xref: /petsc/src/vec/is/sf/utils/sfutils.c (revision 607e733f3db3ee7f6f605a13295c517df8dbb9c9)

#include <petsc/private/sfimpl.h> /*I  "petscsf.h"   I*/
#include <petsc/private/sectionimpl.h>

/*@
  PetscSFSetGraphLayout - Set a `PetscSF` communication pattern using global indices and a `PetscLayout`

  Collective

  Input Parameters:
+ sf        - star forest
. layout    - `PetscLayout` defining the global space for roots, i.e. which roots are owned by each MPI process
. nleaves   - number of leaf vertices on the current process, each of these references a root on any MPI process
. ilocal    - locations of leaves in leafdata buffers, pass `NULL` for contiguous storage, that is the locations are in [0,`nleaves`)
. localmode - copy mode for `ilocal`
- gremote   - root vertices in global numbering corresponding to the leaves

  Level: intermediate

  Note:
  Global indices must lie in [0, N) where N is the global size of `layout`.
  Leaf indices in `ilocal` get sorted; this means the user-provided array gets sorted if localmode is `PETSC_OWN_POINTER`.

  Developer Notes:
  Local indices which are the identity permutation in the range [0,`nleaves`) are discarded as they
  encode contiguous storage. In such case, if localmode is `PETSC_OWN_POINTER`, the memory is deallocated as it is not
  needed

.seealso: [](sec_petscsf), `PetscSF`, `PetscSFGetGraphLayout()`, `PetscSFCreate()`, `PetscSFView()`, `PetscSFSetGraph()`, `PetscSFGetGraph()`
@*/
PetscErrorCode PetscSFSetGraphLayout(PetscSF sf, PetscLayout layout, PetscInt nleaves, PetscInt ilocal[], PetscCopyMode localmode, const PetscInt gremote[])
{
  const PetscInt *range;
  PetscInt        i, nroots, ls = -1, ln = -1;
  PetscMPIInt     lr = -1;
  PetscSFNode    *remote;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscAssertPointer(layout, 2);
  if (nleaves > 0 && ilocal) PetscAssertPointer(ilocal, 4);
  if (nleaves > 0) PetscAssertPointer(gremote, 6);
  PetscCall(PetscLayoutSetUp(layout));
  PetscCall(PetscLayoutGetLocalSize(layout, &nroots));
  PetscCall(PetscLayoutGetRanges(layout, &range));
  PetscCall(PetscMalloc1(nleaves, &remote));
  if (nleaves) ls = gremote[0] + 1;
  for (i = 0; i < nleaves; i++) {
    const PetscInt idx = gremote[i] - ls;
    if (idx < 0 || idx >= ln) { /* short-circuit the search */
      PetscCall(PetscLayoutFindOwnerIndex(layout, gremote[i], &lr, &remote[i].index));
      remote[i].rank = lr;
      ls             = range[lr];
      ln             = range[lr + 1] - ls;
    } else {
      remote[i].rank  = lr;
      remote[i].index = idx;
    }
  }
  PetscCall(PetscSFSetGraph(sf, nroots, nleaves, ilocal, localmode, remote, PETSC_OWN_POINTER));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  PetscSFGetGraphLayout - Get the global indices and `PetscLayout` that describe a `PetscSF`

  Collective

  Input Parameter:
. sf - star forest

  Output Parameters:
+ layout  - `PetscLayout` defining the global space for roots
. nleaves - number of leaf vertices on the current process, each of these references a root on any process
. ilocal  - locations of leaves in leafdata buffers, or `NULL` for contiguous storage
- gremote - root vertices in global numbering corresponding to the leaves

  Level: intermediate

  Notes:
  The outputs are such that passing them as inputs to `PetscSFSetGraphLayout()` would lead to the same star forest.
  The outputs `layout` and `gremote` are freshly created each time this function is called,
  so they need to be freed (with `PetscLayoutDestroy()` and `PetscFree()`) by the user.

.seealso: [](sec_petscsf), `PetscSF`, `PetscSFSetGraphLayout()`, `PetscSFCreate()`, `PetscSFView()`, `PetscSFSetGraph()`, `PetscSFGetGraph()`
@*/
PetscErrorCode PetscSFGetGraphLayout(PetscSF sf, PetscLayout *layout, PetscInt *nleaves, const PetscInt *ilocal[], PetscInt *gremote[])
{
  PetscInt           nr, nl;
  const PetscSFNode *ir;
  PetscLayout        lt;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  if (layout) PetscAssertPointer(layout, 2);
  if (nleaves) PetscAssertPointer(nleaves, 3);
  if (ilocal) PetscAssertPointer(ilocal, 4);
  if (gremote) PetscAssertPointer(gremote, 5);
  PetscCall(PetscSFGetGraph(sf, &nr, &nl, ilocal, &ir));
  PetscCall(PetscLayoutCreateFromSizes(PetscObjectComm((PetscObject)sf), nr, PETSC_DECIDE, 1, &lt));
  if (gremote) {
    PetscInt        i;
    const PetscInt *range;
    PetscInt       *gr;

    PetscCall(PetscLayoutGetRanges(lt, &range));
    PetscCall(PetscMalloc1(nl, &gr));
    for (i = 0; i < nl; i++) gr[i] = range[ir[i].rank] + ir[i].index;
    *gremote = gr;
  }
  if (nleaves) *nleaves = nl;
  if (layout) *layout = lt;
  else PetscCall(PetscLayoutDestroy(&lt));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  PetscSFSetGraphSection - Sets the `PetscSF` graph (communication pattern) encoding the parallel dof overlap based upon the `PetscSection` describing the data layout.

  Input Parameters:
+ sf            - The `PetscSF`
. localSection  - `PetscSection` describing the local data layout
- globalSection - `PetscSection` describing the global data layout

  Level: developer

.seealso: [](sec_petscsf), `PetscSF`, `PetscSFSetGraph()`, `PetscSFSetGraphLayout()`
@*/
PetscErrorCode PetscSFSetGraphSection(PetscSF sf, PetscSection localSection, PetscSection globalSection)
{
  MPI_Comm        comm;
  PetscLayout     layout;
  const PetscInt *ranges;
  PetscInt       *local;
  PetscSFNode    *remote;
  PetscInt        pStart, pEnd, p, nroots, nleaves = 0, l;
  PetscMPIInt     size, rank;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscValidHeaderSpecific(localSection, PETSC_SECTION_CLASSID, 2);
  PetscValidHeaderSpecific(globalSection, PETSC_SECTION_CLASSID, 3);

