/*
    Provides an interface to the FFTW package.
    Testing examples can be found in ~src/mat/tests
*/

#include <../src/mat/impls/fft/fft.h> /*I "petscmat.h" I*/
EXTERN_C_BEGIN
#if !PetscDefined(HAVE_MPIUNI)
  #include <fftw3-mpi.h>
#else
  #include <fftw3.h>
#endif
EXTERN_C_END

typedef struct {
  ptrdiff_t ndim_fftw, *dim_fftw;
#if defined(PETSC_USE_64BIT_INDICES)
  fftw_iodim64 *iodims;
#else
  fftw_iodim *iodims;
#endif
  PetscInt     partial_dim;
  fftw_plan    p_forward, p_backward;
  unsigned     p_flag;                                      /* planner flags, FFTW_ESTIMATE,FFTW_MEASURE, FFTW_PATIENT, FFTW_EXHAUSTIVE */
  PetscScalar *finarray, *foutarray, *binarray, *boutarray; /* keep track of arrays because fftw plan should be
                                                            executed for the arrays with which the plan was created */
} Mat_FFTW;

extern PetscErrorCode MatMult_SeqFFTW(Mat, Vec, Vec);
extern PetscErrorCode MatMultTranspose_SeqFFTW(Mat, Vec, Vec);
extern PetscErrorCode MatDestroy_FFTW(Mat);
extern PetscErrorCode MatCreateVecsFFTW_FFTW(Mat, Vec *, Vec *, Vec *);
#if !PetscDefined(HAVE_MPIUNI)
extern PetscErrorCode MatMult_MPIFFTW(Mat, Vec, Vec);
extern PetscErrorCode MatMultTranspose_MPIFFTW(Mat, Vec, Vec);
extern PetscErrorCode VecDestroy_MPIFFTW(Vec);
#endif

PetscErrorCode MatMult_SeqFFTW(Mat A, Vec x, Vec y)
{
  Mat_FFT           *fft  = (Mat_FFT *)A->data;
  Mat_FFTW          *fftw = (Mat_FFTW *)fft->data;
  const PetscScalar *x_array;
  PetscScalar       *y_array;
#if defined(PETSC_USE_COMPLEX)
  #if defined(PETSC_USE_64BIT_INDICES)
  fftw_iodim64 *iodims;
  #else
  fftw_iodim *iodims;
  #endif
  PetscInt i;
#endif
  PetscInt ndim = fft->ndim, *dim = fft->dim;

  PetscFunctionBegin;
  PetscCall(VecGetArrayRead(x, &x_array));
  PetscCall(VecGetArray(y, &y_array));
  if (!fftw->p_forward) { /* create a plan, then execute it */
    switch (ndim) {
    case 1:
#if defined(PETSC_USE_COMPLEX)
      fftw->p_forward = fftw_plan_dft_1d(dim[0], (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_FORWARD, fftw->p_flag);
#else
      fftw->p_forward = fftw_plan_dft_r2c_1d(dim[0], (double *)x_array, (fftw_complex *)y_array, fftw->p_flag);
#endif
      break;
    case 2:
#if defined(PETSC_USE_COMPLEX)
      fftw->p_forward = fftw_plan_dft_2d(dim[0], dim[1], (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_FORWARD, fftw->p_flag);
#else
      fftw->p_forward = fftw_plan_dft_r2c_2d(dim[0], dim[1], (double *)x_array, (fftw_complex *)y_array, fftw->p_flag);
#endif
      break;
    case 3:
#if defined(PETSC_USE_COMPLEX)
      fftw->p_forward = fftw_plan_dft_3d(dim[0], dim[1], dim[2], (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_FORWARD, fftw->p_flag);
#else
      fftw->p_forward = fftw_plan_dft_r2c_3d(dim[0], dim[1], dim[2], (double *)x_array, (fftw_complex *)y_array, fftw->p_flag);
#endif
      break;
    default:
#if defined(PETSC_USE_COMPLEX)
      iodims = fftw->iodims;
  #if defined(PETSC_USE_64BIT_INDICES)
      if (ndim) {
        iodims[ndim - 1].n  = (ptrdiff_t)dim[ndim - 1];
        iodims[ndim - 1].is = iodims[ndim - 1].os = 1;
        for (i = ndim - 2; i >= 0; --i) {
          iodims[i].n  = (ptrdiff_t)dim[i];
          iodims[i].is = iodims[i].os = iodims[i + 1].is * iodims[i + 1].n;
        }
      }
      fftw->p_forward = fftw_plan_guru64_dft((int)ndim, (fftw_iodim64 *)iodims, 0, NULL, (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_FORWARD, fftw->p_flag);
  #else
      if (ndim) {
        iodims[ndim - 1].n  = (int)dim[ndim - 1];
        iodims[ndim - 1].is = iodims[ndim - 1].os = 1;
        for (i = ndim - 2; i >= 0; --i) {
          iodims[i].n  = (int)dim[i];
          iodims[i].is = iodims[i].os = iodims[i + 1].is * iodims[i + 1].n;
        }
      }
      fftw->p_forward = fftw_plan_guru_dft((int)ndim, (fftw_iodim *)iodims, 0, NULL, (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_FORWARD, fftw->p_flag);
  #endif

#else
      fftw->p_forward = fftw_plan_dft_r2c(ndim, (int *)dim, (double *)x_array, (fftw_complex *)y_array, fftw->p_flag);
#endif
      break;
    }
    fftw->finarray  = (PetscScalar *)x_array;
    fftw->foutarray = y_array;
    /* Warning: if (fftw->p_flag!==FFTW_ESTIMATE) The data in the in/out arrays is overwritten!
                planning should be done before x is initialized! See FFTW manual sec2.1 or sec4 */
    fftw_execute(fftw->p_forward);
  } else {                                                         /* use existing plan */
    if (fftw->finarray != x_array || fftw->foutarray != y_array) { /* use existing plan on new arrays */
#if defined(PETSC_USE_COMPLEX)
      fftw_execute_dft(fftw->p_forward, (fftw_complex *)x_array, (fftw_complex *)y_array);
#else
      fftw_execute_dft_r2c(fftw->p_forward, (double *)x_array, (fftw_complex *)y_array);
#endif
    } else {
      fftw_execute(fftw->p_forward);
    }
  }
  PetscCall(VecRestoreArray(y, &y_array));
  PetscCall(VecRestoreArrayRead(x, &x_array));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode MatMultTranspose_SeqFFTW(Mat A, Vec x, Vec y)
{
  Mat_FFT           *fft  = (Mat_FFT *)A->data;
  Mat_FFTW          *fftw = (Mat_FFTW *)fft->data;
  const PetscScalar *x_array;
  PetscScalar       *y_array;
  PetscInt           ndim = fft->ndim, *dim = fft->dim;
#if defined(PETSC_USE_COMPLEX)
  #if defined(PETSC_USE_64BIT_INDICES)
  fftw_iodim64 *iodims = fftw->iodims;
  #else
  fftw_iodim *iodims = fftw->iodims;
  #endif
#endif

