/*

 * 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.io.Serializable;

import java.lang.invoke.MethodHandles;

import java.lang.invoke.VarHandle;

import java.lang.reflect.Constructor;

import java.util.Collection;

import java.util.List;

import java.util.RandomAccess;

import java.util.concurrent.locks.LockSupport;

/**

 * Abstract base class for tasks that run within a {@link ForkJoinPool}.

 * A {@code ForkJoinTask} is a thread-like entity that is much

 * lighter weight than a normal thread.  Huge numbers of tasks and

 * subtasks may be hosted by a small number of actual threads in a

 * ForkJoinPool, at the price of some usage limitations.

 *

 * <p>A "main" {@code ForkJoinTask} begins execution when it is

 * explicitly submitted to a {@link ForkJoinPool}, or, if not already

 * engaged in a ForkJoin computation, commenced in the {@link

 * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or

 * related methods.  Once started, it will usually in turn start other

 * subtasks.  As indicated by the name of this class, many programs

 * using {@code ForkJoinTask} employ only methods {@link #fork} and

 * {@link #join}, or derivatives such as {@link

 * #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also

 * provides a number of other methods that can come into play in

 * advanced usages, as well as extension mechanics that allow support

 * of new forms of fork/join processing.

 *

 * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.

 * The efficiency of {@code ForkJoinTask}s stems from a set of

 * restrictions (that are only partially statically enforceable)

 * reflecting their main use as computational tasks calculating pure

 * functions or operating on purely isolated objects.  The primary

 * coordination mechanisms are {@link #fork}, that arranges

 * asynchronous execution, and {@link #join}, that doesn't proceed

 * until the task's result has been computed.  Computations should

 * ideally avoid {@code synchronized} methods or blocks, and should

 * minimize other blocking synchronization apart from joining other

 * tasks or using synchronizers such as Phasers that are advertised to

 * cooperate with fork/join scheduling. Subdividable tasks should also

 * not perform blocking I/O, and should ideally access variables that

 * are completely independent of those accessed by other running

 * tasks. These guidelines are loosely enforced by not permitting

 * checked exceptions such as {@code IOExceptions} to be

 * thrown. However, computations may still encounter unchecked

 * exceptions, that are rethrown to callers attempting to join

 * them. These exceptions may additionally include {@link

 * RejectedExecutionException} stemming from internal resource

 * exhaustion, such as failure to allocate internal task

 * queues. Rethrown exceptions behave in the same way as regular

 * exceptions, but, when possible, contain stack traces (as displayed

 * for example using {@code ex.printStackTrace()}) of both the thread

 * that initiated the computation as well as the thread actually

 * encountering the exception; minimally only the latter.

 *

 * <p>It is possible to define and use ForkJoinTasks that may block,

 * but doing so requires three further considerations: (1) Completion

 * of few if any <em>other</em> tasks should be dependent on a task

 * that blocks on external synchronization or I/O. Event-style async

 * tasks that are never joined (for example, those subclassing {@link

 * CountedCompleter}) often fall into this category.  (2) To minimize

 * resource impact, tasks should be small; ideally performing only the

 * (possibly) blocking action. (3) Unless the {@link

 * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly

 * blocked tasks is known to be less than the pool's {@link

 * ForkJoinPool#getParallelism} level, the pool cannot guarantee that

 * enough threads will be available to ensure progress or good

 * performance.

 *

 * <p>The primary method for awaiting completion and extracting

 * results of a task is {@link #join}, but there are several variants:

 * The {@link Future#get} methods support interruptible and/or timed

 * waits for completion and report results using {@code Future}

 * conventions. Method {@link #invoke} is semantically

 * equivalent to {@code fork(); join()} but always attempts to begin

 * execution in the current thread. The "<em>quiet</em>" forms of

 * these methods do not extract results or report exceptions. These

 * may be useful when a set of tasks are being executed, and you need

 * to delay processing of results or exceptions until all complete.

 * Method {@code invokeAll} (available in multiple versions)

 * performs the most common form of parallel invocation: forking a set

 * of tasks and joining them all.

 *

 * <p>In the most typical usages, a fork-join pair act like a call

 * (fork) and return (join) from a parallel recursive function. As is

 * the case with other forms of recursive calls, returns (joins)

 * should be performed innermost-first. For example, {@code a.fork();

 * b.fork(); b.join(); a.join();} is likely to be substantially more

 * efficient than joining {@code a} before {@code b}.

 *

 * <p>The execution status of tasks may be queried at several levels

 * of detail: {@link #isDone} is true if a task completed in any way

 * (including the case where a task was cancelled without executing);

 * {@link #isCompletedNormally} is true if a task completed without

 * cancellation or encountering an exception; {@link #isCancelled} is

 * true if the task was cancelled (in which case {@link #getException}

 * returns a {@link CancellationException}); and

 * {@link #isCompletedAbnormally} is true if a task was either

 * cancelled or encountered an exception, in which case {@link

 * #getException} will return either the encountered exception or

 * {@link CancellationException}.

 *

 * <p>The ForkJoinTask class is not usually directly subclassed.

