    // Copyright (C) 2008  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_THREAD_POOl_CPPh_
    #define DLIB_THREAD_POOl_CPPh_

    #include "thread_pool_extension.h"
    #include <memory>

    namespace dlib
    {

    // ----------------------------------------------------------------------------------------

        thread_pool_implementation::
        thread_pool_implementation (
            unsigned long num_threads
        ) :
            task_done_signaler(m),
            task_ready_signaler(m),
            we_are_destructing(false)
        {
            tasks.resize(num_threads);
            threads.resize(num_threads);
            for (unsigned long i = 0; i < num_threads; ++i)
            {
                threads[i] = std::thread([&](){this->thread();});
            }
        }

    // ----------------------------------------------------------------------------------------

        void thread_pool_implementation::
        shutdown_pool (
        )
        {
            {
                auto_mutex M(m);

                // first wait for all pending tasks to finish
                bool found_task = true;
                while (found_task)
                {
                    found_task = false;
                    for (unsigned long i = 0; i < tasks.size(); ++i)
                    {
                        // If task bucket i has a task that is currently supposed to be processed
                        if (tasks[i].is_empty() == false)
                        {
                            found_task = true;
                            break;
                        }
                    }

                    if (found_task)
                        task_done_signaler.wait();
                }

                // now tell the threads to kill themselves
                we_are_destructing = true;
                task_ready_signaler.broadcast();
            }

            // wait for all threads to terminate
            for (auto& t : threads)
                t.join();
            threads.clear();

            // Throw any unhandled exceptions.  Since shutdown_pool() is only called in the
            // destructor this will kill the program.
            for (auto&& task : tasks)
                task.propagate_exception();
        }

    // ----------------------------------------------------------------------------------------

        thread_pool_implementation::
        ~thread_pool_implementation()
        {
            shutdown_pool();
        }

    // ----------------------------------------------------------------------------------------

        unsigned long thread_pool_implementation::
        num_threads_in_pool (
        ) const
        {
            auto_mutex M(m);
            return tasks.size();
        }

    // ----------------------------------------------------------------------------------------

        void thread_pool_implementation::
        wait_for_task (
            uint64 task_id
        ) const
        {
            auto_mutex M(m);
            if (tasks.size() != 0)
            {
                const unsigned long idx = task_id_to_index(task_id);
                while (tasks[idx].task_id == task_id)
                    task_done_signaler.wait();

                for (auto&& task : tasks)
                    task.propagate_exception();
            }
        }

    // ----------------------------------------------------------------------------------------

        void thread_pool_implementation::
        wait_for_all_tasks (
        ) const
        {
            const thread_id_type thread_id = get_thread_id();

            auto_mutex M(m);
            bool found_task = true;
            while (found_task)
            {
                found_task = false;
                for (unsigned long i = 0; i < tasks.size(); ++i)
                {
                    // If task bucket i has a task that is currently supposed to be processed
                    // and it originated from the calling thread
                    if (tasks[i].is_empty() == false && tasks[i].thread_id == thread_id)
                    {
                        found_task = true;
                        break;
                    }
                }

                if (found_task)
                    task_done_signaler.wait();
            }

            // throw any exceptions generated by the tasks
            for (auto&& task : tasks)
                task.propagate_exception();
        }

    // ----------------------------------------------------------------------------------------

        bool thread_pool_implementation::
        is_worker_thread (
            const thread_id_type id
        ) const
        {
            for (unsigned long i = 0; i < worker_thread_ids.size(); ++i)
            {
                if (worker_thread_ids[i] == id)
                    return true;
            }

            // if there aren't any threads in the pool then we consider all threads
            // to be worker threads
            if (tasks.size() == 0)
                return true;
            else
                return false;
        }

    // ----------------------------------------------------------------------------------------

        void thread_pool_implementation::
        thread (
        )
        {
            {
                // save the id of this worker thread into worker_thread_ids
                auto_mutex M(m);
                thread_id_type id = get_thread_id();
                worker_thread_ids.push_back(id);
            }

            task_state_type task;
            while (we_are_destructing == false)
            {
                long idx = 0;

