xref: /petsc/src/mat/impls/nest/matnest.c (revision 2ff79c18c26c94ed8cb599682f680f231dca6444)

#include <../src/mat/impls/nest/matnestimpl.h> /*I   "petscmat.h"   I*/
#include <../src/mat/impls/aij/seq/aij.h>
#include <../src/mat/impls/shell/shell.h>
#include <petscsf.h>

static PetscErrorCode MatSetUp_NestIS_Private(Mat, PetscInt, const IS[], PetscInt, const IS[]);
static PetscErrorCode MatCreateVecs_Nest(Mat, Vec *, Vec *);
static PetscErrorCode MatReset_Nest(Mat);

PETSC_INTERN PetscErrorCode MatConvert_Nest_IS(Mat, MatType, MatReuse, Mat *);

/* private functions */
static PetscErrorCode MatNestGetSizes_Private(Mat A, PetscInt *m, PetscInt *n, PetscInt *M, PetscInt *N)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  PetscInt  i, j;

  PetscFunctionBegin;
  *m = *n = *M = *N = 0;
  for (i = 0; i < bA->nr; i++) { /* rows */
    PetscInt sm, sM;
    PetscCall(ISGetLocalSize(bA->isglobal.row[i], &sm));
    PetscCall(ISGetSize(bA->isglobal.row[i], &sM));
    *m += sm;
    *M += sM;
  }
  for (j = 0; j < bA->nc; j++) { /* cols */
    PetscInt sn, sN;
    PetscCall(ISGetLocalSize(bA->isglobal.col[j], &sn));
    PetscCall(ISGetSize(bA->isglobal.col[j], &sN));
    *n += sn;
    *N += sN;
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* operations */
static PetscErrorCode MatMult_Nest(Mat A, Vec x, Vec y)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  Vec      *bx = bA->right, *by = bA->left;
  PetscInt  i, j, nr = bA->nr, nc = bA->nc;

  PetscFunctionBegin;
  for (i = 0; i < nr; i++) PetscCall(VecGetSubVector(y, bA->isglobal.row[i], &by[i]));
  for (i = 0; i < nc; i++) PetscCall(VecGetSubVector(x, bA->isglobal.col[i], &bx[i]));
  for (i = 0; i < nr; i++) {
    PetscCall(VecZeroEntries(by[i]));
    for (j = 0; j < nc; j++) {
      if (!bA->m[i][j]) continue;
      /* y[i] <- y[i] + A[i][j] * x[j] */
      PetscCall(MatMultAdd(bA->m[i][j], bx[j], by[i], by[i]));
    }
  }
  for (i = 0; i < nr; i++) PetscCall(VecRestoreSubVector(y, bA->isglobal.row[i], &by[i]));
  for (i = 0; i < nc; i++) PetscCall(VecRestoreSubVector(x, bA->isglobal.col[i], &bx[i]));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatMultAdd_Nest(Mat A, Vec x, Vec y, Vec z)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  Vec      *bx = bA->right, *bz = bA->left;
  PetscInt  i, j, nr = bA->nr, nc = bA->nc;

  PetscFunctionBegin;
  for (i = 0; i < nr; i++) PetscCall(VecGetSubVector(z, bA->isglobal.row[i], &bz[i]));
  for (i = 0; i < nc; i++) PetscCall(VecGetSubVector(x, bA->isglobal.col[i], &bx[i]));
  for (i = 0; i < nr; i++) {
    if (y != z) {
      Vec by;
      PetscCall(VecGetSubVector(y, bA->isglobal.row[i], &by));
      PetscCall(VecCopy(by, bz[i]));
      PetscCall(VecRestoreSubVector(y, bA->isglobal.row[i], &by));
    }
    for (j = 0; j < nc; j++) {
      if (!bA->m[i][j]) continue;
      /* y[i] <- y[i] + A[i][j] * x[j] */
      PetscCall(MatMultAdd(bA->m[i][j], bx[j], bz[i], bz[i]));
    }
  }
  for (i = 0; i < nr; i++) PetscCall(VecRestoreSubVector(z, bA->isglobal.row[i], &bz[i]));
  for (i = 0; i < nc; i++) PetscCall(VecRestoreSubVector(x, bA->isglobal.col[i], &bx[i]));
  PetscFunctionReturn(PETSC_SUCCESS);
}

typedef struct {
  Mat         *workC;      /* array of Mat with specific containers depending on the underlying MatMatMult implementation */
  PetscScalar *tarray;     /* buffer for storing all temporary products A[i][j] B[j] */
  PetscInt    *dm, *dn, k; /* displacements and number of submatrices */
} Nest_Dense;

static PetscErrorCode MatProductNumeric_Nest_Dense(Mat C)
{
  Mat_Nest          *bA;
  Nest_Dense        *contents;
  Mat                viewB, viewC, productB, workC;
  const PetscScalar *barray;
  PetscScalar       *carray;
  PetscInt           i, j, M, N, nr, nc, ldb, ldc;
  Mat                A, B;

  PetscFunctionBegin;
  MatCheckProduct(C, 1);
  A = C->product->A;
  B = C->product->B;
  PetscCall(MatGetSize(B, NULL, &N));
  if (!N) {
    PetscCall(MatAssemblyBegin(C, MAT_FINAL_ASSEMBLY));
    PetscCall(MatAssemblyEnd(C, MAT_FINAL_ASSEMBLY));
    PetscFunctionReturn(PETSC_SUCCESS);
  }
  contents = (Nest_Dense *)C->product->data;
  PetscCheck(contents, PetscObjectComm((PetscObject)C), PETSC_ERR_PLIB, "Product data empty");
  bA = (Mat_Nest *)A->data;
  nr = bA->nr;
  nc = bA->nc;
  PetscCall(MatDenseGetLDA(B, &ldb));
  PetscCall(MatDenseGetLDA(C, &ldc));
  PetscCall(MatZeroEntries(C));
  PetscCall(MatDenseGetArrayRead(B, &barray));
  PetscCall(MatDenseGetArray(C, &carray));
  for (i = 0; i < nr; i++) {
    PetscCall(ISGetSize(bA->isglobal.row[i], &M));
    PetscCall(MatCreateDense(PetscObjectComm((PetscObject)A), contents->dm[i + 1] - contents->dm[i], PETSC_DECIDE, M, N, PetscSafePointerPlusOffset(carray, contents->dm[i]), &viewC));
    PetscCall(MatDenseSetLDA(viewC, ldc));
    for (j = 0; j < nc; j++) {
      if (!bA->m[i][j]) continue;
      PetscCall(ISGetSize(bA->isglobal.col[j], &M));
      PetscCall(MatCreateDense(PetscObjectComm((PetscObject)A), contents->dn[j + 1] - contents->dn[j], PETSC_DECIDE, M, N, PetscSafePointerPlusOffset((PetscScalar *)barray, contents->dn[j]), &viewB));
      PetscCall(MatDenseSetLDA(viewB, ldb));

      /* MatMatMultNumeric(bA->m[i][j],viewB,contents->workC[i*nc + j]); */
      workC             = contents->workC[i * nc + j];
      productB          = workC->product->B;
      workC->product->B = viewB; /* use newly created dense matrix viewB */
      PetscCall(MatProductNumeric(workC));
      PetscCall(MatDestroy(&viewB));
      workC->product->B = productB; /* resume original B */

