/*

  comm_create.f -- builds a communicator from the first q processes
      in a communicator containing p = q^2 processes.

  Input: none
  Output: q -- program tests correct creation of new communicator
      by broadcasting the value 1 to its members -- all other
      processes have the value 0 -- global sum computed across
      all the processes.

  Note:  Assumes that MPI_COMM_WORLD contains p = q^2 processes

  This example can be found in "Parallel Programming with MPI" by Peter Pacheco, 
  Morgan Kaufmann Publishers.
  See Chap 7, pp. 117 & ff in PPMPI

*/
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <mpi.h> 

#define MAX_PROCS 100
int process_ranks[MAX_PROCS];

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

      int        p;
      float      p_real;
      int        q   ;
      int        my_rank;
      MPI_Group        group_world;
      MPI_Group        first_row_group;
      MPI_Comm        first_row_comm;
      int        proc;
      int        test;
      int        sum;
      int        my_rank_in_first_row;
      int        ierr;


      test = 0;
      ierr = MPI_Init(&argc, &argv); 
      ierr = MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
      ierr = MPI_Comm_size(MPI_COMM_WORLD, &p);

      p_real = p;
      q =  sqrt(p_real);
      if ( q*q != p ) {
      	if ( my_rank == 0 ) fprintf(stderr,"%d^2 != %d: program terminates\n",q,p);
      	ierr = MPI_Finalize();
      	exit(-1);
      }
 
/*  Make a list of the processes in the new
        communicator    */
      for ( proc = 0; proc < q; proc++ ) 
          process_ranks[proc] = proc;
 
/*   Get the group underlying MPI_COMM_WORLD    */
      ierr = MPI_Comm_group(MPI_COMM_WORLD, &group_world );
 
/*  Create the new group    */
      ierr = MPI_Group_incl(group_world, q, process_ranks,
                        &first_row_group);
 
/*  Create the new communicator    */
      ierr = MPI_Comm_create(MPI_COMM_WORLD, first_row_group,
           &first_row_comm);
 
/*  Now check whether we can do collective ops in first_row_comm    */
      if (my_rank < q)  {
          ierr = MPI_Comm_rank(first_row_comm, &my_rank_in_first_row);
          if (my_rank_in_first_row == 0) test = 1;
          ierr = MPI_Bcast( &test, 1, MPI_INTEGER, 0,
                         first_row_comm);
      }
      ierr = MPI_Reduce( &test,  &sum, 1, MPI_INTEGER, MPI_SUM, 0,
                      MPI_COMM_WORLD );
 
      if (my_rank == 0)  {
          fprintf(stdout,"q = %d, sum = %d \n",q,sum);
      }
 
      ierr = MPI_Finalize();
      return(0);
}