/*

 * 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.util.Collection;

import java.util.List;

import java.util.RandomAccess;

import java.lang.ref.WeakReference;

import java.lang.ref.ReferenceQueue;

import java.util.concurrent.Callable;

import java.util.concurrent.CancellationException;

import java.util.concurrent.ExecutionException;

import java.util.concurrent.Future;

import java.util.concurrent.RejectedExecutionException;

import java.util.concurrent.RunnableFuture;

import java.util.concurrent.TimeUnit;

import java.util.concurrent.TimeoutException;

import java.util.concurrent.locks.ReentrantLock;

import java.lang.reflect.Constructor;

/**

 * 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 submitted

 * to a {@link ForkJoinPool}.  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 intended 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

 * 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. Tasks should also not perform

 * blocking IO, and should ideally access variables that are

 * completely independent of those accessed by other running

 * tasks. Minor breaches of these restrictions, for example using

 * shared output streams, may be tolerable in practice, but frequent

 * use may result in poor performance, and the potential to

 * indefinitely stall if the number of threads not waiting for IO or

 * other external synchronization becomes exhausted. This usage

 * restriction is in part 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>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>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 java.util.concurrent.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 java.util.concurrent.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 computations that do not return results, or

 * {@link RecursiveTask} for those that do.  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. While these methods have {@code public} access (to allow

 * instances of different task subclasses to call each other's

 * methods), some of them may only be called from within other

 * ForkJoinTasks (as may be determined using method {@link

 * #inForkJoinPool}).  Attempts to invoke them in other contexts

 * result in exceptions or errors, possibly including

 * {@code ClassCastException}.

 *

 * <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.

 *

 * <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.

     */

    /*

     * The status field holds run control status bits packed into a

     * single int to minimize footprint and to ensure atomicity (via

     * CAS).  Status is initially zero, and takes on nonnegative

     * values until completed, upon which status holds value

     * NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking

     * waits by other threads have the SIGNAL bit set.  Completion of

     * a stolen task with SIGNAL set awakens any waiters via

     * notifyAll. Even though suboptimal for some purposes, we use

     * basic builtin wait/notify to take advantage of "monitor

     * inflation" in JVMs that we would otherwise need to emulate to

     * avoid adding further per-task bookkeeping overhead.  We want

     * these monitors to be "fat", i.e., not use biasing or thin-lock

     * techniques, so use some odd coding idioms that tend to avoid

     * them.

     */

    /** The run status of this task */

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

    private static final int NORMAL      = -1;

    private static final int CANCELLED   = -2;

    private static final int EXCEPTIONAL = -3;

    private static final int SIGNAL      =  1;

    /**

     * Marks completion and wakes up threads waiting to join this task,

     * also clearing signal request bits.

     *

     * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL

     * @return completion status on exit

     */

    private int setCompletion(int completion) {

        for (int s;;) {

            if ((s = status) < 0)

                return s;

            if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {

                if (s != 0)

                    synchronized (this) { notifyAll(); }

                return completion;

            }

        }

    }

    /**

     * Tries to block a worker thread until completed or timed out.

     * Uses Object.wait time argument conventions.

     * May fail on contention or interrupt.

     *

     * @param millis if > 0, wait time.

     */

    final void tryAwaitDone(long millis) {

        int s;

        try {

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

                 (s == 0 &&

                  UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL))) &&

                status > 0) {

                synchronized (this) {

                    if (status > 0)

                        wait(millis);

                }

            }

        } catch (InterruptedException ie) {

            // caller must check termination

        }

    }

    /**

     * Blocks a non-worker-thread until completion.

     * @return status upon completion

     */

    private int externalAwaitDone() {

        int s;

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

            boolean interrupted = false;

            synchronized (this) {

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

                    if (s == 0)

                        UNSAFE.compareAndSwapInt(this, statusOffset,

                                                 0, SIGNAL);

                    else {

                        try {

                            wait();

                        } catch (InterruptedException ie) {

                            interrupted = true;

                        }

                    }

                }

            }

            if (interrupted)

                Thread.currentThread().interrupt();

        }

        return s;

    }

    /**

     * Blocks a non-worker-thread until completion or interruption or timeout.

