/*

 * 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 {@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>

 */

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;

    }

    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;

    }

    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);

    }

}