#include <petsc/private/dmpleximpl.h> /*I      "petscdmplex.h"   I*/
#include <petsc/private/isimpl.h>
#include <petsc/private/petscfeimpl.h>
#include <petscsf.h>
#include <petscds.h>

/* hierarchy routines */

/*@
  DMPlexSetReferenceTree - set the reference tree for hierarchically non-conforming meshes.

  Not Collective

  Input Parameters:
+ dm  - The `DMPLEX` object
- ref - The reference tree `DMPLEX` object

  Level: intermediate

.seealso: [](ch_unstructured), `DM`, `DMPLEX`, `DMPlexGetReferenceTree()`, `DMPlexCreateDefaultReferenceTree()`
@*/
PetscErrorCode DMPlexSetReferenceTree(DM dm, DM ref)
{
  DM_Plex *mesh = (DM_Plex *)dm->data;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  if (ref) PetscValidHeaderSpecific(ref, DM_CLASSID, 2);
  PetscCall(PetscObjectReference((PetscObject)ref));
  PetscCall(DMDestroy(&mesh->referenceTree));
  mesh->referenceTree = ref;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  DMPlexGetReferenceTree - get the reference tree for hierarchically non-conforming meshes.

  Not Collective

  Input Parameter:
. dm - The `DMPLEX` object

  Output Parameter:
. ref - The reference tree `DMPLEX` object

  Level: intermediate

  Developer Notes:
  The reference tree is shallow copied during `DMClone()`, thus it is may be shared by different `DM`s.
  It is not a topological-only object, since some parts of the library use its local section to compute
  interpolation and injection matrices. This may lead to unexpected failures during those calls.

.seealso: [](ch_unstructured), `DM`, `DMPLEX`, `DMPlexSetReferenceTree()`, `DMPlexCreateDefaultReferenceTree()`
@*/
PetscErrorCode DMPlexGetReferenceTree(DM dm, DM *ref)
{
  DM_Plex *mesh = (DM_Plex *)dm->data;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  PetscAssertPointer(ref, 2);
  *ref = mesh->referenceTree;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexReferenceTreeGetChildSymmetry_Default(DM dm, PetscInt parent, PetscInt parentOrientA, PetscInt childOrientA, PetscInt childA, PetscInt parentOrientB, PetscInt *childOrientB, PetscInt *childB)
{
  PetscInt coneSize, dStart, dEnd, dim, ABswap, oAvert, oBvert, ABswapVert;

  PetscFunctionBegin;
  if (parentOrientA == parentOrientB) {
    if (childOrientB) *childOrientB = childOrientA;
    if (childB) *childB = childA;
    PetscFunctionReturn(PETSC_SUCCESS);
  }
  for (dim = 0; dim < 3; dim++) {
    PetscCall(DMPlexGetDepthStratum(dm, dim, &dStart, &dEnd));
    if (parent >= dStart && parent <= dEnd) break;
  }
  PetscCheck(dim <= 2, PETSC_COMM_SELF, PETSC_ERR_SUP, "Cannot perform child symmetry for %" PetscInt_FMT "-cells", dim);
  PetscCheck(dim, PETSC_COMM_SELF, PETSC_ERR_PLIB, "A vertex has no children");
  if (childA < dStart || childA >= dEnd) {
    /* this is a lower-dimensional child: bootstrap */
    PetscInt        size, i, sA = -1, sB, sOrientB, sConeSize;
    const PetscInt *supp, *coneA, *coneB, *oA, *oB;

    PetscCall(DMPlexGetSupportSize(dm, childA, &size));
    PetscCall(DMPlexGetSupport(dm, childA, &supp));

    /* find a point sA in supp(childA) that has the same parent */
    for (i = 0; i < size; i++) {
      PetscInt sParent;

      sA = supp[i];
      if (sA == parent) continue;
      PetscCall(DMPlexGetTreeParent(dm, sA, &sParent, NULL));
      if (sParent == parent) break;
    }
    PetscCheck(i != size, PETSC_COMM_SELF, PETSC_ERR_PLIB, "could not find support in children");
    /* find out which point sB is in an equivalent position to sA under
     * parentOrientB */
    PetscCall(DMPlexReferenceTreeGetChildSymmetry_Default(dm, parent, parentOrientA, 0, sA, parentOrientB, &sOrientB, &sB));
    PetscCall(DMPlexGetConeSize(dm, sA, &sConeSize));
    PetscCall(DMPlexGetCone(dm, sA, &coneA));
    PetscCall(DMPlexGetCone(dm, sB, &coneB));
    PetscCall(DMPlexGetConeOrientation(dm, sA, &oA));
    PetscCall(DMPlexGetConeOrientation(dm, sB, &oB));
    /* step through the cone of sA in natural order */
    for (i = 0; i < sConeSize; i++) {
      if (coneA[i] == childA) {
        /* if childA is at position i in coneA,
         * then we want the point that is at sOrientB*i in coneB */
        PetscInt j = (sOrientB >= 0) ? ((sOrientB + i) % sConeSize) : ((sConeSize - (sOrientB + 1) - i) % sConeSize);
        if (childB) *childB = coneB[j];
        if (childOrientB) {
          DMPolytopeType ct;
          PetscInt       oBtrue;

          PetscCall(DMPlexGetConeSize(dm, childA, &coneSize));
          /* compose sOrientB and oB[j] */
          PetscCheck(coneSize == 0 || coneSize == 2, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Expected a vertex or an edge");
          ct = coneSize ? DM_POLYTOPE_SEGMENT : DM_POLYTOPE_POINT;
          /* we may have to flip an edge */
          oBtrue        = (sOrientB >= 0) ? oB[j] : DMPolytopeTypeComposeOrientation(ct, -1, oB[j]);
          oBtrue        = DMPolytopeConvertNewOrientation_Internal(ct, oBtrue);
          ABswap        = DihedralSwap(coneSize, DMPolytopeConvertNewOrientation_Internal(ct, oA[i]), oBtrue);
          *childOrientB = DihedralCompose(coneSize, childOrientA, ABswap);
        }
        break;
      }
    }
    PetscCheck(i != sConeSize, PETSC_COMM_SELF, PETSC_ERR_PLIB, "support cone mismatch");
    PetscFunctionReturn(PETSC_SUCCESS);
  }
  /* get the cone size and symmetry swap */
  PetscCall(DMPlexGetConeSize(dm, parent, &coneSize));
  ABswap = DihedralSwap(coneSize, parentOrientA, parentOrientB);
  if (dim == 2) {
    /* orientations refer to cones: we want them to refer to vertices:
     * if it's a rotation, they are the same, but if the order is reversed, a
     * permutation that puts side i first does *not* put vertex i first */
    oAvert     = (parentOrientA >= 0) ? parentOrientA : -((-parentOrientA % coneSize) + 1);
    oBvert     = (parentOrientB >= 0) ? parentOrientB : -((-parentOrientB % coneSize) + 1);
    ABswapVert = DihedralSwap(coneSize, oAvert, oBvert);
  } else {
    ABswapVert = ABswap;
  }
  if (childB) {
    /* assume that each child corresponds to a vertex, in the same order */
    PetscInt        p, posA = -1, numChildren, i;
    const PetscInt *children;

    /* count which position the child is in */
    PetscCall(DMPlexGetTreeChildren(dm, parent, &numChildren, &children));
    for (i = 0; i < numChildren; i++) {
      p = children[i];
      if (p == childA) {
        posA = i;
        break;
      }
    }
    if (posA >= coneSize) {
      /* this is the triangle in the middle of a uniformly refined triangle: it
       * is invariant */
      PetscCheck(dim == 2 && posA == 3, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Expected a middle triangle, got something else");
      *childB = childA;
    } else {
      /* figure out position B by applying ABswapVert */
      PetscInt posB;

      posB = (ABswapVert >= 0) ? ((ABswapVert + posA) % coneSize) : ((coneSize - (ABswapVert + 1) - posA) % coneSize);
      if (childB) *childB = children[posB];
    }
  }
  if (childOrientB) *childOrientB = DihedralCompose(coneSize, childOrientA, ABswap);
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  DMPlexReferenceTreeGetChildSymmetry - Given a reference tree, transform a childid and orientation from one parent frame to another

  Input Parameters:
+ dm            - the reference tree `DMPLEX` object
. parent        - the parent point
. parentOrientA - the reference orientation for describing the parent
. childOrientA  - the reference orientation for describing the child
. childA        - the reference childID for describing the child
- parentOrientB - the new orientation for describing the parent

  Output Parameters:
+ childOrientB - if not `NULL`, set to the new orientation for describing the child
- childB       - if not `NULL`, the new childID for describing the child

  Level: developer

.seealso: [](ch_unstructured), `DM`, `DMPLEX`, `DMPlexGetReferenceTree()`, `DMPlexSetReferenceTree()`, `DMPlexSetTree()`
@*/
PetscErrorCode DMPlexReferenceTreeGetChildSymmetry(DM dm, PetscInt parent, PetscInt parentOrientA, PetscInt childOrientA, PetscInt childA, PetscInt parentOrientB, PetscInt *childOrientB, PetscInt *childB)
{
  DM_Plex *mesh = (DM_Plex *)dm->data;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  PetscCheck(mesh->getchildsymmetry, PETSC_COMM_SELF, PETSC_ERR_SUP, "DMPlexReferenceTreeGetChildSymmetry not implemented");
  PetscCall(mesh->getchildsymmetry(dm, parent, parentOrientA, childOrientA, childA, parentOrientB, childOrientB, childB));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexSetTree_Internal(DM, PetscSection, PetscInt *, PetscInt *, PetscBool, PetscBool);

PetscErrorCode DMPlexCreateReferenceTree_SetTree(DM dm, PetscSection parentSection, PetscInt parents[], PetscInt childIDs[])
{
  PetscFunctionBegin;
  PetscCall(DMPlexSetTree_Internal(dm, parentSection, parents, childIDs, PETSC_TRUE, PETSC_FALSE));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode DMPlexCreateReferenceTree_Union(DM K, DM Kref, const char *labelName, DM *ref)
{
  MPI_Comm     comm;
  PetscInt     dim, p, pStart, pEnd, pRefStart, pRefEnd, d, offset, parentSize, *parents, *childIDs;
  PetscInt    *permvals, *unionCones, *coneSizes, *unionOrientations, numUnionPoints, *numDimPoints, numCones, numVerts;
  DMLabel      identity, identityRef;
  PetscSection unionSection, unionConeSection, parentSection;
  PetscScalar *unionCoords;
  IS           perm;

  PetscFunctionBegin;
  comm = PetscObjectComm((PetscObject)K);
  PetscCall(DMGetDimension(K, &dim));
  PetscCall(DMPlexGetChart(K, &pStart, &pEnd));
  PetscCall(DMGetLabel(K, labelName, &identity));
  PetscCall(DMGetLabel(Kref, labelName, &identityRef));
  PetscCall(DMPlexGetChart(Kref, &pRefStart, &pRefEnd));
  PetscCall(PetscSectionCreate(comm, &unionSection));
  PetscCall(PetscSectionSetChart(unionSection, 0, (pEnd - pStart) + (pRefEnd - pRefStart)));
  /* count points that will go in the union */
  for (p = pStart; p < pEnd; p++) PetscCall(PetscSectionSetDof(unionSection, p - pStart, 1));
  for (p = pRefStart; p < pRefEnd; p++) {
    PetscInt q, qSize;
    PetscCall(DMLabelGetValue(identityRef, p, &q));
    PetscCall(DMLabelGetStratumSize(identityRef, q, &qSize));
    if (qSize > 1) PetscCall(PetscSectionSetDof(unionSection, p - pRefStart + (pEnd - pStart), 1));
  }
  PetscCall(PetscMalloc1(pEnd - pStart + pRefEnd - pRefStart, &permvals));
  offset = 0;
  /* stratify points in the union by topological dimension */
  for (d = 0; d <= dim; d++) {
    PetscInt cStart, cEnd, c;

    PetscCall(DMPlexGetHeightStratum(K, d, &cStart, &cEnd));
    for (c = cStart; c < cEnd; c++) permvals[offset++] = c;

    PetscCall(DMPlexGetHeightStratum(Kref, d, &cStart, &cEnd));
    for (c = cStart; c < cEnd; c++) permvals[offset++] = c + (pEnd - pStart);
  }
  PetscCall(ISCreateGeneral(comm, (pEnd - pStart) + (pRefEnd - pRefStart), permvals, PETSC_OWN_POINTER, &perm));
  PetscCall(PetscSectionSetPermutation(unionSection, perm));
  PetscCall(PetscSectionSetUp(unionSection));
  PetscCall(PetscSectionGetStorageSize(unionSection, &numUnionPoints));
  PetscCall(PetscMalloc2(numUnionPoints, &coneSizes, dim + 1, &numDimPoints));
  /* count dimension points */
  for (d = 0; d <= dim; d++) {
    PetscInt cStart, cOff, cOff2;
    PetscCall(DMPlexGetHeightStratum(K, d, &cStart, NULL));
    PetscCall(PetscSectionGetOffset(unionSection, cStart - pStart, &cOff));
    if (d < dim) {
      PetscCall(DMPlexGetHeightStratum(K, d + 1, &cStart, NULL));
      PetscCall(PetscSectionGetOffset(unionSection, cStart - pStart, &cOff2));
    } else {
      cOff2 = numUnionPoints;
    }
    numDimPoints[dim - d] = cOff2 - cOff;
  }
  PetscCall(PetscSectionCreate(comm, &unionConeSection));
  PetscCall(PetscSectionSetChart(unionConeSection, 0, numUnionPoints));
  /* count the cones in the union */
  for (p = pStart; p < pEnd; p++) {
    PetscInt dof, uOff;

    PetscCall(DMPlexGetConeSize(K, p, &dof));
    PetscCall(PetscSectionGetOffset(unionSection, p - pStart, &uOff));
    PetscCall(PetscSectionSetDof(unionConeSection, uOff, dof));
    coneSizes[uOff] = dof;
  }
  for (p = pRefStart; p < pRefEnd; p++) {
    PetscInt dof, uDof, uOff;

    PetscCall(DMPlexGetConeSize(Kref, p, &dof));
    PetscCall(PetscSectionGetDof(unionSection, p - pRefStart + (pEnd - pStart), &uDof));
    PetscCall(PetscSectionGetOffset(unionSection, p - pRefStart + (pEnd - pStart), &uOff));
    if (uDof) {
      PetscCall(PetscSectionSetDof(unionConeSection, uOff, dof));
      coneSizes[uOff] = dof;
    }
  }
  PetscCall(PetscSectionSetUp(unionConeSection));
  PetscCall(PetscSectionGetStorageSize(unionConeSection, &numCones));
  PetscCall(PetscMalloc2(numCones, &unionCones, numCones, &unionOrientations));
  /* write the cones in the union */
  for (p = pStart; p < pEnd; p++) {
    PetscInt        dof, uOff, c, cOff;
    const PetscInt *cone, *orientation;

    PetscCall(DMPlexGetConeSize(K, p, &dof));
    PetscCall(DMPlexGetCone(K, p, &cone));
    PetscCall(DMPlexGetConeOrientation(K, p, &orientation));
    PetscCall(PetscSectionGetOffset(unionSection, p - pStart, &uOff));
    PetscCall(PetscSectionGetOffset(unionConeSection, uOff, &cOff));
    for (c = 0; c < dof; c++) {
      PetscInt e, eOff;
      e = cone[c];
      PetscCall(PetscSectionGetOffset(unionSection, e - pStart, &eOff));
      unionCones[cOff + c]        = eOff;
      unionOrientations[cOff + c] = orientation[c];
    }
  }
  for (p = pRefStart; p < pRefEnd; p++) {
    PetscInt        dof, uDof, uOff, c, cOff;
    const PetscInt *cone, *orientation;

    PetscCall(DMPlexGetConeSize(Kref, p, &dof));
    PetscCall(DMPlexGetCone(Kref, p, &cone));
    PetscCall(DMPlexGetConeOrientation(Kref, p, &orientation));
    PetscCall(PetscSectionGetDof(unionSection, p - pRefStart + (pEnd - pStart), &uDof));
    PetscCall(PetscSectionGetOffset(unionSection, p - pRefStart + (pEnd - pStart), &uOff));
    if (uDof) {
      PetscCall(PetscSectionGetOffset(unionConeSection, uOff, &cOff));
      for (c = 0; c < dof; c++) {
        PetscInt e, eOff, eDof;

        e = cone[c];
        PetscCall(PetscSectionGetDof(unionSection, e - pRefStart + (pEnd - pStart), &eDof));
        if (eDof) {
          PetscCall(PetscSectionGetOffset(unionSection, e - pRefStart + (pEnd - pStart), &eOff));
        } else {
          PetscCall(DMLabelGetValue(identityRef, e, &e));
          PetscCall(PetscSectionGetOffset(unionSection, e - pStart, &eOff));
        }
        unionCones[cOff + c]        = eOff;
        unionOrientations[cOff + c] = orientation[c];
      }
    }
  }
  /* get the coordinates */
  {
    PetscInt     vStart, vEnd, vRefStart, vRefEnd, v, vDof, vOff;
    PetscSection KcoordsSec, KrefCoordsSec;
    Vec          KcoordsVec, KrefCoordsVec;
    PetscScalar *Kcoords;

    PetscCall(DMGetCoordinateSection(K, &KcoordsSec));
    PetscCall(DMGetCoordinatesLocal(K, &KcoordsVec));
    PetscCall(DMGetCoordinateSection(Kref, &KrefCoordsSec));
    PetscCall(DMGetCoordinatesLocal(Kref, &KrefCoordsVec));

    numVerts = numDimPoints[0];
    PetscCall(PetscMalloc1(numVerts * dim, &unionCoords));
    PetscCall(DMPlexGetDepthStratum(K, 0, &vStart, &vEnd));

    offset = 0;
    for (v = vStart; v < vEnd; v++) {
      PetscCall(PetscSectionGetOffset(unionSection, v - pStart, &vOff));
      PetscCall(VecGetValuesSection(KcoordsVec, KcoordsSec, v, &Kcoords));
      for (d = 0; d < dim; d++) unionCoords[offset * dim + d] = Kcoords[d];
      offset++;
    }
    PetscCall(DMPlexGetDepthStratum(Kref, 0, &vRefStart, &vRefEnd));
    for (v = vRefStart; v < vRefEnd; v++) {
      PetscCall(PetscSectionGetDof(unionSection, v - pRefStart + (pEnd - pStart), &vDof));
      PetscCall(PetscSectionGetOffset(unionSection, v - pRefStart + (pEnd - pStart), &vOff));
      PetscCall(VecGetValuesSection(KrefCoordsVec, KrefCoordsSec, v, &Kcoords));
      if (vDof) {
        for (d = 0; d < dim; d++) unionCoords[offset * dim + d] = Kcoords[d];
        offset++;
      }
    }
  }
  PetscCall(DMCreate(comm, ref));
  PetscCall(DMSetType(*ref, DMPLEX));
  PetscCall(DMSetDimension(*ref, dim));
  PetscCall(DMPlexCreateFromDAG(*ref, dim, numDimPoints, coneSizes, unionCones, unionOrientations, unionCoords));
  /* set the tree */
  PetscCall(PetscSectionCreate(comm, &parentSection));
  PetscCall(PetscSectionSetChart(parentSection, 0, numUnionPoints));
  for (p = pRefStart; p < pRefEnd; p++) {
    PetscInt uDof, uOff;

    PetscCall(PetscSectionGetDof(unionSection, p - pRefStart + (pEnd - pStart), &uDof));
    PetscCall(PetscSectionGetOffset(unionSection, p - pRefStart + (pEnd - pStart), &uOff));
    if (uDof) PetscCall(PetscSectionSetDof(parentSection, uOff, 1));
  }
  PetscCall(PetscSectionSetUp(parentSection));
  PetscCall(PetscSectionGetStorageSize(parentSection, &parentSize));
  PetscCall(PetscMalloc2(parentSize, &parents, parentSize, &childIDs));
  for (p = pRefStart; p < pRefEnd; p++) {
    PetscInt uDof, uOff;

    PetscCall(PetscSectionGetDof(unionSection, p - pRefStart + (pEnd - pStart), &uDof));
    PetscCall(PetscSectionGetOffset(unionSection, p - pRefStart + (pEnd - pStart), &uOff));
    if (uDof) {
      PetscInt pOff, parent, parentU;
      PetscCall(PetscSectionGetOffset(parentSection, uOff, &pOff));
      PetscCall(DMLabelGetValue(identityRef, p, &parent));
      PetscCall(PetscSectionGetOffset(unionSection, parent - pStart, &parentU));
      parents[pOff]  = parentU;
      childIDs[pOff] = uOff;
    }
  }
  PetscCall(DMPlexCreateReferenceTree_SetTree(*ref, parentSection, parents, childIDs));
  PetscCall(PetscSectionDestroy(&parentSection));
  PetscCall(PetscFree2(parents, childIDs));

  /* clean up */
  PetscCall(PetscSectionDestroy(&unionSection));
  PetscCall(PetscSectionDestroy(&unionConeSection));
  PetscCall(ISDestroy(&perm));
  PetscCall(PetscFree(unionCoords));
  PetscCall(PetscFree2(unionCones, unionOrientations));
  PetscCall(PetscFree2(coneSizes, numDimPoints));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  DMPlexCreateDefaultReferenceTree - create a reference tree for isotropic hierarchical mesh refinement.

  Collective

  Input Parameters:
+ comm    - the MPI communicator
. dim     - the spatial dimension
- simplex - Flag for simplex, otherwise use a tensor-product cell

  Output Parameter:
. ref - the reference tree `DMPLEX` object

  Level: intermediate

.seealso: `DMPlexSetReferenceTree()`, `DMPlexGetReferenceTree()`
@*/
PetscErrorCode DMPlexCreateDefaultReferenceTree(MPI_Comm comm, PetscInt dim, PetscBool simplex, DM *ref)
{
  DM_Plex *mesh;
  DM       K, Kref;
  PetscInt p, pStart, pEnd;
  DMLabel  identity;

  PetscFunctionBegin;
#if 1
  comm = PETSC_COMM_SELF;
#endif
  /* create a reference element */
  PetscCall(DMPlexCreateReferenceCell(comm, DMPolytopeTypeSimpleShape(dim, simplex), &K));
  PetscCall(DMCreateLabel(K, "identity"));
  PetscCall(DMGetLabel(K, "identity", &identity));
  PetscCall(DMPlexGetChart(K, &pStart, &pEnd));
  for (p = pStart; p < pEnd; p++) PetscCall(DMLabelSetValue(identity, p, p));
  /* refine it */
  PetscCall(DMRefine(K, comm, &Kref));

  /* the reference tree is the union of these two, without duplicating
   * points that appear in both */
  PetscCall(DMPlexCreateReferenceTree_Union(K, Kref, "identity", ref));
  mesh                   = (DM_Plex *)(*ref)->data;
  mesh->getchildsymmetry = DMPlexReferenceTreeGetChildSymmetry_Default;
  PetscCall(DMDestroy(&K));
  PetscCall(DMDestroy(&Kref));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexTreeSymmetrize(DM dm)
{
  DM_Plex     *mesh = (DM_Plex *)dm->data;
  PetscSection childSec, pSec;
  PetscInt     p, pSize, cSize, parMax = PETSC_INT_MIN, parMin = PETSC_INT_MAX;
  PetscInt    *offsets, *children, pStart, pEnd;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  PetscCall(PetscSectionDestroy(&mesh->childSection));
  PetscCall(PetscFree(mesh->children));
  pSec = mesh->parentSection;
  if (!pSec) PetscFunctionReturn(PETSC_SUCCESS);
  PetscCall(PetscSectionGetStorageSize(pSec, &pSize));
  for (p = 0; p < pSize; p++) {
    PetscInt par = mesh->parents[p];

    parMax = PetscMax(parMax, par + 1);
    parMin = PetscMin(parMin, par);
  }
  if (parMin > parMax) {
    parMin = -1;
    parMax = -1;
  }
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)pSec), &childSec));
  PetscCall(PetscSectionSetChart(childSec, parMin, parMax));
  for (p = 0; p < pSize; p++) {
    PetscInt par = mesh->parents[p];

    PetscCall(PetscSectionAddDof(childSec, par, 1));
  }
  PetscCall(PetscSectionSetUp(childSec));
  PetscCall(PetscSectionGetStorageSize(childSec, &cSize));
  PetscCall(PetscMalloc1(cSize, &children));
  PetscCall(PetscCalloc1(parMax - parMin, &offsets));
  PetscCall(PetscSectionGetChart(pSec, &pStart, &pEnd));
  for (p = pStart; p < pEnd; p++) {
    PetscInt dof, off, i;

    PetscCall(PetscSectionGetDof(pSec, p, &dof));
    PetscCall(PetscSectionGetOffset(pSec, p, &off));
    for (i = 0; i < dof; i++) {
      PetscInt par = mesh->parents[off + i], cOff;

      PetscCall(PetscSectionGetOffset(childSec, par, &cOff));
      children[cOff + offsets[par - parMin]++] = p;
    }
  }
  mesh->childSection = childSec;
  mesh->children     = children;
  PetscCall(PetscFree(offsets));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode AnchorsFlatten(PetscSection section, IS is, PetscSection *sectionNew, IS *isNew)
{
  PetscInt        pStart, pEnd, size, sizeNew, i, p, *valsNew = NULL;
  const PetscInt *vals;
  PetscSection    secNew;
  PetscBool       anyNew, globalAnyNew;
  PetscBool       compress;

  PetscFunctionBegin;
  PetscCall(PetscSectionGetChart(section, &pStart, &pEnd));
  PetscCall(ISGetLocalSize(is, &size));
  PetscCall(ISGetIndices(is, &vals));
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)section), &secNew));
  PetscCall(PetscSectionSetChart(secNew, pStart, pEnd));
  for (i = 0; i < size; i++) {
    PetscInt dof;

    p = vals[i];
    if (p < pStart || p >= pEnd) continue;
    PetscCall(PetscSectionGetDof(section, p, &dof));
    if (dof) break;
  }
  if (i == size) {
    PetscCall(PetscSectionSetUp(secNew));
    anyNew   = PETSC_FALSE;
    compress = PETSC_FALSE;
    sizeNew  = 0;
  } else {
    anyNew = PETSC_TRUE;
    for (p = pStart; p < pEnd; p++) {
      PetscInt dof, off;

      PetscCall(PetscSectionGetDof(section, p, &dof));
      PetscCall(PetscSectionGetOffset(section, p, &off));
      for (i = 0; i < dof; i++) {
        PetscInt q = vals[off + i], qDof = 0;

        if (q >= pStart && q < pEnd) PetscCall(PetscSectionGetDof(section, q, &qDof));
        if (qDof) PetscCall(PetscSectionAddDof(secNew, p, qDof));
        else PetscCall(PetscSectionAddDof(secNew, p, 1));
      }
    }
    PetscCall(PetscSectionSetUp(secNew));
    PetscCall(PetscSectionGetStorageSize(secNew, &sizeNew));
    PetscCall(PetscMalloc1(sizeNew, &valsNew));
    compress = PETSC_FALSE;
    for (p = pStart; p < pEnd; p++) {
      PetscInt dof, off, count, offNew, dofNew;

