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 Phaser} and {@link
 * java.util.concurrent.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 pended 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:
 *
 * <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]);
 *         tryComplete();
 *     }
 * }</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);
    }

    /**
     * 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;
        }
    }

    /**
     * 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:	1.3
Committed:	Thu Aug 16 12:25:03 2012 UTC (11 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.2:	+55 -39 lines
Log Message:	Parameterize CountedCompleters
#	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 Phaser} and {@link
26	* java.util.concurrent.Semaphore} these methods affect only internal
27	* counts; they do not establish any further internal bookkeeping. In
28	* particular, the identities of pending tasks are not maintained. As
29	* illustrated below, you can create subclasses that do record some or
30	* all pended tasks or their results when needed.
31	*
32	* <p>A concrete CountedCompleter class must define method {@link
33	* #compute}, that should, in almost all use cases, invoke {@code
34	* tryComplete()} once before returning. The class may also optionally
35	* override method {@link #onCompletion} to perform an action upon
36	* normal completion, and method {@link #onExceptionalCompletion} to
37	* perform an action upon any exception.
38	*
39	* <p>CountedCompleters most often do not bear results, in which case
40	* they are normally declared as {@code CountedCompleter<Void>}, and
41	* will always return {@code null} as a result value. In other cases,
42	* you should override method {@link #getRawResult} to provide a
43	* result from {@code join(), invoke()}, and related methods. (Method
44	* {@link #setRawResult} by default plays no role in CountedCompleters
45	* but may be overridden for example to maintain fields holding result
46	* data.)
47	*
48	* <p>A CountedCompleter that does not itself have a completer (i.e.,
49	* one for which {@link #getCompleter} returns {@code null}) can be
50	* used as a regular ForkJoinTask with this added functionality.
51	* However, any completer that in turn has another completer serves
52	* only as an internal helper for other computations, so its own task
53	* status (as reported in methods such as {@link ForkJoinTask#isDone})
54	* is arbitrary; this status changes only upon explicit invocations of
55	* {@link #complete}, {@link ForkJoinTask#cancel}, {@link
56	* ForkJoinTask#completeExceptionally} or upon exceptional completion
57	* of method {@code compute}. Upon any exceptional completion, the
58	* exception may be relayed to a task's completer (and its completer,
59	* and so on), if one exists and it has not otherwise already
60	* completed.
61	*
62	* <p><b>Sample Usages.</b>
63	*
64	* <p><b>Parallel recursive decomposition.</b> CountedCompleters may
65	* be arranged in trees similar to those often used with {@link
66	* RecursiveAction}s, although the constructions involved in setting
67	* them up typically vary. Here, the completer of each task is its
68	* parent in the computation tree. Even though they entail a bit more
69	* bookkeeping, CountedCompleters may be better choices when applying
70	* a possibly time-consuming operation (that cannot be further
71	* subdivided) to each element of an array or collection; especially
72	* when the operation takes a significantly different amount of time
73	* to complete for some elements than others, either because of
74	* intrinsic variation (for example IO) or auxiliary effects such as
75	* garbage collection. Because CountedCompleters provide their own
76	* continuations, other threads need not block waiting to perform
77	* them.
78	*
79	* <p> For example, here is an initial version of a class that uses
80	* divide-by-two recursive decomposition to divide work into single
81	* pieces (leaf tasks). Even when work is split into individual calls,
82	* tree-based techniques are usually preferable to directly forking
83	* leaf tasks, because they reduce inter-thread communication and
84	* improve load balancing. In the recursive case, the second of each
85	* pair of subtasks to finish triggers completion of its parent
86	* (because no result combination is performed, the default no-op
87	* implementation of method {@code onCompletion} is not overridden). A
88	* static utility method sets up the base task and invokes it:
89	*
90	* <pre> {@code
91	* class MyOperation<E> { void apply(E e) { ... } }
92	*
93	* class ForEach<E> extends CountedCompleter<Void> {
94	*
95	* public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) {
96	* pool.invoke(new ForEach<E>(null, array, op, 0, array.length));
97	* }
98	*
99	* final E[] array; final MyOperation<E> op; final int lo, hi;
100	* ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
101	* super(p);
102	* this.array = array; this.op = op; this.lo = lo; this.hi = hi;
103	* }
104	*
105	* public void compute() { // version 1
106	* if (hi - lo >= 2) {
107	* int mid = (lo + hi) >>> 1;
108	* setPendingCount(2); // must set pending count before fork
109	* new ForEach(this, array, op, mid, hi).fork(); // right child
110	* new ForEach(this, array, op, lo, mid).fork(); // left child
111	* }
112	* else if (hi > lo)
113	* op.apply(array[lo]);
114	* tryComplete();
115	* }
116	* } }</pre>
117	*
118	* This design can be improved by noticing that in the recursive case,
119	* the task has nothing to do after forking its right task, so can
120	* directly invoke its left task before returning. (This is an analog
121	* of tail recursion removal.) Also, because the task returns upon
122	* executing its left task (rather than falling through to invoke
123	* tryComplete) the pending count is set to one:
124	*
125	* <pre> {@code
126	* class ForEach<E> ...