 * Instead, you subclass one of the abstract classes that support a

 * particular style of fork/join processing, typically {@link

 * RecursiveAction} for most computations that do not return results,

 * {@link RecursiveTask} for those that do, and {@link

 * CountedCompleter} for those in which completed actions trigger

 * other actions.  Normally, a concrete ForkJoinTask subclass declares

 * fields comprising its parameters, established in a constructor, and

 * then defines a {@code compute} method that somehow uses the control

 * methods supplied by this base class.

 *

 * <p>Method {@link #join} and its variants are appropriate for use

 * only when completion dependencies are acyclic; that is, the

 * parallel computation can be described as a directed acyclic graph

 * (DAG). Otherwise, executions may encounter a form of deadlock as

 * tasks cyclically wait for each other.  However, this framework

 * supports other methods and techniques (for example the use of

 * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that

 * may be of use in constructing custom subclasses for problems that

 * are not statically structured as DAGs. To support such usages, a

 * ForkJoinTask may be atomically <em>tagged</em> with a {@code short}

 * value using {@link #setForkJoinTaskTag} or {@link

 * #compareAndSetForkJoinTaskTag} and checked using {@link

 * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use

 * these {@code protected} methods or tags for any purpose, but they

 * may be of use in the construction of specialized subclasses.  For

 * example, parallel graph traversals can use the supplied methods to

 * avoid revisiting nodes/tasks that have already been processed.

 * (Method names for tagging are bulky in part to encourage definition

 * of methods that reflect their usage patterns.)

 *

 * <p>Most base support methods are {@code final}, to prevent

 * overriding of implementations that are intrinsically tied to the

 * underlying lightweight task scheduling framework.  Developers

 * creating new basic styles of fork/join processing should minimally

 * implement {@code protected} methods {@link #exec}, {@link

 * #setRawResult}, and {@link #getRawResult}, while also introducing

 * an abstract computational method that can be implemented in its

 * subclasses, possibly relying on other {@code protected} methods

 * provided by this class.

 *

 * <p>ForkJoinTasks should perform relatively small amounts of

 * computation. Large tasks should be split into smaller subtasks,

 * usually via recursive decomposition. As a very rough rule of thumb,

 * a task should perform more than 100 and less than 10000 basic

 * computational steps, and should avoid indefinite looping. If tasks

 * are too big, then parallelism cannot improve throughput. If too

 * small, then memory and internal task maintenance overhead may

 * overwhelm processing.

 *

 * <p>This class provides {@code adapt} methods for {@link Runnable}

 * and {@link Callable}, that may be of use when mixing execution of

 * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are

 * of this form, consider using a pool constructed in <em>asyncMode</em>.

 *

 * <p>ForkJoinTasks are {@code Serializable}, which enables them to be

 * used in extensions such as remote execution frameworks. It is

 * sensible to serialize tasks only before or after, but not during,

 * execution. Serialization is not relied on during execution itself.

 *

 * @since 1.7

 * @author Doug Lea

 */

public abstract class ForkJoinTask<V> implements Future<V>, Serializable {

    /*

     * See the internal documentation of class ForkJoinPool for a

     * general implementation overview.  ForkJoinTasks are mainly

     * responsible for maintaining their "status" field amidst relays

     * to methods in ForkJoinWorkerThread and ForkJoinPool.

     *

     * The methods of this class are more-or-less layered into

     * (1) basic status maintenance

     * (2) execution and awaiting completion

     * (3) user-level methods that additionally report results.

     * This is sometimes hard to see because this file orders exported

     * methods in a way that flows well in javadocs.

     *

     * Revision notes: The use of "Aux" field replaces previous

     * reliance on a table to hold exceptions and synchronized blocks

     * and monitors to wait for completion.

     */

    /**

     * Nodes for threads waiting for completion, or holding a thrown

     * exception (never both). Waiting threads prepend nodes

     * Treiber-stack-style.  Signallers detach and unpark

     * waiters. Cancelled waiters try to unsplice.

     */

    static final class Aux {

        final Thread thread;

        final Throwable ex;  // null if a waiter

        Aux next;            // accessed only via memory-acquire chains

        Aux(Thread thread, Throwable ex) {

            this.thread = thread;

            this.ex = ex;

        }

        final boolean casNext(Aux c, Aux v) { // used only in cancellation

            return NEXT.compareAndSet(this, c, v);

        }

        private static final VarHandle NEXT;

        static {

            try {

                NEXT = MethodHandles.lookup()

                    .findVarHandle(Aux.class, "next", Aux.class);

            } catch (ReflectiveOperationException e) {

                throw new ExceptionInInitializerError(e);

            }

        }

    }

    /*

     * The status field holds bits packed into a single int to ensure

     * atomicity.  Status is initially zero, and takes on nonnegative

     * values until completed, upon which it holds (sign bit) DONE,

     * possibly with ABNORMAL (cancelled or exceptional) and THROWN

     * (in which case an exception has been stored). A value of

     * ABNORMAL without DONE signifies an interrupted wait.  These

     * control bits occupy only (some of) the upper half (16 bits) of

     * status field. The lower bits are used for user-defined tags.

