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

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

  int value;
  int my_rank, size;
  int previous, next;
  MPI_Status status;

  MPI_Init(NULL, NULL);

  // Get number of processes
  MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
  MPI_Comm_size(MPI_COMM_WORLD, &size);

  // determine my neighbours according to my rank
  if (my_rank == 0)
  {
    previous = size - 1;
    next = my_rank + 1;
  }
  else if (my_rank == size - 1)
  {
    previous = my_rank - 1;
    next = 0;
  }
  else
  {
    previous = my_rank - 1;
    next = my_rank + 1;
  }

  value = 1;

  // The nodes, starting with node 0, transmit the value to each other,
  // each time multiplying it by 2.
  // At the end of the transmission, node 0 receives the value 2^(size-1)
  //
  // Instruction: before each send and after each receive, each node displays
  //   - its rank
  //   - the type communication (send, recv)
  //   - the value

  // receive value from previous node
  if (my_rank != 0)
  {
    MPI_Recv(&value, 1, MPI_INT, previous, 0, MPI_COMM_WORLD, &status);
    printf("RECEIVED from process %d of %d, value = %d\n", my_rank, size, value);
    value = value * 2;
  }
  else
  {
    printf("START, value = %d\n", value);
  }

  printf("SENDING from process %d of %d, value = %d\n", my_rank, size, value);

  // send value to next node
  if (my_rank != size - 1)
  {
    MPI_Send(&value, 1, MPI_INT, next, 0, MPI_COMM_WORLD);
  }
  else
  {
    printf("The End, value = %d\n", value);
  }

  MPI_Finalize();

  return EXIT_SUCCESS;
}