src/jsr166y/CountedCompleter.java

/*
 * 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 jsr166y;

/**
 * A {@link ForkJoinTask} with a completion action
 * performed when triggered and there are no remaining pending
 * actions. Uses of CountedCompleter are similar to those of other
 * completion based components (such as {@link
 * java.nio.channels.CompletionHandler}) except that multiple
 * <em>pending</em> completions may be necessary to trigger the {@link
 * #onCompletion} action, not just one. Unless initialized otherwise,
 * the {@link #getPendingCount pending count} starts at zero, but may
 * be (atomically) changed using methods {@link #setPendingCount},
 * {@link #addToPendingCount}, and {@link
 * #compareAndSetPendingCount}. Upon invocation of {@link
 * #tryComplete}, if the pending action count is nonzero, it is
 * decremented; otherwise, the completion action is performed, and if
 * this completer itself has a completer, the process is continued
 * with its completer.  As is the case with related synchronization
 * components such as {@link java.util.concurrent.Phaser Phaser} and
 * {@link java.util.concurrent.Semaphore Semaphore}, these methods
 * affect only internal counts; they do not establish any further
 * internal bookkeeping. In particular, the identities of pending
 * tasks are not maintained. As illustrated below, you can create
 * subclasses that do record some or all pending tasks or their
 * results when needed.
 *
 * <p>A concrete CountedCompleter class must define method {@link
 * #compute}, that should, in almost all use cases, invoke {@code
 * tryComplete()} once before returning. The class may also optionally
 * override method {@link #onCompletion} to perform an action upon
 * normal completion, and method {@link #onExceptionalCompletion} to
 * perform an action upon any exception.
 *
 * <p>CountedCompleters most often do not bear results, in which case
 * they are normally declared as {@code CountedCompleter<Void>}, and
 * will always return {@code null} as a result value.  In other cases,
 * you should override method {@link #getRawResult} to provide a
 * result from {@code join(), invoke()}, and related methods. (Method
 * {@link #setRawResult} by default plays no role in CountedCompleters
 * but may be overridden for example to maintain fields holding result
 * data.)
 *
 * <p>A CountedCompleter that does not itself have a completer (i.e.,
 * one for which {@link #getCompleter} returns {@code null}) can be
 * used as a regular ForkJoinTask with this added functionality.
 * However, any completer that in turn has another completer serves
 * only as an internal helper for other computations, so its own task
 * status (as reported in methods such as {@link ForkJoinTask#isDone})
 * is arbitrary; this status changes only upon explicit invocations of
 * {@link #complete}, {@link ForkJoinTask#cancel}, {@link
 * ForkJoinTask#completeExceptionally} or upon exceptional completion
 * of method {@code compute}. Upon any exceptional completion, the
 * exception may be relayed to a task's completer (and its completer,
 * and so on), if one exists and it has not otherwise already
 * completed.
 *
 * <p><b>Sample Usages.</b>
 *
 * <p><b>Parallel recursive decomposition.</b> CountedCompleters may
 * be arranged in trees similar to those often used with {@link
 * RecursiveAction}s, although the constructions involved in setting
 * them up typically vary. Here, the completer of each task is its
 * parent in the computation tree. Even though they entail a bit more
 * bookkeeping, CountedCompleters may be better choices when applying
 * a possibly time-consuming operation (that cannot be further
 * subdivided) to each element of an array or collection; especially
 * when the operation takes a significantly different amount of time
 * to complete for some elements than others, either because of
 * intrinsic variation (for example IO) or auxiliary effects such as
 * garbage collection.  Because CountedCompleters provide their own
 * continuations, other threads need not block waiting to perform
 * them.
 *
 * <p> For example, here is an initial version of a class that uses
 * divide-by-two recursive decomposition to divide work into single
 * pieces (leaf tasks). Even when work is split into individual calls,
 * tree-based techniques are usually preferable to directly forking
 * leaf tasks, because they reduce inter-thread communication and
 * improve load balancing. In the recursive case, the second of each
 * pair of subtasks to finish triggers completion of its parent
 * (because no result combination is performed, the default no-op
 * implementation of method {@code onCompletion} is not overridden). A
 * static utility method sets up the base task and invokes it:
 *
 * <pre> {@code
 * class MyOperation<E> { void apply(E e) { ... }  }
 *
 * class ForEach<E> extends CountedCompleter<Void> {
 *
 *     public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) {
 *         pool.invoke(new ForEach<E>(null, array, op, 0, array.length));
 *     }
 *
 *     final E[] array; final MyOperation<E> op; final int lo, hi;
 *     ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
 *         super(p);
 *         this.array = array; this.op = op; this.lo = lo; this.hi = hi;
 *     }
 *
 *     public void compute() { // version 1
 *         if (hi - lo >= 2) {
 *             int mid = (lo + hi) >>> 1;
 *             setPendingCount(2); // must set pending count before fork
 *             new ForEach(this, array, op, mid, hi).fork(); // right child
 *             new ForEach(this, array, op, lo, mid).fork(); // left child
 *         }
 *         else if (hi > lo)
 *             op.apply(array[lo]);
 *         tryComplete();