      PetscCall(PetscSectionGetDof(section, p, &dof));
      PetscCall(PetscSectionGetOffset(section, p, &off));
      PetscCall(PetscSectionGetDof(secNew, p, &dofNew));
      PetscCall(PetscSectionGetOffset(secNew, p, &offNew));
      count = 0;
      for (i = 0; i < dof; i++) {
        PetscInt q = vals[off + i], qDof = 0, qOff = 0, j;

        if (q >= pStart && q < pEnd) {
          PetscCall(PetscSectionGetDof(section, q, &qDof));
          PetscCall(PetscSectionGetOffset(section, q, &qOff));
        }
        if (qDof) {
          PetscInt oldCount = count;

          for (j = 0; j < qDof; j++) {
            PetscInt k, r = vals[qOff + j];

            for (k = 0; k < oldCount; k++) {
              if (valsNew[offNew + k] == r) break;
            }
            if (k == oldCount) valsNew[offNew + count++] = r;
          }
        } else {
          PetscInt k, oldCount = count;

          for (k = 0; k < oldCount; k++) {
            if (valsNew[offNew + k] == q) break;
          }
          if (k == oldCount) valsNew[offNew + count++] = q;
        }
      }
      if (count < dofNew) {
        PetscCall(PetscSectionSetDof(secNew, p, count));
        compress = PETSC_TRUE;
      }
    }
  }
  PetscCall(ISRestoreIndices(is, &vals));
  PetscCallMPI(MPIU_Allreduce(&anyNew, &globalAnyNew, 1, MPI_C_BOOL, MPI_LOR, PetscObjectComm((PetscObject)secNew)));
  if (!globalAnyNew) {
    PetscCall(PetscSectionDestroy(&secNew));
    *sectionNew = NULL;
    *isNew      = NULL;
  } else {
    if (compress) {
      PetscSection secComp;
      PetscInt    *valsComp = NULL;

      PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)section), &secComp));
      PetscCall(PetscSectionSetChart(secComp, pStart, pEnd));
      for (p = pStart; p < pEnd; p++) {
        PetscInt dof;

        PetscCall(PetscSectionGetDof(secNew, p, &dof));
        PetscCall(PetscSectionSetDof(secComp, p, dof));
      }
      PetscCall(PetscSectionSetUp(secComp));
      PetscCall(PetscSectionGetStorageSize(secComp, &sizeNew));
      PetscCall(PetscMalloc1(sizeNew, &valsComp));
      for (p = pStart; p < pEnd; p++) {
        PetscInt dof, off, offNew, j;

        PetscCall(PetscSectionGetDof(secNew, p, &dof));
        PetscCall(PetscSectionGetOffset(secNew, p, &off));
        PetscCall(PetscSectionGetOffset(secComp, p, &offNew));
        for (j = 0; j < dof; j++) valsComp[offNew + j] = valsNew[off + j];
      }
      PetscCall(PetscSectionDestroy(&secNew));
      secNew = secComp;
      PetscCall(PetscFree(valsNew));
      valsNew = valsComp;
    }
    PetscCall(ISCreateGeneral(PetscObjectComm((PetscObject)is), sizeNew, valsNew, PETSC_OWN_POINTER, isNew));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexCreateAnchors_Tree(DM dm)
{
  PetscInt     p, pStart, pEnd, *anchors, size;
  PetscInt     aMin = PETSC_INT_MAX, aMax = PETSC_INT_MIN;
  PetscSection aSec;
  DMLabel      canonLabel;
  IS           aIS;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  PetscCall(DMPlexGetChart(dm, &pStart, &pEnd));
  PetscCall(DMGetLabel(dm, "canonical", &canonLabel));
  for (p = pStart; p < pEnd; p++) {
    PetscInt parent;

    if (canonLabel) {
      PetscInt canon;

      PetscCall(DMLabelGetValue(canonLabel, p, &canon));
      if (p != canon) continue;
    }
    PetscCall(DMPlexGetTreeParent(dm, p, &parent, NULL));
    if (parent != p) {
      aMin = PetscMin(aMin, p);
      aMax = PetscMax(aMax, p + 1);
    }
  }
  if (aMin > aMax) {
    aMin = -1;
    aMax = -1;
  }
  PetscCall(PetscSectionCreate(PETSC_COMM_SELF, &aSec));
  PetscCall(PetscSectionSetChart(aSec, aMin, aMax));
  for (p = aMin; p < aMax; p++) {
    PetscInt parent, ancestor = p;

    if (canonLabel) {
      PetscInt canon;

      PetscCall(DMLabelGetValue(canonLabel, p, &canon));
      if (p != canon) continue;
    }
    PetscCall(DMPlexGetTreeParent(dm, p, &parent, NULL));
    while (parent != ancestor) {
      ancestor = parent;
      PetscCall(DMPlexGetTreeParent(dm, ancestor, &parent, NULL));
    }
    if (ancestor != p) {
      PetscInt closureSize, *closure = NULL;

      PetscCall(DMPlexGetTransitiveClosure(dm, ancestor, PETSC_TRUE, &closureSize, &closure));
      PetscCall(PetscSectionSetDof(aSec, p, closureSize));
      PetscCall(DMPlexRestoreTransitiveClosure(dm, ancestor, PETSC_TRUE, &closureSize, &closure));
    }
  }
  PetscCall(PetscSectionSetUp(aSec));
  PetscCall(PetscSectionGetStorageSize(aSec, &size));
  PetscCall(PetscMalloc1(size, &anchors));
  for (p = aMin; p < aMax; p++) {
    PetscInt parent, ancestor = p;

    if (canonLabel) {
      PetscInt canon;

      PetscCall(DMLabelGetValue(canonLabel, p, &canon));
      if (p != canon) continue;
    }
    PetscCall(DMPlexGetTreeParent(dm, p, &parent, NULL));
    while (parent != ancestor) {
      ancestor = parent;
      PetscCall(DMPlexGetTreeParent(dm, ancestor, &parent, NULL));
    }
    if (ancestor != p) {
      PetscInt j, closureSize, *closure = NULL, aOff;

      PetscCall(PetscSectionGetOffset(aSec, p, &aOff));

      PetscCall(DMPlexGetTransitiveClosure(dm, ancestor, PETSC_TRUE, &closureSize, &closure));
      for (j = 0; j < closureSize; j++) anchors[aOff + j] = closure[2 * j];
      PetscCall(DMPlexRestoreTransitiveClosure(dm, ancestor, PETSC_TRUE, &closureSize, &closure));
    }
  }
  PetscCall(ISCreateGeneral(PETSC_COMM_SELF, size, anchors, PETSC_OWN_POINTER, &aIS));
  {
    PetscSection aSecNew = aSec;
    IS           aISNew  = aIS;

    PetscCall(PetscObjectReference((PetscObject)aSec));
    PetscCall(PetscObjectReference((PetscObject)aIS));
    while (aSecNew) {
      PetscCall(PetscSectionDestroy(&aSec));
      PetscCall(ISDestroy(&aIS));
      aSec    = aSecNew;
      aIS     = aISNew;
      aSecNew = NULL;
      aISNew  = NULL;
      PetscCall(AnchorsFlatten(aSec, aIS, &aSecNew, &aISNew));
    }
  }
  PetscCall(DMPlexSetAnchors(dm, aSec, aIS));
  PetscCall(PetscSectionDestroy(&aSec));
  PetscCall(ISDestroy(&aIS));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexGetTrueSupportSize(DM dm, PetscInt p, PetscInt *dof, PetscInt *numTrueSupp)
{
  PetscFunctionBegin;
  if (numTrueSupp[p] == -1) {
    PetscInt        i, alldof;
    const PetscInt *supp;
    PetscInt        count = 0;

    PetscCall(DMPlexGetSupportSize(dm, p, &alldof));
    PetscCall(DMPlexGetSupport(dm, p, &supp));
    for (i = 0; i < alldof; i++) {
      PetscInt        q = supp[i], numCones, j;
      const PetscInt *cone;

      PetscCall(DMPlexGetConeSize(dm, q, &numCones));
      PetscCall(DMPlexGetCone(dm, q, &cone));
      for (j = 0; j < numCones; j++) {
        if (cone[j] == p) break;
      }
      if (j < numCones) count++;
    }
    numTrueSupp[p] = count;
  }
  *dof = numTrueSupp[p];
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexTreeExchangeSupports(DM dm)
{
  DM_Plex     *mesh = (DM_Plex *)dm->data;
  PetscSection newSupportSection;
  PetscInt     newSize, *newSupports, pStart, pEnd, p, d, depth;
  PetscInt    *numTrueSupp;
  PetscInt    *offsets;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  /* symmetrize the hierarchy */
  PetscCall(DMPlexGetDepth(dm, &depth));
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)mesh->supportSection), &newSupportSection));
  PetscCall(DMPlexGetChart(dm, &pStart, &pEnd));
  PetscCall(PetscSectionSetChart(newSupportSection, pStart, pEnd));
  PetscCall(PetscCalloc1(pEnd, &offsets));
  PetscCall(PetscMalloc1(pEnd, &numTrueSupp));
  for (p = 0; p < pEnd; p++) numTrueSupp[p] = -1;
  /* if a point is in the (true) support of q, it should be in the support of
   * parent(q) */
  for (d = 0; d <= depth; d++) {
    PetscCall(DMPlexGetHeightStratum(dm, d, &pStart, &pEnd));
    for (p = pStart; p < pEnd; ++p) {
      PetscInt dof, q, qdof, parent;

      PetscCall(DMPlexGetTrueSupportSize(dm, p, &dof, numTrueSupp));
      PetscCall(PetscSectionAddDof(newSupportSection, p, dof));
      q = p;
      PetscCall(DMPlexGetTreeParent(dm, q, &parent, NULL));
      while (parent != q && parent >= pStart && parent < pEnd) {
        q = parent;

        PetscCall(DMPlexGetTrueSupportSize(dm, q, &qdof, numTrueSupp));
        PetscCall(PetscSectionAddDof(newSupportSection, p, qdof));
        PetscCall(PetscSectionAddDof(newSupportSection, q, dof));
        PetscCall(DMPlexGetTreeParent(dm, q, &parent, NULL));
      }
    }
  }
  PetscCall(PetscSectionSetUp(newSupportSection));
  PetscCall(PetscSectionGetStorageSize(newSupportSection, &newSize));
  PetscCall(PetscMalloc1(newSize, &newSupports));
  for (d = 0; d <= depth; d++) {
    PetscCall(DMPlexGetHeightStratum(dm, d, &pStart, &pEnd));
    for (p = pStart; p < pEnd; p++) {
      PetscInt dof, off, q, qdof, qoff, newDof, newOff, newqOff, i, parent;

      PetscCall(PetscSectionGetDof(mesh->supportSection, p, &dof));
      PetscCall(PetscSectionGetOffset(mesh->supportSection, p, &off));
      PetscCall(PetscSectionGetDof(newSupportSection, p, &newDof));
      PetscCall(PetscSectionGetOffset(newSupportSection, p, &newOff));
      for (i = 0; i < dof; i++) {
        PetscInt        numCones, j;
        const PetscInt *cone;
        PetscInt        q = mesh->supports[off + i];

        PetscCall(DMPlexGetConeSize(dm, q, &numCones));
        PetscCall(DMPlexGetCone(dm, q, &cone));
        for (j = 0; j < numCones; j++) {
          if (cone[j] == p) break;
        }
        if (j < numCones) newSupports[newOff + offsets[p]++] = q;
      }

      q = p;
      PetscCall(DMPlexGetTreeParent(dm, q, &parent, NULL));
      while (parent != q && parent >= pStart && parent < pEnd) {
        q = parent;
        PetscCall(PetscSectionGetDof(mesh->supportSection, q, &qdof));
        PetscCall(PetscSectionGetOffset(mesh->supportSection, q, &qoff));
        PetscCall(PetscSectionGetOffset(newSupportSection, q, &newqOff));
        for (i = 0; i < qdof; i++) {
          PetscInt        numCones, j;
          const PetscInt *cone;
          PetscInt        r = mesh->supports[qoff + i];

          PetscCall(DMPlexGetConeSize(dm, r, &numCones));
          PetscCall(DMPlexGetCone(dm, r, &cone));
          for (j = 0; j < numCones; j++) {
            if (cone[j] == q) break;
          }
          if (j < numCones) newSupports[newOff + offsets[p]++] = r;
        }
        for (i = 0; i < dof; i++) {
          PetscInt        numCones, j;
          const PetscInt *cone;
          PetscInt        r = mesh->supports[off + i];

          PetscCall(DMPlexGetConeSize(dm, r, &numCones));
          PetscCall(DMPlexGetCone(dm, r, &cone));
          for (j = 0; j < numCones; j++) {
            if (cone[j] == p) break;
          }
          if (j < numCones) newSupports[newqOff + offsets[q]++] = r;
        }
        PetscCall(DMPlexGetTreeParent(dm, q, &parent, NULL));
      }
    }
  }
  PetscCall(PetscSectionDestroy(&mesh->supportSection));
  mesh->supportSection = newSupportSection;
  PetscCall(PetscFree(mesh->supports));
  mesh->supports = newSupports;
  PetscCall(PetscFree(offsets));
  PetscCall(PetscFree(numTrueSupp));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexComputeAnchorMatrix_Tree_Direct(DM, PetscSection, PetscSection, Mat);
static PetscErrorCode DMPlexComputeAnchorMatrix_Tree_FromReference(DM, PetscSection, PetscSection, Mat);

static PetscErrorCode DMPlexSetTree_Internal(DM dm, PetscSection parentSection, PetscInt *parents, PetscInt *childIDs, PetscBool computeCanonical, PetscBool exchangeSupports)
{
  DM_Plex *mesh = (DM_Plex *)dm->data;
  DM       refTree;
  PetscInt size;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  PetscValidHeaderSpecific(parentSection, PETSC_SECTION_CLASSID, 2);
  PetscCall(PetscObjectReference((PetscObject)parentSection));
  PetscCall(PetscSectionDestroy(&mesh->parentSection));
  mesh->parentSection = parentSection;
  PetscCall(PetscSectionGetStorageSize(parentSection, &size));
  if (parents != mesh->parents) {
    PetscCall(PetscFree(mesh->parents));
    PetscCall(PetscMalloc1(size, &mesh->parents));
    PetscCall(PetscArraycpy(mesh->parents, parents, size));
  }
  if (childIDs != mesh->childIDs) {
    PetscCall(PetscFree(mesh->childIDs));
    PetscCall(PetscMalloc1(size, &mesh->childIDs));
    PetscCall(PetscArraycpy(mesh->childIDs, childIDs, size));
  }
  PetscCall(DMPlexGetReferenceTree(dm, &refTree));
  if (refTree) {
    DMLabel canonLabel;

    PetscCall(DMGetLabel(refTree, "canonical", &canonLabel));
    if (canonLabel) {
      PetscInt i;

      for (i = 0; i < size; i++) {
        PetscInt canon;
        PetscCall(DMLabelGetValue(canonLabel, mesh->childIDs[i], &canon));
        if (canon >= 0) mesh->childIDs[i] = canon;
      }
    }
    mesh->computeanchormatrix = DMPlexComputeAnchorMatrix_Tree_FromReference;
  } else {
    mesh->computeanchormatrix = DMPlexComputeAnchorMatrix_Tree_Direct;
  }
  PetscCall(DMPlexTreeSymmetrize(dm));
  if (computeCanonical) {
    PetscInt d, dim;

    /* add the canonical label */
    PetscCall(DMGetDimension(dm, &dim));
    PetscCall(DMCreateLabel(dm, "canonical"));
    for (d = 0; d <= dim; d++) {
      PetscInt        p, dStart, dEnd, canon = -1, cNumChildren;
      const PetscInt *cChildren;

      PetscCall(DMPlexGetDepthStratum(dm, d, &dStart, &dEnd));
      for (p = dStart; p < dEnd; p++) {
        PetscCall(DMPlexGetTreeChildren(dm, p, &cNumChildren, &cChildren));
        if (cNumChildren) {
          canon = p;
          break;
        }
      }
      if (canon == -1) continue;
      for (p = dStart; p < dEnd; p++) {
        PetscInt        numChildren, i;
        const PetscInt *children;

        PetscCall(DMPlexGetTreeChildren(dm, p, &numChildren, &children));
        if (numChildren) {
          PetscCheck(numChildren == cNumChildren, PetscObjectComm((PetscObject)dm), PETSC_ERR_PLIB, "All parent points in a stratum should have the same number of children: %" PetscInt_FMT " != %" PetscInt_FMT, numChildren, cNumChildren);
          PetscCall(DMSetLabelValue(dm, "canonical", p, canon));
          for (i = 0; i < numChildren; i++) PetscCall(DMSetLabelValue(dm, "canonical", children[i], cChildren[i]));
        }
      }
    }
  }
  if (exchangeSupports) PetscCall(DMPlexTreeExchangeSupports(dm));
  mesh->createanchors = DMPlexCreateAnchors_Tree;
  /* reset anchors */
  PetscCall(DMPlexSetAnchors(dm, NULL, NULL));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  DMPlexSetTree - set the tree that describes the hierarchy of non-conforming mesh points.  This routine also creates
  the point-to-point constraints determined by the tree: a point is constrained to the points in the closure of its
  tree root.

  Collective

  Input Parameters:
+ dm            - the `DMPLEX` object
. parentSection - a section describing the tree: a point has a parent if it has 1 dof in the section; the section
                  offset indexes the parent and childID list; the reference count of parentSection is incremented
. parents       - a list of the point parents; copied, can be destroyed
- childIDs      - identifies the relationship of the child point to the parent point; if there is a reference tree, then
             the child corresponds to the point in the reference tree with index childIDs; copied, can be destroyed

  Level: intermediate

.seealso: [](ch_unstructured), `DM`, `DMPLEX`, `DMPlexGetTree()`, `DMPlexSetReferenceTree()`, `DMPlexSetAnchors()`, `DMPlexGetTreeParent()`, `DMPlexGetTreeChildren()`
@*/
PetscErrorCode DMPlexSetTree(DM dm, PetscSection parentSection, PetscInt parents[], PetscInt childIDs[])
{
  PetscFunctionBegin;
  PetscCall(DMPlexSetTree_Internal(dm, parentSection, parents, childIDs, PETSC_FALSE, PETSC_TRUE));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  DMPlexGetTree - get the tree that describes the hierarchy of non-conforming mesh points.
  Collective

  Input Parameter:
. dm - the `DMPLEX` object

  Output Parameters:
+ parentSection - a section describing the tree: a point has a parent if it has 1 dof in the section; the section
                  offset indexes the parent and childID list
. parents       - a list of the point parents
. childIDs      - identifies the relationship of the child point to the parent point; if there is a reference tree, then
             the child corresponds to the point in the reference tree with index childID
. childSection  - the inverse of the parent section
- children      - a list of the point children

  Level: intermediate

.seealso: [](ch_unstructured), `DM`, `DMPLEX`, `DMPlexSetTree()`, `DMPlexSetReferenceTree()`, `DMPlexSetAnchors()`, `DMPlexGetTreeParent()`, `DMPlexGetTreeChildren()`
@*/
PetscErrorCode DMPlexGetTree(DM dm, PetscSection *parentSection, PetscInt *parents[], PetscInt *childIDs[], PetscSection *childSection, PetscInt *children[])
{
  DM_Plex *mesh = (DM_Plex *)dm->data;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  if (parentSection) *parentSection = mesh->parentSection;
  if (parents) *parents = mesh->parents;
  if (childIDs) *childIDs = mesh->childIDs;
  if (childSection) *childSection = mesh->childSection;
  if (children) *children = mesh->children;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  DMPlexGetTreeParent - get the parent of a point in the tree describing the point hierarchy (not the DAG)

  Input Parameters:
+ dm    - the `DMPLEX` object
- point - the query point

  Output Parameters:
+ parent  - if not `NULL`, set to the parent of the point, or the point itself if the point does not have a parent
- childID - if not `NULL`, set to the child ID of the point with respect to its parent, or 0 if the point
            does not have a parent

  Level: intermediate

.seealso: [](ch_unstructured), `DM`, `DMPLEX`, `DMPlexSetTree()`, `DMPlexGetTree()`, `DMPlexGetTreeChildren()`
@*/
PetscErrorCode DMPlexGetTreeParent(DM dm, PetscInt point, PetscInt *parent, PetscInt *childID)
{
  DM_Plex     *mesh = (DM_Plex *)dm->data;
  PetscSection pSec;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  pSec = mesh->parentSection;
  if (pSec && point >= pSec->pStart && point < pSec->pEnd) {
    PetscInt dof;

    PetscCall(PetscSectionGetDof(pSec, point, &dof));
    if (dof) {
      PetscInt off;

      PetscCall(PetscSectionGetOffset(pSec, point, &off));
      if (parent) *parent = mesh->parents[off];
      if (childID) *childID = mesh->childIDs[off];
      PetscFunctionReturn(PETSC_SUCCESS);
    }
  }
  if (parent) *parent = point;
  if (childID) *childID = 0;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  DMPlexGetTreeChildren - get the children of a point in the tree describing the point hierarchy (not the DAG)

  Input Parameters:
+ dm    - the `DMPLEX` object
- point - the query point

  Output Parameters:
+ numChildren - if not `NULL`, set to the number of children
- children    - if not `NULL`, set to a list children, or set to `NULL` if the point has no children

  Level: intermediate

.seealso: [](ch_unstructured), `DM`, `DMPLEX`, `DMPlexSetTree()`, `DMPlexGetTree()`, `DMPlexGetTreeParent()`
@*/
PetscErrorCode DMPlexGetTreeChildren(DM dm, PetscInt point, PetscInt *numChildren, const PetscInt *children[])
{
  DM_Plex     *mesh = (DM_Plex *)dm->data;
  PetscSection childSec;
  PetscInt     dof = 0;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  childSec = mesh->childSection;
  if (childSec && point >= childSec->pStart && point < childSec->pEnd) PetscCall(PetscSectionGetDof(childSec, point, &dof));
  if (numChildren) *numChildren = dof;
  if (children) {
    if (dof) {
      PetscInt off;

      PetscCall(PetscSectionGetOffset(childSec, point, &off));
      *children = &mesh->children[off];
    } else {
      *children = NULL;
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode EvaluateBasis(PetscSpace space, PetscInt nBasis, PetscInt nFunctionals, PetscInt nComps, PetscInt nPoints, const PetscInt *pointsPerFn, const PetscReal *points, const PetscReal *weights, PetscReal *work, Mat basisAtPoints)
{
  PetscInt f, b, p, c, offset, qPoints;

  PetscFunctionBegin;
  PetscCall(PetscSpaceEvaluate(space, nPoints, points, work, NULL, NULL));
  for (f = 0, offset = 0; f < nFunctionals; f++) {
    qPoints = pointsPerFn[f];
    for (b = 0; b < nBasis; b++) {
      PetscScalar val = 0.;

      for (p = 0; p < qPoints; p++) {
        for (c = 0; c < nComps; c++) val += work[((offset + p) * nBasis + b) * nComps + c] * weights[(offset + p) * nComps + c];
      }
      PetscCall(MatSetValue(basisAtPoints, b, f, val, INSERT_VALUES));
    }
    offset += qPoints;
  }
  PetscCall(MatAssemblyBegin(basisAtPoints, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(basisAtPoints, MAT_FINAL_ASSEMBLY));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexComputeAnchorMatrix_Tree_Direct(DM dm, PetscSection section, PetscSection cSec, Mat cMat)
{
  PetscDS         ds;
  PetscInt        spdim;
  PetscInt        numFields, f, c, cStart, cEnd, pStart, pEnd, conStart, conEnd;
  const PetscInt *anchors;
  PetscSection    aSec;
  PetscReal      *v0, *v0parent, *vtmp, *J, *Jparent, *invJparent, detJ, detJparent;
  IS              aIS;

  PetscFunctionBegin;
  PetscCall(DMPlexGetChart(dm, &pStart, &pEnd));
  PetscCall(DMGetDS(dm, &ds));
  PetscCall(PetscDSGetNumFields(ds, &numFields));
  PetscCall(DMPlexGetHeightStratum(dm, 0, &cStart, &cEnd));
  PetscCall(DMPlexGetAnchors(dm, &aSec, &aIS));
  PetscCall(ISGetIndices(aIS, &anchors));
  PetscCall(PetscSectionGetChart(cSec, &conStart, &conEnd));
  PetscCall(DMGetDimension(dm, &spdim));
  PetscCall(PetscMalloc6(spdim, &v0, spdim, &v0parent, spdim, &vtmp, spdim * spdim, &J, spdim * spdim, &Jparent, spdim * spdim, &invJparent));

  for (f = 0; f < numFields; f++) {
    PetscObject          disc;
    PetscClassId         id;
    PetscSpace           bspace;
    PetscDualSpace       dspace;
    PetscInt             i, j, k, nPoints, Nc, offset;
    PetscInt             fSize, maxDof;
    PetscReal           *weights, *pointsRef, *pointsReal, *work;
    PetscScalar         *scwork;
    const PetscScalar   *X;
    PetscInt            *sizes, *workIndRow, *workIndCol;
    Mat                  Amat, Bmat, Xmat;
    const PetscInt      *numDof = NULL;
    const PetscInt    ***perms  = NULL;
    const PetscScalar ***flips  = NULL;

    PetscCall(PetscDSGetDiscretization(ds, f, &disc));
    PetscCall(PetscObjectGetClassId(disc, &id));
    if (id == PETSCFE_CLASSID) {
      PetscFE fe = (PetscFE)disc;

      PetscCall(PetscFEGetBasisSpace(fe, &bspace));
      PetscCall(PetscFEGetDualSpace(fe, &dspace));
      PetscCall(PetscDualSpaceGetDimension(dspace, &fSize));
      PetscCall(PetscFEGetNumComponents(fe, &Nc));
    } else if (id == PETSCFV_CLASSID) {
      PetscFV fv = (PetscFV)disc;

      PetscCall(PetscFVGetNumComponents(fv, &Nc));
      PetscCall(PetscSpaceCreate(PetscObjectComm((PetscObject)fv), &bspace));
      PetscCall(PetscSpaceSetType(bspace, PETSCSPACEPOLYNOMIAL));
      PetscCall(PetscSpaceSetDegree(bspace, 0, PETSC_DETERMINE));
      PetscCall(PetscSpaceSetNumComponents(bspace, Nc));
      PetscCall(PetscSpaceSetNumVariables(bspace, spdim));
      PetscCall(PetscSpaceSetUp(bspace));
      PetscCall(PetscFVGetDualSpace(fv, &dspace));
      PetscCall(PetscDualSpaceGetDimension(dspace, &fSize));
    } else SETERRQ(PetscObjectComm(disc), PETSC_ERR_ARG_UNKNOWN_TYPE, "PetscDS discretization id %d not recognized.", id);
    PetscCall(PetscDualSpaceGetNumDof(dspace, &numDof));
    for (i = 0, maxDof = 0; i <= spdim; i++) maxDof = PetscMax(maxDof, numDof[i]);
    PetscCall(PetscDualSpaceGetSymmetries(dspace, &perms, &flips));