                // wait for a task to do
                { auto_mutex M(m);
                    while ( (idx = find_ready_task()) == -1 && we_are_destructing == false)
                        task_ready_signaler.wait();

                    if (we_are_destructing)
                        break;

                    tasks[idx].is_being_processed = true;
                    task = tasks[idx];
                }

                std::exception_ptr eptr = nullptr;
                try
                {
                    // now do the task
                    if (task.bfp)
                        task.bfp();
                    else if (task.mfp0)
                        task.mfp0();
                    else if (task.mfp1)
                        task.mfp1(task.arg1);
                    else if (task.mfp2)
                        task.mfp2(task.arg1, task.arg2);
                }
                catch(...)
                {
                    eptr = std::current_exception();
                }

                // Now let others know that we finished the task.  We do this
                // by clearing out the state of this task
                { auto_mutex M(m);
                    tasks[idx].is_being_processed = false;
                    tasks[idx].task_id = 0;
                    tasks[idx].bfp.clear();
                    tasks[idx].mfp0.clear();
                    tasks[idx].mfp1.clear();
                    tasks[idx].mfp2.clear();
                    tasks[idx].arg1 = 0;
                    tasks[idx].arg2 = 0;
                    tasks[idx].eptr = eptr;
                    task_done_signaler.broadcast();
                }

            }
        }

    // ----------------------------------------------------------------------------------------

        long thread_pool_implementation::
        find_empty_task_slot (
        ) const
        {
            for (auto&& task : tasks)
                task.propagate_exception();

            for (unsigned long i = 0; i < tasks.size(); ++i)
            {
                if (tasks[i].is_empty())
                    return i;
            }

            return -1;
        }

    // ----------------------------------------------------------------------------------------

        long thread_pool_implementation::
        find_ready_task (
        ) const
        {
            for (unsigned long i = 0; i < tasks.size(); ++i)
            {
                if (tasks[i].is_ready())
                    return i;
            }

            return -1;
        }

    // ----------------------------------------------------------------------------------------

        uint64 thread_pool_implementation::
        make_next_task_id (
            long idx
        )
        {
            uint64 id = tasks[idx].next_task_id * tasks.size() + idx;
            tasks[idx].next_task_id += 1;
            return id;
        }

    // ----------------------------------------------------------------------------------------

        unsigned long thread_pool_implementation::
        task_id_to_index (
            uint64 id
        ) const
        {
            return static_cast<unsigned long>(id%tasks.size());
        }

    // ----------------------------------------------------------------------------------------

        uint64 thread_pool_implementation::
        add_task_internal (
            const bfp_type& bfp,
            std::shared_ptr<function_object_copy>& item
        )
        {
            auto_mutex M(m);
            const thread_id_type my_thread_id = get_thread_id();

            // find a thread that isn't doing anything
            long idx = find_empty_task_slot();
            if (idx == -1 && is_worker_thread(my_thread_id))
            {
                // this function is being called from within a worker thread and there
                // aren't any other worker threads free so just perform the task right
                // here

                M.unlock();
                bfp();

                // return a task id that is both non-zero and also one
                // that is never normally returned.  This way calls
                // to wait_for_task() will never block given this id.
                return 1;
            }

            // wait until there is a thread that isn't doing anything
            while (idx == -1)
            {
                task_done_signaler.wait();
                idx = find_empty_task_slot();
            }

            tasks[idx].thread_id = my_thread_id;
            tasks[idx].task_id = make_next_task_id(idx);
            tasks[idx].bfp = bfp;
            tasks[idx].function_copy.swap(item);

            task_ready_signaler.signal();

            return tasks[idx].task_id;
        }

    // ----------------------------------------------------------------------------------------

        bool thread_pool_implementation::
        is_task_thread (
        ) const
        {
            auto_mutex M(m);
            return is_worker_thread(get_thread_id());
        }

    // ----------------------------------------------------------------------------------------

    }

    #endif // DLIB_THREAD_POOl_CPPh_