############ Tamim Jabban #############
############   Recitation 9   #############

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

#define NUM_ELEMS 100
#define NUM_PROCS 4
#define NUM_ELEMS_PER_PROC (NUM_ELEMS / NUM_PROCS)

#define TAG_SPREAD 5
#define TAG_JOIN 10

#define ROOT 0

int main(int argc, char** argv) {

	int my_pid, num_proc;
	
	MPI_Init(&argc, &argv);
	
	MPI_Comm_size(MPI_COMM_WORLD, &num_proc);
	MPI_Comm_rank(MPI_COMM_WORLD, &my_pid);
	MPI_Status status;
	
	int sendBuffer[NUM_ELEMS];
	int recvBuffer[NUM_ELEMS_PER_PROC];
	
	if (my_pid == ROOT) { // Master
					
		double start_time = MPI_Wtime();
		
		int i;
		for (i = 0; i < NUM_ELEMS; i++)
			sendBuffer[i] = i+1;
		
		int j;
		for (j = 1; j < NUM_PROCS; j++) 
			MPI_Send(&sendBuffer[j*NUM_ELEMS_PER_PROC], NUM_ELEMS_PER_PROC,
							MPI_INT, j, TAG_SPREAD, MPI_COMM_WORLD);
		
		int total_sum = 0;
		
		int k;
		for (k = 0; k < NUM_ELEMS_PER_PROC; k++)
			total_sum += sendBuffer[k];
		
		int partial_sum;
		
		int m;
		for (m = 1; m < NUM_PROCS; m++) {
			MPI_Recv(&partial_sum, 1, MPI_INT, m, TAG_JOIN, MPI_COMM_WORLD, &status);
			total_sum += partial_sum;
		}
		
		double end_time = MPI_Wtime();

		printf("Total sum: %d, and it took %f\n", total_sum, (end_time - start_time));

	} else { // Slave
				
		MPI_Recv(&recvBuffer, NUM_ELEMS_PER_PROC, MPI_INT, ROOT, 
				TAG_SPREAD, MPI_COMM_WORLD, &status);
		
		int partial_sum = 0;
		
		int n;
		for (n = 0; n < NUM_ELEMS_PER_PROC; n++)
			partial_sum += recvBuffer[n];
		
		MPI_Send(&partial_sum, 1, MPI_INT, ROOT, TAG_JOIN, MPI_COMM_WORLD);	
	}
	MPI_Finalize();	
}