      /* C[i] <- workC + C[i] */
      PetscCall(MatAXPY(viewC, 1.0, contents->workC[i * nc + j], SAME_NONZERO_PATTERN));
    }
    PetscCall(MatDestroy(&viewC));
  }
  PetscCall(MatDenseRestoreArray(C, &carray));
  PetscCall(MatDenseRestoreArrayRead(B, &barray));

  PetscCall(MatSetOption(C, MAT_NO_OFF_PROC_ENTRIES, PETSC_TRUE));
  PetscCall(MatAssemblyBegin(C, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(C, MAT_FINAL_ASSEMBLY));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNest_DenseDestroy(void *ctx)
{
  Nest_Dense *contents = (Nest_Dense *)ctx;
  PetscInt    i;

  PetscFunctionBegin;
  PetscCall(PetscFree(contents->tarray));
  for (i = 0; i < contents->k; i++) PetscCall(MatDestroy(contents->workC + i));
  PetscCall(PetscFree3(contents->dm, contents->dn, contents->workC));
  PetscCall(PetscFree(contents));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatProductSymbolic_Nest_Dense(Mat C)
{
  Mat_Nest          *bA;
  Mat                viewB, workC;
  const PetscScalar *barray;
  PetscInt           i, j, M, N, m, n, nr, nc, maxm = 0, ldb;
  Nest_Dense        *contents = NULL;
  PetscBool          cisdense;
  Mat                A, B;
  PetscReal          fill;

  PetscFunctionBegin;
  MatCheckProduct(C, 1);
  PetscCheck(!C->product->data, PetscObjectComm((PetscObject)C), PETSC_ERR_PLIB, "Product data not empty");
  A    = C->product->A;
  B    = C->product->B;
  fill = C->product->fill;
  bA   = (Mat_Nest *)A->data;
  nr   = bA->nr;
  nc   = bA->nc;
  PetscCall(MatGetLocalSize(C, &m, &n));
  PetscCall(MatGetSize(C, &M, &N));
  if (m == PETSC_DECIDE || n == PETSC_DECIDE || M == PETSC_DECIDE || N == PETSC_DECIDE) {
    PetscCall(MatGetLocalSize(B, NULL, &n));
    PetscCall(MatGetSize(B, NULL, &N));
    PetscCall(MatGetLocalSize(A, &m, NULL));
    PetscCall(MatGetSize(A, &M, NULL));
    PetscCall(MatSetSizes(C, m, n, M, N));
  }
  PetscCall(PetscObjectTypeCompareAny((PetscObject)C, &cisdense, MATSEQDENSE, MATMPIDENSE, MATSEQDENSECUDA, MATMPIDENSECUDA, ""));
  if (!cisdense) PetscCall(MatSetType(C, ((PetscObject)B)->type_name));
  PetscCall(MatSetUp(C));
  if (!N) {
    C->ops->productnumeric = MatProductNumeric_Nest_Dense;
    PetscFunctionReturn(PETSC_SUCCESS);
  }

  PetscCall(PetscNew(&contents));
  C->product->data    = contents;
  C->product->destroy = MatNest_DenseDestroy;
  PetscCall(PetscCalloc3(nr + 1, &contents->dm, nc + 1, &contents->dn, nr * nc, &contents->workC));
  contents->k = nr * nc;
  for (i = 0; i < nr; i++) {
    PetscCall(ISGetLocalSize(bA->isglobal.row[i], contents->dm + i + 1));
    maxm = PetscMax(maxm, contents->dm[i + 1]);
    contents->dm[i + 1] += contents->dm[i];
  }
  for (i = 0; i < nc; i++) {
    PetscCall(ISGetLocalSize(bA->isglobal.col[i], contents->dn + i + 1));
    contents->dn[i + 1] += contents->dn[i];
  }
  PetscCall(PetscMalloc1(maxm * N, &contents->tarray));
  PetscCall(MatDenseGetLDA(B, &ldb));
  PetscCall(MatGetSize(B, NULL, &N));
  PetscCall(MatDenseGetArrayRead(B, &barray));
  /* loops are permuted compared to MatMatMultNumeric so that viewB is created only once per column of A */
  for (j = 0; j < nc; j++) {
    PetscCall(ISGetSize(bA->isglobal.col[j], &M));
    PetscCall(MatCreateDense(PetscObjectComm((PetscObject)A), contents->dn[j + 1] - contents->dn[j], PETSC_DECIDE, M, N, PetscSafePointerPlusOffset((PetscScalar *)barray, contents->dn[j]), &viewB));
    PetscCall(MatDenseSetLDA(viewB, ldb));
    for (i = 0; i < nr; i++) {
      if (!bA->m[i][j]) continue;
      /* MatMatMultSymbolic may attach a specific container (depending on MatType of bA->m[i][j]) to workC[i][j] */

      PetscCall(MatProductCreate(bA->m[i][j], viewB, NULL, &contents->workC[i * nc + j]));
      workC = contents->workC[i * nc + j];
      PetscCall(MatProductSetType(workC, MATPRODUCT_AB));
      PetscCall(MatProductSetAlgorithm(workC, "default"));
      PetscCall(MatProductSetFill(workC, fill));
      PetscCall(MatProductSetFromOptions(workC));
      PetscCall(MatProductSymbolic(workC));

      /* since tarray will be shared by all Mat */
      PetscCall(MatSeqDenseSetPreallocation(workC, contents->tarray));
      PetscCall(MatMPIDenseSetPreallocation(workC, contents->tarray));
    }
    PetscCall(MatDestroy(&viewB));
  }
  PetscCall(MatDenseRestoreArrayRead(B, &barray));

  C->ops->productnumeric = MatProductNumeric_Nest_Dense;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatProductSetFromOptions_Nest_Dense(Mat C)
{
  Mat_Product *product = C->product;

  PetscFunctionBegin;
  if (product->type == MATPRODUCT_AB) C->ops->productsymbolic = MatProductSymbolic_Nest_Dense;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatMultTransposeKernel_Nest(Mat A, Vec x, Vec y, PetscBool herm)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  Vec      *bx = bA->left, *by = bA->right;
  PetscInt  i, j, nr = bA->nr, nc = bA->nc;