     */

    private static final int DONE         = 1 << 31; // must be negative

    private static final int ABNORMAL     = 1 << 16;

    private static final int THROWN       = 1 << 17;

    private static final int SMASK        = 0xffff;  // short bits for tags

    private static final int UNCOMPENSATE = 1 << 16; // helpJoin return sentinel

    // Fields

    volatile int status;                // accessed directly by pool and workers

    private transient volatile Aux aux; // either waiters or thrown Exception

    // Support for atomic operations

    private static final VarHandle STATUS;

    private static final VarHandle AUX;

    private int getAndBitwiseOrStatus(int v) {

        return (int)STATUS.getAndBitwiseOr(this, v);

    }

    private boolean casStatus(int c, int v) {

        return STATUS.compareAndSet(this, c, v);

    }

    private boolean casAux(Aux c, Aux v) {

        return AUX.compareAndSet(this, c, v);

    }

    /** Removes and unparks waiters */

    private void signalWaiters() {

        for (Aux a; (a = aux) != null && a.ex == null; ) {

            if (casAux(a, null)) {             // detach entire list

                for (Thread t; a != null; a = a.next) {

                    if ((t = a.thread) != Thread.currentThread() && t != null)

                        LockSupport.unpark(t); // don't self-signal

                }

                break;

            }

        }

    }

    /**

     * Sets DONE status and wakes up threads waiting to join this task.

     * @return status on exit

     */

    private int setDone() {

        int s = getAndBitwiseOrStatus(DONE) | DONE;

        signalWaiters();

        return s;

    }

    /**

     * Sets ABNORMAL DONE status unless already done, and wakes up threads

     * waiting to join this task.

     * @return status on exit

     */

    private int trySetCancelled() {

        int s;

        do {} while ((s = status) >= 0 && !casStatus(s, s |= (DONE | ABNORMAL)));

        signalWaiters();

        return s;

    }

    /**

     * Records exception and sets ABNORMAL THROWN DONE status unless

     * already done, and wakes up threads waiting to join this task.

     * If losing a race with setDone or trySetCancelled, the exception

     * may be recorded but not reported.

     *

     * @return status on exit

     */

    final int trySetThrown(Throwable ex) {

        Aux h = new Aux(Thread.currentThread(), ex), p = null;

        boolean installed = false;

        int s;

        while ((s = status) >= 0) {

            Aux a;

            if (!installed && ((a = aux) == null || a.ex == null) &&

                (installed = casAux(a, h)))

                p = a; // list of waiters replaced by h

            if (installed && casStatus(s, s |= (DONE | ABNORMAL | THROWN)))

                break;

        }

        for (; p != null; p = p.next)

            LockSupport.unpark(p.thread);

        return s;

    }

    /**

     * Records exception unless already done. Overridable in subclasses.

     *

     * @return status on exit

     */

    int trySetException(Throwable ex) {

        return trySetThrown(ex);

    }

    /**

     * Constructor for subclasses to call.

     */

    public ForkJoinTask() {}

    static boolean isExceptionalStatus(int s) {  // needed by subclasses

        return (s & THROWN) != 0;

    }

    /**

     * Unless done, calls exec and records status if completed, but

     * doesn't wait for completion otherwise.

     *

     * @return status on exit from this method

     */

    final int doExec() {

        int s; boolean completed;

        if ((s = status) >= 0) {

            try {

                completed = exec();

            } catch (Throwable rex) {

                s = trySetException(rex);

                completed = false;

            }

            if (completed)

                s = setDone();

        }

        return s;

    }

    /**

     * Helps and/or waits for completion from join, get, or invoke;

     * called from either internal or external threads.

     *

     * @param pool if nonnull, known submitted pool, else assumes current pool

     * @param ran true if task known to have been exec'd

     * @param interruptible true if park interruptibly when external

     * @param timed true if use timed wait

     * @param nanos if timed, timeout value

     * @return ABNORMAL if interrupted, else status on exit

     */

    private int awaitDone(ForkJoinPool pool, boolean ran,

                          boolean interruptible, boolean timed,

                          long nanos) {

        ForkJoinPool p; boolean internal; int s; Thread t;

        ForkJoinPool.WorkQueue q = null;

        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {

            ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;

            p = wt.pool;

            if (pool == null)

                pool = p;

            if (internal = (pool == p))

                q = wt.workQueue;

        }

        else {

            internal = false;

            p = ForkJoinPool.common;

            if (pool == null)

                pool = p;

            if (pool == p && p != null)

                q = p.externalQueue();

        }

        if (interruptible && Thread.interrupted())

            return ABNORMAL;

        if ((s = status) < 0)

            return s;

        long deadline = 0L;

        if (timed) {

            if (nanos <= 0L)

                return 0;

            else if ((deadline = nanos + System.nanoTime()) == 0L)

                deadline = 1L;

        }

        boolean uncompensate = false;

        if (q != null && p != null) {  // try helping

            // help even in timed mode if pool has no parallelism

            boolean canHelp = !timed || (p.mode & SMASK) == 0;

            if (canHelp) {

                if ((this instanceof CountedCompleter) &&

                    (s = p.helpComplete(this, q, internal)) < 0)

                    return s;

                if (!ran && ((!internal && q.externalTryUnpush(this)) ||

                             q.tryRemove(this, internal)) && (s = doExec()) < 0)

                    return s;

            }

            if (internal) {

                if ((s = p.helpJoin(this, q, canHelp)) < 0)

                    return s;

                if (s == UNCOMPENSATE)

                    uncompensate = true;

            }

        }

        // block until done or cancelled wait

        boolean interrupted = false, queued = false;

        boolean parked = false, fail = false;

