/*
    * Copyright 2013 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   http://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  
  /*
   * 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 io.netty.util.internal.chmv8;
  
  import java.io.Serializable;
  import java.lang.ref.ReferenceQueue;
  import java.lang.ref.WeakReference;
  import java.lang.reflect.Constructor;
  import java.util.Collection;
  import java.util.List;
  import java.util.RandomAccess;
  import java.util.concurrent.Callable;
  import java.util.concurrent.CancellationException;
  import java.util.concurrent.ExecutionException;
  import java.util.concurrent.Future;
  import java.util.concurrent.Phaser;
  import java.util.concurrent.RecursiveAction;
  import java.util.concurrent.RecursiveTask;
  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;
  
  /**
   * Abstract base class for tasks that run within a [email protected] ForkJoinPool}.
   * A [email protected] 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" [email protected] ForkJoinTask} begins execution when it is
   * explicitly submitted to a [email protected] ForkJoinPool}, or, if not already
   * engaged in a ForkJoin computation, commenced in the [email protected]
   * ForkJoinPool#commonPool()} via [email protected] #fork}, [email protected] #invoke}, or
   * related methods.  Once started, it will usually in turn start other
   * subtasks.  As indicated by the name of this class, many programs
   * using [email protected] ForkJoinTask} employ only methods [email protected] #fork} and
   * [email protected] #join}, or derivatives such as [email protected]
   * #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 [email protected] ForkJoinTask} is a lightweight form of [email protected] Future}.
   * The efficiency of [email protected] ForkJoinTask}s stems from a set of
   * restrictions (that are only partially statically enforceable)
   * reflecting their main use as computational tasks calculating pure
   * functions or operating on purely isolated objects.  The primary
   * coordination mechanisms are [email protected] #fork}, that arranges
   * asynchronous execution, and [email protected] #join}, that doesn't proceed
   * until the task's result has been computed.  Computations should
   * ideally avoid [email protected] synchronized} methods or blocks, and should
   * minimize other blocking synchronization apart from joining other
   * tasks or using synchronizers such as Phasers that are advertised to
   * cooperate with fork/join scheduling. Subdividable tasks should also
   * not perform blocking I/O, and should ideally access variables that
   * are completely independent of those accessed by other running
   * tasks. These guidelines are loosely enforced by not permitting
   * checked exceptions such as [email protected] 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 [email protected]
   * 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 [email protected] ex.printStackTrace()}) of both the thread
   * that initiated the computation as well as the thread actually
   * encountering the exception; minimally only the latter.
   *
   * <p>It is possible to define and use ForkJoinTasks that may block,
   * but doing do requires three further considerations: (1) Completion
   * of few if any <em>other</em> tasks should be dependent on a task
   * that blocks on external synchronization or I/O. Event-style async
   * tasks that are never joined (for example, those subclassing [email protected]
   * CountedCompleter}) often fall into this category.  (2) To minimize
   * resource impact, tasks should be small; ideally performing only the
   * (possibly) blocking action. (3) Unless the [email protected]
  * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly
  * blocked tasks is known to be less than the pool's [email protected]
  * ForkJoinPool#getParallelism} level, the pool cannot guarantee that
  * enough threads will be available to ensure progress or good
  * performance.
  *
  * <p>The primary method for awaiting completion and extracting
  * results of a task is [email protected] #join}, but there are several variants:
  * The [email protected] Future#get} methods support interruptible and/or timed
  * waits for completion and report results using [email protected] Future}
  * conventions. Method [email protected] #invoke} is semantically
  * equivalent to [email protected] 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 [email protected] invokeAll} (available in multiple versions)
  * performs the most common form of parallel invocation: forking a set
  * of tasks and joining them all.
  *
  * <p>In the most typical usages, a fork-join pair act like a call
  * (fork) and return (join) from a parallel recursive function. As is
  * the case with other forms of recursive calls, returns (joins)
  * should be performed innermost-first. For example, [email protected] a.fork();
  * b.fork(); b.join(); a.join();} is likely to be substantially more
  * efficient than joining [email protected] a} before [email protected] b}.
  *
  * <p>The execution status of tasks may be queried at several levels
  * of detail: [email protected] #isDone} is true if a task completed in any way
  * (including the case where a task was cancelled without executing);
  * [email protected] #isCompletedNormally} is true if a task completed without
  * cancellation or encountering an exception; [email protected] #isCancelled} is
  * true if the task was cancelled (in which case [email protected] #getException}
  * returns a [email protected] java.util.concurrent.CancellationException}); and
  * [email protected] #isCompletedAbnormally} is true if a task was either
  * cancelled or encountered an exception, in which case [email protected]
  * #getException} will return either the encountered exception or
  * [email protected] 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 [email protected]
  * RecursiveAction} for most computations that do not return results,
  * [email protected] RecursiveTask} for those that do, and [email protected]
  * CountedCompleter} for those in which completed actions trigger
  * other actions.  Normally, a concrete ForkJoinTask subclass declares
  * fields comprising its parameters, established in a constructor, and
  * then defines a [email protected] compute} method that somehow uses the control
  * methods supplied by this base class.
  *
  * <p>Method [email protected] #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
  * [email protected] Phaser}, [email protected] #helpQuiesce}, and [email protected] #complete}) that
  * may be of use in constructing custom subclasses for problems that
  * are not statically structured as DAGs. To support such usages, a
  * ForkJoinTask may be atomically <em>tagged</em> with a [email protected] short}
  * value using [email protected] #setForkJoinTaskTag} or [email protected]
  * #compareAndSetForkJoinTaskTag} and checked using [email protected]
  * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use
  * these [email protected] protected} methods or tags for any purpose, but they
  * may be of use in the construction of specialized subclasses.  For
  * example, parallel graph traversals can use the supplied methods to
  * avoid revisiting nodes/tasks that have already been processed.
  * (Method names for tagging are bulky in part to encourage definition
  * of methods that reflect their usage patterns.)
  *
  * <p>Most base support methods are [email protected] 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 [email protected] protected} methods [email protected] #exec}, [email protected]
  * #setRawResult}, and [email protected] #getRawResult}, while also introducing
  * an abstract computational method that can be implemented in its
  * subclasses, possibly relying on other [email protected] 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 [email protected] adapt} methods for [email protected] Runnable}
  * and [email protected] Callable}, that may be of use when mixing execution of
  * [email protected] 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 [email protected] 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
  */
 @SuppressWarnings("all")
 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 (anded with
      * DONE_MASK) 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, mainly by arranging that every synchronized
      * block performs a wait, notifyAll or both.
      *
      * These control bits occupy only (some of) the upper half (16
      * bits) of status field. The lower bits are used for user-defined
      * tags.
      */
 