  PetscCall(PetscObjectGetComm((PetscObject)sf, &comm));
  PetscCallMPI(MPI_Comm_size(comm, &size));
  PetscCallMPI(MPI_Comm_rank(comm, &rank));
  PetscCall(PetscSectionGetChart(globalSection, &pStart, &pEnd));
  PetscCall(PetscSectionGetConstrainedStorageSize(globalSection, &nroots));
  PetscCall(PetscLayoutCreate(comm, &layout));
  PetscCall(PetscLayoutSetBlockSize(layout, 1));
  PetscCall(PetscLayoutSetLocalSize(layout, nroots));
  PetscCall(PetscLayoutSetUp(layout));
  PetscCall(PetscLayoutGetRanges(layout, &ranges));
  for (p = pStart; p < pEnd; ++p) {
    PetscInt gdof, gcdof;

    PetscCall(PetscSectionGetDof(globalSection, p, &gdof));
    PetscCall(PetscSectionGetConstraintDof(globalSection, p, &gcdof));
    PetscCheck(gcdof <= (gdof < 0 ? -(gdof + 1) : gdof), PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Point %" PetscInt_FMT " has %" PetscInt_FMT " constraints > %" PetscInt_FMT " dof", p, gcdof, gdof < 0 ? -(gdof + 1) : gdof);
    nleaves += gdof < 0 ? -(gdof + 1) - gcdof : gdof - gcdof;
  }
  PetscCall(PetscMalloc1(nleaves, &local));
  PetscCall(PetscMalloc1(nleaves, &remote));
  for (p = pStart, l = 0; p < pEnd; ++p) {
    const PetscInt *cind;
    PetscInt        dof, cdof, off, gdof, gcdof, goff, gsize, d, c;

    PetscCall(PetscSectionGetDof(localSection, p, &dof));
    PetscCall(PetscSectionGetOffset(localSection, p, &off));
    PetscCall(PetscSectionGetConstraintDof(localSection, p, &cdof));
    PetscCall(PetscSectionGetConstraintIndices(localSection, p, &cind));
    PetscCall(PetscSectionGetDof(globalSection, p, &gdof));
    PetscCall(PetscSectionGetConstraintDof(globalSection, p, &gcdof));
    PetscCall(PetscSectionGetOffset(globalSection, p, &goff));
    if (!gdof) continue; /* Censored point */
    gsize = gdof < 0 ? -(gdof + 1) - gcdof : gdof - gcdof;
    if (gsize != dof - cdof) {
      PetscCheck(gsize == dof, comm, PETSC_ERR_ARG_WRONG, "Global dof %" PetscInt_FMT " for point %" PetscInt_FMT " is neither the constrained size %" PetscInt_FMT ", nor the unconstrained %" PetscInt_FMT, gsize, p, dof - cdof, dof);
      cdof = 0; /* Ignore constraints */
    }
    for (d = 0, c = 0; d < dof; ++d) {
      if ((c < cdof) && (cind[c] == d)) {
        ++c;
        continue;
      }
      local[l + d - c] = off + d;
    }
    PetscCheck(d - c == gsize, comm, PETSC_ERR_ARG_WRONG, "Point %" PetscInt_FMT ": Global dof %" PetscInt_FMT " != %" PetscInt_FMT " size - number of constraints", p, gsize, d - c);
    if (gdof < 0) {
      for (d = 0; d < gsize; ++d, ++l) {
        PetscInt    offset = -(goff + 1) + d, ir;
        PetscMPIInt r;

        PetscCall(PetscFindInt(offset, size + 1, ranges, &ir));
        PetscCall(PetscMPIIntCast(ir, &r));
        if (r < 0) r = -(r + 2);
        PetscCheck(!(r < 0) && !(r >= size), PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Point %" PetscInt_FMT " mapped to invalid process %d (%" PetscInt_FMT ", %" PetscInt_FMT ")", p, r, gdof, goff);
        remote[l].rank  = r;
        remote[l].index = offset - ranges[r];
      }
    } else {
      for (d = 0; d < gsize; ++d, ++l) {
        remote[l].rank  = rank;
        remote[l].index = goff + d - ranges[rank];
      }
    }
  }
  PetscCheck(l == nleaves, comm, PETSC_ERR_PLIB, "Iteration error, l %" PetscInt_FMT " != nleaves %" PetscInt_FMT, l, nleaves);
  PetscCall(PetscLayoutDestroy(&layout));
  PetscCall(PetscSFSetGraph(sf, nroots, nleaves, local, PETSC_OWN_POINTER, remote, PETSC_OWN_POINTER));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  PetscSFDistributeSection - Create a new `PetscSection` reorganized, moving from the root to the leaves of the `PetscSF`

  Collective

  Input Parameters:
+ sf          - The `PetscSF`
- rootSection - Section defined on root space

  Output Parameters:
+ remoteOffsets - root offsets in leaf storage, or `NULL`, its length will be the size of the chart of `leafSection`
- leafSection   - Section defined on the leaf space

  Level: advanced

  Note:
  Caller must `PetscFree()` `remoteOffsets` if it was requested

  To distribute data from the `rootSection` to the `leafSection`, see  `PetscSFCreateSectionSF()` or `PetscSectionMigrateData()`.

  Fortran Note:
  Use `PetscSFDestroyRemoteOffsets()` when `remoteOffsets` is no longer needed.

.seealso: [](sec_petscsf), `PetscSF`, `PetscSFCreate()`, `PetscSFCreateSectionSF()`
@*/
PetscErrorCode PetscSFDistributeSection(PetscSF sf, PetscSection rootSection, PetscInt *remoteOffsets[], PetscSection leafSection)
{
  PetscSF         embedSF;
  const PetscInt *indices;
  IS              selected;
  PetscInt        numFields, nroots, rpStart, rpEnd, lpStart = PETSC_INT_MAX, lpEnd = -1, f, c;
  PetscBool      *sub, hasc;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscValidHeaderSpecific(rootSection, PETSC_SECTION_CLASSID, 2);
  if (remoteOffsets) PetscAssertPointer(remoteOffsets, 3);
  PetscValidHeaderSpecific(leafSection, PETSC_SECTION_CLASSID, 4);
  PetscCall(PetscLogEventBegin(PETSCSF_DistSect, sf, 0, 0, 0));
  PetscCall(PetscSectionGetNumFields(rootSection, &numFields));
  if (numFields) {
    IS perm;

