    // Copyright (C) 2006  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_PIPE_KERNEl_1_
    #define DLIB_PIPE_KERNEl_1_

    #include "../algs.h"
    #include "../threads.h"
    #include "pipe_kernel_abstract.h"

    namespace dlib
    {

        template <
            typename T
            >
        class pipe
        {
            /*!
                INITIAL VALUE
                    - pipe_size == 0
                    - pipe_max_size == defined by constructor
                    - enabled == true
                    - data == a pointer to an array of ((pipe_max_size>0)?pipe_max_size:1) T objects.
                    - dequeue_waiters == 0
                    - enqueue_waiters == 0
                    - first == 1
                    - last == 1
                    - unblock_sig_waiters == 0

                CONVENTION
                    - size() == pipe_size
                    - max_size() == pipe_max_size
                    - is_enabled() == enabled

                    - m == the mutex used to lock access to all the members of this class

                    - dequeue_waiters == the number of threads blocked on calls to dequeue()
                    - enqueue_waiters == the number of threads blocked on calls to enqueue() and
                      wait_until_empty()
                    - unblock_sig_waiters == the number of threads blocked on calls to
                      wait_for_num_blocked_dequeues() and the destructor.  (i.e. the number of
                      blocking calls to unblock_sig.wait())

                    - dequeue_sig == the signaler that threads blocked on calls to dequeue() wait on
                    - enqueue_sig == the signaler that threads blocked on calls to enqueue()
                      or wait_until_empty() wait on.
                    - unblock_sig == the signaler that is signaled when a thread stops blocking on a call
                      to enqueue() or dequeue().  It is also signaled when a dequeue that will probably
                      block is called.  The destructor and wait_for_num_blocked_dequeues are the only
                      things that will wait on this signaler.

                    - if (pipe_size > 0) then
                        - data[first] == the next item to dequeue
                        - data[last] == the item most recently added via enqueue, so the last to dequeue.
                    - else if (pipe_max_size == 0)
                        - if (first == 0 && last == 0) then
                            - data[0] == the next item to dequeue
                        - else if (first == 0 && last == 1) then
                            - data[0] has been taken out already by a dequeue
            !*/

        public:
            // this is here for backwards compatibility with older versions of dlib.
            typedef pipe kernel_1a;

            typedef T type;

            explicit pipe (
                unsigned long maximum_size
            );

            virtual ~pipe (
            );

            void empty (
            );

            void wait_until_empty (
            ) const;

            void wait_for_num_blocked_dequeues (
                unsigned long num
            )const;

            void enable (
            );

            void disable (
            );

            bool is_enqueue_enabled (
            ) const;

            void disable_enqueue (
            );

            void enable_enqueue (
            );

            bool is_dequeue_enabled (
            ) const;

            void disable_dequeue (
            );

            void enable_dequeue (
            );

            bool is_enabled (
            ) const;

            unsigned long max_size (
            ) const;

            unsigned long size (
            ) const;

            bool enqueue (
                T& item
            );

            bool dequeue (
                T& item
            );

            bool enqueue_or_timeout (
                T& item,
                unsigned long timeout
            );

            bool dequeue_or_timeout (
                T& item,
                unsigned long timeout
            );

        private:

            unsigned long pipe_size;
            const unsigned long pipe_max_size;
            bool enabled;

            T* const data;

            unsigned long first;
            unsigned long last;

            mutex m;
            signaler dequeue_sig;
            signaler enqueue_sig;
            signaler unblock_sig;

            unsigned long dequeue_waiters;
            mutable unsigned long enqueue_waiters;
            mutable unsigned long unblock_sig_waiters;
            bool enqueue_enabled;
            bool dequeue_enabled;

            // restricted functions
            pipe(const pipe&);        // copy constructor
            pipe& operator=(const pipe&);    // assignment operator

        };