 *     }
 * } }</pre>
 *
 * This design can be improved by noticing that in the recursive case,
 * the task has nothing to do after forking its right task, so can
 * directly invoke its left task before returning. (This is an analog
 * of tail recursion removal.)  Also, because the task returns upon
 * executing its left task (rather than falling through to invoke
 * tryComplete) the pending count is set to one:
 *
 * <pre> {@code
 * class ForEach<E> ...
 *     public void compute() { // version 2
 *         if (hi - lo >= 2) {
 *             int mid = (lo + hi) >>> 1;
 *             setPendingCount(1); // only one pending
 *             new ForEach(this, array, op, mid, hi).fork(); // right child
 *             new ForEach(this, array, op, lo, mid).compute(); // direct invoke
 *         }
 *         else {
 *             if (hi > lo)
 *                 op.apply(array[lo]);
 *             tryComplete();
 *         }
 *     }
 * }</pre>
 *
 * As a further improvement, notice that the left task need not even
 * exist.  Instead of creating a new one, we can iterate using the
 * original task, and add a pending count for each fork. Additionally,
 * this version uses {@code helpComplete} to streamline assistance in
 * the execution of forked tasks.
 *
 * <pre> {@code
 * class ForEach<E> ...
 *     public void compute() { // version 3
 *         int l = lo,  h = hi;
 *         while (h - l >= 2) {
 *             int mid = (l + h) >>> 1;
 *             addToPendingCount(1);
 *             new ForEach(this, array, op, mid, h).fork(); // right child
 *             h = mid;
 *         }
 *         if (h > l)
 *             op.apply(array[l]);
 *         helpComplete();
 *     }
 * }</pre>
 *
 * Additional improvements of such classes might entail precomputing
 * pending counts so that they can be established in constructors,
 * specializing classes for leaf steps, subdividing by say, four,
 * instead of two per iteration, and using an adaptive threshold
 * instead of always subdividing down to single elements.
 *
 * <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
 * results of multiple subtasks usually need to access these results
 * in method {@link #onCompletion}. As illustrated in the following
 * class (that performs a simplified form of map-reduce where mappings
 * and reductions are all of type {@code E}), one way to do this in
 * divide and conquer designs is to have each subtask record its
 * sibling, so that it can be accessed in method {@code onCompletion}.
 * This technique applies to reductions in which the order of
 * combining left and right results does not matter; ordered
 * reductions require explicit left/right designations.  Variants of
 * other streamlinings seen in the above examples may also apply.
 *
 * <pre> {@code
 * class MyMapper<E> { E apply(E v) {  ...  } }
 * class MyReducer<E> { E apply(E x, E y) {  ...  } }
 * class MapReducer<E> extends CountedCompleter<E> {
 *     final E[] array; final MyMapper<E> mapper;
 *     final MyReducer<E> reducer; final int lo, hi;
 *     MapReducer<E> sibling;
 *     E result;
 *     MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper,
 *                MyReducer<E> reducer, int lo, int hi) {
 *         super(p);
 *         this.array = array; this.mapper = mapper;
 *         this.reducer = reducer; this.lo = lo; this.hi = hi;
 *     }
 *     public void compute() {
 *         if (hi - lo >= 2) {
 *             int mid = (lo + hi) >>> 1;
 *             MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
 *             MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
 *             left.sibling = right;
 *             right.sibling = left;
 *             setPendingCount(1); // only right is pending
 *             right.fork();
 *             left.compute();     // directly execute left
 *         }
 *         else {
 *             if (hi > lo)
 *                 result = mapper.apply(array[lo]);
 *             tryComplete();
 *         }
 *     }
 *     public void onCompletion(CountedCompleter caller) {
 *         if (caller != this) {
 *            MapReducer<E> child = (MapReducer<E>)caller;
 *            MapReducer<E> sib = child.sibling;
 *            if (sib == null || sib.result == null)
 *                result = child.result;
 *            else
 *                result = reducer.apply(child.result, sib.result);
 *         }
 *     }
 *     public E getRawResult() { return result; }
 *
 *     public static <E> E mapReduce(ForkJoinPool pool, E[] array,
 *                                   MyMapper<E> mapper, MyReducer<E> reducer) {
 *         return pool.invoke(new MapReducer<E>(null, array, mapper,
 *                                              reducer, 0, array.length));
 *     }
 * } }</pre>
 *
 * <p><b>Triggers.</b> Some CountedCompleters are themselves never
 * forked, but instead serve as bits of plumbing in other designs;
 * including those in which the completion of one of more async tasks
 * triggers another async task. For example:
 *
 * <pre> {@code
 * class HeaderBuilder extends CountedCompleter<...> { ... }
 * class BodyBuilder extends CountedCompleter<...> { ... }
 * class PacketSender extends CountedCompleter<...> {
 *     PacketSender(...) { super(null, 1); ... } // trigger on second completion
 *     public void compute() { } // never called
 *     public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
 * }
 * // sample use:
 * PacketSender p = new PacketSender();
 * new HeaderBuilder(p, ...).fork();
 * new BodyBuilder(p, ...).fork();
 * }</pre>
 *
 * @since 1.8
 * @author Doug Lea
 */
public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
    private static final long serialVersionUID = 5232453752276485070L;