  PetscFunctionBegin;
  for (i = 0; i < nr; i++) PetscCall(VecGetSubVector(x, bA->isglobal.row[i], &bx[i]));
  for (i = 0; i < nc; i++) PetscCall(VecGetSubVector(y, bA->isglobal.col[i], &by[i]));
  for (j = 0; j < nc; j++) {
    PetscCall(VecZeroEntries(by[j]));
    for (i = 0; i < nr; i++) {
      if (!bA->m[i][j]) continue;
      if (herm) PetscCall(MatMultHermitianTransposeAdd(bA->m[i][j], bx[i], by[j], by[j])); /* y[j] <- y[j] + (A[i][j])^H * x[i] */
      else PetscCall(MatMultTransposeAdd(bA->m[i][j], bx[i], by[j], by[j]));               /* y[j] <- y[j] + (A[i][j])^T * x[i] */
    }
  }
  for (i = 0; i < nr; i++) PetscCall(VecRestoreSubVector(x, bA->isglobal.row[i], &bx[i]));
  for (i = 0; i < nc; i++) PetscCall(VecRestoreSubVector(y, bA->isglobal.col[i], &by[i]));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatMultTranspose_Nest(Mat A, Vec x, Vec y)
{
  PetscFunctionBegin;
  PetscCall(MatMultTransposeKernel_Nest(A, x, y, PETSC_FALSE));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatMultHermitianTranspose_Nest(Mat A, Vec x, Vec y)
{
  PetscFunctionBegin;
  PetscCall(MatMultTransposeKernel_Nest(A, x, y, PETSC_TRUE));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatMultTransposeAddKernel_Nest(Mat A, Vec x, Vec y, Vec z, PetscBool herm)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  Vec      *bx = bA->left, *bz = bA->right;
  PetscInt  i, j, nr = bA->nr, nc = bA->nc;

  PetscFunctionBegin;
  for (i = 0; i < nr; i++) PetscCall(VecGetSubVector(x, bA->isglobal.row[i], &bx[i]));
  for (i = 0; i < nc; i++) PetscCall(VecGetSubVector(z, bA->isglobal.col[i], &bz[i]));
  for (j = 0; j < nc; j++) {
    if (y != z) {
      Vec by;
      PetscCall(VecGetSubVector(y, bA->isglobal.col[j], &by));
      PetscCall(VecCopy(by, bz[j]));
      PetscCall(VecRestoreSubVector(y, bA->isglobal.col[j], &by));
    }
    for (i = 0; i < nr; i++) {
      if (!bA->m[i][j]) continue;
      if (herm) PetscCall(MatMultHermitianTransposeAdd(bA->m[i][j], bx[i], bz[j], bz[j])); /* z[j] <- y[j] + (A[i][j])^H * x[i] */
      else PetscCall(MatMultTransposeAdd(bA->m[i][j], bx[i], bz[j], bz[j]));               /* z[j] <- y[j] + (A[i][j])^T * x[i] */
    }
  }
  for (i = 0; i < nr; i++) PetscCall(VecRestoreSubVector(x, bA->isglobal.row[i], &bx[i]));
  for (i = 0; i < nc; i++) PetscCall(VecRestoreSubVector(z, bA->isglobal.col[i], &bz[i]));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatMultTransposeAdd_Nest(Mat A, Vec x, Vec y, Vec z)
{
  PetscFunctionBegin;
  PetscCall(MatMultTransposeAddKernel_Nest(A, x, y, z, PETSC_FALSE));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatMultHermitianTransposeAdd_Nest(Mat A, Vec x, Vec y, Vec z)
{
  PetscFunctionBegin;
  PetscCall(MatMultTransposeAddKernel_Nest(A, x, y, z, PETSC_TRUE));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatTranspose_Nest(Mat A, MatReuse reuse, Mat *B)
{
  Mat_Nest *bA = (Mat_Nest *)A->data, *bC;
  Mat       C;
  PetscInt  i, j, nr = bA->nr, nc = bA->nc;

  PetscFunctionBegin;
  if (reuse == MAT_REUSE_MATRIX) PetscCall(MatTransposeCheckNonzeroState_Private(A, *B));
  PetscCheck(reuse != MAT_INPLACE_MATRIX || nr == nc, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_SIZ, "Square nested matrix only for in-place");

  if (reuse == MAT_INITIAL_MATRIX || reuse == MAT_INPLACE_MATRIX) {
    Mat *subs;
    IS  *is_row, *is_col;

    PetscCall(PetscCalloc1(nr * nc, &subs));
    PetscCall(PetscMalloc2(nr, &is_row, nc, &is_col));
    PetscCall(MatNestGetISs(A, is_row, is_col));
    if (reuse == MAT_INPLACE_MATRIX) {
      for (i = 0; i < nr; i++) {
        for (j = 0; j < nc; j++) subs[i + nr * j] = bA->m[i][j];
      }
    }

    PetscCall(MatCreateNest(PetscObjectComm((PetscObject)A), nc, is_col, nr, is_row, subs, &C));
    PetscCall(PetscFree(subs));
    PetscCall(PetscFree2(is_row, is_col));
  } else {
    C = *B;
  }

  bC = (Mat_Nest *)C->data;
  for (i = 0; i < nr; i++) {
    for (j = 0; j < nc; j++) {
      if (bA->m[i][j]) {
        PetscCall(MatTranspose(bA->m[i][j], reuse, &bC->m[j][i]));
      } else {
        bC->m[j][i] = NULL;
      }
    }
  }

  if (reuse == MAT_INITIAL_MATRIX || reuse == MAT_REUSE_MATRIX) {
    *B = C;
  } else {
    PetscCall(MatHeaderMerge(A, &C));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestDestroyISList(PetscInt n, IS **list)
{
  IS      *lst = *list;
  PetscInt i;

  PetscFunctionBegin;
  if (!lst) PetscFunctionReturn(PETSC_SUCCESS);
  for (i = 0; i < n; i++)
    if (lst[i]) PetscCall(ISDestroy(&lst[i]));
  PetscCall(PetscFree(lst));
  *list = NULL;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatReset_Nest(Mat A)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  i, j;

  PetscFunctionBegin;
  /* release the matrices and the place holders */
  PetscCall(MatNestDestroyISList(vs->nr, &vs->isglobal.row));
  PetscCall(MatNestDestroyISList(vs->nc, &vs->isglobal.col));
  PetscCall(MatNestDestroyISList(vs->nr, &vs->islocal.row));
  PetscCall(MatNestDestroyISList(vs->nc, &vs->islocal.col));

  PetscCall(PetscFree(vs->row_len));
  PetscCall(PetscFree(vs->col_len));
  PetscCall(PetscFree(vs->nnzstate));

  PetscCall(PetscFree2(vs->left, vs->right));

  /* release the matrices and the place holders */
  if (vs->m) {
    for (i = 0; i < vs->nr; i++) {
      for (j = 0; j < vs->nc; j++) PetscCall(MatDestroy(&vs->m[i][j]));
    }
    PetscCall(PetscFree(vs->m[0]));
    PetscCall(PetscFree(vs->m));
  }

  /* restore defaults */
  vs->nr            = 0;
  vs->nc            = 0;
  vs->splitassembly = PETSC_FALSE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatDestroy_Nest(Mat A)
{
  PetscFunctionBegin;
  PetscCall(MatReset_Nest(A));
  PetscCall(PetscFree(A->data));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetSubMat_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestSetSubMat_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetSubMats_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetSize_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetISs_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetLocalISs_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestSetVecType_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestSetSubMats_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_mpiaij_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_seqaij_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_aij_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_is_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_mpidense_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_seqdense_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatProductSetFromOptions_nest_seqdense_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatProductSetFromOptions_nest_mpidense_C", NULL));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatMissingDiagonal_Nest(Mat mat, PetscBool *missing, PetscInt *dd)
{
  Mat_Nest *vs = (Mat_Nest *)mat->data;
  PetscInt  i;

