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

#define START 0
#define END 100

int main(int argc, char** argv) {
    int rank, size;

    /* initialize MPI */
    MPI_Init(&argc, &argv);

    /* get the rank (process id) and size (number of processes) */
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    /* calculate the start and end points by evenly dividing the range */
    int start = ((END - START) / size) * rank;
    int end = start + ((END - START) / size) - 1;

    /* the last process needs to do all remaining ones */
    if (rank == (size - 1)) {
        end = END;
    }

    /* do the calculation */
    int sum = 0, i;
    for (i = start; i <= end; i++) {
        sum += i;
    }

    /* debugging output */
    printf("Process %d: sum(%d, %d) = %d\n", rank, start, end, sum);

    /* MPI communication: process 0 receives everyone elses sum */
    if (rank == 0) {
        /* parent process: receive each partial sum and add it to ours -
         * now we no longer care about the order we receive them in */
        int partial, i;
        MPI_Status status;
        for (i = 1; i < size; i++) {
            MPI_Recv(&partial, 1, MPI_INT, MPI_ANY_SOURCE, 0, MPI_COMM_WORLD, &status);

            /* see which process we are reading from */
            printf("Process 0 got data from process %d.\n", status.MPI_SOURCE);

            sum += partial;
        }
    } else {
        /* worker process: send sum to process 0 */
        MPI_Send(&sum, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
    } 

    /* now have process 0 display the results */
    if (rank == 0) {
        printf("The final sum = %d.\n", sum);
    }

    /* quit MPI */
    MPI_Finalize();
    return 0;
}