/// @file
/// @brief platform-independent fast "raw" thread-safe event queue interface
/// @ingroup qf
/// @cond
///***************************************************************************
/// Last updated for version 5.8.0
/// Last updated on  2016-11-19
///
///                    Q u a n t u m     L e a P s
///                    ---------------------------
///                    innovating embedded systems
///
/// Copyright (C) Quantum Leaps, LLC. All rights reserved.
///
/// This program is open source software: you can redistribute it and/or
/// modify it under the terms of the GNU General Public License as published
/// by the Free Software Foundation, either version 3 of the License, or
/// (at your option) any later version.
///
/// Alternatively, this program may be distributed and modified under the
/// terms of Quantum Leaps commercial licenses, which expressly supersede
/// the GNU General Public License and are specifically designed for
/// licensees interested in retaining the proprietary status of their code.
///
/// This program 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 for more details.
///
/// You should have received a copy of the GNU General Public License
/// along with this program. If not, see <http://www.gnu.org/licenses/>.
///
/// Contact information:
/// https://state-machine.com
/// mailto:info@state-machine.com
///***************************************************************************
/// @endcond

#ifndef qequeue_h
#define qequeue_h

/// @description
/// This header file must be included in all QF ports that use native QF
/// event queue for active objects. Also, this file needs to be included
/// in the QP/C++ library when the application uses QP::QMActive::defer() /
/// QP::QMActive::recall(). Finally, this file is also needed when the "raw"
/// thread-safe queues are used for communication between active objects
/// and non-framework entities, such as ISRs, device drivers, or legacy
/// code.

#ifndef QF_EQUEUE_CTR_SIZE

    //! The size (in bytes) of the ring-buffer counters used in the
    //! native QF event queue implementation. Valid values: 1, 2, or 4;
    //! default 1.
    /// @description
    /// This macro can be defined in the QF port file (qf_port.h) to
    /// configure the QP::QEQueueCtr type. Here the macro is not defined
    /// so the default of 1 byte is chosen.
    #define QF_EQUEUE_CTR_SIZE 1
#endif


namespace QP {

#if (QF_EQUEUE_CTR_SIZE == 1)
    //! The data type to store the ring-buffer counters based on
    //! the macro #QF_EQUEUE_CTR_SIZE.
    /// @description
    /// The dynamic range of this data type determines the maximum length
    /// of the ring buffer managed by the native QF event queue.
    typedef uint_fast8_t QEQueueCtr;
#elif (QF_EQUEUE_CTR_SIZE == 2)
    typedef uint_fast16_t QEQueueCtr;
#elif (QF_EQUEUE_CTR_SIZE == 4)
    typedef uint_fast32_t QEQueueCtr;
#else
    #error "QF_EQUEUE_CTR_SIZE defined incorrectly, expected 1, 2, or 4"
#endif


//****************************************************************************
//! Native QF Event Queue class
/// @description
/// This structure describes the native QF event queue, which can be used as
/// the event queue for active objects, or as a simple "raw" event queue for
/// thread-safe event passing among non-framework entities, such as ISRs,
/// device drivers, or other third-party components.@n
/// @n
/// The native QF event queue is configured by defining the macro
/// #QF_EQUEUE_TYPE as QP::QEQueue in the specific QF port header file.@n
/// @n
/// The QP::QEQueue class contains only data members for managing an event
/// queue, but does not contain the storage for the queue buffer, which must
/// be provided externally during the queue initialization.@n
/// @n
/// The event queue can store only event pointers, not the whole events. The
/// internal implementation uses the standard ring-buffer plus one external
/// location that optimizes the queue operation for the most frequent case
/// of empty queue.@n
/// @n
/// The QP::QEQueue class is used with two sets of functions. One set is for
/// the active object event queue, which needs to block the active object
/// task when the event queue is empty and unblock it when events are posted
/// to the queue. The interface for the native active object event queue
/// consists of the following functions: QP::QMActive::post(),
/// QP::QMActive::postLIFO(), and QP::QMActive::get_(). Additionally the
/// function QP::QEQueue::init() is used to initialize the queue.@n
/// @n
/// The other set of functions, uses this class as a simple "raw" event
/// queue to pass events between entities other than active objects, such as
/// ISRs. The "raw" event queue is not capable of blocking on the get()
/// operation, but is still thread-safe because it uses QF critical section
/// to protect its integrity. The interface for the "raw" thread-safe queue
/// consists of the following functions: QP::QEQueue::post(),
/// QP::QEQueue::postLIFO(), and QP::QEQueue::get(). Additionally the
/// function QP::QEQueue::init() is used to initialize the queue.@n
/// @n
/// @note Most event queue operations (both the active object queues and
/// the "raw" queues) internally use  the QF critical section. You should be
/// careful not to invoke those operations from other critical sections when
/// nesting of critical sections is not supported.
class QEQueue {
private:

