/*
 * 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.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.concurrent.locks.LockSupport;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;

A Future that may be explicitly completed (setting its value and status), and may be used as a CompletionStage, supporting dependent functions and actions that trigger upon its completion. 
When two or more threads attempt to complete, completeExceptionally, or cancel a CompletableFuture, only one of them succeeds. 
In addition to these and related methods for directly manipulating status and results, CompletableFuture implements interface CompletionStage with the following policies: 

Actions supplied for dependent completions of
non-async methods may be performed by the thread that
completes the current CompletableFuture, or by any other caller of
a completion method.
All async methods without an explicit Executor argument are performed using the ForkJoinPool.commonPool() (unless it does not support a parallelism level of at least two, in which case, a new Thread is created to run each task). This may be overridden for non-static methods in subclasses by defining method defaultExecutor(). To simplify monitoring, debugging, and tracking, all generated asynchronous tasks are instances of the marker interface AsynchronousCompletionTask. Operations with time-delays can use adapter methods defined in this class, for example: supplyAsync(supplier, delayedExecutor(timeout,
timeUnit)). To support methods with delays and timeouts, this class maintains at most one daemon thread for triggering and cancelling actions, not for running them. 
All CompletionStage methods are implemented independently of
other public methods, so the behavior of one method is not impacted
by overrides of others in subclasses.
All CompletionStage methods return CompletableFutures. To restrict usages to only those methods defined in interface CompletionStage, use method minimalCompletionStage. Or to ensure only that clients do not themselves modify a future, use method copy. 
CompletableFuture also implements Future with the following policies: 

Since (unlike FutureTask) this class has no direct control over the computation that causes it to be completed, cancellation is treated as just another form of exceptional completion. Method cancel has the same effect as completeExceptionally(new CancellationException()). Method isCompletedExceptionally can be used to determine if a CompletableFuture completed in any exceptional fashion. 
In case of exceptional completion with a CompletionException, methods get() and get(long, TimeUnit) throw an ExecutionException with the same cause as held in the corresponding CompletionException. To simplify usage in most contexts, this class also defines methods join() and getNow that instead throw the CompletionException directly in these cases. 
Arguments used to pass a completion result (that is, for parameters of type T) for methods accepting them may be null, but passing a null value for any other parameter will result in a NullPointerException being thrown. 
Subclasses of this class should normally override the "virtual constructor" method newIncompleteFuture, which establishes the concrete type returned by CompletionStage methods. For example, here is a class that substitutes a different default Executor and disables the obtrude methods: 
 
class MyCompletableFuture<T> extends CompletableFuture<T> {
  static final Executor myExecutor = ...;
  public MyCompletableFuture() { }
  public <U> CompletableFuture<U> newIncompleteFuture() {
    return new MyCompletableFuture<U>(); }
  public Executor defaultExecutor() {
    return myExecutor; }
  public void obtrudeValue(T value) {
    throw new UnsupportedOperationException(); }
  public void obtrudeException(Throwable ex) {
    throw new UnsupportedOperationException(); }
 }
Author:
Doug Lea
Type parameters:
<T> – The result type returned by this future's join and get methods
Since:
1.8/**
 * A {@link Future} that may be explicitly completed (setting its
 * value and status), and may be used as a {@link CompletionStage},
 * supporting dependent functions and actions that trigger upon its
 * completion.
 *
 * <p>When two or more threads attempt to
 * {@link #complete complete},
 * {@link #completeExceptionally completeExceptionally}, or
 * {@link #cancel cancel}
 * a CompletableFuture, only one of them succeeds.
 *
 * <p>In addition to these and related methods for directly
 * manipulating status and results, CompletableFuture implements
 * interface {@link CompletionStage} with the following policies: <ul>
 *
 * <li>Actions supplied for dependent completions of
 * <em>non-async</em> methods may be performed by the thread that
 * completes the current CompletableFuture, or by any other caller of
 * a completion method.
 *
 * <li>All <em>async</em> methods without an explicit Executor
 * argument are performed using the {@link ForkJoinPool#commonPool()}
 * (unless it does not support a parallelism level of at least two, in
 * which case, a new Thread is created to run each task).  This may be
 * overridden for non-static methods in subclasses by defining method
 * {@link #defaultExecutor()}. To simplify monitoring, debugging,
 * and tracking, all generated asynchronous tasks are instances of the
 * marker interface {@link AsynchronousCompletionTask}.  Operations
 * with time-delays can use adapter methods defined in this class, for
 * example: {@code supplyAsync(supplier, delayedExecutor(timeout,
 * timeUnit))}.  To support methods with delays and timeouts, this
 * class maintains at most one daemon thread for triggering and
 * cancelling actions, not for running them.
 *
 * <li>All CompletionStage methods are implemented independently of
 * other public methods, so the behavior of one method is not impacted
 * by overrides of others in subclasses.
 *
 * <li>All CompletionStage methods return CompletableFutures.  To
 * restrict usages to only those methods defined in interface
 * CompletionStage, use method {@link #minimalCompletionStage}. Or to
 * ensure only that clients do not themselves modify a future, use
 * method {@link #copy}.
 * </ul>
 *
 * <p>CompletableFuture also implements {@link Future} with the following
 * policies: <ul>
 *
 * <li>Since (unlike {@link FutureTask}) this class has no direct
 * control over the computation that causes it to be completed,
 * cancellation is treated as just another form of exceptional
 * completion.  Method {@link #cancel cancel} has the same effect as
 * {@code completeExceptionally(new CancellationException())}. Method
 * {@link #isCompletedExceptionally} can be used to determine if a
 * CompletableFuture completed in any exceptional fashion.
 *
 * <li>In case of exceptional completion with a CompletionException,
 * methods {@link #get()} and {@link #get(long, TimeUnit)} throw an
 * {@link ExecutionException} with the same cause as held in the
 * corresponding CompletionException.  To simplify usage in most
 * contexts, this class also defines methods {@link #join()} and
 * {@link #getNow} that instead throw the CompletionException directly
 * in these cases.
 * </ul>
 *
 * <p>Arguments used to pass a completion result (that is, for
 * parameters of type {@code T}) for methods accepting them may be
 * null, but passing a null value for any other parameter will result
 * in a {@link NullPointerException} being thrown.
 *
 * <p>Subclasses of this class should normally override the "virtual
 * constructor" method {@link #newIncompleteFuture}, which establishes
 * the concrete type returned by CompletionStage methods. For example,
 * here is a class that substitutes a different default Executor and
 * disables the {@code obtrude} methods:
 *
 * <pre> {@code
 * class MyCompletableFuture<T> extends CompletableFuture<T> {
 *   static final Executor myExecutor = ...;
 *   public MyCompletableFuture() { }
 *   public <U> CompletableFuture<U> newIncompleteFuture() {
 *     return new MyCompletableFuture<U>(); }
 *   public Executor defaultExecutor() {
 *     return myExecutor; }
 *   public void obtrudeValue(T value) {
 *     throw new UnsupportedOperationException(); }
 *   public void obtrudeException(Throwable ex) {
 *     throw new UnsupportedOperationException(); }
 * }}</pre>
 *
 * @author Doug Lea
 * @param <T> The result type returned by this future's {@code join}
 * and {@code get} methods
 * @since 1.8
 */
public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {

    /*
     * Overview:
     *
     * A CompletableFuture may have dependent completion actions,
     * collected in a linked stack. It atomically completes by CASing
     * a result field, and then pops off and runs those actions. This
     * applies across normal vs exceptional outcomes, sync vs async
     * actions, binary triggers, and various forms of completions.
     *
     * Non-nullness of volatile field "result" indicates done.  It may
     * be set directly if known to be thread-confined, else via CAS.
     * An AltResult is used to box null as a result, as well as to
     * hold exceptions.  Using a single field makes completion simple
     * to detect and trigger.  Result encoding and decoding is
     * straightforward but tedious and adds to the sprawl of trapping
     * and associating exceptions with targets.  Minor simplifications
     * rely on (static) NIL (to box null results) being the only
     * AltResult with a null exception field, so we don't usually need
     * explicit comparisons.  Even though some of the generics casts
     * are unchecked (see SuppressWarnings annotations), they are
     * placed to be appropriate even if checked.
     *
     * Dependent actions are represented by Completion objects linked
     * as Treiber stacks headed by field "stack". There are Completion
     * classes for each kind of action, grouped into:
     * - single-input (UniCompletion),
     * - two-input (BiCompletion),
     * - projected (BiCompletions using exactly one of two inputs),
     * - shared (CoCompletion, used by the second of two sources),
     * - zero-input source actions,
     * - Signallers that unblock waiters.
     * Class Completion extends ForkJoinTask to enable async execution
     * (adding no space overhead because we exploit its "tag" methods
     * to maintain claims). It is also declared as Runnable to allow
     * usage with arbitrary executors.
     *
     * Support for each kind of CompletionStage relies on a separate
     * class, along with two CompletableFuture methods:
     *
     * * A Completion class with name X corresponding to function,
     *   prefaced with "Uni", "Bi", or "Or". Each class contains
     *   fields for source(s), actions, and dependent. They are
     *   boringly similar, differing from others only with respect to
     *   underlying functional forms. We do this so that users don't
     *   encounter layers of adapters in common usages.
     *
     * * Boolean CompletableFuture method x(...) (for example
     *   biApply) takes all of the arguments needed to check that an
     *   action is triggerable, and then either runs the action or
     *   arranges its async execution by executing its Completion
     *   argument, if present. The method returns true if known to be
     *   complete.
     *
     * * Completion method tryFire(int mode) invokes the associated x
     *   method with its held arguments, and on success cleans up.
     *   The mode argument allows tryFire to be called twice (SYNC,
     *   then ASYNC); the first to screen and trap exceptions while
     *   arranging to execute, and the second when called from a task.
     *   (A few classes are not used async so take slightly different
     *   forms.)  The claim() callback suppresses function invocation
     *   if already claimed by another thread.
     *
     * * Some classes (for example UniApply) have separate handling
     *   code for when known to be thread-confined ("now" methods) and
     *   for when shared (in tryFire), for efficiency.
     *
     * * CompletableFuture method xStage(...) is called from a public
     *   stage method of CompletableFuture f. It screens user
     *   arguments and invokes and/or creates the stage object.  If
     *   not async and already triggerable, the action is run
     *   immediately.  Otherwise a Completion c is created, and
     *   submitted to the executor if triggerable, or pushed onto f's
     *   stack if not.  Completion actions are started via c.tryFire.
     *   We recheck after pushing to a source future's stack to cover
     *   possible races if the source completes while pushing.
     *   Classes with two inputs (for example BiApply) deal with races
     *   across both while pushing actions.  The second completion is
     *   a CoCompletion pointing to the first, shared so that at most
     *   one performs the action.  The multiple-arity methods allOf
     *   does this pairwise to form trees of completions.  Method
     *   anyOf is handled differently from allOf because completion of
     *   any source should trigger a cleanStack of other sources.
     *   Each AnyOf completion can reach others via a shared array.
     *
     * Note that the generic type parameters of methods vary according
     * to whether "this" is a source, dependent, or completion.
     *
     * Method postComplete is called upon completion unless the target
     * is guaranteed not to be observable (i.e., not yet returned or
     * linked). Multiple threads can call postComplete, which
     * atomically pops each dependent action, and tries to trigger it
     * via method tryFire, in NESTED mode.  Triggering can propagate
     * recursively, so NESTED mode returns its completed dependent (if
     * one exists) for further processing by its caller (see method
     * postFire).
     *
     * Blocking methods get() and join() rely on Signaller Completions
     * that wake up waiting threads.  The mechanics are similar to
     * Treiber stack wait-nodes used in FutureTask, Phaser, and
     * SynchronousQueue. See their internal documentation for
     * algorithmic details.
     *
     * Without precautions, CompletableFutures would be prone to
     * garbage accumulation as chains of Completions build up, each
     * pointing back to its sources. So we null out fields as soon as
     * possible.  The screening checks needed anyway harmlessly ignore
     * null arguments that may have been obtained during races with
     * threads nulling out fields.  We also try to unlink non-isLive
     * (fired or cancelled) Completions from stacks that might
     * otherwise never be popped: Method cleanStack always unlinks non
     * isLive completions from the head of stack; others may
     * occasionally remain if racing with other cancellations or
     * removals.
     *
     * Completion fields need not be declared as final or volatile
     * because they are only visible to other threads upon safe
     * publication.
     */

    volatile Object result;       // Either the result or boxed AltResult
    volatile Completion stack;    // Top of Treiber stack of dependent actions

    final boolean internalComplete(Object r) { // CAS from null to r
        return RESULT.compareAndSet(this, null, r);
    }

    
Returns true if successfully pushed c onto stack. /** Returns true if successfully pushed c onto stack. */
    final boolean tryPushStack(Completion c) {
        Completion h = stack;
        NEXT.set(c, h);         // CAS piggyback
        return STACK.compareAndSet(this, h, c);
    }

    
Unconditionally pushes c onto stack, retrying if necessary. /** Unconditionally pushes c onto stack, retrying if necessary. */
    final void pushStack(Completion c) {
        do {} while (!tryPushStack(c));
    }

    /* ------------- Encoding and decoding outcomes -------------- */

    static final class AltResult { // See above
        final Throwable ex;        // null only for NIL
        AltResult(Throwable x) { this.ex = x; }
    }

    
The encoding of the null value. /** The encoding of the null value. */
    static final AltResult NIL = new AltResult(null);

    
Completes with the null value, unless already completed. /** Completes with the null value, unless already completed. */
    final boolean completeNull() {
        return RESULT.compareAndSet(this, null, NIL);
    }