    PetscCall(MatCreate(PETSC_COMM_SELF, &Amat));
    PetscCall(MatSetSizes(Amat, fSize, fSize, fSize, fSize));
    PetscCall(MatSetType(Amat, MATSEQDENSE));
    PetscCall(MatSetUp(Amat));
    PetscCall(MatDuplicate(Amat, MAT_DO_NOT_COPY_VALUES, &Bmat));
    PetscCall(MatDuplicate(Amat, MAT_DO_NOT_COPY_VALUES, &Xmat));
    nPoints = 0;
    for (i = 0; i < fSize; i++) {
      PetscInt        qPoints, thisNc;
      PetscQuadrature quad;

      PetscCall(PetscDualSpaceGetFunctional(dspace, i, &quad));
      PetscCall(PetscQuadratureGetData(quad, NULL, &thisNc, &qPoints, NULL, NULL));
      PetscCheck(thisNc == Nc, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Functional dim %" PetscInt_FMT " does not much basis dim %" PetscInt_FMT, thisNc, Nc);
      nPoints += qPoints;
    }
    PetscCall(PetscMalloc7(fSize, &sizes, nPoints * Nc, &weights, spdim * nPoints, &pointsRef, spdim * nPoints, &pointsReal, nPoints * fSize * Nc, &work, maxDof, &workIndRow, maxDof, &workIndCol));
    PetscCall(PetscMalloc1(maxDof * maxDof, &scwork));
    offset = 0;
    for (i = 0; i < fSize; i++) {
      PetscInt         qPoints;
      const PetscReal *p, *w;
      PetscQuadrature  quad;

      PetscCall(PetscDualSpaceGetFunctional(dspace, i, &quad));
      PetscCall(PetscQuadratureGetData(quad, NULL, NULL, &qPoints, &p, &w));
      PetscCall(PetscArraycpy(weights + Nc * offset, w, Nc * qPoints));
      PetscCall(PetscArraycpy(pointsRef + spdim * offset, p, spdim * qPoints));
      sizes[i] = qPoints;
      offset += qPoints;
    }
    PetscCall(EvaluateBasis(bspace, fSize, fSize, Nc, nPoints, sizes, pointsRef, weights, work, Amat));
    PetscCall(MatLUFactor(Amat, NULL, NULL, NULL));
    for (c = cStart; c < cEnd; c++) {
      PetscInt  parent;
      PetscInt  closureSize, closureSizeP, *closure = NULL, *closureP = NULL;
      PetscInt *childOffsets, *parentOffsets;

      PetscCall(DMPlexGetTreeParent(dm, c, &parent, NULL));
      if (parent == c) continue;
      PetscCall(DMPlexGetTransitiveClosure(dm, c, PETSC_TRUE, &closureSize, &closure));
      for (i = 0; i < closureSize; i++) {
        PetscInt p = closure[2 * i];
        PetscInt conDof;

        if (p < conStart || p >= conEnd) continue;
        if (numFields) PetscCall(PetscSectionGetFieldDof(cSec, p, f, &conDof));
        else PetscCall(PetscSectionGetDof(cSec, p, &conDof));
        if (conDof) break;
      }
      if (i == closureSize) {
        PetscCall(DMPlexRestoreTransitiveClosure(dm, c, PETSC_TRUE, &closureSize, &closure));
        continue;
      }

      PetscCall(DMPlexComputeCellGeometryFEM(dm, c, NULL, v0, J, NULL, &detJ));
      PetscCall(DMPlexComputeCellGeometryFEM(dm, parent, NULL, v0parent, Jparent, invJparent, &detJparent));
      for (i = 0; i < nPoints; i++) {
        const PetscReal xi0[3] = {-1., -1., -1.};

        CoordinatesRefToReal(spdim, spdim, xi0, v0, J, &pointsRef[i * spdim], vtmp);
        CoordinatesRealToRef(spdim, spdim, xi0, v0parent, invJparent, vtmp, &pointsReal[i * spdim]);
      }
      PetscCall(EvaluateBasis(bspace, fSize, fSize, Nc, nPoints, sizes, pointsReal, weights, work, Bmat));
      PetscCall(MatMatSolve(Amat, Bmat, Xmat));
      PetscCall(MatDenseGetArrayRead(Xmat, &X));
      PetscCall(DMPlexGetTransitiveClosure(dm, parent, PETSC_TRUE, &closureSizeP, &closureP));
      PetscCall(PetscMalloc2(closureSize + 1, &childOffsets, closureSizeP + 1, &parentOffsets));
      childOffsets[0] = 0;
      for (i = 0; i < closureSize; i++) {
        PetscInt p = closure[2 * i];
        PetscInt dof;

        if (numFields) PetscCall(PetscSectionGetFieldDof(section, p, f, &dof));
        else PetscCall(PetscSectionGetDof(section, p, &dof));
        childOffsets[i + 1] = childOffsets[i] + dof;
      }
      parentOffsets[0] = 0;
      for (i = 0; i < closureSizeP; i++) {
        PetscInt p = closureP[2 * i];
        PetscInt dof;

        if (numFields) PetscCall(PetscSectionGetFieldDof(section, p, f, &dof));
        else PetscCall(PetscSectionGetDof(section, p, &dof));
        parentOffsets[i + 1] = parentOffsets[i] + dof;
      }
      for (i = 0; i < closureSize; i++) {
        PetscInt           conDof, conOff, aDof, aOff, nWork;
        PetscInt           p = closure[2 * i];
        PetscInt           o = closure[2 * i + 1];
        const PetscInt    *perm;
        const PetscScalar *flip;

        if (p < conStart || p >= conEnd) continue;
        if (numFields) {
          PetscCall(PetscSectionGetFieldDof(cSec, p, f, &conDof));
          PetscCall(PetscSectionGetFieldOffset(cSec, p, f, &conOff));
        } else {
          PetscCall(PetscSectionGetDof(cSec, p, &conDof));
          PetscCall(PetscSectionGetOffset(cSec, p, &conOff));
        }
        if (!conDof) continue;
        perm = (perms && perms[i]) ? perms[i][o] : NULL;
        flip = (flips && flips[i]) ? flips[i][o] : NULL;
        PetscCall(PetscSectionGetDof(aSec, p, &aDof));
        PetscCall(PetscSectionGetOffset(aSec, p, &aOff));
        nWork = childOffsets[i + 1] - childOffsets[i];
        for (k = 0; k < aDof; k++) {
          PetscInt a = anchors[aOff + k];
          PetscInt aSecDof, aSecOff;

          if (numFields) {
            PetscCall(PetscSectionGetFieldDof(section, a, f, &aSecDof));
            PetscCall(PetscSectionGetFieldOffset(section, a, f, &aSecOff));
          } else {
            PetscCall(PetscSectionGetDof(section, a, &aSecDof));
            PetscCall(PetscSectionGetOffset(section, a, &aSecOff));
          }
          if (!aSecDof) continue;

          for (j = 0; j < closureSizeP; j++) {
            PetscInt q  = closureP[2 * j];
            PetscInt oq = closureP[2 * j + 1];

            if (q == a) {
              PetscInt           r, s, nWorkP;
              const PetscInt    *permP;
              const PetscScalar *flipP;

              permP  = (perms && perms[j]) ? perms[j][oq] : NULL;
              flipP  = (flips && flips[j]) ? flips[j][oq] : NULL;
              nWorkP = parentOffsets[j + 1] - parentOffsets[j];
              /* get a copy of the child-to-anchor portion of the matrix, and transpose so that rows correspond to the
               * child and columns correspond to the anchor: BUT the maxrix returned by MatDenseGetArrayRead() is
               * column-major, so transpose-transpose = do nothing */
              for (r = 0; r < nWork; r++) {
                for (s = 0; s < nWorkP; s++) scwork[r * nWorkP + s] = X[fSize * (r + childOffsets[i]) + (s + parentOffsets[j])];
              }
              for (r = 0; r < nWork; r++) workIndRow[perm ? perm[r] : r] = conOff + r;
              for (s = 0; s < nWorkP; s++) workIndCol[permP ? permP[s] : s] = aSecOff + s;
              if (flip) {
                for (r = 0; r < nWork; r++) {
                  for (s = 0; s < nWorkP; s++) scwork[r * nWorkP + s] *= flip[r];
                }
              }
              if (flipP) {
                for (r = 0; r < nWork; r++) {
                  for (s = 0; s < nWorkP; s++) scwork[r * nWorkP + s] *= flipP[s];
                }
              }
              PetscCall(MatSetValues(cMat, nWork, workIndRow, nWorkP, workIndCol, scwork, INSERT_VALUES));
              break;
            }
          }
        }
      }
      PetscCall(MatDenseRestoreArrayRead(Xmat, &X));
      PetscCall(PetscFree2(childOffsets, parentOffsets));
      PetscCall(DMPlexRestoreTransitiveClosure(dm, c, PETSC_TRUE, &closureSize, &closure));
      PetscCall(DMPlexRestoreTransitiveClosure(dm, parent, PETSC_TRUE, &closureSizeP, &closureP));
    }
    PetscCall(MatDestroy(&Amat));
    PetscCall(MatDestroy(&Bmat));
    PetscCall(MatDestroy(&Xmat));
    PetscCall(PetscFree(scwork));
    PetscCall(PetscFree7(sizes, weights, pointsRef, pointsReal, work, workIndRow, workIndCol));
    if (id == PETSCFV_CLASSID) PetscCall(PetscSpaceDestroy(&bspace));
  }
  PetscCall(MatAssemblyBegin(cMat, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(cMat, MAT_FINAL_ASSEMBLY));
  PetscCall(PetscFree6(v0, v0parent, vtmp, J, Jparent, invJparent));
  PetscCall(ISRestoreIndices(aIS, &anchors));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexReferenceTreeGetChildrenMatrices(DM refTree, PetscScalar ****childrenMats, PetscInt ***childrenN)
{
  Mat                 refCmat;
  PetscDS             ds;
  PetscInt            numFields, maxFields, f, pRefStart, pRefEnd, p, *rows, *cols, maxDof, maxAnDof, **refPointFieldN;
  PetscScalar      ***refPointFieldMats;
  PetscSection        refConSec, refAnSec, refSection;
  IS                  refAnIS;
  const PetscInt     *refAnchors;
  const PetscInt    **perms;
  const PetscScalar **flips;

  PetscFunctionBegin;
  PetscCall(DMGetDS(refTree, &ds));
  PetscCall(PetscDSGetNumFields(ds, &numFields));
  maxFields = PetscMax(1, numFields);
  PetscCall(DMGetDefaultConstraints(refTree, &refConSec, &refCmat, NULL));
  PetscCall(DMPlexGetAnchors(refTree, &refAnSec, &refAnIS));
  PetscCall(ISGetIndices(refAnIS, &refAnchors));
  PetscCall(DMGetLocalSection(refTree, &refSection));
  PetscCall(PetscSectionGetChart(refConSec, &pRefStart, &pRefEnd));
  PetscCall(PetscMalloc1(pRefEnd - pRefStart, &refPointFieldMats));
  PetscCall(PetscMalloc1(pRefEnd - pRefStart, &refPointFieldN));
  PetscCall(PetscSectionGetMaxDof(refConSec, &maxDof));
  PetscCall(PetscSectionGetMaxDof(refAnSec, &maxAnDof));
  PetscCall(PetscMalloc1(maxDof, &rows));
  PetscCall(PetscMalloc1(maxDof * maxAnDof, &cols));
  for (p = pRefStart; p < pRefEnd; p++) {
    PetscInt parent, closureSize, *closure = NULL, pDof;

    PetscCall(DMPlexGetTreeParent(refTree, p, &parent, NULL));
    PetscCall(PetscSectionGetDof(refConSec, p, &pDof));
    if (!pDof || parent == p) continue;

    PetscCall(PetscMalloc1(maxFields, &refPointFieldMats[p - pRefStart]));
    PetscCall(PetscCalloc1(maxFields, &refPointFieldN[p - pRefStart]));
    PetscCall(DMPlexGetTransitiveClosure(refTree, parent, PETSC_TRUE, &closureSize, &closure));
    for (f = 0; f < maxFields; f++) {
      PetscInt cDof, cOff, numCols, r, i;

      if (f < numFields) {
        PetscCall(PetscSectionGetFieldDof(refConSec, p, f, &cDof));
        PetscCall(PetscSectionGetFieldOffset(refConSec, p, f, &cOff));
        PetscCall(PetscSectionGetFieldPointSyms(refSection, f, closureSize, closure, &perms, &flips));
      } else {
        PetscCall(PetscSectionGetDof(refConSec, p, &cDof));
        PetscCall(PetscSectionGetOffset(refConSec, p, &cOff));
        PetscCall(PetscSectionGetPointSyms(refSection, closureSize, closure, &perms, &flips));
      }

      for (r = 0; r < cDof; r++) rows[r] = cOff + r;
      numCols = 0;
      for (i = 0; i < closureSize; i++) {
        PetscInt        q = closure[2 * i];
        PetscInt        aDof, aOff, j;
        const PetscInt *perm = perms ? perms[i] : NULL;

        if (numFields) {
          PetscCall(PetscSectionGetFieldDof(refSection, q, f, &aDof));
          PetscCall(PetscSectionGetFieldOffset(refSection, q, f, &aOff));
        } else {
          PetscCall(PetscSectionGetDof(refSection, q, &aDof));
          PetscCall(PetscSectionGetOffset(refSection, q, &aOff));
        }

        for (j = 0; j < aDof; j++) cols[numCols++] = aOff + (perm ? perm[j] : j);
      }
      refPointFieldN[p - pRefStart][f] = numCols;
      PetscCall(PetscMalloc1(cDof * numCols, &refPointFieldMats[p - pRefStart][f]));
      PetscCall(MatGetValues(refCmat, cDof, rows, numCols, cols, refPointFieldMats[p - pRefStart][f]));
      if (flips) {
        PetscInt colOff = 0;

        for (i = 0; i < closureSize; i++) {
          PetscInt           q = closure[2 * i];
          PetscInt           aDof, aOff, j;
          const PetscScalar *flip = flips ? flips[i] : NULL;

          if (numFields) {
            PetscCall(PetscSectionGetFieldDof(refSection, q, f, &aDof));
            PetscCall(PetscSectionGetFieldOffset(refSection, q, f, &aOff));
          } else {
            PetscCall(PetscSectionGetDof(refSection, q, &aDof));
            PetscCall(PetscSectionGetOffset(refSection, q, &aOff));
          }
          if (flip) {
            PetscInt k;
            for (k = 0; k < cDof; k++) {
              for (j = 0; j < aDof; j++) refPointFieldMats[p - pRefStart][f][k * numCols + colOff + j] *= flip[j];
            }
          }
          colOff += aDof;
        }
      }
      if (numFields) {
        PetscCall(PetscSectionRestoreFieldPointSyms(refSection, f, closureSize, closure, &perms, &flips));
      } else {
        PetscCall(PetscSectionRestorePointSyms(refSection, closureSize, closure, &perms, &flips));
      }
    }
    PetscCall(DMPlexRestoreTransitiveClosure(refTree, parent, PETSC_TRUE, &closureSize, &closure));
  }
  *childrenMats = refPointFieldMats;
  *childrenN    = refPointFieldN;
  PetscCall(ISRestoreIndices(refAnIS, &refAnchors));
  PetscCall(PetscFree(rows));
  PetscCall(PetscFree(cols));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexReferenceTreeRestoreChildrenMatrices(DM refTree, PetscScalar ****childrenMats, PetscInt ***childrenN)
{
  PetscDS        ds;
  PetscInt     **refPointFieldN;
  PetscScalar ***refPointFieldMats;
  PetscInt       numFields, maxFields, pRefStart, pRefEnd, p, f;
  PetscSection   refConSec;

  PetscFunctionBegin;
  refPointFieldN    = *childrenN;
  *childrenN        = NULL;
  refPointFieldMats = *childrenMats;
  *childrenMats     = NULL;
  PetscCall(DMGetDS(refTree, &ds));
  PetscCall(PetscDSGetNumFields(ds, &numFields));
  maxFields = PetscMax(1, numFields);
  PetscCall(DMGetDefaultConstraints(refTree, &refConSec, NULL, NULL));
  PetscCall(PetscSectionGetChart(refConSec, &pRefStart, &pRefEnd));
  for (p = pRefStart; p < pRefEnd; p++) {
    PetscInt parent, pDof;

    PetscCall(DMPlexGetTreeParent(refTree, p, &parent, NULL));
    PetscCall(PetscSectionGetDof(refConSec, p, &pDof));
    if (!pDof || parent == p) continue;

    for (f = 0; f < maxFields; f++) {
      PetscInt cDof;

      if (numFields) {
        PetscCall(PetscSectionGetFieldDof(refConSec, p, f, &cDof));
      } else {
        PetscCall(PetscSectionGetDof(refConSec, p, &cDof));
      }

      PetscCall(PetscFree(refPointFieldMats[p - pRefStart][f]));
    }
    PetscCall(PetscFree(refPointFieldMats[p - pRefStart]));
    PetscCall(PetscFree(refPointFieldN[p - pRefStart]));
  }
  PetscCall(PetscFree(refPointFieldMats));
  PetscCall(PetscFree(refPointFieldN));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexComputeAnchorMatrix_Tree_FromReference(DM dm, PetscSection section, PetscSection conSec, Mat cMat)
{
  DM              refTree;
  PetscDS         ds;
  Mat             refCmat;
  PetscInt        numFields, maxFields, f, pRefStart, pRefEnd, p, maxDof, maxAnDof, *perm, *iperm, pStart, pEnd, conStart, conEnd, **refPointFieldN;
  PetscScalar  ***refPointFieldMats, *pointWork;
  PetscSection    refConSec, refAnSec, anSec;
  IS              refAnIS, anIS;
  const PetscInt *anchors;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(dm, DM_CLASSID, 1);
  PetscCall(DMGetDS(dm, &ds));
  PetscCall(PetscDSGetNumFields(ds, &numFields));
  maxFields = PetscMax(1, numFields);
  PetscCall(DMPlexGetReferenceTree(dm, &refTree));
  PetscCall(DMCopyDisc(dm, refTree));
  PetscCall(DMSetLocalSection(refTree, NULL));
  PetscCall(DMSetDefaultConstraints(refTree, NULL, NULL, NULL));
  PetscCall(DMGetDefaultConstraints(refTree, &refConSec, &refCmat, NULL));
  PetscCall(DMPlexGetAnchors(refTree, &refAnSec, &refAnIS));
  PetscCall(DMPlexGetAnchors(dm, &anSec, &anIS));
  PetscCall(ISGetIndices(anIS, &anchors));
  PetscCall(PetscSectionGetChart(refConSec, &pRefStart, &pRefEnd));
  PetscCall(PetscSectionGetChart(conSec, &conStart, &conEnd));
  PetscCall(PetscSectionGetMaxDof(refConSec, &maxDof));
  PetscCall(PetscSectionGetMaxDof(refAnSec, &maxAnDof));
  PetscCall(PetscMalloc1(maxDof * maxDof * maxAnDof, &pointWork));

  /* step 1: get submats for every constrained point in the reference tree */
  PetscCall(DMPlexReferenceTreeGetChildrenMatrices(refTree, &refPointFieldMats, &refPointFieldN));

  /* step 2: compute the preorder */
  PetscCall(DMPlexGetChart(dm, &pStart, &pEnd));
  PetscCall(PetscMalloc2(pEnd - pStart, &perm, pEnd - pStart, &iperm));
  for (p = pStart; p < pEnd; p++) {
    perm[p - pStart]  = p;
    iperm[p - pStart] = p - pStart;
  }
  for (p = 0; p < pEnd - pStart;) {
    PetscInt point = perm[p];
    PetscInt parent;

    PetscCall(DMPlexGetTreeParent(dm, point, &parent, NULL));
    if (parent == point) {
      p++;
    } else {
      PetscInt size, closureSize, *closure = NULL, i;

      PetscCall(DMPlexGetTransitiveClosure(dm, parent, PETSC_TRUE, &closureSize, &closure));
      for (i = 0; i < closureSize; i++) {
        PetscInt q = closure[2 * i];
        if (iperm[q - pStart] > iperm[point - pStart]) {
          /* swap */
          perm[p]                 = q;
          perm[iperm[q - pStart]] = point;
          iperm[point - pStart]   = iperm[q - pStart];
          iperm[q - pStart]       = p;
          break;
        }
      }
      size = closureSize;
      PetscCall(DMPlexRestoreTransitiveClosure(dm, parent, PETSC_TRUE, &closureSize, &closure));
      if (i == size) p++;
    }
  }

  /* step 3: fill the constraint matrix */
  /* we are going to use a preorder progressive fill strategy.  Mat doesn't
   * allow progressive fill without assembly, so we are going to set up the
   * values outside of the Mat first.
   */
  {
    PetscInt        nRows, row, nnz;
    PetscBool       done;
    PetscInt        secStart, secEnd;
    const PetscInt *ia, *ja;
    PetscScalar    *vals;

    PetscCall(PetscSectionGetChart(section, &secStart, &secEnd));
    PetscCall(MatGetRowIJ(cMat, 0, PETSC_FALSE, PETSC_FALSE, &nRows, &ia, &ja, &done));
    PetscCheck(done, PetscObjectComm((PetscObject)cMat), PETSC_ERR_PLIB, "Could not get RowIJ of constraint matrix");
    nnz = ia[nRows];
    /* malloc and then zero rows right before we fill them: this way valgrind
     * can tell if we are doing progressive fill in the wrong order */
    PetscCall(PetscMalloc1(nnz, &vals));
    for (p = 0; p < pEnd - pStart; p++) {
      PetscInt parent, childid, closureSize, *closure = NULL;
      PetscInt point = perm[p], pointDof;

      PetscCall(DMPlexGetTreeParent(dm, point, &parent, &childid));
      if ((point < conStart) || (point >= conEnd) || (parent == point)) continue;
      PetscCall(PetscSectionGetDof(conSec, point, &pointDof));
      if (!pointDof) continue;
      PetscCall(DMPlexGetTransitiveClosure(dm, parent, PETSC_TRUE, &closureSize, &closure));
      for (f = 0; f < maxFields; f++) {
        PetscInt            cDof, cOff, numCols, numFillCols, i, r, matOffset, offset;
        PetscScalar        *pointMat;
        const PetscInt    **perms;
        const PetscScalar **flips;

        if (numFields) {
          PetscCall(PetscSectionGetFieldDof(conSec, point, f, &cDof));
          PetscCall(PetscSectionGetFieldOffset(conSec, point, f, &cOff));
        } else {
          PetscCall(PetscSectionGetDof(conSec, point, &cDof));
          PetscCall(PetscSectionGetOffset(conSec, point, &cOff));
        }
        if (!cDof) continue;
        if (numFields) PetscCall(PetscSectionGetFieldPointSyms(section, f, closureSize, closure, &perms, &flips));
        else PetscCall(PetscSectionGetPointSyms(section, closureSize, closure, &perms, &flips));

        /* make sure that every row for this point is the same size */
        if (PetscDefined(USE_DEBUG)) {
          for (r = 0; r < cDof; r++) {
            if (cDof > 1 && r) {
              PetscCheck((ia[cOff + r + 1] - ia[cOff + r]) == (ia[cOff + r] - ia[cOff + r - 1]), PETSC_COMM_SELF, PETSC_ERR_PLIB, "Two point rows have different nnz: %" PetscInt_FMT " vs. %" PetscInt_FMT, ia[cOff + r + 1] - ia[cOff + r], ia[cOff + r] - ia[cOff + r - 1]);
            }
          }
        }
        /* zero rows */
        for (i = ia[cOff]; i < ia[cOff + cDof]; i++) vals[i] = 0.;
        matOffset   = ia[cOff];
        numFillCols = ia[cOff + 1] - matOffset;
        pointMat    = refPointFieldMats[childid - pRefStart][f];
        numCols     = refPointFieldN[childid - pRefStart][f];
        offset      = 0;
        for (i = 0; i < closureSize; i++) {
          PetscInt           q = closure[2 * i];
          PetscInt           aDof, aOff, j, k, qConDof, qConOff;
          const PetscInt    *perm = perms ? perms[i] : NULL;
          const PetscScalar *flip = flips ? flips[i] : NULL;

          qConDof = qConOff = 0;
          if (q < secStart || q >= secEnd) continue;
          if (numFields) {
            PetscCall(PetscSectionGetFieldDof(section, q, f, &aDof));
            PetscCall(PetscSectionGetFieldOffset(section, q, f, &aOff));
            if (q >= conStart && q < conEnd) {
              PetscCall(PetscSectionGetFieldDof(conSec, q, f, &qConDof));
              PetscCall(PetscSectionGetFieldOffset(conSec, q, f, &qConOff));
            }
          } else {
            PetscCall(PetscSectionGetDof(section, q, &aDof));
            PetscCall(PetscSectionGetOffset(section, q, &aOff));
            if (q >= conStart && q < conEnd) {
              PetscCall(PetscSectionGetDof(conSec, q, &qConDof));
              PetscCall(PetscSectionGetOffset(conSec, q, &qConOff));
            }
          }
          if (!aDof) continue;
          if (qConDof) {
            /* this point has anchors: its rows of the matrix should already
             * be filled, thanks to preordering */
            /* first multiply into pointWork, then set in matrix */
            PetscInt aMatOffset   = ia[qConOff];
            PetscInt aNumFillCols = ia[qConOff + 1] - aMatOffset;
            for (r = 0; r < cDof; r++) {
              for (j = 0; j < aNumFillCols; j++) {
                PetscScalar inVal = 0;
                for (k = 0; k < aDof; k++) {
                  PetscInt col = perm ? perm[k] : k;

                  inVal += pointMat[r * numCols + offset + col] * vals[aMatOffset + aNumFillCols * k + j] * (flip ? flip[col] : 1.);
                }
                pointWork[r * aNumFillCols + j] = inVal;
              }
            }
            /* assume that the columns are sorted, spend less time searching */
            for (j = 0, k = 0; j < aNumFillCols; j++) {
              PetscInt col = ja[aMatOffset + j];
              for (; k < numFillCols; k++) {
                if (ja[matOffset + k] == col) break;
              }
              PetscCheck(k != numFillCols, PETSC_COMM_SELF, PETSC_ERR_PLIB, "No nonzero space for (%" PetscInt_FMT ", %" PetscInt_FMT ")", cOff, col);
              for (r = 0; r < cDof; r++) vals[matOffset + numFillCols * r + k] = pointWork[r * aNumFillCols + j];
            }
          } else {
            /* find where to put this portion of pointMat into the matrix */
            for (k = 0; k < numFillCols; k++) {
              if (ja[matOffset + k] == aOff) break;
            }
            PetscCheck(k != numFillCols, PETSC_COMM_SELF, PETSC_ERR_PLIB, "No nonzero space for (%" PetscInt_FMT ", %" PetscInt_FMT ")", cOff, aOff);
            for (r = 0; r < cDof; r++) {
              for (j = 0; j < aDof; j++) {
                PetscInt col = perm ? perm[j] : j;

                vals[matOffset + numFillCols * r + k + col] += pointMat[r * numCols + offset + j] * (flip ? flip[col] : 1.);
              }
            }
          }
          offset += aDof;
        }
        if (numFields) {
          PetscCall(PetscSectionRestoreFieldPointSyms(section, f, closureSize, closure, &perms, &flips));
        } else {
          PetscCall(PetscSectionRestorePointSyms(section, closureSize, closure, &perms, &flips));
        }
      }
      PetscCall(DMPlexRestoreTransitiveClosure(dm, parent, PETSC_TRUE, &closureSize, &closure));
    }
    for (row = 0; row < nRows; row++) PetscCall(MatSetValues(cMat, 1, &row, ia[row + 1] - ia[row], &ja[ia[row]], &vals[ia[row]], INSERT_VALUES));
    PetscCall(MatRestoreRowIJ(cMat, 0, PETSC_FALSE, PETSC_FALSE, &nRows, &ia, &ja, &done));
    PetscCheck(done, PetscObjectComm((PetscObject)cMat), PETSC_ERR_PLIB, "Could not restore RowIJ of constraint matrix");
    PetscCall(MatAssemblyBegin(cMat, MAT_FINAL_ASSEMBLY));
    PetscCall(MatAssemblyEnd(cMat, MAT_FINAL_ASSEMBLY));
    PetscCall(PetscFree(vals));
  }