     /** The run status of this task */
     volatile int status; // accessed directly by pool and workers
     static final int DONE_MASK   = 0xf0000000;  // mask out non-completion bits
     static final int NORMAL      = 0xf0000000;  // must be negative
     static final int CANCELLED   = 0xc0000000;  // must be < NORMAL
     static final int EXCEPTIONAL = 0x80000000;  // must be < CANCELLED
     static final int SIGNAL      = 0x00010000;  // must be >= 1 << 16
     static final int SMASK       = 0x0000ffff;  // short bits for tags
 
     /**
      * Marks completion and wakes up threads waiting to join this
      * task.
      *
      * @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 (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
                 if ((s >>> 16) != 0)
                     synchronized (this) { notifyAll(); }
                 return completion;
             }
         }
     }
 
     /**
      * Primary execution method for stolen tasks. Unless done, calls
      * exec and records status if completed, but doesn't wait for
      * completion otherwise.
      *
      * @return status on exit from this method
      */
     final int doExec() {
         int s; boolean completed;
         if ((s = status) >= 0) {
             try {
                 completed = exec();
             } catch (Throwable rex) {
                 return setExceptionalCompletion(rex);
             }
             if (completed)
                 s = setCompletion(NORMAL);
         }
         return s;
     }
 
     /**
      * Tries to set SIGNAL status unless already completed. Used by
      * ForkJoinPool. Other variants are directly incorporated into
      * externalAwaitDone etc.
      *
      * @return true if successful
      */
     final boolean trySetSignal() {
         int s = status;
         return s >= 0 && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL);
     }
 
