lin_alg.c

        /* initiate recvs */
        nbr = &system->my_nt_nbrs[d];
        if ( nbr->atoms_cnt )
        {
            /* calculate the total data that will be received */
            cnt = 0;
            for ( i = nbr->atoms_str; i < (nbr->atoms_str + nbr->atoms_cnt); ++i )
            {
                cnt += row_nnz[i];
            }

            /* initiate Irecv */
            if ( cnt )
            {
                count += 2;

                j_recv[d] = (int *) malloc( sizeof(int) * cnt );
                val_recv[d] = (real *) malloc( sizeof(real) * cnt );

                fprintf( stdout,"Dist communication receive phase direction %d will receive %d\n", d, cnt);
                fflush( stdout );
                t_start = Get_Time( );
                ret = MPI_Irecv( j_recv + d, cnt, MPI_INT, nbr->receive_rank, d, comm, &req[2 * d] );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                ret = MPI_Irecv( val_recv + d, cnt, MPI_DOUBLE, nbr->receive_rank, d, comm, &req[2 * d + 1] );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                t_comm += Get_Time( ) - t_start;
            }
        }
    }
    /////////////////////
    for( d = 0; d < 6; ++d)
    {
        nbr = &system->my_nt_nbrs[d];
        /* send both messages in dimension d */
        if ( out_bufs[d].cnt )
        {
            cnt = 0;
            for ( i = 0; i < out_bufs[d].cnt; ++i )
            {
                cnt += A->end[ out_bufs[d].index[i] ] - A->start[ out_bufs[d].index[i] ];
                if ( out_bufs[d].index[i] < 0 || out_bufs[d].index[i] >= A->n )
                {
                    fprintf( stdout, "INDEXING ERROR %d > %d\n", out_bufs[d].index[i], A->n );
                    fflush( stdout );
                }
               //     row_nnz[ out_bufs[d].index[i] ];
            }
            fprintf( stdout,"Dist communication    send phase direction %d should  send %d\n", d, cnt);
            fflush( stdout );

            if ( cnt )
            {
                j_send = (int *) malloc( sizeof(int) * cnt );
                val_send = (real *) malloc( sizeof(real) * cnt );

                cnt = 0;
                for ( i = 0; i < out_bufs[d].cnt; ++i )
                {
                    for ( pj = A->start[ out_bufs[d].index[i] ]; pj < A->end[ out_bufs[d].index[i] ]; ++pj )
                    {
                        atom = &system->my_atoms[ A->j[pj] ];
                        j_send[cnt] = atom->orig_id;
                        val_send[cnt] = A->val[pj];
                        cnt++;
                    }
                }

                fprintf( stdout,"Dist communication    send phase direction %d will    send %d\n", d, cnt );
                fflush( stdout );

                t_start = Get_Time( );
                ret = MPI_Send( j_send, cnt, MPI_INT, nbr->rank, d, comm );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                fprintf( stdout,"Dist communication send phase direction %d cnt = %d\n", d, cnt);
                fflush( stdout );
                ret = MPI_Send( val_send, cnt, MPI_DOUBLE, nbr->rank, d, comm );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                fprintf( stdout,"Dist communication send phase direction %d cnt = %d\n", d, cnt);
                fflush( stdout );
                t_comm += Get_Time( ) - t_start;
            }
        }
    }
    fprintf( stdout," Dist communication for sending row info before waitany\n");
    fflush( stdout );
    ///////////////////////
    for ( d = 0; d < count; ++d )
    {
        t_start = Get_Time( );
        ret = MPI_Waitany( MAX_NT_NBRS, req, &index, stat);
        Check_MPI_Error( ret, __FILE__, __LINE__ );
        t_comm += Get_Time( ) - t_start;

        nbr = &system->my_nt_nbrs[index / 2];
        cnt = 0;
        for ( i = nbr->atoms_str; i < (nbr->atoms_str + nbr->atoms_cnt); ++i )
        {
            if ( index % 2 == 0 )
            {
                j_list[i] = (int *) malloc( sizeof(int) *  row_nnz[i] );
                for ( pj = 0; pj < row_nnz[i]; ++pj )
                {
                    j_list[i][pj] = j_recv[index / 2][cnt];
                    cnt++;
                }
            }
            else
            {
                val_list[i] = (real *) malloc( sizeof(real) * row_nnz[i] );
                for ( pj = 0; pj < row_nnz[i]; ++pj )
                {
                    val_list[i][pj] = val_recv[index / 2][cnt];
                    cnt++;
                }
            }

