/* A simple MPI example program that uses MPI_Type_vector to build a    */
/* derived datatype that describes a column of a matrix.                */

/* The program should be run with two processes.                        */
/* The program first builds a derived datatype consisting of a column   */
/* of elements from a square matrix. Process zero then initializes a    */
/* matrix with integers 0 .. LENGTH*LENGTH-1 and sends the first and    */
/* last columns of the matrix x to process 1. This recieves the columns */
/* and places them into a local matrix y, which is initialized with all */
/* zeros. Process one then sends back a message with its own hostname   */
/* and the modified matrix y. Process zero receives thes and prints out */
/* the modified matrix, where all but the first and last columns are    */
/* zeros.                                                               */

/* Compile the program with 'mpicc sendcol.c -o sendcol'                */
/* Run the program with 'mpirun -machinefile hostfile -np 2 sendcol'    */


#include <stdio.h>
#include "mpi.h"
#define MAXPROC 8    /* Max number of procsses */
#define NAMELEN 80   /* Max length of machine name */
#define LENGTH 8    /*  Size of matrix in LENGTH*LENGTH */

main(int argc, char* argv[]) {
  int i, j, np, me, count;
  const int nametag  = 42;    /* Tag value for sending name */
  const int datatag  = 43;    /* Tag value for sending data */
  MPI_Status status;          /* Status object for receive */
  MPI_Datatype Column_type;   /* Data type for a column of the matrix */

  char myname[NAMELEN];       /* Local host name string */
  char hostname[NAMELEN];     /* Received host names */

  int x[LENGTH][LENGTH];      /* Send buffer */
  int y[LENGTH][LENGTH];      /* Receive buffer */

  MPI_Init(&argc, &argv);                /* Initialize MPI */
  MPI_Comm_size(MPI_COMM_WORLD, &np);    /* Get nr of processes */
  MPI_Comm_rank(MPI_COMM_WORLD, &me);    /* Get own identifier */
  
  gethostname(&myname, NAMELEN);         /* Get host name */

  /* Define the column data type */
  MPI_Type_vector(LENGTH, 1, LENGTH, MPI_INT, &Column_type);
  MPI_Type_commit(&Column_type);

  if (np != 2) {
    if (me == 0) printf("You have to run this program with 2 processes\n");
    MPI_Finalize();
    exit(0);
  }

  if (me == 0) {    /* Process 0 does this */
    
    /* Initialize the matrix x with values 0 .. LENGTH*LENGTH-1 */
    for (i=0; i<LENGTH; i++) {
      for (j=0; j<LENGTH; j++) {
	x[i][j] = i*LENGTH+j;
      }
    }

    /* Print out the matrix before we send columns to process 1 */
    printf("The original matrix is\n");
    for (i=0; i<LENGTH; i++) {
      for (j=0; j<LENGTH; j++) {
	printf("%4d ", x[i][j]);
      }
      printf("\n");
    }

    /* Send first column of array x */
    MPI_Send(&x[0][0], 1, Column_type, 1, datatag, MPI_COMM_WORLD);

    /* Send last column of array x */
    MPI_Send(&x[0][LENGTH-1], 1, Column_type, 1, datatag, MPI_COMM_WORLD);

    /* Receive messages with hostname and the matrix */
    MPI_Recv (&hostname, NAMELEN, MPI_CHAR, 1, nametag, MPI_COMM_WORLD, \
	      &status);
    MPI_Recv (&y, LENGTH*LENGTH, MPI_INT, 1, datatag, MPI_COMM_WORLD, &status);

    /* Print out the matrixes that we received from process 1 */
    printf("\nThe recived matrix from process 1 on host %s is\n", hostname);
    for (i=0; i<LENGTH; i++) {
      for (j=0; j<LENGTH; j++) {
	printf("%4d ", y[i][j]);
      }
      printf("\n");
    }

    printf("Ready\n");


  } else { /* Processes 1 does this */

    /* Initialize the matrix y with zeros */
    for (i=0; i<LENGTH; i++) {
      for (j=0; j<LENGTH; j++) {
	y[i][j] = 0;
      }
    }

    /* Receive the first and last columns from process 0, place them in array y */
    MPI_Recv(&y[0][0], 1, Column_type, 0, datatag, MPI_COMM_WORLD, &status);
    MPI_Recv(&y[0][LENGTH-1], 1, Column_type, 0, datatag, MPI_COMM_WORLD, &status);

        /* Send own name back to process 0 */
    MPI_Send (&myname, NAMELEN, MPI_CHAR, 0, nametag, MPI_COMM_WORLD);
    /* Send the modified matrix back to process 0 */
    MPI_Send (&y, LENGTH*LENGTH, MPI_INT, 0, datatag, MPI_COMM_WORLD);

  }

  MPI_Finalize();
  exit(0);
}