  /* clean up */
  PetscCall(ISRestoreIndices(anIS, &anchors));
  PetscCall(PetscFree2(perm, iperm));
  PetscCall(PetscFree(pointWork));
  PetscCall(DMPlexReferenceTreeRestoreChildrenMatrices(refTree, &refPointFieldMats, &refPointFieldN));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* refine a single cell on rank 0: this is not intended to provide good local refinement, only to create an example of
 * a non-conforming mesh.  Local refinement comes later */
PetscErrorCode DMPlexTreeRefineCell(DM dm, PetscInt cell, DM *ncdm)
{
  DM           K;
  PetscMPIInt  rank;
  PetscInt     dim, *pNewStart, *pNewEnd, *pNewCount, *pOldStart, *pOldEnd, offset, d, pStart, pEnd;
  PetscInt     numNewCones, *newConeSizes, *newCones, *newOrientations;
  PetscInt    *Kembedding;
  PetscInt    *cellClosure = NULL, nc;
  PetscScalar *newVertexCoords;
  PetscInt     numPointsWithParents, *parents, *childIDs, *perm, *iperm, *preOrient, pOffset;
  PetscSection parentSection;

  PetscFunctionBegin;
  PetscCallMPI(MPI_Comm_rank(PetscObjectComm((PetscObject)dm), &rank));
  PetscCall(DMGetDimension(dm, &dim));
  PetscCall(DMPlexCreate(PetscObjectComm((PetscObject)dm), ncdm));
  PetscCall(DMSetDimension(*ncdm, dim));

  PetscCall(DMPlexGetChart(dm, &pStart, &pEnd));
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)dm), &parentSection));
  PetscCall(DMPlexGetReferenceTree(dm, &K));
  PetscCall(DMGetCoordinatesLocalSetUp(dm));
  if (rank == 0) {
    /* compute the new charts */
    PetscCall(PetscMalloc5(dim + 1, &pNewCount, dim + 1, &pNewStart, dim + 1, &pNewEnd, dim + 1, &pOldStart, dim + 1, &pOldEnd));
    offset = 0;
    for (d = 0; d <= dim; d++) {
      PetscInt pOldCount, kStart, kEnd, k;

      pNewStart[d] = offset;
      PetscCall(DMPlexGetHeightStratum(dm, d, &pOldStart[d], &pOldEnd[d]));
      PetscCall(DMPlexGetHeightStratum(K, d, &kStart, &kEnd));
      pOldCount = pOldEnd[d] - pOldStart[d];
      /* adding the new points */
      pNewCount[d] = pOldCount + kEnd - kStart;
      if (!d) {
        /* removing the cell */
        pNewCount[d]--;
      }
      for (k = kStart; k < kEnd; k++) {
        PetscInt parent;
        PetscCall(DMPlexGetTreeParent(K, k, &parent, NULL));
        if (parent == k) {
          /* avoid double counting points that won't actually be new */
          pNewCount[d]--;
        }
      }
      pNewEnd[d] = pNewStart[d] + pNewCount[d];
      offset     = pNewEnd[d];
    }
    PetscCheck(cell >= pOldStart[0] && cell < pOldEnd[0], PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "%" PetscInt_FMT " not in cell range [%" PetscInt_FMT ", %" PetscInt_FMT ")", cell, pOldStart[0], pOldEnd[0]);
    /* get the current closure of the cell that we are removing */
    PetscCall(DMPlexGetTransitiveClosure(dm, cell, PETSC_TRUE, &nc, &cellClosure));

    PetscCall(PetscMalloc1(pNewEnd[dim], &newConeSizes));
    {
      DMPolytopeType pct, qct;
      PetscInt       kStart, kEnd, k, closureSizeK, *closureK = NULL, j;

      PetscCall(DMPlexGetChart(K, &kStart, &kEnd));
      PetscCall(PetscMalloc4(kEnd - kStart, &Kembedding, kEnd - kStart, &perm, kEnd - kStart, &iperm, kEnd - kStart, &preOrient));

      for (k = kStart; k < kEnd; k++) {
        perm[k - kStart]      = k;
        iperm[k - kStart]     = k - kStart;
        preOrient[k - kStart] = 0;
      }

      PetscCall(DMPlexGetTransitiveClosure(K, 0, PETSC_TRUE, &closureSizeK, &closureK));
      for (j = 1; j < closureSizeK; j++) {
        PetscInt parentOrientA = closureK[2 * j + 1];
        PetscInt parentOrientB = cellClosure[2 * j + 1];
        PetscInt p, q;

        p = closureK[2 * j];
        q = cellClosure[2 * j];
        PetscCall(DMPlexGetCellType(K, p, &pct));
        PetscCall(DMPlexGetCellType(dm, q, &qct));
        for (d = 0; d <= dim; d++) {
          if (q >= pOldStart[d] && q < pOldEnd[d]) Kembedding[p] = (q - pOldStart[d]) + pNewStart[d];
        }
        parentOrientA = DMPolytopeConvertNewOrientation_Internal(pct, parentOrientA);
        parentOrientB = DMPolytopeConvertNewOrientation_Internal(qct, parentOrientB);
        if (parentOrientA != parentOrientB) {
          PetscInt        numChildren, i;
          const PetscInt *children;

          PetscCall(DMPlexGetTreeChildren(K, p, &numChildren, &children));
          for (i = 0; i < numChildren; i++) {
            PetscInt kPerm, oPerm;

            k = children[i];
            PetscCall(DMPlexReferenceTreeGetChildSymmetry(K, p, parentOrientA, 0, k, parentOrientB, &oPerm, &kPerm));
            /* perm = what refTree position I'm in */
            perm[kPerm - kStart] = k;
            /* iperm = who is at this position */
            iperm[k - kStart]         = kPerm - kStart;
            preOrient[kPerm - kStart] = oPerm;
          }
        }
      }
      PetscCall(DMPlexRestoreTransitiveClosure(K, 0, PETSC_TRUE, &closureSizeK, &closureK));
    }
    PetscCall(PetscSectionSetChart(parentSection, 0, pNewEnd[dim]));
    offset      = 0;
    numNewCones = 0;
    for (d = 0; d <= dim; d++) {
      PetscInt kStart, kEnd, k;
      PetscInt p;
      PetscInt size;

      for (p = pOldStart[d]; p < pOldEnd[d]; p++) {
        /* skip cell 0 */
        if (p == cell) continue;
        /* old cones to new cones */
        PetscCall(DMPlexGetConeSize(dm, p, &size));
        newConeSizes[offset++] = size;
        numNewCones += size;
      }

      PetscCall(DMPlexGetHeightStratum(K, d, &kStart, &kEnd));
      for (k = kStart; k < kEnd; k++) {
        PetscInt kParent;

        PetscCall(DMPlexGetTreeParent(K, k, &kParent, NULL));
        if (kParent != k) {
          Kembedding[k] = offset;
          PetscCall(DMPlexGetConeSize(K, k, &size));
          newConeSizes[offset++] = size;
          numNewCones += size;
          if (kParent != 0) PetscCall(PetscSectionSetDof(parentSection, Kembedding[k], 1));
        }
      }
    }

    PetscCall(PetscSectionSetUp(parentSection));
    PetscCall(PetscSectionGetStorageSize(parentSection, &numPointsWithParents));
    PetscCall(PetscMalloc2(numNewCones, &newCones, numNewCones, &newOrientations));
    PetscCall(PetscMalloc2(numPointsWithParents, &parents, numPointsWithParents, &childIDs));

    /* fill new cones */
    offset = 0;
    for (d = 0; d <= dim; d++) {
      PetscInt        kStart, kEnd, k, l;
      PetscInt        p;
      PetscInt        size;
      const PetscInt *cone, *orientation;

      for (p = pOldStart[d]; p < pOldEnd[d]; p++) {
        /* skip cell 0 */
        if (p == cell) continue;
        /* old cones to new cones */
        PetscCall(DMPlexGetConeSize(dm, p, &size));
        PetscCall(DMPlexGetCone(dm, p, &cone));
        PetscCall(DMPlexGetConeOrientation(dm, p, &orientation));
        for (l = 0; l < size; l++) {
          newCones[offset]          = (cone[l] - pOldStart[d + 1]) + pNewStart[d + 1];
          newOrientations[offset++] = orientation[l];
        }
      }

      PetscCall(DMPlexGetHeightStratum(K, d, &kStart, &kEnd));
      for (k = kStart; k < kEnd; k++) {
        PetscInt kPerm = perm[k], kParent;
        PetscInt preO  = preOrient[k];

        PetscCall(DMPlexGetTreeParent(K, k, &kParent, NULL));
        if (kParent != k) {
          /* embed new cones */
          PetscCall(DMPlexGetConeSize(K, k, &size));
          PetscCall(DMPlexGetCone(K, kPerm, &cone));
          PetscCall(DMPlexGetConeOrientation(K, kPerm, &orientation));
          for (l = 0; l < size; l++) {
            PetscInt       q, m = (preO >= 0) ? ((preO + l) % size) : ((size - (preO + 1) - l) % size);
            PetscInt       newO, lSize, oTrue;
            DMPolytopeType ct = DM_NUM_POLYTOPES;

            q                = iperm[cone[m]];
            newCones[offset] = Kembedding[q];
            PetscCall(DMPlexGetConeSize(K, q, &lSize));
            if (lSize == 2) ct = DM_POLYTOPE_SEGMENT;
            else if (lSize == 4) ct = DM_POLYTOPE_QUADRILATERAL;
            oTrue                     = DMPolytopeConvertNewOrientation_Internal(ct, orientation[m]);
            oTrue                     = ((!lSize) || (preOrient[k] >= 0)) ? oTrue : -(oTrue + 2);
            newO                      = DihedralCompose(lSize, oTrue, preOrient[q]);
            newOrientations[offset++] = DMPolytopeConvertOldOrientation_Internal(ct, newO);
          }
          if (kParent != 0) {
            PetscInt newPoint = Kembedding[kParent];
            PetscCall(PetscSectionGetOffset(parentSection, Kembedding[k], &pOffset));
            parents[pOffset]  = newPoint;
            childIDs[pOffset] = k;
          }
        }
      }
    }

    PetscCall(PetscMalloc1(dim * (pNewEnd[dim] - pNewStart[dim]), &newVertexCoords));

    /* fill coordinates */
    offset = 0;
    {
      PetscInt     kStart, kEnd, l;
      PetscSection vSection;
      PetscInt     v;
      Vec          coords;
      PetscScalar *coordvals;
      PetscInt     dof, off;
      PetscReal    v0[3], J[9], detJ;

      if (PetscDefined(USE_DEBUG)) {
        PetscInt k;
        PetscCall(DMPlexGetHeightStratum(K, 0, &kStart, &kEnd));
        for (k = kStart; k < kEnd; k++) {
          PetscCall(DMPlexComputeCellGeometryFEM(K, k, NULL, v0, J, NULL, &detJ));
          PetscCheck(detJ > 0., PETSC_COMM_SELF, PETSC_ERR_PLIB, "reference tree cell %" PetscInt_FMT " has bad determinant", k);
        }
      }
      PetscCall(DMPlexComputeCellGeometryFEM(dm, cell, NULL, v0, J, NULL, &detJ));
      PetscCall(DMGetCoordinateSection(dm, &vSection));
      PetscCall(DMGetCoordinatesLocal(dm, &coords));
      PetscCall(VecGetArray(coords, &coordvals));
      for (v = pOldStart[dim]; v < pOldEnd[dim]; v++) {
        PetscCall(PetscSectionGetDof(vSection, v, &dof));
        PetscCall(PetscSectionGetOffset(vSection, v, &off));
        for (l = 0; l < dof; l++) newVertexCoords[offset++] = coordvals[off + l];
      }
      PetscCall(VecRestoreArray(coords, &coordvals));

      PetscCall(DMGetCoordinateSection(K, &vSection));
      PetscCall(DMGetCoordinatesLocal(K, &coords));
      PetscCall(VecGetArray(coords, &coordvals));
      PetscCall(DMPlexGetDepthStratum(K, 0, &kStart, &kEnd));
      for (v = kStart; v < kEnd; v++) {
        PetscReal       coord[3], newCoord[3];
        PetscInt        vPerm = perm[v];
        PetscInt        kParent;
        const PetscReal xi0[3] = {-1., -1., -1.};

        PetscCall(DMPlexGetTreeParent(K, v, &kParent, NULL));
        if (kParent != v) {
          /* this is a new vertex */
          PetscCall(PetscSectionGetOffset(vSection, vPerm, &off));
          for (l = 0; l < dim; ++l) coord[l] = PetscRealPart(coordvals[off + l]);
          CoordinatesRefToReal(dim, dim, xi0, v0, J, coord, newCoord);
          for (l = 0; l < dim; ++l) newVertexCoords[offset + l] = newCoord[l];
          offset += dim;
        }
      }
      PetscCall(VecRestoreArray(coords, &coordvals));
    }

    /* need to reverse the order of pNewCount: vertices first, cells last */
    for (d = 0; d < (dim + 1) / 2; d++) {
      PetscInt tmp;

      tmp                = pNewCount[d];
      pNewCount[d]       = pNewCount[dim - d];
      pNewCount[dim - d] = tmp;
    }

    PetscCall(DMPlexCreateFromDAG(*ncdm, dim, pNewCount, newConeSizes, newCones, newOrientations, newVertexCoords));
    PetscCall(DMPlexSetReferenceTree(*ncdm, K));
    PetscCall(DMPlexSetTree(*ncdm, parentSection, parents, childIDs));

    /* clean up */
    PetscCall(DMPlexRestoreTransitiveClosure(dm, cell, PETSC_TRUE, &nc, &cellClosure));
    PetscCall(PetscFree5(pNewCount, pNewStart, pNewEnd, pOldStart, pOldEnd));
    PetscCall(PetscFree(newConeSizes));
    PetscCall(PetscFree2(newCones, newOrientations));
    PetscCall(PetscFree(newVertexCoords));
    PetscCall(PetscFree2(parents, childIDs));
    PetscCall(PetscFree4(Kembedding, perm, iperm, preOrient));
  } else {
    PetscInt     p, counts[4];
    PetscInt    *coneSizes, *cones, *orientations;
    Vec          coordVec;
    PetscScalar *coords;

    for (d = 0; d <= dim; d++) {
      PetscInt dStart, dEnd;

      PetscCall(DMPlexGetDepthStratum(dm, d, &dStart, &dEnd));
      counts[d] = dEnd - dStart;
    }
    PetscCall(PetscMalloc1(pEnd - pStart, &coneSizes));
    for (p = pStart; p < pEnd; p++) PetscCall(DMPlexGetConeSize(dm, p, &coneSizes[p - pStart]));
    PetscCall(DMPlexGetCones(dm, &cones));
    PetscCall(DMPlexGetConeOrientations(dm, &orientations));
    PetscCall(DMGetCoordinatesLocal(dm, &coordVec));
    PetscCall(VecGetArray(coordVec, &coords));

    PetscCall(PetscSectionSetChart(parentSection, pStart, pEnd));
    PetscCall(PetscSectionSetUp(parentSection));
    PetscCall(DMPlexCreateFromDAG(*ncdm, dim, counts, coneSizes, cones, orientations, NULL));
    PetscCall(DMPlexSetReferenceTree(*ncdm, K));
    PetscCall(DMPlexSetTree(*ncdm, parentSection, NULL, NULL));
    PetscCall(VecRestoreArray(coordVec, &coords));
  }
  PetscCall(PetscSectionDestroy(&parentSection));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode DMPlexComputeInterpolatorTree(DM coarse, DM fine, PetscSF coarseToFine, PetscInt *childIds, Mat mat)
{
  PetscSF              coarseToFineEmbedded;
  PetscSection         globalCoarse, globalFine;
  PetscSection         localCoarse, localFine;
  PetscSection         aSec, cSec;
  PetscSection         rootIndicesSec, rootMatricesSec;
  PetscSection         leafIndicesSec, leafMatricesSec;
  PetscInt            *rootIndices, *leafIndices;
  PetscScalar         *rootMatrices, *leafMatrices;
  IS                   aIS;
  const PetscInt      *anchors;
  Mat                  cMat;
  PetscInt             numFields, maxFields;
  PetscInt             pStartC, pEndC, pStartF, pEndF, p;
  PetscInt             aStart, aEnd, cStart, cEnd;
  PetscInt            *maxChildIds;
  PetscInt            *offsets, *newOffsets, *offsetsCopy, *newOffsetsCopy, *rowOffsets, *numD, *numO;
  const PetscInt    ***perms;
  const PetscScalar ***flips;

  PetscFunctionBegin;
  PetscCall(DMPlexGetChart(coarse, &pStartC, &pEndC));
  PetscCall(DMPlexGetChart(fine, &pStartF, &pEndF));
  PetscCall(DMGetGlobalSection(fine, &globalFine));
  { /* winnow fine points that don't have global dofs out of the sf */
    PetscInt        dof, cdof, numPointsWithDofs, offset, *pointsWithDofs, nleaves, l;
    const PetscInt *leaves;

    PetscCall(PetscSFGetGraph(coarseToFine, NULL, &nleaves, &leaves, NULL));
    for (l = 0, numPointsWithDofs = 0; l < nleaves; l++) {
      p = leaves ? leaves[l] : l;
      PetscCall(PetscSectionGetDof(globalFine, p, &dof));
      PetscCall(PetscSectionGetConstraintDof(globalFine, p, &cdof));
      if ((dof - cdof) > 0) numPointsWithDofs++;
    }
    PetscCall(PetscMalloc1(numPointsWithDofs, &pointsWithDofs));
    for (l = 0, offset = 0; l < nleaves; l++) {
      p = leaves ? leaves[l] : l;
      PetscCall(PetscSectionGetDof(globalFine, p, &dof));
      PetscCall(PetscSectionGetConstraintDof(globalFine, p, &cdof));
      if ((dof - cdof) > 0) pointsWithDofs[offset++] = l;
    }
    PetscCall(PetscSFCreateEmbeddedLeafSF(coarseToFine, numPointsWithDofs, pointsWithDofs, &coarseToFineEmbedded));
    PetscCall(PetscFree(pointsWithDofs));
  }
  /* communicate back to the coarse mesh which coarse points have children (that may require interpolation) */
  PetscCall(PetscMalloc1(pEndC - pStartC, &maxChildIds));
  for (p = pStartC; p < pEndC; p++) maxChildIds[p - pStartC] = -2;
  PetscCall(PetscSFReduceBegin(coarseToFineEmbedded, MPIU_INT, childIds, maxChildIds, MPI_MAX));
  PetscCall(PetscSFReduceEnd(coarseToFineEmbedded, MPIU_INT, childIds, maxChildIds, MPI_MAX));

  PetscCall(DMGetLocalSection(coarse, &localCoarse));
  PetscCall(DMGetGlobalSection(coarse, &globalCoarse));

  PetscCall(DMPlexGetAnchors(coarse, &aSec, &aIS));
  PetscCall(ISGetIndices(aIS, &anchors));
  PetscCall(PetscSectionGetChart(aSec, &aStart, &aEnd));

  PetscCall(DMGetDefaultConstraints(coarse, &cSec, &cMat, NULL));
  PetscCall(PetscSectionGetChart(cSec, &cStart, &cEnd));

  /* create sections that will send to children the indices and matrices they will need to construct the interpolator */
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)coarse), &rootIndicesSec));
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)coarse), &rootMatricesSec));
  PetscCall(PetscSectionSetChart(rootIndicesSec, pStartC, pEndC));
  PetscCall(PetscSectionSetChart(rootMatricesSec, pStartC, pEndC));
  PetscCall(PetscSectionGetNumFields(localCoarse, &numFields));
  maxFields = PetscMax(1, numFields);
  PetscCall(PetscMalloc7(maxFields + 1, &offsets, maxFields + 1, &offsetsCopy, maxFields + 1, &newOffsets, maxFields + 1, &newOffsetsCopy, maxFields + 1, &rowOffsets, maxFields + 1, &numD, maxFields + 1, &numO));
  PetscCall(PetscMalloc2(maxFields + 1, (PetscInt ****)&perms, maxFields + 1, (PetscScalar ****)&flips));
  PetscCall(PetscMemzero((void *)perms, (maxFields + 1) * sizeof(const PetscInt **)));
  PetscCall(PetscMemzero((void *)flips, (maxFields + 1) * sizeof(const PetscScalar **)));

  for (p = pStartC; p < pEndC; p++) { /* count the sizes of the indices and matrices */
    PetscInt dof, matSize = 0;
    PetscInt aDof          = 0;
    PetscInt cDof          = 0;
    PetscInt maxChildId    = maxChildIds[p - pStartC];
    PetscInt numRowIndices = 0;
    PetscInt numColIndices = 0;
    PetscInt f;

    PetscCall(PetscSectionGetDof(globalCoarse, p, &dof));
    if (dof < 0) dof = -(dof + 1);
    if (p >= aStart && p < aEnd) PetscCall(PetscSectionGetDof(aSec, p, &aDof));
    if (p >= cStart && p < cEnd) PetscCall(PetscSectionGetDof(cSec, p, &cDof));
    for (f = 0; f <= numFields; f++) offsets[f] = 0;
    for (f = 0; f <= numFields; f++) newOffsets[f] = 0;
    if (maxChildId >= 0) { /* this point has children (with dofs) that will need to be interpolated from the closure of p */
      PetscInt *closure = NULL, closureSize, cl;

      PetscCall(DMPlexGetTransitiveClosure(coarse, p, PETSC_TRUE, &closureSize, &closure));
      for (cl = 0; cl < closureSize; cl++) { /* get the closure */
        PetscInt c = closure[2 * cl], clDof;

        PetscCall(PetscSectionGetDof(localCoarse, c, &clDof));
        numRowIndices += clDof;
        for (f = 0; f < numFields; f++) {
          PetscCall(PetscSectionGetFieldDof(localCoarse, c, f, &clDof));
          offsets[f + 1] += clDof;
        }
      }
      for (f = 0; f < numFields; f++) {
        offsets[f + 1] += offsets[f];
        newOffsets[f + 1] = offsets[f + 1];
      }
      /* get the number of indices needed and their field offsets */
      PetscCall(DMPlexAnchorsModifyMat(coarse, localCoarse, closureSize, numRowIndices, closure, NULL, NULL, NULL, &numColIndices, NULL, NULL, newOffsets, PETSC_FALSE));
      PetscCall(DMPlexRestoreTransitiveClosure(coarse, p, PETSC_TRUE, &closureSize, &closure));
      if (!numColIndices) { /* there are no hanging constraint modifications, so the matrix is just the identity: do not send it */
        numColIndices = numRowIndices;
        matSize       = 0;
      } else if (numFields) { /* we send one submat for each field: sum their sizes */
        matSize = 0;
        for (f = 0; f < numFields; f++) {
          PetscInt numRow, numCol;

          numRow = offsets[f + 1] - offsets[f];
          numCol = newOffsets[f + 1] - newOffsets[f];
          matSize += numRow * numCol;
        }
      } else {
        matSize = numRowIndices * numColIndices;
      }
    } else if (maxChildId == -1) {
      if (cDof > 0) { /* this point's dofs are interpolated via cMat: get the submatrix of cMat */
        PetscInt aOff, a;

        PetscCall(PetscSectionGetOffset(aSec, p, &aOff));
        for (f = 0; f < numFields; f++) {
          PetscInt fDof;

          PetscCall(PetscSectionGetFieldDof(localCoarse, p, f, &fDof));
          offsets[f + 1] = fDof;
        }
        for (a = 0; a < aDof; a++) {
          PetscInt anchor = anchors[a + aOff], aLocalDof;

          PetscCall(PetscSectionGetDof(localCoarse, anchor, &aLocalDof));
          numColIndices += aLocalDof;
          for (f = 0; f < numFields; f++) {
            PetscInt fDof;

            PetscCall(PetscSectionGetFieldDof(localCoarse, anchor, f, &fDof));
            newOffsets[f + 1] += fDof;
          }
        }
        if (numFields) {
          matSize = 0;
          for (f = 0; f < numFields; f++) matSize += offsets[f + 1] * newOffsets[f + 1];
        } else {
          matSize = numColIndices * dof;
        }
      } else { /* no children, and no constraints on dofs: just get the global indices */
        numColIndices = dof;
        matSize       = 0;
      }
    }
    /* we will pack the column indices with the field offsets */
    PetscCall(PetscSectionSetDof(rootIndicesSec, p, numColIndices ? numColIndices + 2 * numFields : 0));
    PetscCall(PetscSectionSetDof(rootMatricesSec, p, matSize));
  }
  PetscCall(PetscSectionSetUp(rootIndicesSec));
  PetscCall(PetscSectionSetUp(rootMatricesSec));
  {
    PetscInt numRootIndices, numRootMatrices;

    PetscCall(PetscSectionGetStorageSize(rootIndicesSec, &numRootIndices));
    PetscCall(PetscSectionGetStorageSize(rootMatricesSec, &numRootMatrices));
    PetscCall(PetscMalloc2(numRootIndices, &rootIndices, numRootMatrices, &rootMatrices));
    for (p = pStartC; p < pEndC; p++) {
      PetscInt     numRowIndices = 0, numColIndices, matSize, dof;
      PetscInt     pIndOff, pMatOff, f;
      PetscInt    *pInd;
      PetscInt     maxChildId = maxChildIds[p - pStartC];
      PetscScalar *pMat       = NULL;