     */

    private int externalInterruptibleAwaitDone(long millis)

        throws InterruptedException {

        int s;

        if (Thread.interrupted())

            throw new InterruptedException();

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

            synchronized (this) {

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

                    if (s == 0)

                        UNSAFE.compareAndSwapInt(this, statusOffset,

                                                 0, SIGNAL);

                    else {

                        wait(millis);

                        if (millis > 0L)

                            break;

                    }

                }

            }

        }

        return s;

    }

    /**

     * Primary execution method for stolen tasks. Unless done, calls

     * exec and records status if completed, but doesn't wait for

     * completion otherwise.

     */

    final void doExec() {

        if (status >= 0) {

            boolean completed;

            try {

                completed = exec();

            } catch (Throwable rex) {

                setExceptionalCompletion(rex);

                return;

            }

            if (completed)

                setCompletion(NORMAL); // must be outside try block

        }

    }

    /**

     * Primary mechanics for join, get, quietlyJoin.

     * @return status upon completion

     */

    private int doJoin() {

        Thread t; ForkJoinWorkerThread w; int s; boolean completed;

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

            if ((s = status) < 0)

                return s;

            if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {

                try {

                    completed = exec();

                } catch (Throwable rex) {

                    return setExceptionalCompletion(rex);

                }

                if (completed)

                    return setCompletion(NORMAL);

            }

            return w.joinTask(this);

        }

        else

            return externalAwaitDone();

    }

    /**

     * Primary mechanics for invoke, quietlyInvoke.

     * @return status upon completion

     */

    private int doInvoke() {

        int s; boolean completed;

        if ((s = status) < 0)

            return s;

        try {

            completed = exec();

        } catch (Throwable rex) {

            return setExceptionalCompletion(rex);

        }

        if (completed)

            return setCompletion(NORMAL);

        else

            return doJoin();

    }

    // Exception table support

    /**

     * Table of exceptions thrown by tasks, to enable reporting by

     * callers. Because exceptions are rare, we don't directly keep

     * them with task objects, but instead use a weak ref table.  Note

     * that cancellation exceptions don't appear in the table, but are

     * instead recorded as status values.

     *

     * Note: These statics are initialized below in static block.

     */

    private static final ExceptionNode[] exceptionTable;

    private static final ReentrantLock exceptionTableLock;

    private static final ReferenceQueue<Object> exceptionTableRefQueue;

    /**

     * Fixed capacity for exceptionTable.

     */

    private static final int EXCEPTION_MAP_CAPACITY = 32;

    /**

     * Key-value nodes for exception table.  The chained hash table

     * uses identity comparisons, full locking, and weak references

     * for keys. The table has a fixed capacity because it only

     * maintains task exceptions long enough for joiners to access

     * them, so should never become very large for sustained

     * periods. However, since we do not know when the last joiner

     * completes, we must use weak references and expunge them. We do

     * so on each operation (hence full locking). Also, some thread in

     * any ForkJoinPool will call helpExpungeStaleExceptions when its

     * pool becomes isQuiescent.

     */

    static final class ExceptionNode extends WeakReference<ForkJoinTask<?>>{

        final Throwable ex;

        ExceptionNode next;

        final long thrower;  // use id not ref to avoid weak cycles

        ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {

            super(task, exceptionTableRefQueue);

            this.ex = ex;

            this.next = next;

            this.thrower = Thread.currentThread().getId();

        }

    }

    /**

     * Records exception and sets exceptional completion.

     *

     * @return status on exit

     */

    private int setExceptionalCompletion(Throwable ex) {

        int h = System.identityHashCode(this);

        final ReentrantLock lock = exceptionTableLock;

        lock.lock();

        try {

            expungeStaleExceptions();

            ExceptionNode[] t = exceptionTable;

            int i = h & (t.length - 1);

            for (ExceptionNode e = t[i]; ; e = e.next) {

                if (e == null) {

                    t[i] = new ExceptionNode(this, ex, t[i]);

                    break;

                }

                if (e.get() == this) // already present

                    break;

            }

        } finally {

            lock.unlock();

        }

        return setCompletion(EXCEPTIONAL);