    /* PetscSectionSetNumFields() calls PetscSectionReset(), which destroys
       leafSection->perm. To keep this permutation set by the user, we grab
       the reference before calling PetscSectionSetNumFields() and set it
       back after. */
    PetscCall(PetscSectionGetPermutation(leafSection, &perm));
    PetscCall(PetscObjectReference((PetscObject)perm));
    PetscCall(PetscSectionSetNumFields(leafSection, numFields));
    PetscCall(PetscSectionSetPermutation(leafSection, perm));
    PetscCall(ISDestroy(&perm));
  }
  PetscCall(PetscMalloc1(numFields + 2, &sub));
  sub[1] = rootSection->bc ? PETSC_TRUE : PETSC_FALSE;
  for (f = 0; f < numFields; ++f) {
    PetscSectionSym sym, dsym = NULL;
    const char     *name    = NULL;
    PetscInt        numComp = 0;

    sub[2 + f] = rootSection->field[f]->bc ? PETSC_TRUE : PETSC_FALSE;
    PetscCall(PetscSectionGetFieldComponents(rootSection, f, &numComp));
    PetscCall(PetscSectionGetFieldName(rootSection, f, &name));
    PetscCall(PetscSectionGetFieldSym(rootSection, f, &sym));
    if (sym) PetscCall(PetscSectionSymDistribute(sym, sf, &dsym));
    PetscCall(PetscSectionSetFieldComponents(leafSection, f, numComp));
    PetscCall(PetscSectionSetFieldName(leafSection, f, name));
    PetscCall(PetscSectionSetFieldSym(leafSection, f, dsym));
    PetscCall(PetscSectionSymDestroy(&dsym));
    for (c = 0; c < rootSection->numFieldComponents[f]; ++c) {
      PetscCall(PetscSectionGetComponentName(rootSection, f, c, &name));
      PetscCall(PetscSectionSetComponentName(leafSection, f, c, name));
    }
  }
  PetscCall(PetscSectionGetChart(rootSection, &rpStart, &rpEnd));
  PetscCall(PetscSFGetGraph(sf, &nroots, NULL, NULL, NULL));
  rpEnd = PetscMin(rpEnd, nroots);
  rpEnd = PetscMax(rpStart, rpEnd);
  /* see if we can avoid creating the embedded SF, since it can cost more than an allreduce */
  sub[0] = (PetscBool)(nroots != rpEnd - rpStart);
  PetscCallMPI(MPIU_Allreduce(MPI_IN_PLACE, sub, 2 + numFields, MPI_C_BOOL, MPI_LOR, PetscObjectComm((PetscObject)sf)));
  if (sub[0]) {
    PetscCall(ISCreateStride(PETSC_COMM_SELF, rpEnd - rpStart, rpStart, 1, &selected));
    PetscCall(ISGetIndices(selected, &indices));
    PetscCall(PetscSFCreateEmbeddedRootSF(sf, rpEnd - rpStart, indices, &embedSF));
    PetscCall(ISRestoreIndices(selected, &indices));
    PetscCall(ISDestroy(&selected));
  } else {
    PetscCall(PetscObjectReference((PetscObject)sf));
    embedSF = sf;
  }
  PetscCall(PetscSFGetLeafRange(embedSF, &lpStart, &lpEnd));
  lpEnd++;

  PetscCall(PetscSectionSetChart(leafSection, lpStart, lpEnd));

  /* Constrained dof section */
  hasc = sub[1];
  for (f = 0; f < numFields; ++f) hasc = (PetscBool)(hasc || sub[2 + f]);

  /* Could fuse these at the cost of copies and extra allocation */
  PetscCall(PetscSFBcastBegin(embedSF, MPIU_INT, PetscSafePointerPlusOffset(rootSection->atlasDof, -rpStart), PetscSafePointerPlusOffset(leafSection->atlasDof, -lpStart), MPI_REPLACE));
  PetscCall(PetscSFBcastEnd(embedSF, MPIU_INT, PetscSafePointerPlusOffset(rootSection->atlasDof, -rpStart), PetscSafePointerPlusOffset(leafSection->atlasDof, -lpStart), MPI_REPLACE));
  if (sub[1]) {
    PetscCall(PetscSectionCheckConstraints_Private(rootSection));
    PetscCall(PetscSectionCheckConstraints_Private(leafSection));
    PetscCall(PetscSFBcastBegin(embedSF, MPIU_INT, &rootSection->bc->atlasDof[-rpStart], &leafSection->bc->atlasDof[-lpStart], MPI_REPLACE));
    PetscCall(PetscSFBcastEnd(embedSF, MPIU_INT, &rootSection->bc->atlasDof[-rpStart], &leafSection->bc->atlasDof[-lpStart], MPI_REPLACE));
  }
  for (f = 0; f < numFields; ++f) {
    PetscCall(PetscSFBcastBegin(embedSF, MPIU_INT, PetscSafePointerPlusOffset(rootSection->field[f]->atlasDof, -rpStart), PetscSafePointerPlusOffset(leafSection->field[f]->atlasDof, -lpStart), MPI_REPLACE));
    PetscCall(PetscSFBcastEnd(embedSF, MPIU_INT, PetscSafePointerPlusOffset(rootSection->field[f]->atlasDof, -rpStart), PetscSafePointerPlusOffset(leafSection->field[f]->atlasDof, -lpStart), MPI_REPLACE));
    if (sub[2 + f]) {
      PetscCall(PetscSectionCheckConstraints_Private(rootSection->field[f]));
      PetscCall(PetscSectionCheckConstraints_Private(leafSection->field[f]));
      PetscCall(PetscSFBcastBegin(embedSF, MPIU_INT, &rootSection->field[f]->bc->atlasDof[-rpStart], &leafSection->field[f]->bc->atlasDof[-lpStart], MPI_REPLACE));
      PetscCall(PetscSFBcastEnd(embedSF, MPIU_INT, &rootSection->field[f]->bc->atlasDof[-rpStart], &leafSection->field[f]->bc->atlasDof[-lpStart], MPI_REPLACE));
    }
  }
  if (remoteOffsets) {
    PetscCall(PetscMalloc1(lpEnd - lpStart, remoteOffsets));
    PetscCall(PetscSFBcastBegin(embedSF, MPIU_INT, PetscSafePointerPlusOffset(rootSection->atlasOff, -rpStart), PetscSafePointerPlusOffset(*remoteOffsets, -lpStart), MPI_REPLACE));
    PetscCall(PetscSFBcastEnd(embedSF, MPIU_INT, PetscSafePointerPlusOffset(rootSection->atlasOff, -rpStart), PetscSafePointerPlusOffset(*remoteOffsets, -lpStart), MPI_REPLACE));
  }
  PetscCall(PetscSectionInvalidateMaxDof_Internal(leafSection));
  PetscCall(PetscSectionSetUp(leafSection));
  if (hasc) { /* need to communicate bcIndices */
    PetscSF   bcSF;
    PetscInt *rOffBc;

