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	/*
	* Copyright (c) 2012, 2021, 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.Objects;
	import java.util.Spliterator;
	import java.util.function.IntFunction;
	import java.util.function.Supplier;

	/**
	* Abstract base class for "pipeline" classes, which are the core
	* implementations of the Stream interface and its primitive specializations.
	* Manages construction and evaluation of stream pipelines.
	*
	* <p>An {@code AbstractPipeline} represents an initial portion of a stream
	* pipeline, encapsulating a stream source and zero or more intermediate
	* operations. The individual {@code AbstractPipeline} objects are often
	* referred to as <em>stages</em>, where each stage describes either the stream
	* source or an intermediate operation.
	*
	* <p>A concrete intermediate stage is generally built from an
	* {@code AbstractPipeline}, a shape-specific pipeline class which extends it
	* (e.g., {@code IntPipeline}) which is also abstract, and an operation-specific
	* concrete class which extends that. {@code AbstractPipeline} contains most of
	* the mechanics of evaluating the pipeline, and implements methods that will be
	* used by the operation; the shape-specific classes add helper methods for
	* dealing with collection of results into the appropriate shape-specific
	* containers.
	*
	* <p>After chaining a new intermediate operation, or executing a terminal
	* operation, the stream is considered to be consumed, and no more intermediate
	* or terminal operations are permitted on this stream instance.
	*
	* @implNote
	* <p>For sequential streams, and parallel streams without
	* <a href="package-summary.html#StreamOps">stateful intermediate
	* operations</a>, parallel streams, pipeline evaluation is done in a single
	* pass that "jams" all the operations together. For parallel streams with
	* stateful operations, execution is divided into segments, where each
	* stateful operations marks the end of a segment, and each segment is
	* evaluated separately and the result used as the input to the next
	* segment. In all cases, the source data is not consumed until a terminal
	* operation begins.
	*
	* @param <E_IN> type of input elements
	* @param <E_OUT> type of output elements
	* @param <S> type of the subclass implementing {@code BaseStream}
	* @since 1.8
	*/
	abstract class AbstractPipeline<E_IN, E_OUT, S extends BaseStream<E_OUT, S>>
	extends PipelineHelper<E_OUT> implements BaseStream<E_OUT, S> {
	private static final String MSG_STREAM_LINKED = "stream has already been operated upon or closed";
	private static final String MSG_CONSUMED = "source already consumed or closed";

	/**
	* Backlink to the head of the pipeline chain (self if this is the source
	* stage).
	*/
	@SuppressWarnings("rawtypes")
	private final AbstractPipeline sourceStage;

	/**
	* The "upstream" pipeline, or null if this is the source stage.
	*/
	@SuppressWarnings("rawtypes")
	private final AbstractPipeline previousStage;

	/**
	* The operation flags for the intermediate operation represented by this
	* pipeline object.
	*/
	protected final int sourceOrOpFlags;

	/**
	* The next stage in the pipeline, or null if this is the last stage.
	* Effectively final at the point of linking to the next pipeline.
	*/
	@SuppressWarnings("rawtypes")
	private AbstractPipeline nextStage;

	/**
	* The number of intermediate operations between this pipeline object
	* and the stream source if sequential, or the previous stateful if parallel.
	* Valid at the point of pipeline preparation for evaluation.
	*/
	private int depth;

	/**
	* The combined source and operation flags for the source and all operations
	* up to and including the operation represented by this pipeline object.
	* Valid at the point of pipeline preparation for evaluation.
	*/
	private int combinedFlags;

	/**
	* The source spliterator. Only valid for the head pipeline.
	* Before the pipeline is consumed if non-null then {@code sourceSupplier}
	* must be null. After the pipeline is consumed if non-null then is set to
	* null.
	*/
	private Spliterator<?> sourceSpliterator;

	/**
	* The source supplier. Only valid for the head pipeline. Before the
	* pipeline is consumed if non-null then {@code sourceSpliterator} must be
	* null. After the pipeline is consumed if non-null then is set to null.
	*/
	private Supplier<? extends Spliterator<?>> sourceSupplier;

	/**
	* True if this pipeline has been linked or consumed
	*/
	private boolean linkedOrConsumed;

	/**
	* True if there are any stateful ops in the pipeline; only valid for the
	* source stage.
	*/
	private boolean sourceAnyStateful;

	private Runnable sourceCloseAction;

	/**
	* True if pipeline is parallel, otherwise the pipeline is sequential; only
	* valid for the source stage.
	*/
	private boolean parallel;