    /** This task's completer, or null if none */
    final CountedCompleter<?> completer;
    /** The number of pending tasks until completion */
    volatile int pending;

    /**
     * Creates a new CountedCompleter with the given completer
     * and initial pending count.
     *
     * @param completer this tasks completer, or {@code null} if none
     * @param initialPendingCount the initial pending count
     */
    protected CountedCompleter(CountedCompleter<?> completer,
                               int initialPendingCount) {
        this.completer = completer;
        this.pending = initialPendingCount;
    }

    /**
     * Creates a new CountedCompleter with the given completer
     * and an initial pending count of zero.
     *
     * @param completer this tasks completer, or {@code null} if none
     */
    protected CountedCompleter(CountedCompleter<?> completer) {
        this.completer = completer;
    }

    /**
     * Creates a new CountedCompleter with no completer
     * and an initial pending count of zero.
     */
    protected CountedCompleter() {
        this.completer = null;
    }

    /**
     * The main computation performed by this task.
     */
    public abstract void compute();

    /**
     * Performs an action when method {@link #tryComplete} is invoked
     * and there are no pending counts, or when the unconditional
     * method {@link #complete} is invoked.  By default, this method
     * does nothing.
     *
     * @param caller the task invoking this method (which may
     * be this task itself).
     */
    public void onCompletion(CountedCompleter<?> caller) {
    }

    /**
     * Performs an action when method {@link #completeExceptionally}
     * is invoked or method {@link #compute} throws an exception, and
     * this task has not otherwise already completed normally. On
     * entry to this method, this task {@link
     * ForkJoinTask#isCompletedAbnormally}.  The return value of this
     * method controls further propagation: If {@code true} and this
     * task has a completer, then this completer is also completed
     * exceptionally.  The default implementation of this method does
     * nothing except return {@code true}.
     *
     * @param ex the exception
     * @param caller the task invoking this method (which may
     * be this task itself).
     * @return true if this exception should be propagated to this
     * tasks completer, if one exists.
     */
    public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
        return true;
    }

    /**
     * Returns the completer established in this task's constructor,
     * or {@code null} if none.
     *
     * @return the completer
     */
    public final CountedCompleter<?> getCompleter() {
        return completer;
    }

    /**
     * Returns the current pending count.
     *
     * @return the current pending count
     */
    public final int getPendingCount() {
        return pending;
    }

    /**
     * Sets the pending count to the given value.
     *
     * @param count the count
     */
    public final void setPendingCount(int count) {
        pending = count;
    }

    /**
     * Adds (atomically) the given value to the pending count.
     *
     * @param delta the value to add
     */
    public final void addToPendingCount(int delta) {
        int c; // note: can replace with intrinsic in jdk8
        do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
    }

    /**
     * Sets (atomically) the pending count to the given count only if
     * it currently holds the given expected value.
     *
     * @param expected the expected value
     * @param count the new value
     * @return true is successful
     */
    public final boolean compareAndSetPendingCount(int expected, int count) {
        return U.compareAndSwapInt(this, PENDING, expected, count);
    }

    /**
     * Returns the root of the current computation; i.e., this
     * task if it has no completer, else its completer's root.
     *
     * @return the root of the current computation
     */
    public final CountedCompleter<?> getRoot() {
        CountedCompleter<?> a = this, p;
        while ((p = a.completer) != null)
            a = p;
        return a;
    }