    // ----------------------------------------------------------------------------------------
    // ----------------------------------------------------------------------------------------
    //                      member function definitions
    // ----------------------------------------------------------------------------------------
    // ----------------------------------------------------------------------------------------

        template <
            typename T
            >
        pipe<T>::
        pipe (
            unsigned long maximum_size
        ) :
            pipe_size(0),
            pipe_max_size(maximum_size),
            enabled(true),
            data(new T[(maximum_size>0) ? maximum_size : 1]),
            first(1),
            last(1),
            dequeue_sig(m),
            enqueue_sig(m),
            unblock_sig(m),
            dequeue_waiters(0),
            enqueue_waiters(0),
            unblock_sig_waiters(0),
            enqueue_enabled(true),
            dequeue_enabled(true)
        {
        }

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

        template <
            typename T
            >
        pipe<T>::
        ~pipe (
        )
        {
            auto_mutex M(m);
            ++unblock_sig_waiters;

            // first make sure no one is blocked on any calls to enqueue() or dequeue()
            enabled = false;
            dequeue_sig.broadcast();
            enqueue_sig.broadcast();
            unblock_sig.broadcast();

            // wait for all threads to unblock
            while (dequeue_waiters > 0 || enqueue_waiters > 0 || unblock_sig_waiters > 1)
                unblock_sig.wait();

            delete [] data;
            --unblock_sig_waiters;
        }

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

        template <
            typename T
            >
        void pipe<T>::
        empty (
        )
        {
            auto_mutex M(m);
            pipe_size = 0;

            // let any calls to enqueue() know that the pipe is now empty
            if (enqueue_waiters > 0)
                enqueue_sig.broadcast();
        }

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

        template <
            typename T
            >
        void pipe<T>::
        wait_until_empty (
        ) const
        {
            auto_mutex M(m);
            // this function is sort of like a call to enqueue so treat it like that
            ++enqueue_waiters;

            while (pipe_size > 0 && enabled && dequeue_enabled )
                enqueue_sig.wait();

            // let the destructor know we are ending if it is blocked waiting
            if (enabled == false)
                unblock_sig.broadcast();

            --enqueue_waiters;
        }

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

        template <
            typename T
            >
        void pipe<T>::
        enable (
        )
        {
            auto_mutex M(m);
            enabled = true;
        }

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

        template <
            typename T
            >
        void pipe<T>::
        disable (
        )
        {
            auto_mutex M(m);
            enabled = false;
            dequeue_sig.broadcast();
            enqueue_sig.broadcast();
            unblock_sig.broadcast();
        }

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

        template <
            typename T
            >
        bool pipe<T>::
        is_enabled (
        ) const
        {
            auto_mutex M(m);
            return enabled;
        }

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

        template <
            typename T
            >
        unsigned long pipe<T>::
        max_size (
        ) const
        {
            auto_mutex M(m);
            return pipe_max_size;
        }

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

        template <
            typename T
            >
        unsigned long pipe<T>::
        size (
        ) const
        {
            auto_mutex M(m);
            return pipe_size;
        }

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

        template <
            typename T
            >
        bool pipe<T>::
        enqueue (
            T& item
        )
        {
            auto_mutex M(m);
            ++enqueue_waiters;

            // wait until there is room or we are disabled
            while (pipe_size == pipe_max_size && enabled && enqueue_enabled &&
                   !(pipe_max_size == 0 && first == 1) )
                enqueue_sig.wait();

            if (enabled == false || enqueue_enabled == false)
            {
                --enqueue_waiters;
                // let the destructor know we are unblocking
                unblock_sig.broadcast();
                return false;
            }

            // set the appropriate values for first and last
            if (pipe_size == 0)
            {
                first = 0;
                last = 0;
            }
            else
            {
                last = (last+1)%pipe_max_size;
            }

            exchange(item,data[last]);