    //! pointer to event at the front of the queue
    /// @description
    /// All incoming and outgoing events pass through the m_frontEvt location.
    /// When the queue is empty (which is most of the time), the extra
    /// m_frontEvt location allows to bypass the ring buffer altogether,
    /// greatly optimizing the performance of the queue. Only bursts of events
    /// engage the ring buffer.@n
    /// @n
    /// The additional role of this attribute is to indicate the empty status
    /// of the queue. The queue is empty if the m_frontEvt location is NULL.
    QEvt const * volatile m_frontEvt;

    //! pointer to the start of the ring buffer
    QEvt const **m_ring;

    //! offset of the end of the ring buffer from the start of the buffer
    QEQueueCtr m_end;

    //! offset to where next event will be inserted into the buffer
    QEQueueCtr volatile m_head;

    //! offset of where next event will be extracted from the buffer
    QEQueueCtr volatile m_tail;

    //! number of free events in the ring buffer
    QEQueueCtr volatile m_nFree;

    //! minimum number of free events ever in the ring buffer.
    /// @note this attribute remembers the low-watermark of the ring buffer,
    /// which provides a valuable information for sizing event queues.
    /// @sa QP::QF::getQueueMin().
    QEQueueCtr m_nMin;

public:
    //! public default constructor
    QEQueue(void);

    //! Initializes the native QF event queue
    /// @description
    /// The parameters are as follows: @p qSto[] is the ring buffer storage,
    /// @p qLen is the length of the ring buffer in the units of event-
    /// pointers.
    ///
    /// @note The actual capacity of the queue is qLen + 1, because of the
    /// extra location fornEvt_.
    void init(QEvt const *qSto[], uint_fast16_t const qLen);

    //! "raw" thread-safe QF event queue implementation for the event
    //! posting (FIFO). You can call this function from any task context or
    //! ISR context. This function uses internally a critical section.
    /// @description
    /// The argument @p margin specifies the minimum number of free entries
    /// in the queue that must be available for posting to succeed. The
    /// function returns true (success) if the posting succeeded (with the
    /// provided margin) and false (failure) when the posting fails.
    ///
    /// @note
    /// The function raises an assertion if the @p margin is zero and the
    /// queue becomes full and cannot accept the event.
    ///
    /// @sa QP::QEQueue::postLIFO(), QP::QEQueue::get()
    bool post(QEvt const * const e, uint_fast16_t const margin);

    //! "raw" thread-safe QF event queue implementation for the
    //! First-In-First-Out (FIFO) event posting. You can call this function
    //! from any task context or ISR context. Please note that this function
    //! uses internally a critical section.
    /// @note The function raises an assertion if the native QF queue becomes
    /// full and cannot accept the event.
    ///
    /// @sa QP::QEQueue::postLIFO(), QP::QEQueue::get()
    void postLIFO(QEvt const * const e);

    //! "raw" thread-safe QF event queue implementation for the
    //! Last-In-First-Out (LIFO) event posting.
    /// @note
    /// The LIFO policy should be used only with great caution because it
    /// alters order of events in the queue.
    /// @note
    /// The function raises an assertion if the native QF queue becomes
    /// full and cannot accept the event. You can call this function from
    /// any task context or ISR context. Please note that this function uses
    /// internally a critical section.
    ///
    /// @sa QP::QEQueue::post(), QP::QEQueue::postLIFO(), QP::QEQueue::get()
    QEvt const *get(void);

    //! "raw" thread-safe QF event queue operation for obtaining the number
    //! of free entries still available in the queue.
    /// @note
    /// This operation needs to be used with caution because the
    /// number of free entries can change unexpectedly. The main intent for
    /// using this operation is in conjunction with event deferral. In this
    /// case the queue is accessed only from a single thread (by a single AO),
    /// so the number of free entries cannot change unexpectedly.
    ///
    /// @sa QP::QMActive::defer(), QP::QMActive::recall()
    QEQueueCtr getNFree(void) const {
        return m_nFree;
    }

    //! "raw" thread-safe QF event queue operation to find out if the queue
    //! is empty
    /// @note
    /// This operation needs to be used with caution because the
    /// queue status can change unexpectedly. The main intent for using
    /// this operation is in conjunction with event deferral. In this case
    /// the queue is accessed only from a single thread (by a single AO),
    /// so no other entity can post events to the queue.
    ///
    /// @sa QP::QMActive::defer(), QP::QMActive::recall()
    bool isEmpty(void) const {
        return m_frontEvt == static_cast<QEvt const *>(0);
    }

private:
    //! disallow copying of QEQueue
    QEQueue(QEQueue const &);

    //! disallow assignment of QEQueue
    QEQueue & operator=(QEQueue const &);

    friend class QF;
    friend class QActive;
    friend class QXThread;
    friend class QTicker;
};

} // namespace QP

#endif // qequeue_h