    PetscCall(PetscMalloc1(lpEnd - lpStart, &rOffBc));
    if (sub[1]) {
      PetscCall(PetscSFBcastBegin(embedSF, MPIU_INT, &rootSection->bc->atlasOff[-rpStart], &rOffBc[-lpStart], MPI_REPLACE));
      PetscCall(PetscSFBcastEnd(embedSF, MPIU_INT, &rootSection->bc->atlasOff[-rpStart], &rOffBc[-lpStart], MPI_REPLACE));
      PetscCall(PetscSFCreateSectionSF(embedSF, rootSection->bc, rOffBc, leafSection->bc, &bcSF));
      PetscCall(PetscSFBcastBegin(bcSF, MPIU_INT, rootSection->bcIndices, leafSection->bcIndices, MPI_REPLACE));
      PetscCall(PetscSFBcastEnd(bcSF, MPIU_INT, rootSection->bcIndices, leafSection->bcIndices, MPI_REPLACE));
      PetscCall(PetscSFDestroy(&bcSF));
    }
    for (f = 0; f < numFields; ++f) {
      if (sub[2 + f]) {
        PetscCall(PetscSFBcastBegin(embedSF, MPIU_INT, &rootSection->field[f]->bc->atlasOff[-rpStart], &rOffBc[-lpStart], MPI_REPLACE));
        PetscCall(PetscSFBcastEnd(embedSF, MPIU_INT, &rootSection->field[f]->bc->atlasOff[-rpStart], &rOffBc[-lpStart], MPI_REPLACE));
        PetscCall(PetscSFCreateSectionSF(embedSF, rootSection->field[f]->bc, rOffBc, leafSection->field[f]->bc, &bcSF));
        PetscCall(PetscSFBcastBegin(bcSF, MPIU_INT, rootSection->field[f]->bcIndices, leafSection->field[f]->bcIndices, MPI_REPLACE));
        PetscCall(PetscSFBcastEnd(bcSF, MPIU_INT, rootSection->field[f]->bcIndices, leafSection->field[f]->bcIndices, MPI_REPLACE));
        PetscCall(PetscSFDestroy(&bcSF));
      }
    }
    PetscCall(PetscFree(rOffBc));
  }
  PetscCall(PetscSFDestroy(&embedSF));
  PetscCall(PetscFree(sub));
  PetscCall(PetscLogEventEnd(PETSCSF_DistSect, sf, 0, 0, 0));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  PetscSFCreateRemoteOffsets - Create offsets for point data on remote processes

  Collective

  Input Parameters:
+ sf          - The `PetscSF`
. rootSection - Data layout of remote points for outgoing data (this is layout for roots)
- leafSection - Data layout of local points for incoming data  (this is layout for leaves)

  Output Parameter:
. remoteOffsets - Offsets for point data on remote processes (these are offsets from the root section), or `NULL`

  Level: developer

  Note:
  Caller must `PetscFree()` `remoteOffsets` if it was requested

  Fortran Note:
  Use `PetscSFDestroyRemoteOffsets()` when `remoteOffsets` is no longer needed.

.seealso: [](sec_petscsf), `PetscSF`, `PetscSFCreate()`
@*/
PetscErrorCode PetscSFCreateRemoteOffsets(PetscSF sf, PetscSection rootSection, PetscSection leafSection, PetscInt *remoteOffsets[])
{
  PetscSF         embedSF;
  const PetscInt *indices;
  IS              selected;
  PetscInt        numRoots, rpStart = 0, rpEnd = 0, lpStart = 0, lpEnd = 0;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscValidHeaderSpecific(rootSection, PETSC_SECTION_CLASSID, 2);
  PetscValidHeaderSpecific(leafSection, PETSC_SECTION_CLASSID, 3);
  PetscAssertPointer(remoteOffsets, 4);
  *remoteOffsets = NULL;
  PetscCall(PetscSFGetGraph(sf, &numRoots, NULL, NULL, NULL));
  if (numRoots < 0) PetscFunctionReturn(PETSC_SUCCESS);
  PetscCall(PetscLogEventBegin(PETSCSF_RemoteOff, sf, 0, 0, 0));
  PetscCall(PetscSectionGetChart(rootSection, &rpStart, &rpEnd));
  PetscCall(PetscSectionGetChart(leafSection, &lpStart, &lpEnd));
  PetscCall(ISCreateStride(PETSC_COMM_SELF, rpEnd - rpStart, rpStart, 1, &selected));
  PetscCall(ISGetIndices(selected, &indices));
  PetscCall(PetscSFCreateEmbeddedRootSF(sf, rpEnd - rpStart, indices, &embedSF));
  PetscCall(ISRestoreIndices(selected, &indices));
  PetscCall(ISDestroy(&selected));
  PetscCall(PetscCalloc1(lpEnd - lpStart, remoteOffsets));
  PetscCall(PetscSFBcastBegin(embedSF, MPIU_INT, PetscSafePointerPlusOffset(rootSection->atlasOff, -rpStart), PetscSafePointerPlusOffset(*remoteOffsets, -lpStart), MPI_REPLACE));
  PetscCall(PetscSFBcastEnd(embedSF, MPIU_INT, PetscSafePointerPlusOffset(rootSection->atlasOff, -rpStart), PetscSafePointerPlusOffset(*remoteOffsets, -lpStart), MPI_REPLACE));
  PetscCall(PetscSFDestroy(&embedSF));
  PetscCall(PetscLogEventEnd(PETSCSF_RemoteOff, sf, 0, 0, 0));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  PetscSFCreateSectionSF - Create an expanded `PetscSF` of dofs, assuming the input `PetscSF` relates points

  Collective

  Input Parameters:
+ sf            - The `PetscSF`
. rootSection   - Data layout of remote points for outgoing data (this is usually the serial section)
. remoteOffsets - Offsets for point data on remote processes (these are offsets from the root section), or `NULL`
- leafSection   - Data layout of local points for incoming data  (this is the distributed section)

  Output Parameter:
. sectionSF - The new `PetscSF`

  Level: advanced

  Notes:
  `remoteOffsets` can be `NULL` if `sf` does not reference any points in `leafSection`

.seealso: [](sec_petscsf), `PetscSF`, `PetscSFCreate()`, `PetscSFDistributeSection()`
@*/
PetscErrorCode PetscSFCreateSectionSF(PetscSF sf, PetscSection rootSection, PetscInt remoteOffsets[], PetscSection leafSection, PetscSF *sectionSF)
{
  MPI_Comm           comm;
  const PetscInt    *localPoints;
  const PetscSFNode *remotePoints;
  PetscInt           lpStart, lpEnd;
  PetscInt           numRoots, numSectionRoots, numPoints, numIndices = 0;
  PetscInt          *localIndices;
  PetscSFNode       *remoteIndices;
  PetscInt           i, ind;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscAssertPointer(rootSection, 2);
  /* Cannot check PetscAssertPointer(remoteOffsets,3) because it can be NULL if sf does not reference any points in leafSection */
  PetscAssertPointer(leafSection, 4);
  PetscAssertPointer(sectionSF, 5);
  PetscCall(PetscObjectGetComm((PetscObject)sf, &comm));
  PetscCall(PetscSFCreate(comm, sectionSF));
  PetscCall(PetscSectionGetChart(leafSection, &lpStart, &lpEnd));
  PetscCall(PetscSectionGetStorageSize(rootSection, &numSectionRoots));
  PetscCall(PetscSFGetGraph(sf, &numRoots, &numPoints, &localPoints, &remotePoints));
  if (numRoots < 0) PetscFunctionReturn(PETSC_SUCCESS);
  PetscCall(PetscLogEventBegin(PETSCSF_SectSF, sf, 0, 0, 0));
  for (i = 0; i < numPoints; ++i) {
    PetscInt localPoint = localPoints ? localPoints[i] : i;
    PetscInt dof;

    if ((localPoint >= lpStart) && (localPoint < lpEnd)) {
      PetscCall(PetscSectionGetDof(leafSection, localPoint, &dof));
      numIndices += dof < 0 ? 0 : dof;
    }
  }
  PetscCall(PetscMalloc1(numIndices, &localIndices));
  PetscCall(PetscMalloc1(numIndices, &remoteIndices));
  /* Create new index graph */
  for (i = 0, ind = 0; i < numPoints; ++i) {
    PetscInt localPoint = localPoints ? localPoints[i] : i;
    PetscInt rank       = remotePoints[i].rank;

    if ((localPoint >= lpStart) && (localPoint < lpEnd)) {
      PetscInt remoteOffset = remoteOffsets[localPoint - lpStart];
      PetscInt loff, dof, d;