        }
    }
    //////////////////////
    fprintf( stdout," wow wow wow, Dist communication for sending row info worked\n");
    fflush( stdout );
    //TODO: size?
    X = (int *) malloc( sizeof(int) * (system->bigN + 1) );
    pos_x = (int *) malloc( sizeof(int) * (system->bigN + 1) );

    for ( i = 0; i < A_spar_patt->NT; ++i )
    {
        N = 0;
        M = 0;
        for ( k = 0; k <= system->bigN; ++k )
        {
            X[k] = 0;
            pos_x[k] = 0;
        }

        /* find column indices of nonzeros (which will be the columns indices of the dense matrix) */
        for ( pj = A_spar_patt->start[i]; pj < A_spar_patt->end[i]; ++pj )
        {
            j_temp = A_spar_patt->j[pj];
            atom = &system->my_atoms[j_temp];
            ++N;

            /* for each of those indices
             * search through the row of full A of that index */

            /* the case where the local matrix has that index's row */
            if ( j_temp < A->NT )
            {
                for ( k = A->start[ j_temp ]; k < A->end[ j_temp ]; ++k )
                {
                    /* and accumulate the nonzero column indices to serve as the row indices of the dense matrix */
                    atom = &system->my_atoms[ A->j[k] ];
                    X[atom->orig_id] = 1;
                }
            }

            /* the case where we communicated that index's row */
            else
            {
                for ( k = 0; k < row_nnz[j_temp]; ++k )
                {
                    /* and accumulate the nonzero column indices to serve as the row indices of the dense matrix */
                    X[ j_list[j_temp][k] ] = 1;
                }
            }
        }

        /* enumerate the row indices from 0 to (# of nonzero rows - 1) for the dense matrix */
        identity_pos = M;
        atom = &system->my_atoms[ i ];
        atom_pos = atom->orig_id;

        for ( k = 0; k <= system->bigN; k++)
        {
            if ( X[k] != 0 )
            {
                pos_x[k] = M;
                if ( k == atom_pos )
                {
                    identity_pos = M;
                }
                ++M;
            }
        }

        /* allocate memory for NxM dense matrix */
        dense_matrix = (real *) malloc( sizeof(real) * N * M );

        /* fill in the entries of dense matrix */
        for ( d_j = 0; d_j < N; ++d_j)
        {
            /* all rows are initialized to zero */
            for ( d_i = 0; d_i < M; ++d_i )
            {
                dense_matrix[d_i * N + d_j] = 0.0;
            }
            /* change the value if any of the column indices is seen */

            /* it is in the original list */
            local_pos = A_spar_patt->j[ A_spar_patt->start[i] + d_j ];
            if ( local_pos < 0 || local_pos >= system->N )
            {
                fprintf( stderr, "THE LOCAL POSITION OF THE ATOM IS NOT VALID, STOP THE EXECUTION\n");
                fflush( stderr );

            }
            /////////////////////////////
            if ( local_pos < A->NT )
            {
                for ( d_i = A->start[local_pos]; d_i < A->end[local_pos]; ++d_i )
                {
                    atom = &system->my_atoms[ A->j[d_i] ];
                    if ( pos_x[ atom->orig_id ] >= M || d_j >=  N )
                    {
                        fprintf( stderr, "CANNOT MAP IT TO THE DENSE MATRIX, STOP THE EXECUTION, orig_id = %d, i =  %d, j = %d, M = %d N = %d\n", atom->orig_id, pos_x[ atom->orig_id ], d_j, M, N );
                        fflush( stderr );
                    }
                    if ( X[ atom->orig_id ] == 1 )
                    {
                        dense_matrix[ pos_x[ atom->orig_id ] * N + d_j ] = A->val[d_i];
                    }
                }
            }
            else
            {
                for ( d_i = 0; d_i < row_nnz[ local_pos ]; ++d_i )
                {
                    if ( pos_x[ j_list[local_pos][d_i] ] >= M || d_j  >= N )
                    {
                        fprintf( stderr, "CANNOT MAP IT TO THE DENSE MATRIX, STOP THE EXECUTION, %d %d\n", pos_x[ j_list[local_pos][d_i] ], d_j);
                        fflush( stderr );
                    }
                    if ( X[ j_list[local_pos][d_i] ] == 1 )
                    {
                        dense_matrix[ pos_x[ j_list[local_pos][d_i] ] * N + d_j ] = val_list[local_pos][d_i];
                    }
                }
            }
        }