            // wake up a call to dequeue() if there are any currently blocked
            if (dequeue_waiters > 0)
                dequeue_sig.signal();

            if (pipe_max_size > 0)
            {
                ++pipe_size;
            }
            else
            {
                // wait for a dequeue to take the item out
                while (last == 0 && enabled && enqueue_enabled)
                    enqueue_sig.wait();

                if (last == 0 && (enabled == false || enqueue_enabled == false))
                {
                    last = 1;
                    first = 1;

                    // no one dequeued this object to put it back into item
                    exchange(item,data[0]);

                    --enqueue_waiters;
                    // let the destructor know we are unblocking
                    if (unblock_sig_waiters > 0)
                        unblock_sig.broadcast();
                    return false;
                }

                last = 1;
                first = 1;

                // tell any waiting calls to enqueue() that one of them can proceed
                if (enqueue_waiters > 1)
                    enqueue_sig.broadcast();

                // let the destructor know we are unblocking
                if (enabled == false && unblock_sig_waiters > 0)
                    unblock_sig.broadcast();
            }

            --enqueue_waiters;
            return true;
        }

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

        template <
            typename T
            >
        bool pipe<T>::
        dequeue (
            T& item
        )
        {
            auto_mutex M(m);
            ++dequeue_waiters;

            if (pipe_size == 0)
            {
                // notify wait_for_num_blocked_dequeues()
                if (unblock_sig_waiters > 0)
                    unblock_sig.broadcast();

                // notify any blocked enqueue_or_timeout() calls
                if (enqueue_waiters > 0)
                    enqueue_sig.broadcast();
            }

            // wait until there is something in the pipe or we are disabled
            while (pipe_size == 0 && enabled && dequeue_enabled &&
                   !(pipe_max_size == 0 && first == 0 && last == 0) )
                dequeue_sig.wait();

            if (enabled == false || dequeue_enabled == false)
            {
                --dequeue_waiters;
                // let the destructor know we are unblocking
                unblock_sig.broadcast();
                return false;
            }

            exchange(item,data[first]);

            if (pipe_max_size > 0)
            {
                // set the appropriate values for first
                first = (first+1)%pipe_max_size;

                --pipe_size;
            }
            else
            {
                // let the enqueue waiting on us know that we took the
                // item out already.
                last = 1;
            }

            // wake up a call to enqueue() if there are any currently blocked
            if (enqueue_waiters > 0)
                enqueue_sig.broadcast();

            --dequeue_waiters;
            return true;
        }

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

        template <
            typename T
            >
        bool pipe<T>::
        enqueue_or_timeout (
            T& item,
            unsigned long timeout
        )
        {
            auto_mutex M(m);
            ++enqueue_waiters;

            // wait until there is room or we are disabled or
            // we run out of time.
            bool timed_out = false;
            while (pipe_size == pipe_max_size && enabled && enqueue_enabled &&
                   !(pipe_max_size == 0 && dequeue_waiters > 0 && first == 1) )
            {
                if (timeout == 0 || enqueue_sig.wait_or_timeout(timeout) == false)
                {
                    timed_out = true;
                    break;
                }
            }

            if (enabled == false || timed_out || enqueue_enabled == false)
            {
                --enqueue_waiters;
                // let the destructor know we are unblocking
                unblock_sig.broadcast();
                return false;
            }

            // set the appropriate values for first and last
            if (pipe_size == 0)
            {
                first = 0;
                last = 0;
            }
            else
            {
                last = (last+1)%pipe_max_size;
            }

            exchange(item,data[last]);

            // wake up a call to dequeue() if there are any currently blocked
            if (dequeue_waiters > 0)
                dequeue_sig.signal();

            if (pipe_max_size > 0)
            {
                ++pipe_size;
            }
            else
            {
                // wait for a dequeue to take the item out
                while (last == 0 && enabled && enqueue_enabled)
                    enqueue_sig.wait();

                if (last == 0 && (enabled == false || enqueue_enabled == false))
                {
                    last = 1;
                    first = 1;

