src/jsr166y/RecursiveAction.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 recursive resultless {@link ForkJoinTask}.  This class
 * establishes conventions to parameterize resultless actions as
 * {@code Void} {@code ForkJoinTask}s. Because {@code null} is the
 * only valid value of type {@code Void}, methods such as {@code join}
 * always return {@code null} upon completion.
 *
 * <p><b>Sample Usages.</b> Here is a simple but complete ForkJoin
 * sort that sorts a given {@code long[]} array:
 *
 *  <pre> {@code
 * static class SortTask extends RecursiveAction {
 *   final long[] array; final int lo, hi;
 *   SortTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   SortTask(long[] array) { this(array, 0, array.length); }
 *   protected void compute() {
 *     if (hi - lo < THRESHOLD)
 *       sortSequentially(lo, hi);
 *     else {
 *       int mid = (lo + hi) >>> 1;
 *       invokeAll(new SortTask(array, lo, mid),
 *                 new SortTask(array, mid, hi));
 *       merge(lo, mid, hi);
 *     }
 *   }
 *   // implementation details follow:
 *   final static int THRESHOLD = 1000;
 *   void sortSequentially(int lo, int hi) {
 *     Arrays.sort(array, lo, hi);
 *   }
 *   void merge(int lo, int mid, int hi) {
 *     long[] buf = Arrays.copyOfRange(array, lo, mid);
 *     for (int i = 0, j = lo, k = mid; i < buf.length; j++)
 *       array[j] = (k == hi || buf[i] < array[k]) ?
 *         buf[i++] : array[k++];
 *   }
 * }}</pre>
 *
 * You could then sort {@code anArray} by creating {@code new
 * SortTask(anArray)} and invoking it in a ForkJoinPool.  As a more
 * concrete simple example, the following task increments each element
 * of an array:
 *  <pre> {@code
 * class IncrementTask extends RecursiveAction {
 *   final long[] array; final int lo, hi;
 *   IncrementTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   protected void compute() {
 *     if (hi - lo < THRESHOLD) {
 *       for (int i = lo; i < hi; ++i)
 *         array[i]++;
 *     }
 *     else {
 *       int mid = (lo + hi) >>> 1;
 *       invokeAll(new IncrementTask(array, lo, mid),
 *                 new IncrementTask(array, mid, hi));
 *     }
 *   }
 * }}</pre>
 *
 * <p>The following example illustrates some refinements and idioms
 * that may lead to better performance: RecursiveActions need not be
 * fully recursive, so long as they maintain the basic
 * divide-and-conquer approach. Here is a class that sums the squares
 * of each element of a double array, by subdividing out only the
 * right-hand-sides of repeated divisions by two, and keeping track of
 * them with a chain of {@code next} references. It uses a dynamic
 * threshold based on method {@code getSurplusQueuedTaskCount}, but
 * counterbalances potential excess partitioning by directly
 * performing leaf actions on unstolen tasks rather than further
 * subdividing.
 *
 *  <pre> {@code
 * double sumOfSquares(ForkJoinPool pool, double[] array) {
 *   int n = array.length;
 *   Applyer a = new Applyer(array, 0, n, null);
 *   pool.invoke(a);
 *   return a.result;
 * }
 *
 * class Applyer extends RecursiveAction {
 *   final double[] array;
 *   final int lo, hi;
 *   double result;
 *   Applyer next; // keeps track of right-hand-side tasks
 *   Applyer(double[] array, int lo, int hi, Applyer next) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *     this.next = next;
 *   }
 *
 *   double atLeaf(int l, int h) {
 *     double sum = 0;
 *     for (int i = l; i < h; ++i) // perform leftmost base step
 *       sum += array[i] * array[i];
 *     return sum;
 *   }
 *
 *   protected void compute() {
 *     int l = lo;
 *     int h = hi;
 *     Applyer right = null;
 *     while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) {
 *        int mid = (l + h) >>> 1;
 *        right = new Applyer(array, mid, h, right);
 *        right.fork();
 *        h = mid;
 *     }
 *     double sum = atLeaf(l, h);
 *     while (right != null) {
 *        if (right.tryUnfork()) // directly calculate if not stolen
 *          sum += right.atLeaf(right.lo, right.hi);
 *       else {
 *          right.join();
 *          sum += right.result;
 *        }
 *        right = right.next;
 *      }
 *     result = sum;
 *   }
 * }}</pre>
 *
 * @since 1.7
 * @author Doug Lea
 */
public abstract class RecursiveAction extends ForkJoinTask<Void> {
    private static final long serialVersionUID = 5232453952276485070L;