        /* create the right hand side of the linear equation
         * that is the full column of the identity matrix */
        e_j = (real *) malloc( sizeof(real) * M );
        //////////////////////
        for ( k = 0; k < M; ++k )
        {
            e_j[k] = 0.0;
        }
        e_j[identity_pos] = 1.0;

        /* Solve the overdetermined system AX = B through the least-squares problem:
         * min ||B - AX||_2 */
        m = M;
        n = N;
        nrhs = 1;
        lda = N;
        ldb = nrhs;

        info = LAPACKE_dgels( LAPACK_ROW_MAJOR, 'N', m, n, nrhs, dense_matrix, lda,
                e_j, ldb );

        /* Check for the full rank */
        if ( info > 0 )
        {
            fprintf( stderr, "The diagonal element %i of the triangular factor ", info );
            fprintf( stderr, "of A is zero, so that A does not have full rank;\n" );
            fprintf( stderr, "the least squares solution could not be computed.\n" );
            exit( INVALID_INPUT );
        }

        /* accumulate the resulting vector to build A_app_inv */
        (*A_app_inv)->start[i] = A_spar_patt->start[i];
        (*A_app_inv)->end[i] = A_spar_patt->end[i];
        for ( k = (*A_app_inv)->start[i]; k < (*A_app_inv)->end[i]; ++k)
        {
            (*A_app_inv)->j[k] = A_spar_patt->j[k];
            (*A_app_inv)->val[k] = e_j[k - A_spar_patt->start[i]];
        }
        free( dense_matrix );
        free( e_j );
    }

    free( pos_x);
    free( X );
    /////////////////////
    ret = MPI_Reduce( &t_comm, &total_comm, 1, MPI_DOUBLE, MPI_SUM,
            MASTER_NODE, MPI_COMM_WORLD );
    Check_MPI_Error( ret, __FILE__, __LINE__ );

    if ( system->my_rank == MASTER_NODE )
    {
        data->timing.cm_solver_comm += total_comm / nprocs;
    }

    return Get_Time( ) - start;
}


#else
real sparse_approx_inverse( reax_system const * const system,
        simulation_data * const data,
        storage * const workspace, mpi_datatypes * const mpi_data, 
        sparse_matrix * const A, sparse_matrix * const A_spar_patt,
        sparse_matrix **A_app_inv, int nprocs )
{
    int i, k, pj, j_temp, push, ret;
    int N, M, d_i, d_j, mark;
    int local_pos, atom_pos, identity_pos;
    int *X, *q;
    int size_e, size_dense;
    int cnt;
    int *row_nnz;
    int **j_list;
    int d;
    int flag1, flag2;
    int *j_send, *j_recv1, *j_recv2;
    int size_send, size_recv1, size_recv2;
    real *e_j, *dense_matrix;
    real **val_list;
    real *val_send, *val_recv1, *val_recv2;
    real start, t_start, t_comm, total_comm;
    reax_atom *atom;
    mpi_out_data *out_bufs;
    const neighbor_proc *nbr1, *nbr2;
    MPI_Request req1, req2, req3, req4;
    MPI_Status stat1, stat2, stat3, stat4;
    lapack_int m, n, nrhs, lda, ldb, info;

    start = Get_Time( );
    t_comm = 0.0;

    if ( *A_app_inv == NULL)
    {
        Allocate_Matrix( A_app_inv, A_spar_patt->n, system->local_cap, A_spar_patt->m,
                SYM_FULL_MATRIX );
    }
    else /* if ( (*A_app_inv)->m < A_spar_patt->m ) */
    {
        Deallocate_Matrix( *A_app_inv );
        Allocate_Matrix( A_app_inv, A_spar_patt->n, system->local_cap, A_spar_patt->m,
                SYM_FULL_MATRIX );
    }

    X = NULL;
    j_send = NULL;
    val_send = NULL;
    j_recv1 = NULL;
    j_recv2 = NULL;
    val_recv1 = NULL;
    val_recv2 = NULL;
    size_send = 0;
    size_recv1 = 0;
    size_recv2 = 0;

    e_j = NULL;
    dense_matrix = NULL;
    size_e = 0;
    size_dense = 0;


    row_nnz = smalloc( sizeof(int) * system->total_cap,
           "sparse_approx_inverse::row_nnz" );
    j_list = smalloc( sizeof(int *) * system->N,
           "sparse_approx_inverse::j_list" );
    val_list = smalloc( sizeof(real *) * system->N,
           "sparse_approx_inverse::val_list" );

    for ( i = 0; i < system->total_cap; ++i )
    {
        row_nnz[i] = 0;
    }

    /* mark the atoms that already have their row stored in the local matrix */
    for ( i = 0; i < system->n; ++i )
    {
        row_nnz[i] = A->end[i] - A->start[i];
    }