      PetscCall(PetscSectionGetDof(rootIndicesSec, p, &numColIndices));
      if (!numColIndices) continue;
      for (f = 0; f <= numFields; f++) {
        offsets[f]        = 0;
        newOffsets[f]     = 0;
        offsetsCopy[f]    = 0;
        newOffsetsCopy[f] = 0;
      }
      numColIndices -= 2 * numFields;
      PetscCall(PetscSectionGetOffset(rootIndicesSec, p, &pIndOff));
      pInd = &rootIndices[pIndOff];
      PetscCall(PetscSectionGetDof(rootMatricesSec, p, &matSize));
      if (matSize) {
        PetscCall(PetscSectionGetOffset(rootMatricesSec, p, &pMatOff));
        pMat = &rootMatrices[pMatOff];
      }
      PetscCall(PetscSectionGetDof(globalCoarse, p, &dof));
      if (dof < 0) dof = -(dof + 1);
      if (maxChildId >= 0) { /* build an identity matrix, apply matrix constraints on the right */
        PetscInt i, j;

        if (matSize == 0) { /* don't need to calculate the mat, just the indices */
          PetscInt numIndices, *indices;
          PetscCall(DMPlexGetClosureIndices(coarse, localCoarse, globalCoarse, p, PETSC_TRUE, &numIndices, &indices, offsets, NULL));
          PetscCheck(numIndices == numColIndices, PETSC_COMM_SELF, PETSC_ERR_PLIB, "mismatching constraint indices calculations");
          for (i = 0; i < numColIndices; i++) pInd[i] = indices[i];
          for (i = 0; i < numFields; i++) {
            pInd[numColIndices + i]             = offsets[i + 1];
            pInd[numColIndices + numFields + i] = offsets[i + 1];
          }
          PetscCall(DMPlexRestoreClosureIndices(coarse, localCoarse, globalCoarse, p, PETSC_TRUE, &numIndices, &indices, offsets, NULL));
        } else {
          PetscInt     closureSize, *closure = NULL, cl;
          PetscScalar *pMatIn, *pMatModified;
          PetscInt     numPoints, *points;

          {
            PetscInt *closure = NULL, closureSize, cl;

            PetscCall(DMPlexGetTransitiveClosure(coarse, p, PETSC_TRUE, &closureSize, &closure));
            for (cl = 0; cl < closureSize; cl++) { /* get the closure */
              PetscInt c = closure[2 * cl], clDof;

              PetscCall(PetscSectionGetDof(localCoarse, c, &clDof));
              numRowIndices += clDof;
            }
            PetscCall(DMPlexRestoreTransitiveClosure(coarse, p, PETSC_TRUE, &closureSize, &closure));
          }

          PetscCall(DMGetWorkArray(coarse, numRowIndices * numRowIndices, MPIU_SCALAR, &pMatIn));
          for (i = 0; i < numRowIndices; i++) { /* initialize to the identity */
            for (j = 0; j < numRowIndices; j++) pMatIn[i * numRowIndices + j] = (i == j) ? 1. : 0.;
          }
          PetscCall(DMPlexGetTransitiveClosure(coarse, p, PETSC_TRUE, &closureSize, &closure));
          for (f = 0; f < maxFields; f++) {
            if (numFields) PetscCall(PetscSectionGetFieldPointSyms(localCoarse, f, closureSize, closure, &perms[f], &flips[f]));
            else PetscCall(PetscSectionGetPointSyms(localCoarse, closureSize, closure, &perms[f], &flips[f]));
          }
          if (numFields) {
            for (cl = 0; cl < closureSize; cl++) {
              PetscInt c = closure[2 * cl];

              for (f = 0; f < numFields; f++) {
                PetscInt fDof;

                PetscCall(PetscSectionGetFieldDof(localCoarse, c, f, &fDof));
                offsets[f + 1] += fDof;
              }
            }
            for (f = 0; f < numFields; f++) {
              offsets[f + 1] += offsets[f];
              newOffsets[f + 1] = offsets[f + 1];
            }
          }
          /* TODO : flips here ? */
          /* apply hanging node constraints on the right, get the new points and the new offsets */
          PetscCall(DMPlexAnchorsModifyMat(coarse, localCoarse, closureSize, numRowIndices, closure, perms, pMatIn, &numPoints, NULL, &points, &pMatModified, newOffsets, PETSC_FALSE));
          for (f = 0; f < maxFields; f++) {
            if (numFields) PetscCall(PetscSectionRestoreFieldPointSyms(localCoarse, f, closureSize, closure, &perms[f], &flips[f]));
            else PetscCall(PetscSectionRestorePointSyms(localCoarse, closureSize, closure, &perms[f], &flips[f]));
          }
          for (f = 0; f < maxFields; f++) {
            if (numFields) PetscCall(PetscSectionGetFieldPointSyms(localCoarse, f, numPoints, points, &perms[f], &flips[f]));
            else PetscCall(PetscSectionGetPointSyms(localCoarse, numPoints, points, &perms[f], &flips[f]));
          }
          if (!numFields) {
            for (i = 0; i < numRowIndices * numColIndices; i++) pMat[i] = pMatModified[i];
          } else {
            PetscInt i, j, count;
            for (f = 0, count = 0; f < numFields; f++) {
              for (i = offsets[f]; i < offsets[f + 1]; i++) {
                for (j = newOffsets[f]; j < newOffsets[f + 1]; j++, count++) pMat[count] = pMatModified[i * numColIndices + j];
              }
            }
          }
          PetscCall(DMRestoreWorkArray(coarse, numRowIndices * numColIndices, MPIU_SCALAR, &pMatModified));
          PetscCall(DMPlexRestoreTransitiveClosure(coarse, p, PETSC_TRUE, &closureSize, &closure));
          PetscCall(DMRestoreWorkArray(coarse, numRowIndices * numColIndices, MPIU_SCALAR, &pMatIn));
          if (numFields) {
            for (f = 0; f < numFields; f++) {
              pInd[numColIndices + f]             = offsets[f + 1];
              pInd[numColIndices + numFields + f] = newOffsets[f + 1];
            }
            for (cl = 0; cl < numPoints; cl++) {
              PetscInt globalOff, c = points[2 * cl];
              PetscCall(PetscSectionGetOffset(globalCoarse, c, &globalOff));
              PetscCall(DMPlexGetIndicesPointFields_Internal(localCoarse, PETSC_FALSE, c, globalOff < 0 ? -(globalOff + 1) : globalOff, newOffsets, PETSC_FALSE, perms, cl, NULL, pInd));
            }
          } else {
            for (cl = 0; cl < numPoints; cl++) {
              PetscInt        c    = points[2 * cl], globalOff;
              const PetscInt *perm = perms[0] ? perms[0][cl] : NULL;

              PetscCall(PetscSectionGetOffset(globalCoarse, c, &globalOff));
              PetscCall(DMPlexGetIndicesPoint_Internal(localCoarse, PETSC_FALSE, c, globalOff < 0 ? -(globalOff + 1) : globalOff, newOffsets, PETSC_FALSE, perm, NULL, pInd));
            }
          }
          for (f = 0; f < maxFields; f++) {
            if (numFields) PetscCall(PetscSectionRestoreFieldPointSyms(localCoarse, f, numPoints, points, &perms[f], &flips[f]));
            else PetscCall(PetscSectionRestorePointSyms(localCoarse, numPoints, points, &perms[f], &flips[f]));
          }
          PetscCall(DMRestoreWorkArray(coarse, numPoints, MPIU_SCALAR, &points));
        }
      } else if (matSize) {
        PetscInt  cOff;
        PetscInt *rowIndices, *colIndices, a, aDof = 0, aOff;

        numRowIndices = dof;
        PetscCall(DMGetWorkArray(coarse, numRowIndices, MPIU_INT, &rowIndices));
        PetscCall(DMGetWorkArray(coarse, numColIndices, MPIU_INT, &colIndices));
        PetscCall(PetscSectionGetOffset(cSec, p, &cOff));
        PetscCall(PetscSectionGetDof(aSec, p, &aDof));
        PetscCall(PetscSectionGetOffset(aSec, p, &aOff));
        if (numFields) {
          for (f = 0; f < numFields; f++) {
            PetscInt fDof;

            PetscCall(PetscSectionGetFieldDof(cSec, p, f, &fDof));
            offsets[f + 1] = fDof;
            for (a = 0; a < aDof; a++) {
              PetscInt anchor = anchors[a + aOff];
              PetscCall(PetscSectionGetFieldDof(localCoarse, anchor, f, &fDof));
              newOffsets[f + 1] += fDof;
            }
          }
          for (f = 0; f < numFields; f++) {
            offsets[f + 1] += offsets[f];
            offsetsCopy[f + 1] = offsets[f + 1];
            newOffsets[f + 1] += newOffsets[f];
            newOffsetsCopy[f + 1] = newOffsets[f + 1];
          }
          PetscCall(DMPlexGetIndicesPointFields_Internal(cSec, PETSC_TRUE, p, cOff, offsetsCopy, PETSC_TRUE, NULL, -1, NULL, rowIndices));
          for (a = 0; a < aDof; a++) {
            PetscInt anchor = anchors[a + aOff], lOff;
            PetscCall(PetscSectionGetOffset(localCoarse, anchor, &lOff));
            PetscCall(DMPlexGetIndicesPointFields_Internal(localCoarse, PETSC_TRUE, anchor, lOff, newOffsetsCopy, PETSC_TRUE, NULL, -1, NULL, colIndices));
          }
        } else {
          PetscCall(DMPlexGetIndicesPoint_Internal(cSec, PETSC_TRUE, p, cOff, offsetsCopy, PETSC_TRUE, NULL, NULL, rowIndices));
          for (a = 0; a < aDof; a++) {
            PetscInt anchor = anchors[a + aOff], lOff;
            PetscCall(PetscSectionGetOffset(localCoarse, anchor, &lOff));
            PetscCall(DMPlexGetIndicesPoint_Internal(localCoarse, PETSC_TRUE, anchor, lOff, newOffsetsCopy, PETSC_TRUE, NULL, NULL, colIndices));
          }
        }
        if (numFields) {
          PetscInt count, a;

          for (f = 0, count = 0; f < numFields; f++) {
            PetscInt iSize = offsets[f + 1] - offsets[f];
            PetscInt jSize = newOffsets[f + 1] - newOffsets[f];
            PetscCall(MatGetValues(cMat, iSize, &rowIndices[offsets[f]], jSize, &colIndices[newOffsets[f]], &pMat[count]));
            count += iSize * jSize;
            pInd[numColIndices + f]             = offsets[f + 1];
            pInd[numColIndices + numFields + f] = newOffsets[f + 1];
          }
          for (a = 0; a < aDof; a++) {
            PetscInt anchor = anchors[a + aOff];
            PetscInt gOff;
            PetscCall(PetscSectionGetOffset(globalCoarse, anchor, &gOff));
            PetscCall(DMPlexGetIndicesPointFields_Internal(localCoarse, PETSC_FALSE, anchor, gOff < 0 ? -(gOff + 1) : gOff, newOffsets, PETSC_FALSE, NULL, -1, NULL, pInd));
          }
        } else {
          PetscInt a;
          PetscCall(MatGetValues(cMat, numRowIndices, rowIndices, numColIndices, colIndices, pMat));
          for (a = 0; a < aDof; a++) {
            PetscInt anchor = anchors[a + aOff];
            PetscInt gOff;
            PetscCall(PetscSectionGetOffset(globalCoarse, anchor, &gOff));
            PetscCall(DMPlexGetIndicesPoint_Internal(localCoarse, PETSC_FALSE, anchor, gOff < 0 ? -(gOff + 1) : gOff, newOffsets, PETSC_FALSE, NULL, NULL, pInd));
          }
        }
        PetscCall(DMRestoreWorkArray(coarse, numColIndices, MPIU_INT, &colIndices));
        PetscCall(DMRestoreWorkArray(coarse, numRowIndices, MPIU_INT, &rowIndices));
      } else {
        PetscInt gOff;

        PetscCall(PetscSectionGetOffset(globalCoarse, p, &gOff));
        if (numFields) {
          for (f = 0; f < numFields; f++) {
            PetscInt fDof;
            PetscCall(PetscSectionGetFieldDof(localCoarse, p, f, &fDof));
            offsets[f + 1] = fDof + offsets[f];
          }
          for (f = 0; f < numFields; f++) {
            pInd[numColIndices + f]             = offsets[f + 1];
            pInd[numColIndices + numFields + f] = offsets[f + 1];
          }
          PetscCall(DMPlexGetIndicesPointFields_Internal(localCoarse, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsets, PETSC_FALSE, NULL, -1, NULL, pInd));
        } else {
          PetscCall(DMPlexGetIndicesPoint_Internal(localCoarse, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsets, PETSC_FALSE, NULL, NULL, pInd));
        }
      }
    }
    PetscCall(PetscFree(maxChildIds));
  }
  {
    PetscSF   indicesSF, matricesSF;
    PetscInt *remoteOffsetsIndices, *remoteOffsetsMatrices, numLeafIndices, numLeafMatrices;

    PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)fine), &leafIndicesSec));
    PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)fine), &leafMatricesSec));
    PetscCall(PetscSFDistributeSection(coarseToFineEmbedded, rootIndicesSec, &remoteOffsetsIndices, leafIndicesSec));
    PetscCall(PetscSFDistributeSection(coarseToFineEmbedded, rootMatricesSec, &remoteOffsetsMatrices, leafMatricesSec));
    PetscCall(PetscSFCreateSectionSF(coarseToFineEmbedded, rootIndicesSec, remoteOffsetsIndices, leafIndicesSec, &indicesSF));
    PetscCall(PetscSFCreateSectionSF(coarseToFineEmbedded, rootMatricesSec, remoteOffsetsMatrices, leafMatricesSec, &matricesSF));
    PetscCall(PetscSFDestroy(&coarseToFineEmbedded));
    PetscCall(PetscFree(remoteOffsetsIndices));
    PetscCall(PetscFree(remoteOffsetsMatrices));
    PetscCall(PetscSectionGetStorageSize(leafIndicesSec, &numLeafIndices));
    PetscCall(PetscSectionGetStorageSize(leafMatricesSec, &numLeafMatrices));
    PetscCall(PetscMalloc2(numLeafIndices, &leafIndices, numLeafMatrices, &leafMatrices));
    PetscCall(PetscSFBcastBegin(indicesSF, MPIU_INT, rootIndices, leafIndices, MPI_REPLACE));
    PetscCall(PetscSFBcastBegin(matricesSF, MPIU_SCALAR, rootMatrices, leafMatrices, MPI_REPLACE));
    PetscCall(PetscSFBcastEnd(indicesSF, MPIU_INT, rootIndices, leafIndices, MPI_REPLACE));
    PetscCall(PetscSFBcastEnd(matricesSF, MPIU_SCALAR, rootMatrices, leafMatrices, MPI_REPLACE));
    PetscCall(PetscSFDestroy(&matricesSF));
    PetscCall(PetscSFDestroy(&indicesSF));
    PetscCall(PetscFree2(rootIndices, rootMatrices));
    PetscCall(PetscSectionDestroy(&rootIndicesSec));
    PetscCall(PetscSectionDestroy(&rootMatricesSec));
  }
  /* count to preallocate */
  PetscCall(DMGetLocalSection(fine, &localFine));
  {
    PetscInt       nGlobal;
    PetscInt      *dnnz, *onnz;
    PetscLayout    rowMap, colMap;
    PetscInt       rowStart, rowEnd, colStart, colEnd;
    PetscInt       maxDof;
    PetscInt      *rowIndices;
    DM             refTree;
    PetscInt     **refPointFieldN;
    PetscScalar ***refPointFieldMats;
    PetscSection   refConSec, refAnSec;
    PetscInt       pRefStart, pRefEnd, maxConDof, maxColumns, leafStart, leafEnd;
    PetscScalar   *pointWork;

    PetscCall(PetscSectionGetConstrainedStorageSize(globalFine, &nGlobal));
    PetscCall(PetscCalloc2(nGlobal, &dnnz, nGlobal, &onnz));
    PetscCall(MatGetLayouts(mat, &rowMap, &colMap));
    PetscCall(PetscLayoutSetUp(rowMap));
    PetscCall(PetscLayoutSetUp(colMap));
    PetscCall(PetscLayoutGetRange(rowMap, &rowStart, &rowEnd));
    PetscCall(PetscLayoutGetRange(colMap, &colStart, &colEnd));
    PetscCall(PetscSectionGetMaxDof(localFine, &maxDof));
    PetscCall(PetscSectionGetChart(leafIndicesSec, &leafStart, &leafEnd));
    PetscCall(DMGetWorkArray(fine, maxDof, MPIU_INT, &rowIndices));
    for (p = leafStart; p < leafEnd; p++) {
      PetscInt gDof, gcDof, gOff;
      PetscInt numColIndices, pIndOff, *pInd;
      PetscInt matSize;
      PetscInt i;

      PetscCall(PetscSectionGetDof(globalFine, p, &gDof));
      PetscCall(PetscSectionGetConstraintDof(globalFine, p, &gcDof));
      if ((gDof - gcDof) <= 0) continue;
      PetscCall(PetscSectionGetOffset(globalFine, p, &gOff));
      PetscCheck(gOff >= 0, PETSC_COMM_SELF, PETSC_ERR_PLIB, "I though having global dofs meant a non-negative offset");
      PetscCheck(gOff >= rowStart && (gOff + gDof - gcDof) <= rowEnd, PETSC_COMM_SELF, PETSC_ERR_PLIB, "I thought the row map would constrain the global dofs");
      PetscCall(PetscSectionGetDof(leafIndicesSec, p, &numColIndices));
      PetscCall(PetscSectionGetOffset(leafIndicesSec, p, &pIndOff));
      numColIndices -= 2 * numFields;
      PetscCheck(numColIndices > 0, PETSC_COMM_SELF, PETSC_ERR_PLIB, "global fine dof with no dofs to interpolate from");
      pInd              = &leafIndices[pIndOff];
      offsets[0]        = 0;
      offsetsCopy[0]    = 0;
      newOffsets[0]     = 0;
      newOffsetsCopy[0] = 0;
      if (numFields) {
        PetscInt f;
        for (f = 0; f < numFields; f++) {
          PetscInt rowDof;

          PetscCall(PetscSectionGetFieldDof(localFine, p, f, &rowDof));
          offsets[f + 1]     = offsets[f] + rowDof;
          offsetsCopy[f + 1] = offsets[f + 1];
          newOffsets[f + 1]  = pInd[numColIndices + numFields + f];
          numD[f]            = 0;
          numO[f]            = 0;
        }
        PetscCall(DMPlexGetIndicesPointFields_Internal(localFine, PETSC_FALSE, p, gOff, offsetsCopy, PETSC_FALSE, NULL, -1, NULL, rowIndices));
        for (f = 0; f < numFields; f++) {
          PetscInt colOffset    = newOffsets[f];
          PetscInt numFieldCols = newOffsets[f + 1] - newOffsets[f];

          for (i = 0; i < numFieldCols; i++) {
            PetscInt gInd = pInd[i + colOffset];

            if (gInd >= colStart && gInd < colEnd) {
              numD[f]++;
            } else if (gInd >= 0) { /* negative means non-entry */
              numO[f]++;
            }
          }
        }
      } else {
        PetscCall(DMPlexGetIndicesPoint_Internal(localFine, PETSC_FALSE, p, gOff, offsetsCopy, PETSC_FALSE, NULL, NULL, rowIndices));
        numD[0] = 0;
        numO[0] = 0;
        for (i = 0; i < numColIndices; i++) {
          PetscInt gInd = pInd[i];

          if (gInd >= colStart && gInd < colEnd) {
            numD[0]++;
          } else if (gInd >= 0) { /* negative means non-entry */
            numO[0]++;
          }
        }
      }
      PetscCall(PetscSectionGetDof(leafMatricesSec, p, &matSize));
      if (!matSize) { /* incoming matrix is identity */
        PetscInt childId;

        childId = childIds[p - pStartF];
        if (childId < 0) { /* no child interpolation: one nnz per */
          if (numFields) {
            PetscInt f;
            for (f = 0; f < numFields; f++) {
              PetscInt numRows = offsets[f + 1] - offsets[f], row;
              for (row = 0; row < numRows; row++) {
                PetscInt gIndCoarse = pInd[newOffsets[f] + row];
                PetscInt gIndFine   = rowIndices[offsets[f] + row];
                if (gIndCoarse >= colStart && gIndCoarse < colEnd) { /* local */
                  PetscCheck(gIndFine >= rowStart && gIndFine < rowEnd, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
                  dnnz[gIndFine - rowStart] = 1;
                } else if (gIndCoarse >= 0) { /* remote */
                  PetscCheck(gIndFine >= rowStart && gIndFine < rowEnd, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
                  onnz[gIndFine - rowStart] = 1;
                } else { /* constrained */
                  PetscCheck(gIndFine < 0, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
                }
              }
            }
          } else {
            PetscInt i;
            for (i = 0; i < gDof; i++) {
              PetscInt gIndCoarse = pInd[i];
              PetscInt gIndFine   = rowIndices[i];
              if (gIndCoarse >= colStart && gIndCoarse < colEnd) { /* local */
                PetscCheck(gIndFine >= rowStart && gIndFine < rowEnd, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
                dnnz[gIndFine - rowStart] = 1;
              } else if (gIndCoarse >= 0) { /* remote */
                PetscCheck(gIndFine >= rowStart && gIndFine < rowEnd, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
                onnz[gIndFine - rowStart] = 1;
              } else { /* constrained */
                PetscCheck(gIndFine < 0, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
              }
            }
          }
        } else { /* interpolate from all */
          if (numFields) {
            PetscInt f;
            for (f = 0; f < numFields; f++) {
              PetscInt numRows = offsets[f + 1] - offsets[f], row;
              for (row = 0; row < numRows; row++) {
                PetscInt gIndFine = rowIndices[offsets[f] + row];
                if (gIndFine >= 0) {
                  PetscCheck(gIndFine >= rowStart && gIndFine < rowEnd, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
                  dnnz[gIndFine - rowStart] = numD[f];
                  onnz[gIndFine - rowStart] = numO[f];
                }
              }
            }
          } else {
            PetscInt i;
            for (i = 0; i < gDof; i++) {
              PetscInt gIndFine = rowIndices[i];
              if (gIndFine >= 0) {
                PetscCheck(gIndFine >= rowStart && gIndFine < rowEnd, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
                dnnz[gIndFine - rowStart] = numD[0];
                onnz[gIndFine - rowStart] = numO[0];
              }
            }
          }
        }
      } else { /* interpolate from all */
        if (numFields) {
          PetscInt f;
          for (f = 0; f < numFields; f++) {
            PetscInt numRows = offsets[f + 1] - offsets[f], row;
            for (row = 0; row < numRows; row++) {
              PetscInt gIndFine = rowIndices[offsets[f] + row];
              if (gIndFine >= 0) {
                PetscCheck(gIndFine >= rowStart && gIndFine < rowEnd, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
                dnnz[gIndFine - rowStart] = numD[f];
                onnz[gIndFine - rowStart] = numO[f];
              }
            }
          }
        } else { /* every dof get a full row */
          PetscInt i;
          for (i = 0; i < gDof; i++) {
            PetscInt gIndFine = rowIndices[i];
            if (gIndFine >= 0) {
              PetscCheck(gIndFine >= rowStart && gIndFine < rowEnd, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Mismatched number of constrained dofs");
              dnnz[gIndFine - rowStart] = numD[0];
              onnz[gIndFine - rowStart] = numO[0];
            }
          }
        }
      }
    }
    PetscCall(MatXAIJSetPreallocation(mat, 1, dnnz, onnz, NULL, NULL));
    PetscCall(PetscFree2(dnnz, onnz));

    PetscCall(DMPlexGetReferenceTree(fine, &refTree));
    PetscCall(DMCopyDisc(fine, refTree));
    PetscCall(DMSetLocalSection(refTree, NULL));
    PetscCall(DMSetDefaultConstraints(refTree, NULL, NULL, NULL));
    PetscCall(DMPlexReferenceTreeGetChildrenMatrices(refTree, &refPointFieldMats, &refPointFieldN));
    PetscCall(DMGetDefaultConstraints(refTree, &refConSec, NULL, NULL));
    PetscCall(DMPlexGetAnchors(refTree, &refAnSec, NULL));
    PetscCall(PetscSectionGetChart(refConSec, &pRefStart, &pRefEnd));
    PetscCall(PetscSectionGetMaxDof(refConSec, &maxConDof));
    PetscCall(PetscSectionGetMaxDof(leafIndicesSec, &maxColumns));
    PetscCall(PetscMalloc1(maxConDof * maxColumns, &pointWork));
    for (p = leafStart; p < leafEnd; p++) {
      PetscInt gDof, gcDof, gOff;
      PetscInt numColIndices, pIndOff, *pInd;
      PetscInt matSize;
      PetscInt childId;

      PetscCall(PetscSectionGetDof(globalFine, p, &gDof));
      PetscCall(PetscSectionGetConstraintDof(globalFine, p, &gcDof));
      if ((gDof - gcDof) <= 0) continue;
      childId = childIds[p - pStartF];
      PetscCall(PetscSectionGetOffset(globalFine, p, &gOff));
      PetscCall(PetscSectionGetDof(leafIndicesSec, p, &numColIndices));
      PetscCall(PetscSectionGetOffset(leafIndicesSec, p, &pIndOff));
      numColIndices -= 2 * numFields;
      pInd              = &leafIndices[pIndOff];
      offsets[0]        = 0;
      offsetsCopy[0]    = 0;
      newOffsets[0]     = 0;
      newOffsetsCopy[0] = 0;
      rowOffsets[0]     = 0;
      if (numFields) {
        PetscInt f;
        for (f = 0; f < numFields; f++) {
          PetscInt rowDof;

          PetscCall(PetscSectionGetFieldDof(localFine, p, f, &rowDof));
          offsets[f + 1]     = offsets[f] + rowDof;
          offsetsCopy[f + 1] = offsets[f + 1];
          rowOffsets[f + 1]  = pInd[numColIndices + f];
          newOffsets[f + 1]  = pInd[numColIndices + numFields + f];
        }
        PetscCall(DMPlexGetIndicesPointFields_Internal(localFine, PETSC_FALSE, p, gOff, offsetsCopy, PETSC_FALSE, NULL, -1, NULL, rowIndices));
      } else {
        PetscCall(DMPlexGetIndicesPoint_Internal(localFine, PETSC_FALSE, p, gOff, offsetsCopy, PETSC_FALSE, NULL, NULL, rowIndices));
      }
      PetscCall(PetscSectionGetDof(leafMatricesSec, p, &matSize));
      if (!matSize) {      /* incoming matrix is identity */
        if (childId < 0) { /* no child interpolation: scatter */
          if (numFields) {
            PetscInt f;
            for (f = 0; f < numFields; f++) {
              PetscInt numRows = offsets[f + 1] - offsets[f], row;
              for (row = 0; row < numRows; row++) PetscCall(MatSetValue(mat, rowIndices[offsets[f] + row], pInd[newOffsets[f] + row], 1., INSERT_VALUES));
            }
          } else {
            PetscInt numRows = gDof, row;
            for (row = 0; row < numRows; row++) PetscCall(MatSetValue(mat, rowIndices[row], pInd[row], 1., INSERT_VALUES));
          }
        } else { /* interpolate from all */
          if (numFields) {
            PetscInt f;
            for (f = 0; f < numFields; f++) {
              PetscInt numRows = offsets[f + 1] - offsets[f];
              PetscInt numCols = newOffsets[f + 1] - newOffsets[f];
              PetscCall(MatSetValues(mat, numRows, &rowIndices[offsets[f]], numCols, &pInd[newOffsets[f]], refPointFieldMats[childId - pRefStart][f], INSERT_VALUES));
            }
          } else {
            PetscCall(MatSetValues(mat, gDof, rowIndices, numColIndices, pInd, refPointFieldMats[childId - pRefStart][0], INSERT_VALUES));
          }
        }
      } else { /* interpolate from all */
        PetscInt     pMatOff;
        PetscScalar *pMat;