        Aux node = null;

        while ((s = status) >= 0) {

            Aux a; long ns;

            if (fail || (fail = (pool != null && pool.mode < 0)))

                casStatus(s, s | (DONE | ABNORMAL)); // try to cancel

            else if (parked && Thread.interrupted()) {

                if (interruptible) {

                    s = ABNORMAL;

                    break;

                }

                interrupted = true;

            }

            else if (queued) {

                if (deadline != 0L) {

                    if ((ns = deadline - System.nanoTime()) <= 0L)

                        break;

                    LockSupport.parkNanos(ns);

                }

                else

                    LockSupport.park();

                parked = true;

            }

            else if (node != null) {

                if ((a = aux) != null && a.ex != null)

                    Thread.onSpinWait();     // exception in progress

                else if (queued = casAux(node.next = a, node))

                    LockSupport.setCurrentBlocker(this);

            }

            else {

                try {

                    node = new Aux(Thread.currentThread(), null);

                } catch (Throwable ex) {     // cannot create

                    fail = true;

                }

            }

        }

        if (pool != null && uncompensate)

            pool.uncompensate();

        if (queued) {

            LockSupport.setCurrentBlocker(null);

            if (s >= 0) { // cancellation similar to AbstractQueuedSynchronizer

                outer: for (Aux a; (a = aux) != null && a.ex == null; ) {

                    for (Aux trail = null;;) {

                        Aux next = a.next;

                        if (a == node) {

                            if (trail != null)

                                trail.casNext(trail, next);

                            else if (casAux(a, next))

                                break outer; // cannot be re-encountered

                            break;           // restart

                        } else {

                            trail = a;

                            if ((a = next) == null)

                                break outer;

                        }

                    }

                }

            }

            else {

                signalWaiters();             // help clean or signal

                if (interrupted)

                    Thread.currentThread().interrupt();

            }

        }

        return s;

    }

    /**

     * Cancels, ignoring any exceptions thrown by cancel.  Cancel is

     * spec'ed not to throw any exceptions, but if it does anyway, we

     * have no recourse, so guard against this case.

     */

    static final void cancelIgnoringExceptions(Future<?> t) {

        if (t != null) {

            try {

                t.cancel(true);

            } catch (Throwable ignore) {

            }

        }

    }

    /**

     * Returns a rethrowable exception for this task, if available.

     * To provide accurate stack traces, if the exception was not

     * thrown by the current thread, we try to create a new exception

     * of the same type as the one thrown, but with the recorded

     * exception as its cause. If there is no such constructor, we

     * instead try to use a no-arg constructor, followed by initCause,

     * to the same effect. If none of these apply, or any fail due to

     * other exceptions, we return the recorded exception, which is

     * still correct, although it may contain a misleading stack

     * trace.

     *

     * @return the exception, or null if none

     */

    private Throwable getThrowableException() {

        Throwable ex; Aux a;

        if ((a = aux) == null)

            ex = null;

        else if ((ex = a.ex) != null && a.thread != Thread.currentThread()) {

            try {

                Constructor<?> noArgCtor = null, oneArgCtor = null;

                for (Constructor<?> c : ex.getClass().getConstructors()) {

                    Class<?>[] ps = c.getParameterTypes();

                    if (ps.length == 0)

                        noArgCtor = c;

                    else if (ps.length == 1 && ps[0] == Throwable.class) {

                        oneArgCtor = c;

                        break;

                    }

                }

                if (oneArgCtor != null)

                    ex = (Throwable)oneArgCtor.newInstance(ex);

                else if (noArgCtor != null) {

                    Throwable rx = (Throwable)noArgCtor.newInstance();

                    rx.initCause(ex);

                    ex = rx;

                }

            } catch (Exception ignore) {

            }

        }

        return ex;

    }

    /**

     * Returns exception associated with the given status, or null if none.

     */

    private Throwable getException(int s) {

        Throwable ex = null;

        if ((s & ABNORMAL) != 0 &&

            ((s & THROWN) == 0 || (ex = getThrowableException()) == null))

            ex = new CancellationException();

        return ex;

    }

    /**

     * Throws exception associated with the given status, or

     * CancellationException if none recorded.

     */

    private void reportException(int s) {

        ForkJoinTask.<RuntimeException>uncheckedThrow(

            (s & THROWN) != 0 ? getThrowableException() : null);

    }

    /**

     * Throws exception for (timed or untimed) get, wrapping if

     * necessary in an ExecutionException.

     */

    private void reportExecutionException(int s) {

        Throwable ex = null;

        if (s == ABNORMAL)

            ex = new InterruptedException();

        else if (s >= 0)

            ex = new TimeoutException();

        else if ((s & THROWN) != 0 && (ex = getThrowableException()) != null)

            ex = new ExecutionException(ex);

        ForkJoinTask.<RuntimeException>uncheckedThrow(ex);

    }

    /**

     * A version of "sneaky throw" to relay exceptions in other

     * contexts.

     */

    static void rethrow(Throwable ex) {

        ForkJoinTask.<RuntimeException>uncheckedThrow(ex);

    }

    /**

     * The sneaky part of sneaky throw, relying on generics

     * limitations to evade compiler complaints about rethrowing

     * unchecked exceptions. If argument null, throws

     * CancellationException.