    /* Announce the nnz's in each row that will be communicated later */
    t_start = Get_Time( );
    Dist( system, mpi_data, row_nnz, INT_PTR_TYPE, MPI_INT );
    t_comm += Get_Time( ) - t_start;

    comm = mpi_data->comm_mesh3D;
    out_bufs = mpi_data->out_buffers;

    /* use a Dist-like approach to send the row information */
    for ( d = 0; d < 3; ++d)
    {
        flag1 = 0;
        flag2 = 0;
        cnt = 0;

        /* initiate recvs */
        nbr1 = &system->my_nbrs[2 * d];
        if ( nbr1->atoms_cnt )
        {
            cnt = 0;

            /* calculate the total data that will be received */
            for( i = nbr1->atoms_str; i < (nbr1->atoms_str + nbr1->atoms_cnt); ++i )
            {
                cnt += row_nnz[i];
            }

            /* initiate Irecv */
            if( cnt )
            {
                flag1 = 1;
                
                if ( size_recv1 < cnt )
                {
                    if ( size_recv1 )
                    {
                        sfree( j_recv1, "sparse_approx_inverse::j_recv1" );
                        sfree( val_recv1, "sparse_approx_inverse::val_recv1" );
                    }

                    size_recv1 = cnt * SAFE_ZONE;

                    j_recv1 = smalloc( sizeof(int) * size_recv1,
                            "sparse_approx_inverse::j_recv1" );
                    val_recv1 = smalloc( sizeof(real) * size_recv1,
                            "sparse_approx_inverse::val_recv1" );
                }

                t_start = Get_Time( );
                ret = MPI_Irecv( j_recv1, cnt, MPI_INT, nbr1->rank, 2 * d + 1, comm, &req1 );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                ret = MPI_Irecv( val_recv1, cnt, MPI_DOUBLE, nbr1->rank, 2 * d + 1, comm, &req2 );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                t_comm += Get_Time( ) - t_start;
            }
        }

        nbr2 = &system->my_nbrs[2 * d + 1];
        if ( nbr2->atoms_cnt )
        {
            /* calculate the total data that will be received */
            cnt = 0;
            for( i = nbr2->atoms_str; i < (nbr2->atoms_str + nbr2->atoms_cnt); ++i )
            {
                cnt += row_nnz[i];
            }

            /* initiate Irecv */
            if( cnt )
            {
                flag2 = 1;

                if ( size_recv2 < cnt )
                {
                    if ( size_recv2 )
                    {
                        sfree( j_recv2, "sparse_approx_inverse::j_recv2" );
                        sfree( val_recv2, "sparse_approx_inverse::val_recv2" );
                    }

                    size_recv2 = cnt * SAFE_ZONE;

                    j_recv2 = smalloc( sizeof(int) * size_recv2,
                            "sparse_approx_inverse::j_recv2" );
                    val_recv2 = smalloc( sizeof(real) * size_recv2,
                            "sparse_approx_inverse::val_recv2" );
                }

                t_start = Get_Time( );
                ret = MPI_Irecv( j_recv2, cnt, MPI_INT, nbr2->rank, 2 * d, comm, &req3 );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                ret = MPI_Irecv( val_recv2, cnt, MPI_DOUBLE, nbr2->rank, 2 * d, comm, &req4 );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                t_comm += Get_Time( ) - t_start;
            }
        }

        /* send both messages in dimension d */
        if ( out_bufs[2 * d].cnt )
        {
            cnt = 0;
            for ( i = 0; i < out_bufs[2 * d].cnt; ++i )
            {
                cnt += row_nnz[ out_bufs[2 * d].index[i] ];
            }

            if ( cnt > 0 )
            {
                if ( size_send < cnt )
                {
                    if ( size_send )
                    {
                        sfree( j_send, "sparse_approx_inverse::j_send" );
                        sfree( val_send, "sparse_approx_inverse::val_send" );
                    }

                    size_send = cnt * SAFE_ZONE;

                    j_send = smalloc( sizeof(int) * size_send,
                            "sparse_approx_inverse::j_send" );
                    val_send = smalloc( sizeof(real) * size_send,
                            "sparse_approx_inverse::j_send" );
                }