    
Returns the encoding of the given non-exceptional value. /** Returns the encoding of the given non-exceptional value. */
    final Object encodeValue(T t) {
        return (t == null) ? NIL : t;
    }

    
Completes with a non-exceptional result, unless already completed. /** Completes with a non-exceptional result, unless already completed. */
    final boolean completeValue(T t) {
        return RESULT.compareAndSet(this, null, (t == null) ? NIL : t);
    }

    
Returns the encoding of the given (non-null) exception as a
wrapped CompletionException unless it is one already.
/**
     * Returns the encoding of the given (non-null) exception as a
     * wrapped CompletionException unless it is one already.
     */
    static AltResult encodeThrowable(Throwable x) {
        return new AltResult((x instanceof CompletionException) ? x :
                             new CompletionException(x));
    }

    
Completes with an exceptional result, unless already completed. /** Completes with an exceptional result, unless already completed. */
    final boolean completeThrowable(Throwable x) {
        return RESULT.compareAndSet(this, null, encodeThrowable(x));
    }

    
Returns the encoding of the given (non-null) exception as a
wrapped CompletionException unless it is one already.  May
return the given Object r (which must have been the result of a
source future) if it is equivalent, i.e. if this is a simple
relay of an existing CompletionException.
/**
     * Returns the encoding of the given (non-null) exception as a
     * wrapped CompletionException unless it is one already.  May
     * return the given Object r (which must have been the result of a
     * source future) if it is equivalent, i.e. if this is a simple
     * relay of an existing CompletionException.
     */
    static Object encodeThrowable(Throwable x, Object r) {
        if (!(x instanceof CompletionException))
            x = new CompletionException(x);
        else if (r instanceof AltResult && x == ((AltResult)r).ex)
            return r;
        return new AltResult(x);
    }

    
Completes with the given (non-null) exceptional result as a
wrapped CompletionException unless it is one already, unless
already completed.  May complete with the given Object r
(which must have been the result of a source future) if it is
equivalent, i.e. if this is a simple propagation of an
existing CompletionException.
/**
     * Completes with the given (non-null) exceptional result as a
     * wrapped CompletionException unless it is one already, unless
     * already completed.  May complete with the given Object r
     * (which must have been the result of a source future) if it is
     * equivalent, i.e. if this is a simple propagation of an
     * existing CompletionException.
     */
    final boolean completeThrowable(Throwable x, Object r) {
        return RESULT.compareAndSet(this, null, encodeThrowable(x, r));
    }

    
Returns the encoding of the given arguments: if the exception
is non-null, encodes as AltResult.  Otherwise uses the given
value, boxed as NIL if null.
/**
     * Returns the encoding of the given arguments: if the exception
     * is non-null, encodes as AltResult.  Otherwise uses the given
     * value, boxed as NIL if null.
     */
    Object encodeOutcome(T t, Throwable x) {
        return (x == null) ? (t == null) ? NIL : t : encodeThrowable(x);
    }

    
Returns the encoding of a copied outcome; if exceptional,
rewraps as a CompletionException, else returns argument.
/**
     * Returns the encoding of a copied outcome; if exceptional,
     * rewraps as a CompletionException, else returns argument.
     */
    static Object encodeRelay(Object r) {
        Throwable x;
        if (r instanceof AltResult
            && (x = ((AltResult)r).ex) != null
            && !(x instanceof CompletionException))
            r = new AltResult(new CompletionException(x));
        return r;
    }

    
Completes with r or a copy of r, unless already completed.
If exceptional, r is first coerced to a CompletionException.
/**
     * Completes with r or a copy of r, unless already completed.
     * If exceptional, r is first coerced to a CompletionException.
     */
    final boolean completeRelay(Object r) {
        return RESULT.compareAndSet(this, null, encodeRelay(r));
    }

    
Reports result using Future.get conventions.
/**
     * Reports result using Future.get conventions.
     */
    private static Object reportGet(Object r)
        throws InterruptedException, ExecutionException {
        if (r == null) // by convention below, null means interrupted
            throw new InterruptedException();
        if (r instanceof AltResult) {
            Throwable x, cause;
            if ((x = ((AltResult)r).ex) == null)
                return null;
            if (x instanceof CancellationException)
                throw (CancellationException)x;
            if ((x instanceof CompletionException) &&
                (cause = x.getCause()) != null)
                x = cause;
            throw new ExecutionException(x);
        }
        return r;
    }

    
Decodes outcome to return result or throw unchecked exception.
/**
     * Decodes outcome to return result or throw unchecked exception.
     */
    private static Object reportJoin(Object r) {
        if (r instanceof AltResult) {
            Throwable x;
            if ((x = ((AltResult)r).ex) == null)
                return null;
            if (x instanceof CancellationException)
                throw (CancellationException)x;
            if (x instanceof CompletionException)
                throw (CompletionException)x;
            throw new CompletionException(x);
        }
        return r;
    }

    /* ------------- Async task preliminaries -------------- */

    
A marker interface identifying asynchronous tasks produced by async methods. This may be useful for monitoring, debugging, and tracking asynchronous activities. 
Since:
1.8/**
     * A marker interface identifying asynchronous tasks produced by
     * {@code async} methods. This may be useful for monitoring,
     * debugging, and tracking asynchronous activities.
     *
     * @since 1.8
     */
    public static interface AsynchronousCompletionTask {
    }

    private static final boolean USE_COMMON_POOL =
        (ForkJoinPool.getCommonPoolParallelism() > 1);

    
Default executor -- ForkJoinPool.commonPool() unless it cannot
support parallelism.
/**
     * Default executor -- ForkJoinPool.commonPool() unless it cannot
     * support parallelism.
     */
    private static final Executor ASYNC_POOL = USE_COMMON_POOL ?
        ForkJoinPool.commonPool() : new ThreadPerTaskExecutor();

    
Fallback if ForkJoinPool.commonPool() cannot support parallelism /** Fallback if ForkJoinPool.commonPool() cannot support parallelism */
    static final class ThreadPerTaskExecutor implements Executor {
        public void execute(Runnable r) { new Thread(r).start(); }
    }

    
Null-checks user executor argument, and translates uses of
commonPool to ASYNC_POOL in case parallelism disabled.
/**
     * Null-checks user executor argument, and translates uses of
     * commonPool to ASYNC_POOL in case parallelism disabled.
     */
    static Executor screenExecutor(Executor e) {
        if (!USE_COMMON_POOL && e == ForkJoinPool.commonPool())
            return ASYNC_POOL;
        if (e == null) throw new NullPointerException();
        return e;
    }

    // Modes for Completion.tryFire. Signedness matters.
    static final int SYNC   =  0;
    static final int ASYNC  =  1;
    static final int NESTED = -1;

    /* ------------- Base Completion classes and operations -------------- */

    @SuppressWarnings("serial")
    abstract static class Completion extends ForkJoinTask<Void>
        implements Runnable, AsynchronousCompletionTask {
        volatile Completion next;      // Treiber stack link

        
Performs completion action if triggered, returning a
dependent that may need propagation, if one exists.
Params:
mode – SYNC, ASYNC, or NESTED/**
         * Performs completion action if triggered, returning a
         * dependent that may need propagation, if one exists.
         *
         * @param mode SYNC, ASYNC, or NESTED
         */
        abstract CompletableFuture<?> tryFire(int mode);

        
Returns true if possibly still triggerable. Used by cleanStack. /** Returns true if possibly still triggerable. Used by cleanStack. */
        abstract boolean isLive();

        public final void run()                { tryFire(ASYNC); }
        public final boolean exec()            { tryFire(ASYNC); return false; }
        public final Void getRawResult()       { return null; }
        public final void setRawResult(Void v) {}
    }

    
Pops and tries to trigger all reachable dependents.  Call only
when known to be done.
/**
     * Pops and tries to trigger all reachable dependents.  Call only
     * when known to be done.
     */
    final void postComplete() {
        /*
         * On each step, variable f holds current dependents to pop
         * and run.  It is extended along only one path at a time,
         * pushing others to avoid unbounded recursion.
         */
        CompletableFuture<?> f = this; Completion h;
        while ((h = f.stack) != null ||
               (f != this && (h = (f = this).stack) != null)) {
            CompletableFuture<?> d; Completion t;
            if (STACK.compareAndSet(f, h, t = h.next)) {
                if (t != null) {
                    if (f != this) {
                        pushStack(h);
                        continue;
                    }
                    NEXT.compareAndSet(h, t, null); // try to detach
                }
                f = (d = h.tryFire(NESTED)) == null ? this : d;
            }
        }
    }

    
Traverses stack and unlinks one or more dead Completions, if found. /** Traverses stack and unlinks one or more dead Completions, if found. */
    final void cleanStack() {
        Completion p = stack;
        // ensure head of stack live
        for (boolean unlinked = false;;) {
            if (p == null)
                return;
            else if (p.isLive()) {
                if (unlinked)
                    return;
                else
                    break;
            }
            else if (STACK.weakCompareAndSet(this, p, (p = p.next)))
                unlinked = true;
            else
                p = stack;
        }
        // try to unlink first non-live
        for (Completion q = p.next; q != null;) {
            Completion s = q.next;
            if (q.isLive()) {
                p = q;
                q = s;
            } else if (NEXT.weakCompareAndSet(p, q, s))
                break;
            else
                q = p.next;
        }
    }

    /* ------------- One-input Completions -------------- */

    
A Completion with a source, dependent, and executor. /** A Completion with a source, dependent, and executor. */
    @SuppressWarnings("serial")
    abstract static class UniCompletion<T,V> extends Completion {
        Executor executor;                 // executor to use (null if none)
        CompletableFuture<V> dep;          // the dependent to complete
        CompletableFuture<T> src;          // source for action

        UniCompletion(Executor executor, CompletableFuture<V> dep,
                      CompletableFuture<T> src) {
            this.executor = executor; this.dep = dep; this.src = src;
        }

        
Returns true if action can be run. Call only when known to
be triggerable. Uses FJ tag bit to ensure that only one
thread claims ownership.  If async, starts as task -- a
later call to tryFire will run action.
/**
         * Returns true if action can be run. Call only when known to
         * be triggerable. Uses FJ tag bit to ensure that only one
         * thread claims ownership.  If async, starts as task -- a
         * later call to tryFire will run action.
         */
        final boolean claim() {
            Executor e = executor;
            if (compareAndSetForkJoinTaskTag((short)0, (short)1)) {
                if (e == null)
                    return true;
                executor = null; // disable
                e.execute(this);
            }
            return false;
        }

        final boolean isLive() { return dep != null; }
    }

    
Pushes the given completion unless it completes while trying.
Caller should first check that result is null.
/**
     * Pushes the given completion unless it completes while trying.
     * Caller should first check that result is null.
     */
    final void unipush(Completion c) {
        if (c != null) {
            while (!tryPushStack(c)) {
                if (result != null) {
                    NEXT.set(c, null);
                    break;
                }
            }
            if (result != null)
                c.tryFire(SYNC);
        }
    }

    
Post-processing by dependent after successful UniCompletion tryFire.
Tries to clean stack of source a, and then either runs postComplete
or returns this to caller, depending on mode.
/**
     * Post-processing by dependent after successful UniCompletion tryFire.
     * Tries to clean stack of source a, and then either runs postComplete
     * or returns this to caller, depending on mode.
     */
    final CompletableFuture<T> postFire(CompletableFuture<?> a, int mode) {
        if (a != null && a.stack != null) {
            Object r;
            if ((r = a.result) == null)
                a.cleanStack();
            if (mode >= 0 && (r != null || a.result != null))
                a.postComplete();
        }
        if (result != null && stack != null) {
            if (mode < 0)
                return this;
            else
                postComplete();
        }
        return null;
    }

    @SuppressWarnings("serial")
    static final class UniApply<T,V> extends UniCompletion<T,V> {
        Function<? super T,? extends V> fn;
        UniApply(Executor executor, CompletableFuture<V> dep,
                 CompletableFuture<T> src,
                 Function<? super T,? extends V> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<V> tryFire(int mode) {
            CompletableFuture<V> d; CompletableFuture<T> a;
            Object r; Throwable x; Function<? super T,? extends V> f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null)
                return null;
            tryComplete: if (d.result == null) {
                if (r instanceof AltResult) {
                    if ((x = ((AltResult)r).ex) != null) {
                        d.completeThrowable(x, r);
                        break tryComplete;
                    }
                    r = null;
                }
                try {
                    if (mode <= 0 && !claim())
                        return null;
                    else {
                        @SuppressWarnings("unchecked") T t = (T) r;
                        d.completeValue(f.apply(t));
                    }
                } catch (Throwable ex) {
                    d.completeThrowable(ex);
                }
            }
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    private <V> CompletableFuture<V> uniApplyStage(
        Executor e, Function<? super T,? extends V> f) {
        if (f == null) throw new NullPointerException();
        Object r;
        if ((r = result) != null)
            return uniApplyNow(r, e, f);
        CompletableFuture<V> d = newIncompleteFuture();
        unipush(new UniApply<T,V>(e, d, this, f));
        return d;
    }

