mat/matfd/fdmatrix.c

/*
   This is where the abstract matrix operations are defined that are
  used for finite difference computations of Jacobians using coloring.
*/

#include <petsc/private/matimpl.h> /*I "petscmat.h" I*/
#include <petsc/private/isimpl.h>

PetscErrorCode MatFDColoringSetF(MatFDColoring fd, Vec F)
{
  PetscFunctionBegin;
  if (F) {
    PetscCall(VecCopy(F, fd->w1));
    fd->fset = PETSC_TRUE;
  } else {
    fd->fset = PETSC_FALSE;
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

#include <petscdraw.h>
static PetscErrorCode MatFDColoringView_Draw_Zoom(PetscDraw draw, void *Aa)
{
  MatFDColoring fd = (MatFDColoring)Aa;
  PetscInt      i, j, nz, row;
  PetscReal     x, y;
  MatEntry     *Jentry = fd->matentry;

  PetscFunctionBegin;
  /* loop over colors  */
  nz = 0;
  for (i = 0; i < fd->ncolors; i++) {
    for (j = 0; j < fd->nrows[i]; j++) {
      row = Jentry[nz].row;
      y   = fd->M - row - fd->rstart;
      x   = (PetscReal)Jentry[nz++].col;
      PetscCall(PetscDrawRectangle(draw, x, y, x + 1, y + 1, i + 1, i + 1, i + 1, i + 1));
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatFDColoringView_Draw(MatFDColoring fd, PetscViewer viewer)
{
  PetscBool isnull;
  PetscDraw draw;
  PetscReal xr, yr, xl, yl, h, w;

  PetscFunctionBegin;
  PetscCall(PetscViewerDrawGetDraw(viewer, 0, &draw));
  PetscCall(PetscDrawIsNull(draw, &isnull));
  if (isnull) PetscFunctionReturn(PETSC_SUCCESS);

  xr = fd->N;
  yr = fd->M;
  h  = yr / 10.0;
  w  = xr / 10.0;
  xr += w;
  yr += h;
  xl = -w;
  yl = -h;
  PetscCall(PetscDrawSetCoordinates(draw, xl, yl, xr, yr));
  PetscCall(PetscObjectCompose((PetscObject)fd, "Zoomviewer", (PetscObject)viewer));
  PetscCall(PetscDrawZoom(draw, MatFDColoringView_Draw_Zoom, fd));
  PetscCall(PetscObjectCompose((PetscObject)fd, "Zoomviewer", NULL));
  PetscCall(PetscDrawSave(draw));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   MatFDColoringView - Views a finite difference coloring context.

   Collective

   Input Parameters:
+  c - the coloring context
-  viewer - visualization context

   Level: intermediate

   Notes:
   The available visualization contexts include
+     `PETSC_VIEWER_STDOUT_SELF` - standard output (default)
.     `PETSC_VIEWER_STDOUT_WORLD` - synchronized standard
        output where only the first processor opens
        the file.  All other processors send their
        data to the first processor to print.
-     `PETSC_VIEWER_DRAW_WORLD` - graphical display of nonzero structure

     Since PETSc uses only a small number of basic colors (currently 33), if the coloring
   involves more than 33 then some seemingly identical colors are displayed making it look
   like an illegal coloring. This is just a graphical artifact.

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`
@*/
PetscErrorCode MatFDColoringView(MatFDColoring c, PetscViewer viewer)
{
  PetscInt          i, j;
  PetscBool         isdraw, iascii;
  PetscViewerFormat format;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(c, MAT_FDCOLORING_CLASSID, 1);
  if (!viewer) PetscCall(PetscViewerASCIIGetStdout(PetscObjectComm((PetscObject)c), &viewer));
  PetscValidHeaderSpecific(viewer, PETSC_VIEWER_CLASSID, 2);
  PetscCheckSameComm(c, 1, viewer, 2);

  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERDRAW, &isdraw));
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &iascii));
  if (isdraw) {
    PetscCall(MatFDColoringView_Draw(c, viewer));
  } else if (iascii) {
    PetscCall(PetscObjectPrintClassNamePrefixType((PetscObject)c, viewer));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  Error tolerance=%g\n", (double)c->error_rel));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  Umin=%g\n", (double)c->umin));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  Number of colors=%" PetscInt_FMT "\n", c->ncolors));