	/**
	* Constructor for the head of a stream pipeline.
	*
	* @param source {@code Supplier<Spliterator>} describing the stream source
	* @param sourceFlags The source flags for the stream source, described in
	* {@link StreamOpFlag}
	* @param parallel True if the pipeline is parallel
	*/
	AbstractPipeline(Supplier<? extends Spliterator<?>> source,
	int sourceFlags, boolean parallel) {
	this.previousStage = null;
	this.sourceSupplier = source;
	this.sourceStage = this;
	this.sourceOrOpFlags = sourceFlags & StreamOpFlag.STREAM_MASK;
	// The following is an optimization of:
	// StreamOpFlag.combineOpFlags(sourceOrOpFlags, StreamOpFlag.INITIAL_OPS_VALUE);
	this.combinedFlags = (~(sourceOrOpFlags << 1)) & StreamOpFlag.INITIAL_OPS_VALUE;
	this.depth = 0;
	this.parallel = parallel;
	}

	/**
	* Constructor for the head of a stream pipeline.
	*
	* @param source {@code Spliterator} describing the stream source
	* @param sourceFlags the source flags for the stream source, described in
	* {@link StreamOpFlag}
	* @param parallel {@code true} if the pipeline is parallel
	*/
	AbstractPipeline(Spliterator<?> source,
	int sourceFlags, boolean parallel) {
	this.previousStage = null;
	this.sourceSpliterator = source;
	this.sourceStage = this;
	this.sourceOrOpFlags = sourceFlags & StreamOpFlag.STREAM_MASK;
	// The following is an optimization of:
	// StreamOpFlag.combineOpFlags(sourceOrOpFlags, StreamOpFlag.INITIAL_OPS_VALUE);
	this.combinedFlags = (~(sourceOrOpFlags << 1)) & StreamOpFlag.INITIAL_OPS_VALUE;
	this.depth = 0;
	this.parallel = parallel;
	}

	/**
	* Constructor for appending an intermediate operation stage onto an
	* existing pipeline.
	*
	* @param previousStage the upstream pipeline stage
	* @param opFlags the operation flags for the new stage, described in
	* {@link StreamOpFlag}
	*/
	AbstractPipeline(AbstractPipeline<?, E_IN, ?> previousStage, int opFlags) {
	if (previousStage.linkedOrConsumed)
	throw new IllegalStateException(MSG_STREAM_LINKED);
	previousStage.linkedOrConsumed = true;
	previousStage.nextStage = this;

	this.previousStage = previousStage;
	this.sourceOrOpFlags = opFlags & StreamOpFlag.OP_MASK;
	this.combinedFlags = StreamOpFlag.combineOpFlags(opFlags, previousStage.combinedFlags);
	this.sourceStage = previousStage.sourceStage;
	if (opIsStateful())
	sourceStage.sourceAnyStateful = true;
	this.depth = previousStage.depth + 1;
	}


	// Terminal evaluation methods

	/**
	* Evaluate the pipeline with a terminal operation to produce a result.
	*
	* @param <R> the type of result
	* @param terminalOp the terminal operation to be applied to the pipeline.
	* @return the result
	*/
	final <R> R evaluate(TerminalOp<E_OUT, R> terminalOp) {
	assert getOutputShape() == terminalOp.inputShape();
	if (linkedOrConsumed)
	throw new IllegalStateException(MSG_STREAM_LINKED);
	linkedOrConsumed = true;

	return isParallel()
	? terminalOp.evaluateParallel(this, sourceSpliterator(terminalOp.getOpFlags()))
	: terminalOp.evaluateSequential(this, sourceSpliterator(terminalOp.getOpFlags()));
	}

	/**
	* Collect the elements output from the pipeline stage.
	*
	* @param generator the array generator to be used to create array instances
	* @return a flat array-backed Node that holds the collected output elements
	*/
	@SuppressWarnings("unchecked")
	final Node<E_OUT> evaluateToArrayNode(IntFunction<E_OUT[]> generator) {
	if (linkedOrConsumed)
	throw new IllegalStateException(MSG_STREAM_LINKED);
	linkedOrConsumed = true;

	// If the last intermediate operation is stateful then
	// evaluate directly to avoid an extra collection step
	if (isParallel() && previousStage != null && opIsStateful()) {
	// Set the depth of this, last, pipeline stage to zero to slice the
	// pipeline such that this operation will not be included in the
	// upstream slice and upstream operations will not be included
	// in this slice
	depth = 0;
	return opEvaluateParallel(previousStage, previousStage.sourceSpliterator(0), generator);
	}
	else {
	return evaluate(sourceSpliterator(0), true, generator);
	}
	}