     /**
      * Blocks a non-worker-thread until completion.
      * @return status upon completion
      */
     private int externalAwaitDone() {
         int s;
         ForkJoinPool cp = ForkJoinPool.common;
         if ((s = status) >= 0) {
             if (cp != null) {
                 if (this instanceof CountedCompleter)
                     s = cp.externalHelpComplete((CountedCompleter<?>)this);
                 else if (cp.tryExternalUnpush(this))
                     s = doExec();
             }
             if (s >= 0 && (s = status) >= 0) {
                 boolean interrupted = false;
                 do {
                     if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                         synchronized (this) {
                             if (status >= 0) {
                                 try {
                                     wait();
                                 } catch (InterruptedException ie) {
                                     interrupted = true;
                                 }
                             }
                             else
                                 notifyAll();
                         }
                     }
                 } while ((s = status) >= 0);
                 if (interrupted)
                     Thread.currentThread().interrupt();
             }
         }
         return s;
     }
 
     /**
      * Blocks a non-worker-thread until completion or interruption.
      */
     private int externalInterruptibleAwaitDone() throws InterruptedException {
         int s;
         ForkJoinPool cp = ForkJoinPool.common;
         if (Thread.interrupted())
             throw new InterruptedException();
         if ((s = status) >= 0 && cp != null) {
             if (this instanceof CountedCompleter)
                 cp.externalHelpComplete((CountedCompleter<?>)this);
             else if (cp.tryExternalUnpush(this))
                 doExec();
         }
         while ((s = status) >= 0) {
             if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                 synchronized (this) {
                     if (status >= 0)
                         wait();
                     else
                         notifyAll();
                 }
             }
         }
         return s;
     }
 
 
     /**
      * Implementation for join, get, quietlyJoin. Directly handles
      * only cases of already-completed, external wait, and
      * unfork+exec.  Others are relayed to ForkJoinPool.awaitJoin.
      *
      * @return status upon completion
      */
     private int doJoin() {
         int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
         return (s = status) < 0 ? s :
                 ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                         (w = (wt = (ForkJoinWorkerThread)t).workQueue).
                                 tryUnpush(this) && (s = doExec()) < 0 ? s :
                                 wt.pool.awaitJoin(w, this) :
                         externalAwaitDone();
     }
 
     /**
      * Implementation for invoke, quietlyInvoke.
      *
      * @return status upon completion
      */
     private int doInvoke() {
         int s; Thread t; ForkJoinWorkerThread wt;
         return (s = doExec()) < 0 ? s :
                 ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                         (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this) :
                         externalAwaitDone();
     }
 
     // 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 status.
      *
      * @return status on exit
      */
     final int recordExceptionalCompletion(Throwable ex) {
         int s;
         if ((s = status) >= 0) {
             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();
             }
             s = setCompletion(EXCEPTIONAL);
         }
         return s;
     }
 
     /**
      * Records exception and possibly propagates.
      *
      * @return status on exit
      */
     private int setExceptionalCompletion(Throwable ex) {
         int s = recordExceptionalCompletion(ex);
         if ((s & DONE_MASK) == EXCEPTIONAL)
             internalPropagateException(ex);
         return s;
     }
 
     /**
      * Hook for exception propagation support for tasks with completers.
      */
     void internalPropagateException(Throwable ex) {
     }
 
     /**
      * 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.
      */
     static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
         if (t != null && t.status >= 0) {
             try {
                 t.cancel(false);
             } catch (Throwable ignore) {
             }
         }
     }
 
     /**
      * 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 & DONE_MASK) != 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 (false && e.thrower != Thread.currentThread().getId()) {
             Class<? extends Throwable> 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();
             }
         }
     }
 
     /**
      * A version of "sneaky throw" to relay exceptions
      */
     static void rethrow(Throwable ex) {
         if (ex != null)
             ForkJoinTask.<RuntimeException>uncheckedThrow(ex);
     }
 
     /**
      * The sneaky part of sneaky throw, relying on generics
      * limitations to evade compiler complaints about rethrowing
      * unchecked exceptions
      */
     @SuppressWarnings("unchecked") static <T extends Throwable>
     void uncheckedThrow(Throwable t) throws T {
         throw (T)t; // rely on vacuous cast
     }
 
     /**
      * Throws exception, if any, associated with the given status.
      */
     private void reportException(int s) {
         if (s == CANCELLED)
             throw new CancellationException();
         if (s == EXCEPTIONAL)
             rethrow(getThrowableException());
     }
 