    PetscCall(PetscViewerGetFormat(viewer, &format));
    if (format != PETSC_VIEWER_ASCII_INFO) {
      PetscInt row, col, nz;
      nz = 0;
      for (i = 0; i < c->ncolors; i++) {
        PetscCall(PetscViewerASCIIPrintf(viewer, "  Information for color %" PetscInt_FMT "\n", i));
        PetscCall(PetscViewerASCIIPrintf(viewer, "    Number of columns %" PetscInt_FMT "\n", c->ncolumns[i]));
        for (j = 0; j < c->ncolumns[i]; j++) PetscCall(PetscViewerASCIIPrintf(viewer, "      %" PetscInt_FMT "\n", c->columns[i][j]));
        PetscCall(PetscViewerASCIIPrintf(viewer, "    Number of rows %" PetscInt_FMT "\n", c->nrows[i]));
        if (c->matentry) {
          for (j = 0; j < c->nrows[i]; j++) {
            row = c->matentry[nz].row;
            col = c->matentry[nz++].col;
            PetscCall(PetscViewerASCIIPrintf(viewer, "      %" PetscInt_FMT " %" PetscInt_FMT " \n", row, col));
          }
        }
      }
    }
    PetscCall(PetscViewerFlush(viewer));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   MatFDColoringSetParameters - Sets the parameters for the sparse approximation of
   a Jacobian matrix using finite differences.

   Logically Collective

   Input Parameters:
+  matfd - the coloring context
.  error - relative error
-  umin - minimum allowable u-value magnitude

   Level: advanced

   Note:
     The Jacobian is estimated with the differencing approximation
.vb
       F'(u)_{:,i} = [F(u+h*dx_{i}) - F(u)]/h where
       htype = 'ds':
         h = error_rel*u[i]                 if  abs(u[i]) > umin
           = +/- error_rel*umin             otherwise, with +/- determined by the sign of u[i]
         dx_{i} = (0, ... 1, .... 0)

       htype = 'wp':
         h = error_rel * sqrt(1 + ||u||)
.ve

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`, `MatFDColoringSetFromOptions()`
@*/
PetscErrorCode MatFDColoringSetParameters(MatFDColoring matfd, PetscReal error, PetscReal umin)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(matfd, MAT_FDCOLORING_CLASSID, 1);
  PetscValidLogicalCollectiveReal(matfd, error, 2);
  PetscValidLogicalCollectiveReal(matfd, umin, 3);
  if (error != (PetscReal)PETSC_DEFAULT) matfd->error_rel = error;
  if (umin != (PetscReal)PETSC_DEFAULT) matfd->umin = umin;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   MatFDColoringSetBlockSize - Sets block size for efficient inserting entries of Jacobian matrix.

   Logically Collective

   Input Parameters:
+  coloring - the coloring context
.  brows - number of rows in the block
-  bcols - number of columns in the block

   Level: intermediate

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`, `MatFDColoringSetFromOptions()`
@*/
PetscErrorCode MatFDColoringSetBlockSize(MatFDColoring matfd, PetscInt brows, PetscInt bcols)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(matfd, MAT_FDCOLORING_CLASSID, 1);
  PetscValidLogicalCollectiveInt(matfd, brows, 2);
  PetscValidLogicalCollectiveInt(matfd, bcols, 3);
  if (brows != PETSC_DEFAULT) matfd->brows = brows;
  if (bcols != PETSC_DEFAULT) matfd->bcols = bcols;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   MatFDColoringSetUp - Sets up the internal data structures of matrix coloring context for the later use.

   Collective

   Input Parameters:
+  mat - the matrix containing the nonzero structure of the Jacobian
.  iscoloring - the coloring of the matrix; usually obtained with `MatGetColoring()` or `DMCreateColoring()`
-  color - the matrix coloring context

   Level: beginner

   Notes:
   When the coloring type is `IS_COLORING_LOCAL` the coloring is in the local ordering of the unknowns.