        PetscCall(PetscSectionGetOffset(leafMatricesSec, p, &pMatOff));
        pMat = &leafMatrices[pMatOff];
        if (childId < 0) { /* copy the incoming matrix */
          if (numFields) {
            PetscInt f, count;
            for (f = 0, count = 0; f < numFields; f++) {
              PetscInt     numRows   = offsets[f + 1] - offsets[f];
              PetscInt     numCols   = newOffsets[f + 1] - newOffsets[f];
              PetscInt     numInRows = rowOffsets[f + 1] - rowOffsets[f];
              PetscScalar *inMat     = &pMat[count];

              PetscCall(MatSetValues(mat, numRows, &rowIndices[offsets[f]], numCols, &pInd[newOffsets[f]], inMat, INSERT_VALUES));
              count += numCols * numInRows;
            }
          } else {
            PetscCall(MatSetValues(mat, gDof, rowIndices, numColIndices, pInd, pMat, INSERT_VALUES));
          }
        } else { /* multiply the incoming matrix by the child interpolation */
          if (numFields) {
            PetscInt f, count;
            for (f = 0, count = 0; f < numFields; f++) {
              PetscInt     numRows   = offsets[f + 1] - offsets[f];
              PetscInt     numCols   = newOffsets[f + 1] - newOffsets[f];
              PetscInt     numInRows = rowOffsets[f + 1] - rowOffsets[f];
              PetscScalar *inMat     = &pMat[count];
              PetscInt     i, j, k;
              PetscCheck(refPointFieldN[childId - pRefStart][f] == numInRows, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Point constraint matrix multiply dimension mismatch");
              for (i = 0; i < numRows; i++) {
                for (j = 0; j < numCols; j++) {
                  PetscScalar val = 0.;
                  for (k = 0; k < numInRows; k++) val += refPointFieldMats[childId - pRefStart][f][i * numInRows + k] * inMat[k * numCols + j];
                  pointWork[i * numCols + j] = val;
                }
              }
              PetscCall(MatSetValues(mat, numRows, &rowIndices[offsets[f]], numCols, &pInd[newOffsets[f]], pointWork, INSERT_VALUES));
              count += numCols * numInRows;
            }
          } else { /* every dof gets a full row */
            PetscInt numRows   = gDof;
            PetscInt numCols   = numColIndices;
            PetscInt numInRows = matSize / numColIndices;
            PetscInt i, j, k;
            PetscCheck(refPointFieldN[childId - pRefStart][0] == numInRows, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Point constraint matrix multiply dimension mismatch");
            for (i = 0; i < numRows; i++) {
              for (j = 0; j < numCols; j++) {
                PetscScalar val = 0.;
                for (k = 0; k < numInRows; k++) val += refPointFieldMats[childId - pRefStart][0][i * numInRows + k] * pMat[k * numCols + j];
                pointWork[i * numCols + j] = val;
              }
            }
            PetscCall(MatSetValues(mat, numRows, rowIndices, numCols, pInd, pointWork, INSERT_VALUES));
          }
        }
      }
    }
    PetscCall(DMPlexReferenceTreeRestoreChildrenMatrices(refTree, &refPointFieldMats, &refPointFieldN));
    PetscCall(DMRestoreWorkArray(fine, maxDof, MPIU_INT, &rowIndices));
    PetscCall(PetscFree(pointWork));
  }
  PetscCall(MatAssemblyBegin(mat, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(mat, MAT_FINAL_ASSEMBLY));
  PetscCall(PetscSectionDestroy(&leafIndicesSec));
  PetscCall(PetscSectionDestroy(&leafMatricesSec));
  PetscCall(PetscFree2(leafIndices, leafMatrices));
  PetscCall(PetscFree2(*(PetscInt ****)&perms, *(PetscScalar ****)&flips));
  PetscCall(PetscFree7(offsets, offsetsCopy, newOffsets, newOffsetsCopy, rowOffsets, numD, numO));
  PetscCall(ISRestoreIndices(aIS, &anchors));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
 * Assuming a nodal basis (w.r.t. the dual basis) basis:
 *
 * for each coarse dof \phi^c_i:
 *   for each quadrature point (w_l,x_l) in the dual basis definition of \phi^c_i:
 *     for each fine dof \phi^f_j;
 *       a_{i,j} = 0;
 *       for each fine dof \phi^f_k:
 *         a_{i,j} += interp_{i,k} * \phi^f_k(x_l) * \phi^f_j(x_l) * w_l
 *                    [^^^ this is = \phi^c_i ^^^]
 */
PetscErrorCode DMPlexComputeInjectorReferenceTree(DM refTree, Mat *inj)
{
  PetscDS      ds;
  PetscSection section, cSection;
  DMLabel      canonical, depth;
  Mat          cMat, mat;
  PetscInt    *nnz;
  PetscInt     f, dim, numFields, numSecFields, p, pStart, pEnd, cStart, cEnd;
  PetscInt     m, n;
  PetscScalar *pointScalar;
  PetscReal   *v0, *v0parent, *vtmp, *J, *Jparent, *invJ, *pointRef, detJ, detJparent;

  PetscFunctionBegin;
  PetscCall(DMGetLocalSection(refTree, &section));
  PetscCall(DMGetDimension(refTree, &dim));
  PetscCall(PetscMalloc6(dim, &v0, dim, &v0parent, dim, &vtmp, dim * dim, &J, dim * dim, &Jparent, dim * dim, &invJ));
  PetscCall(PetscMalloc2(dim, &pointScalar, dim, &pointRef));
  PetscCall(DMGetDS(refTree, &ds));
  PetscCall(PetscDSGetNumFields(ds, &numFields));
  PetscCall(PetscSectionGetNumFields(section, &numSecFields));
  PetscCall(DMGetLabel(refTree, "canonical", &canonical));
  PetscCall(DMGetLabel(refTree, "depth", &depth));
  PetscCall(DMGetDefaultConstraints(refTree, &cSection, &cMat, NULL));
  PetscCall(DMPlexGetChart(refTree, &pStart, &pEnd));
  PetscCall(DMPlexGetHeightStratum(refTree, 0, &cStart, &cEnd));
  PetscCall(MatGetSize(cMat, &n, &m)); /* the injector has transpose sizes from the constraint matrix */
  /* Step 1: compute non-zero pattern.  A proper subset of constraint matrix non-zero */
  PetscCall(PetscCalloc1(m, &nnz));
  for (p = pStart; p < pEnd; p++) { /* a point will have non-zeros if it is canonical, it has dofs, and its children have dofs */
    const PetscInt *children;
    PetscInt        numChildren;
    PetscInt        i, numChildDof, numSelfDof;

    if (canonical) {
      PetscInt pCanonical;
      PetscCall(DMLabelGetValue(canonical, p, &pCanonical));
      if (p != pCanonical) continue;
    }
    PetscCall(DMPlexGetTreeChildren(refTree, p, &numChildren, &children));
    if (!numChildren) continue;
    for (i = 0, numChildDof = 0; i < numChildren; i++) {
      PetscInt child = children[i];
      PetscInt dof;

      PetscCall(PetscSectionGetDof(section, child, &dof));
      numChildDof += dof;
    }
    PetscCall(PetscSectionGetDof(section, p, &numSelfDof));
    if (!numChildDof || !numSelfDof) continue;
    for (f = 0; f < numFields; f++) {
      PetscInt selfOff;

      if (numSecFields) { /* count the dofs for just this field */
        for (i = 0, numChildDof = 0; i < numChildren; i++) {
          PetscInt child = children[i];
          PetscInt dof;

          PetscCall(PetscSectionGetFieldDof(section, child, f, &dof));
          numChildDof += dof;
        }
        PetscCall(PetscSectionGetFieldDof(section, p, f, &numSelfDof));
        PetscCall(PetscSectionGetFieldOffset(section, p, f, &selfOff));
      } else {
        PetscCall(PetscSectionGetOffset(section, p, &selfOff));
      }
      for (i = 0; i < numSelfDof; i++) nnz[selfOff + i] = numChildDof;
    }
  }
  PetscCall(MatCreateAIJ(PETSC_COMM_SELF, m, n, m, n, -1, nnz, -1, NULL, &mat));
  PetscCall(PetscFree(nnz));
  /* Setp 2: compute entries */
  for (p = pStart; p < pEnd; p++) {
    const PetscInt *children;
    PetscInt        numChildren;
    PetscInt        i, numChildDof, numSelfDof;

    /* same conditions about when entries occur */
    if (canonical) {
      PetscInt pCanonical;
      PetscCall(DMLabelGetValue(canonical, p, &pCanonical));
      if (p != pCanonical) continue;
    }
    PetscCall(DMPlexGetTreeChildren(refTree, p, &numChildren, &children));
    if (!numChildren) continue;
    for (i = 0, numChildDof = 0; i < numChildren; i++) {
      PetscInt child = children[i];
      PetscInt dof;

      PetscCall(PetscSectionGetDof(section, child, &dof));
      numChildDof += dof;
    }
    PetscCall(PetscSectionGetDof(section, p, &numSelfDof));
    if (!numChildDof || !numSelfDof) continue;

    for (f = 0; f < numFields; f++) {
      PetscInt        pI = -1, cI = -1;
      PetscInt        selfOff, Nc, parentCell;
      PetscInt        cellShapeOff;
      PetscObject     disc;
      PetscDualSpace  dsp;
      PetscClassId    classId;
      PetscScalar    *pointMat;
      PetscInt       *matRows, *matCols;
      PetscInt        pO = PETSC_INT_MIN;
      const PetscInt *depthNumDof;

      if (numSecFields) {
        for (i = 0, numChildDof = 0; i < numChildren; i++) {
          PetscInt child = children[i];
          PetscInt dof;

          PetscCall(PetscSectionGetFieldDof(section, child, f, &dof));
          numChildDof += dof;
        }
        PetscCall(PetscSectionGetFieldDof(section, p, f, &numSelfDof));
        PetscCall(PetscSectionGetFieldOffset(section, p, f, &selfOff));
      } else {
        PetscCall(PetscSectionGetOffset(section, p, &selfOff));
      }

      /* find a cell whose closure contains p */
      if (p >= cStart && p < cEnd) {
        parentCell = p;
      } else {
        PetscInt *star = NULL;
        PetscInt  numStar;

        parentCell = -1;
        PetscCall(DMPlexGetTransitiveClosure(refTree, p, PETSC_FALSE, &numStar, &star));
        for (i = numStar - 1; i >= 0; i--) {
          PetscInt c = star[2 * i];

          if (c >= cStart && c < cEnd) {
            parentCell = c;
            break;
          }
        }
        PetscCall(DMPlexRestoreTransitiveClosure(refTree, p, PETSC_FALSE, &numStar, &star));
      }
      /* determine the offset of p's shape functions within parentCell's shape functions */
      PetscCall(PetscDSGetDiscretization(ds, f, &disc));
      PetscCall(PetscObjectGetClassId(disc, &classId));
      if (classId == PETSCFE_CLASSID) PetscCall(PetscFEGetDualSpace((PetscFE)disc, &dsp));
      else {
        PetscCheck(classId == PETSCFV_CLASSID, PETSC_COMM_SELF, PETSC_ERR_SUP, "Unsupported discretization object");
        PetscCall(PetscFVGetDualSpace((PetscFV)disc, &dsp));
      }
      PetscCall(PetscDualSpaceGetNumDof(dsp, &depthNumDof));
      PetscCall(PetscDualSpaceGetNumComponents(dsp, &Nc));
      {
        PetscInt *closure = NULL;
        PetscInt  numClosure;

        PetscCall(DMPlexGetTransitiveClosure(refTree, parentCell, PETSC_TRUE, &numClosure, &closure));
        for (i = 0, pI = -1, cellShapeOff = 0; i < numClosure; i++) {
          PetscInt point = closure[2 * i], pointDepth;

          pO = closure[2 * i + 1];
          if (point == p) {
            pI = i;
            break;
          }
          PetscCall(DMLabelGetValue(depth, point, &pointDepth));
          cellShapeOff += depthNumDof[pointDepth];
        }
        PetscCall(DMPlexRestoreTransitiveClosure(refTree, parentCell, PETSC_TRUE, &numClosure, &closure));
      }

      PetscCall(DMGetWorkArray(refTree, numSelfDof * numChildDof, MPIU_SCALAR, &pointMat));
      PetscCall(DMGetWorkArray(refTree, numSelfDof + numChildDof, MPIU_INT, &matRows));
      matCols = matRows + numSelfDof;
      for (i = 0; i < numSelfDof; i++) matRows[i] = selfOff + i;
      for (i = 0; i < numSelfDof * numChildDof; i++) pointMat[i] = 0.;
      {
        PetscInt colOff = 0;

        for (i = 0; i < numChildren; i++) {
          PetscInt child = children[i];
          PetscInt dof, off, j;

          if (numSecFields) {
            PetscCall(PetscSectionGetFieldDof(cSection, child, f, &dof));
            PetscCall(PetscSectionGetFieldOffset(cSection, child, f, &off));
          } else {
            PetscCall(PetscSectionGetDof(cSection, child, &dof));
            PetscCall(PetscSectionGetOffset(cSection, child, &off));
          }

          for (j = 0; j < dof; j++) matCols[colOff++] = off + j;
        }
      }
      if (classId == PETSCFE_CLASSID) {
        PetscFE              fe = (PetscFE)disc;
        PetscInt             fSize;
        const PetscInt    ***perms;
        const PetscScalar ***flips;
        const PetscInt      *pperms;

        PetscCall(PetscFEGetDualSpace(fe, &dsp));
        PetscCall(PetscDualSpaceGetDimension(dsp, &fSize));
        PetscCall(PetscDualSpaceGetSymmetries(dsp, &perms, &flips));
        pperms = perms ? perms[pI] ? perms[pI][pO] : NULL : NULL;
        for (i = 0; i < numSelfDof; i++) { /* for every shape function */
          PetscQuadrature  q;
          PetscInt         dim, thisNc, numPoints, j, k;
          const PetscReal *points;
          const PetscReal *weights;
          PetscInt        *closure = NULL;
          PetscInt         numClosure;
          PetscInt         iCell              = pperms ? pperms[i] : i;
          PetscInt         parentCellShapeDof = cellShapeOff + iCell;
          PetscTabulation  Tparent;

          PetscCall(PetscDualSpaceGetFunctional(dsp, parentCellShapeDof, &q));
          PetscCall(PetscQuadratureGetData(q, &dim, &thisNc, &numPoints, &points, &weights));
          PetscCheck(thisNc == Nc, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Functional dim %" PetscInt_FMT " does not much basis dim %" PetscInt_FMT, thisNc, Nc);
          PetscCall(PetscFECreateTabulation(fe, 1, numPoints, points, 0, &Tparent)); /* I'm expecting a nodal basis: weights[:]' * Bparent[:,cellShapeDof] = 1. */
          for (j = 0; j < numPoints; j++) {
            PetscInt           childCell = -1;
            PetscReal         *parentValAtPoint;
            const PetscReal    xi0[3]    = {-1., -1., -1.};
            const PetscReal   *pointReal = &points[dim * j];
            const PetscScalar *point;
            PetscTabulation    Tchild;
            PetscInt           childCellShapeOff, pointMatOff;
#if defined(PETSC_USE_COMPLEX)
            PetscInt d;

            for (d = 0; d < dim; d++) pointScalar[d] = points[dim * j + d];
            point = pointScalar;
#else
            point = pointReal;
#endif

            parentValAtPoint = &Tparent->T[0][(fSize * j + parentCellShapeDof) * Nc];

            for (k = 0; k < numChildren; k++) { /* locate the point in a child's star cell*/
              PetscInt  child = children[k];
              PetscInt *star  = NULL;
              PetscInt  numStar, s;

              PetscCall(DMPlexGetTransitiveClosure(refTree, child, PETSC_FALSE, &numStar, &star));
              for (s = numStar - 1; s >= 0; s--) {
                PetscInt c = star[2 * s];

                if (c < cStart || c >= cEnd) continue;
                PetscCall(DMPlexLocatePoint_Internal(refTree, dim, point, c, &childCell));
                if (childCell >= 0) break;
              }
              PetscCall(DMPlexRestoreTransitiveClosure(refTree, child, PETSC_FALSE, &numStar, &star));
              if (childCell >= 0) break;
            }
            PetscCheck(childCell >= 0, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Could not locate quadrature point");
            PetscCall(DMPlexComputeCellGeometryFEM(refTree, childCell, NULL, v0, J, invJ, &detJ));
            PetscCall(DMPlexComputeCellGeometryFEM(refTree, parentCell, NULL, v0parent, Jparent, NULL, &detJparent));
            CoordinatesRefToReal(dim, dim, xi0, v0parent, Jparent, pointReal, vtmp);
            CoordinatesRealToRef(dim, dim, xi0, v0, invJ, vtmp, pointRef);

            PetscCall(PetscFECreateTabulation(fe, 1, 1, pointRef, 0, &Tchild));
            PetscCall(DMPlexGetTransitiveClosure(refTree, childCell, PETSC_TRUE, &numClosure, &closure));
            for (k = 0, pointMatOff = 0; k < numChildren; k++) { /* point is located in cell => child dofs support at point are in closure of cell */
              PetscInt        child = children[k], childDepth, childDof, childO = PETSC_INT_MIN;
              PetscInt        l;
              const PetscInt *cperms;

              PetscCall(DMLabelGetValue(depth, child, &childDepth));
              childDof = depthNumDof[childDepth];
              for (l = 0, cI = -1, childCellShapeOff = 0; l < numClosure; l++) {
                PetscInt point = closure[2 * l];
                PetscInt pointDepth;

                childO = closure[2 * l + 1];
                if (point == child) {
                  cI = l;
                  break;
                }
                PetscCall(DMLabelGetValue(depth, point, &pointDepth));
                childCellShapeOff += depthNumDof[pointDepth];
              }
              if (l == numClosure) {
                pointMatOff += childDof;
                continue; /* child is not in the closure of the cell: has nothing to contribute to this point */
              }
              cperms = perms ? perms[cI] ? perms[cI][childO] : NULL : NULL;
              for (l = 0; l < childDof; l++) {
                PetscInt   lCell        = cperms ? cperms[l] : l;
                PetscInt   childCellDof = childCellShapeOff + lCell;
                PetscReal *childValAtPoint;
                PetscReal  val = 0.;

                childValAtPoint = &Tchild->T[0][childCellDof * Nc];
                for (m = 0; m < Nc; m++) val += weights[j * Nc + m] * parentValAtPoint[m] * childValAtPoint[m];

                pointMat[i * numChildDof + pointMatOff + l] += val;
              }
              pointMatOff += childDof;
            }
            PetscCall(DMPlexRestoreTransitiveClosure(refTree, childCell, PETSC_TRUE, &numClosure, &closure));
            PetscCall(PetscTabulationDestroy(&Tchild));
          }
          PetscCall(PetscTabulationDestroy(&Tparent));
        }
      } else { /* just the volume-weighted averages of the children */
        PetscReal parentVol;
        PetscInt  childCell;

        PetscCall(DMPlexComputeCellGeometryFVM(refTree, p, &parentVol, NULL, NULL));
        for (i = 0, childCell = 0; i < numChildren; i++) {
          PetscInt  child = children[i], j;
          PetscReal childVol;

          if (child < cStart || child >= cEnd) continue;
          PetscCall(DMPlexComputeCellGeometryFVM(refTree, child, &childVol, NULL, NULL));
          for (j = 0; j < Nc; j++) pointMat[j * numChildDof + Nc * childCell + j] = childVol / parentVol;
          childCell++;
        }
      }
      /* Insert pointMat into mat */
      PetscCall(MatSetValues(mat, numSelfDof, matRows, numChildDof, matCols, pointMat, INSERT_VALUES));
      PetscCall(DMRestoreWorkArray(refTree, numSelfDof + numChildDof, MPIU_INT, &matRows));
      PetscCall(DMRestoreWorkArray(refTree, numSelfDof * numChildDof, MPIU_SCALAR, &pointMat));
    }
  }
  PetscCall(PetscFree6(v0, v0parent, vtmp, J, Jparent, invJ));
  PetscCall(PetscFree2(pointScalar, pointRef));
  PetscCall(MatAssemblyBegin(mat, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(mat, MAT_FINAL_ASSEMBLY));
  *inj = mat;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexReferenceTreeGetChildrenMatrices_Injection(DM refTree, Mat inj, PetscScalar ****childrenMats)
{
  PetscDS        ds;
  PetscInt       numFields, f, pRefStart, pRefEnd, p, *rows, *cols, maxDof;
  PetscScalar ***refPointFieldMats;
  PetscSection   refConSec, refSection;

  PetscFunctionBegin;
  PetscCall(DMGetDS(refTree, &ds));
  PetscCall(PetscDSGetNumFields(ds, &numFields));
  PetscCall(DMGetDefaultConstraints(refTree, &refConSec, NULL, NULL));
  PetscCall(DMGetLocalSection(refTree, &refSection));
  PetscCall(PetscSectionGetChart(refConSec, &pRefStart, &pRefEnd));
  PetscCall(PetscMalloc1(pRefEnd - pRefStart, &refPointFieldMats));
  PetscCall(PetscSectionGetMaxDof(refConSec, &maxDof));
  PetscCall(PetscMalloc1(maxDof, &rows));
  PetscCall(PetscMalloc1(maxDof * maxDof, &cols));
  for (p = pRefStart; p < pRefEnd; p++) {
    PetscInt parent, pDof, parentDof;

    PetscCall(DMPlexGetTreeParent(refTree, p, &parent, NULL));
    PetscCall(PetscSectionGetDof(refConSec, p, &pDof));
    PetscCall(PetscSectionGetDof(refSection, parent, &parentDof));
    if (!pDof || !parentDof || parent == p) continue;

    PetscCall(PetscMalloc1(numFields, &refPointFieldMats[p - pRefStart]));
    for (f = 0; f < numFields; f++) {
      PetscInt cDof, cOff, numCols, r;

      if (numFields > 1) {
        PetscCall(PetscSectionGetFieldDof(refConSec, p, f, &cDof));
        PetscCall(PetscSectionGetFieldOffset(refConSec, p, f, &cOff));
      } else {
        PetscCall(PetscSectionGetDof(refConSec, p, &cDof));
        PetscCall(PetscSectionGetOffset(refConSec, p, &cOff));
      }

      for (r = 0; r < cDof; r++) rows[r] = cOff + r;
      numCols = 0;
      {
        PetscInt aDof, aOff, j;

        if (numFields > 1) {
          PetscCall(PetscSectionGetFieldDof(refSection, parent, f, &aDof));
          PetscCall(PetscSectionGetFieldOffset(refSection, parent, f, &aOff));
        } else {
          PetscCall(PetscSectionGetDof(refSection, parent, &aDof));
          PetscCall(PetscSectionGetOffset(refSection, parent, &aOff));
        }

        for (j = 0; j < aDof; j++) cols[numCols++] = aOff + j;
      }
      PetscCall(PetscMalloc1(cDof * numCols, &refPointFieldMats[p - pRefStart][f]));
      /* transpose of constraint matrix */
      PetscCall(MatGetValues(inj, numCols, cols, cDof, rows, refPointFieldMats[p - pRefStart][f]));
    }
  }
  *childrenMats = refPointFieldMats;
  PetscCall(PetscFree(rows));
  PetscCall(PetscFree(cols));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexReferenceTreeRestoreChildrenMatrices_Injection(DM refTree, Mat inj, PetscScalar ****childrenMats)
{
  PetscDS        ds;
  PetscScalar ***refPointFieldMats;
  PetscInt       numFields, pRefStart, pRefEnd, p, f;
  PetscSection   refConSec, refSection;

  PetscFunctionBegin;
  refPointFieldMats = *childrenMats;
  *childrenMats     = NULL;
  PetscCall(DMGetDS(refTree, &ds));
  PetscCall(DMGetLocalSection(refTree, &refSection));
  PetscCall(PetscDSGetNumFields(ds, &numFields));
  PetscCall(DMGetDefaultConstraints(refTree, &refConSec, NULL, NULL));
  PetscCall(PetscSectionGetChart(refConSec, &pRefStart, &pRefEnd));
  for (p = pRefStart; p < pRefEnd; p++) {
    PetscInt parent, pDof, parentDof;

    PetscCall(DMPlexGetTreeParent(refTree, p, &parent, NULL));
    PetscCall(PetscSectionGetDof(refConSec, p, &pDof));
    PetscCall(PetscSectionGetDof(refSection, parent, &parentDof));
    if (!pDof || !parentDof || parent == p) continue;

    for (f = 0; f < numFields; f++) {
      PetscInt cDof;

      if (numFields > 1) {
        PetscCall(PetscSectionGetFieldDof(refConSec, p, f, &cDof));
      } else {
        PetscCall(PetscSectionGetDof(refConSec, p, &cDof));
      }

      PetscCall(PetscFree(refPointFieldMats[p - pRefStart][f]));
    }
    PetscCall(PetscFree(refPointFieldMats[p - pRefStart]));
  }
  PetscCall(PetscFree(refPointFieldMats));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexReferenceTreeGetInjector(DM refTree, Mat *injRef)
{
  Mat         cMatRef;
  PetscObject injRefObj;

  PetscFunctionBegin;
  PetscCall(DMGetDefaultConstraints(refTree, NULL, &cMatRef, NULL));
  PetscCall(PetscObjectQuery((PetscObject)cMatRef, "DMPlexComputeInjectorTree_refTree", &injRefObj));
  *injRef = (Mat)injRefObj;
  if (!*injRef) {
    PetscCall(DMPlexComputeInjectorReferenceTree(refTree, injRef));
    PetscCall(PetscObjectCompose((PetscObject)cMatRef, "DMPlexComputeInjectorTree_refTree", (PetscObject)*injRef));
    /* there is now a reference in cMatRef, which should be the only one for symmetry with the above case */
    PetscCall(PetscObjectDereference((PetscObject)*injRef));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexTransferInjectorTree(DM coarse, DM fine, PetscSF coarseToFine, const PetscInt *childIds, Vec fineVec, PetscInt numFields, PetscInt *offsets, PetscSection *rootMultiSec, PetscSection *multiLeafSec, PetscInt **gatheredIndices, PetscScalar **gatheredValues)
{
  PetscInt        pStartF, pEndF, pStartC, pEndC, p, maxDof, numMulti;
  PetscSection    globalCoarse, globalFine;
  PetscSection    localCoarse, localFine, leafIndicesSec;
  PetscSection    multiRootSec, rootIndicesSec;
  PetscInt       *leafInds, *rootInds = NULL;
  const PetscInt *rootDegrees;
  PetscScalar    *leafVals = NULL, *rootVals = NULL;
  PetscSF         coarseToFineEmbedded;