     // public methods
 
     /**
      * Arranges to asynchronously execute this task in the pool the
      * current task is running in, if applicable, or using the [email protected]
      * ForkJoinPool#commonPool()} if not [email protected] #inForkJoinPool}.  While
      * it is not necessarily enforced, it is a usage error to fork a
      * task more than once unless it has completed and been
      * reinitialized.  Subsequent modifications to the state of this
      * task or any data it operates on are not necessarily
      * consistently observable by any thread other than the one
      * executing it unless preceded by a call to [email protected] #join} or
      * related methods, or a call to [email protected] #isDone} returning [email protected]
      * true}.
      *
      * @return [email protected] this}, to simplify usage
      */
     public final ForkJoinTask<V> fork() {
         Thread t;
         if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
             ((ForkJoinWorkerThread)t).workQueue.push(this);
         else
             ForkJoinPool.common.externalPush(this);
         return this;
     }
 
     /**
      * Returns the result of the computation when it [email protected] #isDone is
      * done}.  This method differs from [email protected] #get()} in that
      * abnormal completion results in [email protected] RuntimeException} or
      * [email protected] Error}, not [email protected] ExecutionException}, and that
      * interrupts of the calling thread do <em>not</em> cause the
      * method to abruptly return by throwing [email protected]
      * InterruptedException}.
      *
      * @return the computed result
      */
     public final V join() {
         int s;
         if ((s = doJoin() & DONE_MASK) != NORMAL)
             reportException(s);
         return getRawResult();
     }
 
     /**
      * Commences performing this task, awaits its completion if
      * necessary, and returns its result, or throws an (unchecked)
      * [email protected] RuntimeException} or [email protected] Error} if the underlying
      * computation did so.
      *
      * @return the computed result
      */
     public final V invoke() {
         int s;
         if ((s = doInvoke() & DONE_MASK) != NORMAL)
             reportException(s);
         return getRawResult();
     }
 
     /**
      * Forks the given tasks, returning when [email protected] 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 [email protected]
      * #getException()} and related methods to check if they have been
      * cancelled, completed normally or exceptionally, or left
      * unprocessed.
      *
      * @param t1 the first task
      * @param t2 the second task
      * @throws NullPointerException if any task is null
      */
     public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
         int s1, s2;
         t2.fork();
         if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL)
             t1.reportException(s1);
         if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL)
             t2.reportException(s2);
     }
 
     /**
      * Forks the given tasks, returning when [email protected] 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 [email protected] #getException()} and
      * related methods to check if they have been cancelled, completed
      * normally or exceptionally, or left unprocessed.
      *
      * @param tasks the tasks
      * @throws NullPointerException if any task is null
      */
     public static void invokeAll(ForkJoinTask<?>... tasks) {
         Throwable ex = null;
         int last = tasks.length - 1;
         for (int i = last; i >= 0; --i) {
             ForkJoinTask<?> t = 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 = t.getException();
             }
         }
         if (ex != null)
             rethrow(ex);
     }
 
     /**
      * Forks all tasks in the specified collection, returning when
      * [email protected] 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 [email protected]
      * #getException()} and related methods to check if they have been
      * cancelled, completed normally or exceptionally, or left
      * unprocessed.
      *
      * @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 = t.getException();
             }
         }
         if (ex != null)
             rethrow(ex);
         return tasks;
     }
 
     /**
      * Attempts to cancel execution of this task. This attempt will
      * fail if the task has already completed or could not be
      * cancelled for some other reason. If successful, and this task
      * has not started when [email protected] cancel} is called, execution of
      * this task is suppressed. After this method returns
      * successfully, unless there is an intervening call to [email protected]
      * #reinitialize}, subsequent calls to [email protected] #isCancelled},
      * [email protected] #isDone}, and [email protected] cancel} will return [email protected] true}
      * and calls to [email protected] #join} and related methods will result in
      * [email protected] CancellationException}.
      *
      * <p>This method may be overridden in subclasses, but if so, must
      * still ensure that these properties hold. In particular, the
      * [email protected] 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 [email protected] #completeExceptionally(Throwable)}.
      *
      * @param mayInterruptIfRunning this value has no effect in the
      * default implementation because interrupts are not used to
      * control cancellation.
      *
      * @return [email protected] true} if this task is now cancelled
      */
     public boolean cancel(boolean mayInterruptIfRunning) {
         return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
     }
 
     public final boolean isDone() {
         return status < 0;
     }
 
     public final boolean isCancelled() {
         return (status & DONE_MASK) == CANCELLED;
     }
 
