/*

  * 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.Collections;

 import java.util.List;

 import java.util.RandomAccess;

 import java.util.Map;

 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.Executor;

 import java.util.concurrent.ExecutorService;

 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();

     }

     /**

      * 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() {

         doJoin();

     }

     /**

      * Commences performing this task and awaits its completion if

      * necessary, without returning its result or throwing its

      * exception.

      */

     public final void quietlyInvoke() {

         doInvoke();

     }

     /**

      * Possibly executes tasks until the pool hosting the current task

      * {@link 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.

      *

      * <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}.

      */

     public static void helpQuiesce() {

         ((ForkJoinWorkerThread) Thread.currentThread())

             .helpQuiescePool();

     }

     /**

      * 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() {

         if (status == EXCEPTIONAL)

             clearExceptionalCompletion();

         else

             status = 0;

     }

     /**

      * Returns the pool hosting the current task execution, or null

      * if this task is executing outside of any ForkJoinPool.

      *

      * @see #inForkJoinPool

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

      */

     public static ForkJoinPool getPool() {

         Thread t = Thread.currentThread();

         return (t 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 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.

      *

      * <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 true} if unforked

      */

     public boolean tryUnfork() {

         return ((ForkJoinWorkerThread) Thread.currentThread())

             .unpushTask(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.

      *

      * <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 the number of tasks

      */

     public static int getQueuedTaskCount() {

         return ((ForkJoinWorkerThread) Thread.currentThread())

             .getQueueSize();

     }

     /**

      * 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.  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.

      *

      * <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 the surplus number of tasks, which may be negative

      */

     public static int getSurplusQueuedTaskCount() {

         return ((ForkJoinWorkerThread) Thread.currentThread())

             .getEstimatedSurplusTaskCount();

     }

     // 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.  This method

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

      * called otherwise. The return value controls whether this task

      * is considered to be done normally. It may return false in

      * asynchronous actions that require explicit invocations of

      * {@link #complete} to become joinable. It may also throw an

      * (unchecked) exception to indicate abnormal exit.

      *

      * @return {@code true} if 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.

      *

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

      */

     protected static ForkJoinTask<?> peekNextLocalTask() {

         return ((ForkJoinWorkerThread) Thread.currentThread())

             .peekTask();

     }

     /**

      * Unschedules and returns, without executing, the next task

      * queued by the current thread but not yet executed.  This method

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

      * be useful otherwise.

      *

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

      */

     protected static ForkJoinTask<?> pollNextLocalTask() {

         return ((ForkJoinWorkerThread) Thread.currentThread())

             .pollLocalTask();

     }

     /**

      * Unschedules and returns, without executing, the next task

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

      * available, or if not available, a task that was forked by some

      * other thread, if available. Availability may be transient, so a

      * {@code null} result does not necessarily imply quiescence

      * of the pool this task is operating in.  This method is designed

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

      * otherwise.

      *

      * <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 a task, or {@code null} if none are available

      */

     protected static ForkJoinTask<?> pollTask() {

         return ((ForkJoinWorkerThread) Thread.currentThread())

             .pollTask();

     }

     /**

      * Adaptor for Runnables. This implements RunnableFuture

      * to be compliant with AbstractExecutorService constraints

      * when used in ForkJoinPool.

      */

     static final class AdaptedRunnable<T> extends ForkJoinTask<T>

         implements RunnableFuture<T> {

         final Runnable runnable;

         final T resultOnCompletion;

         T result;

         AdaptedRunnable(Runnable runnable, T result) {

             if (runnable == null) throw new NullPointerException();

             this.runnable = runnable;

             this.resultOnCompletion = result;

         }

         public T getRawResult() { return result; }

         public void setRawResult(T v) { result = v; }

         public boolean exec() {

             runnable.run();

             result = resultOnCompletion;

             return true;

         }

         public void run() { invoke(); }

         private static final long serialVersionUID = 5232453952276885070L;

     }

     /**

      * Adaptor for Callables

      */

     static final class AdaptedCallable<T> extends ForkJoinTask<T>

         implements RunnableFuture<T> {

         final Callable<? extends T> callable;

         T result;

         AdaptedCallable(Callable<? extends T> callable) {

             if (callable == null) throw new NullPointerException();

             this.callable = callable;

         }

         public T getRawResult() { return result; }

         public void setRawResult(T v) { result = v; }

         public boolean exec() {

             try {

                 result = callable.call();

                 return true;

             } catch (Error err) {

                 throw err;

             } catch (RuntimeException rex) {

                 throw rex;

             } catch (Exception ex) {

                 throw new RuntimeException(ex);

             }

         }

         public void run() { invoke(); }

         private static final long serialVersionUID = 2838392045355241008L;

     }

     /**

      * Returns a new {@code ForkJoinTask} that performs the {@code run}

      * method of the given {@code Runnable} as its action, and returns

      * a null result upon {@link #join}.

      *

      * @param runnable the runnable action

      * @return the task

      */

     public static ForkJoinTask<?> adapt(Runnable runnable) {

         return new AdaptedRunnable<Void>(runnable, null);

     }

     /**

      * Returns a new {@code ForkJoinTask} that performs the {@code run}

      * method of the given {@code Runnable} as its action, and returns

      * the given result upon {@link #join}.

      *

      * @param runnable the runnable action

      * @param result the result upon completion

      * @return the task

      */

     public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {

         return new AdaptedRunnable<T>(runnable, result);

     }

     /**

      * Returns a new {@code ForkJoinTask} that performs the {@code call}

      * method of the given {@code Callable} as its action, and returns

      * its result upon {@link #join}, translating any checked exceptions

      * encountered into {@code RuntimeException}.

      *

      * @param callable the callable action

      * @return the task

      */

     public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {

         return new AdaptedCallable<T>(callable);

     }

     // Serialization support

     private static final long serialVersionUID = -7721805057305804111L;

     /**

      * Saves the state to a stream (that is, serializes it).

      *

      * @serialData the current run status and the exception thrown

      * during execution, or {@code null} if none

      * @param s the stream

      */

     private void writeObject(java.io.ObjectOutputStream s)

         throws java.io.IOException {

         s.defaultWriteObject();

         s.writeObject(getException());

     }

     /**

      * Reconstitutes the instance from a stream (that is, deserializes it).

      *

      * @param s the stream

      */

     private void readObject(java.io.ObjectInputStream s)

         throws java.io.IOException, ClassNotFoundException {

         s.defaultReadObject();

         Object ex = s.readObject();

         if (ex != null)

             setExceptionalCompletion((Throwable)ex);

     }

     // Unsafe mechanics

     private static final sun.misc.Unsafe UNSAFE;

     private static final long statusOffset;

     static {

         exceptionTableLock = new ReentrantLock();

         exceptionTableRefQueue = new ReferenceQueue<Object>();

         exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];

         try {

             UNSAFE = sun.misc.Unsafe.getUnsafe();

             statusOffset = UNSAFE.objectFieldOffset

                 (ForkJoinTask.class.getDeclaredField("status"));

         } catch (Exception e) {

             throw new Error(e);

         }

     }

 }