    /**
     * If the pending count is nonzero, decrements the count;
     * otherwise invokes {@link #onCompletion} and then similarly
     * tries to complete this task's completer, if one exists,
     * else marks this task as complete.
     */
    public final void tryComplete() {
        CountedCompleter<?> a = this, s = a;
        for (int c;;) {
            if ((c = a.pending) == 0) {
                a.onCompletion(s);
                if ((a = (s = a).completer) == null) {
                    s.quietlyComplete();
                    return;
                }
            }
            else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
                return;
        }
    }

    /**
     * Identical to {@link #tryComplete}, but may additionally execute
     * other tasks within the current computation (i.e., those
     * with the same {@link #getRoot}.
     */
    public final void helpComplete() {
        CountedCompleter<?> a = this, s = a;
        for (int c;;) {
            if ((c = a.pending) == 0) {
                a.onCompletion(s);
                if ((a = (s = a).completer) == null) {
                    s.quietlyComplete();
                    return;
                }
            }
            else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) {
                if (!(Thread.currentThread() instanceof ForkJoinWorkerThread))
                    ForkJoinPool.popAndExecCCFromCommonPool(a);
                return;
            }
        }
    }

    /**
     * Regardless of pending count, invokes {@link #onCompletion},
     * marks this task as complete and further triggers {@link
     * #tryComplete} on this task's completer, if one exists. This
     * method may be useful when forcing completion as soon as any one
     * (versus all) of several subtask results are obtained.  The
     * given rawResult is used as an argument to {@link #setRawResult}
     * before marking this task as complete; its value is meaningful
     * only for classes overriding {@code setRawResult}.
     *
     * @param rawResult the raw result
     */
    public void complete(T rawResult) {
        CountedCompleter<?> p;
        onCompletion(this);
        setRawResult(rawResult);
        quietlyComplete();
        if ((p = completer) != null)
            p.tryComplete();
    }

    /**
     * Support for FJT exception propagation
     */
    void internalPropagateException(Throwable ex) {
        CountedCompleter<?> a = this, s = a;
        while (a.onExceptionalCompletion(ex, s) &&
               (a = (s = a).completer) != null && a.status >= 0)
            a.recordExceptionalCompletion(ex);
    }

    /**
     * Implements execution conventions for CountedCompleters
     */
    protected final boolean exec() {
        compute();
        return false;
    }

    /**
     * Returns the result of the computation. By default
     * returns {@code null}, which is appropriate for {@code Void}
     * actions, but in other cases should be overridden.
     *
     * @return the result of the computation
     */
    public T getRawResult() { return null; }

    /**
     * A method that result-bearing CountedCompleters may optionally
     * use to help maintain result data.  By default, does nothing.
     */
    protected void setRawResult(T t) { }

    // Unsafe mechanics
    private static final sun.misc.Unsafe U;
    private static final long PENDING;
    static {
        try {
            U = getUnsafe();
            PENDING = U.objectFieldOffset
                (CountedCompleter.class.getDeclaredField("pending"));
        } 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 se) {
            try {
                return java.security.AccessController.doPrivileged
                    (new java.security
                     .PrivilegedExceptionAction<sun.misc.Unsafe>() {
                        public sun.misc.Unsafe run() throws Exception {
                            java.lang.reflect.Field f = sun.misc
                                .Unsafe.class.getDeclaredField("theUnsafe");
                            f.setAccessible(true);
                            return (sun.misc.Unsafe) f.get(null);
                        }});
            } catch (java.security.PrivilegedActionException e) {
                throw new RuntimeException("Could not initialize intrinsics",
                                           e.getCause());
            }
        }
    }