  PetscFunctionBegin;
  if (dd) *dd = 0;
  if (!vs->nr) {
    *missing = PETSC_TRUE;
    PetscFunctionReturn(PETSC_SUCCESS);
  }
  *missing = PETSC_FALSE;
  for (i = 0; i < vs->nr && !*missing; i++) {
    *missing = PETSC_TRUE;
    if (vs->m[i][i]) {
      PetscCall(MatMissingDiagonal(vs->m[i][i], missing, NULL));
      PetscCheck(!*missing || !dd, PetscObjectComm((PetscObject)mat), PETSC_ERR_SUP, "First missing entry not yet implemented");
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatAssemblyBegin_Nest(Mat A, MatAssemblyType type)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  i, j;
  PetscBool nnzstate = PETSC_FALSE;

  PetscFunctionBegin;
  for (i = 0; i < vs->nr; i++) {
    for (j = 0; j < vs->nc; j++) {
      PetscObjectState subnnzstate = 0;
      if (vs->m[i][j]) {
        PetscCall(MatAssemblyBegin(vs->m[i][j], type));
        if (!vs->splitassembly) {
          /* Note: split assembly will fail if the same block appears more than once (even indirectly through a nested
           * sub-block). This could be fixed by adding a flag to Mat so that there was a way to check if a Mat was
           * already performing an assembly, but the result would by more complicated and appears to offer less
           * potential for diagnostics and correctness checking. Split assembly should be fixed once there is an
           * interface for libraries to make asynchronous progress in "user-defined non-blocking collectives".
           */
          PetscCall(MatAssemblyEnd(vs->m[i][j], type));
          PetscCall(MatGetNonzeroState(vs->m[i][j], &subnnzstate));
        }
      }
      nnzstate                     = (PetscBool)(nnzstate || vs->nnzstate[i * vs->nc + j] != subnnzstate);
      vs->nnzstate[i * vs->nc + j] = subnnzstate;
    }
  }
  if (nnzstate) A->nonzerostate++;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatAssemblyEnd_Nest(Mat A, MatAssemblyType type)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  i, j;

  PetscFunctionBegin;
  for (i = 0; i < vs->nr; i++) {
    for (j = 0; j < vs->nc; j++) {
      if (vs->m[i][j]) {
        if (vs->splitassembly) PetscCall(MatAssemblyEnd(vs->m[i][j], type));
      }
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestFindNonzeroSubMatRow(Mat A, PetscInt row, Mat *B)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  j;
  Mat       sub;

  PetscFunctionBegin;
  sub = (row < vs->nc) ? vs->m[row][row] : (Mat)NULL; /* Prefer to find on the diagonal */
  for (j = 0; !sub && j < vs->nc; j++) sub = vs->m[row][j];
  if (sub) PetscCall(MatSetUp(sub)); /* Ensure that the sizes are available */
  *B = sub;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestFindNonzeroSubMatCol(Mat A, PetscInt col, Mat *B)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  i;
  Mat       sub;

  PetscFunctionBegin;
  sub = (col < vs->nr) ? vs->m[col][col] : (Mat)NULL; /* Prefer to find on the diagonal */
  for (i = 0; !sub && i < vs->nr; i++) sub = vs->m[i][col];
  if (sub) PetscCall(MatSetUp(sub)); /* Ensure that the sizes are available */
  *B = sub;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestFindISRange(Mat A, PetscInt n, const IS list[], IS is, PetscInt *begin, PetscInt *end)
{
  PetscInt  i, j, size, m;
  PetscBool flg;
  IS        out, concatenate[2];

  PetscFunctionBegin;
  PetscAssertPointer(list, 3);
  PetscValidHeaderSpecific(is, IS_CLASSID, 4);
  if (begin) {
    PetscAssertPointer(begin, 5);
    *begin = -1;
  }
  if (end) {
    PetscAssertPointer(end, 6);
    *end = -1;
  }
  for (i = 0; i < n; i++) {
    if (!list[i]) continue;
    PetscCall(ISEqualUnsorted(list[i], is, &flg));
    if (flg) {
      if (begin) *begin = i;
      if (end) *end = i + 1;
      PetscFunctionReturn(PETSC_SUCCESS);
    }
  }
  PetscCall(ISGetSize(is, &size));
  for (i = 0; i < n - 1; i++) {
    if (!list[i]) continue;
    m = 0;
    PetscCall(ISConcatenate(PetscObjectComm((PetscObject)A), 2, list + i, &out));
    PetscCall(ISGetSize(out, &m));
    for (j = i + 2; j < n && m < size; j++) {
      if (list[j]) {
        concatenate[0] = out;
        concatenate[1] = list[j];
        PetscCall(ISConcatenate(PetscObjectComm((PetscObject)A), 2, concatenate, &out));
        PetscCall(ISDestroy(concatenate));
        PetscCall(ISGetSize(out, &m));
      }
    }
    if (m == size) {
      PetscCall(ISEqualUnsorted(out, is, &flg));
      if (flg) {
        if (begin) *begin = i;
        if (end) *end = j;
        PetscCall(ISDestroy(&out));
        PetscFunctionReturn(PETSC_SUCCESS);
      }
    }
    PetscCall(ISDestroy(&out));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestFillEmptyMat_Private(Mat A, PetscInt i, PetscInt j, Mat *B)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  lr, lc;

  PetscFunctionBegin;
  PetscCall(MatCreate(PetscObjectComm((PetscObject)A), B));
  PetscCall(ISGetLocalSize(vs->isglobal.row[i], &lr));
  PetscCall(ISGetLocalSize(vs->isglobal.col[j], &lc));
  PetscCall(MatSetSizes(*B, lr, lc, PETSC_DECIDE, PETSC_DECIDE));
  PetscCall(MatSetType(*B, MATAIJ));
  PetscCall(MatSeqAIJSetPreallocation(*B, 0, NULL));
  PetscCall(MatMPIAIJSetPreallocation(*B, 0, NULL, 0, NULL));
  PetscCall(MatSetUp(*B));
  PetscCall(MatSetOption(*B, MAT_NO_OFF_PROC_ENTRIES, PETSC_TRUE));
  PetscCall(MatAssemblyBegin(*B, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(*B, MAT_FINAL_ASSEMBLY));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestGetBlock_Private(Mat A, PetscInt rbegin, PetscInt rend, PetscInt cbegin, PetscInt cend, Mat *B)
{
  Mat_Nest  *vs = (Mat_Nest *)A->data;
  Mat       *a;
  PetscInt   i, j, k, l, nr = rend - rbegin, nc = cend - cbegin;
  char       keyname[256];
  PetscBool *b;
  PetscBool  flg;

  PetscFunctionBegin;
  *B = NULL;
  PetscCall(PetscSNPrintf(keyname, sizeof(keyname), "NestBlock_%" PetscInt_FMT "-%" PetscInt_FMT "x%" PetscInt_FMT "-%" PetscInt_FMT, rbegin, rend, cbegin, cend));
  PetscCall(PetscObjectQuery((PetscObject)A, keyname, (PetscObject *)B));
  if (*B) PetscFunctionReturn(PETSC_SUCCESS);

