/* L-5 MCS 572 Friday 20 January 2012 : parallel sum with scatter/gather 
 * This program illustrates the MPI_Scatter and MPI_Gather to calculate
 * the sum of the first positive 100 numbers on four processors,
 * so run this program using "mpirun -np 4 par_sum". */

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

#define v 1 /* verbose flag, output if 1,
                          no output if 0 */

int main ( int argc, char *argv[] )
{
   int myid,j,*data,tosum[25],sums[4];

   MPI_Init(&argc,&argv);
   MPI_Comm_rank(MPI_COMM_WORLD,&myid);

   if(myid==0) /* manager allocates 
                  and initializes the data */
   {
      data = (int*)calloc(100,sizeof(int));
      for (j=0; j<100; j++) data[j] = j+1;
      if(v>0)
      {
         printf("The data to sum : ");
         for (j=0; j<100; j++) printf(" %d",data[j]); printf("\n");
      }
   }

   MPI_Scatter(data,25,MPI_INT,tosum,25,MPI_INT,0,
               MPI_COMM_WORLD);

   if(v>0) /* after the scatter,
              every node has 25 numbers to sum */
   {
      printf("Node %d has numbers to sum :",myid);
      for(j=0; j<25; j++) printf(" %d", tosum[j]);
      printf("\n");
   }
   sums[myid] = 0;
   for(j=0; j<25; j++) sums[myid] += tosum[j];
   if(v>0) printf("Node %d computes the sum %d\n",
                   myid,sums[myid]);

   MPI_Gather(&sums[myid],1,MPI_INT,sums,1,MPI_INT,0,
              MPI_COMM_WORLD);

   if(myid==0) /* after the gather,
                  sums contains the four sums */
   {
      printf("The four sums : ");
      printf("%d",sums[0]);
      for(j=1; j<4; j++) printf(" + %d", sums[j]);
      for(j=1; j<4; j++) sums[0] += sums[j];
      printf(" = %d, which should be 5050.\n",
             sums[0]);
   }
   MPI_Finalize();
   return 0;
}