/**
 * Test MPI_Reduce with a user-defined reduction function
 *
 * Author: Thomas Moschny <moschny@ipd.uni-karlsruhe.de>
 */
#include <mpi.h>
#include <stdio.h>
#include <assert.h>

int const vlength=8;
void reduction(int *in, int *inout, int *len, MPI_Datatype *dptr);
MPI_Datatype datatype;

int main(int argc, char** argv)
{
  /* initialization */
  int rank = -1;
  MPI_Init(&argc, &argv);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);

  /* create our own type */
  MPI_Type_vector(vlength/2, 1, 2, MPI_INT, &datatype);
  MPI_Type_commit(&datatype);

  /* create our own operator */
  MPI_Op op;
  MPI_Op_create((MPI_User_function*)reduction, 1, &op);

  /* create the arrays */
  int src[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
  int dst[] = { 0, 0, 0, 0, 0, 0, 0, 0 };

  /* reduce */
  MPI_Reduce(src, dst, 1, datatype, op, 0, MPI_COMM_WORLD);

  /* print */
  if (rank==0){
    for (int i=0; i<vlength; i++)
      printf("%d ", dst[i]);
    printf("\n");
  }

  /* that's it */
  MPI_Finalize();
  
}

void reduction(int *in, int *inout, int *len, MPI_Datatype *dptr)
{
  assert(*dptr == datatype);
  for (int l=0; l<(*len); l++)
    for (int i=0; i<vlength; i++)
      if (i%2==0)
	inout[i] += in[i];
}