    }

    /**

     * Removes exception node and clears status

     */

    private void clearExceptionalCompletion() {

        int h = System.identityHashCode(this);

        final ReentrantLock lock = exceptionTableLock;

        lock.lock();

        try {

            ExceptionNode[] t = exceptionTable;

            int i = h & (t.length - 1);

            ExceptionNode e = t[i];

            ExceptionNode pred = null;

            while (e != null) {

                ExceptionNode next = e.next;

                if (e.get() == this) {

                    if (pred == null)

                        t[i] = next;

                    else

                        pred.next = next;

                    break;

                }

                pred = e;

                e = next;

            }

            expungeStaleExceptions();

            status = 0;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Returns a rethrowable exception for the given 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() {

        if (status != EXCEPTIONAL)

            return null;

        int h = System.identityHashCode(this);

        ExceptionNode e;

        final ReentrantLock lock = exceptionTableLock;

        lock.lock();

        try {

            expungeStaleExceptions();

            ExceptionNode[] t = exceptionTable;

            e = t[h & (t.length - 1)];

            while (e != null && e.get() != this)

                e = e.next;

        } finally {

            lock.unlock();

        }

        Throwable ex;

        if (e == null || (ex = e.ex) == null)

            return null;

        if (e.thrower != Thread.currentThread().getId()) {

            Class ec = ex.getClass();

            try {

                Constructor<?> noArgCtor = null;

                Constructor<?>[] cs = ec.getConstructors();// public ctors only

                for (int i = 0; i < cs.length; ++i) {

                    Constructor<?> c = cs[i];

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

                    if (ps.length == 0)

                        noArgCtor = c;

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

                        return (Throwable)(c.newInstance(ex));

                }

                if (noArgCtor != null) {

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

                    wx.initCause(ex);

                    return wx;

                }

            } catch (Exception ignore) {

            }

        }

        return ex;

    }

    /**

     * Poll stale refs and remove them. Call only while holding lock.

     */

    private static void expungeStaleExceptions() {

        for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {

            if (x instanceof ExceptionNode) {

                ForkJoinTask<?> key = ((ExceptionNode)x).get();

                ExceptionNode[] t = exceptionTable;

                int i = System.identityHashCode(key) & (t.length - 1);

                ExceptionNode e = t[i];

                ExceptionNode pred = null;

                while (e != null) {

                    ExceptionNode next = e.next;

                    if (e == x) {

                        if (pred == null)

                            t[i] = next;

                        else

                            pred.next = next;

                        break;

                    }

                    pred = e;

                    e = next;

                }

            }

        }

    }

    /**

     * If lock is available, poll stale refs and remove them.

     * Called from ForkJoinPool when pools become quiescent.

     */

    static final void helpExpungeStaleExceptions() {

        final ReentrantLock lock = exceptionTableLock;

        if (lock.tryLock()) {

            try {

                expungeStaleExceptions();

            } finally {

                lock.unlock();

            }

        }

    }

    /**

     * Report the result of invoke or join; called only upon

     * non-normal return of internal versions.

     */

    private V reportResult() {

        int s; Throwable ex;

        if ((s = status) == CANCELLED)

            throw new CancellationException();

        if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)

            UNSAFE.throwException(ex);

        return getRawResult();

    }

    // public methods

    /**

     * Arranges to asynchronously execute this task.  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}.

     *

     * <p>This method may be invoked only from within {@code

     * ForkJoinPool} computations (as may be determined using method

     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts

     * result in exceptions or errors, possibly including {@code

     * ClassCastException}.

     *

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

     */

    public final ForkJoinTask<V> fork() {

        ((ForkJoinWorkerThread) Thread.currentThread())

            .pushTask(this);

        return this;

    }

    /**

     * Returns the result of the computation when it {@link #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() {

        if (doJoin() != NORMAL)

            return reportResult();

        else

            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() {

        if (doInvoke() != NORMAL)

            return reportResult();

        else

            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.

     *

     * <p>This method may be invoked only from within {@code

     * ForkJoinPool} computations (as may be determined using method

     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts

     * result in exceptions or errors, possibly including {@code

     * ClassCastException}.

     *

     * @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) {

        t2.fork();

        t1.invoke();

        t2.join();

    }

    /**

     * 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.