  PetscCall(PetscMalloc2(nr * nc, &a, nr * nc, &b));
  for (i = 0; i < nr; i++) {
    for (j = 0; j < nc; j++) {
      a[i * nc + j] = vs->m[rbegin + i][cbegin + j];
      b[i * nc + j] = PETSC_FALSE;
    }
  }
  if (nc != vs->nc && nr != vs->nr) {
    for (i = 0; i < nr; i++) {
      for (j = 0; j < nc; j++) {
        flg = PETSC_FALSE;
        for (k = 0; (k < nr && !flg); k++) {
          if (a[j + k * nc]) flg = PETSC_TRUE;
        }
        if (flg) {
          flg = PETSC_FALSE;
          for (l = 0; (l < nc && !flg); l++) {
            if (a[i * nc + l]) flg = PETSC_TRUE;
          }
        }
        if (!flg) {
          b[i * nc + j] = PETSC_TRUE;
          PetscCall(MatNestFillEmptyMat_Private(A, rbegin + i, cbegin + j, a + i * nc + j));
        }
      }
    }
  }
  PetscCall(MatCreateNest(PetscObjectComm((PetscObject)A), nr, nr != vs->nr ? NULL : vs->isglobal.row, nc, nc != vs->nc ? NULL : vs->isglobal.col, a, B));
  for (i = 0; i < nr; i++) {
    for (j = 0; j < nc; j++) {
      if (b[i * nc + j]) PetscCall(MatDestroy(a + i * nc + j));
    }
  }
  PetscCall(PetscFree2(a, b));
  (*B)->assembled = A->assembled;
  PetscCall(PetscObjectCompose((PetscObject)A, keyname, (PetscObject)*B));
  PetscCall(PetscObjectDereference((PetscObject)*B)); /* Leave the only remaining reference in the composition */
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestFindSubMat(Mat A, struct MatNestISPair *is, IS isrow, IS iscol, Mat *B)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  rbegin, rend, cbegin, cend;

  PetscFunctionBegin;
  PetscCall(MatNestFindISRange(A, vs->nr, is->row, isrow, &rbegin, &rend));
  PetscCall(MatNestFindISRange(A, vs->nc, is->col, iscol, &cbegin, &cend));
  if (rend == rbegin + 1 && cend == cbegin + 1) {
    if (!vs->m[rbegin][cbegin]) PetscCall(MatNestFillEmptyMat_Private(A, rbegin, cbegin, vs->m[rbegin] + cbegin));
    *B = vs->m[rbegin][cbegin];
  } else if (rbegin != -1 && cbegin != -1) {
    PetscCall(MatNestGetBlock_Private(A, rbegin, rend, cbegin, cend, B));
  } else SETERRQ(PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_INCOMP, "Could not find index set");
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
   TODO: This does not actually returns a submatrix we can modify
*/
static PetscErrorCode MatCreateSubMatrix_Nest(Mat A, IS isrow, IS iscol, MatReuse reuse, Mat *B)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  Mat       sub;

  PetscFunctionBegin;
  PetscCall(MatNestFindSubMat(A, &vs->isglobal, isrow, iscol, &sub));
  switch (reuse) {
  case MAT_INITIAL_MATRIX:
    if (sub) PetscCall(PetscObjectReference((PetscObject)sub));
    *B = sub;
    break;
  case MAT_REUSE_MATRIX:
    PetscCheck(sub == *B, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_WRONGSTATE, "Submatrix was not used before in this call");
    break;
  case MAT_IGNORE_MATRIX: /* Nothing to do */
    break;
  case MAT_INPLACE_MATRIX: /* Nothing to do */
    SETERRQ(PetscObjectComm((PetscObject)A), PETSC_ERR_SUP, "MAT_INPLACE_MATRIX is not supported yet");
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatGetLocalSubMatrix_Nest(Mat A, IS isrow, IS iscol, Mat *B)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  Mat       sub;

  PetscFunctionBegin;
  PetscCall(MatNestFindSubMat(A, &vs->islocal, isrow, iscol, &sub));
  /* We allow the submatrix to be NULL, perhaps it would be better for the user to return an empty matrix instead */
  if (sub) PetscCall(PetscObjectReference((PetscObject)sub));
  *B = sub;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatRestoreLocalSubMatrix_Nest(Mat A, IS isrow, IS iscol, Mat *B)
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  Mat       sub;

  PetscFunctionBegin;
  PetscCall(MatNestFindSubMat(A, &vs->islocal, isrow, iscol, &sub));
  PetscCheck(*B == sub, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_WRONGSTATE, "Local submatrix has not been gotten");
  if (sub) {
    PetscCheck(((PetscObject)sub)->refct > 1, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_WRONGSTATE, "Local submatrix has had reference count decremented too many times");
    PetscCall(MatDestroy(B));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatGetDiagonal_Nest(Mat A, Vec v)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  PetscInt  i;

  PetscFunctionBegin;
  for (i = 0; i < bA->nr; i++) {
    Vec bv;
    PetscCall(VecGetSubVector(v, bA->isglobal.row[i], &bv));
    if (bA->m[i][i]) {
      PetscCall(MatGetDiagonal(bA->m[i][i], bv));
    } else {
      PetscCall(VecSet(bv, 0.0));
    }
    PetscCall(VecRestoreSubVector(v, bA->isglobal.row[i], &bv));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatDiagonalScale_Nest(Mat A, Vec l, Vec r)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  Vec       bl, *br;
  PetscInt  i, j;

  PetscFunctionBegin;
  PetscCall(PetscCalloc1(bA->nc, &br));
  if (r) {
    for (j = 0; j < bA->nc; j++) PetscCall(VecGetSubVector(r, bA->isglobal.col[j], &br[j]));
  }
  bl = NULL;
  for (i = 0; i < bA->nr; i++) {
    if (l) PetscCall(VecGetSubVector(l, bA->isglobal.row[i], &bl));
    for (j = 0; j < bA->nc; j++) {
      if (bA->m[i][j]) PetscCall(MatDiagonalScale(bA->m[i][j], bl, br[j]));
    }
    if (l) PetscCall(VecRestoreSubVector(l, bA->isglobal.row[i], &bl));
  }
  if (r) {
    for (j = 0; j < bA->nc; j++) PetscCall(VecRestoreSubVector(r, bA->isglobal.col[j], &br[j]));
  }
  PetscCall(PetscFree(br));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatScale_Nest(Mat A, PetscScalar a)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  PetscInt  i, j;

  PetscFunctionBegin;
  for (i = 0; i < bA->nr; i++) {
    for (j = 0; j < bA->nc; j++) {
      if (bA->m[i][j]) PetscCall(MatScale(bA->m[i][j], a));
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatShift_Nest(Mat A, PetscScalar a)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  PetscInt  i;
  PetscBool nnzstate = PETSC_FALSE;

  PetscFunctionBegin;
  for (i = 0; i < bA->nr; i++) {
    PetscObjectState subnnzstate = 0;
    PetscCheck(bA->m[i][i], PetscObjectComm((PetscObject)A), PETSC_ERR_SUP, "No support for shifting an empty diagonal block, insert a matrix in block (%" PetscInt_FMT ",%" PetscInt_FMT ")", i, i);
    PetscCall(MatShift(bA->m[i][i], a));
    PetscCall(MatGetNonzeroState(bA->m[i][i], &subnnzstate));
    nnzstate                     = (PetscBool)(nnzstate || bA->nnzstate[i * bA->nc + i] != subnnzstate);
    bA->nnzstate[i * bA->nc + i] = subnnzstate;
  }
  if (nnzstate) A->nonzerostate++;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatDiagonalSet_Nest(Mat A, Vec D, InsertMode is)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  PetscInt  i;
  PetscBool nnzstate = PETSC_FALSE;