  PetscFunctionBegin;
  PetscCall(VecGetArrayRead(x, &x_array));
  PetscCall(VecGetArray(y, &y_array));
  if (!fftw->p_backward) { /* create a plan, then execute it */
    switch (ndim) {
    case 1:
#if defined(PETSC_USE_COMPLEX)
      fftw->p_backward = fftw_plan_dft_1d(dim[0], (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_BACKWARD, fftw->p_flag);
#else
      fftw->p_backward = fftw_plan_dft_c2r_1d(dim[0], (fftw_complex *)x_array, (double *)y_array, fftw->p_flag);
#endif
      break;
    case 2:
#if defined(PETSC_USE_COMPLEX)
      fftw->p_backward = fftw_plan_dft_2d(dim[0], dim[1], (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_BACKWARD, fftw->p_flag);
#else
      fftw->p_backward = fftw_plan_dft_c2r_2d(dim[0], dim[1], (fftw_complex *)x_array, (double *)y_array, fftw->p_flag);
#endif
      break;
    case 3:
#if defined(PETSC_USE_COMPLEX)
      fftw->p_backward = fftw_plan_dft_3d(dim[0], dim[1], dim[2], (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_BACKWARD, fftw->p_flag);
#else
      fftw->p_backward = fftw_plan_dft_c2r_3d(dim[0], dim[1], dim[2], (fftw_complex *)x_array, (double *)y_array, fftw->p_flag);
#endif
      break;
    default:
#if defined(PETSC_USE_COMPLEX)
  #if defined(PETSC_USE_64BIT_INDICES)
      fftw->p_backward = fftw_plan_guru64_dft((int)ndim, (fftw_iodim64 *)iodims, 0, NULL, (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_BACKWARD, fftw->p_flag);
  #else
      fftw->p_backward = fftw_plan_guru_dft((int)ndim, iodims, 0, NULL, (fftw_complex *)x_array, (fftw_complex *)y_array, FFTW_BACKWARD, fftw->p_flag);
  #endif
#else
      fftw->p_backward = fftw_plan_dft_c2r((int)ndim, (int *)dim, (fftw_complex *)x_array, (double *)y_array, fftw->p_flag);
#endif
      break;
    }
    fftw->binarray  = (PetscScalar *)x_array;
    fftw->boutarray = y_array;
    fftw_execute(fftw->p_backward);
  } else {                                                         /* use existing plan */
    if (fftw->binarray != x_array || fftw->boutarray != y_array) { /* use existing plan on new arrays */
#if defined(PETSC_USE_COMPLEX)
      fftw_execute_dft(fftw->p_backward, (fftw_complex *)x_array, (fftw_complex *)y_array);
#else
      fftw_execute_dft_c2r(fftw->p_backward, (fftw_complex *)x_array, (double *)y_array);
#endif
    } else {
      fftw_execute(fftw->p_backward);
    }
  }
  PetscCall(VecRestoreArray(y, &y_array));
  PetscCall(VecRestoreArrayRead(x, &x_array));
  PetscFunctionReturn(PETSC_SUCCESS);
}

#if !PetscDefined(HAVE_MPIUNI)
PetscErrorCode MatMult_MPIFFTW(Mat A, Vec x, Vec y)
{
  Mat_FFT           *fft  = (Mat_FFT *)A->data;
  Mat_FFTW          *fftw = (Mat_FFTW *)fft->data;
  const PetscScalar *x_array;
  PetscScalar       *y_array;
  PetscInt           ndim = fft->ndim, *dim = fft->dim;
  MPI_Comm           comm;

  PetscFunctionBegin;
  PetscCall(PetscObjectGetComm((PetscObject)A, &comm));
  PetscCall(VecGetArrayRead(x, &x_array));
  PetscCall(VecGetArray(y, &y_array));
  if (!fftw->p_forward) { /* create a plan, then execute it */
    switch (ndim) {
    case 1:
  #if defined(PETSC_USE_COMPLEX)
      fftw->p_forward = fftw_mpi_plan_dft_1d(dim[0], (fftw_complex *)x_array, (fftw_complex *)y_array, comm, FFTW_FORWARD, fftw->p_flag);
  #else
      SETERRQ(comm, PETSC_ERR_SUP, "not support for real numbers yet");
  #endif
      break;
    case 2:
  #if defined(PETSC_USE_COMPLEX) /* For complex transforms call fftw_mpi_plan_dft, for real transforms call fftw_mpi_plan_dft_r2c */
      fftw->p_forward = fftw_mpi_plan_dft_2d(dim[0], dim[1], (fftw_complex *)x_array, (fftw_complex *)y_array, comm, FFTW_FORWARD, fftw->p_flag);
  #else
      fftw->p_forward = fftw_mpi_plan_dft_r2c_2d(dim[0], dim[1], (double *)x_array, (fftw_complex *)y_array, comm, FFTW_ESTIMATE);
  #endif
      break;
    case 3:
  #if defined(PETSC_USE_COMPLEX)
      fftw->p_forward = fftw_mpi_plan_dft_3d(dim[0], dim[1], dim[2], (fftw_complex *)x_array, (fftw_complex *)y_array, comm, FFTW_FORWARD, fftw->p_flag);
  #else
      fftw->p_forward = fftw_mpi_plan_dft_r2c_3d(dim[0], dim[1], dim[2], (double *)x_array, (fftw_complex *)y_array, comm, FFTW_ESTIMATE);
  #endif
      break;
    default:
  #if defined(PETSC_USE_COMPLEX)
      fftw->p_forward = fftw_mpi_plan_dft(fftw->ndim_fftw, fftw->dim_fftw, (fftw_complex *)x_array, (fftw_complex *)y_array, comm, FFTW_FORWARD, fftw->p_flag);
  #else
      fftw->p_forward = fftw_mpi_plan_dft_r2c(fftw->ndim_fftw, fftw->dim_fftw, (double *)x_array, (fftw_complex *)y_array, comm, FFTW_ESTIMATE);
  #endif
      break;
    }
    fftw->finarray  = (PetscScalar *)x_array;
    fftw->foutarray = y_array;
    /* Warning: if (fftw->p_flag!==FFTW_ESTIMATE) The data in the in/out arrays is overwritten!
                planning should be done before x is initialized! See FFTW manual sec2.1 or sec4 */
    fftw_execute(fftw->p_forward);
  } else {                                                         /* use existing plan */
    if (fftw->finarray != x_array || fftw->foutarray != y_array) { /* use existing plan on new arrays */
      fftw_execute_dft(fftw->p_forward, (fftw_complex *)x_array, (fftw_complex *)y_array);
    } else {
      fftw_execute(fftw->p_forward);
    }
  }
  PetscCall(VecRestoreArray(y, &y_array));
  PetscCall(VecRestoreArrayRead(x, &x_array));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode MatMultTranspose_MPIFFTW(Mat A, Vec x, Vec y)
{
  Mat_FFT           *fft  = (Mat_FFT *)A->data;
  Mat_FFTW          *fftw = (Mat_FFTW *)fft->data;
  const PetscScalar *x_array;
  PetscScalar       *y_array;
  PetscInt           ndim = fft->ndim, *dim = fft->dim;
  MPI_Comm           comm;

