From 635b673492b23fcc343b23e42b05036d54a06959 Mon Sep 17 00:00:00 2001
From: kevin <kevin@zonk.verizon.net>
Date: Fri, 30 Oct 2020 09:41:01 -0400
Subject: [PATCH] cwMpScQueue.h : Initial commit.

---
 cwMpScQueue.h | 102 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 102 insertions(+)
 create mode 100644 cwMpScQueue.h

diff --git a/cwMpScQueue.h b/cwMpScQueue.h
new file mode 100644
index 0000000..7eb480a
--- /dev/null
+++ b/cwMpScQueue.h
@@ -0,0 +1,102 @@
+#ifndef cwMpScQueue_h
+#define cwMpScQueue_h
+
+namespace cw
+{
+  
+  template<typename T>
+  class MpScQueue
+  {
+  public:
+    
+    typedef struct node_str
+    {
+      struct node_str* next     = nullptr;
+      T*               payload  = nullptr;
+    } node_t;
+    
+    MpScQueue()
+    {
+      node_t* stub = mem::allocZ<node_t>();
+      _head = stub;   // last-in
+      _tail = stub;   // first-out
+      mutex::create( _mH );
+    }
+    
+    virtual ~MpScQueue()
+    {
+      mem::free(_tail);
+      mutex::destroy( _mH );
+    }
+
+    MpScQueue( const MpScQueue& ) = delete;
+    MpScQueue( const MpScQueue&& ) = delete;
+    MpScQueue& operator=(const MpScQueue& ) = delete;
+    MpScQueue& operator=(const MpScQueue&& ) = delete;
+    
+
+    void    push( T* payload )
+    {
+      node_t* new_node  = mem::allocZ<node_t>(1);
+      new_node->payload = payload;
+      new_node->next    = nullptr;
+
+      if( mutex::lock(_mH) == kOkRC )
+      {
+      
+        // Note that the elements of the queue are only accessed from the end of the queue (tail).
+        // New nodes can therefore safely be updated in two steps:
+
+        // 1. Atomically set _head to the new node and return 'old-head'
+        node_t* prev   = _head.exchange(new_node,std::memory_order_acq_rel);  
+
+        // Note that at this point only the new node may have the 'old-head' as it's predecssor.
+        // Other threads may therefore safely interrupt at this point.
+      
+        // 2. Set the old-head next pointer to the new node (thereby adding the new node to the list)
+        prev->next = new_node; // RELEASE 'next' to consumer
+
+        mutex::unlock(_mH);
+      }
+      
+    }
+    
+    T*      pop()
+    {
+      T*      payload = nullptr;
+      node_t* t       = _tail;
+
+      if( mutex::lock(_mH) == kOkRC )
+      {
+        node_t* next    = t->next;  //  ACQUIRE 'next' from producer
+        if( next != nullptr )
+        {
+          _tail   = next;    
+          payload = next->payload;
+          mem::free(t);
+        }
+        
+        mutex::unlock(_mH);
+      }
+      
+      return payload;
+      
+    }
+    
+    bool    isempty() const
+    {
+      return _tail->next == nullptr;  //  ACQUIRE 'next'  from producer
+    }
+
+  private:
+
+    node_t*         _stub;
+    node_t*         _tail;
+    node_t*         _head;
+    mutex::handle_t _mH;
+  };
+
+}
+
+
+#endif