}
Revision:	1.9
Committed:	Fri Nov 9 03:30:03 2012 UTC (11 years, 5 months ago) by jsr166
Branch:	MAIN
Changes since 1.8:	+1 -1 lines
Log Message:	whitespace
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package jsr166y;
8
9	/**
10	* A {@link ForkJoinTask} with a completion action
11	* performed when triggered and there are no remaining pending
12	* actions. Uses of CountedCompleter are similar to those of other
13	* completion based components (such as {@link
14	* java.nio.channels.CompletionHandler}) except that multiple
15	* <em>pending</em> completions may be necessary to trigger the {@link
16	* #onCompletion} action, not just one. Unless initialized otherwise,
17	* the {@link #getPendingCount pending count} starts at zero, but may
18	* be (atomically) changed using methods {@link #setPendingCount},
19	* {@link #addToPendingCount}, and {@link
20	* #compareAndSetPendingCount}. Upon invocation of {@link
21	* #tryComplete}, if the pending action count is nonzero, it is
22	* decremented; otherwise, the completion action is performed, and if
23	* this completer itself has a completer, the process is continued
24	* with its completer. As is the case with related synchronization
25	* components such as {@link java.util.concurrent.Phaser Phaser} and
26	* {@link java.util.concurrent.Semaphore Semaphore}, these methods
27	* affect only internal counts; they do not establish any further
28	* internal bookkeeping. In particular, the identities of pending
29	* tasks are not maintained. As illustrated below, you can create
30	* subclasses that do record some or all pending tasks or their
31	* results when needed.
32	*
33	* <p>A concrete CountedCompleter class must define method {@link
34	* #compute}, that should, in almost all use cases, invoke {@code
35	* tryComplete()} once before returning. The class may also optionally
36	* override method {@link #onCompletion} to perform an action upon
37	* normal completion, and method {@link #onExceptionalCompletion} to
38	* perform an action upon any exception.
39	*
40	* <p>CountedCompleters most often do not bear results, in which case
41	* they are normally declared as {@code CountedCompleter<Void>}, and
42	* will always return {@code null} as a result value. In other cases,
43	* you should override method {@link #getRawResult} to provide a
44	* result from {@code join(), invoke()}, and related methods. (Method
45	* {@link #setRawResult} by default plays no role in CountedCompleters
46	* but may be overridden for example to maintain fields holding result
47	* data.)
48	*
49	* <p>A CountedCompleter that does not itself have a completer (i.e.,
50	* one for which {@link #getCompleter} returns {@code null}) can be
51	* used as a regular ForkJoinTask with this added functionality.
52	* However, any completer that in turn has another completer serves
53	* only as an internal helper for other computations, so its own task
54	* status (as reported in methods such as {@link ForkJoinTask#isDone})
55	* is arbitrary; this status changes only upon explicit invocations of
56	* {@link #complete}, {@link ForkJoinTask#cancel}, {@link
57	* ForkJoinTask#completeExceptionally} or upon exceptional completion
58	* of method {@code compute}. Upon any exceptional completion, the
59	* exception may be relayed to a task's completer (and its completer,
60	* and so on), if one exists and it has not otherwise already
61	* completed.
62	*
63	* <p><b>Sample Usages.</b>
64	*
65	* <p><b>Parallel recursive decomposition.</b> CountedCompleters may
66	* be arranged in trees similar to those often used with {@link
67	* RecursiveAction}s, although the constructions involved in setting
68	* them up typically vary. Here, the completer of each task is its
69	* parent in the computation tree. Even though they entail a bit more
70	* bookkeeping, CountedCompleters may be better choices when applying
71	* a possibly time-consuming operation (that cannot be further
72	* subdivided) to each element of an array or collection; especially
73	* when the operation takes a significantly different amount of time
74	* to complete for some elements than others, either because of
75	* intrinsic variation (for example IO) or auxiliary effects such as
76	* garbage collection. Because CountedCompleters provide their own
77	* continuations, other threads need not block waiting to perform
78	* them.
79	*
80	* <p> For example, here is an initial version of a class that uses
81	* divide-by-two recursive decomposition to divide work into single
82	* pieces (leaf tasks). Even when work is split into individual calls,
83	* tree-based techniques are usually preferable to directly forking
84	* leaf tasks, because they reduce inter-thread communication and
85	* improve load balancing. In the recursive case, the second of each
86	* pair of subtasks to finish triggers completion of its parent
87	* (because no result combination is performed, the default no-op
