text: #pragma once
  
  #include "../C/Baselib_Memory.h"
  #include "mpsc_node.h"
  
  namespace baselib
  {
      BASELIB_CPP_INTERFACE
      {
          // In computer science, a queue is a collection in which the entities in the collection are kept in order and the principal (or only) operations on the
          // collection are the addition of entities to the rear terminal position, known as enqueue, and removal of entities from the front terminal position, known
          // as dequeue. This makes the queue a First-In-First-Out (FIFO) data structure. In a FIFO data structure, the first element added to the queue will be the
          // first one to be removed. This is equivalent to the requirement that once a new element is added, all elements that were added before have to be removed
          // before the new element can be removed. Often a peek or front operation is also entered, returning the value of the front element without dequeuing it.
          // A queue is an example of a linear data structure, or more abstractly a sequential collection.
          //
          // "Queue (abstract data type)", Wikipedia: The Free Encyclopedia
          // https://en.wikipedia.org/w/index.php?title=Queue_(abstract_data_type)&oldid=878671332
          //
  
          // This implementation is a lockless node queue capable of handling multiple producers and a single consumer (exclusive access)
          //
          // Node types are required to inherit the mpsc_node class. No data from the inherited class is modified/copied, so no restrictions apply.
          // The node memory is allocated and destroyed by the user (user owned).
          // Dequeued nodes may be deleted, overwritten/discarded and/or reused.
          //
          // Notes consumer threads:
          //  Only one consumer thread will exclusively access the front node. Other consumer threads will always progress, either by failing to dequeue or
          //  successfully dequeuing the next node once the current thread thread opens access. As opposed to the parallel consumer implementation,
          //  this is significantly more performant as no DCAS-operations/loops are involved, but if the consumer thread with current exclusive access falls asleep
          //  when dequeuing, no other threads will successfully dequeue until the thread wakes up.
          //  Producer threads always progress independently.
          //
          // Notes on producer threads:
          //  A producer thread swaps the back and writes the link information in two consecutive atomic operations. If a producer thread falls asleep after the
          //  swap and before the link information has been written, the consumer thread(s) will not advance past this point since it doesn't have
          //  the information yet. Therefore the consumer threads calls will yield null until that particular producer thread wakes back up.
          //
          template<typename T>
          class alignas(PLATFORM_CACHE_LINE_SIZE)mpsc_node_queue
          {
          public:
              // Create a new queue instance.
              mpsc_node_queue()
              {
                  m_Front.obj = 0;
                  m_Back.obj = 0;
                  atomic_thread_fence(memory_order_seq_cst);
              }
  
              // Returns true if queue is empty.
              bool empty() const
              {
                  return m_Back.load(memory_order_relaxed) == 0;
              }
  
              // Push a node to the back of the queue.
              void push_back(T* node)
              {
                  node->next.store(0, memory_order_relaxed);
                  if (T* prev = m_Back.exchange(node, memory_order_release))
                      prev->next.store(node, memory_order_release);
                  else
                      m_Front.store(node, memory_order_release);
              }
  
              // Push a linked list of nodes to the back of the queue.
              void push_back(T* first_node, T* last_node)
              {
                  last_node->next.store(0, memory_order_relaxed);
                  if (T* prev = m_Back.exchange(last_node, memory_order_release))
                      prev->next.store(first_node, memory_order_release);
                  else
                      m_Front.store(first_node, memory_order_release);
              }
  
              // Try to pop frontmost node of the queue.
              //
              // Note that if null is returned, there may still be push operations in progress in a producer thread.
              // Use the "empty" function to check if a queue is empty.
              //
              // \returns front node of the queue or null.
              T* try_pop_front()
              {
                  T* node, *next, *expected;
  
                  // acquire thread exclusive access of front node, return 0 if fail or queue is empty
                  intptr_t front = m_FrontIntPtr.fetch_or(1, memory_order_acquire);
                  if ((front & 1) | !(front >> 1))
                      return 0;
  
                  node = (T*)front;
                  next = static_cast<T*>(node->next.load(memory_order_relaxed));
                  if (!next)
                  {
                      // Set to zero, assuming we got the head. Exclusive access maintained as only producer can write zero.
                      m_Front.store(0, memory_order_release);
  
                      // - filters incomplete nodes
                      // - check if node is back == retrigger new back
                      expected = node;
                      if (!m_Back.compare_exchange_strong(expected, 0, memory_order_acquire, memory_order_relaxed))
                      {
                          // Back progressed or node is incomplete, reset front ptr and return 0.
                          m_Front.store(node, memory_order_release);
                          return 0;
                      }
  
                      // Successfully got the back, so just return node.
                      return node;
                  }
  
                  // Store next (clear block) and return node
                  m_Front.store(next, memory_order_release);
                  return node;
              }
  
          private:
              union
              {
                  atomic<T*> m_Front;
                  atomic<intptr_t> m_FrontIntPtr;
              };
              alignas(PLATFORM_CACHE_LINE_SIZE) atomic<T*> m_Back;
  
              // Verify mpsc_node is base of T
              static_assert(std::is_base_of<baselib::mpsc_node, T>::value, "Node class/struct used with baselib::mpsc_node_queue must derive from baselib::mpsc_node.");
          };
      }
  }
  