     */

    @SuppressWarnings("unchecked") static <T extends Throwable>

    void uncheckedThrow(Throwable t) throws T {

        if (t == null)

            t = new CancellationException();

        throw (T)t; // rely on vacuous cast

    }

    // public methods

    /**

     * Arranges to asynchronously execute this task in the pool the

     * current task is running in, if applicable, or using the {@link

     * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.  While

     * it is not necessarily enforced, it is a usage error to fork a

     * task more than once unless it has completed and been

     * reinitialized.  Subsequent modifications to the state of this

     * task or any data it operates on are not necessarily

     * consistently observable by any thread other than the one

     * executing it unless preceded by a call to {@link #join} or

     * related methods, or a call to {@link #isDone} returning {@code

     * true}.

     *

     * @return {@code this}, to simplify usage

     */

    public final ForkJoinTask<V> fork() {

        Thread t; ForkJoinWorkerThread w;

        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)

            (w = (ForkJoinWorkerThread)t).workQueue.push(this, w.pool);

        else

            ForkJoinPool.common.externalPush(this);

        return this;

    }

    /**

     * Returns the result of the computation when it

     * {@linkplain #isDone is done}.

     * This method differs from {@link #get()} in that abnormal

     * completion results in {@code RuntimeException} or {@code Error},

     * not {@code ExecutionException}, and that interrupts of the

     * calling thread do <em>not</em> cause the method to abruptly

     * return by throwing {@code InterruptedException}.

     *

     * @return the computed result

     */

    public final V join() {

        int s;

        if ((s = status) >= 0)

            s = awaitDone(null, false, false, false, 0L);

        if ((s & ABNORMAL) != 0)

            reportException(s);

        return getRawResult();

    }

    /**

     * Commences performing this task, awaits its completion if

     * necessary, and returns its result, or throws an (unchecked)

     * {@code RuntimeException} or {@code Error} if the underlying

     * computation did so.

     *

     * @return the computed result

     */

    public final V invoke() {

        int s;

        if ((s = doExec()) >= 0)

            s = awaitDone(null, true, false, false, 0L);

        if ((s & ABNORMAL) != 0)

            reportException(s);

        return getRawResult();

    }

    /**

     * Forks the given tasks, returning when {@code isDone} holds for

     * each task or an (unchecked) exception is encountered, in which

     * case the exception is rethrown. If more than one task

     * encounters an exception, then this method throws any one of

     * these exceptions. If any task encounters an exception, the

     * other may be cancelled. However, the execution status of

     * individual tasks is not guaranteed upon exceptional return. The

     * status of each task may be obtained using {@link

     * #getException()} and related methods to check if they have been

     * cancelled, completed normally or exceptionally, or left

     * unprocessed.

     *

     * @param t1 the first task

     * @param t2 the second task

     * @throws NullPointerException if any task is null

     */

    public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {

        int s1, s2;

        if (t1 == null || t2 == null)

            throw new NullPointerException();

        t2.fork();

        if ((s1 = t1.doExec()) >= 0)

            s1 = t1.awaitDone(null, true, false, false, 0L);

        if ((s1 & ABNORMAL) != 0) {

            cancelIgnoringExceptions(t2);

            t1.reportException(s1);

        }

        else if (((s2 = t2.awaitDone(null, false, false, false, 0L)) & ABNORMAL) != 0)

            t2.reportException(s2);

    }

    /**

     * Forks the given tasks, returning when {@code isDone} holds for

     * each task or an (unchecked) exception is encountered, in which

     * case the exception is rethrown. If more than one task

     * encounters an exception, then this method throws any one of

     * these exceptions. If any task encounters an exception, others

     * may be cancelled. However, the execution status of individual

     * tasks is not guaranteed upon exceptional return. The status of

     * each task may be obtained using {@link #getException()} and

     * related methods to check if they have been cancelled, completed

     * normally or exceptionally, or left unprocessed.

     *

     * @param tasks the tasks

     * @throws NullPointerException if any task is null

     */

    public static void invokeAll(ForkJoinTask<?>... tasks) {

        Throwable ex = null;

        int last = tasks.length - 1;

        for (int i = last; i >= 0; --i) {

            ForkJoinTask<?> t;

            if ((t = tasks[i]) == null) {

                ex = new NullPointerException();

                break;

            }

            if (i == 0) {

                int s;

                if ((s = t.doExec()) >= 0)

                    s = t.awaitDone(null, true, false, false, 0L);

                if ((s & ABNORMAL) != 0)

                    ex = t.getException(s);

                break;

            }

            t.fork();

        }

        if (ex == null) {

            for (int i = 1; i <= last; ++i) {

                ForkJoinTask<?> t;

                if ((t = tasks[i]) != null) {

                    int s;

                    if ((s = t.status) >= 0)

                        s = t.awaitDone(null, false, false, false, 0L);

                    if ((s & ABNORMAL) != 0 && (ex = t.getException(s)) != null)

                        break;

                }

            }

        }

        if (ex != null) {

            for (int i = 1; i <= last; ++i)

                cancelIgnoringExceptions(tasks[i]);

            rethrow(ex);

        }

    }

    /**

     * Forks all tasks in the specified collection, returning when

     * {@code isDone} holds for each task or an (unchecked) exception

     * is encountered, in which case the exception is rethrown. If

     * more than one task encounters an exception, then this method

     * throws any one of these exceptions. If any task encounters an

     * exception, others may be cancelled. However, the execution

     * status of individual tasks is not guaranteed upon exceptional

     * return. The status of each task may be obtained using {@link

     * #getException()} and related methods to check if they have been

     * cancelled, completed normally or exceptionally, or left

     * unprocessed.