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`, `MatFDColoringDestroy()`
@*/
PetscErrorCode MatFDColoringSetUp(Mat mat, ISColoring iscoloring, MatFDColoring color)
{
  PetscBool eq;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(mat, MAT_CLASSID, 1);
  PetscValidHeaderSpecific(color, MAT_FDCOLORING_CLASSID, 3);
  if (color->setupcalled) PetscFunctionReturn(PETSC_SUCCESS);
  PetscCall(PetscObjectCompareId((PetscObject)mat, color->matid, &eq));
  PetscCheck(eq, PetscObjectComm((PetscObject)mat), PETSC_ERR_ARG_WRONG, "Matrix used with MatFDColoringSetUp() must be that used with MatFDColoringCreate()");

  PetscCall(PetscLogEventBegin(MAT_FDColoringSetUp, mat, 0, 0, 0));
  PetscUseTypeMethod(mat, fdcoloringsetup, iscoloring, color);

  color->setupcalled = PETSC_TRUE;
  PetscCall(PetscLogEventEnd(MAT_FDColoringSetUp, mat, 0, 0, 0));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   MatFDColoringGetFunction - Gets the function to use for computing the Jacobian.

   Not Collective

   Input Parameter:
.  coloring - the coloring context

   Output Parameters:
+  f - the function
-  fctx - the optional user-defined function context

   Level: intermediate

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`, `MatFDColoringSetFunction()`, `MatFDColoringSetFromOptions()`
@*/
PetscErrorCode MatFDColoringGetFunction(MatFDColoring matfd, PetscErrorCode (**f)(void), void **fctx)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(matfd, MAT_FDCOLORING_CLASSID, 1);
  if (f) *f = matfd->f;
  if (fctx) *fctx = matfd->fctx;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   MatFDColoringSetFunction - Sets the function to use for computing the Jacobian.

   Logically Collective

   Input Parameters:
+  coloring - the coloring context
.  f - the function
-  fctx - the optional user-defined function context

   Calling sequence with `SNES` of `f`:
$   PetscErrorCode f(SNES, Vec in, Vec out, void *fctx)

   Calling sequence without `SNES` of `f`:
$   PetscErrorCode f(void *dummy, Vec in, Vec out, void *fctx)

   Level: advanced

   Note:
    This function is usually used automatically by `SNES` (when one uses `SNESSetJacobian()` with the argument
     `SNESComputeJacobianDefaultColor()`) and only needs to be used by someone computing a matrix via coloring directly by
     calling `MatFDColoringApply()`

   Fortran Note:
    In Fortran you must call `MatFDColoringSetFunction()` for a coloring object to
  be used without `SNES` or within the `SNES` solvers.

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`, `MatFDColoringGetFunction()`, `MatFDColoringSetFromOptions()`
@*/
PetscErrorCode MatFDColoringSetFunction(MatFDColoring matfd, PetscErrorCode (*f)(void), void *fctx)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(matfd, MAT_FDCOLORING_CLASSID, 1);
  matfd->f    = f;
  matfd->fctx = fctx;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   MatFDColoringSetFromOptions - Sets coloring finite difference parameters from
   the options database.

   Collective

   The Jacobian, F'(u), is estimated with the differencing approximation
.vb
       F'(u)_{:,i} = [F(u+h*dx_{i}) - F(u)]/h where
       h = error_rel*u[i]                 if  abs(u[i]) > umin
         = +/- error_rel*umin             otherwise, with +/- determined by the sign of u[i]
       dx_{i} = (0, ... 1, .... 0)
.ve

   Input Parameter:
.  coloring - the coloring context

   Options Database Keys:
+  -mat_fd_coloring_err <err> - Sets <err> (square root of relative error in the function)
.  -mat_fd_coloring_umin <umin> - Sets umin, the minimum allowable u-value magnitude
.  -mat_fd_type - "wp" or "ds" (see MATMFFD_WP or MATMFFD_DS)
.  -mat_fd_coloring_view - Activates basic viewing
.  -mat_fd_coloring_view ::ascii_info - Activates viewing info
-  -mat_fd_coloring_view draw - Activates drawing

    Level: intermediate

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`, `MatFDColoringView()`, `MatFDColoringSetParameters()`
@*/
PetscErrorCode MatFDColoringSetFromOptions(MatFDColoring matfd)
{
  PetscBool flg;
  char      value[3];