127	* public void compute() { // version 2
128	* if (hi - lo >= 2) {
129	* int mid = (lo + hi) >>> 1;
130	* setPendingCount(1); // only one pending
131	* new ForEach(this, array, op, mid, hi).fork(); // right child
132	* new ForEach(this, array, op, lo, mid).compute(); // direct invoke
133	* }
134	* else {
135	* if (hi > lo)
136	* op.apply(array[lo]);
137	* tryComplete();
138	* }
139	* }
140	* }</pre>
141	*
142	* As a further improvement, notice that the left task need not even
143	* exist. Instead of creating a new one, we can iterate using the
144	* original task, and add a pending count for each fork:
145	*
146	* <pre> {@code
147	* class ForEach<E> ...
148	* public void compute() { // version 3
149	* int l = lo, h = hi;
150	* while (h - l >= 2) {
151	* int mid = (l + h) >>> 1;
152	* addToPendingCount(1);
153	* new ForEach(this, array, op, mid, h).fork(); // right child
154	* h = mid;
155	* }
156	* if (h > l)
157	* op.apply(array[l]);
158	* tryComplete();
159	* }
160	* }</pre>
161	*
162	* Additional improvements of such classes might entail precomputing
163	* pending counts so that they can be established in constructors,
164	* specializing classes for leaf steps, subdividing by say, four,
165	* instead of two per iteration, and using an adaptive threshold
166	* instead of always subdividing down to single elements.
167	*
168	* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
169	* results of multiple subtasks usually need to access these results
170	* in method {@link #onCompletion}. As illustrated in the following
171	* class (that performs a simplified form of map-reduce where mappings
172	* and reductions are all of type {@code E}), one way to do this in
173	* divide and conquer designs is to have each subtask record its
174	* sibling, so that it can be accessed in method {@code onCompletion}.
175	* This technique applies to reductions in which the order of
176	* combining left and right results does not matter; ordered
177	* reductions require explicit left/right designations. Variants of
178	* other streamlinings seen in the above examples may also apply.
179	*
180	* <pre> {@code
181	* class MyMapper<E> { E apply(E v) { ... } }
182	* class MyReducer<E> { E apply(E x, E y) { ... } }
183	* class MapReducer<E> extends CountedCompleter<E> {
184	* final E[] array; final MyMapper<E> mapper;
185	* final MyReducer<E> reducer; final int lo, hi;
186	* MapReducer<E> sibling;
187	* E result;
188	* MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper,
189	* MyReducer<E> reducer, int lo, int hi) {
190	* super(p);
191	* this.array = array; this.mapper = mapper;
192	* this.reducer = reducer; this.lo = lo; this.hi = hi;
193	* }
194	* public void compute() {
195	* if (hi - lo >= 2) {
196	* int mid = (lo + hi) >>> 1;
197	* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
198	* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
199	* left.sibling = right;
200	* right.sibling = left;
201	* setPendingCount(1); // only right is pending
202	* right.fork();
203	* left.compute(); // directly execute left
204	* }
205	* else {
206	* if (hi > lo)
207	* result = mapper.apply(array[lo]);
208	* tryComplete();
209	* }
210	* }
211	* public void onCompletion(CountedCompleter caller) {
212	* if (caller != this) {
213	* MapReducer<E> child = (MapReducer<E>)caller;
214	* MapReducer<E> sib = child.sibling;
215	* if (sib == null \|\| sib.result == null)
216	* result = child.result;
217	* else
218	* result = reducer.apply(child.result, sib.result);
219	* }
220	* }
221	* public E getRawResult() { return result; }
222	*
223	* public static <E> E mapReduce(ForkJoinPool pool, E[] array,
224	* MyMapper<E> mapper, MyReducer<E> reducer) {
225	* return pool.invoke(new MapReducer<E>(null, array, mapper,
226	* reducer, 0, array.length));
227	* }
228	* } }</pre>
229	*
230	* <p><b>Triggers.</b> Some CountedCompleters are themselves never