      PetscCall(PetscSectionGetOffset(leafSection, localPoint, &loff));
      PetscCall(PetscSectionGetDof(leafSection, localPoint, &dof));
      for (d = 0; d < dof; ++d, ++ind) {
        localIndices[ind]        = loff + d;
        remoteIndices[ind].rank  = rank;
        remoteIndices[ind].index = remoteOffset + d;
      }
    }
  }
  PetscCheck(numIndices == ind, comm, PETSC_ERR_PLIB, "Inconsistency in indices, %" PetscInt_FMT " should be %" PetscInt_FMT, ind, numIndices);
  PetscCall(PetscSFSetGraph(*sectionSF, numSectionRoots, numIndices, localIndices, PETSC_OWN_POINTER, remoteIndices, PETSC_OWN_POINTER));
  PetscCall(PetscSFSetUp(*sectionSF));
  PetscCall(PetscLogEventEnd(PETSCSF_SectSF, sf, 0, 0, 0));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  PetscSFCreateFromLayouts - Creates a parallel star forest mapping between two `PetscLayout` objects

  Collective

  Input Parameters:
+ rmap - `PetscLayout` defining the global root space
- lmap - `PetscLayout` defining the global leaf space

  Output Parameter:
. sf - The parallel star forest

  Level: intermediate

  Notes:
  If the global length of `lmap` differs from the global length of `rmap` then the excess entries are ignored.

  The resulting `sf` used with `PetscSFBcastBegin()` and `PetscSFBcastEnd()` merely copies the array entries of `rootdata` to
  `leafdata`; moving them between MPI processes if needed. For example,
  if rmap is [0, 3, 5) and lmap is [0, 2, 6) and `rootdata` is (1, 2, 3) on MPI rank 0 and (4, 5) on MPI rank 1 then the
  `leafdata` would become (1, 2) on MPI rank 0 and (3, 4, 5, x) on MPI rank 1.

.seealso: [](sec_petscsf), `PetscSF`, `PetscLayout`, `PetscSFCreate()`, `PetscSFSetGraph()`, `PetscLayoutCreate()`, `PetscSFSetGraphLayout()`
@*/
PetscErrorCode PetscSFCreateFromLayouts(PetscLayout rmap, PetscLayout lmap, PetscSF *sf)
{
  PetscInt     i, nroots, nleaves = 0;
  PetscInt     rN, lst, len;
  PetscMPIInt  owner = -1;
  PetscSFNode *remote;
  MPI_Comm     rcomm = rmap->comm;
  MPI_Comm     lcomm = lmap->comm;
  PetscMPIInt  flg;

  PetscFunctionBegin;
  PetscAssertPointer(rmap, 1);
  PetscAssertPointer(lmap, 2);
  PetscAssertPointer(sf, 3);
  PetscCheck(rmap->setupcalled, rcomm, PETSC_ERR_ARG_WRONGSTATE, "Root layout not setup");
  PetscCheck(lmap->setupcalled, lcomm, PETSC_ERR_ARG_WRONGSTATE, "Leaf layout not setup");
  PetscCallMPI(MPI_Comm_compare(rcomm, lcomm, &flg));
  PetscCheck(flg == MPI_CONGRUENT || flg == MPI_IDENT, rcomm, PETSC_ERR_SUP, "cannot map two layouts with non-matching communicators");
  PetscCall(PetscSFCreate(rcomm, sf));
  PetscCall(PetscLayoutGetLocalSize(rmap, &nroots));
  PetscCall(PetscLayoutGetSize(rmap, &rN));
  PetscCall(PetscLayoutGetRange(lmap, &lst, &len));
  PetscCall(PetscMalloc1(len - lst, &remote));
  for (i = lst; i < len && i < rN; i++) {
    if (owner < -1 || i >= rmap->range[owner + 1]) PetscCall(PetscLayoutFindOwner(rmap, i, &owner));
    remote[nleaves].rank  = owner;
    remote[nleaves].index = i - rmap->range[owner];
    nleaves++;
  }
  PetscCall(PetscSFSetGraph(*sf, nroots, nleaves, NULL, PETSC_OWN_POINTER, remote, PETSC_COPY_VALUES));
  PetscCall(PetscFree(remote));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* TODO: handle nooffprocentries like MatZeroRowsMapLocal_Private, since this code is the same */
PetscErrorCode PetscLayoutMapLocal(PetscLayout map, PetscInt N, const PetscInt idxs[], PetscInt *on, PetscInt *oidxs[], PetscInt *ogidxs[])
{
  PetscInt    *owners = map->range;
  PetscInt     n      = map->n;
  PetscSF      sf;
  PetscInt    *lidxs, *work = NULL, *ilocal;
  PetscSFNode *ridxs;
  PetscMPIInt  rank, p = 0;
  PetscInt     r, len = 0, nleaves = 0;

  PetscFunctionBegin;
  if (on) *on = 0; /* squelch -Wmaybe-uninitialized */
  /* Create SF where leaves are input idxs and roots are owned idxs */
  PetscCallMPI(MPI_Comm_rank(map->comm, &rank));
  PetscCall(PetscMalloc1(n, &lidxs));
  for (r = 0; r < n; ++r) lidxs[r] = -1;
  PetscCall(PetscMalloc1(N, &ridxs));
  PetscCall(PetscMalloc1(N, &ilocal));
  for (r = 0; r < N; ++r) {
    const PetscInt idx = idxs[r];

    if (idx < 0) continue;
    PetscCheck(idx < map->N, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Index %" PetscInt_FMT " out of range [0,%" PetscInt_FMT ")", idx, map->N);
    if (idx < owners[p] || owners[p + 1] <= idx) { /* short-circuit the search if the last p owns this idx too */
      PetscCall(PetscLayoutFindOwner(map, idx, &p));
    }
    ridxs[nleaves].rank  = p;
    ridxs[nleaves].index = idxs[r] - owners[p];
    ilocal[nleaves]      = r;
    nleaves++;
  }
  PetscCall(PetscSFCreate(map->comm, &sf));
  PetscCall(PetscSFSetGraph(sf, n, nleaves, ilocal, PETSC_OWN_POINTER, ridxs, PETSC_OWN_POINTER));
  PetscCall(PetscSFReduceBegin(sf, MPIU_INT, (PetscInt *)idxs, lidxs, MPI_LOR));
  PetscCall(PetscSFReduceEnd(sf, MPIU_INT, (PetscInt *)idxs, lidxs, MPI_LOR));
  if (ogidxs) { /* communicate global idxs */
    PetscInt cum = 0, start, *work2;

