/*

 * 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

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/*

 * 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/licenses/publicdomain

 */

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.util.WeakHashMap;

/**

 * 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.

 *

 * <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>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

 *

 * <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. In particular, most

     * join mechanics are in method quietlyJoin, below.

     */

    /*

     * 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;

    /**

     * 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.

     * TODO: Use ConcurrentReferenceHashMap

     */

    static final Map<ForkJoinTask<?>, Throwable> exceptionMap =

        Collections.synchronizedMap

        (new WeakHashMap<ForkJoinTask<?>, Throwable>());

    // Maintaining completion status

    /**

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

     * also clearing signal request bits.

     *

     * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL

     */

    private void setCompletion(int completion) {

        int s;

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

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

                if (s != 0)

                    synchronized (this) { notifyAll(); }

                break;

            }

        }

    }

    /**

     * Records exception and sets exceptional completion.

     *

     * @return status on exit

     */

    private void setExceptionalCompletion(Throwable rex) {

        exceptionMap.put(this, rex);

        setCompletion(EXCEPTIONAL);

    }

    /**

     */

                synchronized (this) {

                }

            } catch (InterruptedException ie) {

                cancelIfTerminating();

            }

        }

    }

    /**

     * Blocks a non-worker-thread until completion.

     */

    private void externalAwaitDone() {

            synchronized (this) {

                        try {

                            wait();

                        } catch (InterruptedException ie) {

                            interrupted = true;

                        }

                    }

                }

            }

        }

    }

    /**

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

     * doesn't wait for completion otherwise. Primary execution method

     * for ForkJoinWorkerThread.

     */

    final void quietlyExec() {

        try {

            if (status < 0 || !exec())

                return;

        } catch (Throwable rex) {

            setExceptionalCompletion(rex);

            return;

        }

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

    }

    // 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

     * {@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;

    }

    /**

     * abnormal completion results in {@code RuntimeException} or

     *

     * @return the computed result

     */

    public final V join() {

        quietlyJoin();

        Throwable ex;

        if (status < NORMAL && (ex = getException()) != null)

            UNSAFE.throwException(ex);

        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() {

        quietlyInvoke();

        Throwable ex;

        if (status < NORMAL && (ex = getException()) != null)

            UNSAFE.throwException(ex);

        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

     * {@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

     * {@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 {

                t.quietlyInvoke();

                if (ex == null && t.status < NORMAL)

                    ex = t.getException();

            }

        }

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

            ForkJoinTask<?> t = tasks[i];

            if (t != null) {

                if (ex != null)

                    t.cancel(false);

                else {

                    t.quietlyJoin();

                    if (ex == null && t.status < NORMAL)

                        ex = t.getException();

                }

            }

        }

        if (ex != null)

            UNSAFE.throwException(ex);

    }

    /**

     * Forks all tasks in the specified collection, returning when