88	* implementation of method {@code onCompletion} is not overridden). A
89	* static utility method sets up the base task and invokes it:
90	*
91	* <pre> {@code
92	* class MyOperation<E> { void apply(E e) { ... } }
93	*
94	* class ForEach<E> extends CountedCompleter<Void> {
95	*
96	* public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) {
97	* pool.invoke(new ForEach<E>(null, array, op, 0, array.length));
98	* }
99	*
100	* final E[] array; final MyOperation<E> op; final int lo, hi;
101	* ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
102	* super(p);
103	* this.array = array; this.op = op; this.lo = lo; this.hi = hi;
104	* }
105	*
106	* public void compute() { // version 1
107	* if (hi - lo >= 2) {
108	* int mid = (lo + hi) >>> 1;
109	* setPendingCount(2); // must set pending count before fork
110	* new ForEach(this, array, op, mid, hi).fork(); // right child
111	* new ForEach(this, array, op, lo, mid).fork(); // left child
112	* }
113	* else if (hi > lo)
114	* op.apply(array[lo]);
115	* tryComplete();
116	* }
117	* } }</pre>
118	*
119	* This design can be improved by noticing that in the recursive case,
120	* the task has nothing to do after forking its right task, so can
121	* directly invoke its left task before returning. (This is an analog
122	* of tail recursion removal.) Also, because the task returns upon
123	* executing its left task (rather than falling through to invoke
124	* tryComplete) the pending count is set to one:
125	*
126	* <pre> {@code
127	* class ForEach<E> ...
128	* public void compute() { // version 2
129	* if (hi - lo >= 2) {
130	* int mid = (lo + hi) >>> 1;
131	* setPendingCount(1); // only one pending
132	* new ForEach(this, array, op, mid, hi).fork(); // right child
133	* new ForEach(this, array, op, lo, mid).compute(); // direct invoke
134	* }
135	* else {
136	* if (hi > lo)
137	* op.apply(array[lo]);
138	* tryComplete();
139	* }
140	* }
141	* }</pre>
142	*
143	* As a further improvement, notice that the left task need not even
144	* exist. Instead of creating a new one, we can iterate using the
145	* original task, and add a pending count for each fork. Additionally,
146	* this version uses {@code helpComplete} to streamline assistance in
147	* the execution of forked tasks.
148	*
149	* <pre> {@code
150	* class ForEach<E> ...
151	* public void compute() { // version 3
152	* int l = lo, h = hi;
153	* while (h - l >= 2) {
154	* int mid = (l + h) >>> 1;
155	* addToPendingCount(1);
156	* new ForEach(this, array, op, mid, h).fork(); // right child
157	* h = mid;
158	* }
159	* if (h > l)
160	* op.apply(array[l]);
161	* helpComplete();
162	* }
163	* }</pre>
164	*
165	* Additional improvements of such classes might entail precomputing
166	* pending counts so that they can be established in constructors,
167	* specializing classes for leaf steps, subdividing by say, four,
168	* instead of two per iteration, and using an adaptive threshold
169	* instead of always subdividing down to single elements.
170	*
171	* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
172	* results of multiple subtasks usually need to access these results
173	* in method {@link #onCompletion}. As illustrated in the following
174	* class (that performs a simplified form of map-reduce where mappings
175	* and reductions are all of type {@code E}), one way to do this in
176	* divide and conquer designs is to have each subtask record its
177	* sibling, so that it can be accessed in method {@code onCompletion}.
178	* This technique applies to reductions in which the order of
179	* combining left and right results does not matter; ordered
180	* reductions require explicit left/right designations. Variants of
181	* other streamlinings seen in the above examples may also apply.
182	*
183	* <pre> {@code
184	* class MyMapper<E> { E apply(E v) { ... } }
185	* class MyReducer<E> { E apply(E x, E y) { ... } }
186	* class MapReducer<E> extends CountedCompleter<E> {
187	* final E[] array; final MyMapper<E> mapper;
188	* final MyReducer<E> reducer; final int lo, hi;
189	* MapReducer<E> sibling;
190	* E result;
191	* MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper,
192	* MyReducer<E> reducer, int lo, int hi) {
193	* super(p);
194	* this.array = array; this.mapper = mapper;
195	* this.reducer = reducer; this.lo = lo; this.hi = hi;
196	* }
197	* public void compute() {
198	* if (hi - lo >= 2) {
199	* int mid = (lo + hi) >>> 1;
200	* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
201	* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
202	* left.sibling = right;
203	* right.sibling = left;
204	* setPendingCount(1); // only right is pending
205	* right.fork();
206	* left.compute(); // directly execute left
207	* }
208	* else {
209	* if (hi > lo)
210	* result = mapper.apply(array[lo]);
211	* tryComplete();
212	* }
213	* }
214	* public void onCompletion(CountedCompleter caller) {
215	* if (caller != this) {
