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

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

    MPI_Comm GlobalComm, LocComm, InterComm=MPI_COMM_NULL, IntraComm=MPI_COMM_NULL;
    MPI_Status stat;
    int high, colour, numproc, id, lid, root, idinter;

    int val, lsum, gsum;

    MPI_Init(&argc, &argv);
    MPI_Comm_dup(MPI_COMM_WORLD,&GlobalComm);
    MPI_Comm_size(GlobalComm, &numproc);
    MPI_Comm_rank(GlobalComm, &id);

    /* Divide processes in 3 groups */
    colour = id % 3;
    if ( id == 0 && numproc < 3 ) {
        fprintf(stderr,"Less than 3 processes have been activated: error exit\n");
        MPI_Finalize();
        return(1);
    }

    MPI_Comm_split(GlobalComm, colour, id, &LocComm);
    MPI_Comm_rank(LocComm, &lid);

    printf("Process %d in global comm is process %d in local comm\n",id,lid);

    /* Generate inter communicator between 2 leaders of first two groups
       the value of colour identifies the group to be considered */
    if ( colour < 2 ) {
       if ( colour == 0 ) {
          MPI_Intercomm_create(LocComm, 0, GlobalComm, 1, 01, &InterComm);
       } else {
          MPI_Intercomm_create(LocComm, 0, GlobalComm, 0, 01, &InterComm);
       }
       high = colour;
       /* merge first two groups */
       MPI_Intercomm_merge(InterComm,high,&IntraComm);
       LocComm = IntraComm;
    }

    /* Sum of values:  each group computes a partial sum */
    val = 1; lsum = 0;
    MPI_Comm_rank(LocComm, &lid);
    MPI_Reduce(&val,&lsum,1,MPI_INT,MPI_SUM,0,LocComm);
    if ( lid == 0 ) printf("Local sum = %d\n",lsum);

    MPI_Barrier(GlobalComm);

    MPI_Finalize();

    return(0);
}