    private <V> CompletableFuture<V> uniApplyNow(
        Object r, Executor e, Function<? super T,? extends V> f) {
        Throwable x;
        CompletableFuture<V> d = newIncompleteFuture();
        if (r instanceof AltResult) {
            if ((x = ((AltResult)r).ex) != null) {
                d.result = encodeThrowable(x, r);
                return d;
            }
            r = null;
        }
        try {
            if (e != null) {
                e.execute(new UniApply<T,V>(null, d, this, f));
            } else {
                @SuppressWarnings("unchecked") T t = (T) r;
                d.result = d.encodeValue(f.apply(t));
            }
        } catch (Throwable ex) {
            d.result = encodeThrowable(ex);
        }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniAccept<T> extends UniCompletion<T,Void> {
        Consumer<? super T> fn;
        UniAccept(Executor executor, CompletableFuture<Void> dep,
                  CompletableFuture<T> src, Consumer<? super T> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<Void> tryFire(int mode) {
            CompletableFuture<Void> d; CompletableFuture<T> a;
            Object r; Throwable x; Consumer<? super T> f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null)
                return null;
            tryComplete: if (d.result == null) {
                if (r instanceof AltResult) {
                    if ((x = ((AltResult)r).ex) != null) {
                        d.completeThrowable(x, r);
                        break tryComplete;
                    }
                    r = null;
                }
                try {
                    if (mode <= 0 && !claim())
                        return null;
                    else {
                        @SuppressWarnings("unchecked") T t = (T) r;
                        f.accept(t);
                        d.completeNull();
                    }
                } catch (Throwable ex) {
                    d.completeThrowable(ex);
                }
            }
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    private CompletableFuture<Void> uniAcceptStage(Executor e,
                                                   Consumer<? super T> f) {
        if (f == null) throw new NullPointerException();
        Object r;
        if ((r = result) != null)
            return uniAcceptNow(r, e, f);
        CompletableFuture<Void> d = newIncompleteFuture();
        unipush(new UniAccept<T>(e, d, this, f));
        return d;
    }

    private CompletableFuture<Void> uniAcceptNow(
        Object r, Executor e, Consumer<? super T> f) {
        Throwable x;
        CompletableFuture<Void> d = newIncompleteFuture();
        if (r instanceof AltResult) {
            if ((x = ((AltResult)r).ex) != null) {
                d.result = encodeThrowable(x, r);
                return d;
            }
            r = null;
        }
        try {
            if (e != null) {
                e.execute(new UniAccept<T>(null, d, this, f));
            } else {
                @SuppressWarnings("unchecked") T t = (T) r;
                f.accept(t);
                d.result = NIL;
            }
        } catch (Throwable ex) {
            d.result = encodeThrowable(ex);
        }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniRun<T> extends UniCompletion<T,Void> {
        Runnable fn;
        UniRun(Executor executor, CompletableFuture<Void> dep,
               CompletableFuture<T> src, Runnable fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<Void> tryFire(int mode) {
            CompletableFuture<Void> d; CompletableFuture<T> a;
            Object r; Throwable x; Runnable f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null)
                return null;
            if (d.result == null) {
                if (r instanceof AltResult && (x = ((AltResult)r).ex) != null)
                    d.completeThrowable(x, r);
                else
                    try {
                        if (mode <= 0 && !claim())
                            return null;
                        else {
                            f.run();
                            d.completeNull();
                        }
                    } catch (Throwable ex) {
                        d.completeThrowable(ex);
                    }
            }
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    private CompletableFuture<Void> uniRunStage(Executor e, Runnable f) {
        if (f == null) throw new NullPointerException();
        Object r;
        if ((r = result) != null)
            return uniRunNow(r, e, f);
        CompletableFuture<Void> d = newIncompleteFuture();
        unipush(new UniRun<T>(e, d, this, f));
        return d;
    }

    private CompletableFuture<Void> uniRunNow(Object r, Executor e, Runnable f) {
        Throwable x;
        CompletableFuture<Void> d = newIncompleteFuture();
        if (r instanceof AltResult && (x = ((AltResult)r).ex) != null)
            d.result = encodeThrowable(x, r);
        else
            try {
                if (e != null) {
                    e.execute(new UniRun<T>(null, d, this, f));
                } else {
                    f.run();
                    d.result = NIL;
                }
            } catch (Throwable ex) {
                d.result = encodeThrowable(ex);
            }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniWhenComplete<T> extends UniCompletion<T,T> {
        BiConsumer<? super T, ? super Throwable> fn;
        UniWhenComplete(Executor executor, CompletableFuture<T> dep,
                        CompletableFuture<T> src,
                        BiConsumer<? super T, ? super Throwable> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<T> tryFire(int mode) {
            CompletableFuture<T> d; CompletableFuture<T> a;
            Object r; BiConsumer<? super T, ? super Throwable> f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null
                || !d.uniWhenComplete(r, f, mode > 0 ? null : this))
                return null;
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    final boolean uniWhenComplete(Object r,
                                  BiConsumer<? super T,? super Throwable> f,
                                  UniWhenComplete<T> c) {
        T t; Throwable x = null;
        if (result == null) {
            try {
                if (c != null && !c.claim())
                    return false;
                if (r instanceof AltResult) {
                    x = ((AltResult)r).ex;
                    t = null;
                } else {
                    @SuppressWarnings("unchecked") T tr = (T) r;
                    t = tr;
                }
                f.accept(t, x);
                if (x == null) {
                    internalComplete(r);
                    return true;
                }
            } catch (Throwable ex) {
                if (x == null)
                    x = ex;
                else if (x != ex)
                    x.addSuppressed(ex);
            }
            completeThrowable(x, r);
        }
        return true;
    }

    private CompletableFuture<T> uniWhenCompleteStage(
        Executor e, BiConsumer<? super T, ? super Throwable> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<T> d = newIncompleteFuture();
        Object r;
        if ((r = result) == null)
            unipush(new UniWhenComplete<T>(e, d, this, f));
        else if (e == null)
            d.uniWhenComplete(r, f, null);
        else {
            try {
                e.execute(new UniWhenComplete<T>(null, d, this, f));
            } catch (Throwable ex) {
                d.result = encodeThrowable(ex);
            }
        }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniHandle<T,V> extends UniCompletion<T,V> {
        BiFunction<? super T, Throwable, ? extends V> fn;
        UniHandle(Executor executor, CompletableFuture<V> dep,
                  CompletableFuture<T> src,
                  BiFunction<? super T, Throwable, ? extends V> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<V> tryFire(int mode) {
            CompletableFuture<V> d; CompletableFuture<T> a;
            Object r; BiFunction<? super T, Throwable, ? extends V> f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null
                || !d.uniHandle(r, f, mode > 0 ? null : this))
                return null;
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    final <S> boolean uniHandle(Object r,
                                BiFunction<? super S, Throwable, ? extends T> f,
                                UniHandle<S,T> c) {
        S s; Throwable x;
        if (result == null) {
            try {
                if (c != null && !c.claim())
                    return false;
                if (r instanceof AltResult) {
                    x = ((AltResult)r).ex;
                    s = null;
                } else {
                    x = null;
                    @SuppressWarnings("unchecked") S ss = (S) r;
                    s = ss;
                }
                completeValue(f.apply(s, x));
            } catch (Throwable ex) {
                completeThrowable(ex);
            }
        }
        return true;
    }

    private <V> CompletableFuture<V> uniHandleStage(
        Executor e, BiFunction<? super T, Throwable, ? extends V> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<V> d = newIncompleteFuture();
        Object r;
        if ((r = result) == null)
            unipush(new UniHandle<T,V>(e, d, this, f));
        else if (e == null)
            d.uniHandle(r, f, null);
        else {
            try {
                e.execute(new UniHandle<T,V>(null, d, this, f));
            } catch (Throwable ex) {
                d.result = encodeThrowable(ex);
            }
        }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniExceptionally<T> extends UniCompletion<T,T> {
        Function<? super Throwable, ? extends T> fn;
        UniExceptionally(Executor executor,
                         CompletableFuture<T> dep, CompletableFuture<T> src,
                         Function<? super Throwable, ? extends T> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<T> tryFire(int mode) {
            CompletableFuture<T> d; CompletableFuture<T> a;
            Object r; Function<? super Throwable, ? extends T> f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null
                || !d.uniExceptionally(r, f, mode > 0 ? null : this))
                return null;
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    final boolean uniExceptionally(Object r,
                                   Function<? super Throwable, ? extends T> f,
                                   UniExceptionally<T> c) {
        Throwable x;
        if (result == null) {
            try {
                if (c != null && !c.claim())
                    return false;
                if (r instanceof AltResult && (x = ((AltResult)r).ex) != null)
                    completeValue(f.apply(x));
                else
                    internalComplete(r);
            } catch (Throwable ex) {
                completeThrowable(ex);
            }
        }
        return true;
    }

    private CompletableFuture<T> uniExceptionallyStage(
        Executor e, Function<Throwable, ? extends T> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<T> d = newIncompleteFuture();
        Object r;
        if ((r = result) == null)
            unipush(new UniExceptionally<T>(e, d, this, f));
        else if (e == null)
            d.uniExceptionally(r, f, null);
        else {
            try {
                e.execute(new UniExceptionally<T>(null, d, this, f));
            } catch (Throwable ex) {
                d.result = encodeThrowable(ex);
            }
        }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniComposeExceptionally<T> extends UniCompletion<T,T> {
        Function<Throwable, ? extends CompletionStage<T>> fn;
        UniComposeExceptionally(Executor executor, CompletableFuture<T> dep,
                                CompletableFuture<T> src,
                                Function<Throwable, ? extends CompletionStage<T>> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<T> tryFire(int mode) {
            CompletableFuture<T> d; CompletableFuture<T> a;
            Function<Throwable, ? extends CompletionStage<T>> f;
            Object r; Throwable x;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null)
                return null;
            if (d.result == null) {
                if ((r instanceof AltResult) &&
                    (x = ((AltResult)r).ex) != null) {
                    try {
                        if (mode <= 0 && !claim())
                            return null;
                        CompletableFuture<T> g = f.apply(x).toCompletableFuture();
                        if ((r = g.result) != null)
                            d.completeRelay(r);
                        else {
                            g.unipush(new UniRelay<T,T>(d, g));
                            if (d.result == null)
                                return null;
                        }
                    } catch (Throwable ex) {
                        d.completeThrowable(ex);
                    }
                }
                else
                    d.internalComplete(r);
            }
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    private CompletableFuture<T> uniComposeExceptionallyStage(
        Executor e, Function<Throwable, ? extends CompletionStage<T>> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<T> d = newIncompleteFuture();
        Object r, s; Throwable x;
        if ((r = result) == null)
            unipush(new UniComposeExceptionally<T>(e, d, this, f));
        else if (!(r instanceof AltResult) || (x = ((AltResult)r).ex) == null)
            d.internalComplete(r);
        else
            try {
                if (e != null)
                    e.execute(new UniComposeExceptionally<T>(null, d, this, f));
                else {
                    CompletableFuture<T> g = f.apply(x).toCompletableFuture();
                    if ((s = g.result) != null)
                        d.result = encodeRelay(s);
                    else
                        g.unipush(new UniRelay<T,T>(d, g));
                }
            } catch (Throwable ex) {
                d.result = encodeThrowable(ex);
            }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniRelay<U, T extends U> extends UniCompletion<T,U> {
        UniRelay(CompletableFuture<U> dep, CompletableFuture<T> src) {
            super(null, dep, src);
        }
        final CompletableFuture<U> tryFire(int mode) {
            CompletableFuture<U> d; CompletableFuture<T> a; Object r;
            if ((d = dep) == null
                || (a = src) == null || (r = a.result) == null)
                return null;
            if (d.result == null)
                d.completeRelay(r);
            src = null; dep = null;
            return d.postFire(a, mode);
        }
    }

    private static <U, T extends U> CompletableFuture<U> uniCopyStage(
        CompletableFuture<T> src) {
        Object r;
        CompletableFuture<U> d = src.newIncompleteFuture();
        if ((r = src.result) != null)
            d.result = encodeRelay(r);
        else
            src.unipush(new UniRelay<U,T>(d, src));
        return d;
    }

    private MinimalStage<T> uniAsMinimalStage() {
        Object r;
        if ((r = result) != null)
            return new MinimalStage<T>(encodeRelay(r));
        MinimalStage<T> d = new MinimalStage<T>();
        unipush(new UniRelay<T,T>(d, this));
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniCompose<T,V> extends UniCompletion<T,V> {
        Function<? super T, ? extends CompletionStage<V>> fn;
        UniCompose(Executor executor, CompletableFuture<V> dep,
                   CompletableFuture<T> src,
                   Function<? super T, ? extends CompletionStage<V>> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<V> tryFire(int mode) {
            CompletableFuture<V> d; CompletableFuture<T> a;
            Function<? super T, ? extends CompletionStage<V>> f;
            Object r; Throwable x;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null)
                return null;
            tryComplete: if (d.result == null) {
                if (r instanceof AltResult) {
                    if ((x = ((AltResult)r).ex) != null) {
                        d.completeThrowable(x, r);
                        break tryComplete;
                    }
                    r = null;
                }
                try {
                    if (mode <= 0 && !claim())
                        return null;
                    @SuppressWarnings("unchecked") T t = (T) r;
                    CompletableFuture<V> g = f.apply(t).toCompletableFuture();
                    if ((r = g.result) != null)
                        d.completeRelay(r);
                    else {
                        g.unipush(new UniRelay<V,V>(d, g));
                        if (d.result == null)
                            return null;
                    }
                } catch (Throwable ex) {
                    d.completeThrowable(ex);
                }
            }
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    private <V> CompletableFuture<V> uniComposeStage(
        Executor e, Function<? super T, ? extends CompletionStage<V>> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<V> d = newIncompleteFuture();
        Object r, s; Throwable x;
        if ((r = result) == null)
            unipush(new UniCompose<T,V>(e, d, this, f));
        else {
            if (r instanceof AltResult) {
                if ((x = ((AltResult)r).ex) != null) {
                    d.result = encodeThrowable(x, r);
                    return d;
                }
                r = null;
            }
            try {
                if (e != null)
                    e.execute(new UniCompose<T,V>(null, d, this, f));
                else {
                    @SuppressWarnings("unchecked") T t = (T) r;
                    CompletableFuture<V> g = f.apply(t).toCompletableFuture();
                    if ((s = g.result) != null)
                        d.result = encodeRelay(s);
                    else
                        g.unipush(new UniRelay<V,V>(d, g));
                }
            } catch (Throwable ex) {
                d.result = encodeThrowable(ex);
            }
        }
        return d;
    }