     /**
      * Returns [email protected] true} if this task threw an exception or was cancelled.
      *
      * @return [email protected] true} if this task threw an exception or was cancelled
      */
     public final boolean isCompletedAbnormally() {
         return status < NORMAL;
     }
 
     /**
      * Returns [email protected] true} if this task completed without throwing an
      * exception and was not cancelled.
      *
      * @return [email protected] true} if this task completed without throwing an
      * exception and was not cancelled
      */
     public final boolean isCompletedNormally() {
         return (status & DONE_MASK) == NORMAL;
     }
 
     /**
      * Returns the exception thrown by the base computation, or a
      * [email protected] 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 & DONE_MASK;
         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);
     }
 
     /**
      * Completes this task normally without setting a value. The most
      * recent value established by {@link #setRawResult} (or {@code
      * null} by default) will be returned as the result of subsequent
      * invocations of {@code join} and related operations.
      *
      * @since 1.8
      */
     public final void quietlyComplete() {
         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();
         Throwable ex;
         if ((s &= DONE_MASK) == 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 {
         if (Thread.interrupted())
             throw new InterruptedException();
         // Messy in part because we measure in nanosecs, but wait in millisecs
         int s; long ms;
         long ns = unit.toNanos(timeout);
         ForkJoinPool cp;
         if ((s = status) >= 0 && ns > 0L) {
             long deadline = System.nanoTime() + ns;
             ForkJoinPool p = null;
             ForkJoinPool.WorkQueue w = null;
             Thread t = Thread.currentThread();
             if (t instanceof ForkJoinWorkerThread) {
                 ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
                 p = wt.pool;
                 w = wt.workQueue;
                 p.helpJoinOnce(w, this); // no retries on failure
             }
             else if ((cp = ForkJoinPool.common) != null) {
                 if (this instanceof CountedCompleter)
                     cp.externalHelpComplete((CountedCompleter<?>)this);
                 else if (cp.tryExternalUnpush(this))
                     doExec();
             }
             boolean canBlock = false;
             boolean interrupted = false;
             try {
                 while ((s = status) >= 0) {
                     if (w != null && w.qlock < 0)
                         cancelIgnoringExceptions(this);
                     else if (!canBlock) {
                         if (p == null || p.tryCompensate(p.ctl))
                             canBlock = true;
                     }
                     else {
                         if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L &&
                                 U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                             synchronized (this) {
                                 if (status >= 0) {
                                     try {
                                         wait(ms);
                                     } catch (InterruptedException ie) {
                                         if (p == null)
                                             interrupted = true;
                                     }
                                 }
                                 else
                                     notifyAll();
                             }
                         }
                         if ((s = status) < 0 || interrupted ||
                                 (ns = deadline - System.nanoTime()) <= 0L)
                             break;
                     }
                 }
             } finally {
                 if (p != null && canBlock)
                     p.incrementActiveCount();
             }
             if (interrupted)
                 throw new InterruptedException();
         }
         if ((s &= DONE_MASK) != 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.
      */
     public static void helpQuiesce() {
         Thread t;
         if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
             ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
             wt.pool.helpQuiescePool(wt.workQueue);
         }
         else
             ForkJoinPool.quiesceCommonPool();
     }
 
     /**
      * 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 & DONE_MASK) == 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 (but is not guaranteed to) succeed if this task is
      * the most recently forked task by the current thread, and has
      * not commenced executing in another thread.  This method may be
      * useful when arranging alternative local processing of tasks
      * that could have been, but were not, stolen.
      *
      * @return {@code true} if unforked
      */
     public boolean tryUnfork() {
         Thread t;
         return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                 ((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) :
                 ForkJoinPool.common.tryExternalUnpush(this));
     }
 
     /**
      * Returns an estimate of the number of tasks that have been
      * forked by the current worker thread but not yet executed. This
      * value may be useful for heuristic decisions about whether to
      * fork other tasks.
      *
      * @return the number of tasks
      */
     public static int getQueuedTaskCount() {
         Thread t; ForkJoinPool.WorkQueue q;
         if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
             q = ((ForkJoinWorkerThread)t).workQueue;
         else
             q = ForkJoinPool.commonSubmitterQueue();
         return (q == null) ? 0 : q.queueSize();
     }
 
