/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.
 */
#ifndef _DECAF_UTIL_CONCURRENT_THREADPOOL_H_
#define _DECAF_UTIL_CONCURRENT_THREADPOOL_H_

#include <decaf/lang/Runnable.h>
#include <decaf/util/concurrent/PooledThread.h>
#include <decaf/util/concurrent/PooledThreadListener.h>
#include <decaf/util/concurrent/TaskListener.h>
#include <decaf/util/concurrent/Mutex.h>
#include <decaf/util/Queue.h>
#include <decaf/util/logging/LoggerDefines.h>
#include <decaf/util/Config.h>

#include <vector>

namespace decaf{
namespace util{
namespace concurrent{

    /**
     * Defines a Thread Pool object that implements the functionality
     * of pooling threads to perform user tasks.  The Thread Poll has
     * max size that it will grow to.  The thread pool allocates threads
     * in blocks.  When there are no waiting worker threads and a task
     * is queued then a new batch is allocated.  The user can specify
     * the size of the blocks, otherwise a default value is used.
     * <P>
     * When the user queues a task they must also queue a listner to
     * be notified when the task has completed, this provides the user
     * with a mechanism to know when a task object can be freed.
     * <P>
     * To have the Thread Pool perform a task, the user enqueue's an
     * object that implements the <code>Runnable</code> insterface and
     * one of the worker threads will executing it in its thread context.
     */
    class DECAF_API ThreadPool : public PooledThreadListener
    {
    public:

        // Constants
        static const size_t DEFAULT_MAX_POOL_SIZE  = 10;
        static const size_t DEFAULT_MAX_BLOCK_SIZE = 3;

        // Types
        typedef std::pair<lang::Runnable*, TaskListener*> Task;

    private:

        // Vector of threads that this object has created for its pool.
        std::vector< PooledThread* > pool;

        // Queue of Task that are in need of completion
        util::Queue<Task> queue;

        // Max number of Threads this Pool can contian
        std::size_t maxThreads;

        // Max number of tasks that can be allocated at a time
        std::size_t blockSize;

        // boolean flag use to indocate that this object is shutting down.
        bool shutdown;

        // Count of threads that are currently free to perfom some work.
        std::size_t freeThreads;

        // Mutex for locking operations that affect the pool.
        Mutex poolLock;

        // Logger Init
        LOGDECAF_DECLARE(logger)
        LOGDECAF_DECLARE(marker)

    private:   // Statics

        // The singleton instance of this class
        static ThreadPool instance;

    public:

        ThreadPool();
        virtual ~ThreadPool();

        /**
         * Queue a task to be completed by one of the Pooled Threads.
         * tasks are serviced as soon as a <code>PooledThread</code>
         * is available to run it.
         * @param task object that derives from Runnable
         * @throws ActiveMQException
         */
        virtual void queueTask( Task task )
            throw ( lang::Exception );

        /**
         * DeQueue a task to be completed by one of the Pooled Threads.
         * A caller of this method will block until there is something
         * in the tasks queue, therefore care must be taken when calling
         * this function.  Normally clients of ThreadPool don't use
         * this, only the <code>PooledThread</code> objects owned by
         * this ThreadPool.
         * @return object that derives from Runnable
         * @throws ActiveMQException
         */
        virtual Task deQueueTask()
            throw ( lang::Exception );

        /**
         * Returns the current number of Threads in the Pool, this is
         * how many there are now, not how many are active or the max
         * number that might exist.
         * @return integer number of threads in existance.
         */
        virtual std::size_t getPoolSize() const { return pool.size(); }

        /**
         * Returns the current backlog of items in the tasks queue, this
         * is how much work is still waiting to get done.
         * @return number of outstanding tasks.
         */
        virtual std::size_t getBacklog() const { return queue.size(); }

        /**
         * Ensures that there is at least the specified number of Threads
         * allocated to the pool.  If the size is greater than the MAX
         * number of threads in the pool, then only MAX threads are
         * reservved.  If the size is smaller than the number of threads
         * currently in the pool, than nothing is done.
         * @param size the number of threads to reserve.
         */
        virtual void reserve( std::size_t size );

        /**
         * Get the Max Number of Threads this Pool can contain
         * @return max size
         */
        virtual std::size_t getMaxThreads() const { return maxThreads; }

        /**
         * Sets the Max number of threads this pool can contian.
         * if this value is smaller than the current size of the
         * pool nothing is done.
         * @param maxThreads total number of threads that can be pooled
         */
        virtual void setMaxThreads( std::size_t maxThreads );

        /**
         * Gets the Max number of threads that can be allocated at a time
         * when new threads are needed.
         * @return max Thread Block Size
         */
        virtual std::size_t getBlockSize() const { return blockSize; }

        /**
         * Sets the Max number of Threads that can be allocated at a time
         * when the Thread Pool determines that more Threads are needed.
         * @param blockSize Max Thread Block Size
         */
        virtual void setBlockSize( std::size_t blockSize );

        /**
         * Returns the current number of available threads in the pool, threads
         * that are performing a user task are considered unavailable.  This value
         * could change immeadiately after calling as Threads could finish right
         * after and be available again.  This is informational only.
         * @return totoal free threads
         */
        virtual std::size_t getFreeThreadCount() const {
            return freeThreads;
        }

    public: // PooledThreadListener Callbacks

        /**
         * Called by a pooled thread when it is about to begin
         * executing a new task.  This will decrement the available
         * threads counter so that this object knows when there are
         * no more free threads and must create new ones.
         * @param thread Pointer to the Pooled Thread that is making this call
         */
        virtual void onTaskStarted( PooledThread* thread );

        /**
         * Called by a pooled thread when it has completed a task
         * and is going back to waiting for another task to run,
         * this will increment the free threads counter.
         * @param thread Pointer the the Pooled Thread that is making this call.
         */
        virtual void onTaskCompleted( PooledThread* thread );

        /**
         * Called by a pooled thread when it has encountered an exception
         * while running a user task, after receiving this notification
         * the callee should assume that the PooledThread is now no longer
         * running.
         * @param thread Pointer to the Pooled Thread that is making this call
         * @param ex The Exception that occured.
         */
        virtual void onTaskException( PooledThread* thread,
                                      lang::Exception& ex );

    public:   // Statics

        /**
         * Return the one and only Thread Pool instance.
         * @return The Thread Pool Pointer
         */
        static ThreadPool* getInstance() {
            return &instance;
        }

    private:

        /**
         * Allocates the requested ammount of Threads, won't exceed
         * <code>maxThreads</code>.
         * @param count the number of threads to create
         */
        void AllocateThreads( std::size_t count );

    };

}}}

#endif /*_DECAF_UTIL_CONCURRENT_THREADPOOL_H_*/