to manage results.
	*
	* @param result must be null, or an exception is thrown (this is a safety
	* tripwire to detect when {@code setRawResult()} is being used
	* instead of {@code setLocalResult()}
	*/
	@Override
	protected void setRawResult(R result) {
	if (result != null)
	throw new IllegalStateException();
	}

	/**
	* Retrieves a result previously stored with {@link #setLocalResult}
	*
	* @return local result for this node previously stored with
	* {@link #setLocalResult}
	*/
	protected R getLocalResult() {
	return localResult;
	}

	/**
	* Associates the result with the task, can be retrieved with
	* {@link #getLocalResult}
	*
	* @param localResult local result for this node
	*/
	protected void setLocalResult(R localResult) {
	this.localResult = localResult;
	}

	/**
	* Indicates whether this task is a leaf node. (Only valid after
	* {@link #compute} has been called on this node). If the node is not a
	* leaf node, then children will be non-null and numChildren will be
	* positive.
	*
	* @return {@code true} if this task is a leaf node
	*/
	protected boolean isLeaf() {
	return leftChild == null;
	}

	/**
	* Indicates whether this task is the root node
	*
	* @return {@code true} if this task is the root node.
	*/
	protected boolean isRoot() {
	return getParent() == null;
	}

	/**
	* Returns the parent of this task, or null if this task is the root
	*
	* @return the parent of this task, or null if this task is the root
	*/
	@SuppressWarnings("unchecked")
	protected K getParent() {
	return (K) getCompleter();
	}

	/**
	* Decides whether or not to split a task further or compute it
	* directly. If computing directly, calls {@code doLeaf} and pass
	* the result to {@code setRawResult}. Otherwise splits off
	* subtasks, forking one and continuing as the other.
	*
	* <p> The method is structured to conserve resources across a
	* range of uses. The loop continues with one of the child tasks
	* when split, to avoid deep recursion. To cope with spliterators
	* that may be systematically biased toward left-heavy or
	* right-heavy splits, we alternate which child is forked versus
	* continued in the loop.
	*/
	@Override
	public void compute() {
	Spliterator<P_IN> rs = spliterator, ls; // right, left spliterators
	long sizeEstimate = rs.estimateSize();
	long sizeThreshold = getTargetSize(sizeEstimate);
	boolean forkRight = false;
	@SuppressWarnings("unchecked") K task = (K) this;
	while (sizeEstimate > sizeThreshold && (ls = rs.trySplit()) != null) {
	K leftChild, rightChild, taskToFork;
	task.leftChild = leftChild = task.makeChild(ls);
	task.rightChild = rightChild = task.makeChild(rs);
	task.setPendingCount(1);
	if (forkRight) {
	forkRight = false;
	rs = ls;
	task = leftChild;
	taskToFork = rightChild;
	}
	else {
	forkRight = true;
	task = rightChild;
	taskToFork = leftChild;
	}
	taskToFork.fork();
	sizeEstimate = rs.estimateSize();
	}
	task.setLocalResult(task.doLeaf());
	task.tryComplete();
	}

	/**
	* {@inheritDoc}
	*
	* @implNote
	* Clears spliterator and children fields. Overriders MUST call
	* {@code super.onCompletion} as the last thing they do if they want these
	* cleared.
	*/
	@Override
	public void onCompletion(CountedCompleter<?> caller) {
	spliterator = null;
	leftChild = rightChild = null;
	}

	/**
	* Returns whether this node is a "leftmost" node -- whether the path from
	* the root to this node involves only traversing leftmost child links. For
	* a leaf node, this means it is the first leaf node in the encounter order.
	*
	* @return {@code true} if this node is a "leftmost" node
	*/
	protected boolean isLeftmostNode() {
	@SuppressWarnings("unchecked")
	K node = (K) this;
	while (node != null) {
	K parent = node.getParent();
	if (parent != null && parent.leftChild != node)
	return false;
	node = parent;
	}
	return true;
	}
	}

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