  PetscFunctionBegin;
  PetscValidHeaderSpecific(matfd, MAT_FDCOLORING_CLASSID, 1);

  PetscObjectOptionsBegin((PetscObject)matfd);
  PetscCall(PetscOptionsReal("-mat_fd_coloring_err", "Square root of relative error in function", "MatFDColoringSetParameters", matfd->error_rel, &matfd->error_rel, NULL));
  PetscCall(PetscOptionsReal("-mat_fd_coloring_umin", "Minimum allowable u magnitude", "MatFDColoringSetParameters", matfd->umin, &matfd->umin, NULL));
  PetscCall(PetscOptionsString("-mat_fd_type", "Algorithm to compute h, wp or ds", "MatFDColoringCreate", matfd->htype, value, sizeof(value), &flg));
  if (flg) {
    if (value[0] == 'w' && value[1] == 'p') matfd->htype = "wp";
    else if (value[0] == 'd' && value[1] == 's') matfd->htype = "ds";
    else SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Unknown finite differencing type %s", value);
  }
  PetscCall(PetscOptionsInt("-mat_fd_coloring_brows", "Number of block rows", "MatFDColoringSetBlockSize", matfd->brows, &matfd->brows, NULL));
  PetscCall(PetscOptionsInt("-mat_fd_coloring_bcols", "Number of block columns", "MatFDColoringSetBlockSize", matfd->bcols, &matfd->bcols, &flg));
  if (flg && matfd->bcols > matfd->ncolors) {
    /* input bcols cannot be > matfd->ncolors, thus set it as ncolors */
    matfd->bcols = matfd->ncolors;
  }

  /* process any options handlers added with PetscObjectAddOptionsHandler() */
  PetscCall(PetscObjectProcessOptionsHandlers((PetscObject)matfd, PetscOptionsObject));
  PetscOptionsEnd();
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   MatFDColoringSetType - Sets the approach for computing the finite difference parameter

   Collective

   Input Parameters:
+  coloring - the coloring context
-  type - either `MATMFFD_WP` or `MATMFFD_DS`

   Options Database Key:
.  -mat_fd_type - "wp" or "ds"

   Level: intermediate

   Note:
   It is goofy that the argument type is `MatMFFDType` since the `MatFDColoring` actually computes the matrix entries
         but the process of computing the entries is the same as as with the `MATMFFD` operation so we should reuse the names instead of
         introducing another one.

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`, `MatFDColoringView()`, `MatFDColoringSetParameters()`
@*/
PetscErrorCode MatFDColoringSetType(MatFDColoring matfd, MatMFFDType type)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(matfd, MAT_FDCOLORING_CLASSID, 1);
  /*
     It is goofy to handle the strings this way but currently there is no code to free a dynamically created matfd->htype
     and this function is being provided as patch for a release so it shouldn't change the implementation
  */
  if (type[0] == 'w' && type[1] == 'p') matfd->htype = "wp";
  else if (type[0] == 'd' && type[1] == 's') matfd->htype = "ds";
  else SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Unknown finite differencing type %s", type);
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode MatFDColoringViewFromOptions(MatFDColoring fd, const char prefix[], const char optionname[])
{
  PetscBool         flg;
  PetscViewer       viewer;
  PetscViewerFormat format;

  PetscFunctionBegin;
  if (prefix) {
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)fd), ((PetscObject)fd)->options, prefix, optionname, &viewer, &format, &flg));
  } else {
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)fd), ((PetscObject)fd)->options, ((PetscObject)fd)->prefix, optionname, &viewer, &format, &flg));
  }
  if (flg) {
    PetscCall(PetscViewerPushFormat(viewer, format));
    PetscCall(MatFDColoringView(fd, viewer));
    PetscCall(PetscViewerPopFormat(viewer));
    PetscCall(PetscViewerDestroy(&viewer));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   MatFDColoringCreate - Creates a matrix coloring context for finite difference
   computation of Jacobians.