	/**
	* Gets the source stage spliterator if this pipeline stage is the source
	* stage. The pipeline is consumed after this method is called and
	* returns successfully.
	*
	* @return the source stage spliterator
	* @throws IllegalStateException if this pipeline stage is not the source
	* stage.
	*/
	@SuppressWarnings("unchecked")
	final Spliterator<E_OUT> sourceStageSpliterator() {
	if (this != sourceStage)
	throw new IllegalStateException();

	if (linkedOrConsumed)
	throw new IllegalStateException(MSG_STREAM_LINKED);
	linkedOrConsumed = true;

	if (sourceStage.sourceSpliterator != null) {
	@SuppressWarnings("unchecked")
	Spliterator<E_OUT> s = sourceStage.sourceSpliterator;
	sourceStage.sourceSpliterator = null;
	return s;
	}
	else if (sourceStage.sourceSupplier != null) {
	@SuppressWarnings("unchecked")
	Spliterator<E_OUT> s = (Spliterator<E_OUT>) sourceStage.sourceSupplier.get();
	sourceStage.sourceSupplier = null;
	return s;
	}
	else {
	throw new IllegalStateException(MSG_CONSUMED);
	}
	}

	// BaseStream

	@Override
	@SuppressWarnings("unchecked")
	public final S sequential() {
	sourceStage.parallel = false;
	return (S) this;
	}

	@Override
	@SuppressWarnings("unchecked")
	public final S parallel() {
	sourceStage.parallel = true;
	return (S) this;
	}

	@Override
	public void close() {
	linkedOrConsumed = true;
	sourceSupplier = null;
	sourceSpliterator = null;
	if (sourceStage.sourceCloseAction != null) {
	Runnable closeAction = sourceStage.sourceCloseAction;
	sourceStage.sourceCloseAction = null;
	closeAction.run();
	}
	}

	@Override
	@SuppressWarnings("unchecked")
	public S onClose(Runnable closeHandler) {
	if (linkedOrConsumed)
	throw new IllegalStateException(MSG_STREAM_LINKED);
	Objects.requireNonNull(closeHandler);
	Runnable existingHandler = sourceStage.sourceCloseAction;
	sourceStage.sourceCloseAction =
	(existingHandler == null)
	? closeHandler
	: Streams.composeWithExceptions(existingHandler, closeHandler);
	return (S) this;
	}

	// Primitive specialization use co-variant overrides, hence is not final
	@Override
	@SuppressWarnings("unchecked")
	public Spliterator<E_OUT> spliterator() {
	if (linkedOrConsumed)
	throw new IllegalStateException(MSG_STREAM_LINKED);
	linkedOrConsumed = true;

	if (this == sourceStage) {
	if (sourceStage.sourceSpliterator != null) {
	@SuppressWarnings("unchecked")
	Spliterator<E_OUT> s = (Spliterator<E_OUT>) sourceStage.sourceSpliterator;
	sourceStage.sourceSpliterator = null;
	return s;
	}
	else if (sourceStage.sourceSupplier != null) {
	@SuppressWarnings("unchecked")
	Supplier<Spliterator<E_OUT>> s = (Supplier<Spliterator<E_OUT>>) sourceStage.sourceSupplier;
	sourceStage.sourceSupplier = null;
	return lazySpliterator(s);
	}
	else {
	throw new IllegalStateException(MSG_CONSUMED);
	}
	}
	else {
	return wrap(this, () -> sourceSpliterator(0), isParallel());
	}
	}

	@Override
	public final boolean isParallel() {
	return sourceStage.parallel;
	}


	/**
	* Returns the composition of stream flags of the stream source and all
	* intermediate operations.
	*
	* @return the composition of stream flags of the stream source and all
	* intermediate operations
	* @see StreamOpFlag
	*/
	final int getStreamFlags() {
	return StreamOpFlag.toStreamFlags(combinedFlags);
	}

