/*

  * 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

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  */

 /*

  * 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/publicdomain/zero/1.0/

  */

 package java.util.concurrent;

 /**

  * 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 join always

  * return {@code null} upon completion.

  *

  * <p><b>Sample Usages.</b> Here is a sketch of a ForkJoin sort that

  * sorts a given {@code long[]} array:

  *

  *  <pre> {@code

  * class SortTask extends RecursiveAction {

  *   final long[] array; final int lo; final int hi;

  *   SortTask(long[] array, int lo, int hi) {

  *     this.array = array; this.lo = lo; this.hi = hi;

  *   }

  *   protected void compute() {

  *     if (hi - lo < THRESHOLD)

  *       sequentiallySort(array, lo, hi);

  *     else {

  *       int mid = (lo + hi) >>> 1;

  *       invokeAll(new SortTask(array, lo, mid),

  *                 new SortTask(array, mid, hi));

  *       merge(array, lo, hi);

  *     }

  *   }

  * }}</pre>

  *

  * You could then sort {@code anArray} by creating {@code new

  * SortTask(anArray, 0, anArray.length-1) } 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; final int 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;

     }

 }