//------------------------------------------------------------------------------
/*
    This file is part of Beast: https://github.com/vinniefalco/Beast
    Copyright 2013, Vinnie Falco <vinnie.falco@gmail.com>

    Permission to use, copy, modify, and/or distribute this software for any
    purpose  with  or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.

    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
//==============================================================================

#ifndef BEAST_LOCKFREEQUEUE_H_INCLUDED
#define BEAST_LOCKFREEQUEUE_H_INCLUDED

/** Default tag for LockFreeQueue

    @ingroup beast_core intrusive
*/
struct LockFreeQueueDefaultTag { };

/** Multiple Producer, Single Consumer (MPSC) intrusive FIFO.

    This container uses the same intrusive interface as List. It is wait-free
    for producers and lock-free for consumers. The caller is responsible for
    preventing the ABA problem (http://en.wikipedia.org/wiki/ABA_problem)

    Invariants:

    - Any thread may call push_back() at any time (Multiple Producer).

    - Only one thread may call try_pop_front() at a time (Single Consumer)

    - The queue is signaled if there are one or more elements.

    @param Tag  A type name used to distinguish lists and nodes, for
                putting objects in multiple lists. If this parameter is
                omitted, the default tag is used.

    @ingroup beast_core intrusive
*/
template <class Element, class Tag = LockFreeQueueDefaultTag>
class LockFreeQueue
{
public:
    class Node : public Uncopyable
    {
    public:
        Node () { }
        explicit Node (Node* next) : m_next (next) { }

        AtomicPointer <Node> m_next;
    };

public:
    /** Create an empty list.
    */
    LockFreeQueue ()
        : m_head (&m_null)
        , m_tail (&m_null)
        , m_null (nullptr)
    {
    }

    /** Determine if the list is empty.

        This is not thread safe, the caller must synchronize.

        @return true if the list is empty.
    */
    bool empty () const
    {
        return (m_head.get () == m_tail);
    }

    /** Put an element into the list.

        This operation is wait-free.

        @param node The element to enqueue.

        @return true if the list was previously empty.
    */
    bool push_back (Node* node)
    {
        node->m_next.set (0);

        Node* prev = m_head.exchange (node);

        // (*) If a try_pop_front() happens at this point, it might not see the
        //     element we are pushing. This only happens when the list is empty,
        //     and furthermore it is detectable.

        prev->m_next.set (node);

        return prev == &m_null;
    }

    /** Retrieve an element from the list.

        This operation is lock-free.

        @return The element, or nullptr if the list was empty.
    */
    Element* pop_front ()
    {
        Element* elem;

        // Avoid the SpinDelay ctor if possible
        if (!try_pop_front (&elem))
        {
            SpinDelay delay;

            do
            {
                delay.pause ();
            }
            while (!try_pop_front (&elem));
        }

        return elem;
    }

    /** Attempt to retrieve an element.

        This attempts to pop an element from the front of the list. The return
        value indicates if the operation was successful. An operation is
        successful if there is no contention for the list. On a successful
        operation, an element is returned if the list was non empty, else nullptr
        is returned. On failure, the returned element is undefined.

        This operation is wait-free.

        @param[out] pElem The element that was retrieved, or nullptr if the
                          list was empty.

        @return           true if the list was uncontended.
    */
    bool try_pop_front (Element** pElem)
    {
        Node* tail = m_tail;
        Node* next = tail->m_next.get ();

        if (tail == &m_null)
        {
            if (next == 0)
            {
                // (*) If a push_back() happens at this point,
                //     we might not see the element.

                if (m_head.get () == tail)
                {
                    *pElem = nullptr;
                    return true; // success, but queue empty
                }
                else
                {
                    return false; // failure: a push_back() caused contention
                }
            }

            m_tail = next;
            tail = next;
            next = next->m_next.get ();
        }

        if (next)
        {
            m_tail = next;
            *pElem = static_cast <Element*> (tail);
            return true;
        }

        Node* head = m_head.get ();

        if (tail == head)
        {
            push_back (&m_null);
            next = tail->m_next.get ();

            if (next)
            {
                m_tail = next;
                *pElem = static_cast <Element*> (tail);
                return true;
            }
        }

        // (*) If a push_back() happens at this point,
        //     we might not see the element.

        if (head == m_tail)
        {
            *pElem = nullptr;
            return true; // success, but queue empty
        }
        else
        {
            return false; // failure: a push_back() caused contention
        }
    }

private:
    // Elements are pushed on to the head and popped from the tail.
    AtomicPointer <Node> m_head;
    Node* m_tail;
    Node m_null;
};

#endif