     /**
      * Returns an estimate of how many more locally queued tasks are
      * held by the current worker thread than there are other worker
      * threads that might steal them, or zero if this thread is not
      * operating in a ForkJoinPool. This value may be useful for
      * heuristic decisions about whether to fork other tasks. In many
      * usages of ForkJoinTasks, at steady state, each worker should
      * aim to maintain a small constant surplus (for example, 3) of
      * tasks, and to process computations locally if this threshold is
      * exceeded.
      *
      * @return the surplus number of tasks, which may be negative
      */
     public static int getSurplusQueuedTaskCount() {
         return ForkJoinPool.getSurplusQueuedTaskCount();
     }
 
     // Extension methods
 
     /**
      * Returns the result that would be returned by {@link #join}, even
      * if this task completed abnormally, or {@code null} if this task
      * is not known to have been completed.  This method is designed
      * to aid debugging, as well as to support extensions. Its use in
      * any other context is discouraged.
      *
      * @return the result, or {@code null} if not completed
      */
     public abstract V getRawResult();
 
     /**
      * Forces the given value to be returned as a result.  This method
      * is designed to support extensions, and should not in general be
      * called otherwise.
      *
      * @param value the value
      */
     protected abstract void setRawResult(V value);
 
     /**
      * Immediately performs the base action of this task and returns
      * true if, upon return from this method, this task is guaranteed
      * to have completed normally. This method may return false
      * otherwise, to indicate that this task is not necessarily
      * complete (or is not known to be complete), for example in
      * asynchronous actions that require explicit invocations of
      * completion methods. This method may also throw an (unchecked)
      * exception to indicate abnormal exit. This method is designed to
      * support extensions, and should not in general be called
      * otherwise.
      *
      * @return {@code true} if this task is known to have completed normally
      */
     protected abstract boolean exec();
 
     /**
      * Returns, but does not unschedule or execute, a task queued by
      * the current thread but not yet executed, if one is immediately
      * available. There is no guarantee that this task will actually
      * be polled or executed next. Conversely, this method may return
      * null even if a task exists but cannot be accessed without
      * contention with other threads.  This method is designed
      * primarily to support extensions, and is unlikely to be useful
      * otherwise.
      *
      * @return the next task, or {@code null} if none are available
      */
     protected static ForkJoinTask<?> peekNextLocalTask() {
         Thread t; ForkJoinPool.WorkQueue q;
         if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
             q = ((ForkJoinWorkerThread)t).workQueue;
         else
             q = ForkJoinPool.commonSubmitterQueue();
         return (q == null) ? null : q.peek();
     }
 
     /**
      * Unschedules and returns, without executing, the next task
      * queued by the current thread but not yet executed, if the
      * current thread is operating in a ForkJoinPool.  This method is
      * designed primarily to support extensions, and is unlikely to be
      * useful otherwise.
      *
      * @return the next task, or {@code null} if none are available
      */
     protected static ForkJoinTask<?> pollNextLocalTask() {
         Thread t;
         return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                 ((ForkJoinWorkerThread)t).workQueue.nextLocalTask() :
                 null;
     }
 
     /**
      * If the current thread is operating in a ForkJoinPool,
      * unschedules and returns, without executing, the next task
      * 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.
      *
      * @return a task, or {@code null} if none are available
      */
     protected static ForkJoinTask<?> pollTask() {
         Thread t; ForkJoinWorkerThread wt;
         return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                 (wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) :
                 null;
     }
 
     // tag operations
 
     /**
      * Returns the tag for this task.
      *
      * @return the tag for this task
      * @since 1.8
      */
     public final short getForkJoinTaskTag() {
         return (short)status;
     }
 
     /**
      * Atomically sets the tag value for this task.
      *
      * @param tag the tag value
      * @return the previous value of the tag
      * @since 1.8
      */
     public final short setForkJoinTaskTag(short tag) {
         for (int s;;) {
             if (U.compareAndSwapInt(this, STATUS, s = status,
                     (s & ~SMASK) | (tag & SMASK)))
                 return (short)s;
         }
     }
 