216	* MapReducer<E> child = (MapReducer<E>)caller;
217	* MapReducer<E> sib = child.sibling;
218	* if (sib == null \|\| sib.result == null)
219	* result = child.result;
220	* else
221	* result = reducer.apply(child.result, sib.result);
222	* }
223	* }
224	* public E getRawResult() { return result; }
225	*
226	* public static <E> E mapReduce(ForkJoinPool pool, E[] array,
227	* MyMapper<E> mapper, MyReducer<E> reducer) {
228	* return pool.invoke(new MapReducer<E>(null, array, mapper,
229	* reducer, 0, array.length));
230	* }
231	* } }</pre>
232	*
233	* <p><b>Triggers.</b> Some CountedCompleters are themselves never
234	* forked, but instead serve as bits of plumbing in other designs;
235	* including those in which the completion of one of more async tasks
236	* triggers another async task. For example:
237	*
238	* <pre> {@code
239	* class HeaderBuilder extends CountedCompleter<...> { ... }
240	* class BodyBuilder extends CountedCompleter<...> { ... }
241	* class PacketSender extends CountedCompleter<...> {
242	* PacketSender(...) { super(null, 1); ... } // trigger on second completion
243	* public void compute() { } // never called
244	* public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
245	* }
246	* // sample use:
247	* PacketSender p = new PacketSender();
248	* new HeaderBuilder(p, ...).fork();
249	* new BodyBuilder(p, ...).fork();
250	* }</pre>
251	*
252	* @since 1.8
253	* @author Doug Lea
254	*/
255	public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
256	private static final long serialVersionUID = 5232453752276485070L;
257
258	/** This task's completer, or null if none */
259	final CountedCompleter<?> completer;
260	/** The number of pending tasks until completion */
261	volatile int pending;
262
263	/**
264	* Creates a new CountedCompleter with the given completer
265	* and initial pending count.
266	*
267	* @param completer this tasks completer, or {@code null} if none
268	* @param initialPendingCount the initial pending count
269	*/
270	protected CountedCompleter(CountedCompleter<?> completer,
271	int initialPendingCount) {
272	this.completer = completer;
273	this.pending = initialPendingCount;
274	}
275
276	/**
277	* Creates a new CountedCompleter with the given completer
278	* and an initial pending count of zero.
279	*
280	* @param completer this tasks completer, or {@code null} if none
281	*/
282	protected CountedCompleter(CountedCompleter<?> completer) {
283	this.completer = completer;
284	}
285
286	/**
287	* Creates a new CountedCompleter with no completer
288	* and an initial pending count of zero.
289	*/
290	protected CountedCompleter() {
291	this.completer = null;
292	}
293
294	/**
295	* The main computation performed by this task.
296	*/
297	public abstract void compute();
298
299	/**
300	* Performs an action when method {@link #tryComplete} is invoked
301	* and there are no pending counts, or when the unconditional
302	* method {@link #complete} is invoked. By default, this method
303	* does nothing.
304	*
305	* @param caller the task invoking this method (which may
306	* be this task itself).
307	*/
308	public void onCompletion(CountedCompleter<?> caller) {
309	}
310
311	/**
312	* Performs an action when method {@link #completeExceptionally}
313	* is invoked or method {@link #compute} throws an exception, and
314	* this task has not otherwise already completed normally. On
315	* entry to this method, this task {@link
316	* ForkJoinTask#isCompletedAbnormally}. The return value of this
317	* method controls further propagation: If {@code true} and this
318	* task has a completer, then this completer is also completed
319	* exceptionally. The default implementation of this method does
320	* nothing except return {@code true}.
321	*
322	* @param ex the exception
323	* @param caller the task invoking this method (which may
324	* be this task itself).
325	* @return true if this exception should be propagated to this
326	* tasks completer, if one exists.
327	*/
328	public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
329	return true;
330	}
331
332	/**
333	* Returns the completer established in this task's constructor,
334	* or {@code null} if none.
335	*
336	* @return the completer
337	*/
338	public final CountedCompleter<?> getCompleter() {
339	return completer;
340	}
341
342	/**
343	* Returns the current pending count.
344	*
345	* @return the current pending count
346	*/
347	public final int getPendingCount() {
348	return pending;
349	}
350
351	/**
352	* Sets the pending count to the given value.
353	*
354	* @param count the count
355	*/
356	public final void setPendingCount(int count) {
357	pending = count;
358	}
359
360	/**
361	* Adds (atomically) the given value to the pending count.
362	*
363	* @param delta the value to add
364	*/
365	public final void addToPendingCount(int delta) {
366	int c; // note: can replace with intrinsic in jdk8
367	do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