   Collective

   Input Parameters:
+  mat - the matrix containing the nonzero structure of the Jacobian
-  iscoloring - the coloring of the matrix; usually obtained with `MatColoringCreate()` or `DMCreateColoring()`

    Output Parameter:
.   color - the new coloring context

    Level: intermediate

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringDestroy()`, `SNESComputeJacobianDefaultColor()`, `ISColoringCreate()`,
          `MatFDColoringSetFunction()`, `MatFDColoringSetFromOptions()`, `MatFDColoringApply()`,
          `MatFDColoringView()`, `MatFDColoringSetParameters()`, `MatColoringCreate()`, `DMCreateColoring()`, `MatFDColoringSetValues()`
@*/
PetscErrorCode MatFDColoringCreate(Mat mat, ISColoring iscoloring, MatFDColoring *color)
{
  MatFDColoring c;
  MPI_Comm      comm;
  PetscInt      M, N;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(mat, MAT_CLASSID, 1);
  PetscCheck(mat->assembled, PetscObjectComm((PetscObject)mat), PETSC_ERR_ARG_WRONGSTATE, "Matrix must be assembled by calls to MatAssemblyBegin/End();");
  PetscCall(PetscLogEventBegin(MAT_FDColoringCreate, mat, 0, 0, 0));
  PetscCall(MatGetSize(mat, &M, &N));
  PetscCheck(M == N, PetscObjectComm((PetscObject)mat), PETSC_ERR_SUP, "Only for square matrices");
  PetscCall(PetscObjectGetComm((PetscObject)mat, &comm));
  PetscCall(PetscHeaderCreate(c, MAT_FDCOLORING_CLASSID, "MatFDColoring", "Jacobian computation via finite differences with coloring", "Mat", comm, MatFDColoringDestroy, MatFDColoringView));

  c->ctype = iscoloring->ctype;
  PetscCall(PetscObjectGetId((PetscObject)mat, &c->matid));

  PetscUseTypeMethod(mat, fdcoloringcreate, iscoloring, c);

  PetscCall(MatCreateVecs(mat, NULL, &c->w1));
  /* Vec is used intensively in particular piece of scalar CPU code; won't benefit from bouncing back and forth to the GPU */
  PetscCall(VecBindToCPU(c->w1, PETSC_TRUE));
  PetscCall(VecDuplicate(c->w1, &c->w2));
  /* Vec is used intensively in particular piece of scalar CPU code; won't benefit from bouncing back and forth to the GPU */
  PetscCall(VecBindToCPU(c->w2, PETSC_TRUE));

  c->error_rel    = PETSC_SQRT_MACHINE_EPSILON;
  c->umin         = 100.0 * PETSC_SQRT_MACHINE_EPSILON;
  c->currentcolor = -1;
  c->htype        = "wp";
  c->fset         = PETSC_FALSE;
  c->setupcalled  = PETSC_FALSE;

  *color = c;
  PetscCall(PetscObjectCompose((PetscObject)mat, "SNESMatFDColoring", (PetscObject)c));
  PetscCall(PetscLogEventEnd(MAT_FDColoringCreate, mat, 0, 0, 0));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
    MatFDColoringDestroy - Destroys a matrix coloring context that was created
    via `MatFDColoringCreate()`.

    Collective

    Input Parameter:
.   c - coloring context

    Level: intermediate

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`
@*/
PetscErrorCode MatFDColoringDestroy(MatFDColoring *c)
{
  PetscInt      i;
  MatFDColoring color = *c;

  PetscFunctionBegin;
  if (!*c) PetscFunctionReturn(PETSC_SUCCESS);
  if (--((PetscObject)color)->refct > 0) {
    *c = NULL;
    PetscFunctionReturn(PETSC_SUCCESS);
  }

  /* we do not free the column arrays since their entries are owned by the ISs in color->isa */
  for (i = 0; i < color->ncolors; i++) PetscCall(ISDestroy(&color->isa[i]));
  PetscCall(PetscFree(color->isa));
  PetscCall(PetscFree2(color->ncolumns, color->columns));
  PetscCall(PetscFree(color->nrows));
  if (color->htype[0] == 'w') {
    PetscCall(PetscFree(color->matentry2));
  } else {
    PetscCall(PetscFree(color->matentry));
  }
  PetscCall(PetscFree(color->dy));
  if (color->vscale) PetscCall(VecDestroy(&color->vscale));
  PetscCall(VecDestroy(&color->w1));
  PetscCall(VecDestroy(&color->w2));
  PetscCall(VecDestroy(&color->w3));
  PetscCall(PetscHeaderDestroy(c));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
    MatFDColoringGetPerturbedColumns - Returns the indices of the columns that
      that are currently being perturbed.

