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

int main ( int argc, char *argv[] );
void ring_io ( int p, int id );

/******************************************************************************/

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

/******************************************************************************/
/*
  Purpose:

    MAIN is the main program for RING_MPI.

  Discussion:

    RING_MPI sends messages of various size from process 0 to 1 to 2 to
    ...the last process and then back to 0.

  Licensing:

    This code is distributed under the MIT license.

  Modified:

    21 October 2012

  Author:

    John Burkardt

  Reference:

    Peter Pacheco,
    Parallel Programming with MPI,
    Morgan Kaufman, 1996,
    ISBN: 1558603395,
    LC: QA76.642.P3.
*/
{
  int id;
  int p;
/*
  Initialize MPI.
*/
  MPI_Init ( &argc, &argv );
/*
  Get the number of processes.
*/
  MPI_Comm_size ( MPI_COMM_WORLD, &p );
/*
  Get the individual process ID.
*/
  MPI_Comm_rank ( MPI_COMM_WORLD, &id );
/*
  Print a message.
*/
  if ( id == 0 )
  {
    printf ( "\n" );
    printf ( "RING_MPI:\n" );
    printf ( "  C/MPI version\n" );
    printf ( "  Measure time required to transmit data around\n" );
    printf ( "  a ring of processes\n" );
    printf ( "\n" );
    printf ( "  The number of processes is %d\n", p );
  }

  ring_io ( p, id );
/*
  Shut down MPI.
*/
  MPI_Finalize ( );
/*
  Terminate.
*/
  if ( id == 0 )
  {
    printf ( "\n" );
    printf ( "RING_MPI:\n" );
    printf ( "  Normal end of execution.\n" );
  }
  return 0;
}
/******************************************************************************/

void ring_io ( int p, int id )

/******************************************************************************/
/*
  Purpose:

    RING_IO carries out the tasks of process ID, of a total of P processes.

  Licensing:

    This code is distributed under the MIT license.

  Modified:

    21 October 2012

  Author:

    John Burkardt

  Reference:

    Peter Pacheco,
    Parallel Programming with MPI,
    Morgan Kaufman, 1996,
    ISBN: 1558603395,
    LC: QA76.642.P3.
*/
{
  int dest;
  int i;
  int j;
  int n;
  int n_test[5] = { 100, 1000, 10000, 100000, 1000000 };
  int n_test_num = 5;
  int source;
  MPI_Status status;
  double tave;
  int test;
  int test_num = 10;
  double tmax;
  double tmin;
  double wtime;
  double *x;

  if ( id == 0 )
  {
    printf ( "\n" );
    printf ( "  Timings based on %d experiments\n", test_num );
    printf ( "  N double precision values were sent\n" );
    printf ( "  in a ring transmission starting and ending at process 0\n" );
    printf ( "  and using a total of %d processes.\n", p );
    printf ( "\n" );
    printf ( "         N           T min           T ave           T max\n" );
    printf ( "\n" );
  }
/*
  Choose message size.
*/
  for ( i = 0; i < n_test_num; i++ )
  {    
    n = n_test[i];

    x = ( double * ) malloc ( n * sizeof ( double ) );
/*
  Process 0 sends very first message, 
  then waits to receive the "echo" that has gone around the world.
*/
    if ( id == 0 )
    {
      dest = 1;
      source = p - 1;

      tave = 0.0;
      tmin = 1.0E+30;
      tmax = 0.0;

      for ( test = 1; test <= test_num; test++ )
      {
/*
  Just in case, set the entries of X in a way that identifies
  which iteration of the test is being carried out.
*/
        for ( j = 0; j < n; j++ )
        {
          x[j] = ( double ) ( test + j );
        }

        wtime = MPI_Wtime ( );
        MPI_Send ( x, n, MPI_DOUBLE, dest,   0, MPI_COMM_WORLD );
        MPI_Recv ( x, n, MPI_DOUBLE, source, 0, MPI_COMM_WORLD, &status );
        wtime = MPI_Wtime ( ) - wtime;
/*
  Record the time it took.
*/
        tave = tave + wtime;
        if ( wtime < tmin )
        {
          tmin = wtime;
        }
        if ( tmax < wtime )
        {
          tmax = wtime;
        }
      }

      tave = tave / ( double ) ( test_num );

      printf ( "  %8d  %14.6g  %14.6g  %14.6g\n", n, tmin, tave, tmax );
    }
/*
  Worker ID must receive first from ID-1, then send to ID+1.
*/
    else
    {
      source = id - 1;
      dest = ( ( id + 1 ) % p );
 
      for ( test = 1; test <= test_num; test++ )
      {
        MPI_Recv ( x, n, MPI_DOUBLE, source, 0, MPI_COMM_WORLD, &status );
        MPI_Send ( x, n, MPI_DOUBLE, dest,   0, MPI_COMM_WORLD );
      }
    }
    free ( x );
  }

  return;
}