for(ii=0;ii<params.ny;ii++) {
        for(jj=0;jj<params.nx;jj++) {

            y_n = (ii + 1) % params.ny;
            x_e = (jj + 1) % params.nx;
            y_s = (ii == 0) ? (ii + params.ny - 1) : (ii - 1);
            x_w = (jj == 0) ? (jj + params.nx - 1) : (jj - 1);

            tmp_cells[INDEX3(ii,x_e,0)] =  cells[INDEX3(ii,jj,0)];
            tmp_cells[INDEX3(y_n,jj,1)] =  cells[INDEX3(ii,jj,1)];
            tmp_cells[INDEX3(ii,x_w,2)] =  cells[INDEX3(ii,jj,2)];
            tmp_cells[INDEX3(y_s,jj,3)] =  cells[INDEX3(ii,jj,3)];
            tmp_cells[INDEX3(y_n,x_e,4)] = cells[INDEX3(ii,jj,4)];
            tmp_cells[INDEX3(y_n,x_w,5)] = cells[INDEX3(ii,jj,5)];
            tmp_cells[INDEX3(y_s,x_w,6)] = cells[INDEX3(ii,jj,6)];
            tmp_cells[INDEX3(y_s,x_e,7)] = cells[INDEX3(ii,jj,7)];
        }
    }