/* comm_test.c -- creates a communicator from the first q processes
 *     in a communicator containing p = q^2 processes.  Broadcasts
 *     an array to the members of the newly created communicator.
 *
 * Input: none
 * Output: Contents of array broadcast to each process in the newly
 *     created communicator
 *
 * Note:  MPI_COMM_WORLD should contain p = q^2 processes.
 *
 * See Chap 7., pp. 117 & ff. in PPMPI
 */
#include <stdio.h>
#include "mpi.h"
#include <math.h>
#include <stdlib.h>

int main(int argc, char* argv[]) {
    int        p;
    int        q; /* = sqrt(p) */
    int        my_rank;
    int        n_bar = 2;
    MPI_Group  group_world;
    MPI_Group  first_row_group;
    MPI_Comm   first_row_comm;
    int*       process_ranks;
    int        proc, i;
    float*     A_00;
    int        my_rank_in_first_row;


    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &p);
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

    q = (int) sqrt((double) p);

    /* Make a list of the processes in the new
     * communicator */
    process_ranks = (int*) malloc(q*sizeof(int));
    for (proc = 0; proc < q; proc++)
        process_ranks[proc] = proc;

    /* Get the group underlying MPI_COMM_WORLD */
    MPI_Comm_group(MPI_COMM_WORLD, &group_world);

    /* Create the new group */
    MPI_Group_incl(group_world, q, process_ranks,
        &first_row_group);

    /* Create the new communicator */
    MPI_Comm_create(MPI_COMM_WORLD, first_row_group,
        &first_row_comm);

    /* Now broadcast across the first row */
    if (my_rank < q) {
        MPI_Comm_rank(first_row_comm, &my_rank_in_first_row);

        /* Allocate space for A_00 */
        A_00 = (float*) malloc (n_bar*n_bar*sizeof(float));
        if (my_rank_in_first_row == 0) {
            /* Initialize A_00 */
            for (i = 0; i < n_bar*n_bar; i++)
                A_00[i] = (float) i;
        }
        MPI_Bcast(A_00, n_bar*n_bar, MPI_FLOAT, 0,
            first_row_comm);

        printf("Process %d > ", my_rank);
        for (i = 0; i < n_bar*n_bar; i++)
            printf("%4.1f ", A_00[i]);
        printf("\n");
    }
    MPI_Finalize();
}  /* main */