368	}
369
370	/**
371	* Sets (atomically) the pending count to the given count only if
372	* it currently holds the given expected value.
373	*
374	* @param expected the expected value
375	* @param count the new value
376	* @return true is successful
377	*/
378	public final boolean compareAndSetPendingCount(int expected, int count) {
379	return U.compareAndSwapInt(this, PENDING, expected, count);
380	}
381
382	/**
383	* Returns the root of the current computation; i.e., this
384	* task if it has no completer, else its completer's root.
385	*
386	* @return the root of the current computation
387	*/
388	public final CountedCompleter<?> getRoot() {
389	CountedCompleter<?> a = this, p;
390	while ((p = a.completer) != null)
391	a = p;
392	return a;
393	}
394
395	/**
396	* If the pending count is nonzero, decrements the count;
397	* otherwise invokes {@link #onCompletion} and then similarly
398	* tries to complete this task's completer, if one exists,
399	* else marks this task as complete.
400	*/
401	public final void tryComplete() {
402	CountedCompleter<?> a = this, s = a;
403	for (int c;;) {
404	if ((c = a.pending) == 0) {
405	a.onCompletion(s);
406	if ((a = (s = a).completer) == null) {
407	s.quietlyComplete();
408	return;
409	}
410	}
411	else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
412	return;
413	}
414	}
415
416	/**
417	* Identical to {@link #tryComplete}, but may additionally execute
418	* other tasks within the current computation (i.e., those
419	* with the same {@link #getRoot}.
420	*/
421	public final void helpComplete() {
422	CountedCompleter<?> a = this, s = a;
423	for (int c;;) {
424	if ((c = a.pending) == 0) {
425	a.onCompletion(s);
426	if ((a = (s = a).completer) == null) {
427	s.quietlyComplete();
428	return;
429	}
430	}
431	else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) {
432	if (!(Thread.currentThread() instanceof ForkJoinWorkerThread))
433	ForkJoinPool.popAndExecCCFromCommonPool(a);
434	return;
435	}
436	}
437	}
438
439	/**
440	* Regardless of pending count, invokes {@link #onCompletion},
441	* marks this task as complete and further triggers {@link
442	* #tryComplete} on this task's completer, if one exists. This
443	* method may be useful when forcing completion as soon as any one
444	* (versus all) of several subtask results are obtained. The
445	* given rawResult is used as an argument to {@link #setRawResult}
446	* before marking this task as complete; its value is meaningful
447	* only for classes overriding {@code setRawResult}.
448	*
449	* @param rawResult the raw result
450	*/
451	public void complete(T rawResult) {
452	CountedCompleter<?> p;
453	onCompletion(this);
454	setRawResult(rawResult);
455	quietlyComplete();
456	if ((p = completer) != null)
457	p.tryComplete();
458	}
459
460	/**
461	* Support for FJT exception propagation
462	*/
463	void internalPropagateException(Throwable ex) {
464	CountedCompleter<?> a = this, s = a;
465	while (a.onExceptionalCompletion(ex, s) &&
466	(a = (s = a).completer) != null && a.status >= 0)
467	a.recordExceptionalCompletion(ex);
468	}
469
470	/**
471	* Implements execution conventions for CountedCompleters
472	*/
473	protected final boolean exec() {
474	compute();
475	return false;
476	}
477
478	/**
479	* Returns the result of the computation. By default
480	* returns {@code null}, which is appropriate for {@code Void}
481	* actions, but in other cases should be overridden.
482	*
483	* @return the result of the computation
484	*/
485	public T getRawResult() { return null; }
486
487	/**
488	* A method that result-bearing CountedCompleters may optionally
489	* use to help maintain result data. By default, does nothing.
490	*/
491	protected void setRawResult(T t) { }
492
493	// Unsafe mechanics
494	private static final sun.misc.Unsafe U;
495	private static final long PENDING;
496	static {
497	try {
498	U = getUnsafe();
499	PENDING = U.objectFieldOffset
500	(CountedCompleter.class.getDeclaredField("pending"));
501	} catch (Exception e) {
502	throw new Error(e);
503	}
504	}
505
506	/**
507	* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
508	* Replace with a simple call to Unsafe.getUnsafe when integrating
509	* into a jdk.
510	*
511	* @return a sun.misc.Unsafe
512	*/
513	private static sun.misc.Unsafe getUnsafe() {
514	try {
515	return sun.misc.Unsafe.getUnsafe();
516	} catch (SecurityException se) {
517	try {
518	return java.security.AccessController.doPrivileged
519	(new java.security
520	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
521	public sun.misc.Unsafe run() throws Exception {
522	java.lang.reflect.Field f = sun.misc
523	.Unsafe.class.getDeclaredField("theUnsafe");
524	f.setAccessible(true);
525	return (sun.misc.Unsafe) f.get(null);
526	}});
527	} catch (java.security.PrivilegedActionException e) {
528	throw new RuntimeException("Could not initialize intrinsics",
529	e.getCause());
530	}
531	}
532	}
533
534	}