                    // no one dequeued this object to put it back into item
                    exchange(item,data[0]);

                    --enqueue_waiters;
                    // let the destructor know we are unblocking
                    if (unblock_sig_waiters > 0)
                        unblock_sig.broadcast();
                    return false;
                }

                last = 1;
                first = 1;

                // tell any waiting calls to enqueue() that one of them can proceed
                if (enqueue_waiters > 1)
                    enqueue_sig.broadcast();

                // let the destructor know we are unblocking
                if (enabled == false && unblock_sig_waiters > 0)
                    unblock_sig.broadcast();
            }

            --enqueue_waiters;
            return true;
        }

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

        template <
            typename T
            >
        bool pipe<T>::
        dequeue_or_timeout (
            T& item,
            unsigned long timeout
        )
        {
            auto_mutex M(m);
            ++dequeue_waiters;

            if (pipe_size == 0)
            {
                // notify wait_for_num_blocked_dequeues()
                if (unblock_sig_waiters > 0)
                    unblock_sig.broadcast();

                // notify any blocked enqueue_or_timeout() calls
                if (enqueue_waiters > 0)
                    enqueue_sig.broadcast();
            }

            bool timed_out = false;
            // wait until there is something in the pipe or we are disabled or we timeout.
            while (pipe_size == 0 && enabled && dequeue_enabled &&
                   !(pipe_max_size == 0 && first == 0 && last == 0) )
            {
                if (timeout == 0 || dequeue_sig.wait_or_timeout(timeout) == false)
                {
                    timed_out = true;
                    break;
                }
            }

            if (enabled == false || timed_out || dequeue_enabled == false)
            {
                --dequeue_waiters;
                // let the destructor know we are unblocking
                unblock_sig.broadcast();
                return false;
            }

            exchange(item,data[first]);

            if (pipe_max_size > 0)
            {
                // set the appropriate values for first
                first = (first+1)%pipe_max_size;

                --pipe_size;
            }
            else
            {
                // let the enqueue waiting on us know that we took the
                // item out already.
                last = 1;
            }

            // wake up a call to enqueue() if there are any currently blocked
            if (enqueue_waiters > 0)
                enqueue_sig.broadcast();

            --dequeue_waiters;
            return true;
        }

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

        template <
            typename T
            >
        void pipe<T>::
        wait_for_num_blocked_dequeues (
            unsigned long num
        )const
        {
            auto_mutex M(m);
            ++unblock_sig_waiters;

            while ( (dequeue_waiters < num || pipe_size != 0) && enabled && dequeue_enabled)
                unblock_sig.wait();

            // let the destructor know we are ending if it is blocked waiting
            if (enabled == false)
                unblock_sig.broadcast();

            --unblock_sig_waiters;
        }

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

        template <
            typename T
            >
        bool pipe<T>::
        is_enqueue_enabled (
        ) const
        {
            auto_mutex M(m);
            return enqueue_enabled;
        }

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

        template <
            typename T
            >
        void pipe<T>::
        disable_enqueue (
        )
        {
            auto_mutex M(m);
            enqueue_enabled = false;
            enqueue_sig.broadcast();
        }

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

        template <
            typename T
            >
        void pipe<T>::
        enable_enqueue (
        )
        {
            auto_mutex M(m);
            enqueue_enabled = true;
        }

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

        template <
            typename T
            >
        bool pipe<T>::
        is_dequeue_enabled (
        ) const
        {
            auto_mutex M(m);
            return dequeue_enabled;
        }

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

        template <
            typename T
            >
        void pipe<T>::
        disable_dequeue (
        )
        {
            auto_mutex M(m);
            dequeue_enabled = false;
            dequeue_sig.broadcast();
        }

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

        template <
            typename T
            >
        void pipe<T>::
        enable_dequeue (
        )
        {
            auto_mutex M(m);
            dequeue_enabled = true;
        }

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

    }

    #endif // DLIB_PIPE_KERNEl_1_