                cnt = 0;
                for ( i = 0; i < out_bufs[2 * d].cnt; ++i )
                {
                    if ( out_bufs[2 * d].index[i] < A->n )
                    {
                        for ( pj = A->start[ out_bufs[2 * d].index[i] ]; pj < A->end[ out_bufs[2 * d].index[i] ]; ++pj )
                        {
                            atom = &system->my_atoms[ A->j[pj] ];
                            j_send[cnt] = atom->orig_id;
                            val_send[cnt] = A->val[pj];
                            cnt++;
                        }
                    }
                    else
                    {
                        for ( pj = 0; pj < row_nnz[ out_bufs[2 * d].index[i] ]; ++pj )
                        {
                            j_send[cnt] = j_list[ out_bufs[2 * d].index[i] ][pj];
                            val_send[cnt] = val_list[ out_bufs[2 * d].index[i] ][pj];
                            cnt++;
                        }
                    }
                }

                t_start = Get_Time( );
                ret = MPI_Send( j_send, cnt, MPI_INT, nbr1->rank, 2 * d, comm );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                ret = MPI_Send( val_send, cnt, MPI_DOUBLE, nbr1->rank, 2 * d, comm );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                t_comm += Get_Time( ) - t_start;
            }
        }

        if ( out_bufs[2 * d + 1].cnt )
        {
            cnt = 0;
            for ( i = 0; i < out_bufs[2 * d + 1].cnt; ++i )
            {
                cnt += row_nnz[ out_bufs[2 * d + 1].index[i] ];
            }

            if ( cnt > 0 )
            {

                if ( size_send < cnt )
                {
                    if ( size_send )
                    {
                        sfree( j_send, "sparse_approx_inverse::j_send" );
                        sfree( val_send, "sparse_approx_inverse::j_send" );
                    }

                    size_send = cnt * SAFE_ZONE;

                    j_send = smalloc( sizeof(int) * size_send,
                            "sparse_approx_inverse::j_send" );
                    val_send = smalloc( sizeof(real) * size_send,
                            "sparse_approx_inverse::val_send" );
                }

                cnt = 0;
                for ( i = 0; i < out_bufs[2 * d + 1].cnt; ++i )
                {
                    if ( out_bufs[2 * d + 1].index[i] < A->n )
                    {
                        for ( pj = A->start[ out_bufs[2 * d + 1].index[i] ]; pj < A->end[ out_bufs[2 * d + 1].index[i] ]; ++pj )
                        {
                            atom = &system->my_atoms[ A->j[pj] ];
                            j_send[cnt] = atom->orig_id;
                            val_send[cnt] = A->val[pj];
                            cnt++;
                        }
                    }
                    else
                    {
                        for ( pj = 0; pj < row_nnz[ out_bufs[2 * d + 1].index[i] ]; ++pj )
                        {
                            j_send[cnt] = j_list[ out_bufs[2 * d + 1].index[i] ][pj];
                            val_send[cnt] = val_list[ out_bufs[2 * d + 1].index[i] ][pj];
                            cnt++;
                        }
                    }
                }

                t_start = Get_Time( );
                ret = MPI_Send( j_send, cnt, MPI_INT, nbr2->rank, 2 * d + 1, comm );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                ret = MPI_Send( val_send, cnt, MPI_DOUBLE, nbr2->rank, 2 * d + 1, comm );
                Check_MPI_Error( ret, __FILE__, __LINE__ );
                t_comm += Get_Time( ) - t_start;
            }

        }

        if ( flag1 )
        {
            t_start = Get_Time( );
            ret = MPI_Wait( &req1, &stat1 );
            Check_MPI_Error( ret, __FILE__, __LINE__ );
            ret = MPI_Wait( &req2, &stat2 );
            Check_MPI_Error( ret, __FILE__, __LINE__ );
            t_comm += Get_Time( ) - t_start;

            cnt = 0;
            for ( i = nbr1->atoms_str; i < (nbr1->atoms_str + nbr1->atoms_cnt); ++i )
            {
                j_list[i] = smalloc( sizeof(int) *  row_nnz[i],
                       "sparse_approx_inverse::j_list[i]" );
                val_list[i] = smalloc( sizeof(real) * row_nnz[i],
                       "sparse_approx_inverse::val_list[i]" );

                for ( pj = 0; pj < row_nnz[i]; ++pj )
                {
                    j_list[i][pj] = j_recv1[cnt];
                    val_list[i][pj] = val_recv1[cnt];
                    cnt++;
                }
            }
        }