  PetscFunctionBegin;
  PetscCall(DMPlexGetChart(coarse, &pStartC, &pEndC));
  PetscCall(DMPlexGetChart(fine, &pStartF, &pEndF));
  PetscCall(DMGetLocalSection(fine, &localFine));
  PetscCall(DMGetGlobalSection(fine, &globalFine));
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)fine), &leafIndicesSec));
  PetscCall(PetscSectionSetChart(leafIndicesSec, pStartF, pEndF));
  PetscCall(PetscSectionGetMaxDof(localFine, &maxDof));
  { /* winnow fine points that don't have global dofs out of the sf */
    PetscInt        l, nleaves, dof, cdof, numPointsWithDofs, offset, *pointsWithDofs, numIndices;
    const PetscInt *leaves;

    PetscCall(PetscSFGetGraph(coarseToFine, NULL, &nleaves, &leaves, NULL));
    for (l = 0, numPointsWithDofs = 0; l < nleaves; l++) {
      p = leaves ? leaves[l] : l;
      PetscCall(PetscSectionGetDof(globalFine, p, &dof));
      PetscCall(PetscSectionGetConstraintDof(globalFine, p, &cdof));
      if ((dof - cdof) > 0) {
        numPointsWithDofs++;

        PetscCall(PetscSectionGetDof(localFine, p, &dof));
        PetscCall(PetscSectionSetDof(leafIndicesSec, p, dof + 1));
      }
    }
    PetscCall(PetscMalloc1(numPointsWithDofs, &pointsWithDofs));
    PetscCall(PetscSectionSetUp(leafIndicesSec));
    PetscCall(PetscSectionGetStorageSize(leafIndicesSec, &numIndices));
    PetscCall(PetscMalloc1((gatheredIndices ? numIndices : (maxDof + 1)), &leafInds));
    if (gatheredValues) PetscCall(PetscMalloc1(numIndices, &leafVals));
    for (l = 0, offset = 0; l < nleaves; l++) {
      p = leaves ? leaves[l] : l;
      PetscCall(PetscSectionGetDof(globalFine, p, &dof));
      PetscCall(PetscSectionGetConstraintDof(globalFine, p, &cdof));
      if ((dof - cdof) > 0) {
        PetscInt     off, gOff;
        PetscInt    *pInd;
        PetscScalar *pVal = NULL;

        pointsWithDofs[offset++] = l;

        PetscCall(PetscSectionGetOffset(leafIndicesSec, p, &off));

        pInd = gatheredIndices ? (&leafInds[off + 1]) : leafInds;
        if (gatheredValues) {
          PetscInt i;

          pVal = &leafVals[off + 1];
          for (i = 0; i < dof; i++) pVal[i] = 0.;
        }
        PetscCall(PetscSectionGetOffset(globalFine, p, &gOff));

        offsets[0] = 0;
        if (numFields) {
          PetscInt f;

          for (f = 0; f < numFields; f++) {
            PetscInt fDof;
            PetscCall(PetscSectionGetFieldDof(localFine, p, f, &fDof));
            offsets[f + 1] = fDof + offsets[f];
          }
          PetscCall(DMPlexGetIndicesPointFields_Internal(localFine, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsets, PETSC_FALSE, NULL, -1, NULL, pInd));
        } else {
          PetscCall(DMPlexGetIndicesPoint_Internal(localFine, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsets, PETSC_FALSE, NULL, NULL, pInd));
        }
        if (gatheredValues) PetscCall(VecGetValues(fineVec, dof, pInd, pVal));
      }
    }
    PetscCall(PetscSFCreateEmbeddedLeafSF(coarseToFine, numPointsWithDofs, pointsWithDofs, &coarseToFineEmbedded));
    PetscCall(PetscFree(pointsWithDofs));
  }

  PetscCall(DMPlexGetChart(coarse, &pStartC, &pEndC));
  PetscCall(DMGetLocalSection(coarse, &localCoarse));
  PetscCall(DMGetGlobalSection(coarse, &globalCoarse));

  { /* there may be the case where an sf root has a parent: broadcast parents back to children */
    MPI_Datatype threeInt;
    PetscMPIInt  rank;
    PetscInt (*parentNodeAndIdCoarse)[3];
    PetscInt (*parentNodeAndIdFine)[3];
    PetscInt           p, nleaves, nleavesToParents;
    PetscSF            pointSF, sfToParents;
    const PetscInt    *ilocal;
    const PetscSFNode *iremote;
    PetscSFNode       *iremoteToParents;
    PetscInt          *ilocalToParents;

    PetscCallMPI(MPI_Comm_rank(PetscObjectComm((PetscObject)coarse), &rank));
    PetscCallMPI(MPI_Type_contiguous(3, MPIU_INT, &threeInt));
    PetscCallMPI(MPI_Type_commit(&threeInt));
    PetscCall(PetscMalloc2(pEndC - pStartC, &parentNodeAndIdCoarse, pEndF - pStartF, &parentNodeAndIdFine));
    PetscCall(DMGetPointSF(coarse, &pointSF));
    PetscCall(PetscSFGetGraph(pointSF, NULL, &nleaves, &ilocal, &iremote));
    for (p = pStartC; p < pEndC; p++) {
      PetscInt parent, childId;
      PetscCall(DMPlexGetTreeParent(coarse, p, &parent, &childId));
      parentNodeAndIdCoarse[p - pStartC][0] = rank;
      parentNodeAndIdCoarse[p - pStartC][1] = parent - pStartC;
      parentNodeAndIdCoarse[p - pStartC][2] = (p == parent) ? -1 : childId;
      if (nleaves > 0) {
        PetscInt leaf = -1;

        if (ilocal) {
          PetscCall(PetscFindInt(parent, nleaves, ilocal, &leaf));
        } else {
          leaf = p - pStartC;
        }
        if (leaf >= 0) {
          parentNodeAndIdCoarse[p - pStartC][0] = iremote[leaf].rank;
          parentNodeAndIdCoarse[p - pStartC][1] = iremote[leaf].index;
        }
      }
    }
    for (p = pStartF; p < pEndF; p++) {
      parentNodeAndIdFine[p - pStartF][0] = -1;
      parentNodeAndIdFine[p - pStartF][1] = -1;
      parentNodeAndIdFine[p - pStartF][2] = -1;
    }
    PetscCall(PetscSFBcastBegin(coarseToFineEmbedded, threeInt, parentNodeAndIdCoarse, parentNodeAndIdFine, MPI_REPLACE));
    PetscCall(PetscSFBcastEnd(coarseToFineEmbedded, threeInt, parentNodeAndIdCoarse, parentNodeAndIdFine, MPI_REPLACE));
    for (p = pStartF, nleavesToParents = 0; p < pEndF; p++) {
      PetscInt dof;

      PetscCall(PetscSectionGetDof(leafIndicesSec, p, &dof));
      if (dof) {
        PetscInt off;

        PetscCall(PetscSectionGetOffset(leafIndicesSec, p, &off));
        if (gatheredIndices) {
          leafInds[off] = PetscMax(childIds[p - pStartF], parentNodeAndIdFine[p - pStartF][2]);
        } else if (gatheredValues) {
          leafVals[off] = (PetscScalar)PetscMax(childIds[p - pStartF], parentNodeAndIdFine[p - pStartF][2]);
        }
      }
      if (parentNodeAndIdFine[p - pStartF][0] >= 0) nleavesToParents++;
    }
    PetscCall(PetscMalloc1(nleavesToParents, &ilocalToParents));
    PetscCall(PetscMalloc1(nleavesToParents, &iremoteToParents));
    for (p = pStartF, nleavesToParents = 0; p < pEndF; p++) {
      if (parentNodeAndIdFine[p - pStartF][0] >= 0) {
        ilocalToParents[nleavesToParents] = p - pStartF;
        // FIXME PetscCall(PetscMPIIntCast(parentNodeAndIdFine[p - pStartF][0],&iremoteToParents[nleavesToParents].rank));
        iremoteToParents[nleavesToParents].rank  = parentNodeAndIdFine[p - pStartF][0];
        iremoteToParents[nleavesToParents].index = parentNodeAndIdFine[p - pStartF][1];
        nleavesToParents++;
      }
    }
    PetscCall(PetscSFCreate(PetscObjectComm((PetscObject)coarse), &sfToParents));
    PetscCall(PetscSFSetGraph(sfToParents, pEndC - pStartC, nleavesToParents, ilocalToParents, PETSC_OWN_POINTER, iremoteToParents, PETSC_OWN_POINTER));
    PetscCall(PetscSFDestroy(&coarseToFineEmbedded));

    coarseToFineEmbedded = sfToParents;

    PetscCall(PetscFree2(parentNodeAndIdCoarse, parentNodeAndIdFine));
    PetscCallMPI(MPI_Type_free(&threeInt));
  }

  { /* winnow out coarse points that don't have dofs */
    PetscInt dof, cdof, numPointsWithDofs, offset, *pointsWithDofs;
    PetscSF  sfDofsOnly;

    for (p = pStartC, numPointsWithDofs = 0; p < pEndC; p++) {
      PetscCall(PetscSectionGetDof(globalCoarse, p, &dof));
      PetscCall(PetscSectionGetConstraintDof(globalCoarse, p, &cdof));
      if ((dof - cdof) > 0) numPointsWithDofs++;
    }
    PetscCall(PetscMalloc1(numPointsWithDofs, &pointsWithDofs));
    for (p = pStartC, offset = 0; p < pEndC; p++) {
      PetscCall(PetscSectionGetDof(globalCoarse, p, &dof));
      PetscCall(PetscSectionGetConstraintDof(globalCoarse, p, &cdof));
      if ((dof - cdof) > 0) pointsWithDofs[offset++] = p - pStartC;
    }
    PetscCall(PetscSFCreateEmbeddedRootSF(coarseToFineEmbedded, numPointsWithDofs, pointsWithDofs, &sfDofsOnly));
    PetscCall(PetscSFDestroy(&coarseToFineEmbedded));
    PetscCall(PetscFree(pointsWithDofs));
    coarseToFineEmbedded = sfDofsOnly;
  }

  /* communicate back to the coarse mesh which coarse points have children (that may require injection) */
  PetscCall(PetscSFComputeDegreeBegin(coarseToFineEmbedded, &rootDegrees));
  PetscCall(PetscSFComputeDegreeEnd(coarseToFineEmbedded, &rootDegrees));
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)coarse), &multiRootSec));
  PetscCall(PetscSectionSetChart(multiRootSec, pStartC, pEndC));
  for (p = pStartC; p < pEndC; p++) PetscCall(PetscSectionSetDof(multiRootSec, p, rootDegrees[p - pStartC]));
  PetscCall(PetscSectionSetUp(multiRootSec));
  PetscCall(PetscSectionGetStorageSize(multiRootSec, &numMulti));
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)coarse), &rootIndicesSec));
  { /* distribute the leaf section */
    PetscSF   multi, multiInv, indicesSF;
    PetscInt *remoteOffsets, numRootIndices;

    PetscCall(PetscSFGetMultiSF(coarseToFineEmbedded, &multi));
    PetscCall(PetscSFCreateInverseSF(multi, &multiInv));
    PetscCall(PetscSFDistributeSection(multiInv, leafIndicesSec, &remoteOffsets, rootIndicesSec));
    PetscCall(PetscSFCreateSectionSF(multiInv, leafIndicesSec, remoteOffsets, rootIndicesSec, &indicesSF));
    PetscCall(PetscFree(remoteOffsets));
    PetscCall(PetscSFDestroy(&multiInv));
    PetscCall(PetscSectionGetStorageSize(rootIndicesSec, &numRootIndices));
    if (gatheredIndices) {
      PetscCall(PetscMalloc1(numRootIndices, &rootInds));
      PetscCall(PetscSFBcastBegin(indicesSF, MPIU_INT, leafInds, rootInds, MPI_REPLACE));
      PetscCall(PetscSFBcastEnd(indicesSF, MPIU_INT, leafInds, rootInds, MPI_REPLACE));
    }
    if (gatheredValues) {
      PetscCall(PetscMalloc1(numRootIndices, &rootVals));
      PetscCall(PetscSFBcastBegin(indicesSF, MPIU_SCALAR, leafVals, rootVals, MPI_REPLACE));
      PetscCall(PetscSFBcastEnd(indicesSF, MPIU_SCALAR, leafVals, rootVals, MPI_REPLACE));
    }
    PetscCall(PetscSFDestroy(&indicesSF));
  }
  PetscCall(PetscSectionDestroy(&leafIndicesSec));
  PetscCall(PetscFree(leafInds));
  PetscCall(PetscFree(leafVals));
  PetscCall(PetscSFDestroy(&coarseToFineEmbedded));
  *rootMultiSec = multiRootSec;
  *multiLeafSec = rootIndicesSec;
  if (gatheredIndices) *gatheredIndices = rootInds;
  if (gatheredValues) *gatheredValues = rootVals;
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode DMPlexComputeInjectorTree(DM coarse, DM fine, PetscSF coarseToFine, PetscInt *childIds, Mat mat)
{
  DM             refTree;
  PetscSection   multiRootSec, rootIndicesSec;
  PetscSection   globalCoarse, globalFine;
  PetscSection   localCoarse, localFine;
  PetscSection   cSecRef;
  PetscInt      *rootIndices = NULL, *parentIndices, pRefStart, pRefEnd;
  Mat            injRef;
  PetscInt       numFields, maxDof;
  PetscInt       pStartC, pEndC, pStartF, pEndF, p;
  PetscInt      *offsets, *offsetsCopy, *rowOffsets;
  PetscLayout    rowMap, colMap;
  PetscInt       rowStart, rowEnd, colStart, colEnd, *nnzD, *nnzO;
  PetscScalar ***childrenMats = NULL; /* gcc -O gives 'may be used uninitialized' warning'. Initializing to suppress this warning */

  PetscFunctionBegin;
  /* get the templates for the fine-to-coarse injection from the reference tree */
  PetscCall(DMPlexGetReferenceTree(coarse, &refTree));
  PetscCall(DMCopyDisc(coarse, refTree));
  PetscCall(DMSetLocalSection(refTree, NULL));
  PetscCall(DMSetDefaultConstraints(refTree, NULL, NULL, NULL));
  PetscCall(DMGetDefaultConstraints(refTree, &cSecRef, NULL, NULL));
  PetscCall(PetscSectionGetChart(cSecRef, &pRefStart, &pRefEnd));
  PetscCall(DMPlexReferenceTreeGetInjector(refTree, &injRef));

  PetscCall(DMPlexGetChart(fine, &pStartF, &pEndF));
  PetscCall(DMGetLocalSection(fine, &localFine));
  PetscCall(DMGetGlobalSection(fine, &globalFine));
  PetscCall(PetscSectionGetNumFields(localFine, &numFields));
  PetscCall(DMPlexGetChart(coarse, &pStartC, &pEndC));
  PetscCall(DMGetLocalSection(coarse, &localCoarse));
  PetscCall(DMGetGlobalSection(coarse, &globalCoarse));
  PetscCall(PetscSectionGetMaxDof(localCoarse, &maxDof));
  {
    PetscInt maxFields = PetscMax(1, numFields) + 1;
    PetscCall(PetscMalloc3(maxFields, &offsets, maxFields, &offsetsCopy, maxFields, &rowOffsets));
  }

  PetscCall(DMPlexTransferInjectorTree(coarse, fine, coarseToFine, childIds, NULL, numFields, offsets, &multiRootSec, &rootIndicesSec, &rootIndices, NULL));

  PetscCall(PetscMalloc1(maxDof, &parentIndices));

  /* count indices */
  PetscCall(MatGetLayouts(mat, &rowMap, &colMap));
  PetscCall(PetscLayoutSetUp(rowMap));
  PetscCall(PetscLayoutSetUp(colMap));
  PetscCall(PetscLayoutGetRange(rowMap, &rowStart, &rowEnd));
  PetscCall(PetscLayoutGetRange(colMap, &colStart, &colEnd));
  PetscCall(PetscCalloc2(rowEnd - rowStart, &nnzD, rowEnd - rowStart, &nnzO));
  for (p = pStartC; p < pEndC; p++) {
    PetscInt numLeaves, leafStart, leafEnd, l, dof, cdof, gOff;

    PetscCall(PetscSectionGetDof(globalCoarse, p, &dof));
    PetscCall(PetscSectionGetConstraintDof(globalCoarse, p, &cdof));
    if ((dof - cdof) <= 0) continue;
    PetscCall(PetscSectionGetOffset(globalCoarse, p, &gOff));

    rowOffsets[0]  = 0;
    offsetsCopy[0] = 0;
    if (numFields) {
      PetscInt f;

      for (f = 0; f < numFields; f++) {
        PetscInt fDof;
        PetscCall(PetscSectionGetFieldDof(localCoarse, p, f, &fDof));
        rowOffsets[f + 1] = offsetsCopy[f + 1] = fDof + rowOffsets[f];
      }
      PetscCall(DMPlexGetIndicesPointFields_Internal(localCoarse, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsetsCopy, PETSC_FALSE, NULL, -1, NULL, parentIndices));
    } else {
      PetscCall(DMPlexGetIndicesPoint_Internal(localCoarse, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsetsCopy, PETSC_FALSE, NULL, NULL, parentIndices));
      rowOffsets[1] = offsetsCopy[0];
    }

    PetscCall(PetscSectionGetDof(multiRootSec, p, &numLeaves));
    PetscCall(PetscSectionGetOffset(multiRootSec, p, &leafStart));
    leafEnd = leafStart + numLeaves;
    for (l = leafStart; l < leafEnd; l++) {
      PetscInt        numIndices, childId, offset;
      const PetscInt *childIndices;

      PetscCall(PetscSectionGetDof(rootIndicesSec, l, &numIndices));
      PetscCall(PetscSectionGetOffset(rootIndicesSec, l, &offset));
      childId      = rootIndices[offset++];
      childIndices = &rootIndices[offset];
      numIndices--;

      if (childId == -1) { /* equivalent points: scatter */
        PetscInt i;

        for (i = 0; i < numIndices; i++) {
          PetscInt colIndex = childIndices[i];
          PetscInt rowIndex = parentIndices[i];
          if (rowIndex < 0) continue;
          PetscCheck(colIndex >= 0, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Unconstrained fine and constrained coarse");
          if (colIndex >= colStart && colIndex < colEnd) {
            nnzD[rowIndex - rowStart] = 1;
          } else {
            nnzO[rowIndex - rowStart] = 1;
          }
        }
      } else {
        PetscInt parentId, f, lim;

        PetscCall(DMPlexGetTreeParent(refTree, childId, &parentId, NULL));

        lim        = PetscMax(1, numFields);
        offsets[0] = 0;
        if (numFields) {
          PetscInt f;

          for (f = 0; f < numFields; f++) {
            PetscInt fDof;
            PetscCall(PetscSectionGetFieldDof(cSecRef, childId, f, &fDof));

            offsets[f + 1] = fDof + offsets[f];
          }
        } else {
          PetscInt cDof;

          PetscCall(PetscSectionGetDof(cSecRef, childId, &cDof));
          offsets[1] = cDof;
        }
        for (f = 0; f < lim; f++) {
          PetscInt parentStart = rowOffsets[f], parentEnd = rowOffsets[f + 1];
          PetscInt childStart = offsets[f], childEnd = offsets[f + 1];
          PetscInt i, numD = 0, numO = 0;

          for (i = childStart; i < childEnd; i++) {
            PetscInt colIndex = childIndices[i];

            if (colIndex < 0) continue;
            if (colIndex >= colStart && colIndex < colEnd) {
              numD++;
            } else {
              numO++;
            }
          }
          for (i = parentStart; i < parentEnd; i++) {
            PetscInt rowIndex = parentIndices[i];

            if (rowIndex < 0) continue;
            nnzD[rowIndex - rowStart] += numD;
            nnzO[rowIndex - rowStart] += numO;
          }
        }
      }
    }
  }
  /* preallocate */
  PetscCall(MatXAIJSetPreallocation(mat, 1, nnzD, nnzO, NULL, NULL));
  PetscCall(PetscFree2(nnzD, nnzO));
  /* insert values */
  PetscCall(DMPlexReferenceTreeGetChildrenMatrices_Injection(refTree, injRef, &childrenMats));
  for (p = pStartC; p < pEndC; p++) {
    PetscInt numLeaves, leafStart, leafEnd, l, dof, cdof, gOff;

    PetscCall(PetscSectionGetDof(globalCoarse, p, &dof));
    PetscCall(PetscSectionGetConstraintDof(globalCoarse, p, &cdof));
    if ((dof - cdof) <= 0) continue;
    PetscCall(PetscSectionGetOffset(globalCoarse, p, &gOff));

    rowOffsets[0]  = 0;
    offsetsCopy[0] = 0;
    if (numFields) {
      PetscInt f;

      for (f = 0; f < numFields; f++) {
        PetscInt fDof;
        PetscCall(PetscSectionGetFieldDof(localCoarse, p, f, &fDof));
        rowOffsets[f + 1] = offsetsCopy[f + 1] = fDof + rowOffsets[f];
      }
      PetscCall(DMPlexGetIndicesPointFields_Internal(localCoarse, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsetsCopy, PETSC_FALSE, NULL, -1, NULL, parentIndices));
    } else {
      PetscCall(DMPlexGetIndicesPoint_Internal(localCoarse, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsetsCopy, PETSC_FALSE, NULL, NULL, parentIndices));
      rowOffsets[1] = offsetsCopy[0];
    }

    PetscCall(PetscSectionGetDof(multiRootSec, p, &numLeaves));
    PetscCall(PetscSectionGetOffset(multiRootSec, p, &leafStart));
    leafEnd = leafStart + numLeaves;
    for (l = leafStart; l < leafEnd; l++) {
      PetscInt        numIndices, childId, offset;
      const PetscInt *childIndices;

      PetscCall(PetscSectionGetDof(rootIndicesSec, l, &numIndices));
      PetscCall(PetscSectionGetOffset(rootIndicesSec, l, &offset));
      childId      = rootIndices[offset++];
      childIndices = &rootIndices[offset];
      numIndices--;

      if (childId == -1) { /* equivalent points: scatter */
        PetscInt i;

        for (i = 0; i < numIndices; i++) PetscCall(MatSetValue(mat, parentIndices[i], childIndices[i], 1., INSERT_VALUES));
      } else {
        PetscInt parentId, f, lim;

        PetscCall(DMPlexGetTreeParent(refTree, childId, &parentId, NULL));

        lim        = PetscMax(1, numFields);
        offsets[0] = 0;
        if (numFields) {
          PetscInt f;

          for (f = 0; f < numFields; f++) {
            PetscInt fDof;
            PetscCall(PetscSectionGetFieldDof(cSecRef, childId, f, &fDof));

            offsets[f + 1] = fDof + offsets[f];
          }
        } else {
          PetscInt cDof;

          PetscCall(PetscSectionGetDof(cSecRef, childId, &cDof));
          offsets[1] = cDof;
        }
        for (f = 0; f < lim; f++) {
          PetscScalar    *childMat   = &childrenMats[childId - pRefStart][f][0];
          PetscInt       *rowIndices = &parentIndices[rowOffsets[f]];
          const PetscInt *colIndices = &childIndices[offsets[f]];

          PetscCall(MatSetValues(mat, rowOffsets[f + 1] - rowOffsets[f], rowIndices, offsets[f + 1] - offsets[f], colIndices, childMat, INSERT_VALUES));
        }
      }
    }
  }
  PetscCall(PetscSectionDestroy(&multiRootSec));
  PetscCall(PetscSectionDestroy(&rootIndicesSec));
  PetscCall(PetscFree(parentIndices));
  PetscCall(DMPlexReferenceTreeRestoreChildrenMatrices_Injection(refTree, injRef, &childrenMats));
  PetscCall(PetscFree(rootIndices));
  PetscCall(PetscFree3(offsets, offsetsCopy, rowOffsets));

  PetscCall(MatAssemblyBegin(mat, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(mat, MAT_FINAL_ASSEMBLY));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexTransferVecTree_Interpolate(DM coarse, Vec vecCoarseLocal, DM fine, Vec vecFine, PetscSF coarseToFine, PetscInt *cids, Vec grad, Vec cellGeom)
{
  PetscSF            coarseToFineEmbedded;
  PetscSection       globalCoarse, globalFine;
  PetscSection       localCoarse, localFine;
  PetscSection       aSec, cSec;
  PetscSection       rootValuesSec;
  PetscSection       leafValuesSec;
  PetscScalar       *rootValues, *leafValues;
  IS                 aIS;
  const PetscInt    *anchors;
  Mat                cMat;
  PetscInt           numFields;
  PetscInt           pStartC, pEndC, pStartF, pEndF, p, cellStart, cellEnd;
  PetscInt           aStart, aEnd, cStart, cEnd;
  PetscInt          *maxChildIds;
  PetscInt          *offsets, *newOffsets, *offsetsCopy, *newOffsetsCopy, *rowOffsets, *numD, *numO;
  PetscFV            fv = NULL;
  PetscInt           dim, numFVcomps = -1, fvField = -1;
  DM                 cellDM = NULL, gradDM = NULL;
  const PetscScalar *cellGeomArray = NULL;
  const PetscScalar *gradArray     = NULL;

  PetscFunctionBegin;
  PetscCall(VecSetOption(vecFine, VEC_IGNORE_NEGATIVE_INDICES, PETSC_TRUE));
  PetscCall(DMPlexGetChart(coarse, &pStartC, &pEndC));
  PetscCall(DMPlexGetSimplexOrBoxCells(coarse, 0, &cellStart, &cellEnd));
  PetscCall(DMPlexGetChart(fine, &pStartF, &pEndF));
  PetscCall(DMGetGlobalSection(fine, &globalFine));
  PetscCall(DMGetCoordinateDim(coarse, &dim));
  { /* winnow fine points that don't have global dofs out of the sf */
    PetscInt        nleaves, l;
    const PetscInt *leaves;
    PetscInt        dof, cdof, numPointsWithDofs, offset, *pointsWithDofs;

    PetscCall(PetscSFGetGraph(coarseToFine, NULL, &nleaves, &leaves, NULL));

    for (l = 0, numPointsWithDofs = 0; l < nleaves; l++) {
      PetscInt p = leaves ? leaves[l] : l;

      PetscCall(PetscSectionGetDof(globalFine, p, &dof));
      PetscCall(PetscSectionGetConstraintDof(globalFine, p, &cdof));
      if ((dof - cdof) > 0) numPointsWithDofs++;
    }
    PetscCall(PetscMalloc1(numPointsWithDofs, &pointsWithDofs));
    for (l = 0, offset = 0; l < nleaves; l++) {
      PetscInt p = leaves ? leaves[l] : l;