     *

     * @param tasks the collection of tasks

     * @param <T> the type of the values returned from the tasks

     * @return the tasks argument, to simplify usage

     * @throws NullPointerException if tasks or any element are null

     */

    public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {

        if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {

            invokeAll(tasks.toArray(new ForkJoinTask<?>[0]));

            return tasks;

        }

        @SuppressWarnings("unchecked")

        List<? extends ForkJoinTask<?>> ts =

            (List<? extends ForkJoinTask<?>>) tasks;

        Throwable ex = null;

        int last = ts.size() - 1;  // nearly same as array version

        for (int i = last; i >= 0; --i) {

            ForkJoinTask<?> t;

            if ((t = ts.get(i)) == null) {

                ex = new NullPointerException();

                break;

            }

            if (i == 0) {

                int s;

                if ((s = t.doExec()) >= 0)

                    s = t.awaitDone(null, true, false, false, 0L);

                if ((s & ABNORMAL) != 0)

                    ex = t.getException(s);

                break;

            }

            t.fork();

        }

        if (ex == null) {

            for (int i = 1; i <= last; ++i) {

                ForkJoinTask<?> t;

                if ((t = ts.get(i)) != null) {

                    int s;

                    if ((s = t.status) >= 0)

                        s = t.awaitDone(null, false, false, false, 0L);

                    if ((s & ABNORMAL) != 0 && (ex = t.getException(s)) != null)

                        break;

                }

            }

        }

        if (ex != null) {

            for (int i = 1; i <= last; ++i)

                cancelIgnoringExceptions(ts.get(i));

            rethrow(ex);

        }

        return tasks;

    }

    /**

     * Attempts to cancel execution of this task. This attempt will

     * fail if the task has already completed or could not be

     * cancelled for some other reason. If successful, and this task

     * has not started when {@code cancel} is called, execution of

     * this task is suppressed. After this method returns

     * successfully, unless there is an intervening call to {@link

     * #reinitialize}, subsequent calls to {@link #isCancelled},

     * {@link #isDone}, and {@code cancel} will return {@code true}

     * and calls to {@link #join} and related methods will result in

     * {@code CancellationException}.

     *

     * <p>This method may be overridden in subclasses, but if so, must

     * still ensure that these properties hold. In particular, the

     * {@code cancel} method itself must not throw exceptions.

     *

     * <p>This method is designed to be invoked by <em>other</em>

     * tasks. To terminate the current task, you can just return or

     * throw an unchecked exception from its computation method, or

     * invoke {@link #completeExceptionally(Throwable)}.

     *

     * @param mayInterruptIfRunning this value has no effect in the

     * default implementation because interrupts are not used to

     * control cancellation.

     *

     * @return {@code true} if this task is now cancelled

     */

    public boolean cancel(boolean mayInterruptIfRunning) {

        return (trySetCancelled() & (ABNORMAL | THROWN)) == ABNORMAL;

    }

    public final boolean isDone() {

        return status < 0;

    }

    public final boolean isCancelled() {

        return (status & (ABNORMAL | THROWN)) == ABNORMAL;

    }

    /**

     * Returns {@code true} if this task threw an exception or was cancelled.

     *

     * @return {@code true} if this task threw an exception or was cancelled

     */

    public final boolean isCompletedAbnormally() {

        return (status & ABNORMAL) != 0;

    }

    /**

     * Returns {@code true} if this task completed without throwing an

     * exception and was not cancelled.

     *

     * @return {@code true} if this task completed without throwing an

     * exception and was not cancelled

     */

    public final boolean isCompletedNormally() {

        return (status & (DONE | ABNORMAL)) == DONE;

    }

    /**

     * Returns the exception thrown by the base computation, or a

     * {@code CancellationException} if cancelled, or {@code null} if

     * none or if the method has not yet completed.

     *

     * @return the exception, or {@code null} if none

     */

    public final Throwable getException() {

        return getException(status);

    }

    /**

     * Completes this task abnormally, and if not already aborted or

     * cancelled, causes it to throw the given exception upon

     * {@code join} and related operations. This method may be used

     * to induce exceptions in asynchronous tasks, or to force

     * completion of tasks that would not otherwise complete.  Its use

     * in other situations is discouraged.  This method is

     * overridable, but overridden versions must invoke {@code super}

     * implementation to maintain guarantees.

     *

     * @param ex the exception to throw. If this exception is not a

     * {@code RuntimeException} or {@code Error}, the actual exception

     * thrown will be a {@code RuntimeException} with cause {@code ex}.

     */

    public void completeExceptionally(Throwable ex) {

        trySetException((ex instanceof RuntimeException) ||

                        (ex instanceof Error) ? ex :

                        new RuntimeException(ex));

    }

    /**

     * Completes this task, and if not already aborted or cancelled,

     * returning the given value as the result of subsequent

     * invocations of {@code join} and related operations. This method

     * may be used to provide results for asynchronous tasks, or to

     * provide alternative handling for tasks that would not otherwise

     * complete normally. Its use in other situations is

     * discouraged. This method is overridable, but overridden

     * versions must invoke {@code super} implementation to maintain

     * guarantees.

