// reduce.c
// This program uses MPI_Reduce to collect data from all the processors -
// including the root process - and apply some operation on all of this data.
// In this program the operation is to sum all of the data collected from all
// processes.  The first parameter in MPI_Reduce is the value being sent from
// each process.  The second parameter is the variable storing the sum of
// the data collected from all processes (in this case, the sum of all the
// PIs).  The fifth parameter, MPI_SUM, is the operation to be done on all
// of the data collected from all processors.
// The total sum, piSum, has a valid value only in the root process.
// The root process is defined in the sixth parameter of MPI_Reduce ('source').

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

int main(int argc, char **argv)
{
	int rank, size;
	MPI_Status status;
	double pi, piSum;
	int i, count, source;
	
	MPI_Init(&argc, &argv);
	MPI_Comm_rank(MPI_COMM_WORLD, &rank); 
	MPI_Comm_size(MPI_COMM_WORLD, &size); 

	count = 1;
	source = 0;
	if (rank == source) {
		pi = M_PI;
		printf("Rank %d is broadcasting %.10f\n", rank, pi);
	}
	
	MPI_Bcast(&pi, 1, MPI_DOUBLE, source, MPI_COMM_WORLD);
	if (rank != source) {
		printf("Rank %d receives %.10f from process %d\n", rank, pi, source);
	}

	MPI_Reduce(&pi, &piSum, 1, MPI_DOUBLE, MPI_SUM, source, MPI_COMM_WORLD);
	if (rank == source) {
		printf("Inside process %d, sum of pi's = %.10f\n", source, piSum);
	}
	
	MPI_Finalize();
	return 0;
}

/*
CRAY SV1:

cc reduce.c   
mpirun -np 8 a.out

Rank 0 is broadcasting 3.1415926536
Rank 2 receives 3.1415926536 from process 0
Rank 1 receives 3.1415926536 from process 0
Rank 3 receives 3.1415926536 from process 0
Rank 4 receives 3.1415926536 from process 0
Rank 5 receives 3.1415926536 from process 0
Rank 6 receives 3.1415926536 from process 0
Rank 7 receives 3.1415926536 from process 0
Inside process 0, sum of pi's = 25.1327412287

mpirun -np 4 a.out
Rank 0 is broadcasting 3.1415926536
Rank 2 receives 3.1415926536 from process 0
Rank 3 receives 3.1415926536 from process 0
Rank 1 receives 3.1415926536 from process 0
Inside process 0, sum of pi's = 12.5663706144

*/