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// Lesser General Public License for more details.
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// Solve a 2D Laplacian problem
// This example builds the matrix and solves it with AztecOO.

#include "Epetra_ConfigDefs.h"
#ifdef HAVE_MPI
#include "mpi.h"
#include "Epetra_MpiComm.h"
#else
#include "Epetra_SerialComm.h"
#endif
#include "Epetra_Map.h"
#include "Epetra_Vector.h"
#include "Epetra_CrsMatrix.h"
#include "AztecOO.h"

// external function
void  get_neighbours( const int i, const int nx, const int ny,
int & left, int & right,
int & lower, int & upper);

// =========== //
// main driver //
// =========== //

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

#ifdef HAVE_MPI
MPI_Init(&argc, &argv);
Epetra_MpiComm Comm(MPI_COMM_WORLD);
#else
Epetra_SerialComm Comm;
#endif

// number of nodes along the x- and y-axis
int nx = 5;
int ny = 6;
int NumGlobalElements = nx * ny;

// create a linear map
Epetra_Map Map(NumGlobalElements,0,Comm);

// local number of rows
int NumMyElements = Map.NumMyElements();
// get update list
int * MyGlobalElements = new int [NumMyElements];
Map.MyGlobalElements( MyGlobalElements );

// Create a Epetra_Matrix with 5 nonzero per rows

Epetra_CrsMatrix A(Copy,Map,5);

// Need some vectors to help

double Values[4];
int Indices[4];
int left, right, lower, upper;
double diag = 4.0;

for( int i=0 ; i<NumMyElements; ++i ) {
int NumEntries=0;
get_neighbours(  MyGlobalElements[i], nx, ny,
left, right, lower, upper);
if( left != -1 ) {
Indices[NumEntries] = left;
Values[NumEntries] = -1.0;
++NumEntries;
}
if( right != -1 ) {
Indices[NumEntries] = right;
Values[NumEntries] = -1.0;
++NumEntries;
}
if( lower != -1 ) {
Indices[NumEntries] = lower;
Values[NumEntries] = -1.0;
++NumEntries;
}
if( upper != -1 ) {
Indices[NumEntries] = upper;
Values[NumEntries] = -1.0;
++NumEntries;
}
// put the off-diagonal entries
A.InsertGlobalValues(MyGlobalElements[i], NumEntries, Values, Indices);
// Put in the diagonal entry
A.InsertGlobalValues(MyGlobalElements[i], 1, &diag, MyGlobalElements+i);
}

// Finish up
A.FillComplete();

// create x and b vectors
Epetra_Vector x(Map);
Epetra_Vector b(Map);
b.PutScalar(1.0);

// ==================== AZTECOO INTERFACE ======================

// create linear problem
Epetra_LinearProblem Problem(&A,&x,&b);
// create AztecOO instance
AztecOO Solver(Problem);

Solver.SetAztecOption( AZ_precond, AZ_Jacobi );

Solver.Iterate(1000,1E-9);

// ==================== END OF AZTECOO INTERFACE ================

if( Comm.MyPID() == 0 ) {
cout << "Solver performed " << Solver.NumIters()
<< "iterations.\n";
cout << "Norm of the true residual = " << Solver.TrueResidual() << endl;
}

#ifdef HAVE_MPI
MPI_Finalize();
#endif

return(EXIT_SUCCESS);

}

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

void  get_neighbours( const int i, const int nx, const int ny,
int & left, int & right,
int & lower, int & upper)
{

int ix, iy;
ix = i%nx;
iy = (i - ix)/nx;

if( ix == 0 )
left = -1;
else
left = i-1;
if( ix == nx-1 )
right = -1;
else
right = i+1;
if( iy == 0 )
lower = -1;
else
lower = i-nx;
if( iy == ny-1 )
upper = -1;
else
upper = i+nx;

return;

}

#else

#include <stdlib.h>
#include <stdio.h>
#ifdef HAVE_MPI
#include "mpi.h"
#endif

int main(int argc, char *argv[])
{
#ifdef HAVE_MPI
MPI_Init(&argc,&argv);
#endif