231	* forked, but instead serve as bits of plumbing in other designs;
232	* including those in which the completion of one of more async tasks
233	* triggers another async task. For example:
234	*
235	* <pre> {@code
236	* class HeaderBuilder extends CountedCompleter<...> { ... }
237	* class BodyBuilder extends CountedCompleter<...> { ... }
238	* class PacketSender extends CountedCompleter<...> {
239	* PacketSender(...) { super(null, 1); ... } // trigger on second completion
240	* public void compute() { } // never called
241	* public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
242	* }
243	* // sample use:
244	* PacketSender p = new PacketSender();
245	* new HeaderBuilder(p, ...).fork();
246	* new BodyBuilder(p, ...).fork();
247	* }</pre>
248	*
249	* @since 1.8
250	* @author Doug Lea
251	*/
252	public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
253	private static final long serialVersionUID = 5232453752276485070L;
254
255	/** This task's completer, or null if none */
256	final CountedCompleter<?> completer;
257	/** The number of pending tasks until completion */
258	volatile int pending;
259
260	/**
261	* Creates a new CountedCompleter with the given completer
262	* and initial pending count.
263	*
264	* @param completer this tasks completer, or {@code null} if none
265	* @param initialPendingCount the initial pending count
266	*/
267	protected CountedCompleter(CountedCompleter<?> completer,
268	int initialPendingCount) {
269	this.completer = completer;
270	this.pending = initialPendingCount;
271	}
272
273	/**
274	* Creates a new CountedCompleter with the given completer
275	* and an initial pending count of zero.
276	*
277	* @param completer this tasks completer, or {@code null} if none
278	*/
279	protected CountedCompleter(CountedCompleter<?> completer) {
280	this.completer = completer;
281	}
282
283	/**
284	* Creates a new CountedCompleter with no completer
285	* and an initial pending count of zero.
286	*/
287	protected CountedCompleter() {
288	this.completer = null;
289	}
290
291	/**
292	* The main computation performed by this task.
293	*/
294	public abstract void compute();
295
296	/**
297	* Performs an action when method {@link #tryComplete} is invoked
298	* and there are no pending counts, or when the unconditional
299	* method {@link #complete} is invoked. By default, this method
300	* does nothing.
301	*
302	* @param caller the task invoking this method (which may
303	* be this task itself).
304	*/
305	public void onCompletion(CountedCompleter<?> caller) {
306	}
307
308	/**
309	* Performs an action when method {@link #completeExceptionally}
310	* is invoked or method {@link #compute} throws an exception, and
311	* this task has not otherwise already completed normally. On
312	* entry to this method, this task {@link
313	* ForkJoinTask#isCompletedAbnormally}. The return value of this
314	* method controls further propagation: If {@code true} and this
315	* task has a completer, then this completer is also completed
316	* exceptionally. The default implementation of this method does
317	* nothing except return {@code true}.
318	*
319	* @param ex the exception
320	* @param caller the task invoking this method (which may
321	* be this task itself).
322	* @return true if this exception should be propagated to this
323	* tasks completer, if one exists.
324	*/
325	public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
326	return true;
327	}
328
329	/**
330	* Returns the completer established in this task's constructor,
331	* or {@code null} if none.
332	*
333	* @return the completer
334	*/
335	public final CountedCompleter<?> getCompleter() {
336	return completer;
337	}
338
339	/**
340	* Returns the current pending count.
341	*
342	* @return the current pending count
343	*/
344	public final int getPendingCount() {
345	return pending;
346	}
347
348	/**
349	* Sets the pending count to the given value.