     *

     * @param value the result value for this task

     */

    public void complete(V value) {

        try {

            setRawResult(value);

        } catch (Throwable rex) {

            trySetException(rex);

            return;

        }

        setDone();

    }

    /**

     * Completes this task normally without setting a value. The most

     * recent value established by {@link #setRawResult} (or {@code

     * null} by default) will be returned as the result of subsequent

     * invocations of {@code join} and related operations.

     *

     * @since 1.8

     */

    public final void quietlyComplete() {

        setDone();

    }

    /**

     * Waits if necessary for the computation to complete, and then

     * retrieves its result.

     *

     * @return the computed result

     * @throws CancellationException if the computation was cancelled

     * @throws ExecutionException if the computation threw an

     * exception

     * @throws InterruptedException if the current thread is not a

     * member of a ForkJoinPool and was interrupted while waiting

     */

    public final V get() throws InterruptedException, ExecutionException {

        int s = awaitDone(null, false, true, false, 0L);

        if ((s & ABNORMAL) != 0)

            reportExecutionException(s);

        return getRawResult();

    }

    /**

     * Waits if necessary for at most the given time for the computation

     * to complete, and then retrieves its result, if available.

     *

     * @param timeout the maximum time to wait

     * @param unit the time unit of the timeout argument

     * @return the computed result

     * @throws CancellationException if the computation was cancelled

     * @throws ExecutionException if the computation threw an

     * exception

     * @throws InterruptedException if the current thread is not a

     * member of a ForkJoinPool and was interrupted while waiting

     * @throws TimeoutException if the wait timed out

     */

    public final V get(long timeout, TimeUnit unit)

        throws InterruptedException, ExecutionException, TimeoutException {

        long nanos = unit.toNanos(timeout);

        int s = awaitDone(null, false, true, true, nanos);

        if (s >= 0 || (s & ABNORMAL) != 0)

            reportExecutionException(s);

        return getRawResult();

    }

    /**

     * Joins this task, without returning its result or throwing its

     * exception. This method may be useful when processing

     * collections of tasks when some have been cancelled or otherwise

     * known to have aborted.

     */

    public final void quietlyJoin() {

        if (status >= 0)

            awaitDone(null, false, false, false, 0L);

    }

    /**

     * Commences performing this task and awaits its completion if

     * necessary, without returning its result or throwing its

     * exception.

     */

    public final void quietlyInvoke() {

        if (doExec() >= 0)

            awaitDone(null, true, false, false, 0L);

    }

    // Versions of join/get for pool.invoke* methods that use external,

    // possibly-non-commonPool submits

    final void awaitPoolInvoke(ForkJoinPool pool) {

        awaitDone(pool, false, false, false, 0L);

    }

    final void awaitPoolInvoke(ForkJoinPool pool, long nanos) {

        awaitDone(pool, false, true, true, nanos);

    }

    final V joinForPoolInvoke(ForkJoinPool pool) {

        int s = awaitDone(pool, false, false, false, 0L);

        if ((s & ABNORMAL) != 0)

            reportException(s);

        return getRawResult();

    }

    final V getForPoolInvoke(ForkJoinPool pool)

        throws InterruptedException, ExecutionException {

        int s = awaitDone(pool, false, true, false, 0L);

        if ((s & ABNORMAL) != 0)

            reportExecutionException(s);

        return getRawResult();

    }

    final V getForPoolInvoke(ForkJoinPool pool, long nanos)

        throws InterruptedException, ExecutionException, TimeoutException {

        int s = awaitDone(pool, false, true, true, nanos);

        if (s >= 0 || (s & ABNORMAL) != 0)

            reportExecutionException(s);

        return getRawResult();

    }

    /**

     * Possibly executes tasks until the pool hosting the current task

     * {@linkplain ForkJoinPool#isQuiescent is quiescent}.  This

     * method may be of use in designs in which many tasks are forked,

     * but none are explicitly joined, instead executing them until

     * all are processed.

     */

    public static void helpQuiesce() {

        Thread t; ForkJoinWorkerThread w; ForkJoinPool p;

        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread &&

            (p = (w = (ForkJoinWorkerThread)t).pool) != null)

            p.helpQuiescePool(w.workQueue, Long.MAX_VALUE, false);

        else

            ForkJoinPool.common.externalHelpQuiescePool(Long.MAX_VALUE, false);

    }

    /**

     * Resets the internal bookkeeping state of this task, allowing a

     * subsequent {@code fork}. This method allows repeated reuse of

     * this task, but only if reuse occurs when this task has either

     * never been forked, or has been forked, then completed and all

     * outstanding joins of this task have also completed. Effects

     * under any other usage conditions are not guaranteed.

     * This method may be useful when executing

     * pre-constructed trees of subtasks in loops.

     *

     * <p>Upon completion of this method, {@code isDone()} reports

     * {@code false}, and {@code getException()} reports {@code

     * null}. However, the value returned by {@code getRawResult} is

     * unaffected. To clear this value, you can invoke {@code

     * setRawResult(null)}.