	/**
	* Get the source spliterator for this pipeline stage. For a sequential or
	* stateless parallel pipeline, this is the source spliterator. For a
	* stateful parallel pipeline, this is a spliterator describing the results
	* of all computations up to and including the most recent stateful
	* operation.
	*/
	@SuppressWarnings("unchecked")
	private Spliterator<?> sourceSpliterator(int terminalFlags) {
	// Get the source spliterator of the pipeline
	Spliterator<?> spliterator = null;
	if (sourceStage.sourceSpliterator != null) {
	spliterator = sourceStage.sourceSpliterator;
	sourceStage.sourceSpliterator = null;
	}
	else if (sourceStage.sourceSupplier != null) {
	spliterator = (Spliterator<?>) sourceStage.sourceSupplier.get();
	sourceStage.sourceSupplier = null;
	}
	else {
	throw new IllegalStateException(MSG_CONSUMED);
	}

	if (isParallel() && sourceStage.sourceAnyStateful) {
	// Adapt the source spliterator, evaluating each stateful op
	// in the pipeline up to and including this pipeline stage.
	// The depth and flags of each pipeline stage are adjusted accordingly.
	int depth = 1;
	for (@SuppressWarnings("rawtypes") AbstractPipeline u = sourceStage, p = sourceStage.nextStage, e = this;
	u != e;
	u = p, p = p.nextStage) {

	int thisOpFlags = p.sourceOrOpFlags;
	if (p.opIsStateful()) {
	depth = 0;

	if (StreamOpFlag.SHORT_CIRCUIT.isKnown(thisOpFlags)) {
	// Clear the short circuit flag for next pipeline stage
	// This stage encapsulates short-circuiting, the next
	// stage may not have any short-circuit operations, and
	// if so spliterator.forEachRemaining should be used
	// for traversal
	thisOpFlags = thisOpFlags & ~StreamOpFlag.IS_SHORT_CIRCUIT;
	}

	spliterator = p.opEvaluateParallelLazy(u, spliterator);

	// Inject or clear SIZED on the source pipeline stage
	// based on the stage's spliterator
	thisOpFlags = spliterator.hasCharacteristics(Spliterator.SIZED)
	? (thisOpFlags & ~StreamOpFlag.NOT_SIZED) \| StreamOpFlag.IS_SIZED
	: (thisOpFlags & ~StreamOpFlag.IS_SIZED) \| StreamOpFlag.NOT_SIZED;
	}
	p.depth = depth++;
	p.combinedFlags = StreamOpFlag.combineOpFlags(thisOpFlags, u.combinedFlags);
	}
	}

	if (terminalFlags != 0) {
	// Apply flags from the terminal operation to last pipeline stage
	combinedFlags = StreamOpFlag.combineOpFlags(terminalFlags, combinedFlags);
	}

	return spliterator;
	}

	// PipelineHelper

	@Override
	final StreamShape getSourceShape() {
	@SuppressWarnings("rawtypes")
	AbstractPipeline p = AbstractPipeline.this;
	while (p.depth > 0) {
	p = p.previousStage;
	}
	return p.getOutputShape();
	}

	@Override
	final <P_IN> long exactOutputSizeIfKnown(Spliterator<P_IN> spliterator) {
	int flags = getStreamAndOpFlags();
	long size = StreamOpFlag.SIZED.isKnown(flags) ? spliterator.getExactSizeIfKnown() : -1;
	// Currently, we have no stateless SIZE_ADJUSTING intermediate operations,
	// so we can simply ignore SIZE_ADJUSTING in parallel streams, since adjustments
	// are already accounted in the input spliterator.
	//
	// If we ever have a stateless SIZE_ADJUSTING intermediate operation,
	// we would need step back until depth == 0, then call exactOutputSize() for
	// the subsequent stages.
	if (size != -1 && StreamOpFlag.SIZE_ADJUSTING.isKnown(flags) && !isParallel()) {
	// Skip the source stage as it's never SIZE_ADJUSTING
	for (AbstractPipeline<?, ?, ?> stage = sourceStage.nextStage; stage != null; stage = stage.nextStage) {
	size = stage.exactOutputSize(size);
	}
	}
	return size;
	}

	/**
	* Returns the exact output size of the pipeline given the exact size reported by the previous stage.
	*
	* @param previousSize the exact size reported by the previous stage
	* @return the output size of this stage
	*/
	long exactOutputSize(long previousSize) {
	return previousSize;
	}

	@Override
	final <P_IN, S extends Sink<E_OUT>> S wrapAndCopyInto(S sink, Spliterator<P_IN> spliterator) {
	copyInto(wrapSink(Objects.requireNonNull(sink)), spliterator);
	return sink;
	}