    /* ------------- Two-input Completions -------------- */

    
A Completion for an action with two sources /** A Completion for an action with two sources */
    @SuppressWarnings("serial")
    abstract static class BiCompletion<T,U,V> extends UniCompletion<T,V> {
        CompletableFuture<U> snd; // second source for action
        BiCompletion(Executor executor, CompletableFuture<V> dep,
                     CompletableFuture<T> src, CompletableFuture<U> snd) {
            super(executor, dep, src); this.snd = snd;
        }
    }

    
A Completion delegating to a BiCompletion /** A Completion delegating to a BiCompletion */
    @SuppressWarnings("serial")
    static final class CoCompletion extends Completion {
        BiCompletion<?,?,?> base;
        CoCompletion(BiCompletion<?,?,?> base) { this.base = base; }
        final CompletableFuture<?> tryFire(int mode) {
            BiCompletion<?,?,?> c; CompletableFuture<?> d;
            if ((c = base) == null || (d = c.tryFire(mode)) == null)
                return null;
            base = null; // detach
            return d;
        }
        final boolean isLive() {
            BiCompletion<?,?,?> c;
            return (c = base) != null
                // && c.isLive()
                && c.dep != null;
        }
    }

    
Pushes completion to this and b unless both done.
Caller should first check that either result or b.result is null.
/**
     * Pushes completion to this and b unless both done.
     * Caller should first check that either result or b.result is null.
     */
    final void bipush(CompletableFuture<?> b, BiCompletion<?,?,?> c) {
        if (c != null) {
            while (result == null) {
                if (tryPushStack(c)) {
                    if (b.result == null)
                        b.unipush(new CoCompletion(c));
                    else if (result != null)
                        c.tryFire(SYNC);
                    return;
                }
            }
            b.unipush(c);
        }
    }

    
Post-processing after successful BiCompletion tryFire. /** Post-processing after successful BiCompletion tryFire. */
    final CompletableFuture<T> postFire(CompletableFuture<?> a,
                                        CompletableFuture<?> b, int mode) {
        if (b != null && b.stack != null) { // clean second source
            Object r;
            if ((r = b.result) == null)
                b.cleanStack();
            if (mode >= 0 && (r != null || b.result != null))
                b.postComplete();
        }
        return postFire(a, mode);
    }

    @SuppressWarnings("serial")
    static final class BiApply<T,U,V> extends BiCompletion<T,U,V> {
        BiFunction<? super T,? super U,? extends V> fn;
        BiApply(Executor executor, CompletableFuture<V> dep,
                CompletableFuture<T> src, CompletableFuture<U> snd,
                BiFunction<? super T,? super U,? extends V> fn) {
            super(executor, dep, src, snd); this.fn = fn;
        }
        final CompletableFuture<V> tryFire(int mode) {
            CompletableFuture<V> d;
            CompletableFuture<T> a;
            CompletableFuture<U> b;
            Object r, s; BiFunction<? super T,? super U,? extends V> f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null
                || (b = snd) == null || (s = b.result) == null
                || !d.biApply(r, s, f, mode > 0 ? null : this))
                return null;
            dep = null; src = null; snd = null; fn = null;
            return d.postFire(a, b, mode);
        }
    }

    final <R,S> boolean biApply(Object r, Object s,
                                BiFunction<? super R,? super S,? extends T> f,
                                BiApply<R,S,T> c) {
        Throwable x;
        tryComplete: if (result == null) {
            if (r instanceof AltResult) {
                if ((x = ((AltResult)r).ex) != null) {
                    completeThrowable(x, r);
                    break tryComplete;
                }
                r = null;
            }
            if (s instanceof AltResult) {
                if ((x = ((AltResult)s).ex) != null) {
                    completeThrowable(x, s);
                    break tryComplete;
                }
                s = null;
            }
            try {
                if (c != null && !c.claim())
                    return false;
                @SuppressWarnings("unchecked") R rr = (R) r;
                @SuppressWarnings("unchecked") S ss = (S) s;
                completeValue(f.apply(rr, ss));
            } catch (Throwable ex) {
                completeThrowable(ex);
            }
        }
        return true;
    }

    private <U,V> CompletableFuture<V> biApplyStage(
        Executor e, CompletionStage<U> o,
        BiFunction<? super T,? super U,? extends V> f) {
        CompletableFuture<U> b; Object r, s;
        if (f == null || (b = o.toCompletableFuture()) == null)
            throw new NullPointerException();
        CompletableFuture<V> d = newIncompleteFuture();
        if ((r = result) == null || (s = b.result) == null)
            bipush(b, new BiApply<T,U,V>(e, d, this, b, f));
        else if (e == null)
            d.biApply(r, s, f, null);
        else
            try {
                e.execute(new BiApply<T,U,V>(null, d, this, b, f));
            } catch (Throwable ex) {
                d.result = encodeThrowable(ex);
            }
        return d;
    }

    @SuppressWarnings("serial")
    static final class BiAccept<T,U> extends BiCompletion<T,U,Void> {
        BiConsumer<? super T,? super U> fn;
        BiAccept(Executor executor, CompletableFuture<Void> dep,
                 CompletableFuture<T> src, CompletableFuture<U> snd,
                 BiConsumer<? super T,? super U> fn) {
            super(executor, dep, src, snd); this.fn = fn;
        }
        final CompletableFuture<Void> tryFire(int mode) {
            CompletableFuture<Void> d;
            CompletableFuture<T> a;
            CompletableFuture<U> b;
            Object r, s; BiConsumer<? super T,? super U> f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null
                || (b = snd) == null || (s = b.result) == null
                || !d.biAccept(r, s, f, mode > 0 ? null : this))
                return null;
            dep = null; src = null; snd = null; fn = null;
            return d.postFire(a, b, mode);
        }
    }

    final <R,S> boolean biAccept(Object r, Object s,
                                 BiConsumer<? super R,? super S> f,
                                 BiAccept<R,S> c) {
        Throwable x;
        tryComplete: if (result == null) {
            if (r instanceof AltResult) {
                if ((x = ((AltResult)r).ex) != null) {
                    completeThrowable(x, r);
                    break tryComplete;
                }
                r = null;
            }
            if (s instanceof AltResult) {
                if ((x = ((AltResult)s).ex) != null) {
                    completeThrowable(x, s);
                    break tryComplete;
                }
                s = null;
            }
            try {
                if (c != null && !c.claim())
                    return false;
                @SuppressWarnings("unchecked") R rr = (R) r;
                @SuppressWarnings("unchecked") S ss = (S) s;
                f.accept(rr, ss);
                completeNull();
            } catch (Throwable ex) {
                completeThrowable(ex);
            }
        }
        return true;
    }

    private <U> CompletableFuture<Void> biAcceptStage(
        Executor e, CompletionStage<U> o,
        BiConsumer<? super T,? super U> f) {
        CompletableFuture<U> b; Object r, s;
        if (f == null || (b = o.toCompletableFuture()) == null)
            throw new NullPointerException();
        CompletableFuture<Void> d = newIncompleteFuture();
        if ((r = result) == null || (s = b.result) == null)
            bipush(b, new BiAccept<T,U>(e, d, this, b, f));
        else if (e == null)
            d.biAccept(r, s, f, null);
        else
            try {
                e.execute(new BiAccept<T,U>(null, d, this, b, f));
            } catch (Throwable ex) {
                d.result = encodeThrowable(ex);
            }
        return d;
    }

    @SuppressWarnings("serial")
    static final class BiRun<T,U> extends BiCompletion<T,U,Void> {
        Runnable fn;
        BiRun(Executor executor, CompletableFuture<Void> dep,
              CompletableFuture<T> src, CompletableFuture<U> snd,
              Runnable fn) {
            super(executor, dep, src, snd); this.fn = fn;
        }
        final CompletableFuture<Void> tryFire(int mode) {
            CompletableFuture<Void> d;
            CompletableFuture<T> a;
            CompletableFuture<U> b;
            Object r, s; Runnable f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (r = a.result) == null
                || (b = snd) == null || (s = b.result) == null
                || !d.biRun(r, s, f, mode > 0 ? null : this))
                return null;
            dep = null; src = null; snd = null; fn = null;
            return d.postFire(a, b, mode);
        }
    }

    final boolean biRun(Object r, Object s, Runnable f, BiRun<?,?> c) {
        Throwable x; Object z;
        if (result == null) {
            if ((r instanceof AltResult
                 && (x = ((AltResult)(z = r)).ex) != null) ||
                (s instanceof AltResult
                 && (x = ((AltResult)(z = s)).ex) != null))
                completeThrowable(x, z);
            else
                try {
                    if (c != null && !c.claim())
                        return false;
                    f.run();
                    completeNull();
                } catch (Throwable ex) {
                    completeThrowable(ex);
                }
        }
        return true;
    }

    private CompletableFuture<Void> biRunStage(Executor e, CompletionStage<?> o,
                                               Runnable f) {
        CompletableFuture<?> b; Object r, s;
        if (f == null || (b = o.toCompletableFuture()) == null)
            throw new NullPointerException();
        CompletableFuture<Void> d = newIncompleteFuture();
        if ((r = result) == null || (s = b.result) == null)
            bipush(b, new BiRun<>(e, d, this, b, f));
        else if (e == null)
            d.biRun(r, s, f, null);
        else
            try {
                e.execute(new BiRun<>(null, d, this, b, f));
            } catch (Throwable ex) {
                d.result = encodeThrowable(ex);
            }
        return d;
    }

    @SuppressWarnings("serial")
    static final class BiRelay<T,U> extends BiCompletion<T,U,Void> { // for And
        BiRelay(CompletableFuture<Void> dep,
                CompletableFuture<T> src, CompletableFuture<U> snd) {
            super(null, dep, src, snd);
        }
        final CompletableFuture<Void> tryFire(int mode) {
            CompletableFuture<Void> d;
            CompletableFuture<T> a;
            CompletableFuture<U> b;
            Object r, s, z; Throwable x;
            if ((d = dep) == null
                || (a = src) == null || (r = a.result) == null
                || (b = snd) == null || (s = b.result) == null)
                return null;
            if (d.result == null) {
                if ((r instanceof AltResult
                     && (x = ((AltResult)(z = r)).ex) != null) ||
                    (s instanceof AltResult
                     && (x = ((AltResult)(z = s)).ex) != null))
                    d.completeThrowable(x, z);
                else
                    d.completeNull();
            }
            src = null; snd = null; dep = null;
            return d.postFire(a, b, mode);
        }
    }

    
Recursively constructs a tree of completions. /** Recursively constructs a tree of completions. */
    static CompletableFuture<Void> andTree(CompletableFuture<?>[] cfs,
                                           int lo, int hi) {
        CompletableFuture<Void> d = new CompletableFuture<Void>();
        if (lo > hi) // empty
            d.result = NIL;
        else {
            CompletableFuture<?> a, b; Object r, s, z; Throwable x;
            int mid = (lo + hi) >>> 1;
            if ((a = (lo == mid ? cfs[lo] :
                      andTree(cfs, lo, mid))) == null ||
                (b = (lo == hi ? a : (hi == mid+1) ? cfs[hi] :
                      andTree(cfs, mid+1, hi))) == null)
                throw new NullPointerException();
            if ((r = a.result) == null || (s = b.result) == null)
                a.bipush(b, new BiRelay<>(d, a, b));
            else if ((r instanceof AltResult
                      && (x = ((AltResult)(z = r)).ex) != null) ||
                     (s instanceof AltResult
                      && (x = ((AltResult)(z = s)).ex) != null))
                d.result = encodeThrowable(x, z);
            else
                d.result = NIL;
        }
        return d;
    }

    /* ------------- Projected (Ored) BiCompletions -------------- */

    
Pushes completion to this and b unless either done.
Caller should first check that result and b.result are both null.
/**
     * Pushes completion to this and b unless either done.
     * Caller should first check that result and b.result are both null.
     */
    final void orpush(CompletableFuture<?> b, BiCompletion<?,?,?> c) {
        if (c != null) {
            while (!tryPushStack(c)) {
                if (result != null) {
                    NEXT.set(c, null);
                    break;
                }
            }
            if (result != null)
                c.tryFire(SYNC);
            else
                b.unipush(new CoCompletion(c));
        }
    }

    @SuppressWarnings("serial")
    static final class OrApply<T,U extends T,V> extends BiCompletion<T,U,V> {
        Function<? super T,? extends V> fn;
        OrApply(Executor executor, CompletableFuture<V> dep,
                CompletableFuture<T> src, CompletableFuture<U> snd,
                Function<? super T,? extends V> fn) {
            super(executor, dep, src, snd); this.fn = fn;
        }
        final CompletableFuture<V> tryFire(int mode) {
            CompletableFuture<V> d;
            CompletableFuture<T> a;
            CompletableFuture<U> b;
            Object r; Throwable x; Function<? super T,? extends V> f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (b = snd) == null
                || ((r = a.result) == null && (r = b.result) == null))
                return null;
            tryComplete: if (d.result == null) {
                try {
                    if (mode <= 0 && !claim())
                        return null;
                    if (r instanceof AltResult) {
                        if ((x = ((AltResult)r).ex) != null) {
                            d.completeThrowable(x, r);
                            break tryComplete;
                        }
                        r = null;
                    }
                    @SuppressWarnings("unchecked") T t = (T) r;
                    d.completeValue(f.apply(t));
                } catch (Throwable ex) {
                    d.completeThrowable(ex);
                }
            }
            dep = null; src = null; snd = null; fn = null;
            return d.postFire(a, b, mode);
        }
    }

    private <U extends T,V> CompletableFuture<V> orApplyStage(
        Executor e, CompletionStage<U> o, Function<? super T, ? extends V> f) {
        CompletableFuture<U> b;
        if (f == null || (b = o.toCompletableFuture()) == null)
            throw new NullPointerException();