  PetscFunctionBegin;
  PetscCall(PetscObjectGetComm((PetscObject)A, &comm));
  PetscCall(VecGetArrayRead(x, &x_array));
  PetscCall(VecGetArray(y, &y_array));
  if (!fftw->p_backward) { /* create a plan, then execute it */
    switch (ndim) {
    case 1:
  #if defined(PETSC_USE_COMPLEX)
      fftw->p_backward = fftw_mpi_plan_dft_1d(dim[0], (fftw_complex *)x_array, (fftw_complex *)y_array, comm, FFTW_BACKWARD, fftw->p_flag);
  #else
      SETERRQ(comm, PETSC_ERR_SUP, "not support for real numbers yet");
  #endif
      break;
    case 2:
  #if defined(PETSC_USE_COMPLEX) /* For complex transforms call fftw_mpi_plan_dft with flag FFTW_BACKWARD, for real transforms call fftw_mpi_plan_dft_c2r */
      fftw->p_backward = fftw_mpi_plan_dft_2d(dim[0], dim[1], (fftw_complex *)x_array, (fftw_complex *)y_array, comm, FFTW_BACKWARD, fftw->p_flag);
  #else
      fftw->p_backward = fftw_mpi_plan_dft_c2r_2d(dim[0], dim[1], (fftw_complex *)x_array, (double *)y_array, comm, FFTW_ESTIMATE);
  #endif
      break;
    case 3:
  #if defined(PETSC_USE_COMPLEX)
      fftw->p_backward = fftw_mpi_plan_dft_3d(dim[0], dim[1], dim[2], (fftw_complex *)x_array, (fftw_complex *)y_array, comm, FFTW_BACKWARD, fftw->p_flag);
  #else
      fftw->p_backward = fftw_mpi_plan_dft_c2r_3d(dim[0], dim[1], dim[2], (fftw_complex *)x_array, (double *)y_array, comm, FFTW_ESTIMATE);
  #endif
      break;
    default:
  #if defined(PETSC_USE_COMPLEX)
      fftw->p_backward = fftw_mpi_plan_dft(fftw->ndim_fftw, fftw->dim_fftw, (fftw_complex *)x_array, (fftw_complex *)y_array, comm, FFTW_BACKWARD, fftw->p_flag);
  #else
      fftw->p_backward = fftw_mpi_plan_dft_c2r(fftw->ndim_fftw, fftw->dim_fftw, (fftw_complex *)x_array, (double *)y_array, comm, FFTW_ESTIMATE);
  #endif
      break;
    }
    fftw->binarray  = (PetscScalar *)x_array;
    fftw->boutarray = y_array;
    fftw_execute(fftw->p_backward);
  } else {                                                         /* use existing plan */
    if (fftw->binarray != x_array || fftw->boutarray != y_array) { /* use existing plan on new arrays */
      fftw_execute_dft(fftw->p_backward, (fftw_complex *)x_array, (fftw_complex *)y_array);
    } else {
      fftw_execute(fftw->p_backward);
    }
  }
  PetscCall(VecRestoreArray(y, &y_array));
  PetscCall(VecRestoreArrayRead(x, &x_array));
  PetscFunctionReturn(PETSC_SUCCESS);
}
#endif

PetscErrorCode MatDestroy_FFTW(Mat A)
{
  Mat_FFT  *fft  = (Mat_FFT *)A->data;
  Mat_FFTW *fftw = (Mat_FFTW *)fft->data;

  PetscFunctionBegin;
  fftw_destroy_plan(fftw->p_forward);
  fftw_destroy_plan(fftw->p_backward);
  if (fftw->iodims) free(fftw->iodims);
  PetscCall(PetscFree(fftw->dim_fftw));
  PetscCall(PetscFree(fft->data));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatCreateVecsFFTW_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "VecScatterPetscToFFTW_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "VecScatterFFTWToPetsc_C", NULL));
#if !PetscDefined(HAVE_MPIUNI)
  fftw_mpi_cleanup();
#endif
  PetscFunctionReturn(PETSC_SUCCESS);
}

#if !PetscDefined(HAVE_MPIUNI)
  #include <../src/vec/vec/impls/mpi/pvecimpl.h> /*I  "petscvec.h"   I*/
PetscErrorCode VecDestroy_MPIFFTW(Vec v)
{
  PetscScalar *array;

  PetscFunctionBegin;
  PetscCall(VecGetArray(v, &array));
  fftw_free((fftw_complex *)array);
  PetscCall(VecRestoreArray(v, &array));
  PetscCall(VecDestroy_MPI(v));
  PetscFunctionReturn(PETSC_SUCCESS);
}
#endif

#if !PetscDefined(HAVE_MPIUNI)
static PetscErrorCode VecDuplicate_FFTW_fin(Vec fin, Vec *fin_new)
{
  Mat A;

  PetscFunctionBegin;
  PetscCall(PetscObjectQuery((PetscObject)fin, "FFTmatrix", (PetscObject *)&A));
  PetscCall(MatCreateVecsFFTW_FFTW(A, fin_new, NULL, NULL));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode VecDuplicate_FFTW_fout(Vec fout, Vec *fout_new)
{
  Mat A;

  PetscFunctionBegin;
  PetscCall(PetscObjectQuery((PetscObject)fout, "FFTmatrix", (PetscObject *)&A));
  PetscCall(MatCreateVecsFFTW_FFTW(A, NULL, fout_new, NULL));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode VecDuplicate_FFTW_bout(Vec bout, Vec *bout_new)
{
  Mat A;

  PetscFunctionBegin;
  PetscCall(PetscObjectQuery((PetscObject)bout, "FFTmatrix", (PetscObject *)&A));
  PetscCall(MatCreateVecsFFTW_FFTW(A, NULL, NULL, bout_new));
  PetscFunctionReturn(PETSC_SUCCESS);
}
#endif

/*@
  MatCreateVecsFFTW - Get vector(s) compatible with the matrix, i.e. with the
  parallel layout determined by `MATFFTW`

  Collective

  Input Parameter:
. A - the matrix

  Output Parameters:
+ x - (optional) input vector of forward FFTW
. y - (optional) output vector of forward FFTW
- z - (optional) output vector of backward FFTW

  Options Database Key:
. -mat_fftw_plannerflags - set FFTW planner flags

  Level: advanced

  Notes:
  The parallel layout of output of forward FFTW is always same as the input
  of the backward FFTW. But parallel layout of the input vector of forward
  FFTW might not be same as the output of backward FFTW.

  We need to provide enough space while doing parallel real transform.
  We need to pad extra zeros at the end of last dimension. For this reason the one needs to
  invoke the routine fftw_mpi_local_size_transposed routines. Remember one has to change the
  last dimension from n to n/2+1 while invoking this routine. The number of zeros to be padded
  depends on if the last dimension is even or odd. If the last dimension is even need to pad two
  zeros if it is odd only one zero is needed.

  Lastly one needs some scratch space at the end of data set in each process. alloc_local
  figures out how much space is needed, i.e. it figures out the data+scratch space for
  each processor and returns that.

  Developer Notes:
  How come `MatCreateVecs()` doesn't produce the correctly padded vectors automatically?