	@Override
	final <P_IN> void copyInto(Sink<P_IN> wrappedSink, Spliterator<P_IN> spliterator) {
	Objects.requireNonNull(wrappedSink);

	if (!StreamOpFlag.SHORT_CIRCUIT.isKnown(getStreamAndOpFlags())) {
	wrappedSink.begin(spliterator.getExactSizeIfKnown());
	spliterator.forEachRemaining(wrappedSink);
	wrappedSink.end();
	}
	else {
	copyIntoWithCancel(wrappedSink, spliterator);
	}
	}

	@Override
	@SuppressWarnings("unchecked")
	final <P_IN> boolean copyIntoWithCancel(Sink<P_IN> wrappedSink, Spliterator<P_IN> spliterator) {
	@SuppressWarnings({"rawtypes","unchecked"})
	AbstractPipeline p = AbstractPipeline.this;
	while (p.depth > 0) {
	p = p.previousStage;
	}

	wrappedSink.begin(spliterator.getExactSizeIfKnown());
	boolean cancelled = p.forEachWithCancel(spliterator, wrappedSink);
	wrappedSink.end();
	return cancelled;
	}

	@Override
	final int getStreamAndOpFlags() {
	return combinedFlags;
	}

	final boolean isOrdered() {
	return StreamOpFlag.ORDERED.isKnown(combinedFlags);
	}

	@Override
	@SuppressWarnings("unchecked")
	final <P_IN> Sink<P_IN> wrapSink(Sink<E_OUT> sink) {
	Objects.requireNonNull(sink);

	for ( @SuppressWarnings("rawtypes") AbstractPipeline p=AbstractPipeline.this; p.depth > 0; p=p.previousStage) {
	sink = p.opWrapSink(p.previousStage.combinedFlags, sink);
	}
	return (Sink<P_IN>) sink;
	}

	@Override
	@SuppressWarnings("unchecked")
	final <P_IN> Spliterator<E_OUT> wrapSpliterator(Spliterator<P_IN> sourceSpliterator) {
	if (depth == 0) {
	return (Spliterator<E_OUT>) sourceSpliterator;
	}
	else {
	return wrap(this, () -> sourceSpliterator, isParallel());
	}
	}

	@Override
	@SuppressWarnings("unchecked")
	final <P_IN> Node<E_OUT> evaluate(Spliterator<P_IN> spliterator,
	boolean flatten,
	IntFunction<E_OUT[]> generator) {
	if (isParallel()) {
	// @@@ Optimize if op of this pipeline stage is a stateful op
	return evaluateToNode(this, spliterator, flatten, generator);
	}
	else {
	Node.Builder<E_OUT> nb = makeNodeBuilder(
	exactOutputSizeIfKnown(spliterator), generator);
	return wrapAndCopyInto(nb, spliterator).build();
	}
	}


	// Shape-specific abstract methods, implemented by XxxPipeline classes

	/**
	* Get the output shape of the pipeline. If the pipeline is the head,
	* then it's output shape corresponds to the shape of the source.
	* Otherwise, it's output shape corresponds to the output shape of the
	* associated operation.
	*
	* @return the output shape
	*/
	abstract StreamShape getOutputShape();

	/**
	* Collect elements output from a pipeline into a Node that holds elements
	* of this shape.
	*
	* @param helper the pipeline helper describing the pipeline stages
	* @param spliterator the source spliterator
	* @param flattenTree true if the returned node should be flattened
	* @param generator the array generator
	* @return a Node holding the output of the pipeline
	*/
	abstract <P_IN> Node<E_OUT> evaluateToNode(PipelineHelper<E_OUT> helper,
	Spliterator<P_IN> spliterator,
	boolean flattenTree,
	IntFunction<E_OUT[]> generator);

	/**
	* Create a spliterator that wraps a source spliterator, compatible with
	* this stream shape, and operations associated with a {@link
	* PipelineHelper}.
	*
	* @param ph the pipeline helper describing the pipeline stages
	* @param supplier the supplier of a spliterator
	* @return a wrapping spliterator compatible with this shape
	*/
	abstract <P_IN> Spliterator<E_OUT> wrap(PipelineHelper<E_OUT> ph,
	Supplier<Spliterator<P_IN>> supplier,
	boolean isParallel);

	/**
	* Create a lazy spliterator that wraps and obtains the supplied the
	* spliterator when a method is invoked on the lazy spliterator.
	* @param supplier the supplier of a spliterator
	*/
	abstract Spliterator<E_OUT> lazySpliterator(Supplier<? extends Spliterator<E_OUT>> supplier);