     */

    public void reinitialize() {

        aux = null;

        status = 0;

    }

    /**

     * Returns the pool hosting the current thread, or {@code null}

     * if the current thread is executing outside of any ForkJoinPool.

     *

     * <p>This method returns {@code null} if and only if {@link

     * #inForkJoinPool} returns {@code false}.

     *

     * @return the pool, or {@code null} if none

     */

    public static ForkJoinPool getPool() {

        Thread t;

        return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?

                ((ForkJoinWorkerThread) t).pool : null);

    }

    /**

     * Returns {@code true} if the current thread is a {@link

     * ForkJoinWorkerThread} executing as a ForkJoinPool computation.

     *

     * @return {@code true} if the current thread is a {@link

     * ForkJoinWorkerThread} executing as a ForkJoinPool computation,

     * or {@code false} otherwise

     */

    public static boolean inForkJoinPool() {

        return Thread.currentThread() instanceof ForkJoinWorkerThread;

    }

    /**

     * Tries to unschedule this task for execution. This method will

     * typically (but is not guaranteed to) succeed if this task is

     * the most recently forked task by the current thread, and has

     * not commenced executing in another thread.  This method may be

     * useful when arranging alternative local processing of tasks

     * that could have been, but were not, stolen.

     *

     * @return {@code true} if unforked

     */

    public boolean tryUnfork() {

        Thread t; ForkJoinPool.WorkQueue q;

        return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)

            ? (q = ((ForkJoinWorkerThread)t).workQueue) != null

               && q.tryUnpush(this)

            : (q = ForkJoinPool.commonQueue()) != null

               && q.externalTryUnpush(this);

    }

    /**

     * Returns an estimate of the number of tasks that have been

     * forked by the current worker thread but not yet executed. This

     * value may be useful for heuristic decisions about whether to

     * fork other tasks.

     *

     * @return the number of tasks

     */

    public static int getQueuedTaskCount() {

        Thread t; ForkJoinPool.WorkQueue q;

        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)

            q = ((ForkJoinWorkerThread)t).workQueue;

        else

            q = ForkJoinPool.commonQueue();

        return (q == null) ? 0 : q.queueSize();

    }

    /**

     * Returns an estimate of how many more locally queued tasks are

     * held by the current worker thread than there are other worker

     * threads that might steal them, or zero if this thread is not

     * operating in a ForkJoinPool. This value may be useful for

     * heuristic decisions about whether to fork other tasks. In many

     * usages of ForkJoinTasks, at steady state, each worker should

     * aim to maintain a small constant surplus (for example, 3) of

     * tasks, and to process computations locally if this threshold is

     * exceeded.

     *

     * @return the surplus number of tasks, which may be negative

     */

    public static int getSurplusQueuedTaskCount() {

        return ForkJoinPool.getSurplusQueuedTaskCount();

    }

    // Extension methods

    /**

     * Returns the result that would be returned by {@link #join}, even

     * if this task completed abnormally, or {@code null} if this task

     * is not known to have been completed.  This method is designed

     * to aid debugging, as well as to support extensions. Its use in

     * any other context is discouraged.

     *

     * @return the result, or {@code null} if not completed

     */

    public abstract V getRawResult();

    /**

     * Forces the given value to be returned as a result.  This method

     * is designed to support extensions, and should not in general be

     * called otherwise.

     *

     * @param value the value

     */

    protected abstract void setRawResult(V value);

    /**

     * Immediately performs the base action of this task and returns

     * true if, upon return from this method, this task is guaranteed

     * to have completed. This method may return false otherwise, to

     * indicate that this task is not necessarily complete (or is not

     * known to be complete), for example in asynchronous actions that

     * require explicit invocations of completion methods. This method

     * may also throw an (unchecked) exception to indicate abnormal

     * exit. This method is designed to support extensions, and should

     * not in general be called otherwise.

     *

     * @return {@code true} if this task is known to have completed normally

     */

    protected abstract boolean exec();

    /**

     * Returns, but does not unschedule or execute, a task queued by

     * the current thread but not yet executed, if one is immediately

     * available. There is no guarantee that this task will actually

     * be polled or executed next. Conversely, this method may return

     * null even if a task exists but cannot be accessed without

     * contention with other threads.  This method is designed

     * primarily to support extensions, and is unlikely to be useful

     * otherwise.

     *

     * @return the next task, or {@code null} if none are available

     */

    protected static ForkJoinTask<?> peekNextLocalTask() {

        Thread t; ForkJoinPool.WorkQueue q;

        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)

            q = ((ForkJoinWorkerThread)t).workQueue;

        else

            q = ForkJoinPool.commonQueue();

        return (q == null) ? null : q.peek();

    }

    /**

     * Unschedules and returns, without executing, the next task

     * queued by the current thread but not yet executed, if the

     * current thread is operating in a ForkJoinPool.  This method is

     * designed primarily to support extensions, and is unlikely to be

     * useful otherwise.

     *

     * @return the next task, or {@code null} if none are available

     */

    protected static ForkJoinTask<?> pollNextLocalTask() {

        Thread t;

        return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?

                ((ForkJoinWorkerThread)t).workQueue.nextLocalTask() : null);

    }

    /**

     * If the current thread is operating in a ForkJoinPool,

     * unschedules and returns, without executing, the next task