/**
\file   mpiExample2.cpp
\brief  MPI programming example #2.
\author george j. grevera, ph.d.

This program is a skeleton of a parallel MPI application using the one
manager/many workers framework.  The process with an MPI rank of 0 is 
considered to be the manager;  processes with MPI ranks of 1..mpiSize-1
are workers.  Messages are defined and are sent from the manager to the
workers.

<pre>
compile:  mpic++ -g  -o mpiExample2.exe mpiExample2.cpp  # debug version
          mpic++ -O3 -o mpiExample2.exe mpiExample2.cpp  # optimized version
run    :  lamboot -v    # to start lam mpi
          mpirun -np 4 ./mpiExample2.exe  # run in parallel w/ 4 processes
          lamhalt       # to stop lam mpi
</pre>
*/
#include  <assert.h>
#include  <mpi.h>
#include  <stdio.h>
#include  <unistd.h>

static char  mpiName[ 1024 ];  ///< host computer name
static int   mpiRank;          ///< number of this process (0..n-1)
static int   mpiSize;          ///< total number of processes (n)
static int   myPID;            ///< process id

/** \brief format of messages sent between manager and worker processes for 
           example 2.
    \todo  define parameters for OP_WORK.
    \todo  define parameters for OP_RESULT.
*/
struct Message {
    enum {
        OP_WORK,     ///< manager to worker - here's your work assignment
        OP_EXIT,     ///< manager to worker - time to exit
        OP_RESULT    ///< worker to manager - here's the result
    };
    int  operation;  ///< one of the above
    /** \todo define operation specific parameters here. */
};
//----------------------------------------------------------------------
/** \brief manager code for example 2.
 */
static void manager ( void ) {
    printf( "manager: my rank=%d, size=%d, pid=%d. \n",
            mpiRank, mpiSize, myPID );
    /** \todo  insert manager code here. */
    //as an example, send an empty work message to each worker
    struct Message  m;
    m.operation = m.OP_WORK;
    assert( mpiSize>3 );
    MPI_Send( &m, sizeof( m ), MPI_UNSIGNED_CHAR, 1, m.operation,
              MPI_COMM_WORLD );
    MPI_Send( &m, sizeof( m ), MPI_UNSIGNED_CHAR, 2, m.operation,
              MPI_COMM_WORLD );
    MPI_Send( &m, sizeof( m ), MPI_UNSIGNED_CHAR, 3, m.operation,
              MPI_COMM_WORLD );
}
//----------------------------------------------------------------------
/** \brief worker code for example 2.
 */
static void worker ( void ) {
    printf( "worker: my rank=%d, size=%d, pid=%d. \n",
            mpiRank, mpiSize, myPID );
    /** \todo  insert worker code here. */
    //as an example, receive a message
    MPI_Status      status;
    struct Message  m;
    MPI_Recv( &m, sizeof( m ), MPI_UNSIGNED_CHAR, MPI_ANY_SOURCE,
              MPI_ANY_TAG, MPI_COMM_WORLD, &status );
    printf( "worker %d (%d): received message. \n", mpiRank, myPID );
}
//----------------------------------------------------------------------
/** \brief   main program entry point for example 2.  execution begins here.
    \param   argc count of command line arguments.
    \param   argv array of command line arguments.
    \returns 0 is always returned.
 */
int main ( int argc, char* argv[] ) {  //not const because MPI_Init may change
    if (MPI_Init( &argc, &argv ) != MPI_SUCCESS) {
        //actually, we'll never get here but it is a good idea to check.
        // if MPI_Init fails, mpi will exit with an error message.
        puts( "mpi init failed." );
        return 0;
    }

    //get the name of this computer
    gethostname( mpiName, sizeof( mpiName ) );
    //determine rank
    MPI_Comm_rank( MPI_COMM_WORLD, &mpiRank );
    //determine the total number of processes
    MPI_Comm_size( MPI_COMM_WORLD, &mpiSize );
    //get the process id
    myPID = getpid();
    printf( "mpi initialized.  my rank=%d, size=%d, pid=%d. \n",
            mpiRank, mpiSize, myPID );

    if (mpiSize<2) {
        puts("this example requires at least 1 manager and 1 worker process.");
        MPI_Finalize();
        return 0;
    }

    if (mpiRank==0)    manager();
    else               worker();

    MPI_Finalize();
    return 0;
}
//----------------------------------------------------------------------