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

typedef enum neighbor {
  UP    = 0,
  DOWN  = 1,
  LEFT  = 2,
  RIGHT = 3

} neighbor_t;

int main(int argc, char **argv) {
  int world_size;
  int rank, cart_rank;

  int dims[2] = {0, 0};
  int periods[2] ={0, 0};
  int reorder = 0;

  int coords[2];

  int neighbors[4];

  MPI_Comm cart_comm;

  MPI_Init(&argc,&argv);
  MPI_Comm_size(MPI_COMM_WORLD, &world_size);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);

  MPI_Dims_create(world_size, 2, dims);

  MPI_Cart_create(MPI_COMM_WORLD, 2, dims, periods, 0, &cart_comm);
  MPI_Comm_rank(cart_comm, &cart_rank);

  MPI_Cart_coords(cart_comm, cart_rank, 2, coords);

  MPI_Cart_shift(cart_comm, 0, 1, 
                  &neighbors[ UP], &neighbors[DOWN]);

  MPI_Cart_shift(cart_comm, 1, 1, 
                  &neighbors[LEFT], &neighbors[RIGHT]);

  printf("Rank = %4d - Coords = (%3d, %3d) - Neighbors (up, down, left, right) = (%3d, %3d, %3d, %3d)\n",
            rank, coords[0], coords[1], 
            neighbors[UP], neighbors[DOWN], neighbors[LEFT], neighbors[RIGHT]);

  MPI_Finalize();

  return 0;
}