.seealso: [](ch_matrices), `Mat`, `MATFFTW`, `MatCreateFFT()`, `MatCreateVecs()`
@*/
PetscErrorCode MatCreateVecsFFTW(Mat A, Vec *x, Vec *y, Vec *z)
{
  PetscFunctionBegin;
  PetscUseMethod(A, "MatCreateVecsFFTW_C", (Mat, Vec *, Vec *, Vec *), (A, x, y, z));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode MatCreateVecsFFTW_FFTW(Mat A, Vec *fin, Vec *fout, Vec *bout)
{
  PetscMPIInt size, rank;
  MPI_Comm    comm;
  Mat_FFT    *fft = (Mat_FFT *)A->data;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, MAT_CLASSID, 1);
  PetscValidType(A, 1);
  PetscCall(PetscObjectGetComm((PetscObject)A, &comm));

  PetscCallMPI(MPI_Comm_size(comm, &size));
  PetscCallMPI(MPI_Comm_rank(comm, &rank));
  if (size == 1) { /* sequential case */
#if defined(PETSC_USE_COMPLEX)
    if (fin) PetscCall(VecCreateSeq(PETSC_COMM_SELF, fft->N, fin));
    if (fout) PetscCall(VecCreateSeq(PETSC_COMM_SELF, fft->N, fout));
    if (bout) PetscCall(VecCreateSeq(PETSC_COMM_SELF, fft->N, bout));
#else
    if (fin) PetscCall(VecCreateSeq(PETSC_COMM_SELF, fft->n, fin));
    if (fout) PetscCall(VecCreateSeq(PETSC_COMM_SELF, fft->n, fout));
    if (bout) PetscCall(VecCreateSeq(PETSC_COMM_SELF, fft->n, bout));
#endif
#if !PetscDefined(HAVE_MPIUNI)
  } else { /* parallel cases */
    Mat_FFTW     *fftw = (Mat_FFTW *)fft->data;
    PetscInt      ndim = fft->ndim, *dim = fft->dim;
    ptrdiff_t     alloc_local, local_n0, local_0_start;
    ptrdiff_t     local_n1;
    fftw_complex *data_fout;
    ptrdiff_t     local_1_start;
  #if defined(PETSC_USE_COMPLEX)
    fftw_complex *data_fin, *data_bout;
  #else
    double   *data_finr, *data_boutr;
    PetscInt  n1, N1;
    ptrdiff_t temp;
  #endif

    switch (ndim) {
    case 1:
  #if !defined(PETSC_USE_COMPLEX)
      SETERRQ(comm, PETSC_ERR_SUP, "FFTW does not allow parallel real 1D transform");
  #else
      alloc_local = fftw_mpi_local_size_1d(dim[0], comm, FFTW_FORWARD, FFTW_ESTIMATE, &local_n0, &local_0_start, &local_n1, &local_1_start);
      if (fin) {
        data_fin = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, local_n0, fft->N, (const PetscScalar *)data_fin, fin));
        PetscCall(PetscObjectCompose((PetscObject)*fin, "FFTmatrix", (PetscObject)A));
        (*fin)->ops->duplicate = VecDuplicate_FFTW_fin;
        (*fin)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (fout) {
        data_fout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, local_n1, fft->N, (const PetscScalar *)data_fout, fout));
        PetscCall(PetscObjectCompose((PetscObject)*fout, "FFTmatrix", (PetscObject)A));
        (*fout)->ops->duplicate = VecDuplicate_FFTW_fout;
        (*fout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      alloc_local = fftw_mpi_local_size_1d(dim[0], comm, FFTW_BACKWARD, FFTW_ESTIMATE, &local_n0, &local_0_start, &local_n1, &local_1_start);
      if (bout) {
        data_bout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, local_n1, fft->N, (const PetscScalar *)data_bout, bout));
        PetscCall(PetscObjectCompose((PetscObject)*bout, "FFTmatrix", (PetscObject)A));
        (*bout)->ops->duplicate = VecDuplicate_FFTW_fout;
        (*bout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      break;
  #endif
    case 2:
  #if !defined(PETSC_USE_COMPLEX) /* Note that N1 is no more the product of individual dimensions */
      alloc_local = fftw_mpi_local_size_2d_transposed(dim[0], dim[1] / 2 + 1, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);
      N1          = 2 * dim[0] * (dim[1] / 2 + 1);
      n1          = 2 * local_n0 * (dim[1] / 2 + 1);
      if (fin) {
        data_finr = (double *)fftw_malloc(sizeof(double) * alloc_local * 2);
        PetscCall(VecCreateMPIWithArray(comm, 1, (PetscInt)n1, N1, (PetscScalar *)data_finr, fin));
        PetscCall(PetscObjectCompose((PetscObject)*fin, "FFTmatrix", (PetscObject)A));
        (*fin)->ops->duplicate = VecDuplicate_FFTW_fin;
        (*fin)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (fout) {
        data_fout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, (PetscInt)n1, N1, (PetscScalar *)data_fout, fout));
        PetscCall(PetscObjectCompose((PetscObject)*fout, "FFTmatrix", (PetscObject)A));
        (*fout)->ops->duplicate = VecDuplicate_FFTW_fout;
        (*fout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (bout) {
        data_boutr = (double *)fftw_malloc(sizeof(double) * alloc_local * 2);
        PetscCall(VecCreateMPIWithArray(comm, 1, (PetscInt)n1, N1, (PetscScalar *)data_boutr, bout));
        PetscCall(PetscObjectCompose((PetscObject)*bout, "FFTmatrix", (PetscObject)A));
        (*bout)->ops->duplicate = VecDuplicate_FFTW_bout;
        (*bout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
  #else
      /* Get local size */
      alloc_local = fftw_mpi_local_size_2d(dim[0], dim[1], comm, &local_n0, &local_0_start);
      if (fin) {
        data_fin = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, fft->N, (const PetscScalar *)data_fin, fin));
        PetscCall(PetscObjectCompose((PetscObject)*fin, "FFTmatrix", (PetscObject)A));
        (*fin)->ops->duplicate = VecDuplicate_FFTW_fin;
        (*fin)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (fout) {
        data_fout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, fft->N, (const PetscScalar *)data_fout, fout));
        PetscCall(PetscObjectCompose((PetscObject)*fout, "FFTmatrix", (PetscObject)A));
        (*fout)->ops->duplicate = VecDuplicate_FFTW_fout;
        (*fout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (bout) {
        data_bout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, fft->N, (const PetscScalar *)data_bout, bout));
        PetscCall(PetscObjectCompose((PetscObject)*bout, "FFTmatrix", (PetscObject)A));
        (*bout)->ops->duplicate = VecDuplicate_FFTW_bout;
        (*bout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
  #endif
      break;
    case 3:
  #if !defined(PETSC_USE_COMPLEX)
      alloc_local = fftw_mpi_local_size_3d_transposed(dim[0], dim[1], dim[2] / 2 + 1, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);
      N1          = 2 * dim[0] * dim[1] * (dim[2] / 2 + 1);
      n1          = 2 * local_n0 * dim[1] * (dim[2] / 2 + 1);
      if (fin) {
        data_finr = (double *)fftw_malloc(sizeof(double) * alloc_local * 2);
        PetscCall(VecCreateMPIWithArray(comm, 1, (PetscInt)n1, N1, (PetscScalar *)data_finr, fin));
        PetscCall(PetscObjectCompose((PetscObject)*fin, "FFTmatrix", (PetscObject)A));
        (*fin)->ops->duplicate = VecDuplicate_FFTW_fin;
        (*fin)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (fout) {
        data_fout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, n1, N1, (PetscScalar *)data_fout, fout));
        PetscCall(PetscObjectCompose((PetscObject)*fout, "FFTmatrix", (PetscObject)A));
        (*fout)->ops->duplicate = VecDuplicate_FFTW_fout;
        (*fout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (bout) {
        data_boutr = (double *)fftw_malloc(sizeof(double) * alloc_local * 2);
        PetscCall(VecCreateMPIWithArray(comm, 1, (PetscInt)n1, N1, (PetscScalar *)data_boutr, bout));
        PetscCall(PetscObjectCompose((PetscObject)*bout, "FFTmatrix", (PetscObject)A));
        (*bout)->ops->duplicate = VecDuplicate_FFTW_bout;
        (*bout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
  #else
      alloc_local = fftw_mpi_local_size_3d(dim[0], dim[1], dim[2], comm, &local_n0, &local_0_start);
      if (fin) {
        data_fin = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, fft->N, (const PetscScalar *)data_fin, fin));
        PetscCall(PetscObjectCompose((PetscObject)*fin, "FFTmatrix", (PetscObject)A));
        (*fin)->ops->duplicate = VecDuplicate_FFTW_fin;
        (*fin)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (fout) {
        data_fout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, fft->N, (const PetscScalar *)data_fout, fout));
        PetscCall(PetscObjectCompose((PetscObject)*fout, "FFTmatrix", (PetscObject)A));
        (*fout)->ops->duplicate = VecDuplicate_FFTW_fout;
        (*fout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (bout) {
        data_bout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, fft->N, (const PetscScalar *)data_bout, bout));
        PetscCall(PetscObjectCompose((PetscObject)*bout, "FFTmatrix", (PetscObject)A));
        (*bout)->ops->duplicate = VecDuplicate_FFTW_bout;
        (*bout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
  #endif
      break;
    default:
  #if !defined(PETSC_USE_COMPLEX)
      temp                                  = (fftw->dim_fftw)[fftw->ndim_fftw - 1];
      (fftw->dim_fftw)[fftw->ndim_fftw - 1] = temp / 2 + 1;
      alloc_local                           = fftw_mpi_local_size_transposed(fftw->ndim_fftw, fftw->dim_fftw, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);
      N1                                    = 2 * fft->N * (PetscInt)(fftw->dim_fftw[fftw->ndim_fftw - 1]) / ((PetscInt)temp);
      (fftw->dim_fftw)[fftw->ndim_fftw - 1] = temp;