      PetscCall(PetscSectionGetDof(globalFine, p, &dof));
      PetscCall(PetscSectionGetConstraintDof(globalFine, p, &cdof));
      if ((dof - cdof) > 0) pointsWithDofs[offset++] = l;
    }
    PetscCall(PetscSFCreateEmbeddedLeafSF(coarseToFine, numPointsWithDofs, pointsWithDofs, &coarseToFineEmbedded));
    PetscCall(PetscFree(pointsWithDofs));
  }
  /* communicate back to the coarse mesh which coarse points have children (that may require interpolation) */
  PetscCall(PetscMalloc1(pEndC - pStartC, &maxChildIds));
  for (p = pStartC; p < pEndC; p++) maxChildIds[p - pStartC] = -2;
  PetscCall(PetscSFReduceBegin(coarseToFineEmbedded, MPIU_INT, cids, maxChildIds, MPIU_MAX));
  PetscCall(PetscSFReduceEnd(coarseToFineEmbedded, MPIU_INT, cids, maxChildIds, MPIU_MAX));

  PetscCall(DMGetLocalSection(coarse, &localCoarse));
  PetscCall(DMGetGlobalSection(coarse, &globalCoarse));

  PetscCall(DMPlexGetAnchors(coarse, &aSec, &aIS));
  PetscCall(ISGetIndices(aIS, &anchors));
  PetscCall(PetscSectionGetChart(aSec, &aStart, &aEnd));

  PetscCall(DMGetDefaultConstraints(coarse, &cSec, &cMat, NULL));
  PetscCall(PetscSectionGetChart(cSec, &cStart, &cEnd));

  /* create sections that will send to children the indices and matrices they will need to construct the interpolator */
  PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)coarse), &rootValuesSec));
  PetscCall(PetscSectionSetChart(rootValuesSec, pStartC, pEndC));
  PetscCall(PetscSectionGetNumFields(localCoarse, &numFields));
  {
    PetscInt maxFields = PetscMax(1, numFields) + 1;
    PetscCall(PetscMalloc7(maxFields, &offsets, maxFields, &offsetsCopy, maxFields, &newOffsets, maxFields, &newOffsetsCopy, maxFields, &rowOffsets, maxFields, &numD, maxFields, &numO));
  }
  if (grad) {
    PetscInt i;

    PetscCall(VecGetDM(cellGeom, &cellDM));
    PetscCall(VecGetArrayRead(cellGeom, &cellGeomArray));
    PetscCall(VecGetDM(grad, &gradDM));
    PetscCall(VecGetArrayRead(grad, &gradArray));
    for (i = 0; i < PetscMax(1, numFields); i++) {
      PetscObject  obj;
      PetscClassId id;

      PetscCall(DMGetField(coarse, i, NULL, &obj));
      PetscCall(PetscObjectGetClassId(obj, &id));
      if (id == PETSCFV_CLASSID) {
        fv = (PetscFV)obj;
        PetscCall(PetscFVGetNumComponents(fv, &numFVcomps));
        fvField = i;
        break;
      }
    }
  }

  for (p = pStartC; p < pEndC; p++) { /* count the sizes of the indices and matrices */
    PetscInt dof;
    PetscInt maxChildId = maxChildIds[p - pStartC];
    PetscInt numValues  = 0;

    PetscCall(PetscSectionGetDof(globalCoarse, p, &dof));
    if (dof < 0) dof = -(dof + 1);
    offsets[0]    = 0;
    newOffsets[0] = 0;
    if (maxChildId >= 0) { /* this point has children (with dofs) that will need to be interpolated from the closure of p */
      PetscInt *closure = NULL, closureSize, cl;

      PetscCall(DMPlexGetTransitiveClosure(coarse, p, PETSC_TRUE, &closureSize, &closure));
      for (cl = 0; cl < closureSize; cl++) { /* get the closure */
        PetscInt c = closure[2 * cl], clDof;

        PetscCall(PetscSectionGetDof(localCoarse, c, &clDof));
        numValues += clDof;
      }
      PetscCall(DMPlexRestoreTransitiveClosure(coarse, p, PETSC_TRUE, &closureSize, &closure));
    } else if (maxChildId == -1) {
      PetscCall(PetscSectionGetDof(localCoarse, p, &numValues));
    }
    /* we will pack the column indices with the field offsets */
    if (maxChildId >= 0 && grad && p >= cellStart && p < cellEnd) {
      /* also send the centroid, and the gradient */
      numValues += dim * (1 + numFVcomps);
    }
    PetscCall(PetscSectionSetDof(rootValuesSec, p, numValues));
  }
  PetscCall(PetscSectionSetUp(rootValuesSec));
  {
    PetscInt           numRootValues;
    const PetscScalar *coarseArray;

    PetscCall(PetscSectionGetStorageSize(rootValuesSec, &numRootValues));
    PetscCall(PetscMalloc1(numRootValues, &rootValues));
    PetscCall(VecGetArrayRead(vecCoarseLocal, &coarseArray));
    for (p = pStartC; p < pEndC; p++) {
      PetscInt     numValues;
      PetscInt     pValOff;
      PetscScalar *pVal;
      PetscInt     maxChildId = maxChildIds[p - pStartC];

      PetscCall(PetscSectionGetDof(rootValuesSec, p, &numValues));
      if (!numValues) continue;
      PetscCall(PetscSectionGetOffset(rootValuesSec, p, &pValOff));
      pVal = &rootValues[pValOff];
      if (maxChildId >= 0) { /* build an identity matrix, apply matrix constraints on the right */
        PetscInt closureSize = numValues;
        PetscCall(DMPlexVecGetClosure(coarse, NULL, vecCoarseLocal, p, &closureSize, &pVal));
        if (grad && p >= cellStart && p < cellEnd) {
          PetscFVCellGeom *cg;
          PetscScalar     *gradVals = NULL;
          PetscInt         i;

          pVal += (numValues - dim * (1 + numFVcomps));

          PetscCall(DMPlexPointLocalRead(cellDM, p, cellGeomArray, (void *)&cg));
          for (i = 0; i < dim; i++) pVal[i] = cg->centroid[i];
          pVal += dim;
          PetscCall(DMPlexPointGlobalRead(gradDM, p, gradArray, (void *)&gradVals));
          for (i = 0; i < dim * numFVcomps; i++) pVal[i] = gradVals[i];
        }
      } else if (maxChildId == -1) {
        PetscInt lDof, lOff, i;

        PetscCall(PetscSectionGetDof(localCoarse, p, &lDof));
        PetscCall(PetscSectionGetOffset(localCoarse, p, &lOff));
        for (i = 0; i < lDof; i++) pVal[i] = coarseArray[lOff + i];
      }
    }
    PetscCall(VecRestoreArrayRead(vecCoarseLocal, &coarseArray));
    PetscCall(PetscFree(maxChildIds));
  }
  {
    PetscSF   valuesSF;
    PetscInt *remoteOffsetsValues, numLeafValues;

    PetscCall(PetscSectionCreate(PetscObjectComm((PetscObject)fine), &leafValuesSec));
    PetscCall(PetscSFDistributeSection(coarseToFineEmbedded, rootValuesSec, &remoteOffsetsValues, leafValuesSec));
    PetscCall(PetscSFCreateSectionSF(coarseToFineEmbedded, rootValuesSec, remoteOffsetsValues, leafValuesSec, &valuesSF));
    PetscCall(PetscSFDestroy(&coarseToFineEmbedded));
    PetscCall(PetscFree(remoteOffsetsValues));
    PetscCall(PetscSectionGetStorageSize(leafValuesSec, &numLeafValues));
    PetscCall(PetscMalloc1(numLeafValues, &leafValues));
    PetscCall(PetscSFBcastBegin(valuesSF, MPIU_SCALAR, rootValues, leafValues, MPI_REPLACE));
    PetscCall(PetscSFBcastEnd(valuesSF, MPIU_SCALAR, rootValues, leafValues, MPI_REPLACE));
    PetscCall(PetscSFDestroy(&valuesSF));
    PetscCall(PetscFree(rootValues));
    PetscCall(PetscSectionDestroy(&rootValuesSec));
  }
  PetscCall(DMGetLocalSection(fine, &localFine));
  {
    PetscInt       maxDof;
    PetscInt      *rowIndices;
    DM             refTree;
    PetscInt     **refPointFieldN;
    PetscScalar ***refPointFieldMats;
    PetscSection   refConSec, refAnSec;
    PetscInt       pRefStart, pRefEnd, leafStart, leafEnd;
    PetscScalar   *pointWork;

    PetscCall(PetscSectionGetMaxDof(localFine, &maxDof));
    PetscCall(DMGetWorkArray(fine, maxDof, MPIU_INT, &rowIndices));
    PetscCall(DMGetWorkArray(fine, maxDof, MPIU_SCALAR, &pointWork));
    PetscCall(DMPlexGetReferenceTree(fine, &refTree));
    PetscCall(DMCopyDisc(fine, refTree));
    PetscCall(DMPlexReferenceTreeGetChildrenMatrices(refTree, &refPointFieldMats, &refPointFieldN));
    PetscCall(DMGetDefaultConstraints(refTree, &refConSec, NULL, NULL));
    PetscCall(DMPlexGetAnchors(refTree, &refAnSec, NULL));
    PetscCall(PetscSectionGetChart(refConSec, &pRefStart, &pRefEnd));
    PetscCall(PetscSectionGetChart(leafValuesSec, &leafStart, &leafEnd));
    PetscCall(DMPlexGetSimplexOrBoxCells(fine, 0, &cellStart, &cellEnd));
    for (p = leafStart; p < leafEnd; p++) {
      PetscInt           gDof, gcDof, gOff, lDof;
      PetscInt           numValues, pValOff;
      PetscInt           childId;
      const PetscScalar *pVal;
      const PetscScalar *fvGradData = NULL;

      PetscCall(PetscSectionGetDof(globalFine, p, &gDof));
      PetscCall(PetscSectionGetDof(localFine, p, &lDof));
      PetscCall(PetscSectionGetConstraintDof(globalFine, p, &gcDof));
      if ((gDof - gcDof) <= 0) continue;
      PetscCall(PetscSectionGetOffset(globalFine, p, &gOff));
      PetscCall(PetscSectionGetDof(leafValuesSec, p, &numValues));
      if (!numValues) continue;
      PetscCall(PetscSectionGetOffset(leafValuesSec, p, &pValOff));
      pVal              = &leafValues[pValOff];
      offsets[0]        = 0;
      offsetsCopy[0]    = 0;
      newOffsets[0]     = 0;
      newOffsetsCopy[0] = 0;
      childId           = cids[p - pStartF];
      if (numFields) {
        PetscInt f;
        for (f = 0; f < numFields; f++) {
          PetscInt rowDof;

          PetscCall(PetscSectionGetFieldDof(localFine, p, f, &rowDof));
          offsets[f + 1]     = offsets[f] + rowDof;
          offsetsCopy[f + 1] = offsets[f + 1];
          /* TODO: closure indices */
          newOffsets[f + 1] = newOffsets[f] + ((childId == -1) ? rowDof : refPointFieldN[childId - pRefStart][f]);
        }
        PetscCall(DMPlexGetIndicesPointFields_Internal(localFine, PETSC_FALSE, p, gOff, offsetsCopy, PETSC_FALSE, NULL, -1, NULL, rowIndices));
      } else {
        offsets[0]    = 0;
        offsets[1]    = lDof;
        newOffsets[0] = 0;
        newOffsets[1] = (childId == -1) ? lDof : refPointFieldN[childId - pRefStart][0];
        PetscCall(DMPlexGetIndicesPoint_Internal(localFine, PETSC_FALSE, p, gOff, offsetsCopy, PETSC_FALSE, NULL, NULL, rowIndices));
      }
      if (childId == -1) { /* no child interpolation: one nnz per */
        PetscCall(VecSetValues(vecFine, numValues, rowIndices, pVal, INSERT_VALUES));
      } else {
        PetscInt f;

        if (grad && p >= cellStart && p < cellEnd) {
          numValues -= (dim * (1 + numFVcomps));
          fvGradData = &pVal[numValues];
        }
        for (f = 0; f < PetscMax(1, numFields); f++) {
          const PetscScalar *childMat = refPointFieldMats[childId - pRefStart][f];
          PetscInt           numRows  = offsets[f + 1] - offsets[f];
          PetscInt           numCols  = newOffsets[f + 1] - newOffsets[f];
          const PetscScalar *cVal     = &pVal[newOffsets[f]];
          PetscScalar       *rVal     = &pointWork[offsets[f]];
          PetscInt           i, j;

#if 0
          PetscCall(PetscInfo(coarse,"childId %" PetscInt_FMT ", numRows %" PetscInt_FMT ", numCols %" PetscInt_FMT ", refPointFieldN %" PetscInt_FMT " maxDof %" PetscInt_FMT "\n",childId,numRows,numCols,refPointFieldN[childId - pRefStart][f], maxDof));
#endif
          for (i = 0; i < numRows; i++) {
            PetscScalar val = 0.;
            for (j = 0; j < numCols; j++) val += childMat[i * numCols + j] * cVal[j];
            rVal[i] = val;
          }
          if (f == fvField && p >= cellStart && p < cellEnd) {
            PetscReal          centroid[3];
            PetscScalar        diff[3];
            const PetscScalar *parentCentroid = &fvGradData[0];
            const PetscScalar *gradient       = &fvGradData[dim];

            PetscCall(DMPlexComputeCellGeometryFVM(fine, p, NULL, centroid, NULL));
            for (i = 0; i < dim; i++) diff[i] = centroid[i] - parentCentroid[i];
            for (i = 0; i < numFVcomps; i++) {
              PetscScalar val = 0.;

              for (j = 0; j < dim; j++) val += gradient[dim * i + j] * diff[j];
              rVal[i] += val;
            }
          }
          PetscCall(VecSetValues(vecFine, numRows, &rowIndices[offsets[f]], rVal, INSERT_VALUES));
        }
      }
    }
    PetscCall(DMPlexReferenceTreeRestoreChildrenMatrices(refTree, &refPointFieldMats, &refPointFieldN));
    PetscCall(DMRestoreWorkArray(fine, maxDof, MPIU_SCALAR, &pointWork));
    PetscCall(DMRestoreWorkArray(fine, maxDof, MPIU_INT, &rowIndices));
  }
  PetscCall(PetscFree(leafValues));
  PetscCall(PetscSectionDestroy(&leafValuesSec));
  PetscCall(PetscFree7(offsets, offsetsCopy, newOffsets, newOffsetsCopy, rowOffsets, numD, numO));
  PetscCall(ISRestoreIndices(aIS, &anchors));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMPlexTransferVecTree_Inject(DM fine, Vec vecFine, DM coarse, Vec vecCoarse, PetscSF coarseToFine, PetscInt *cids)
{
  DM             refTree;
  PetscSection   multiRootSec, rootIndicesSec;
  PetscSection   globalCoarse, globalFine;
  PetscSection   localCoarse, localFine;
  PetscSection   cSecRef;
  PetscInt      *parentIndices, pRefStart, pRefEnd;
  PetscScalar   *rootValues, *parentValues;
  Mat            injRef;
  PetscInt       numFields, maxDof;
  PetscInt       pStartC, pEndC, pStartF, pEndF, p;
  PetscInt      *offsets, *offsetsCopy, *rowOffsets;
  PetscLayout    rowMap, colMap;
  PetscInt       rowStart, rowEnd, colStart, colEnd;
  PetscScalar ***childrenMats = NULL; /* gcc -O gives 'may be used uninitialized' warning'. Initializing to suppress this warning */

  PetscFunctionBegin;
  /* get the templates for the fine-to-coarse injection from the reference tree */
  PetscCall(VecSetOption(vecFine, VEC_IGNORE_NEGATIVE_INDICES, PETSC_TRUE));
  PetscCall(VecSetOption(vecCoarse, VEC_IGNORE_NEGATIVE_INDICES, PETSC_TRUE));
  PetscCall(DMPlexGetReferenceTree(coarse, &refTree));
  PetscCall(DMCopyDisc(coarse, refTree));
  PetscCall(DMGetDefaultConstraints(refTree, &cSecRef, NULL, NULL));
  PetscCall(PetscSectionGetChart(cSecRef, &pRefStart, &pRefEnd));
  PetscCall(DMPlexReferenceTreeGetInjector(refTree, &injRef));

  PetscCall(DMPlexGetChart(fine, &pStartF, &pEndF));
  PetscCall(DMGetLocalSection(fine, &localFine));
  PetscCall(DMGetGlobalSection(fine, &globalFine));
  PetscCall(PetscSectionGetNumFields(localFine, &numFields));
  PetscCall(DMPlexGetChart(coarse, &pStartC, &pEndC));
  PetscCall(DMGetLocalSection(coarse, &localCoarse));
  PetscCall(DMGetGlobalSection(coarse, &globalCoarse));
  PetscCall(PetscSectionGetMaxDof(localCoarse, &maxDof));
  {
    PetscInt maxFields = PetscMax(1, numFields) + 1;
    PetscCall(PetscMalloc3(maxFields, &offsets, maxFields, &offsetsCopy, maxFields, &rowOffsets));
  }

  PetscCall(DMPlexTransferInjectorTree(coarse, fine, coarseToFine, cids, vecFine, numFields, offsets, &multiRootSec, &rootIndicesSec, NULL, &rootValues));

  PetscCall(PetscMalloc2(maxDof, &parentIndices, maxDof, &parentValues));

  /* count indices */
  PetscCall(VecGetLayout(vecFine, &colMap));
  PetscCall(VecGetLayout(vecCoarse, &rowMap));
  PetscCall(PetscLayoutSetUp(rowMap));
  PetscCall(PetscLayoutSetUp(colMap));
  PetscCall(PetscLayoutGetRange(rowMap, &rowStart, &rowEnd));
  PetscCall(PetscLayoutGetRange(colMap, &colStart, &colEnd));
  /* insert values */
  PetscCall(DMPlexReferenceTreeGetChildrenMatrices_Injection(refTree, injRef, &childrenMats));
  for (p = pStartC; p < pEndC; p++) {
    PetscInt  numLeaves, leafStart, leafEnd, l, dof, cdof, gOff;
    PetscBool contribute = PETSC_FALSE;

    PetscCall(PetscSectionGetDof(globalCoarse, p, &dof));
    PetscCall(PetscSectionGetConstraintDof(globalCoarse, p, &cdof));
    if ((dof - cdof) <= 0) continue;
    PetscCall(PetscSectionGetDof(localCoarse, p, &dof));
    PetscCall(PetscSectionGetOffset(globalCoarse, p, &gOff));

    rowOffsets[0]  = 0;
    offsetsCopy[0] = 0;
    if (numFields) {
      PetscInt f;

      for (f = 0; f < numFields; f++) {
        PetscInt fDof;
        PetscCall(PetscSectionGetFieldDof(localCoarse, p, f, &fDof));
        rowOffsets[f + 1] = offsetsCopy[f + 1] = fDof + rowOffsets[f];
      }
      PetscCall(DMPlexGetIndicesPointFields_Internal(localCoarse, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsetsCopy, PETSC_FALSE, NULL, -1, NULL, parentIndices));
    } else {
      PetscCall(DMPlexGetIndicesPoint_Internal(localCoarse, PETSC_FALSE, p, gOff < 0 ? -(gOff + 1) : gOff, offsetsCopy, PETSC_FALSE, NULL, NULL, parentIndices));
      rowOffsets[1] = offsetsCopy[0];
    }

    PetscCall(PetscSectionGetDof(multiRootSec, p, &numLeaves));
    PetscCall(PetscSectionGetOffset(multiRootSec, p, &leafStart));
    leafEnd = leafStart + numLeaves;
    for (l = 0; l < dof; l++) parentValues[l] = 0.;
    for (l = leafStart; l < leafEnd; l++) {
      PetscInt           numIndices, childId, offset;
      const PetscScalar *childValues;

      PetscCall(PetscSectionGetDof(rootIndicesSec, l, &numIndices));
      PetscCall(PetscSectionGetOffset(rootIndicesSec, l, &offset));
      childId     = (PetscInt)PetscRealPart(rootValues[offset++]);
      childValues = &rootValues[offset];
      numIndices--;

      if (childId == -2) { /* skip */
        continue;
      } else if (childId == -1) { /* equivalent points: scatter */
        PetscInt m;

        contribute = PETSC_TRUE;
        for (m = 0; m < numIndices; m++) parentValues[m] = childValues[m];
      } else { /* contributions from children: sum with injectors from reference tree */
        PetscInt parentId, f, lim;

        contribute = PETSC_TRUE;
        PetscCall(DMPlexGetTreeParent(refTree, childId, &parentId, NULL));

        lim        = PetscMax(1, numFields);
        offsets[0] = 0;
        if (numFields) {
          PetscInt f;

          for (f = 0; f < numFields; f++) {
            PetscInt fDof;
            PetscCall(PetscSectionGetFieldDof(cSecRef, childId, f, &fDof));

            offsets[f + 1] = fDof + offsets[f];
          }
        } else {
          PetscInt cDof;

          PetscCall(PetscSectionGetDof(cSecRef, childId, &cDof));
          offsets[1] = cDof;
        }
        for (f = 0; f < lim; f++) {
          PetscScalar       *childMat = &childrenMats[childId - pRefStart][f][0];
          PetscInt           n        = offsets[f + 1] - offsets[f];
          PetscInt           m        = rowOffsets[f + 1] - rowOffsets[f];
          PetscInt           i, j;
          const PetscScalar *colValues = &childValues[offsets[f]];

          for (i = 0; i < m; i++) {
            PetscScalar val = 0.;
            for (j = 0; j < n; j++) val += childMat[n * i + j] * colValues[j];
            parentValues[rowOffsets[f] + i] += val;
          }
        }
      }
    }
    if (contribute) PetscCall(VecSetValues(vecCoarse, dof, parentIndices, parentValues, INSERT_VALUES));
  }
  PetscCall(PetscSectionDestroy(&multiRootSec));
  PetscCall(PetscSectionDestroy(&rootIndicesSec));
  PetscCall(PetscFree2(parentIndices, parentValues));
  PetscCall(DMPlexReferenceTreeRestoreChildrenMatrices_Injection(refTree, injRef, &childrenMats));
  PetscCall(PetscFree(rootValues));
  PetscCall(PetscFree3(offsets, offsetsCopy, rowOffsets));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  DMPlexTransferVecTree - transfer a vector between two meshes that differ from each other by refinement/coarsening
  that can be represented by a common reference tree used by both.  This routine can be used for a combination of
  coarsening and refinement at the same time.

  Collective

  Input Parameters:
+ dmIn        - The `DMPLEX` mesh for the input vector
. dmOut       - The second `DMPLEX` mesh
. vecIn       - The input vector
. sfRefine    - A star forest indicating points in the mesh `dmIn` (roots in the star forest) that are parents to points in
                the mesh `dmOut` (leaves in the star forest), i.e. where `dmOut` is more refined than `dmIn`
. sfCoarsen   - A star forest indicating points in the mesh `dmOut` (roots in the star forest) that are parents to points in
                the mesh `dmIn` (leaves in the star forest), i.e. where `dmOut` is more coarsened than `dmIn`
. cidsRefine  - The childIds of the points in `dmOut`.  These childIds relate back to the reference tree: childid[j] = k implies
                that mesh point j of `dmOut` was refined from a point in `dmIn` just as the mesh point k in the reference
                tree was refined from its parent.  childid[j] = -1 indicates that the point j in `dmOut` is exactly
                equivalent to its root in `dmIn`, so no interpolation is necessary.  childid[j] = -2 indicates that this
                point j in `dmOut` is not a leaf of `sfRefine`.
. cidsCoarsen - The childIds of the points in `dmIn`.  These childIds relate back to the reference tree: childid[j] = k implies
                that mesh point j of dmIn coarsens to a point in `dmOut` just as the mesh point k in the reference
                tree coarsens to its parent.  childid[j] = -2 indicates that point j in `dmOut` is not a leaf in `sfCoarsen`.
. useBCs      - `PETSC_TRUE` indicates that boundary values should be inserted into `vecIn` before transfer.
- time        - Used if boundary values are time dependent.

  Output Parameter:
. vecOut - Using interpolation and injection operators calculated on the reference tree, the transferred
                projection of `vecIn` from `dmIn` to `dmOut`.  Note that any field discretized with a `PetscFV` finite volume
                method that uses gradient reconstruction will use reconstructed gradients when interpolating from
                coarse points to fine points.

  Level: developer

.seealso: [](ch_unstructured), `DM`, `DMPLEX`, `PetscSF`, `Vec`, `PetscFV`, `DMPlexSetReferenceTree()`, `DMPlexGetReferenceTree()`, `PetscFVGetComputeGradients()`
@*/
PetscErrorCode DMPlexTransferVecTree(DM dmIn, Vec vecIn, DM dmOut, Vec vecOut, PetscSF sfRefine, PetscSF sfCoarsen, PetscInt *cidsRefine, PetscInt *cidsCoarsen, PetscBool useBCs, PetscReal time)
{
  PetscFunctionBegin;
  PetscCall(VecSet(vecOut, 0.0));
  if (sfRefine) {
    Vec vecInLocal;
    DM  dmGrad   = NULL;
    Vec faceGeom = NULL, cellGeom = NULL, grad = NULL;

    PetscCall(DMGetLocalVector(dmIn, &vecInLocal));
    PetscCall(VecSet(vecInLocal, 0.0));
    {
      PetscInt numFields, i;

      PetscCall(DMGetNumFields(dmIn, &numFields));
      for (i = 0; i < numFields; i++) {
        PetscObject  obj;
        PetscClassId classid;

        PetscCall(DMGetField(dmIn, i, NULL, &obj));
        PetscCall(PetscObjectGetClassId(obj, &classid));
        if (classid == PETSCFV_CLASSID) {
          PetscCall(DMPlexGetDataFVM(dmIn, (PetscFV)obj, &cellGeom, &faceGeom, &dmGrad));
          break;
        }
      }
    }
    if (useBCs) PetscCall(DMPlexInsertBoundaryValues(dmIn, PETSC_TRUE, vecInLocal, time, faceGeom, cellGeom, NULL));
    PetscCall(DMGlobalToLocalBegin(dmIn, vecIn, INSERT_VALUES, vecInLocal));
    PetscCall(DMGlobalToLocalEnd(dmIn, vecIn, INSERT_VALUES, vecInLocal));
    if (dmGrad) {
      PetscCall(DMGetGlobalVector(dmGrad, &grad));
      PetscCall(DMPlexReconstructGradientsFVM(dmIn, vecInLocal, grad));
    }
    PetscCall(DMPlexTransferVecTree_Interpolate(dmIn, vecInLocal, dmOut, vecOut, sfRefine, cidsRefine, grad, cellGeom));
    PetscCall(DMRestoreLocalVector(dmIn, &vecInLocal));
    if (dmGrad) PetscCall(DMRestoreGlobalVector(dmGrad, &grad));
  }
  if (sfCoarsen) PetscCall(DMPlexTransferVecTree_Inject(dmIn, vecIn, dmOut, vecOut, sfCoarsen, cidsCoarsen));
  PetscCall(VecAssemblyBegin(vecOut));
  PetscCall(VecAssemblyEnd(vecOut));
  PetscFunctionReturn(PETSC_SUCCESS);
}
