/*

 * 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;

    }

}