        Object r; CompletableFuture<? extends T> z;
        if ((r = (z = this).result) != null ||
            (r = (z = b).result) != null)
            return z.uniApplyNow(r, e, f);

        CompletableFuture<V> d = newIncompleteFuture();
        orpush(b, new OrApply<T,U,V>(e, d, this, b, f));
        return d;
    }

    @SuppressWarnings("serial")
    static final class OrAccept<T,U extends T> extends BiCompletion<T,U,Void> {
        Consumer<? super T> fn;
        OrAccept(Executor executor, CompletableFuture<Void> dep,
                 CompletableFuture<T> src, CompletableFuture<U> snd,
                 Consumer<? super T> fn) {
            super(executor, dep, src, snd); this.fn = fn;
        }
        final CompletableFuture<Void> tryFire(int mode) {
            CompletableFuture<Void> d;
            CompletableFuture<T> a;
            CompletableFuture<U> b;
            Object r; Throwable x; Consumer<? super T> f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (b = snd) == null
                || ((r = a.result) == null && (r = b.result) == null))
                return null;
            tryComplete: if (d.result == null) {
                try {
                    if (mode <= 0 && !claim())
                        return null;
                    if (r instanceof AltResult) {
                        if ((x = ((AltResult)r).ex) != null) {
                            d.completeThrowable(x, r);
                            break tryComplete;
                        }
                        r = null;
                    }
                    @SuppressWarnings("unchecked") T t = (T) r;
                    f.accept(t);
                    d.completeNull();
                } catch (Throwable ex) {
                    d.completeThrowable(ex);
                }
            }
            dep = null; src = null; snd = null; fn = null;
            return d.postFire(a, b, mode);
        }
    }

    private <U extends T> CompletableFuture<Void> orAcceptStage(
        Executor e, CompletionStage<U> o, Consumer<? super T> f) {
        CompletableFuture<U> b;
        if (f == null || (b = o.toCompletableFuture()) == null)
            throw new NullPointerException();

        Object r; CompletableFuture<? extends T> z;
        if ((r = (z = this).result) != null ||
            (r = (z = b).result) != null)
            return z.uniAcceptNow(r, e, f);

        CompletableFuture<Void> d = newIncompleteFuture();
        orpush(b, new OrAccept<T,U>(e, d, this, b, f));
        return d;
    }

    @SuppressWarnings("serial")
    static final class OrRun<T,U> extends BiCompletion<T,U,Void> {
        Runnable fn;
        OrRun(Executor executor, CompletableFuture<Void> dep,
              CompletableFuture<T> src, CompletableFuture<U> snd,
              Runnable fn) {
            super(executor, dep, src, snd); this.fn = fn;
        }
        final CompletableFuture<Void> tryFire(int mode) {
            CompletableFuture<Void> d;
            CompletableFuture<T> a;
            CompletableFuture<U> b;
            Object r; Throwable x; Runnable f;
            if ((d = dep) == null || (f = fn) == null
                || (a = src) == null || (b = snd) == null
                || ((r = a.result) == null && (r = b.result) == null))
                return null;
            if (d.result == null) {
                try {
                    if (mode <= 0 && !claim())
                        return null;
                    else if (r instanceof AltResult
                        && (x = ((AltResult)r).ex) != null)
                        d.completeThrowable(x, r);
                    else {
                        f.run();
                        d.completeNull();
                    }
                } catch (Throwable ex) {
                    d.completeThrowable(ex);
                }
            }
            dep = null; src = null; snd = null; fn = null;
            return d.postFire(a, b, mode);
        }
    }

    private CompletableFuture<Void> orRunStage(Executor e, CompletionStage<?> o,
                                               Runnable f) {
        CompletableFuture<?> b;
        if (f == null || (b = o.toCompletableFuture()) == null)
            throw new NullPointerException();

        Object r; CompletableFuture<?> z;
        if ((r = (z = this).result) != null ||
            (r = (z = b).result) != null)
            return z.uniRunNow(r, e, f);

        CompletableFuture<Void> d = newIncompleteFuture();
        orpush(b, new OrRun<>(e, d, this, b, f));
        return d;
    }

    
Completion for an anyOf input future. /** Completion for an anyOf input future. */
    @SuppressWarnings("serial")
    static class AnyOf extends Completion {
        CompletableFuture<Object> dep; CompletableFuture<?> src;
        CompletableFuture<?>[] srcs;
        AnyOf(CompletableFuture<Object> dep, CompletableFuture<?> src,
              CompletableFuture<?>[] srcs) {
            this.dep = dep; this.src = src; this.srcs = srcs;
        }
        final CompletableFuture<Object> tryFire(int mode) {
            // assert mode != ASYNC;
            CompletableFuture<Object> d; CompletableFuture<?> a;
            CompletableFuture<?>[] as;
            Object r;
            if ((d = dep) == null
                || (a = src) == null || (r = a.result) == null
                || (as = srcs) == null)
                return null;
            dep = null; src = null; srcs = null;
            if (d.completeRelay(r)) {
                for (CompletableFuture<?> b : as)
                    if (b != a)
                        b.cleanStack();
                if (mode < 0)
                    return d;
                else
                    d.postComplete();
            }
            return null;
        }
        final boolean isLive() {
            CompletableFuture<Object> d;
            return (d = dep) != null && d.result == null;
        }
    }

    /* ------------- Zero-input Async forms -------------- */

    @SuppressWarnings("serial")
    static final class AsyncSupply<T> extends ForkJoinTask<Void>
        implements Runnable, AsynchronousCompletionTask {
        CompletableFuture<T> dep; Supplier<? extends T> fn;
        AsyncSupply(CompletableFuture<T> dep, Supplier<? extends T> fn) {
            this.dep = dep; this.fn = fn;
        }

        public final Void getRawResult() { return null; }
        public final void setRawResult(Void v) {}
        public final boolean exec() { run(); return false; }

        public void run() {
            CompletableFuture<T> d; Supplier<? extends T> f;
            if ((d = dep) != null && (f = fn) != null) {
                dep = null; fn = null;
                if (d.result == null) {
                    try {
                        d.completeValue(f.get());
                    } catch (Throwable ex) {
                        d.completeThrowable(ex);
                    }
                }
                d.postComplete();
            }
        }
    }

    static <U> CompletableFuture<U> asyncSupplyStage(Executor e,
                                                     Supplier<U> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<U> d = new CompletableFuture<U>();
        e.execute(new AsyncSupply<U>(d, f));
        return d;
    }

    @SuppressWarnings("serial")
    static final class AsyncRun extends ForkJoinTask<Void>
        implements Runnable, AsynchronousCompletionTask {
        CompletableFuture<Void> dep; Runnable fn;
        AsyncRun(CompletableFuture<Void> dep, Runnable fn) {
            this.dep = dep; this.fn = fn;
        }

        public final Void getRawResult() { return null; }
        public final void setRawResult(Void v) {}
        public final boolean exec() { run(); return false; }

        public void run() {
            CompletableFuture<Void> d; Runnable f;
            if ((d = dep) != null && (f = fn) != null) {
                dep = null; fn = null;
                if (d.result == null) {
                    try {
                        f.run();
                        d.completeNull();
                    } catch (Throwable ex) {
                        d.completeThrowable(ex);
                    }
                }
                d.postComplete();
            }
        }
    }

    static CompletableFuture<Void> asyncRunStage(Executor e, Runnable f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<Void> d = new CompletableFuture<Void>();
        e.execute(new AsyncRun(d, f));
        return d;
    }

    /* ------------- Signallers -------------- */

    
Completion for recording and releasing a waiting thread.  This
class implements ManagedBlocker to avoid starvation when
blocking actions pile up in ForkJoinPools.
/**
     * Completion for recording and releasing a waiting thread.  This
     * class implements ManagedBlocker to avoid starvation when
     * blocking actions pile up in ForkJoinPools.
     */
    @SuppressWarnings("serial")
    static final class Signaller extends Completion
        implements ForkJoinPool.ManagedBlocker {
        long nanos;                    // remaining wait time if timed
        final long deadline;           // non-zero if timed
        final boolean interruptible;
        boolean interrupted;
        volatile Thread thread;

        Signaller(boolean interruptible, long nanos, long deadline) {
            this.thread = Thread.currentThread();
            this.interruptible = interruptible;
            this.nanos = nanos;
            this.deadline = deadline;
        }
        final CompletableFuture<?> tryFire(int ignore) {
            Thread w; // no need to atomically claim
            if ((w = thread) != null) {
                thread = null;
                LockSupport.unpark(w);
            }
            return null;
        }
        public boolean isReleasable() {
            if (Thread.interrupted())
                interrupted = true;
            return ((interrupted && interruptible) ||
                    (deadline != 0L &&
                     (nanos <= 0L ||
                      (nanos = deadline - System.nanoTime()) <= 0L)) ||
                    thread == null);
        }
        public boolean block() {
            while (!isReleasable()) {
                if (deadline == 0L)
                    LockSupport.park(this);
                else
                    LockSupport.parkNanos(this, nanos);
            }
            return true;
        }
        final boolean isLive() { return thread != null; }
    }

    
Returns raw result after waiting, or null if interruptible and
interrupted.
/**
     * Returns raw result after waiting, or null if interruptible and
     * interrupted.
     */
    private Object waitingGet(boolean interruptible) {
        Signaller q = null;
        boolean queued = false;
        Object r;
        while ((r = result) == null) {
            if (q == null) {
                q = new Signaller(interruptible, 0L, 0L);
                if (Thread.currentThread() instanceof ForkJoinWorkerThread)
                    ForkJoinPool.helpAsyncBlocker(defaultExecutor(), q);
            }
            else if (!queued)
                queued = tryPushStack(q);
            else {
                try {
                    ForkJoinPool.managedBlock(q);
                } catch (InterruptedException ie) { // currently cannot happen
                    q.interrupted = true;
                }
                if (q.interrupted && interruptible)
                    break;
            }
        }
        if (q != null && queued) {
            q.thread = null;
            if (!interruptible && q.interrupted)
                Thread.currentThread().interrupt();
            if (r == null)
                cleanStack();
        }
        if (r != null || (r = result) != null)
            postComplete();
        return r;
    }

    
Returns raw result after waiting, or null if interrupted, or
throws TimeoutException on timeout.
/**
     * Returns raw result after waiting, or null if interrupted, or
     * throws TimeoutException on timeout.
     */
    private Object timedGet(long nanos) throws TimeoutException {
        if (Thread.interrupted())
            return null;
        if (nanos > 0L) {
            long d = System.nanoTime() + nanos;
            long deadline = (d == 0L) ? 1L : d; // avoid 0
            Signaller q = null;
            boolean queued = false;
            Object r;
            while ((r = result) == null) { // similar to untimed
                if (q == null) {
                    q = new Signaller(true, nanos, deadline);
                    if (Thread.currentThread() instanceof ForkJoinWorkerThread)
                        ForkJoinPool.helpAsyncBlocker(defaultExecutor(), q);
                }
                else if (!queued)
                    queued = tryPushStack(q);
                else if (q.nanos <= 0L)
                    break;
                else {
                    try {
                        ForkJoinPool.managedBlock(q);
                    } catch (InterruptedException ie) {
                        q.interrupted = true;
                    }
                    if (q.interrupted)
                        break;
                }
            }
            if (q != null && queued) {
                q.thread = null;
                if (r == null)
                    cleanStack();
            }
            if (r != null || (r = result) != null)
                postComplete();
            if (r != null || (q != null && q.interrupted))
                return r;
        }
        throw new TimeoutException();
    }

    /* ------------- public methods -------------- */

    
Creates a new incomplete CompletableFuture.
/**
     * Creates a new incomplete CompletableFuture.
     */
    public CompletableFuture() {
    }

    
Creates a new complete CompletableFuture with given encoded result.
/**
     * Creates a new complete CompletableFuture with given encoded result.
     */
    CompletableFuture(Object r) {
        RESULT.setRelease(this, r);
    }

    
Returns a new CompletableFuture that is asynchronously completed by a task running in the ForkJoinPool.commonPool() with the value obtained by calling the given Supplier. 
Params:
supplier – a function returning the value to be used
to complete the returned CompletableFuture
Type parameters:
<U> – the function's return type
Returns:
the new CompletableFuture/**
     * Returns a new CompletableFuture that is asynchronously completed
     * by a task running in the {@link ForkJoinPool#commonPool()} with
     * the value obtained by calling the given Supplier.
     *
     * @param supplier a function returning the value to be used
     * to complete the returned CompletableFuture
     * @param <U> the function's return type
     * @return the new CompletableFuture
     */
    public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier) {
        return asyncSupplyStage(ASYNC_POOL, supplier);
    }

    
Returns a new CompletableFuture that is asynchronously completed
by a task running in the given executor with the value obtained
by calling the given Supplier.
Params:
supplier – a function returning the value to be used
to complete the returned CompletableFuture
executor – the executor to use for asynchronous execution
Type parameters:
<U> – the function's return type
Returns:
the new CompletableFuture/**
     * Returns a new CompletableFuture that is asynchronously completed
     * by a task running in the given executor with the value obtained
     * by calling the given Supplier.
     *
     * @param supplier a function returning the value to be used
     * to complete the returned CompletableFuture
     * @param executor the executor to use for asynchronous execution
     * @param <U> the function's return type
     * @return the new CompletableFuture
     */
    public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier,
                                                       Executor executor) {
        return asyncSupplyStage(screenExecutor(executor), supplier);
    }

