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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package jsr166e; |
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|
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/** |
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* A recursive resultless {@link ForkJoinTask}. This class |
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* establishes conventions to parameterize resultless actions as |
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* {@code Void} {@code ForkJoinTask}s. Because {@code null} is the |
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* only valid value of type {@code Void}, methods such as {@code join} |
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* always return {@code null} upon completion. |
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* |
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* <p><b>Sample Usages.</b> Here is a simple but complete ForkJoin |
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* sort that sorts a given {@code long[]} array: |
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* |
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* <pre> {@code |
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* static class SortTask extends RecursiveAction { |
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* final long[] array; final int lo, hi; |
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* SortTask(long[] array, int lo, int hi) { |
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* this.array = array; this.lo = lo; this.hi = hi; |
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* } |
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* SortTask(long[] array) { this(array, 0, array.length); } |
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* protected void compute() { |
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* if (hi - lo < THRESHOLD) |
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* sortSequentially(lo, hi); |
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* else { |
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* int mid = (lo + hi) >>> 1; |
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* invokeAll(new SortTask(array, lo, mid), |
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* new SortTask(array, mid, hi)); |
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* merge(lo, mid, hi); |
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* } |
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* } |
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* // implementation details follow: |
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* final static int THRESHOLD = 1000; |
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* void sortSequentially(int lo, int hi) { |
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* Arrays.sort(array, lo, hi); |
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* } |
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* void merge(int lo, int mid, int hi) { |
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* long[] buf = Arrays.copyOfRange(array, lo, mid); |
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* for (int i = 0, j = lo, k = mid; i < buf.length; j++) |
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* array[j] = (k == hi || buf[i] < array[k]) ? |
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* buf[i++] : array[k++]; |
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* } |
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* }}</pre> |
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* |
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* You could then sort {@code anArray} by creating {@code new |
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* SortTask(anArray)} and invoking it in a ForkJoinPool. As a more |
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* concrete simple example, the following task increments each element |
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* of an array: |
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* <pre> {@code |
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* class IncrementTask extends RecursiveAction { |
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* final long[] array; final int lo, hi; |
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* IncrementTask(long[] array, int lo, int hi) { |
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* this.array = array; this.lo = lo; this.hi = hi; |
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* } |
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* protected void compute() { |
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* if (hi - lo < THRESHOLD) { |
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* for (int i = lo; i < hi; ++i) |
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* array[i]++; |
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* } |
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* else { |
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* int mid = (lo + hi) >>> 1; |
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* invokeAll(new IncrementTask(array, lo, mid), |
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* new IncrementTask(array, mid, hi)); |
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* } |
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* } |
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* }}</pre> |
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* |
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* <p>The following example illustrates some refinements and idioms |
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* that may lead to better performance: RecursiveActions need not be |
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* fully recursive, so long as they maintain the basic |
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* divide-and-conquer approach. Here is a class that sums the squares |
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* of each element of a double array, by subdividing out only the |
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* right-hand-sides of repeated divisions by two, and keeping track of |
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* them with a chain of {@code next} references. It uses a dynamic |
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* threshold based on method {@code getSurplusQueuedTaskCount}, but |
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* counterbalances potential excess partitioning by directly |
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* performing leaf actions on unstolen tasks rather than further |
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* subdividing. |
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* |
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* <pre> {@code |
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* double sumOfSquares(ForkJoinPool pool, double[] array) { |
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* int n = array.length; |
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* Applyer a = new Applyer(array, 0, n, null); |
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* pool.invoke(a); |
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* return a.result; |
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* } |
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* |
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* class Applyer extends RecursiveAction { |
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* final double[] array; |
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* final int lo, hi; |
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* double result; |
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* Applyer next; // keeps track of right-hand-side tasks |
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* Applyer(double[] array, int lo, int hi, Applyer next) { |
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* this.array = array; this.lo = lo; this.hi = hi; |
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* this.next = next; |
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* } |
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* |
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* double atLeaf(int l, int h) { |
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* double sum = 0; |
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* for (int i = l; i < h; ++i) // perform leftmost base step |
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* sum += array[i] * array[i]; |
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* return sum; |
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* } |
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* |
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* protected void compute() { |
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* int l = lo; |
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* int h = hi; |
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* Applyer right = null; |
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* while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) { |
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* int mid = (l + h) >>> 1; |
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* right = new Applyer(array, mid, h, right); |
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* right.fork(); |
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* h = mid; |
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* } |
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* double sum = atLeaf(l, h); |
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* while (right != null) { |
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* if (right.tryUnfork()) // directly calculate if not stolen |
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* sum += right.atLeaf(right.lo, right.hi); |
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* else { |
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* right.join(); |
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* sum += right.result; |
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* } |
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* right = right.next; |
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* } |
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* result = sum; |
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* } |
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* }}</pre> |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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*/ |
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public abstract class RecursiveAction extends ForkJoinTask<Void> { |
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private static final long serialVersionUID = 5232453952276485070L; |
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|
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/** |
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* The main computation performed by this task. |
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*/ |
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protected abstract void compute(); |
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|
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/** |
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* Always returns {@code null}. |
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* |
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* @return {@code null} always |
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*/ |
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public final Void getRawResult() { return null; } |
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|
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/** |
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* Requires null completion value. |
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*/ |
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protected final void setRawResult(Void mustBeNull) { } |
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|
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/** |
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* Implements execution conventions for RecursiveActions. |
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*/ |
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protected final boolean exec() { |
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compute(); |
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return true; |
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} |
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|
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} |