	/**
	* Traverse the elements of a spliterator compatible with this stream shape,
	* pushing those elements into a sink. If the sink requests cancellation,
	* no further elements will be pulled or pushed.
	*
	* @param spliterator the spliterator to pull elements from
	* @param sink the sink to push elements to
	* @return true if the cancellation was requested
	*/
	abstract boolean forEachWithCancel(Spliterator<E_OUT> spliterator, Sink<E_OUT> sink);

	/**
	* Make a node builder compatible with this stream shape.
	*
	* @param exactSizeIfKnown if {@literal >=0}, then a node builder will be
	* created that has a fixed capacity of at most sizeIfKnown elements. If
	* {@literal < 0}, then the node builder has an unfixed capacity. A fixed
	* capacity node builder will throw exceptions if an element is added after
	* builder has reached capacity, or is built before the builder has reached
	* capacity.
	*
	* @param generator the array generator to be used to create instances of a
	* T[] array. For implementations supporting primitive nodes, this parameter
	* may be ignored.
	* @return a node builder
	*/
	@Override
	abstract Node.Builder<E_OUT> makeNodeBuilder(long exactSizeIfKnown,
	IntFunction<E_OUT[]> generator);


	// Op-specific abstract methods, implemented by the operation class

	/**
	* Returns whether this operation is stateful or not. If it is stateful,
	* then the method
	* {@link #opEvaluateParallel(PipelineHelper, java.util.Spliterator, java.util.function.IntFunction)}
	* must be overridden.
	*
	* @return {@code true} if this operation is stateful
	*/
	abstract boolean opIsStateful();

	/**
	* Accepts a {@code Sink} which will receive the results of this operation,
	* and return a {@code Sink} which accepts elements of the input type of
	* this operation and which performs the operation, passing the results to
	* the provided {@code Sink}.
	*
	* @apiNote
	* The implementation may use the {@code flags} parameter to optimize the
	* sink wrapping. For example, if the input is already {@code DISTINCT},
	* the implementation for the {@code Stream#distinct()} method could just
	* return the sink it was passed.
	*
	* @param flags The combined stream and operation flags up to, but not
	* including, this operation
	* @param sink sink to which elements should be sent after processing
	* @return a sink which accepts elements, perform the operation upon
	* each element, and passes the results (if any) to the provided
	* {@code Sink}.
	*/
	abstract Sink<E_IN> opWrapSink(int flags, Sink<E_OUT> sink);

	/**
	* Performs a parallel evaluation of the operation using the specified
	* {@code PipelineHelper} which describes the upstream intermediate
	* operations. Only called on stateful operations. If {@link
	* #opIsStateful()} returns true then implementations must override the
	* default implementation.
	*
	* @implSpec The default implementation always throw
	* {@code UnsupportedOperationException}.
	*
	* @param helper the pipeline helper describing the pipeline stages
	* @param spliterator the source {@code Spliterator}
	* @param generator the array generator
	* @return a {@code Node} describing the result of the evaluation
	*/
	<P_IN> Node<E_OUT> opEvaluateParallel(PipelineHelper<E_OUT> helper,
	Spliterator<P_IN> spliterator,
	IntFunction<E_OUT[]> generator) {
	throw new UnsupportedOperationException("Parallel evaluation is not supported");
	}

	/**
	* Returns a {@code Spliterator} describing a parallel evaluation of the
	* operation, using the specified {@code PipelineHelper} which describes the
	* upstream intermediate operations. Only called on stateful operations.
	* It is not necessary (though acceptable) to do a full computation of the
	* result here; it is preferable, if possible, to describe the result via a
	* lazily evaluated spliterator.
	*
	* @implSpec The default implementation behaves as if:
	* <pre>{@code
	* return evaluateParallel(helper, i -> (E_OUT[]) new
	* Object[i]).spliterator();
	* }</pre>
	* and is suitable for implementations that cannot do better than a full
	* synchronous evaluation.
	*
	* @param helper the pipeline helper
	* @param spliterator the source {@code Spliterator}
	* @return a {@code Spliterator} describing the result of the evaluation
	*/
	@SuppressWarnings("unchecked")
	<P_IN> Spliterator<E_OUT> opEvaluateParallelLazy(PipelineHelper<E_OUT> helper,
	Spliterator<P_IN> spliterator) {
	return opEvaluateParallel(helper, spliterator, i -> (E_OUT[]) new Object[i]).spliterator();
	}
	}

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