    PetscCall(PetscMalloc1(n, &work));
    PetscCall(PetscCalloc1(N, &work2));
    for (r = 0; r < N; ++r)
      if (idxs[r] >= 0) cum++;
    PetscCallMPI(MPI_Scan(&cum, &start, 1, MPIU_INT, MPI_SUM, map->comm));
    start -= cum;
    cum = 0;
    for (r = 0; r < N; ++r)
      if (idxs[r] >= 0) work2[r] = start + cum++;
    PetscCall(PetscSFReduceBegin(sf, MPIU_INT, work2, work, MPI_REPLACE));
    PetscCall(PetscSFReduceEnd(sf, MPIU_INT, work2, work, MPI_REPLACE));
    PetscCall(PetscFree(work2));
  }
  PetscCall(PetscSFDestroy(&sf));
  /* Compress and put in indices */
  for (r = 0; r < n; ++r)
    if (lidxs[r] >= 0) {
      if (work) work[len] = work[r];
      lidxs[len++] = r;
    }
  if (on) *on = len;
  if (oidxs) *oidxs = lidxs;
  if (ogidxs) *ogidxs = work;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  PetscSFCreateByMatchingIndices - Create `PetscSF` by matching root and leaf indices

  Collective

  Input Parameters:
+ layout           - `PetscLayout` defining the global index space and the MPI rank that brokers each index
. numRootIndices   - size of `rootIndices`
. rootIndices      - array of global indices of which this process requests ownership
. rootLocalIndices - root local index permutation (`NULL` if no permutation)
. rootLocalOffset  - offset to be added to `rootLocalIndices`
. numLeafIndices   - size of `leafIndices`
. leafIndices      - array of global indices with which this process requires data associated
. leafLocalIndices - leaf local index permutation (`NULL` if no permutation)
- leafLocalOffset  - offset to be added to `leafLocalIndices`

  Output Parameters:
+ sfA - star forest representing the communication pattern from the layout space to the leaf space (`NULL` if not needed)
- sf  - star forest representing the communication pattern from the root space to the leaf space

  Level: advanced

  Example 1:
.vb
  rank             : 0            1            2
  rootIndices      : [1 0 2]      [3]          [3]
  rootLocalOffset  : 100          200          300
  layout           : [0 1]        [2]          [3]
  leafIndices      : [0]          [2]          [0 3]
  leafLocalOffset  : 400          500          600

would build the following PetscSF

  [0] 400 <- (0,101)
  [1] 500 <- (0,102)
  [2] 600 <- (0,101)
  [2] 601 <- (2,300)
.ve

  Example 2:
.vb
  rank             : 0               1               2
  rootIndices      : [1 0 2]         [3]             [3]
  rootLocalOffset  : 100             200             300
  layout           : [0 1]           [2]             [3]
  leafIndices      : rootIndices     rootIndices     rootIndices
  leafLocalOffset  : rootLocalOffset rootLocalOffset rootLocalOffset

would build the following PetscSF

  [1] 200 <- (2,300)
.ve

  Example 3:
.vb
  No process requests ownership of global index 1, but no process needs it.

  rank             : 0            1            2
  numRootIndices   : 2            1            1
  rootIndices      : [0 2]        [3]          [3]
  rootLocalOffset  : 100          200          300
  layout           : [0 1]        [2]          [3]
  numLeafIndices   : 1            1            2
  leafIndices      : [0]          [2]          [0 3]
  leafLocalOffset  : 400          500          600

would build the following PetscSF

  [0] 400 <- (0,100)
  [1] 500 <- (0,101)
  [2] 600 <- (0,100)
  [2] 601 <- (2,300)
.ve

  Notes:
  `layout` represents any partitioning of [0, N), where N is the total number of global indices, and its
  local size can be set to `PETSC_DECIDE`.

  If a global index x lies in the partition owned by process i, each process whose `rootIndices` contains x requests
  ownership of x and sends its own rank and the local index of x to process i.
  If multiple processes request ownership of x, the one with the highest rank is to own x.
  Process i then broadcasts the ownership information, so that each process whose `leafIndices` contains x knows the
  ownership information of x.
  The output `sf` is constructed by associating each leaf point to a root point in this way.

  Suppose there is point data ordered according to the global indices and partitioned according to the given layout.
  The optional output `sfA` can be used to push such data to leaf points.

  All indices in `rootIndices` and `leafIndices` must lie in the layout range. The union (over all processes) of `rootIndices`
  must cover that of `leafIndices`, but need not cover the entire layout.

  If (leafIndices, leafLocalIndices, leafLocalOffset) == (rootIndices, rootLocalIndices, rootLocalOffset), the output
  star forest is almost identity, so will only include non-trivial part of the map.

  Developer Notes:
  Current approach of a process of the highest rank gaining the ownership may cause load imbalance; consider using
  hash(rank, root_local_index) as the bid for the ownership determination.

.seealso: [](sec_petscsf), `PetscSF`, `PetscSFCreate()`
@*/
PetscErrorCode PetscSFCreateByMatchingIndices(PetscLayout layout, PetscInt numRootIndices, const PetscInt rootIndices[], const PetscInt rootLocalIndices[], PetscInt rootLocalOffset, PetscInt numLeafIndices, const PetscInt leafIndices[], const PetscInt leafLocalIndices[], PetscInt leafLocalOffset, PetscSF *sfA, PetscSF *sf)
{
  MPI_Comm     comm = layout->comm;
  PetscMPIInt  rank;
  PetscSF      sf1;
  PetscSFNode *owners, *buffer, *iremote;
  PetscInt    *ilocal, nleaves, N, n, i;
  PetscBool    areIndicesSame;