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

    /**
     * Always returns {@code null}.
     *
     * @return {@code null} always
     */
    public final Void getRawResult() { return null; }

    /**
     * Requires null completion value.
     */
    protected final void setRawResult(Void mustBeNull) { }

    /**
     * Implements execution conventions for RecursiveActions.
     */
    protected final boolean exec() {
        compute();
        return true;
    }

}
Revision:	1.19
Committed:	Mon Jun 27 02:47:32 2011 UTC (12 years, 10 months ago) by jsr166
Branch:	MAIN
Changes since 1.18:	+20 -8 lines
Log Message:	even more SortTask improvements
#	User	Rev	Content
1	dl	1.1	/*
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	jsr166	1.16	* http://creativecommons.org/publicdomain/zero/1.0/
5	dl	1.1	*/
6
7			package jsr166y;
8
9			/**
10	jsr166	1.10	* A recursive resultless {@link ForkJoinTask}. This class
11			* establishes conventions to parameterize resultless actions as
12			* {@code Void} {@code ForkJoinTask}s. Because {@code null} is the
13	jsr166	1.17	* only valid value of type {@code Void}, methods such as {@code join}
14			* always return {@code null} upon completion.
15	jsr166	1.2	*
16	jsr166	1.19	* <p><b>Sample Usages.</b> Here is a simple but complete ForkJoin
17			* sort that sorts a given {@code long[]} array:
18	dl	1.1	*
19	jsr166	1.4	* <pre> {@code
20	jsr166	1.19	* static class SortTask extends RecursiveAction {
21	jsr166	1.17	* final long[] array; final int lo, hi;
22	dl	1.1	* SortTask(long[] array, int lo, int hi) {
23			* this.array = array; this.lo = lo; this.hi = hi;
24			* }
25	jsr166	1.19	* SortTask(long[] array) { this(array, 0, array.length); }
26	dl	1.1	* protected void compute() {
27	jsr166	1.4	* if (hi - lo < THRESHOLD)
28	jsr166	1.19	* sortSequentially(lo, hi);
29	dl	1.1	* else {
30	jsr166	1.4	* int mid = (lo + hi) >>> 1;
31	dl	1.1	* invokeAll(new SortTask(array, lo, mid),
32			* new SortTask(array, mid, hi));
33	jsr166	1.19	* merge(lo, mid, hi);
34	dl	1.1	* }
35			* }
36	jsr166	1.19	* // implementation details follow:
37			* final static int THRESHOLD = 1000;
38			* void sortSequentially(int lo, int hi) {
39			* Arrays.sort(array, lo, hi);
40			* }
41			* void merge(int lo, int mid, int hi) {
42			* long[] buf = Arrays.copyOfRange(array, lo, mid);
43			* for (int i = 0, j = lo, k = mid; i < buf.length; j++)
44			* array[j] = (k == hi \|\| buf[i] < array[k]) ?
45			* buf[i++] : array[k++];
46			* }
47	jsr166	1.4	* }}</pre>
48	dl	1.1	*
49	jsr166	1.7	* You could then sort {@code anArray} by creating {@code new
50	jsr166	1.19	* SortTask(anArray)} and invoking it in a ForkJoinPool. As a more
51			* concrete simple example, the following task increments each element
52			* of an array:
53	jsr166	1.4	* <pre> {@code
54	dl	1.1	* class IncrementTask extends RecursiveAction {
55	jsr166	1.17	* final long[] array; final int lo, hi;
56	dl	1.1	* IncrementTask(long[] array, int lo, int hi) {
57			* this.array = array; this.lo = lo; this.hi = hi;
58			* }
59			* protected void compute() {
60	jsr166	1.4	* if (hi - lo < THRESHOLD) {
61			* for (int i = lo; i < hi; ++i)
62	dl	1.1	* array[i]++;
63			* }
64			* else {
65	jsr166	1.4	* int mid = (lo + hi) >>> 1;
66	dl	1.1	* invokeAll(new IncrementTask(array, lo, mid),
