/*
***********************************************************************

                      Example 12 (diag_comm.c) 

  Objective           : Mpi program to create a communicator consisting of  
                        processors that lie on the diagonal in the  square 
                        gridof processors, arranged in the form of a matrix.
                        This example demonstrates the use of 
                        MPI_Init
                        MPI_Comm_rank
                        MPI_Comm_size
                        MPI_Comm_group
                        MPI_Group_incl
                        MPI_Comm_create
                        MPI_Finalize

  Input               : Communicator consisting of all  the processors 

  Necessary Condition : Number of Processes should be perfect square 
                        and  less than 8. 

***********************************************************************
*/



#include <stdio.h>
#include <math.h>
#include "mpi.h"

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

        int	i,iproc, index;
	int	Numprocs, MyRank;
	int	New_numprocs, NewMyRank;
	int	*process_rank, Group_Diag;

	MPI_Group	GroupWorld,new_group;
	MPI_Comm	Diag_Group; 


  /* ....MPI Initialisation....*/

  	MPI_Init(&argc, &argv); 
  	MPI_Comm_rank(MPI_COMM_WORLD, &MyRank);
  	MPI_Comm_size(MPI_COMM_WORLD, &Numprocs);

	Group_Diag= (int)(sqrt((double)Numprocs));

	if((Group_Diag * Group_Diag) != Numprocs) {
	   if (MyRank == 0) 
	       printf("Numprocs %d is not perfect square ...\n", Numprocs);
  	   MPI_Finalize();
	   exit(-1);
	}
	

	process_rank = (int *)malloc((Group_Diag) * sizeof(int));

        iproc=0;	
	for(index=0; index < Group_Diag ; index++)
	{
	    process_rank[index] = iproc;
	    iproc=iproc + Group_Diag + 1;
	}
	if(MyRank==0)
	{	
	    printf("\nProcessors forming the diagonal group are \n"); 	
	    for(i=0;i<Group_Diag;i++)
   		printf("Processor %d\n",process_rank[i]); 
	}

        /*  .... To create new group....*/
	
	MPI_Comm_group(MPI_COMM_WORLD, &GroupWorld);
	MPI_Group_incl(GroupWorld, (Group_Diag),process_rank,&new_group);
	MPI_Comm_create(MPI_COMM_WORLD,new_group,&Diag_Group); 

        if(MyRank%(Group_Diag+1)==0)
	{
	    MPI_Comm_rank(Diag_Group,&NewMyRank);
  	    MPI_Comm_size(Diag_Group,&New_numprocs);

	    printf("newnumprocs=%d\n",New_numprocs);
	    printf("NewRank in the diagonal group formed = %d\n", NewMyRank);
	}

	MPI_Finalize(); 

}