      if (fin) {
        data_finr = (double *)fftw_malloc(sizeof(double) * alloc_local * 2);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, N1, (PetscScalar *)data_finr, fin));
        PetscCall(PetscObjectCompose((PetscObject)*fin, "FFTmatrix", (PetscObject)A));
        (*fin)->ops->duplicate = VecDuplicate_FFTW_fin;
        (*fin)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (fout) {
        data_fout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, N1, (PetscScalar *)data_fout, fout));
        PetscCall(PetscObjectCompose((PetscObject)*fout, "FFTmatrix", (PetscObject)A));
        (*fout)->ops->duplicate = VecDuplicate_FFTW_fout;
        (*fout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (bout) {
        data_boutr = (double *)fftw_malloc(sizeof(double) * alloc_local * 2);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, N1, (PetscScalar *)data_boutr, bout));
        PetscCall(PetscObjectCompose((PetscObject)*bout, "FFTmatrix", (PetscObject)A));
        (*bout)->ops->duplicate = VecDuplicate_FFTW_bout;
        (*bout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
  #else
      alloc_local = fftw_mpi_local_size(fftw->ndim_fftw, fftw->dim_fftw, comm, &local_n0, &local_0_start);
      if (fin) {
        data_fin = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, fft->N, (const PetscScalar *)data_fin, fin));
        PetscCall(PetscObjectCompose((PetscObject)*fin, "FFTmatrix", (PetscObject)A));
        (*fin)->ops->duplicate = VecDuplicate_FFTW_fin;
        (*fin)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (fout) {
        data_fout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, fft->N, (const PetscScalar *)data_fout, fout));
        PetscCall(PetscObjectCompose((PetscObject)*fout, "FFTmatrix", (PetscObject)A));
        (*fout)->ops->duplicate = VecDuplicate_FFTW_fout;
        (*fout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
      if (bout) {
        data_bout = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * alloc_local);
        PetscCall(VecCreateMPIWithArray(comm, 1, fft->n, fft->N, (const PetscScalar *)data_bout, bout));
        PetscCall(PetscObjectCompose((PetscObject)*bout, "FFTmatrix", (PetscObject)A));
        (*bout)->ops->duplicate = VecDuplicate_FFTW_bout;
        (*bout)->ops->destroy   = VecDestroy_MPIFFTW;
      }
  #endif
      break;
    }
    /* fftw vectors have their data array allocated by fftw_malloc, such that v->array=xxx but
       v->array_allocated=NULL. A regular replacearray call won't free the memory and only causes
       memory leaks. We void these pointers here to avoid accident uses.
    */
    if (fin) (*fin)->ops->replacearray = NULL;
    if (fout) (*fout)->ops->replacearray = NULL;
    if (bout) (*bout)->ops->replacearray = NULL;
#endif
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  VecScatterPetscToFFTW - Copies a PETSc vector to the vector that goes into `MATFFTW` calls.

  Collective

  Input Parameters:
+ A - FFTW matrix
- x - the PETSc vector

  Output Parameter:
. y - the FFTW vector

  Level: intermediate

  Note:
  For real parallel FFT, FFTW requires insertion of extra space at the end of last dimension. This required even when
  one is not doing in-place transform. The last dimension size must be changed to 2*(dim[last]/2+1) to accommodate these extra
  zeros. This routine does that job by scattering operation.

.seealso: [](ch_matrices), `Mat`, `MATFFTW`, `VecScatterFFTWToPetsc()`, `MatCreateVecsFFTW()`
@*/
PetscErrorCode VecScatterPetscToFFTW(Mat A, Vec x, Vec y)
{
  PetscFunctionBegin;
  PetscUseMethod(A, "VecScatterPetscToFFTW_C", (Mat, Vec, Vec), (A, x, y));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode VecScatterPetscToFFTW_FFTW(Mat A, Vec x, Vec y)
{
  MPI_Comm    comm;
  Mat_FFT    *fft = (Mat_FFT *)A->data;
  PetscInt    low;
  PetscMPIInt rank, size;
  PetscInt    vsize, vsize1;
  VecScatter  vecscat;
  IS          list1;

  PetscFunctionBegin;
  PetscCall(PetscObjectGetComm((PetscObject)A, &comm));
  PetscCallMPI(MPI_Comm_size(comm, &size));
  PetscCallMPI(MPI_Comm_rank(comm, &rank));
  PetscCall(VecGetOwnershipRange(y, &low, NULL));

  if (size == 1) {
    PetscCall(VecGetSize(x, &vsize));
    PetscCall(VecGetSize(y, &vsize1));
    PetscCall(ISCreateStride(PETSC_COMM_SELF, fft->N, 0, 1, &list1));
    PetscCall(VecScatterCreate(x, list1, y, list1, &vecscat));
    PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
    PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
    PetscCall(VecScatterDestroy(&vecscat));
    PetscCall(ISDestroy(&list1));
#if !PetscDefined(HAVE_MPIUNI)
  } else {
    Mat_FFTW *fftw = (Mat_FFTW *)fft->data;
    PetscInt  ndim = fft->ndim, *dim = fft->dim;
    ptrdiff_t local_n0, local_0_start;
    ptrdiff_t local_n1, local_1_start;
    IS        list2;
  #if !defined(PETSC_USE_COMPLEX)
    PetscInt  i, j, k, partial_dim;
    PetscInt *indx1, *indx2, tempindx, tempindx1;
    PetscInt  NM;
    ptrdiff_t temp;
  #endif

    switch (ndim) {
    case 1:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size_1d(dim[0], comm, FFTW_FORWARD, FFTW_ESTIMATE, &local_n0, &local_0_start, &local_n1, &local_1_start);