        if ( flag2 )
        {
            t_start = Get_Time( );
            ret = MPI_Wait( &req3, &stat3 );
            Check_MPI_Error( ret, __FILE__, __LINE__ );
            ret = MPI_Wait( &req4, &stat4 );
            Check_MPI_Error( ret, __FILE__, __LINE__ );
            t_comm += Get_Time( ) - t_start;

            cnt = 0;
            for ( i = nbr2->atoms_str; i < (nbr2->atoms_str + nbr2->atoms_cnt); ++i )
            {
                j_list[i] = smalloc( sizeof(int) *  row_nnz[i],
                       "sparse_approx_inverse::j_list[i]" );
                val_list[i] = smalloc( sizeof(real) * row_nnz[i],
                       "sparse_approx_inverse::val_list[i]" );

                for ( pj = 0; pj < row_nnz[i]; ++pj )
                {
                    j_list[i][pj] = j_recv2[cnt];
                    val_list[i][pj] = val_recv2[cnt];
                    cnt++;
                }
            }
        }
    }

    sfree( j_send, "sparse_approx_inverse::j_send" );
    sfree( val_send, "sparse_approx_inverse::val_send" );
    sfree( j_recv1, "sparse_approx_inverse::j_recv1" );
    sfree( j_recv2, "sparse_approx_inverse::j_recv2" );
    sfree( val_recv1, "sparse_approx_inverse::val_recv1" );
    sfree( val_recv2, "sparse_approx_inverse::val_recv2" );

    X = smalloc( sizeof(int) * (system->bigN + 1),
            "sparse_approx_inverse::X" );
    //size of q should be equal to the maximum possible cardinalty 
    //of the set formed by neighbors of neighbors of an atom
    //i.e, maximum number of rows of dense matrix
    //for water systems, this number is 34000
    //for silica systems, it is 12000
    q = smalloc( sizeof(int) * 50000,
            "sparse_approx_inverse::q" );

    for ( i = 0; i <= system->bigN; ++i )
    {
        X[i] = -1;
    }

    for ( i = 0; i < A_spar_patt->n; ++i )
    {
        N = 0;
        M = 0;
        push = 0;
        mark = i + system->bigN;
        
        /* find column indices of nonzeros (which will be the columns indices of the dense matrix) */
        for ( pj = A_spar_patt->start[i]; pj < A_spar_patt->end[i]; ++pj )
        {
            j_temp = A_spar_patt->j[pj];
            atom = &system->my_atoms[j_temp];
            ++N;

            /* for each of those indices
             * search through the row of full A of that index */

            /* the case where the local matrix has that index's row */
            if( j_temp < A->n )
            {
                for ( k = A->start[ j_temp ]; k < A->end[ j_temp ]; ++k )
                {
                    /* and accumulate the nonzero column indices to serve as the row indices of the dense matrix */
                    atom = &system->my_atoms[ A->j[k] ];
                    X[atom->orig_id] = mark;
                    q[push++] = atom->orig_id;
                }
            }

            /* the case where we communicated that index's row */
            else
            {
                for ( k = 0; k < row_nnz[j_temp]; ++k )
                {
                    /* and accumulate the nonzero column indices to serve as the row indices of the dense matrix */
                    X[ j_list[j_temp][k] ] = mark;
                    q[push++] = j_list[j_temp][k];
                }
            }
        }

        /* enumerate the row indices from 0 to (# of nonzero rows - 1) for the dense matrix */
        identity_pos = M;
        atom = &system->my_atoms[ i ];
        atom_pos = atom->orig_id;

        for ( k = 0; k < push; k++)
        {
            if ( X[ q[k] ] == mark )
            {
                X[ q[k] ] = M;
                ++M;
            }
        }
        identity_pos = X[atom_pos];

        /* allocate memory for NxM dense matrix */
        if ( size_dense < N * M )
        {
            if ( size_dense )
            {
                sfree( dense_matrix, "sparse_approx_inverse::dense_matrix" );
            }
            
            size_dense = N * M * SAFE_ZONE;

            dense_matrix = smalloc( sizeof(real) * size_dense,
                "sparse_approx_inverse::dense_matrix" );
        }

        /* fill in the entries of dense matrix */
        for ( d_j = 0; d_j < N; ++d_j)
        {
            /* all rows are initialized to zero */
            for ( d_i = 0; d_i < M; ++d_i )
            {
                dense_matrix[d_i * N + d_j] = 0.0;
            }
            /* change the value if any of the column indices is seen */