    Not Collective

    Input Parameter:
.   coloring - coloring context created with `MatFDColoringCreate()`

    Output Parameters:
+   n - the number of local columns being perturbed
-   cols - the column indices, in global numbering

   Level: advanced

   Note:
   IF the matrix type is `MATBAIJ`, then the block column indices are returned

   Fortran Note:
   This routine has a different interface for Fortran
.vb
     #include <petsc/finclude/petscmat.h>
          use petscmat
          PetscInt, pointer :: array(:)
          PetscErrorCode  ierr
          MatFDColoring   i
          call MatFDColoringGetPerturbedColumnsF90(i,array,ierr)
      use the entries of array ...
          call MatFDColoringRestorePerturbedColumnsF90(i,array,ierr)
.ve

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`, `MatFDColoringDestroy()`, `MatFDColoringView()`, `MatFDColoringApply()`
@*/
PetscErrorCode MatFDColoringGetPerturbedColumns(MatFDColoring coloring, PetscInt *n, const PetscInt *cols[])
{
  PetscFunctionBegin;
  if (coloring->currentcolor >= 0) {
    *n    = coloring->ncolumns[coloring->currentcolor];
    *cols = coloring->columns[coloring->currentcolor];
  } else {
    *n = 0;
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
    MatFDColoringApply - Given a matrix for which a `MatFDColoring` context
    has been created, computes the Jacobian for a function via finite differences.

    Collective

    Input Parameters:
+   mat - location to store Jacobian
.   coloring - coloring context created with `MatFDColoringCreate()`
.   x1 - location at which Jacobian is to be computed
-   sctx - context required by function, if this is being used with the SNES solver then it is `SNES` object, otherwise it is null

    Options Database Keys:
+    -mat_fd_type - "wp" or "ds"  (see `MATMFFD_WP` or `MATMFFD_DS`)
.    -mat_fd_coloring_view - Activates basic viewing or coloring
.    -mat_fd_coloring_view draw - Activates drawing of coloring
-    -mat_fd_coloring_view ::ascii_info - Activates viewing of coloring info

    Level: intermediate

.seealso: `Mat`, `MatFDColoring`, `MatFDColoringCreate()`, `MatFDColoringDestroy()`, `MatFDColoringView()`, `MatFDColoringSetFunction()`, `MatFDColoringSetValues()`
@*/
PetscErrorCode MatFDColoringApply(Mat J, MatFDColoring coloring, Vec x1, void *sctx)
{
  PetscBool eq;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(J, MAT_CLASSID, 1);
  PetscValidHeaderSpecific(coloring, MAT_FDCOLORING_CLASSID, 2);
  PetscValidHeaderSpecific(x1, VEC_CLASSID, 3);
  PetscCall(PetscObjectCompareId((PetscObject)J, coloring->matid, &eq));
  PetscCheck(eq, PetscObjectComm((PetscObject)J), PETSC_ERR_ARG_WRONG, "Matrix used with MatFDColoringApply() must be that used with MatFDColoringCreate()");
  PetscCheck(coloring->f, PetscObjectComm((PetscObject)J), PETSC_ERR_ARG_WRONGSTATE, "Must call MatFDColoringSetFunction()");
  PetscCheck(coloring->setupcalled, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONGSTATE, "Must call MatFDColoringSetUp()");

  PetscCall(MatSetUnfactored(J));
  PetscCall(PetscLogEventBegin(MAT_FDColoringApply, coloring, J, x1, 0));
  PetscUseTypeMethod(J, fdcoloringapply, coloring, x1, sctx);
  PetscCall(PetscLogEventEnd(MAT_FDColoringApply, coloring, J, x1, 0));
  if (!coloring->viewed) {
    PetscCall(MatFDColoringViewFromOptions(coloring, NULL, "-mat_fd_coloring_view"));
    coloring->viewed = PETSC_TRUE;
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}