  PetscFunctionBegin;
  PetscAssertPointer(layout, 1);
  if (rootIndices) PetscAssertPointer(rootIndices, 3);
  if (rootLocalIndices) PetscAssertPointer(rootLocalIndices, 4);
  if (leafIndices) PetscAssertPointer(leafIndices, 7);
  if (leafLocalIndices) PetscAssertPointer(leafLocalIndices, 8);
  if (sfA) PetscAssertPointer(sfA, 10);
  PetscAssertPointer(sf, 11);
  PetscCheck(numRootIndices >= 0, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "numRootIndices (%" PetscInt_FMT ") must be non-negative", numRootIndices);
  PetscCheck(numLeafIndices >= 0, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "numLeafIndices (%" PetscInt_FMT ") must be non-negative", numLeafIndices);
  PetscCallMPI(MPI_Comm_rank(comm, &rank));
  PetscCall(PetscLayoutSetUp(layout));
  PetscCall(PetscLayoutGetSize(layout, &N));
  PetscCall(PetscLayoutGetLocalSize(layout, &n));
  areIndicesSame = (PetscBool)(leafIndices == rootIndices);
  PetscCallMPI(MPIU_Allreduce(MPI_IN_PLACE, &areIndicesSame, 1, MPI_C_BOOL, MPI_LAND, comm));
  PetscCheck(!areIndicesSame || numLeafIndices == numRootIndices, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONG, "leafIndices == rootIndices, but numLeafIndices (%" PetscInt_FMT ") != numRootIndices(%" PetscInt_FMT ")", numLeafIndices, numRootIndices);
  if (PetscDefined(USE_DEBUG)) {
    PetscInt N1 = PETSC_INT_MIN;
    for (i = 0; i < numRootIndices; i++)
      if (rootIndices[i] > N1) N1 = rootIndices[i];
    PetscCallMPI(MPIU_Allreduce(MPI_IN_PLACE, &N1, 1, MPIU_INT, MPI_MAX, comm));
    PetscCheck(N1 < N, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONG, "Max. root index (%" PetscInt_FMT ") out of layout range [0,%" PetscInt_FMT ")", N1, N);
    if (!areIndicesSame) {
      N1 = PETSC_INT_MIN;
      for (i = 0; i < numLeafIndices; i++)
        if (leafIndices[i] > N1) N1 = leafIndices[i];
      PetscCallMPI(MPIU_Allreduce(MPI_IN_PLACE, &N1, 1, MPIU_INT, MPI_MAX, comm));
      PetscCheck(N1 < N, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONG, "Max. leaf index (%" PetscInt_FMT ") out of layout range [0,%" PetscInt_FMT ")", N1, N);
    }
  }

  /* Reduce: owners -> buffer */
  PetscCall(PetscMalloc1(n, &buffer));
  PetscCall(PetscSFCreate(comm, &sf1));
  PetscCall(PetscSFSetFromOptions(sf1));
  PetscCall(PetscSFSetGraphLayout(sf1, layout, numRootIndices, NULL, PETSC_OWN_POINTER, rootIndices));
  PetscCall(PetscMalloc1(numRootIndices, &owners));
  for (i = 0; i < numRootIndices; ++i) {
    owners[i].rank  = rank;
    owners[i].index = rootLocalOffset + (rootLocalIndices ? rootLocalIndices[i] : i);
  }
  for (i = 0; i < n; ++i) {
    buffer[i].index = -1;
    buffer[i].rank  = -1;
  }
  PetscCall(PetscSFReduceBegin(sf1, MPIU_SF_NODE, owners, buffer, MPI_MAXLOC));
  PetscCall(PetscSFReduceEnd(sf1, MPIU_SF_NODE, owners, buffer, MPI_MAXLOC));
  /* Bcast: buffer -> owners */
  if (!areIndicesSame) {
    PetscCall(PetscFree(owners));
    PetscCall(PetscSFSetGraphLayout(sf1, layout, numLeafIndices, NULL, PETSC_OWN_POINTER, leafIndices));
    PetscCall(PetscMalloc1(numLeafIndices, &owners));
  }
  PetscCall(PetscSFBcastBegin(sf1, MPIU_SF_NODE, buffer, owners, MPI_REPLACE));
  PetscCall(PetscSFBcastEnd(sf1, MPIU_SF_NODE, buffer, owners, MPI_REPLACE));
  for (i = 0; i < numLeafIndices; ++i) PetscCheck(owners[i].rank >= 0, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONG, "Global point %" PetscInt_FMT " was unclaimed", leafIndices[i]);
  PetscCall(PetscFree(buffer));
  if (sfA) {
    *sfA = sf1;
  } else PetscCall(PetscSFDestroy(&sf1));
  /* Create sf */
  if (areIndicesSame && rootLocalIndices == leafLocalIndices && leafLocalOffset == rootLocalOffset) {
    /* leaf space == root space */
    for (i = 0, nleaves = 0; i < numLeafIndices; ++i)
      if (owners[i].rank != rank) ++nleaves;
    PetscCall(PetscMalloc1(nleaves, &ilocal));
    PetscCall(PetscMalloc1(nleaves, &iremote));
    for (i = 0, nleaves = 0; i < numLeafIndices; ++i) {
      if (owners[i].rank != rank) {
        ilocal[nleaves]        = leafLocalOffset + i;
        iremote[nleaves].rank  = owners[i].rank;
        iremote[nleaves].index = owners[i].index;
        ++nleaves;
      }
    }
    PetscCall(PetscFree(owners));
  } else {
    nleaves = numLeafIndices;
    PetscCall(PetscMalloc1(nleaves, &ilocal));
    for (i = 0; i < nleaves; ++i) ilocal[i] = leafLocalOffset + (leafLocalIndices ? leafLocalIndices[i] : i);
    iremote = owners;
  }
  PetscCall(PetscSFCreate(comm, sf));
  PetscCall(PetscSFSetFromOptions(*sf));
  PetscCall(PetscSFSetGraph(*sf, rootLocalOffset + numRootIndices, nleaves, ilocal, PETSC_OWN_POINTER, iremote, PETSC_OWN_POINTER));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  PetscSFMerge - append/merge indices of `sfb` into `sfa`, with preference for `sfb`

  Collective

  Input Parameters:
+ sfa - default `PetscSF`
- sfb - additional edges to add/replace edges in `sfa`

  Output Parameter:
. merged - new `PetscSF` with combined edges

  Level: intermediate

.seealso: [](sec_petscsf), `PetscSF`, `PetscSFCompose()`
@*/
PetscErrorCode PetscSFMerge(PetscSF sfa, PetscSF sfb, PetscSF *merged)
{
  PetscInt maxleaf;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(sfa, PETSCSF_CLASSID, 1);
  PetscValidHeaderSpecific(sfb, PETSCSF_CLASSID, 2);
  PetscCheckSameComm(sfa, 1, sfb, 2);
  PetscAssertPointer(merged, 3);
  {
    PetscInt aleaf, bleaf;
    PetscCall(PetscSFGetLeafRange(sfa, NULL, &aleaf));
    PetscCall(PetscSFGetLeafRange(sfb, NULL, &bleaf));
    maxleaf = PetscMax(aleaf, bleaf) + 1; // One more than the last index
  }
  PetscInt          *clocal, aroots, aleaves, broots, bleaves;
  PetscSFNode       *cremote;
  const PetscInt    *alocal, *blocal;
  const PetscSFNode *aremote, *bremote;
  PetscCall(PetscMalloc2(maxleaf, &clocal, maxleaf, &cremote));
  for (PetscInt i = 0; i < maxleaf; i++) clocal[i] = -1;
  PetscCall(PetscSFGetGraph(sfa, &aroots, &aleaves, &alocal, &aremote));
  PetscCall(PetscSFGetGraph(sfb, &broots, &bleaves, &blocal, &bremote));
  PetscCheck(aroots == broots, PETSC_COMM_SELF, PETSC_ERR_ARG_INCOMP, "Both sfa and sfb must have the same root space");
  for (PetscInt i = 0; i < aleaves; i++) {
    PetscInt a = alocal ? alocal[i] : i;
    clocal[a]  = a;
    cremote[a] = aremote[i];
  }
  for (PetscInt i = 0; i < bleaves; i++) {
    PetscInt b = blocal ? blocal[i] : i;
    clocal[b]  = b;
    cremote[b] = bremote[i];
  }
  PetscInt nleaves = 0;
  for (PetscInt i = 0; i < maxleaf; i++) {
    if (clocal[i] < 0) continue;
    clocal[nleaves]  = clocal[i];
    cremote[nleaves] = cremote[i];
    nleaves++;
  }
  PetscCall(PetscSFCreate(PetscObjectComm((PetscObject)sfa), merged));
  PetscCall(PetscSFSetGraph(*merged, aroots, nleaves, clocal, PETSC_COPY_VALUES, cremote, PETSC_COPY_VALUES));
  PetscCall(PetscFree2(clocal, cremote));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  PetscSFCreateStridedSF - Create an `PetscSF` to communicate interleaved blocks of data