  PetscFunctionBegin;
  for (i = 0; i < bA->nr; i++) {
    PetscObjectState subnnzstate = 0;
    Vec              bv;
    PetscCall(VecGetSubVector(D, bA->isglobal.row[i], &bv));
    if (bA->m[i][i]) {
      PetscCall(MatDiagonalSet(bA->m[i][i], bv, is));
      PetscCall(MatGetNonzeroState(bA->m[i][i], &subnnzstate));
    }
    PetscCall(VecRestoreSubVector(D, bA->isglobal.row[i], &bv));
    nnzstate                     = (PetscBool)(nnzstate || bA->nnzstate[i * bA->nc + i] != subnnzstate);
    bA->nnzstate[i * bA->nc + i] = subnnzstate;
  }
  if (nnzstate) A->nonzerostate++;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatSetRandom_Nest(Mat A, PetscRandom rctx)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  PetscInt  i, j;

  PetscFunctionBegin;
  for (i = 0; i < bA->nr; i++) {
    for (j = 0; j < bA->nc; j++) {
      if (bA->m[i][j]) PetscCall(MatSetRandom(bA->m[i][j], rctx));
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatCreateVecs_Nest(Mat A, Vec *right, Vec *left)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  Vec      *L, *R;
  MPI_Comm  comm;
  PetscInt  i, j;

  PetscFunctionBegin;
  PetscCall(PetscObjectGetComm((PetscObject)A, &comm));
  if (right) {
    /* allocate R */
    PetscCall(PetscMalloc1(bA->nc, &R));
    /* Create the right vectors */
    for (j = 0; j < bA->nc; j++) {
      for (i = 0; i < bA->nr; i++) {
        if (bA->m[i][j]) {
          PetscCall(MatCreateVecs(bA->m[i][j], &R[j], NULL));
          break;
        }
      }
      PetscCheck(i != bA->nr, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_WRONG, "Mat(Nest) contains a null column.");
    }
    PetscCall(VecCreateNest(comm, bA->nc, bA->isglobal.col, R, right));
    /* hand back control to the nest vector */
    for (j = 0; j < bA->nc; j++) PetscCall(VecDestroy(&R[j]));
    PetscCall(PetscFree(R));
  }

  if (left) {
    /* allocate L */
    PetscCall(PetscMalloc1(bA->nr, &L));
    /* Create the left vectors */
    for (i = 0; i < bA->nr; i++) {
      for (j = 0; j < bA->nc; j++) {
        if (bA->m[i][j]) {
          PetscCall(MatCreateVecs(bA->m[i][j], NULL, &L[i]));
          break;
        }
      }
      PetscCheck(j != bA->nc, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_WRONG, "Mat(Nest) contains a null row.");
    }

    PetscCall(VecCreateNest(comm, bA->nr, bA->isglobal.row, L, left));
    for (i = 0; i < bA->nr; i++) PetscCall(VecDestroy(&L[i]));

    PetscCall(PetscFree(L));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatView_Nest(Mat A, PetscViewer viewer)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  PetscBool isascii, viewSub = PETSC_FALSE;
  PetscInt  i, j;

  PetscFunctionBegin;
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &isascii));
  if (isascii) {
    PetscViewerFormat format;

    PetscCall(PetscViewerGetFormat(viewer, &format));
    if (format == PETSC_VIEWER_ASCII_MATLAB) {
      Mat T;

      PetscCall(MatConvert(A, MATAIJ, MAT_INITIAL_MATRIX, &T));
      PetscCall(MatView(T, viewer));
      PetscCall(MatDestroy(&T));
      PetscFunctionReturn(PETSC_SUCCESS);
    }
    PetscCall(PetscOptionsGetBool(((PetscObject)A)->options, ((PetscObject)A)->prefix, "-mat_view_nest_sub", &viewSub, NULL));
    PetscCall(PetscViewerASCIIPrintf(viewer, "Matrix object:\n"));
    PetscCall(PetscViewerASCIIPushTab(viewer));
    PetscCall(PetscViewerASCIIPrintf(viewer, "type=nest, rows=%" PetscInt_FMT ", cols=%" PetscInt_FMT "\n", bA->nr, bA->nc));

    PetscCall(PetscViewerASCIIPrintf(viewer, "MatNest structure:\n"));
    for (i = 0; i < bA->nr; i++) {
      for (j = 0; j < bA->nc; j++) {
        MatType   type;
        char      name[256] = "", prefix[256] = "";
        PetscInt  NR, NC;
        PetscBool isNest = PETSC_FALSE;

        if (!bA->m[i][j]) {
          PetscCall(PetscViewerASCIIPrintf(viewer, "(%" PetscInt_FMT ",%" PetscInt_FMT ") : NULL\n", i, j));
          continue;
        }
        PetscCall(MatGetSize(bA->m[i][j], &NR, &NC));
        PetscCall(MatGetType(bA->m[i][j], &type));
        if (((PetscObject)bA->m[i][j])->name) PetscCall(PetscSNPrintf(name, sizeof(name), "name=\"%s\", ", ((PetscObject)bA->m[i][j])->name));
        if (((PetscObject)bA->m[i][j])->prefix) PetscCall(PetscSNPrintf(prefix, sizeof(prefix), "prefix=\"%s\", ", ((PetscObject)bA->m[i][j])->prefix));
        PetscCall(PetscObjectTypeCompare((PetscObject)bA->m[i][j], MATNEST, &isNest));

        PetscCall(PetscViewerASCIIPrintf(viewer, "(%" PetscInt_FMT ",%" PetscInt_FMT ") : %s%stype=%s, rows=%" PetscInt_FMT ", cols=%" PetscInt_FMT "\n", i, j, name, prefix, type, NR, NC));

        if (isNest || viewSub) {
          PetscCall(PetscViewerASCIIPushTab(viewer)); /* push1 */
          PetscCall(MatView(bA->m[i][j], viewer));
          PetscCall(PetscViewerASCIIPopTab(viewer)); /* pop1 */
        }
      }
    }
    PetscCall(PetscViewerASCIIPopTab(viewer)); /* pop0 */
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatZeroEntries_Nest(Mat A)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  PetscInt  i, j;

  PetscFunctionBegin;
  for (i = 0; i < bA->nr; i++) {
    for (j = 0; j < bA->nc; j++) {
      if (!bA->m[i][j]) continue;
      PetscCall(MatZeroEntries(bA->m[i][j]));
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatCopy_Nest(Mat A, Mat B, MatStructure str)
{
  Mat_Nest *bA = (Mat_Nest *)A->data, *bB = (Mat_Nest *)B->data;
  PetscInt  i, j, nr = bA->nr, nc = bA->nc;
  PetscBool nnzstate = PETSC_FALSE;