      PetscCall(ISCreateStride(comm, local_n0, local_0_start, 1, &list1));
      PetscCall(ISCreateStride(comm, local_n0, low, 1, &list2));
      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
  #else
      SETERRQ(comm, PETSC_ERR_SUP, "FFTW does not support parallel 1D real transform");
  #endif
      break;
    case 2:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size_2d(dim[0], dim[1], comm, &local_n0, &local_0_start);

      PetscCall(ISCreateStride(comm, local_n0 * dim[1], local_0_start * dim[1], 1, &list1));
      PetscCall(ISCreateStride(comm, local_n0 * dim[1], low, 1, &list2));
      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
  #else
      fftw_mpi_local_size_2d_transposed(dim[0], dim[1] / 2 + 1, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);

      PetscCall(PetscMalloc1(((PetscInt)local_n0) * dim[1], &indx1));
      PetscCall(PetscMalloc1(((PetscInt)local_n0) * dim[1], &indx2));

      if (dim[1] % 2 == 0) {
        NM = dim[1] + 2;
      } else {
        NM = dim[1] + 1;
      }
      for (i = 0; i < local_n0; i++) {
        for (j = 0; j < dim[1]; j++) {
          tempindx  = i * dim[1] + j;
          tempindx1 = i * NM + j;

          indx1[tempindx] = local_0_start * dim[1] + tempindx;
          indx2[tempindx] = low + tempindx1;
        }
      }

      PetscCall(ISCreateGeneral(comm, local_n0 * dim[1], indx1, PETSC_COPY_VALUES, &list1));
      PetscCall(ISCreateGeneral(comm, local_n0 * dim[1], indx2, PETSC_COPY_VALUES, &list2));

      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
      PetscCall(PetscFree(indx1));
      PetscCall(PetscFree(indx2));
  #endif
      break;

    case 3:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size_3d(dim[0], dim[1], dim[2], comm, &local_n0, &local_0_start);

      PetscCall(ISCreateStride(comm, local_n0 * dim[1] * dim[2], local_0_start * dim[1] * dim[2], 1, &list1));
      PetscCall(ISCreateStride(comm, local_n0 * dim[1] * dim[2], low, 1, &list2));
      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
  #else
      /* buggy, needs to be fixed. See src/mat/tests/ex158.c */
      SETERRQ(comm, PETSC_ERR_SUP, "FFTW does not support parallel 3D real transform");
      fftw_mpi_local_size_3d_transposed(dim[0], dim[1], dim[2] / 2 + 1, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);

      PetscCall(PetscMalloc1(((PetscInt)local_n0) * dim[1] * dim[2], &indx1));
      PetscCall(PetscMalloc1(((PetscInt)local_n0) * dim[1] * dim[2], &indx2));

      if (dim[2] % 2 == 0) NM = dim[2] + 2;
      else NM = dim[2] + 1;

      for (i = 0; i < local_n0; i++) {
        for (j = 0; j < dim[1]; j++) {
          for (k = 0; k < dim[2]; k++) {
            tempindx  = i * dim[1] * dim[2] + j * dim[2] + k;
            tempindx1 = i * dim[1] * NM + j * NM + k;

            indx1[tempindx] = local_0_start * dim[1] * dim[2] + tempindx;
            indx2[tempindx] = low + tempindx1;
          }
        }
      }

      PetscCall(ISCreateGeneral(comm, local_n0 * dim[1] * dim[2], indx1, PETSC_COPY_VALUES, &list1));
      PetscCall(ISCreateGeneral(comm, local_n0 * dim[1] * dim[2], indx2, PETSC_COPY_VALUES, &list2));
      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
      PetscCall(PetscFree(indx1));
      PetscCall(PetscFree(indx2));
  #endif
      break;

    default:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size(fftw->ndim_fftw, fftw->dim_fftw, comm, &local_n0, &local_0_start);

      PetscCall(ISCreateStride(comm, local_n0 * (fftw->partial_dim), local_0_start * (fftw->partial_dim), 1, &list1));
      PetscCall(ISCreateStride(comm, local_n0 * (fftw->partial_dim), low, 1, &list2));
      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
  #else
      /* buggy, needs to be fixed. See src/mat/tests/ex158.c */
      SETERRQ(comm, PETSC_ERR_SUP, "FFTW does not support parallel DIM>3 real transform");
      temp = (fftw->dim_fftw)[fftw->ndim_fftw - 1];

      (fftw->dim_fftw)[fftw->ndim_fftw - 1] = temp / 2 + 1;

      fftw_mpi_local_size_transposed(fftw->ndim_fftw, fftw->dim_fftw, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);

      (fftw->dim_fftw)[fftw->ndim_fftw - 1] = temp;

      partial_dim = fftw->partial_dim;

      PetscCall(PetscMalloc1(((PetscInt)local_n0) * partial_dim, &indx1));
      PetscCall(PetscMalloc1(((PetscInt)local_n0) * partial_dim, &indx2));

      if (dim[ndim - 1] % 2 == 0) NM = 2;
      else NM = 1;

      j = low;
      for (i = 0, k = 1; i < ((PetscInt)local_n0) * partial_dim; i++, k++) {
        indx1[i] = local_0_start * partial_dim + i;
        indx2[i] = j;
        if (k % dim[ndim - 1] == 0) j += NM;
        j++;
      }
      PetscCall(ISCreateGeneral(comm, local_n0 * partial_dim, indx1, PETSC_COPY_VALUES, &list1));
      PetscCall(ISCreateGeneral(comm, local_n0 * partial_dim, indx2, PETSC_COPY_VALUES, &list2));
      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
      PetscCall(PetscFree(indx1));
      PetscCall(PetscFree(indx2));
  #endif
      break;
    }
#endif
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  VecScatterFFTWToPetsc - Converts `MATFFTW` output vector to a PETSc vector.

  Collective

  Input Parameters:
+ A - `MATFFTW` matrix
- x - FFTW vector

  Output Parameter:
. y - PETSc vector

  Level: intermediate

  Note:
  While doing real transform the FFTW output of backward DFT contains extra zeros at the end of last dimension.
  `VecScatterFFTWToPetsc()` removes those extra zeros.

.seealso: [](ch_matrices), `Mat`, `VecScatterPetscToFFTW()`, `MATFFTW`, `MatCreateVecsFFTW()`
@*/
PetscErrorCode VecScatterFFTWToPetsc(Mat A, Vec x, Vec y)
{
  PetscFunctionBegin;
  PetscUseMethod(A, "VecScatterFFTWToPetsc_C", (Mat, Vec, Vec), (A, x, y));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode VecScatterFFTWToPetsc_FFTW(Mat A, Vec x, Vec y)
{
  MPI_Comm    comm;
  Mat_FFT    *fft = (Mat_FFT *)A->data;
  PetscInt    low;
  PetscMPIInt rank, size;
  VecScatter  vecscat;
  IS          list1;

  PetscFunctionBegin;
  PetscCall(PetscObjectGetComm((PetscObject)A, &comm));
  PetscCallMPI(MPI_Comm_size(comm, &size));
  PetscCallMPI(MPI_Comm_rank(comm, &rank));
  PetscCall(VecGetOwnershipRange(x, &low, NULL));

  if (size == 1) {
    PetscCall(ISCreateStride(comm, fft->N, 0, 1, &list1));
    PetscCall(VecScatterCreate(x, list1, y, list1, &vecscat));
    PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
    PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
    PetscCall(VecScatterDestroy(&vecscat));
    PetscCall(ISDestroy(&list1));

