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

using std::cout;
using std::endl;

const int MAX_STRING = 100;

int main(int argc, char **argv) {
    char    greeting[MAX_STRING];
    int     comm_sz;    /* number of processes  */
    int     my_rank;    /* process rank         */

    //Initialize the command line arguments on every process
    MPI_Init(&argc, &argv);

    int token = 0;
    token = atoi(argv[1]);

    //Get the number of processes in MPI_COMM_WORLD, and put 
    //it in the 'comm_sz" variable; ie., how many processes
    //are running this program (the same as what you put in the 
    //-np argument).
    MPI_Comm_size(MPI_COMM_WORLD, &comm_sz);

    //Get the rank of this particular process in MPI_COMM_WORLD,
    //and put it in the 'my_rank' variable -- ie., what number
    //is this process
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

    int next_rank = my_rank + 1;
    if (next_rank >= comm_sz) next_rank = 0;

    int prev_rank = my_rank - 1;
    if (prev_rank < 0) prev_rank = comm_sz - 1;

    /**
     *  Status/Requests so we can wait on the asynchronous sends/receives
     */
    MPI_Request request;
    MPI_Status status;

    /**
     * The head process starts the ring by sending the first token
     */
    if (my_rank == 0) {
        if (token > 0) token--;

        cout << "[process " << my_rank << "]: sending  " << token << endl;

        MPI_Isend(&token,
                 1,
                 MPI_INT,
                 next_rank,
                 0,
                 MPI_COMM_WORLD,
                 &request);

        MPI_Wait(&request, &status);
    }

    int flag = 0;

    /**
     *  Each process will receive the token from the previous in the ring, and sending
     *  it along until the token reaches 0.
     */
    while (token - comm_sz >= 0) {
        MPI_Irecv(&token,
                 1,
                 MPI_INT,
                 prev_rank,
                 0,
                 MPI_COMM_WORLD,
                 &request);

        while (flag == 0) {
            MPI_Test(&request, &flag, &status);
        }

        flag = 0;

//        cout << "[process " << my_rank << "]: received " << token << endl;
        token--;

        if (token >= 0) {
            cout << "[process " << my_rank << "]: sending  " << token << endl;
            MPI_Isend(&token,
                     1,
                     MPI_INT,
                     next_rank,
                     0,
                     MPI_COMM_WORLD,
                     &request);

            MPI_Wait(&request, &status);
        }
    }

    /*Inform MPI that this process has finished*/
    MPI_Finalize();
    return 0;
}