67			* new IncrementTask(array, mid, hi));
68			* }
69			* }
70	jsr166	1.4	* }}</pre>
71	dl	1.1	*
72			* <p>The following example illustrates some refinements and idioms
73			* that may lead to better performance: RecursiveActions need not be
74			* fully recursive, so long as they maintain the basic
75			* divide-and-conquer approach. Here is a class that sums the squares
76			* of each element of a double array, by subdividing out only the
77			* right-hand-sides of repeated divisions by two, and keeping track of
78	jsr166	1.3	* them with a chain of {@code next} references. It uses a dynamic
79	dl	1.12	* threshold based on method {@code getSurplusQueuedTaskCount}, but
80	dl	1.11	* counterbalances potential excess partitioning by directly
81			* performing leaf actions on unstolen tasks rather than further
82			* subdividing.
83	dl	1.1	*
84	jsr166	1.4	* <pre> {@code
85	dl	1.1	* double sumOfSquares(ForkJoinPool pool, double[] array) {
86			* int n = array.length;
87	dl	1.13	* Applyer a = new Applyer(array, 0, n, null);
88	dl	1.1	* pool.invoke(a);
89			* return a.result;
90			* }
91			*
92			* class Applyer extends RecursiveAction {
93			* final double[] array;
94	dl	1.13	* final int lo, hi;
95	dl	1.1	* double result;
96			* Applyer next; // keeps track of right-hand-side tasks
97	dl	1.13	* Applyer(double[] array, int lo, int hi, Applyer next) {
98	dl	1.1	* this.array = array; this.lo = lo; this.hi = hi;
99	dl	1.13	* this.next = next;
100	dl	1.1	* }
101			*
102	dl	1.13	* double atLeaf(int l, int h) {
103	dl	1.1	* double sum = 0;
104	jsr166	1.4	* for (int i = l; i < h; ++i) // perform leftmost base step
105	dl	1.1	* sum += array[i] * array[i];
106			* return sum;
107			* }
108			*
109			* protected void compute() {
110			* int l = lo;
111			* int h = hi;
112			* Applyer right = null;
113	dl	1.8	* while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) {
114	jsr166	1.4	* int mid = (l + h) >>> 1;
115	dl	1.13	* right = new Applyer(array, mid, h, right);
116	dl	1.1	* right.fork();
117			* h = mid;
118			* }
119			* double sum = atLeaf(l, h);
120			* while (right != null) {
121			* if (right.tryUnfork()) // directly calculate if not stolen
122			* sum += right.atLeaf(right.lo, right.hi);
123			* else {
124	dl	1.14	* right.join();
125	dl	1.1	* sum += right.result;
126			* }
127			* right = right.next;
128			* }
129			* result = sum;
130			* }
131	jsr166	1.4	* }}</pre>
132	jsr166	1.6	*
133			* @since 1.7
134			* @author Doug Lea
135	dl	1.1	*/
136			public abstract class RecursiveAction extends ForkJoinTask<Void> {
137	dl	1.9	private static final long serialVersionUID = 5232453952276485070L;
138	dl	1.1
139			/**
140	jsr166	1.2	* The main computation performed by this task.
141	dl	1.1	*/
142			protected abstract void compute();
143
144			/**
145	jsr166	1.15	* Always returns {@code null}.
146			*
147			* @return {@code null} always
148	dl	1.1	*/
149			public final Void getRawResult() { return null; }
150
151			/**
152			* Requires null completion value.
153			*/
154			protected final void setRawResult(Void mustBeNull) { }
155
156			/**
157	jsr166	1.5	* Implements execution conventions for RecursiveActions.
158	dl	1.1	*/
159			protected final boolean exec() {
160			compute();
161			return true;
162			}
163
164			}