#if !PetscDefined(HAVE_MPIUNI)
  } else {
    Mat_FFTW *fftw = (Mat_FFTW *)fft->data;
    PetscInt  ndim = fft->ndim, *dim = fft->dim;
    ptrdiff_t local_n0, local_0_start;
    ptrdiff_t local_n1, local_1_start;
    IS        list2;
  #if !defined(PETSC_USE_COMPLEX)
    PetscInt  i, j, k, partial_dim;
    PetscInt *indx1, *indx2, tempindx, tempindx1;
    PetscInt  NM;
    ptrdiff_t temp;
  #endif
    switch (ndim) {
    case 1:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size_1d(dim[0], comm, FFTW_BACKWARD, FFTW_ESTIMATE, &local_n0, &local_0_start, &local_n1, &local_1_start);

      PetscCall(ISCreateStride(comm, local_n1, local_1_start, 1, &list1));
      PetscCall(ISCreateStride(comm, local_n1, low, 1, &list2));
      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
  #else
      SETERRQ(comm, PETSC_ERR_SUP, "No support for real parallel 1D FFT");
  #endif
      break;
    case 2:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size_2d(dim[0], dim[1], comm, &local_n0, &local_0_start);

      PetscCall(ISCreateStride(comm, local_n0 * dim[1], local_0_start * dim[1], 1, &list1));
      PetscCall(ISCreateStride(comm, local_n0 * dim[1], low, 1, &list2));
      PetscCall(VecScatterCreate(x, list2, y, list1, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
  #else
      fftw_mpi_local_size_2d_transposed(dim[0], dim[1] / 2 + 1, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);

      PetscCall(PetscMalloc1(((PetscInt)local_n0) * dim[1], &indx1));
      PetscCall(PetscMalloc1(((PetscInt)local_n0) * dim[1], &indx2));

      if (dim[1] % 2 == 0) NM = dim[1] + 2;
      else NM = dim[1] + 1;

      for (i = 0; i < local_n0; i++) {
        for (j = 0; j < dim[1]; j++) {
          tempindx  = i * dim[1] + j;
          tempindx1 = i * NM + j;

          indx1[tempindx] = local_0_start * dim[1] + tempindx;
          indx2[tempindx] = low + tempindx1;
        }
      }

      PetscCall(ISCreateGeneral(comm, local_n0 * dim[1], indx1, PETSC_COPY_VALUES, &list1));
      PetscCall(ISCreateGeneral(comm, local_n0 * dim[1], indx2, PETSC_COPY_VALUES, &list2));

      PetscCall(VecScatterCreate(x, list2, y, list1, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
      PetscCall(PetscFree(indx1));
      PetscCall(PetscFree(indx2));
  #endif
      break;
    case 3:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size_3d(dim[0], dim[1], dim[2], comm, &local_n0, &local_0_start);

      PetscCall(ISCreateStride(comm, local_n0 * dim[1] * dim[2], local_0_start * dim[1] * dim[2], 1, &list1));
      PetscCall(ISCreateStride(comm, local_n0 * dim[1] * dim[2], low, 1, &list2));
      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
  #else
      fftw_mpi_local_size_3d_transposed(dim[0], dim[1], dim[2] / 2 + 1, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);

      PetscCall(PetscMalloc1(((PetscInt)local_n0) * dim[1] * dim[2], &indx1));
      PetscCall(PetscMalloc1(((PetscInt)local_n0) * dim[1] * dim[2], &indx2));

      if (dim[2] % 2 == 0) NM = dim[2] + 2;
      else NM = dim[2] + 1;

      for (i = 0; i < local_n0; i++) {
        for (j = 0; j < dim[1]; j++) {
          for (k = 0; k < dim[2]; k++) {
            tempindx  = i * dim[1] * dim[2] + j * dim[2] + k;
            tempindx1 = i * dim[1] * NM + j * NM + k;

            indx1[tempindx] = local_0_start * dim[1] * dim[2] + tempindx;
            indx2[tempindx] = low + tempindx1;
          }
        }
      }

      PetscCall(ISCreateGeneral(comm, local_n0 * dim[1] * dim[2], indx1, PETSC_COPY_VALUES, &list1));
      PetscCall(ISCreateGeneral(comm, local_n0 * dim[1] * dim[2], indx2, PETSC_COPY_VALUES, &list2));

      PetscCall(VecScatterCreate(x, list2, y, list1, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
      PetscCall(PetscFree(indx1));
      PetscCall(PetscFree(indx2));
  #endif
      break;
    default:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size(fftw->ndim_fftw, fftw->dim_fftw, comm, &local_n0, &local_0_start);

      PetscCall(ISCreateStride(comm, local_n0 * (fftw->partial_dim), local_0_start * (fftw->partial_dim), 1, &list1));
      PetscCall(ISCreateStride(comm, local_n0 * (fftw->partial_dim), low, 1, &list2));
      PetscCall(VecScatterCreate(x, list1, y, list2, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
  #else
      temp = (fftw->dim_fftw)[fftw->ndim_fftw - 1];

      (fftw->dim_fftw)[fftw->ndim_fftw - 1] = temp / 2 + 1;

      fftw_mpi_local_size_transposed(fftw->ndim_fftw, fftw->dim_fftw, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);

      (fftw->dim_fftw)[fftw->ndim_fftw - 1] = temp;

      partial_dim = fftw->partial_dim;

      PetscCall(PetscMalloc1(((PetscInt)local_n0) * partial_dim, &indx1));
      PetscCall(PetscMalloc1(((PetscInt)local_n0) * partial_dim, &indx2));

      if (dim[ndim - 1] % 2 == 0) NM = 2;
      else NM = 1;

      j = low;
      for (i = 0, k = 1; i < ((PetscInt)local_n0) * partial_dim; i++, k++) {
        indx1[i] = local_0_start * partial_dim + i;
        indx2[i] = j;
        if (k % dim[ndim - 1] == 0) j += NM;
        j++;
      }
      PetscCall(ISCreateGeneral(comm, local_n0 * partial_dim, indx1, PETSC_COPY_VALUES, &list1));
      PetscCall(ISCreateGeneral(comm, local_n0 * partial_dim, indx2, PETSC_COPY_VALUES, &list2));

      PetscCall(VecScatterCreate(x, list2, y, list1, &vecscat));
      PetscCall(VecScatterBegin(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterEnd(vecscat, x, y, INSERT_VALUES, SCATTER_FORWARD));
      PetscCall(VecScatterDestroy(&vecscat));
      PetscCall(ISDestroy(&list1));
      PetscCall(ISDestroy(&list2));
      PetscCall(PetscFree(indx1));
      PetscCall(PetscFree(indx2));
  #endif
      break;
    }
#endif
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*MC
  MATFFTW -  "fftw" - Matrix type that provides FFT via the FFTW external package.