    
Returns a new CompletableFuture that is asynchronously completed by a task running in the ForkJoinPool.commonPool() after it runs the given action. 
Params:
runnable – the action to run before completing the
returned CompletableFuture
Returns:
the new CompletableFuture/**
     * Returns a new CompletableFuture that is asynchronously completed
     * by a task running in the {@link ForkJoinPool#commonPool()} after
     * it runs the given action.
     *
     * @param runnable the action to run before completing the
     * returned CompletableFuture
     * @return the new CompletableFuture
     */
    public static CompletableFuture<Void> runAsync(Runnable runnable) {
        return asyncRunStage(ASYNC_POOL, runnable);
    }

    
Returns a new CompletableFuture that is asynchronously completed
by a task running in the given executor after it runs the given
action.
Params:
runnable – the action to run before completing the
returned CompletableFuture
executor – the executor to use for asynchronous execution
Returns:
the new CompletableFuture/**
     * Returns a new CompletableFuture that is asynchronously completed
     * by a task running in the given executor after it runs the given
     * action.
     *
     * @param runnable the action to run before completing the
     * returned CompletableFuture
     * @param executor the executor to use for asynchronous execution
     * @return the new CompletableFuture
     */
    public static CompletableFuture<Void> runAsync(Runnable runnable,
                                                   Executor executor) {
        return asyncRunStage(screenExecutor(executor), runnable);
    }

    
Returns a new CompletableFuture that is already completed with
the given value.
Params:
value – the value
Type parameters:
<U> – the type of the value
Returns:
the completed CompletableFuture/**
     * Returns a new CompletableFuture that is already completed with
     * the given value.
     *
     * @param value the value
     * @param <U> the type of the value
     * @return the completed CompletableFuture
     */
    public static <U> CompletableFuture<U> completedFuture(U value) {
        return new CompletableFuture<U>((value == null) ? NIL : value);
    }

    
Returns true if completed in any fashion: normally, exceptionally, or via cancellation. 
Returns:
true if completed/**
     * Returns {@code true} if completed in any fashion: normally,
     * exceptionally, or via cancellation.
     *
     * @return {@code true} if completed
     */
    public boolean isDone() {
        return result != null;
    }

    
Waits if necessary for this future to complete, and then
returns its result.
Throws:
CancellationException – if this future was cancelled
ExecutionException – if this future completed exceptionally
InterruptedException – if the current thread was interrupted
while waiting
Returns:
the result value/**
     * Waits if necessary for this future to complete, and then
     * returns its result.
     *
     * @return the result value
     * @throws CancellationException if this future was cancelled
     * @throws ExecutionException if this future completed exceptionally
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     */
    @SuppressWarnings("unchecked")
    public T get() throws InterruptedException, ExecutionException {
        Object r;
        if ((r = result) == null)
            r = waitingGet(true);
        return (T) reportGet(r);
    }

    
Waits if necessary for at most the given time for this future
to complete, and then returns its result, if available.
Params:
timeout – the maximum time to wait
unit – the time unit of the timeout argument
Throws:
CancellationException – if this future was cancelled
ExecutionException – if this future completed exceptionally
InterruptedException – if the current thread was interrupted
while waiting
TimeoutException – if the wait timed out
Returns:
the result value/**
     * Waits if necessary for at most the given time for this future
     * to complete, and then returns its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the result value
     * @throws CancellationException if this future was cancelled
     * @throws ExecutionException if this future completed exceptionally
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    @SuppressWarnings("unchecked")
    public T get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        long nanos = unit.toNanos(timeout);
        Object r;
        if ((r = result) == null)
            r = timedGet(nanos);
        return (T) reportGet(r);
    }

    
Returns the result value when complete, or throws an (unchecked) exception if completed exceptionally. To better conform with the use of common functional forms, if a computation involved in the completion of this CompletableFuture threw an exception, this method throws an (unchecked) CompletionException with the underlying exception as its cause. 
Throws:
CancellationException – if the computation was cancelled
CompletionException – if this future completed
exceptionally or a completion computation threw an exception
Returns:
the result value/**
     * Returns the result value when complete, or throws an
     * (unchecked) exception if completed exceptionally. To better
     * conform with the use of common functional forms, if a
     * computation involved in the completion of this
     * CompletableFuture threw an exception, this method throws an
     * (unchecked) {@link CompletionException} with the underlying
     * exception as its cause.
     *
     * @return the result value
     * @throws CancellationException if the computation was cancelled
     * @throws CompletionException if this future completed
     * exceptionally or a completion computation threw an exception
     */
    @SuppressWarnings("unchecked")
    public T join() {
        Object r;
        if ((r = result) == null)
            r = waitingGet(false);
        return (T) reportJoin(r);
    }

    
Returns the result value (or throws any encountered exception)
if completed, else returns the given valueIfAbsent.
Params:
valueIfAbsent – the value to return if not completed
Throws:
CancellationException – if the computation was cancelled
CompletionException – if this future completed
exceptionally or a completion computation threw an exception
Returns:
the result value, if completed, else the given valueIfAbsent/**
     * Returns the result value (or throws any encountered exception)
     * if completed, else returns the given valueIfAbsent.
     *
     * @param valueIfAbsent the value to return if not completed
     * @return the result value, if completed, else the given valueIfAbsent
     * @throws CancellationException if the computation was cancelled
     * @throws CompletionException if this future completed
     * exceptionally or a completion computation threw an exception
     */
    @SuppressWarnings("unchecked")
    public T getNow(T valueIfAbsent) {
        Object r;
        return ((r = result) == null) ? valueIfAbsent : (T) reportJoin(r);
    }

    
If not already completed, sets the value returned by get() and related methods to the given value. 
Params:
value – the result value
Returns:
true if this invocation caused this CompletableFuture to transition to a completed state, else false/**
     * If not already completed, sets the value returned by {@link
     * #get()} and related methods to the given value.
     *
     * @param value the result value
     * @return {@code true} if this invocation caused this CompletableFuture
     * to transition to a completed state, else {@code false}
     */
    public boolean complete(T value) {
        boolean triggered = completeValue(value);
        postComplete();
        return triggered;
    }

    
If not already completed, causes invocations of get() and related methods to throw the given exception. 
Params:
ex – the exception
Returns:
true if this invocation caused this CompletableFuture to transition to a completed state, else false/**
     * If not already completed, causes invocations of {@link #get()}
     * and related methods to throw the given exception.
     *
     * @param ex the exception
     * @return {@code true} if this invocation caused this CompletableFuture
     * to transition to a completed state, else {@code false}
     */
    public boolean completeExceptionally(Throwable ex) {
        if (ex == null) throw new NullPointerException();
        boolean triggered = internalComplete(new AltResult(ex));
        postComplete();
        return triggered;
    }

    public <U> CompletableFuture<U> thenApply(
        Function<? super T,? extends U> fn) {
        return uniApplyStage(null, fn);
    }

    public <U> CompletableFuture<U> thenApplyAsync(
        Function<? super T,? extends U> fn) {
        return uniApplyStage(defaultExecutor(), fn);
    }

    public <U> CompletableFuture<U> thenApplyAsync(
        Function<? super T,? extends U> fn, Executor executor) {
        return uniApplyStage(screenExecutor(executor), fn);
    }

    public CompletableFuture<Void> thenAccept(Consumer<? super T> action) {
        return uniAcceptStage(null, action);
    }

    public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action) {
        return uniAcceptStage(defaultExecutor(), action);
    }

    public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action,
                                                   Executor executor) {
        return uniAcceptStage(screenExecutor(executor), action);
    }

    public CompletableFuture<Void> thenRun(Runnable action) {
        return uniRunStage(null, action);
    }

    public CompletableFuture<Void> thenRunAsync(Runnable action) {
        return uniRunStage(defaultExecutor(), action);
    }

    public CompletableFuture<Void> thenRunAsync(Runnable action,
                                                Executor executor) {
        return uniRunStage(screenExecutor(executor), action);
    }

    public <U,V> CompletableFuture<V> thenCombine(
        CompletionStage<? extends U> other,
        BiFunction<? super T,? super U,? extends V> fn) {
        return biApplyStage(null, other, fn);
    }

    public <U,V> CompletableFuture<V> thenCombineAsync(
        CompletionStage<? extends U> other,
        BiFunction<? super T,? super U,? extends V> fn) {
        return biApplyStage(defaultExecutor(), other, fn);
    }

    public <U,V> CompletableFuture<V> thenCombineAsync(
        CompletionStage<? extends U> other,
        BiFunction<? super T,? super U,? extends V> fn, Executor executor) {
        return biApplyStage(screenExecutor(executor), other, fn);
    }

    public <U> CompletableFuture<Void> thenAcceptBoth(
        CompletionStage<? extends U> other,
        BiConsumer<? super T, ? super U> action) {
        return biAcceptStage(null, other, action);
    }

    public <U> CompletableFuture<Void> thenAcceptBothAsync(
        CompletionStage<? extends U> other,
        BiConsumer<? super T, ? super U> action) {
        return biAcceptStage(defaultExecutor(), other, action);
    }

    public <U> CompletableFuture<Void> thenAcceptBothAsync(
        CompletionStage<? extends U> other,
        BiConsumer<? super T, ? super U> action, Executor executor) {
        return biAcceptStage(screenExecutor(executor), other, action);
    }

    public CompletableFuture<Void> runAfterBoth(CompletionStage<?> other,
                                                Runnable action) {
        return biRunStage(null, other, action);
    }

    public CompletableFuture<Void> runAfterBothAsync(CompletionStage<?> other,
                                                     Runnable action) {
        return biRunStage(defaultExecutor(), other, action);
    }

    public CompletableFuture<Void> runAfterBothAsync(CompletionStage<?> other,
                                                     Runnable action,
                                                     Executor executor) {
        return biRunStage(screenExecutor(executor), other, action);
    }

    public <U> CompletableFuture<U> applyToEither(
        CompletionStage<? extends T> other, Function<? super T, U> fn) {
        return orApplyStage(null, other, fn);
    }

    public <U> CompletableFuture<U> applyToEitherAsync(
        CompletionStage<? extends T> other, Function<? super T, U> fn) {
        return orApplyStage(defaultExecutor(), other, fn);
    }

    public <U> CompletableFuture<U> applyToEitherAsync(
        CompletionStage<? extends T> other, Function<? super T, U> fn,
        Executor executor) {
        return orApplyStage(screenExecutor(executor), other, fn);
    }

    public CompletableFuture<Void> acceptEither(
        CompletionStage<? extends T> other, Consumer<? super T> action) {
        return orAcceptStage(null, other, action);
    }

    public CompletableFuture<Void> acceptEitherAsync(
        CompletionStage<? extends T> other, Consumer<? super T> action) {
        return orAcceptStage(defaultExecutor(), other, action);
    }

    public CompletableFuture<Void> acceptEitherAsync(
        CompletionStage<? extends T> other, Consumer<? super T> action,
        Executor executor) {
        return orAcceptStage(screenExecutor(executor), other, action);
    }

    public CompletableFuture<Void> runAfterEither(CompletionStage<?> other,
                                                  Runnable action) {
        return orRunStage(null, other, action);
    }

    public CompletableFuture<Void> runAfterEitherAsync(CompletionStage<?> other,
                                                       Runnable action) {
        return orRunStage(defaultExecutor(), other, action);
    }

    public CompletableFuture<Void> runAfterEitherAsync(CompletionStage<?> other,
                                                       Runnable action,
                                                       Executor executor) {
        return orRunStage(screenExecutor(executor), other, action);
    }

    public <U> CompletableFuture<U> thenCompose(
        Function<? super T, ? extends CompletionStage<U>> fn) {
        return uniComposeStage(null, fn);
    }

    public <U> CompletableFuture<U> thenComposeAsync(
        Function<? super T, ? extends CompletionStage<U>> fn) {
        return uniComposeStage(defaultExecutor(), fn);
    }

    public <U> CompletableFuture<U> thenComposeAsync(
        Function<? super T, ? extends CompletionStage<U>> fn,
        Executor executor) {
        return uniComposeStage(screenExecutor(executor), fn);
    }

    public CompletableFuture<T> whenComplete(
        BiConsumer<? super T, ? super Throwable> action) {
        return uniWhenCompleteStage(null, action);
    }

    public CompletableFuture<T> whenCompleteAsync(
        BiConsumer<? super T, ? super Throwable> action) {
        return uniWhenCompleteStage(defaultExecutor(), action);
    }

    public CompletableFuture<T> whenCompleteAsync(
        BiConsumer<? super T, ? super Throwable> action, Executor executor) {
        return uniWhenCompleteStage(screenExecutor(executor), action);
    }

    public <U> CompletableFuture<U> handle(
        BiFunction<? super T, Throwable, ? extends U> fn) {
        return uniHandleStage(null, fn);
    }

    public <U> CompletableFuture<U> handleAsync(
        BiFunction<? super T, Throwable, ? extends U> fn) {
        return uniHandleStage(defaultExecutor(), fn);
    }

    public <U> CompletableFuture<U> handleAsync(
        BiFunction<? super T, Throwable, ? extends U> fn, Executor executor) {
        return uniHandleStage(screenExecutor(executor), fn);
    }

