/*
* 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 {@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:
* static final 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;
}
}