350	*
351	* @param count the count
352	*/
353	public final void setPendingCount(int count) {
354	pending = count;
355	}
356
357	/**
358	* Adds (atomically) the given value to the pending count.
359	*
360	* @param delta the value to add
361	*/
362	public final void addToPendingCount(int delta) {
363	int c; // note: can replace with intrinsic in jdk8
364	do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
365	}
366
367	/**
368	* Sets (atomically) the pending count to the given count only if
369	* it currently holds the given expected value.
370	*
371	* @param expected the expected value
372	* @param count the new value
373	* @return true is successful
374	*/
375	public final boolean compareAndSetPendingCount(int expected, int count) {
376	return U.compareAndSwapInt(this, PENDING, expected, count);
377	}
378
379	/**
380	* If the pending count is nonzero, decrements the count;
381	* otherwise invokes {@link #onCompletion} and then similarly
382	* tries to complete this task's completer, if one exists,
383	* else marks this task as complete.
384	*/
385	public final void tryComplete() {
386	CountedCompleter<?> a = this, s = a;
387	for (int c;;) {
388	if ((c = a.pending) == 0) {
389	a.onCompletion(s);
390	if ((a = (s = a).completer) == null) {
391	s.quietlyComplete();
392	return;
393	}
394	}
395	else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
396	return;
397	}
398	}
399
400	/**
401	* Regardless of pending count, invokes {@link #onCompletion},
402	* marks this task as complete and further triggers {@link
403	* #tryComplete} on this task's completer, if one exists. This
404	* method may be useful when forcing completion as soon as any one
405	* (versus all) of several subtask results are obtained. The
406	* given rawResult is used as an argument to {@link #setRawResult}
407	* before marking this task as complete; its value is meaningful
408	* only for classes overriding {@code setRawResult}.
409	*
410	* @param rawResult the raw result
411	*/
412	public void complete(T rawResult) {
413	CountedCompleter<?> p;
414	onCompletion(this);
415	setRawResult(rawResult);
416	quietlyComplete();
417	if ((p = completer) != null)
418	p.tryComplete();
419	}
420
421	/**
422	* Support for FJT exception propagation
423	*/
424	void internalPropagateException(Throwable ex) {
425	CountedCompleter<?> a = this, s = a;
426	while (a.onExceptionalCompletion(ex, s) &&
427	(a = (s = a).completer) != null && a.status >= 0)
428	a.recordExceptionalCompletion(ex);
429	}
430
431	/**
432	* Implements execution conventions for CountedCompleters
433	*/
434	protected final boolean exec() {
435	compute();
436	return false;
437	}
438
439	/**
440	* Returns the result of the computation. By default
441	* returns {@code null}, which is appropriate for {@code Void}
442	* actions, but in other cases should be overridden.
443	*
444	* @return the result of the computation
445	*/
446	public T getRawResult() { return null; }
447
448	/**
449	* A method that result-bearing CountedCompleters may optionally
450	* use to help maintain result data. By default, does nothing.
451	*/
452	protected void setRawResult(T t) { }
453
454	// Unsafe mechanics
455	private static final sun.misc.Unsafe U;
456	private static final long PENDING;
457	static {
458	try {
459	U = getUnsafe();
460	PENDING = U.objectFieldOffset
461	(CountedCompleter.class.getDeclaredField("pending"));
462	} catch (Exception e) {
463	throw new Error(e);
464	}
465	}
466
467
468	/**
469	* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
470	* Replace with a simple call to Unsafe.getUnsafe when integrating
471	* into a jdk.
472	*
473	* @return a sun.misc.Unsafe
474	*/
475	private static sun.misc.Unsafe getUnsafe() {
476	try {
477	return sun.misc.Unsafe.getUnsafe();
478	} catch (SecurityException se) {
479	try {
480	return java.security.AccessController.doPrivileged
481	(new java.security
482	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
483	public sun.misc.Unsafe run() throws Exception {
484	java.lang.reflect.Field f = sun.misc
485	.Unsafe.class.getDeclaredField("theUnsafe");
486	f.setAccessible(true);
487	return (sun.misc.Unsafe) f.get(null);
488	}});
489	} catch (java.security.PrivilegedActionException e) {
490	throw new RuntimeException("Could not initialize intrinsics",
491	e.getCause());
492	}
493	}
494	}
495
496	}