            /* it is in the original list */
            local_pos = A_spar_patt->j[ A_spar_patt->start[i] + d_j ];

            if ( local_pos < A->n )
            {
                for ( d_i = A->start[local_pos]; d_i < A->end[local_pos]; ++d_i )
                {
                    atom = &system->my_atoms[ A->j[d_i] ];
                    dense_matrix[ X[ atom->orig_id ] * N + d_j ] = A->val[d_i];
                }
            }
            else
            {
                for ( d_i = 0; d_i < row_nnz[ local_pos ]; ++d_i )
                {
                    dense_matrix[ X[ j_list[local_pos][d_i] ] * N + d_j ] = val_list[local_pos][d_i];
                }
            }
        }

        /* create the right hand side of the linear equation
         * that is the full column of the identity matrix */
        if ( size_e < M )
        {
            if ( size_e )
            {
                sfree( e_j, "sparse_approx_inverse::e_j" );
            }

            size_e = M * SAFE_ZONE;

            e_j = smalloc( sizeof(real) * size_e, "sparse_approx_inverse::e_j" );
        }

        for ( k = 0; k < M; ++k )
        {
            e_j[k] = 0.0;
        }
        e_j[identity_pos] = 1.0;

        /* Solve the overdetermined system AX = B through the least-squares problem:
         * min ||B - AX||_2 */
        m = M;
        n = N;
        nrhs = 1;
        lda = N;
        ldb = nrhs;

        info = LAPACKE_dgels( LAPACK_ROW_MAJOR, 'N', m, n, nrhs, dense_matrix, lda,
                e_j, ldb );

        /* Check for the full rank */
        if ( info > 0 )
        {
            fprintf( stderr, "[ERROR] The diagonal element %i of the triangular factor ", info );
            fprintf( stderr, "of A is zero, so that A does not have full rank;\n" );
            fprintf( stderr, "the least squares solution could not be computed.\n" );
            MPI_Abort( MPI_COMM_WORLD, RUNTIME_ERROR );
        }

        /* accumulate the resulting vector to build A_app_inv */
        (*A_app_inv)->start[i] = A_spar_patt->start[i];
        (*A_app_inv)->end[i] = A_spar_patt->end[i];
        for ( k = (*A_app_inv)->start[i]; k < (*A_app_inv)->end[i]; ++k)
        {
            (*A_app_inv)->j[k] = A_spar_patt->j[k];
            (*A_app_inv)->val[k] = e_j[k - A_spar_patt->start[i]];
        }
    }

    sfree( dense_matrix, "sparse_approx_inverse::dense_matrix" );
    sfree( e_j, "sparse_approx_inverse::e_j" );
    sfree( X, "sparse_approx_inverse::X" );
    /*for ( i = 0; i < system->N; ++i )
    {
        sfree( j_list[i], "sparse_approx_inverse::j_list" );
        sfree( val_list[i], "sparse_approx_inverse::val_list" );
    }
    sfree( j_list, "sparse_approx_inverse::j_list" );
    sfree( val_list, "sparse_approx_inverse::val_list" );*/
    sfree( row_nnz, "sparse_approx_inverse::row_nnz" );

    ret = MPI_Reduce( &t_comm, &total_comm, 1, MPI_DOUBLE, MPI_SUM,
            MASTER_NODE, MPI_COMM_WORLD );
    Check_MPI_Error( ret, __FILE__, __LINE__ );

    if ( system->my_rank == MASTER_NODE )
    {
        data->timing.cm_solver_comm += total_comm / nprocs;
    }

    return Get_Time( ) - start;
}
#endif
#endif


static void apply_preconditioner( const reax_system * const system,
        const storage * const workspace, 
        const control_params * const control,
        mpi_datatypes * const  mpi_data,
        const real * const y, real * const x,
        const int fresh_pre, const int side )
{
//    int i, si;
    real t_start, t_pa, t_comm;

    /* no preconditioning */
    if ( control->cm_solver_pre_comp_type == NONE_PC )
    {
        if ( x != y )
        {
            Vector_Copy( x, y, system->n );
        }
    }
    else
    {
        switch ( side )
        {
            case LEFT:
                switch ( control->cm_solver_pre_app_type )
                {
                    case TRI_SOLVE_PA:
                        switch ( control->cm_solver_pre_comp_type )
                        {
                            case JACOBI_PC:
                                jacobi_app( workspace->Hdia_inv, y, x, system->n );
                                break;
//                            case ICHOLT_PC:
//                            case ILUT_PC:
//                            case ILUTP_PC:
//                                  tri_solve( workspace->L, y, x, workspace->L->n, LOWER );
//                                  break;
                            case SAI_PC:
                                t_comm += Sparse_MatVec_Comm_Part1( system, control, mpi_data,
                                        y, REAL_PTR_TYPE, MPI_DOUBLE );
                                