     /**
      * Atomically conditionally sets the tag value for this task.
      * Among other applications, tags can be used as visit markers
      * in tasks operating on graphs, as in methods that check: {@code
      * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))}
      * before processing, otherwise exiting because the node has
      * already been visited.
      *
      * @param e the expected tag value
      * @param tag the new tag value
      * @return {@code true} if successful; i.e., the current value was
      * equal to e and is now tag.
      * @since 1.8
      */
     public final boolean compareAndSetForkJoinTaskTag(short e, short tag) {
         for (int s;;) {
             if ((short)(s = status) != e)
                 return false;
             if (U.compareAndSwapInt(this, STATUS, s,
                     (s & ~SMASK) | (tag & SMASK)))
                 return true;
         }
     }
 
     /**
      * 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;
         T result;
         AdaptedRunnable(Runnable runnable, T result) {
             if (runnable == null) throw new NullPointerException();
             this.runnable = runnable;
             this.result = result; // OK to set this even before completion
         }
         public final T getRawResult() { return result; }
         public final void setRawResult(T v) { result = v; }
         public final boolean exec() { runnable.run(); return true; }
         public final void run() { invoke(); }
         private static final long serialVersionUID = 5232453952276885070L;
     }
 
     /**
      * Adaptor for Runnables without results
      */
     static final class AdaptedRunnableAction extends ForkJoinTask<Void>
             implements RunnableFuture<Void> {
         final Runnable runnable;
         AdaptedRunnableAction(Runnable runnable) {
             if (runnable == null) throw new NullPointerException();
             this.runnable = runnable;
         }
         public final Void getRawResult() { return null; }
         public final void setRawResult(Void v) { }
         public final boolean exec() { runnable.run(); return true; }
         public final void run() { invoke(); }
         private static final long serialVersionUID = 5232453952276885070L;
     }
 
     /**
      * Adaptor for Runnables in which failure forces worker exception
      */
     static final class RunnableExecuteAction extends ForkJoinTask<Void> {
         final Runnable runnable;
         RunnableExecuteAction(Runnable runnable) {
             if (runnable == null) throw new NullPointerException();
             this.runnable = runnable;
         }
         public final Void getRawResult() { return null; }
         public final void setRawResult(Void v) { }
         public final boolean exec() { runnable.run(); return true; }
         void internalPropagateException(Throwable ex) {
             rethrow(ex); // rethrow outside exec() catches.
         }
         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 final T getRawResult() { return result; }
         public final void setRawResult(T v) { result = v; }
         public final 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 final 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 AdaptedRunnableAction(runnable);
     }
 
     /**
      * 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 this task to a stream (that is, serializes it).
      *
      * @serialData the current run status and the exception thrown
      * during execution, or {@code null} if none
      */
     private void writeObject(java.io.ObjectOutputStream s)
             throws java.io.IOException {
         s.defaultWriteObject();
         s.writeObject(getException());
     }
 
     /**
      * Reconstitutes this task from a stream (that is, deserializes it).
      */
     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 U;
     private static final long STATUS;
 
     static {
         exceptionTableLock = new ReentrantLock();
         exceptionTableRefQueue = new ReferenceQueue<Object>();
         exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
         try {
             U = getUnsafe();
             Class<?> k = ForkJoinTask.class;
             STATUS = U.objectFieldOffset
                     (k.getDeclaredField("status"));
         } catch (Exception e) {
             throw new Error(e);
         }
     }
 
     /**
      * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
      * Replace with a simple call to Unsafe.getUnsafe when integrating
      * into a jdk.
      *
      * @return a sun.misc.Unsafe
      */
     private static sun.misc.Unsafe getUnsafe() {
         try {
             return sun.misc.Unsafe.getUnsafe();
         } catch (SecurityException tryReflectionInstead) {}
         try {
             return java.security.AccessController.doPrivileged
                     (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
                         public sun.misc.Unsafe run() throws Exception {
                             Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
                             for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                                 f.setAccessible(true);
                                 Object x = f.get(null);
                                 if (k.isInstance(x))
                                     return k.cast(x);
                             }
                             throw new NoSuchFieldError("the Unsafe");
                         }});
         } catch (java.security.PrivilegedActionException e) {
             throw new RuntimeException("Could not initialize intrinsics",
                     e.getCause());
         }
     }
 }