     *

     * <p>This method may be invoked only from within {@code

     * ForkJoinPool} computations (as may be determined using method

     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts

     * result in exceptions or errors, possibly including {@code

     * ClassCastException}.

     *

     * @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 = tasks[i];

            if (t == null) {

                if (ex == null)

                    ex = new NullPointerException();

            }

            else if (i != 0)

                t.fork();

            else if (t.doInvoke() < NORMAL && ex == null)

                ex = t.getException();

        }

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

            ForkJoinTask<?> t = tasks[i];

            if (t != null) {

                if (ex != null)

                    t.cancel(false);

                else if (t.doJoin() < NORMAL && ex == null)

                    ex = t.getException();

            }

        }

        if (ex != null)

            UNSAFE.throwException(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.

     *

     * <p>This method may be invoked only from within {@code

     * ForkJoinPool} computations (as may be determined using method

     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts

     * result in exceptions or errors, possibly including {@code

     * ClassCastException}.

     *

     * @param tasks the collection of 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<?>[tasks.size()]));

            return tasks;

        }

        @SuppressWarnings("unchecked")

        List<? extends ForkJoinTask<?>> ts =

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

        Throwable ex = null;

        int last = ts.size() - 1;

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

            ForkJoinTask<?> t = ts.get(i);

            if (t == null) {

                if (ex == null)

                    ex = new NullPointerException();

            }

            else if (i != 0)

                t.fork();

            else if (t.doInvoke() < NORMAL && ex == null)

                ex = t.getException();

        }

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

            ForkJoinTask<?> t = ts.get(i);

            if (t != null) {

                if (ex != null)

                    t.cancel(false);

                else if (t.doJoin() < NORMAL && ex == null)

                    ex = t.getException();

            }

        }

        if (ex != null)

            UNSAFE.throwException(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}.

     *

     * @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 setCompletion(CANCELLED) == CANCELLED;

    }

    /**

     * Cancels, ignoring any exceptions thrown by cancel. Used during

     * worker and pool shutdown. Cancel is spec'ed not to throw any

     * exceptions, but if it does anyway, we have no recourse during

     * shutdown, so guard against this case.

     */

    final void cancelIgnoringExceptions() {

        try {

            cancel(false);

        } catch (Throwable ignore) {

        }

    }

    public final boolean isDone() {

        return status < 0;

    }

    public final boolean isCancelled() {

        return status == CANCELLED;

    }

    /**

     * 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 < NORMAL;

    }

    /**

     * 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 == NORMAL;

    }

    /**

     * 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() {

        int s = status;

        return ((s >= NORMAL)    ? null :

                (s == CANCELLED) ? new CancellationException() :

                getThrowableException());

    }

    /**

     * 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) {

        setExceptionalCompletion((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) {

            setExceptionalCompletion(rex);

            return;

        }

        setCompletion(NORMAL);

    }

    /**

     * 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 = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?

            doJoin() : externalInterruptibleAwaitDone(0L);

        Throwable ex;

        if (s == CANCELLED)

            throw new CancellationException();

        if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)

            throw new ExecutionException(ex);

        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 {

        Thread t = Thread.currentThread();

        if (t instanceof ForkJoinWorkerThread) {

            ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;

            long nanos = unit.toNanos(timeout);

            if (status >= 0) {

                boolean completed = false;

                if (w.unpushTask(this)) {

                    try {

                        completed = exec();

                    } catch (Throwable rex) {

                        setExceptionalCompletion(rex);

                    }

                }

                if (completed)

                    setCompletion(NORMAL);

                else if (status >= 0 && nanos > 0)

                    w.pool.timedAwaitJoin(this, nanos);

            }

        }

        else {

            long millis = unit.toMillis(timeout);

            if (millis > 0)

                externalInterruptibleAwaitDone(millis);

        }

        int s = status;

        if (s != NORMAL) {

            Throwable ex;

            if (s == CANCELLED)

                throw new CancellationException();

            if (s != EXCEPTIONAL)

                throw new TimeoutException();

            if ((ex = getThrowableException()) != null)

                throw new ExecutionException(ex);

        }

        return getRawResult();

    }