libcw/cwSpScQueueTmpl.h

//| Copyright: (C) 2020-2024 Kevin Larke <contact AT larke DOT org> 
//| License: GNU GPL version 3.0 or above. See the accompanying LICENSE file.
#ifndef cwSpScQueueTmpl_H
#define cwSpScQueueTmpl_H

namespace cw
{

  template< typename T >
    class spScQueueTmpl
  {
  public:
    spScQueueTmpl( unsigned eleN )
    {
      _aN  = eleN;
      _aV = mem::allocZ<T>(eleN);
      _wi.store(0,std::memory_order_release);
      _ri.store(0,std::memory_order_release);      
    }

    virtual ~spScQueueTmpl()
    {
      mem::release(_aV);
    }


    T* get()
    {
      unsigned ri = _ri.load( std::memory_order_acquire );
      unsigned wi = _wi.load( std::memory_order_relaxed ); // _wi is written in this thread 

      // calc. the count of full elements
      unsigned n =  wi >= ri ? wi-ri : (_aN-ri) + wi;

      // there must always be at least one empty element because
      // wi can never be advanced to equal ri.
      if( n >= _aN-1 )
        return nullptr;

      return _aV + wi;
    }

    void publish()
    {
      unsigned wi = _wi.load( std::memory_order_relaxed ); // _wi is written in this thread 
      
      wi = (wi+1) % _aN;

      // advance the write position
      _wi.store( wi, std::memory_order_release );
    
    }

    T* pop()
    {
      unsigned ri = _ri.load( std::memory_order_relaxed );
      unsigned wi = _wi.load( std::memory_order_acquire );

      unsigned n =  wi >= ri ? wi-ri : (_aN-ri) + wi;

      if( n == 0 )
        return nullptr;

      T* v = _aV + ri;

      ri = (ri+1) % _aN;

      _ri.store( ri, std::memory_order_release);

      return v;
    }

  

    
  private:
    unsigned              _aN = 0;
    T*                    _aV = nullptr;
    std::atomic<unsigned> _wi;   
    std::atomic<unsigned> _ri;

    // Note: // _wi==_ri indicates an empty buffer.
    // _wi may never be advanced such that it equals _ri, however
    // _ri may be advanced such that it equals _wi.
  };


  rc_t testSpScQueueTmpl();

}
#endif