                                t_start = Get_Time( );
#if defined(NEUTRAL_TERRITORY)
                                Sparse_MatVec_local( &workspace->H_app_inv, y, x, H->NT );
#else
                                Sparse_MatVec_local( &workspace->H_app_inv, y, x, system->n );
#endif
                                t_pa += Get_Time( ) - t_start;

                                /* no comm part2 because x is only local portion */
                                break;
                            default:
                                fprintf( stderr, "Unrecognized preconditioner application method. Terminating...\n" );
                                exit( INVALID_INPUT );
                                break;
                        }
                        break;
                    case TRI_SOLVE_LEVEL_SCHED_PA:
                        switch ( control->cm_solver_pre_comp_type )
                        {
                            case JACOBI_PC:
                                jacobi_app( workspace->Hdia_inv, y, x, system->n );
                                break;
//                            case ICHOLT_PC:
//                            case ILUT_PC:
//                            case ILUTP_PC:
//                                  tri_solve_level_sched( (static_storage *) workspace,
//                                          workspace->L, y, x, workspace->L->n, LOWER, fresh_pre );
//                                  break;
                            case SAI_PC:
                                t_comm += Sparse_MatVec_Comm_Part1( system, control, mpi_data,
                                        y, REAL_PTR_TYPE, MPI_DOUBLE );
                                
                                t_start = Get_Time( );
#if defined(NEUTRAL_TERRITORY)
                                Sparse_MatVec_local( &workspace->H_app_inv, y, x, H->NT );
#else
                                Sparse_MatVec_local( &workspace->H_app_inv, y, x, system->n );
#endif
                                t_pa += Get_Time( ) - t_start;

                                /* no comm part2 because x is only local portion */
                                break;
                            default:
                                fprintf( stderr, "Unrecognized preconditioner application method. Terminating...\n" );
                                exit( INVALID_INPUT );
                                break;
                        }
                        break;
                    case TRI_SOLVE_GC_PA:
                        switch ( control->cm_solver_pre_comp_type )
                        {
                            case JACOBI_PC:
                            case SAI_PC:
                                fprintf( stderr, "Unsupported preconditioner computation/application method combination. Terminating...\n" );
                                exit( INVALID_INPUT );
                                break;
//                            case ICHOLT_PC:
//                            case ILUT_PC:
//                            case ILUTP_PC:
//                                  for ( i = 0; i < workspace->H->n; ++i )
//                                  {
//                                      workspace->y_p[i] = y[i];
//                                  }
//
//                                  permute_vector( workspace, workspace->y_p, workspace->H->n, FALSE, LOWER );
//                                  tri_solve_level_sched( (static_storage *) workspace,
//                                  workspace->L, workspace->y_p, x, workspace->L->n, LOWER, fresh_pre );
//                                  break;
                            default:
                                fprintf( stderr, "Unrecognized preconditioner application method. Terminating...\n" );
                                exit( INVALID_INPUT );
                                break;
                        }
                        break;
                    case JACOBI_ITER_PA:
                        switch ( control->cm_solver_pre_comp_type )
                        {
                            case JACOBI_PC:
                            case SAI_PC:
                                fprintf( stderr, "Unsupported preconditioner computation/application method combination. Terminating...\n" );
                                exit( INVALID_INPUT );
                                break;
//                            case ICHOLT_PC:
//                            case ILUT_PC:
//                            case ILUTP_PC:
//                                // construct D^{-1}_L
//                                if ( fresh_pre == TRUE )
//                                {
//                                    for ( i = 0; i < workspace->L->n; ++i )
//                                    {
//                                        si = workspace->L->start[i + 1] - 1;
//                                        workspace->Dinv_L[i] = 1.0 / workspace->L->val[si];
//                                    }
//                                }
//
//                                jacobi_iter( workspace, workspace->L, workspace->Dinv_L,
//                                        y, x, LOWER, control->cm_solver_pre_app_jacobi_iters );
//                                break;
                            default:
                                fprintf( stderr, "Unrecognized preconditioner application method. Terminating...\n" );
                                exit( INVALID_INPUT );
                                break;
                        }
                        break;
                    default:
                        fprintf( stderr, "Unrecognized preconditioner application method. Terminating...\n" );
                        exit( INVALID_INPUT );
                        break;

                }
                break;