  PetscFunctionBegin;
  PetscCheck(nr == bB->nr && nc == bB->nc, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_INCOMP, "Cannot copy a Mat_Nest of block size (%" PetscInt_FMT ",%" PetscInt_FMT ") to a Mat_Nest of block size (%" PetscInt_FMT ",%" PetscInt_FMT ")", bB->nr, bB->nc, nr, nc);
  for (i = 0; i < nr; i++) {
    for (j = 0; j < nc; j++) {
      PetscObjectState subnnzstate = 0;
      if (bA->m[i][j] && bB->m[i][j]) {
        PetscCall(MatCopy(bA->m[i][j], bB->m[i][j], str));
        PetscCall(MatGetNonzeroState(bB->m[i][j], &subnnzstate));
        nnzstate                 = (PetscBool)(nnzstate || bB->nnzstate[i * nc + j] != subnnzstate);
        bB->nnzstate[i * nc + j] = subnnzstate;
      } else if (bA->m[i][j]) { // bB->m[i][j] is NULL
        Mat M;

        PetscCheck(str == DIFFERENT_NONZERO_PATTERN || str == UNKNOWN_NONZERO_PATTERN, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_INCOMP, "Matrix block does not exist at %" PetscInt_FMT ",%" PetscInt_FMT ". Use DIFFERENT_NONZERO_PATTERN or UNKNOWN_NONZERO_PATTERN", i, j);
        PetscCall(MatDuplicate(bA->m[i][j], MAT_COPY_VALUES, &M));
        PetscCall(MatNestSetSubMat(B, i, j, M));
        PetscCall(MatDestroy(&M));
      } else if (bB->m[i][j]) { // bA->m[i][j] is NULL
        PetscCheck(str == DIFFERENT_NONZERO_PATTERN || str == SUBSET_NONZERO_PATTERN || str == UNKNOWN_NONZERO_PATTERN, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_INCOMP, "Matrix block does not exist at %" PetscInt_FMT ",%" PetscInt_FMT ". Use DIFFERENT_NONZERO_PATTERN, SUBSET_NONZERO_PATTERN or UNKNOWN_NONZERO_PATTERN", i, j);
        PetscCall(MatNestSetSubMat(B, i, j, NULL));
      }
    }
  }
  if (nnzstate) B->nonzerostate++;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatAXPY_Nest(Mat Y, PetscScalar a, Mat X, MatStructure str)
{
  Mat_Nest *bY = (Mat_Nest *)Y->data, *bX = (Mat_Nest *)X->data;
  PetscInt  i, j, nr = bY->nr, nc = bY->nc;
  PetscBool nnzstate = PETSC_FALSE;

  PetscFunctionBegin;
  PetscCheck(nr == bX->nr && nc == bX->nc, PetscObjectComm((PetscObject)Y), PETSC_ERR_ARG_INCOMP, "Cannot AXPY a MatNest of block size (%" PetscInt_FMT ",%" PetscInt_FMT ") with a MatNest of block size (%" PetscInt_FMT ",%" PetscInt_FMT ")", bX->nr, bX->nc, nr, nc);
  for (i = 0; i < nr; i++) {
    for (j = 0; j < nc; j++) {
      PetscObjectState subnnzstate = 0;
      if (bY->m[i][j] && bX->m[i][j]) {
        PetscCall(MatAXPY(bY->m[i][j], a, bX->m[i][j], str));
      } else if (bX->m[i][j]) {
        Mat M;

        PetscCheck(str == DIFFERENT_NONZERO_PATTERN || str == UNKNOWN_NONZERO_PATTERN, PetscObjectComm((PetscObject)Y), PETSC_ERR_ARG_INCOMP, "Matrix block does not exist at %" PetscInt_FMT ",%" PetscInt_FMT ". Use DIFFERENT_NONZERO_PATTERN or UNKNOWN_NONZERO_PATTERN", i, j);
        PetscCall(MatDuplicate(bX->m[i][j], MAT_COPY_VALUES, &M));
        PetscCall(MatNestSetSubMat(Y, i, j, M));
        PetscCall(MatDestroy(&M));
      }
      if (bY->m[i][j]) PetscCall(MatGetNonzeroState(bY->m[i][j], &subnnzstate));
      nnzstate                 = (PetscBool)(nnzstate || bY->nnzstate[i * nc + j] != subnnzstate);
      bY->nnzstate[i * nc + j] = subnnzstate;
    }
  }
  if (nnzstate) Y->nonzerostate++;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatDuplicate_Nest(Mat A, MatDuplicateOption op, Mat *B)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  Mat      *b;
  PetscInt  i, j, nr = bA->nr, nc = bA->nc;

  PetscFunctionBegin;
  PetscCall(PetscMalloc1(nr * nc, &b));
  for (i = 0; i < nr; i++) {
    for (j = 0; j < nc; j++) {
      if (bA->m[i][j]) {
        PetscCall(MatDuplicate(bA->m[i][j], op, &b[i * nc + j]));
      } else {
        b[i * nc + j] = NULL;
      }
    }
  }
  PetscCall(MatCreateNest(PetscObjectComm((PetscObject)A), nr, bA->isglobal.row, nc, bA->isglobal.col, b, B));
  /* Give the new MatNest exclusive ownership */
  for (i = 0; i < nr * nc; i++) PetscCall(MatDestroy(&b[i]));
  PetscCall(PetscFree(b));

  PetscCall(MatAssemblyBegin(*B, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(*B, MAT_FINAL_ASSEMBLY));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* nest api */
static PetscErrorCode MatNestGetSubMat_Nest(Mat A, PetscInt idxm, PetscInt jdxm, Mat *mat)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;

  PetscFunctionBegin;
  PetscCheck(idxm < bA->nr, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_OUTOFRANGE, "Row too large: row %" PetscInt_FMT " max %" PetscInt_FMT, idxm, bA->nr - 1);
  PetscCheck(jdxm < bA->nc, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_OUTOFRANGE, "Col too large: row %" PetscInt_FMT " max %" PetscInt_FMT, jdxm, bA->nc - 1);
  *mat = bA->m[idxm][jdxm];
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  MatNestGetSubMat - Returns a single, sub-matrix from a `MATNEST`

  Not Collective

  Input Parameters:
+ A    - `MATNEST` matrix
. idxm - index of the matrix within the nest matrix
- jdxm - index of the matrix within the nest matrix

  Output Parameter:
. sub - matrix at index `idxm`, `jdxm` within the nest matrix

  Level: developer

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatNestGetSize()`, `MatNestGetSubMats()`, `MatCreateNest()`, `MatNestSetSubMat()`,
          `MatNestGetLocalISs()`, `MatNestGetISs()`
@*/
PetscErrorCode MatNestGetSubMat(Mat A, PetscInt idxm, PetscInt jdxm, Mat *sub)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, MAT_CLASSID, 1);
  PetscValidLogicalCollectiveInt(A, idxm, 2);
  PetscValidLogicalCollectiveInt(A, jdxm, 3);
  PetscAssertPointer(sub, 4);
  PetscUseMethod(A, "MatNestGetSubMat_C", (Mat, PetscInt, PetscInt, Mat *), (A, idxm, jdxm, sub));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestSetSubMat_Nest(Mat A, PetscInt idxm, PetscInt jdxm, Mat mat)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;
  PetscInt  m, n, M, N, mi, ni, Mi, Ni;