  Level: intermediate

.seealso: [](ch_matrices), `Mat`, `MatCreate()`, `MatType`, `MatCreateFFT()`
M*/
PETSC_EXTERN PetscErrorCode MatCreate_FFTW(Mat A)
{
  MPI_Comm    comm;
  Mat_FFT    *fft = (Mat_FFT *)A->data;
  Mat_FFTW   *fftw;
  PetscInt    ndim = fft->ndim, *dim = fft->dim;
  const char *plans[]  = {"FFTW_ESTIMATE", "FFTW_MEASURE", "FFTW_PATIENT", "FFTW_EXHAUSTIVE"};
  unsigned    iplans[] = {FFTW_ESTIMATE, FFTW_MEASURE, FFTW_PATIENT, FFTW_EXHAUSTIVE};
  PetscBool   flg;
  PetscInt    p_flag, partial_dim = 1, ctr;
  PetscMPIInt size, rank;
  ptrdiff_t  *pdim;
#if !defined(PETSC_USE_COMPLEX)
  PetscInt tot_dim;
#endif

  PetscFunctionBegin;
  PetscCall(PetscObjectGetComm((PetscObject)A, &comm));
  PetscCallMPI(MPI_Comm_size(comm, &size));
  PetscCallMPI(MPI_Comm_rank(comm, &rank));

#if !PetscDefined(HAVE_MPIUNI)
  fftw_mpi_init();
#endif
  pdim    = (ptrdiff_t *)calloc(ndim, sizeof(ptrdiff_t));
  pdim[0] = dim[0];
#if !defined(PETSC_USE_COMPLEX)
  tot_dim = 2 * dim[0];
#endif
  for (ctr = 1; ctr < ndim; ctr++) {
    partial_dim *= dim[ctr];
    pdim[ctr] = dim[ctr];
#if !defined(PETSC_USE_COMPLEX)
    if (ctr == ndim - 1) tot_dim *= (dim[ctr] / 2 + 1);
    else tot_dim *= dim[ctr];
#endif
  }

  if (size == 1) {
#if defined(PETSC_USE_COMPLEX)
    PetscCall(MatSetSizes(A, fft->N, fft->N, fft->N, fft->N));
    fft->n = fft->N;
#else
    PetscCall(MatSetSizes(A, tot_dim, tot_dim, tot_dim, tot_dim));
    fft->n = tot_dim;
#endif
#if !PetscDefined(HAVE_MPIUNI)
  } else {
    ptrdiff_t local_n0, local_0_start, local_n1, local_1_start;
  #if !defined(PETSC_USE_COMPLEX)
    ptrdiff_t temp;
    PetscInt  N1;
  #endif

    switch (ndim) {
    case 1:
  #if !defined(PETSC_USE_COMPLEX)
      SETERRQ(comm, PETSC_ERR_SUP, "FFTW does not support parallel 1D real transform");
  #else
      fftw_mpi_local_size_1d(dim[0], comm, FFTW_FORWARD, FFTW_ESTIMATE, &local_n0, &local_0_start, &local_n1, &local_1_start);
      fft->n = (PetscInt)local_n0;
      PetscCall(MatSetSizes(A, local_n1, fft->n, fft->N, fft->N));
  #endif
      break;
    case 2:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size_2d(dim[0], dim[1], comm, &local_n0, &local_0_start);
      fft->n = (PetscInt)local_n0 * dim[1];
      PetscCall(MatSetSizes(A, fft->n, fft->n, fft->N, fft->N));
  #else
      fftw_mpi_local_size_2d_transposed(dim[0], dim[1] / 2 + 1, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);

      fft->n = 2 * (PetscInt)local_n0 * (dim[1] / 2 + 1);
      PetscCall(MatSetSizes(A, fft->n, fft->n, 2 * dim[0] * (dim[1] / 2 + 1), 2 * dim[0] * (dim[1] / 2 + 1)));
  #endif
      break;
    case 3:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size_3d(dim[0], dim[1], dim[2], comm, &local_n0, &local_0_start);

      fft->n = (PetscInt)local_n0 * dim[1] * dim[2];
      PetscCall(MatSetSizes(A, fft->n, fft->n, fft->N, fft->N));
  #else
      fftw_mpi_local_size_3d_transposed(dim[0], dim[1], dim[2] / 2 + 1, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);

      fft->n = 2 * (PetscInt)local_n0 * dim[1] * (dim[2] / 2 + 1);
      PetscCall(MatSetSizes(A, fft->n, fft->n, 2 * dim[0] * dim[1] * (dim[2] / 2 + 1), 2 * dim[0] * dim[1] * (dim[2] / 2 + 1)));
  #endif
      break;
    default:
  #if defined(PETSC_USE_COMPLEX)
      fftw_mpi_local_size(ndim, pdim, comm, &local_n0, &local_0_start);

      fft->n = (PetscInt)local_n0 * partial_dim;
      PetscCall(MatSetSizes(A, fft->n, fft->n, fft->N, fft->N));
  #else
      temp = pdim[ndim - 1];

      pdim[ndim - 1] = temp / 2 + 1;

      fftw_mpi_local_size_transposed(ndim, pdim, comm, &local_n0, &local_0_start, &local_n1, &local_1_start);

      fft->n = 2 * (PetscInt)local_n0 * partial_dim * pdim[ndim - 1] / temp;
      N1     = 2 * fft->N * (PetscInt)pdim[ndim - 1] / ((PetscInt)temp);

      pdim[ndim - 1] = temp;

      PetscCall(MatSetSizes(A, fft->n, fft->n, N1, N1));
  #endif
      break;
    }
#endif
  }
  free(pdim);
  PetscCall(PetscObjectChangeTypeName((PetscObject)A, MATFFTW));
  PetscCall(PetscNew(&fftw));
  fft->data = (void *)fftw;

  fftw->ndim_fftw   = (ptrdiff_t)ndim; /* This is dimension of fft */
  fftw->partial_dim = partial_dim;

  PetscCall(PetscMalloc1(ndim, &fftw->dim_fftw));
  if (size == 1) {
#if defined(PETSC_USE_64BIT_INDICES)
    fftw->iodims = (fftw_iodim64 *)malloc(sizeof(fftw_iodim64) * ndim);
#else
    fftw->iodims = (fftw_iodim *)malloc(sizeof(fftw_iodim) * ndim);
#endif
  }

  for (ctr = 0; ctr < ndim; ctr++) (fftw->dim_fftw)[ctr] = dim[ctr];

  fftw->p_forward  = NULL;
  fftw->p_backward = NULL;
  fftw->p_flag     = FFTW_ESTIMATE;

  if (size == 1) {
    A->ops->mult          = MatMult_SeqFFTW;
    A->ops->multtranspose = MatMultTranspose_SeqFFTW;
#if !PetscDefined(HAVE_MPIUNI)
  } else {
    A->ops->mult          = MatMult_MPIFFTW;
    A->ops->multtranspose = MatMultTranspose_MPIFFTW;
#endif
  }
  fft->matdestroy = MatDestroy_FFTW;
  A->assembled    = PETSC_TRUE;
  A->preallocated = PETSC_TRUE;

  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatCreateVecsFFTW_C", MatCreateVecsFFTW_FFTW));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "VecScatterPetscToFFTW_C", VecScatterPetscToFFTW_FFTW));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "VecScatterFFTWToPetsc_C", VecScatterFFTWToPetsc_FFTW));

  /* get runtime options */
  PetscOptionsBegin(PetscObjectComm((PetscObject)A), ((PetscObject)A)->prefix, "FFTW Options", "Mat");
  PetscCall(PetscOptionsEList("-mat_fftw_plannerflags", "Planner Flags", "None", plans, 4, plans[0], &p_flag, &flg));
  if (flg) fftw->p_flag = iplans[p_flag];
  PetscOptionsEnd();
  PetscFunctionReturn(PETSC_SUCCESS);
}