  Collective

  Input Parameters:
+ sf  - star forest
. bs  - stride
. ldr - leading dimension of root space
- ldl - leading dimension of leaf space

  Output Parameter:
. vsf - the new `PetscSF`

  Level: intermediate

  Notes:
  This can be useful to perform communications on multiple right-hand sides stored in a Fortran-style two dimensional array.
  For example, the calling sequence
.vb
  c_datatype *roots, *leaves;
  for i in [0,bs) do
    PetscSFBcastBegin(sf, mpi_datatype, roots + i*ldr, leaves + i*ldl, op)
    PetscSFBcastEnd(sf, mpi_datatype, roots + i*ldr, leaves + i*ldl, op)
.ve
  is equivalent to
.vb
  c_datatype *roots, *leaves;
  PetscSFCreateStridedSF(sf, bs, ldr, ldl, &vsf)
  PetscSFBcastBegin(vsf, mpi_datatype, roots, leaves, op)
  PetscSFBcastEnd(vsf, mpi_datatype, roots, leaves, op)
.ve

  Developer Notes:
  Should this functionality be handled with a new API instead of creating a new object?

.seealso: [](sec_petscsf), `PetscSF`, `PetscSFCreate()`, `PetscSFSetGraph()`
@*/
PetscErrorCode PetscSFCreateStridedSF(PetscSF sf, PetscInt bs, PetscInt ldr, PetscInt ldl, PetscSF *vsf)
{
  PetscSF            rankssf;
  const PetscSFNode *iremote, *sfrremote;
  PetscSFNode       *viremote;
  const PetscInt    *ilocal;
  PetscInt          *vilocal = NULL, *ldrs;
  PetscInt           nranks, nr, nl, vnr, vnl, maxl;
  PetscMPIInt        rank;
  MPI_Comm           comm;
  PetscSFType        sftype;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscValidLogicalCollectiveInt(sf, bs, 2);
  PetscAssertPointer(vsf, 5);
  if (bs == 1) {
    PetscCall(PetscObjectReference((PetscObject)sf));
    *vsf = sf;
    PetscFunctionReturn(PETSC_SUCCESS);
  }
  PetscCall(PetscSFSetUp(sf));
  PetscCall(PetscObjectGetComm((PetscObject)sf, &comm));
  PetscCallMPI(MPI_Comm_rank(comm, &rank));
  PetscCall(PetscSFGetGraph(sf, &nr, &nl, &ilocal, &iremote));
  PetscCall(PetscSFGetLeafRange(sf, NULL, &maxl));
  maxl += 1;
  if (ldl == PETSC_DECIDE) ldl = maxl;
  if (ldr == PETSC_DECIDE) ldr = nr;
  ldl /= PetscMax(1, sf->vscat.bs); // SFs created from VecScatterCreate() may have a nonzero block size. If not 0, we need to scale ldl and ldr
  ldr /= PetscMax(1, sf->vscat.bs);
  PetscCheck(ldr >= nr, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONG, "Invalid leading dimension %" PetscInt_FMT " must be smaller than number of roots %" PetscInt_FMT, ldr, nr);
  PetscCheck(ldl >= maxl, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONG, "Invalid leading dimension %" PetscInt_FMT " must be larger than leaf range %" PetscInt_FMT, ldl, maxl - 1);
  vnr = nr * bs;
  vnl = nl * bs;
  PetscCall(PetscMalloc1(vnl, &viremote));
  PetscCall(PetscMalloc1(vnl, &vilocal));

  /* Communicate root leading dimensions to leaf ranks */
  PetscCall(PetscSFGetRanksSF(sf, &rankssf));
  PetscCall(PetscSFGetGraph(rankssf, NULL, &nranks, NULL, &sfrremote));
  PetscCall(PetscMalloc1(nranks, &ldrs));
  PetscCall(PetscSFBcastBegin(rankssf, MPIU_INT, &ldr, ldrs, MPI_REPLACE));
  PetscCall(PetscSFBcastEnd(rankssf, MPIU_INT, &ldr, ldrs, MPI_REPLACE));

  for (PetscInt i = 0, rold = -1, lda = -1; i < nl; i++) {
    const PetscInt r  = iremote[i].rank;
    const PetscInt ii = iremote[i].index;

    if (r == rank) lda = ldr;
    else if (rold != r) {
      PetscInt j;

      for (j = 0; j < nranks; j++)
        if (sfrremote[j].rank == r) break;
      PetscCheck(j < nranks, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Unable to locate neighbor rank %" PetscInt_FMT, r);
      lda = ldrs[j];
    }
    rold = r;
    for (PetscInt v = 0; v < bs; v++) {
      viremote[v * nl + i].rank  = r;
      viremote[v * nl + i].index = v * lda + ii;
      vilocal[v * nl + i]        = v * ldl + (ilocal ? ilocal[i] : i);
    }
  }
  PetscCall(PetscFree(ldrs));
  PetscCall(PetscSFCreate(comm, vsf));
  if (sf->vscat.bs > 1) {
    (*vsf)->vscat.bs = sf->vscat.bs;
    PetscCallMPI(MPI_Type_dup(sf->vscat.unit, &(*vsf)->vscat.unit));
    (*vsf)->vscat.to_n   = bs * sf->vscat.to_n;
    (*vsf)->vscat.from_n = bs * sf->vscat.from_n;
  }
  PetscCall(PetscSFGetType(sf, &sftype));
  PetscCall(PetscSFSetType(*vsf, sftype));
  PetscCall(PetscSFSetGraph(*vsf, vnr, vnl, vilocal, PETSC_OWN_POINTER, viremote, PETSC_OWN_POINTER));
  PetscFunctionReturn(PETSC_SUCCESS);
}