#include <cassert>
#include <iostream>

#include <mpi.h>

int main(int argc, char** argv)
{
   MPI_Init(&argc, &argv);

   int size = 0;  
   MPI_Comm_size(MPI_COMM_WORLD, &size); 

   // what is the rank of this process?
   int rank = 0;
   MPI_Comm_rank(MPI_COMM_WORLD, &rank);
   
   // should only run if there are exactly two processes
   if(size != 2)
   {
      if(!rank)
         std::cerr << "Run with two parallel processes only. Not with "
                   << size << " processes.\n";
      MPI_Finalize();
      return EXIT_FAILURE;
   }
   
   int64_t limit = std::atoi(argv[1]);
   if(!rank) std::cout << "MPI ping-pong count up to " << limit << ".\n";
   
   // initialize counter to 0
   int64_t counter = 0;
   int64_t buffer = 0;
   
   while(counter < limit)
   {
      // increment counter and send from 0 to 1
      if(rank == 0) MPI_Send(&(++counter), 1, MPI_INT64_T, 1, 1, MPI_COMM_WORLD);
      if(rank == 1)
      {
         MPI_Recv(&buffer, 1, MPI_INT64_T, 0, 1, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
         counter = buffer;
      }
      if(counter == limit) break;
      
      // increment counter and send back from 1 to 0
      if(rank == 1) MPI_Send(&(++counter), 1, MPI_INT64_T, 0, 2, MPI_COMM_WORLD);
      if(rank == 0)
      {
         MPI_Recv(&buffer, 1, MPI_INT64_T, 1, 2, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
         counter = buffer;
      }
      
      if(!rank && (counter % (limit/10) == 0)) std::clog << "\n\tcounter = " << counter << ".";
   }
   if(!rank) std::cout << "\nDone: counter = " << counter << ".\n";
   
   MPI_Finalize();
}