/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
 *
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 *
 * 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 {@link CompletionService} that uses a supplied {@link Executor}
 * to execute tasks. This class arranges that submitted tasks are,
 * upon completion, placed on a queue accessible using {@code take}.
 * The class is lightweight enough to be suitable for transient use
 * when processing groups of tasks.
 *
 * <p>
 *
 * <b>Usage Examples.</b>
 *
 * Suppose you have a set of solvers for a certain problem, each
 * returning a value of some type {@code Result}, and would like to
 * run them concurrently, processing the results of each of them that
 * return a non-null value, in some method {@code use(Result r)}. You
 * could write this as:
 *
 * <pre> {@code
 * void solve(Executor e,
 * Collection<Callable<Result>> solvers)
 * throws InterruptedException, ExecutionException {
 * CompletionService<Result> cs
 * = new ExecutorCompletionService<>(e);
 * solvers.forEach(cs::submit);
 * for (int i = solvers.size(); i > 0; i--) {
 * Result r = cs.take().get();
 * if (r != null)
 * use(r);
 * }
 * }}</pre>
 *
 * Suppose instead that you would like to use the first non-null result
 * of the set of tasks, ignoring any that encounter exceptions,
 * and cancelling all other tasks when the first one is ready:
 *
 * <pre> {@code
 * void solve(Executor e,
 * Collection<Callable<Result>> solvers)
 * throws InterruptedException {
 * CompletionService<Result> cs
 * = new ExecutorCompletionService<>(e);
 * int n = solvers.size();
 * List<Future<Result>> futures = new ArrayList<>(n);
 * Result result = null;
 * try {
 * solvers.forEach(solver -> futures.add(cs.submit(solver)));
 * for (int i = n; i > 0; i--) {
 * try {
 * Result r = cs.take().get();
 * if (r != null) {
 * result = r;
 * break;
 * }
 * } catch (ExecutionException ignore) {}
 * }
 * } finally {
 * futures.forEach(future -> future.cancel(true));
 * }
 *
 * if (result != null)
 * use(result);
 * }}</pre>
 *
 * @since 1.5
 */
public class ExecutorCompletionService<V> implements CompletionService<V> {
 private final Executor executor;
 private final AbstractExecutorService aes;
 private final BlockingQueue<Future<V>> completionQueue;

 /**
 * FutureTask extension to enqueue upon completion.
 */
 private static class QueueingFuture<V> extends FutureTask<Void> {
 QueueingFuture(RunnableFuture<V> task,
 BlockingQueue<Future<V>> completionQueue) {
 super(task, null);
 this.task = task;
 this.completionQueue = completionQueue;
 }
 private final Future<V> task;
 private final BlockingQueue<Future<V>> completionQueue;
 protected void done() { completionQueue.add(task); }
 }

 private RunnableFuture<V> newTaskFor(Callable<V> task) {
 if (aes == null)
 return new FutureTask<V>(task);
 else
 return aes.newTaskFor(task);
 }

 private RunnableFuture<V> newTaskFor(Runnable task, V result) {
 if (aes == null)
 return new FutureTask<V>(task, result);
 else
 return aes.newTaskFor(task, result);
 }

 /**
 * Creates an ExecutorCompletionService using the supplied
 * executor for base task execution and a
 * {@link LinkedBlockingQueue} as a completion queue.
 *
 * @param executor the executor to use
 * @throws NullPointerException if executor is {@code null}
 */
 public ExecutorCompletionService(Executor executor) {
 if (executor == null)
 throw new NullPointerException();
 this.executor = executor;
 this.aes = (executor instanceof AbstractExecutorService) ?
 (AbstractExecutorService) executor : null;
 this.completionQueue = new LinkedBlockingQueue<Future<V>>();
 }

 /**
 * Creates an ExecutorCompletionService using the supplied
 * executor for base task execution and the supplied queue as its
 * completion queue.
 *
 * @param executor the executor to use
 * @param completionQueue the queue to use as the completion queue
 * normally one dedicated for use by this service. This
 * queue is treated as unbounded -- failed attempted
 * {@code Queue.add} operations for completed tasks cause
 * them not to be retrievable.
 * @throws NullPointerException if executor or completionQueue are {@code null}
 */
 public ExecutorCompletionService(Executor executor,
 BlockingQueue<Future<V>> completionQueue) {
 if (executor == null || completionQueue == null)
 throw new NullPointerException();
 this.executor = executor;
 this.aes = (executor instanceof AbstractExecutorService) ?
 (AbstractExecutorService) executor : null;
 this.completionQueue = completionQueue;
 }

 /**
 * @throws RejectedExecutionException {@inheritDoc}
 * @throws NullPointerException {@inheritDoc}
 */
 public Future<V> submit(Callable<V> task) {
 if (task == null) throw new NullPointerException();
 RunnableFuture<V> f = newTaskFor(task);
 executor.execute(new QueueingFuture<V>(f, completionQueue));
 return f;
 }

 /**
 * @throws RejectedExecutionException {@inheritDoc}
 * @throws NullPointerException {@inheritDoc}
 */
 public Future<V> submit(Runnable task, V result) {
 if (task == null) throw new NullPointerException();
 RunnableFuture<V> f = newTaskFor(task, result);
 executor.execute(new QueueingFuture<V>(f, completionQueue));
 return f;
 }

 public Future<V> take() throws InterruptedException {
 return completionQueue.take();
 }

 public Future<V> poll() {
 return completionQueue.poll();
 }

 public Future<V> poll(long timeout, TimeUnit unit)
 throws InterruptedException {
 return completionQueue.poll(timeout, unit);
 }

}