    
Returns this CompletableFuture.
Returns:
this CompletableFuture/**
     * Returns this CompletableFuture.
     *
     * @return this CompletableFuture
     */
    public CompletableFuture<T> toCompletableFuture() {
        return this;
    }

    public CompletableFuture<T> exceptionally(
        Function<Throwable, ? extends T> fn) {
        return uniExceptionallyStage(null, fn);
    }

    public CompletableFuture<T> exceptionallyAsync(
        Function<Throwable, ? extends T> fn) {
        return uniExceptionallyStage(defaultExecutor(), fn);
    }

    public CompletableFuture<T> exceptionallyAsync(
        Function<Throwable, ? extends T> fn, Executor executor) {
        return uniExceptionallyStage(screenExecutor(executor), fn);
    }

    public CompletableFuture<T> exceptionallyCompose(
        Function<Throwable, ? extends CompletionStage<T>> fn) {
        return uniComposeExceptionallyStage(null, fn);
    }

    public CompletableFuture<T> exceptionallyComposeAsync(
        Function<Throwable, ? extends CompletionStage<T>> fn) {
        return uniComposeExceptionallyStage(defaultExecutor(), fn);
    }

    public CompletableFuture<T> exceptionallyComposeAsync(
        Function<Throwable, ? extends CompletionStage<T>> fn,
        Executor executor) {
        return uniComposeExceptionallyStage(screenExecutor(executor), fn);
    }

    /* ------------- Arbitrary-arity constructions -------------- */

    
Returns a new CompletableFuture that is completed when all of the given CompletableFutures complete. If any of the given CompletableFutures complete exceptionally, then the returned CompletableFuture also does so, with a CompletionException holding this exception as its cause. Otherwise, the results, if any, of the given CompletableFutures are not reflected in the returned CompletableFuture, but may be obtained by inspecting them individually. If no CompletableFutures are provided, returns a CompletableFuture completed with the value null. 
Among the applications of this method is to await completion of a set of independent CompletableFutures before continuing a program, as in: CompletableFuture.allOf(c1, c2,
c3).join();. 
Params:
cfs – the CompletableFutures
Throws:
NullPointerException – if the array or any of its elements are null
Returns:
a new CompletableFuture that is completed when all of the
given CompletableFutures complete/**
     * Returns a new CompletableFuture that is completed when all of
     * the given CompletableFutures complete.  If any of the given
     * CompletableFutures complete exceptionally, then the returned
     * CompletableFuture also does so, with a CompletionException
     * holding this exception as its cause.  Otherwise, the results,
     * if any, of the given CompletableFutures are not reflected in
     * the returned CompletableFuture, but may be obtained by
     * inspecting them individually. If no CompletableFutures are
     * provided, returns a CompletableFuture completed with the value
     * {@code null}.
     *
     * <p>Among the applications of this method is to await completion
     * of a set of independent CompletableFutures before continuing a
     * program, as in: {@code CompletableFuture.allOf(c1, c2,
     * c3).join();}.
     *
     * @param cfs the CompletableFutures
     * @return a new CompletableFuture that is completed when all of the
     * given CompletableFutures complete
     * @throws NullPointerException if the array or any of its elements are
     * {@code null}
     */
    public static CompletableFuture<Void> allOf(CompletableFuture<?>... cfs) {
        return andTree(cfs, 0, cfs.length - 1);
    }

    
Returns a new CompletableFuture that is completed when any of
the given CompletableFutures complete, with the same result.
Otherwise, if it completed exceptionally, the returned
CompletableFuture also does so, with a CompletionException
holding this exception as its cause.  If no CompletableFutures
are provided, returns an incomplete CompletableFuture.
Params:
cfs – the CompletableFutures
Throws:
NullPointerException – if the array or any of its elements are null
Returns:
a new CompletableFuture that is completed with the
result or exception of any of the given CompletableFutures when
one completes/**
     * Returns a new CompletableFuture that is completed when any of
     * the given CompletableFutures complete, with the same result.
     * Otherwise, if it completed exceptionally, the returned
     * CompletableFuture also does so, with a CompletionException
     * holding this exception as its cause.  If no CompletableFutures
     * are provided, returns an incomplete CompletableFuture.
     *
     * @param cfs the CompletableFutures
     * @return a new CompletableFuture that is completed with the
     * result or exception of any of the given CompletableFutures when
     * one completes
     * @throws NullPointerException if the array or any of its elements are
     * {@code null}
     */
    public static CompletableFuture<Object> anyOf(CompletableFuture<?>... cfs) {
        int n; Object r;
        if ((n = cfs.length) <= 1)
            return (n == 0)
                ? new CompletableFuture<Object>()
                : uniCopyStage(cfs[0]);
        for (CompletableFuture<?> cf : cfs)
            if ((r = cf.result) != null)
                return new CompletableFuture<Object>(encodeRelay(r));
        cfs = cfs.clone();
        CompletableFuture<Object> d = new CompletableFuture<>();
        for (CompletableFuture<?> cf : cfs)
            cf.unipush(new AnyOf(d, cf, cfs));
        // If d was completed while we were adding completions, we should
        // clean the stack of any sources that may have had completions
        // pushed on their stack after d was completed.
        if (d.result != null)
            for (int i = 0, len = cfs.length; i < len; i++)
                if (cfs[i].result != null)
                    for (i++; i < len; i++)
                        if (cfs[i].result == null)
                            cfs[i].cleanStack();
        return d;
    }

    /* ------------- Control and status methods -------------- */

    
If not already completed, completes this CompletableFuture with a CancellationException. Dependent CompletableFutures that have not already completed will also complete exceptionally, with a CompletionException caused by this CancellationException. 
Params:
mayInterruptIfRunning – this value has no effect in this
implementation because interrupts are not used to control
processing.
Returns:
true if this task is now cancelled/**
     * If not already completed, completes this CompletableFuture with
     * a {@link CancellationException}. Dependent CompletableFutures
     * that have not already completed will also complete
     * exceptionally, with a {@link CompletionException} caused by
     * this {@code CancellationException}.
     *
     * @param mayInterruptIfRunning this value has no effect in this
     * implementation because interrupts are not used to control
     * processing.
     *
     * @return {@code true} if this task is now cancelled
     */
    public boolean cancel(boolean mayInterruptIfRunning) {
        boolean cancelled = (result == null) &&
            internalComplete(new AltResult(new CancellationException()));
        postComplete();
        return cancelled || isCancelled();
    }

    
Returns true if this CompletableFuture was cancelled before it completed normally. 
Returns:
true if this CompletableFuture was cancelled before it completed normally/**
     * Returns {@code true} if this CompletableFuture was cancelled
     * before it completed normally.
     *
     * @return {@code true} if this CompletableFuture was cancelled
     * before it completed normally
     */
    public boolean isCancelled() {
        Object r;
        return ((r = result) instanceof AltResult) &&
            (((AltResult)r).ex instanceof CancellationException);
    }

    
Returns true if this CompletableFuture completed exceptionally, in any way. Possible causes include cancellation, explicit invocation of 
completeExceptionally, and abrupt termination of a CompletionStage action. 
Returns:
true if this CompletableFuture completed exceptionally/**
     * Returns {@code true} if this CompletableFuture completed
     * exceptionally, in any way. Possible causes include
     * cancellation, explicit invocation of {@code
     * completeExceptionally}, and abrupt termination of a
     * CompletionStage action.
     *
     * @return {@code true} if this CompletableFuture completed
     * exceptionally
     */
    public boolean isCompletedExceptionally() {
        Object r;
        return ((r = result) instanceof AltResult) && r != NIL;
    }

    
Forcibly sets or resets the value subsequently returned by method get() and related methods, whether or not already completed. This method is designed for use only in error recovery actions, and even in such situations may result in ongoing dependent completions using established versus overwritten outcomes. 
Params:
value – the completion value/**
     * Forcibly sets or resets the value subsequently returned by
     * method {@link #get()} and related methods, whether or not
     * already completed. This method is designed for use only in
     * error recovery actions, and even in such situations may result
     * in ongoing dependent completions using established versus
     * overwritten outcomes.
     *
     * @param value the completion value
     */
    public void obtrudeValue(T value) {
        result = (value == null) ? NIL : value;
        postComplete();
    }

    
Forcibly causes subsequent invocations of method get() and related methods to throw the given exception, whether or not already completed. This method is designed for use only in error recovery actions, and even in such situations may result in ongoing dependent completions using established versus overwritten outcomes. 
Params:
ex – the exception
Throws:
NullPointerException – if the exception is null/**
     * Forcibly causes subsequent invocations of method {@link #get()}
     * and related methods to throw the given exception, whether or
     * not already completed. This method is designed for use only in
     * error recovery actions, and even in such situations may result
     * in ongoing dependent completions using established versus
     * overwritten outcomes.
     *
     * @param ex the exception
     * @throws NullPointerException if the exception is null
     */
    public void obtrudeException(Throwable ex) {
        if (ex == null) throw new NullPointerException();
        result = new AltResult(ex);
        postComplete();
    }

    
Returns the estimated number of CompletableFutures whose
completions are awaiting completion of this CompletableFuture.
This method is designed for use in monitoring system state, not
for synchronization control.
Returns:
the number of dependent CompletableFutures/**
     * Returns the estimated number of CompletableFutures whose
     * completions are awaiting completion of this CompletableFuture.
     * This method is designed for use in monitoring system state, not
     * for synchronization control.
     *
     * @return the number of dependent CompletableFutures
     */
    public int getNumberOfDependents() {
        int count = 0;
        for (Completion p = stack; p != null; p = p.next)
            ++count;
        return count;
    }

    
Returns a string identifying this CompletableFuture, as well as its completion state. The state, in brackets, contains the String "Completed Normally" or the String 
"Completed Exceptionally", or the String "Not
completed" followed by the number of CompletableFutures dependent upon its completion, if any. 
Returns:
a string identifying this CompletableFuture, as well as its state/**
     * Returns a string identifying this CompletableFuture, as well as
     * its completion state.  The state, in brackets, contains the
     * String {@code "Completed Normally"} or the String {@code
     * "Completed Exceptionally"}, or the String {@code "Not
     * completed"} followed by the number of CompletableFutures
     * dependent upon its completion, if any.
     *
     * @return a string identifying this CompletableFuture, as well as its state
     */
    public String toString() {
        Object r = result;
        int count = 0; // avoid call to getNumberOfDependents in case disabled
        for (Completion p = stack; p != null; p = p.next)
            ++count;
        return super.toString() +
            ((r == null)
             ? ((count == 0)
                ? "[Not completed]"
                : "[Not completed, " + count + " dependents]")
             : (((r instanceof AltResult) && ((AltResult)r).ex != null)
                ? "[Completed exceptionally: " + ((AltResult)r).ex + "]"
                : "[Completed normally]"));
    }

    // jdk9 additions

    
Returns a new incomplete CompletableFuture of the type to be
returned by a CompletionStage method. Subclasses should
normally override this method to return an instance of the same
class as this CompletableFuture. The default implementation
returns an instance of class CompletableFuture.
Type parameters:
<U> – the type of the value
Returns:
a new CompletableFuture
Since:
9/**
     * Returns a new incomplete CompletableFuture of the type to be
     * returned by a CompletionStage method. Subclasses should
     * normally override this method to return an instance of the same
     * class as this CompletableFuture. The default implementation
     * returns an instance of class CompletableFuture.
     *
     * @param <U> the type of the value
     * @return a new CompletableFuture
     * @since 9
     */
    public <U> CompletableFuture<U> newIncompleteFuture() {
        return new CompletableFuture<U>();
    }

    
Returns the default Executor used for async methods that do not specify an Executor. This class uses the ForkJoinPool.commonPool() if it supports more than one parallel thread, or else an Executor using one thread per async task. This method may be overridden in subclasses to return an Executor that provides at least one independent thread. 
Returns:
the executor
Since:
9/**
     * Returns the default Executor used for async methods that do not
     * specify an Executor. This class uses the {@link
     * ForkJoinPool#commonPool()} if it supports more than one
     * parallel thread, or else an Executor using one thread per async
     * task.  This method may be overridden in subclasses to return
     * an Executor that provides at least one independent thread.
     *
     * @return the executor
     * @since 9
     */
    public Executor defaultExecutor() {
        return ASYNC_POOL;
    }

    
Returns a new CompletableFuture that is completed normally with the same value as this CompletableFuture when it completes normally. If this CompletableFuture completes exceptionally, then the returned CompletableFuture completes exceptionally with a CompletionException with this exception as cause. The behavior is equivalent to thenApply(x -> x). This method may be useful as a form of "defensive copying", to prevent clients from completing, while still being able to arrange dependent actions. 
Returns:
the new CompletableFuture
Since:
9/**
     * Returns a new CompletableFuture that is completed normally with
     * the same value as this CompletableFuture when it completes
     * normally. If this CompletableFuture completes exceptionally,
     * then the returned CompletableFuture completes exceptionally
     * with a CompletionException with this exception as cause. The
     * behavior is equivalent to {@code thenApply(x -> x)}. This
     * method may be useful as a form of "defensive copying", to
     * prevent clients from completing, while still being able to
     * arrange dependent actions.
     *
     * @return the new CompletableFuture
     * @since 9
     */
    public CompletableFuture<T> copy() {
        return uniCopyStage(this);
    }

    
Returns a new CompletionStage that is completed normally with the same value as this CompletableFuture when it completes normally, and cannot be independently completed or otherwise used in ways not defined by the methods of interface CompletionStage. If this CompletableFuture completes exceptionally, then the returned CompletionStage completes exceptionally with a CompletionException with this exception as cause. 
Unless overridden by a subclass, a new non-minimal CompletableFuture with all methods available can be obtained from a minimal CompletionStage via toCompletableFuture(). For example, completion of a minimal stage can be awaited by 
 minimalStage.toCompletableFuture().join(); 
Returns:
the new CompletionStage
Since:
9/**
     * Returns a new CompletionStage that is completed normally with
     * the same value as this CompletableFuture when it completes
     * normally, and cannot be independently completed or otherwise
     * used in ways not defined by the methods of interface {@link
     * CompletionStage}.  If this CompletableFuture completes
     * exceptionally, then the returned CompletionStage completes
     * exceptionally with a CompletionException with this exception as
     * cause.
     *
     * <p>Unless overridden by a subclass, a new non-minimal
     * CompletableFuture with all methods available can be obtained from
     * a minimal CompletionStage via {@link #toCompletableFuture()}.
     * For example, completion of a minimal stage can be awaited by
     *
     * <pre> {@code minimalStage.toCompletableFuture().join(); }</pre>
     *
     * @return the new CompletionStage
     * @since 9
     */
    public CompletionStage<T> minimalCompletionStage() {
        return uniAsMinimalStage();
    }

