[petsc-users] Efficient FFTShift-implementation for vectors/matrices

[Roland Richter]

Dear all,

I tried to implement the function fftshift from numpy (i.e. swap the
half-spaces of all axis) for row vectors in a matrix by using the
following code

void fft_shift(Mat &fft_matrix) {
        PetscScalar *mat_ptr;
        MatDenseGetArray (fft_matrix, &mat_ptr);
        PetscInt r_0, r_1;
        MatGetOwnershipRange(fft_matrix, &r_0, &r_1);
        PetscInt local_row_num = r_1 - r_0;
        arma::cx_mat temp_mat(local_row_num, Ntime, arma::fill::zeros);
        for(int i = 0; i < Ntime; ++i) {
            const PetscInt row_shift = i * local_row_num;
            for(int j = 0; j < local_row_num; ++j) {
                const PetscInt cur_pos = j + row_shift;
                if(i < (int)(Ntime / 2))
                    temp_mat(j, i + int(Ntime / 2)) = *(mat_ptr + cur_pos);
                else
                    temp_mat(j, i - int(Ntime / 2)) = *(mat_ptr + cur_pos);
            }
        }
        for(int i = 0; i < Ntime; ++i) {
            const PetscInt row_shift = i * local_row_num;
            for(int j = 0; j < local_row_num; ++j) {
                const PetscInt cur_pos = j + row_shift;
                *(mat_ptr + cur_pos) = temp_mat(j, i);
            }
        }
        MatDenseRestoreArray (fft_matrix, &mat_ptr);
}

but I do not like the approach of having a second matrix as temporary
storage space. Are there more efficient approaches possible using
PETSc-functions?

Thanks!

Regards,

Roland Richter