// Catalano Core Library
// The Catalano Framework
//
// Copyright 2015 Diego Catalano
// diego.catalano at live.com
//
// Copyright 2015 Haifeng Li
// haifeng.hli at gmail.com
//
// Based on Smile (Statistical Machine Intelligence & Learning Engine)
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//

package Catalano.Core.Concurrent;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadPoolExecutor;

/**
 * Utility class to run tasks in a thread pool on multi-core systems.
 * 
 * @author Haifeng Li
 */
public class MulticoreExecutor {

    /**
     * The number of processors.
     */
    private static final int nprocs;
    /**
     * Thread pool.
     */
    private static ThreadPoolExecutor threads = null;

    static {
        nprocs = Runtime.getRuntime().availableProcessors();
        
        if (nprocs > 1) {
            threads = (ThreadPoolExecutor) Executors.newFixedThreadPool(nprocs, new SimpleDeamonThreadFactory());
        }
    }

    /**
     * Returns the number of threads in the thread pool. 0 and 1 mean no thread pool.
     * @return the number of threads in the thread pool
     */
    public static int getThreadPoolSize() {
        return nprocs;
    }
    
    /**
     * Executes the given tasks serially or parallel depending on the number
     * of cores of the system. Returns a list of result objects of each task.
     * The results of this method are undefined if the given collection is
     * modified while this operation is in progress.
     * @param tasks the collection of tasks.
     * @return a list of result objects in the same sequential order as
     * produced by the iterator for the given task list.
     * @throws Exception if unable to compute a result.
     */
    public static  List run(Collection> tasks) throws Exception {
        List results = new ArrayList();
        if (threads == null) {
            for (Callable task : tasks) {
                results.add(task.call());
            }
        } else {
            if (threads.getActiveCount() < nprocs) {
                List> futures = threads.invokeAll(tasks);
                for (Future future : futures) {
                    results.add(future.get());
                }
            } else {
                // Thread pool is busy. Just run in the caller's thread.
                for (Callable task : tasks) {
                    results.add(task.call());
                }
            }
        }
        
        return results;
    }
    
    /**
     * Shutdown the thread pool.
     */
    public static void shutdown() {
        if (threads != null) {
            threads.shutdown();
        }
    }
}