    
Completes this CompletableFuture with the result of
the given Supplier function invoked from an asynchronous
task using the given executor.
Params:
supplier – a function returning the value to be used
to complete this CompletableFuture
executor – the executor to use for asynchronous execution
Returns:
this CompletableFuture
Since:
9/**
     * Completes this CompletableFuture with the result of
     * the given Supplier function invoked from an asynchronous
     * task using the given executor.
     *
     * @param supplier a function returning the value to be used
     * to complete this CompletableFuture
     * @param executor the executor to use for asynchronous execution
     * @return this CompletableFuture
     * @since 9
     */
    public CompletableFuture<T> completeAsync(Supplier<? extends T> supplier,
                                              Executor executor) {
        if (supplier == null || executor == null)
            throw new NullPointerException();
        executor.execute(new AsyncSupply<T>(this, supplier));
        return this;
    }

    
Completes this CompletableFuture with the result of the given
Supplier function invoked from an asynchronous task using the
default executor.
Params:
supplier – a function returning the value to be used
to complete this CompletableFuture
Returns:
this CompletableFuture
Since:
9/**
     * Completes this CompletableFuture with the result of the given
     * Supplier function invoked from an asynchronous task using the
     * default executor.
     *
     * @param supplier a function returning the value to be used
     * to complete this CompletableFuture
     * @return this CompletableFuture
     * @since 9
     */
    public CompletableFuture<T> completeAsync(Supplier<? extends T> supplier) {
        return completeAsync(supplier, defaultExecutor());
    }

    
Exceptionally completes this CompletableFuture with a TimeoutException if not otherwise completed before the given timeout. 
Params:
timeout – how long to wait before completing exceptionally with a TimeoutException, in units of unit
unit – a TimeUnit determining how to interpret the timeout parameter
Returns:
this CompletableFuture
Since:
9/**
     * Exceptionally completes this CompletableFuture with
     * a {@link TimeoutException} if not otherwise completed
     * before the given timeout.
     *
     * @param timeout how long to wait before completing exceptionally
     *        with a TimeoutException, in units of {@code unit}
     * @param unit a {@code TimeUnit} determining how to interpret the
     *        {@code timeout} parameter
     * @return this CompletableFuture
     * @since 9
     */
    public CompletableFuture<T> orTimeout(long timeout, TimeUnit unit) {
        if (unit == null)
            throw new NullPointerException();
        if (result == null)
            whenComplete(new Canceller(Delayer.delay(new Timeout(this),
                                                     timeout, unit)));
        return this;
    }

    
Completes this CompletableFuture with the given value if not
otherwise completed before the given timeout.
Params:
value – the value to use upon timeout
timeout – how long to wait before completing normally with the given value, in units of unit
unit – a TimeUnit determining how to interpret the timeout parameter
Returns:
this CompletableFuture
Since:
9/**
     * Completes this CompletableFuture with the given value if not
     * otherwise completed before the given timeout.
     *
     * @param value the value to use upon timeout
     * @param timeout how long to wait before completing normally
     *        with the given value, in units of {@code unit}
     * @param unit a {@code TimeUnit} determining how to interpret the
     *        {@code timeout} parameter
     * @return this CompletableFuture
     * @since 9
     */
    public CompletableFuture<T> completeOnTimeout(T value, long timeout,
                                                  TimeUnit unit) {
        if (unit == null)
            throw new NullPointerException();
        if (result == null)
            whenComplete(new Canceller(Delayer.delay(
                                           new DelayedCompleter<T>(this, value),
                                           timeout, unit)));
        return this;
    }

    
Returns a new Executor that submits a task to the given base executor after the given delay (or no delay if non-positive). Each delay commences upon invocation of the returned executor's execute method. 
Params:
delay – how long to delay, in units of unit
unit – a TimeUnit determining how to interpret the delay parameter
executor – the base executor
Returns:
the new delayed executor
Since:
9/**
     * Returns a new Executor that submits a task to the given base
     * executor after the given delay (or no delay if non-positive).
     * Each delay commences upon invocation of the returned executor's
     * {@code execute} method.
     *
     * @param delay how long to delay, in units of {@code unit}
     * @param unit a {@code TimeUnit} determining how to interpret the
     *        {@code delay} parameter
     * @param executor the base executor
     * @return the new delayed executor
     * @since 9
     */
    public static Executor delayedExecutor(long delay, TimeUnit unit,
                                           Executor executor) {
        if (unit == null || executor == null)
            throw new NullPointerException();
        return new DelayedExecutor(delay, unit, executor);
    }

    
Returns a new Executor that submits a task to the default executor after the given delay (or no delay if non-positive). Each delay commences upon invocation of the returned executor's execute method. 
Params:
delay – how long to delay, in units of unit
unit – a TimeUnit determining how to interpret the delay parameter
Returns:
the new delayed executor
Since:
9/**
     * Returns a new Executor that submits a task to the default
     * executor after the given delay (or no delay if non-positive).
     * Each delay commences upon invocation of the returned executor's
     * {@code execute} method.
     *
     * @param delay how long to delay, in units of {@code unit}
     * @param unit a {@code TimeUnit} determining how to interpret the
     *        {@code delay} parameter
     * @return the new delayed executor
     * @since 9
     */
    public static Executor delayedExecutor(long delay, TimeUnit unit) {
        if (unit == null)
            throw new NullPointerException();
        return new DelayedExecutor(delay, unit, ASYNC_POOL);
    }

    
Returns a new CompletionStage that is already completed with the given value and supports only those methods in interface CompletionStage. 
Params:
value – the value
Type parameters:
<U> – the type of the value
Returns:
the completed CompletionStage
Since:
9/**
     * Returns a new CompletionStage that is already completed with
     * the given value and supports only those methods in
     * interface {@link CompletionStage}.
     *
     * @param value the value
     * @param <U> the type of the value
     * @return the completed CompletionStage
     * @since 9
     */
    public static <U> CompletionStage<U> completedStage(U value) {
        return new MinimalStage<U>((value == null) ? NIL : value);
    }

    
Returns a new CompletableFuture that is already completed
exceptionally with the given exception.
Params:
ex – the exception
Type parameters:
<U> – the type of the value
Returns:
the exceptionally completed CompletableFuture
Since:
9/**
     * Returns a new CompletableFuture that is already completed
     * exceptionally with the given exception.
     *
     * @param ex the exception
     * @param <U> the type of the value
     * @return the exceptionally completed CompletableFuture
     * @since 9
     */
    public static <U> CompletableFuture<U> failedFuture(Throwable ex) {
        if (ex == null) throw new NullPointerException();
        return new CompletableFuture<U>(new AltResult(ex));
    }

    
Returns a new CompletionStage that is already completed exceptionally with the given exception and supports only those methods in interface CompletionStage. 
Params:
ex – the exception
Type parameters:
<U> – the type of the value
Returns:
the exceptionally completed CompletionStage
Since:
9/**
     * Returns a new CompletionStage that is already completed
     * exceptionally with the given exception and supports only those
     * methods in interface {@link CompletionStage}.
     *
     * @param ex the exception
     * @param <U> the type of the value
     * @return the exceptionally completed CompletionStage
     * @since 9
     */
    public static <U> CompletionStage<U> failedStage(Throwable ex) {
        if (ex == null) throw new NullPointerException();
        return new MinimalStage<U>(new AltResult(ex));
    }

    
Singleton delay scheduler, used only for starting and
cancelling tasks.
/**
     * Singleton delay scheduler, used only for starting and
     * cancelling tasks.
     */
    static final class Delayer {
        static ScheduledFuture<?> delay(Runnable command, long delay,
                                        TimeUnit unit) {
            return delayer.schedule(command, delay, unit);
        }

        static final class DaemonThreadFactory implements ThreadFactory {
            public Thread newThread(Runnable r) {
                Thread t = new Thread(r);
                t.setDaemon(true);
                t.setName("CompletableFutureDelayScheduler");
                return t;
            }
        }

        static final ScheduledThreadPoolExecutor delayer;
        static {
            (delayer = new ScheduledThreadPoolExecutor(
                1, new DaemonThreadFactory())).
                setRemoveOnCancelPolicy(true);
        }
    }

    // Little class-ified lambdas to better support monitoring

    static final class DelayedExecutor implements Executor {
        final long delay;
        final TimeUnit unit;
        final Executor executor;
        DelayedExecutor(long delay, TimeUnit unit, Executor executor) {
            this.delay = delay; this.unit = unit; this.executor = executor;
        }
        public void execute(Runnable r) {
            Delayer.delay(new TaskSubmitter(executor, r), delay, unit);
        }
    }

    
Action to submit user task /** Action to submit user task */
    static final class TaskSubmitter implements Runnable {
        final Executor executor;
        final Runnable action;
        TaskSubmitter(Executor executor, Runnable action) {
            this.executor = executor;
            this.action = action;
        }
        public void run() { executor.execute(action); }
    }

    
Action to completeExceptionally on timeout /** Action to completeExceptionally on timeout */
    static final class Timeout implements Runnable {
        final CompletableFuture<?> f;
        Timeout(CompletableFuture<?> f) { this.f = f; }
        public void run() {
            if (f != null && !f.isDone())
                f.completeExceptionally(new TimeoutException());
        }
    }

    
Action to complete on timeout /** Action to complete on timeout */
    static final class DelayedCompleter<U> implements Runnable {
        final CompletableFuture<U> f;
        final U u;
        DelayedCompleter(CompletableFuture<U> f, U u) { this.f = f; this.u = u; }
        public void run() {
            if (f != null)
                f.complete(u);
        }
    }

    
Action to cancel unneeded timeouts /** Action to cancel unneeded timeouts */
    static final class Canceller implements BiConsumer<Object, Throwable> {
        final Future<?> f;
        Canceller(Future<?> f) { this.f = f; }
        public void accept(Object ignore, Throwable ex) {
            if (ex == null && f != null && !f.isDone())
                f.cancel(false);
        }
    }

    
A subclass that just throws UOE for most non-CompletionStage methods.
/**
     * A subclass that just throws UOE for most non-CompletionStage methods.
     */
    static final class MinimalStage<T> extends CompletableFuture<T> {
        MinimalStage() { }
        MinimalStage(Object r) { super(r); }
        @Override public <U> CompletableFuture<U> newIncompleteFuture() {
            return new MinimalStage<U>(); }
        @Override public T get() {
            throw new UnsupportedOperationException(); }
        @Override public T get(long timeout, TimeUnit unit) {
            throw new UnsupportedOperationException(); }
        @Override public T getNow(T valueIfAbsent) {
            throw new UnsupportedOperationException(); }
        @Override public T join() {
            throw new UnsupportedOperationException(); }
        @Override public boolean complete(T value) {
            throw new UnsupportedOperationException(); }
        @Override public boolean completeExceptionally(Throwable ex) {
            throw new UnsupportedOperationException(); }
        @Override public boolean cancel(boolean mayInterruptIfRunning) {
            throw new UnsupportedOperationException(); }
        @Override public void obtrudeValue(T value) {
            throw new UnsupportedOperationException(); }
        @Override public void obtrudeException(Throwable ex) {
            throw new UnsupportedOperationException(); }
        @Override public boolean isDone() {
            throw new UnsupportedOperationException(); }
        @Override public boolean isCancelled() {
            throw new UnsupportedOperationException(); }
        @Override public boolean isCompletedExceptionally() {
            throw new UnsupportedOperationException(); }
        @Override public int getNumberOfDependents() {
            throw new UnsupportedOperationException(); }
        @Override public CompletableFuture<T> completeAsync
            (Supplier<? extends T> supplier, Executor executor) {
            throw new UnsupportedOperationException(); }
        @Override public CompletableFuture<T> completeAsync
            (Supplier<? extends T> supplier) {
            throw new UnsupportedOperationException(); }
        @Override public CompletableFuture<T> orTimeout
            (long timeout, TimeUnit unit) {
            throw new UnsupportedOperationException(); }
        @Override public CompletableFuture<T> completeOnTimeout
            (T value, long timeout, TimeUnit unit) {
            throw new UnsupportedOperationException(); }
        @Override public CompletableFuture<T> toCompletableFuture() {
            Object r;
            if ((r = result) != null)
                return new CompletableFuture<T>(encodeRelay(r));
            else {
                CompletableFuture<T> d = new CompletableFuture<>();
                unipush(new UniRelay<T,T>(d, this));
                return d;
            }
        }
    }

    // VarHandle mechanics
    private static final VarHandle RESULT;
    private static final VarHandle STACK;
    private static final VarHandle NEXT;
    static {
        try {
            MethodHandles.Lookup l = MethodHandles.lookup();
            RESULT = l.findVarHandle(CompletableFuture.class, "result", Object.class);
            STACK = l.findVarHandle(CompletableFuture.class, "stack", Completion.class);
            NEXT = l.findVarHandle(Completion.class, "next", Completion.class);
        } catch (ReflectiveOperationException e) {
            throw new ExceptionInInitializerError(e);
        }

        // Reduce the risk of rare disastrous classloading in first call to
        // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
        Class<?> ensureLoaded = LockSupport.class;
    }
}