  PetscFunctionBegin;
  PetscCheck(idxm < bA->nr, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_OUTOFRANGE, "Row too large: row %" PetscInt_FMT " max %" PetscInt_FMT, idxm, bA->nr - 1);
  PetscCheck(jdxm < bA->nc, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_OUTOFRANGE, "Col too large: row %" PetscInt_FMT " max %" PetscInt_FMT, jdxm, bA->nc - 1);
  if (mat) {
    PetscCall(MatGetLocalSize(mat, &m, &n));
    PetscCall(MatGetSize(mat, &M, &N));
    PetscCall(ISGetLocalSize(bA->isglobal.row[idxm], &mi));
    PetscCall(ISGetSize(bA->isglobal.row[idxm], &Mi));
    PetscCall(ISGetLocalSize(bA->isglobal.col[jdxm], &ni));
    PetscCall(ISGetSize(bA->isglobal.col[jdxm], &Ni));
    PetscCheck(M == Mi && N == Ni, PetscObjectComm((PetscObject)mat), PETSC_ERR_ARG_INCOMP, "Submatrix dimension (%" PetscInt_FMT ",%" PetscInt_FMT ") incompatible with nest block (%" PetscInt_FMT ",%" PetscInt_FMT ")", M, N, Mi, Ni);
    PetscCheck(m == mi && n == ni, PetscObjectComm((PetscObject)mat), PETSC_ERR_ARG_INCOMP, "Submatrix local dimension (%" PetscInt_FMT ",%" PetscInt_FMT ") incompatible with nest block (%" PetscInt_FMT ",%" PetscInt_FMT ")", m, n, mi, ni);
  }

  /* do not increase object state */
  if (mat == bA->m[idxm][jdxm]) PetscFunctionReturn(PETSC_SUCCESS);

  PetscCall(PetscObjectReference((PetscObject)mat));
  PetscCall(MatDestroy(&bA->m[idxm][jdxm]));
  bA->m[idxm][jdxm] = mat;
  PetscCall(PetscObjectStateIncrease((PetscObject)A));
  if (mat) PetscCall(MatGetNonzeroState(mat, &bA->nnzstate[idxm * bA->nc + jdxm]));
  else bA->nnzstate[idxm * bA->nc + jdxm] = 0;
  A->nonzerostate++;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  MatNestSetSubMat - Set a single submatrix in the `MATNEST`

  Logically Collective

  Input Parameters:
+ A    - `MATNEST` matrix
. idxm - index of the matrix within the nest matrix
. jdxm - index of the matrix within the nest matrix
- sub  - matrix at index `idxm`, `jdxm` within the nest matrix

  Level: developer

  Notes:
  The new submatrix must have the same size and communicator as that block of the nest.

  This increments the reference count of the submatrix.

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatNestSetSubMats()`, `MatNestGetSubMats()`, `MatNestGetLocalISs()`, `MatCreateNest()`,
          `MatNestGetSubMat()`, `MatNestGetISs()`, `MatNestGetSize()`
@*/
PetscErrorCode MatNestSetSubMat(Mat A, PetscInt idxm, PetscInt jdxm, Mat sub)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, MAT_CLASSID, 1);
  PetscValidLogicalCollectiveInt(A, idxm, 2);
  PetscValidLogicalCollectiveInt(A, jdxm, 3);
  if (sub) PetscValidHeaderSpecific(sub, MAT_CLASSID, 4);
  PetscTryMethod(A, "MatNestSetSubMat_C", (Mat, PetscInt, PetscInt, Mat), (A, idxm, jdxm, sub));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestGetSubMats_Nest(Mat A, PetscInt *M, PetscInt *N, Mat ***mat)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;

  PetscFunctionBegin;
  if (M) *M = bA->nr;
  if (N) *N = bA->nc;
  if (mat) *mat = bA->m;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  MatNestGetSubMats - Returns the entire two dimensional array of matrices defining a `MATNEST` matrix.

  Not Collective

  Input Parameter:
. A - nest matrix

  Output Parameters:
+ M   - number of submatrix rows in the nest matrix
. N   - number of submatrix columns in the nest matrix
- mat - array of matrices

  Level: developer

  Note:
  The user should not free the array `mat`.

  Fortran Notes:
  This routine has a calling sequence `call MatNestGetSubMats(A, M, N, mat, ierr)`
  where the space allocated for the optional argument `mat` is assumed large enough (if provided).
  Matrices in `mat` are returned in row-major order, see `MatCreateNest()` for an example.

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatNestGetSize()`, `MatNestGetSubMat()`, `MatNestGetLocalISs()`, `MatCreateNest()`,
          `MatNestSetSubMats()`, `MatNestGetISs()`, `MatNestSetSubMat()`
@*/
PetscErrorCode MatNestGetSubMats(Mat A, PetscInt *M, PetscInt *N, Mat ***mat)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, MAT_CLASSID, 1);
  PetscUseMethod(A, "MatNestGetSubMats_C", (Mat, PetscInt *, PetscInt *, Mat ***), (A, M, N, mat));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestGetSize_Nest(Mat A, PetscInt *M, PetscInt *N)
{
  Mat_Nest *bA = (Mat_Nest *)A->data;

  PetscFunctionBegin;
  if (M) *M = bA->nr;
  if (N) *N = bA->nc;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  MatNestGetSize - Returns the size of the `MATNEST` matrix.

  Not Collective

  Input Parameter:
. A - `MATNEST` matrix

  Output Parameters:
+ M - number of rows in the nested mat
- N - number of cols in the nested mat

  Level: developer

  Note:
  `size` refers to the number of submatrices in the row and column directions of the nested matrix

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatNestGetSubMat()`, `MatNestGetSubMats()`, `MatCreateNest()`, `MatNestGetLocalISs()`,
          `MatNestGetISs()`
@*/
PetscErrorCode MatNestGetSize(Mat A, PetscInt *M, PetscInt *N)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, MAT_CLASSID, 1);
  PetscUseMethod(A, "MatNestGetSize_C", (Mat, PetscInt *, PetscInt *), (A, M, N));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestGetISs_Nest(Mat A, IS rows[], IS cols[])
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  i;

  PetscFunctionBegin;
  if (rows)
    for (i = 0; i < vs->nr; i++) rows[i] = vs->isglobal.row[i];
  if (cols)
    for (i = 0; i < vs->nc; i++) cols[i] = vs->isglobal.col[i];
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  MatNestGetISs - Returns the index sets partitioning the row and column spaces of a `MATNEST`

  Not Collective

  Input Parameter:
. A - `MATNEST` matrix

  Output Parameters:
+ rows - array of row index sets (pass `NULL` to ignore)
- cols - array of column index sets (pass `NULL` to ignore)

  Level: advanced

  Note:
  The user must have allocated arrays of the correct size. The reference count is not increased on the returned `IS`s.

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatNestGetSubMat()`, `MatNestGetSubMats()`, `MatNestGetSize()`, `MatNestGetLocalISs()`,
          `MatCreateNest()`, `MatNestSetSubMats()`
@*/
PetscErrorCode MatNestGetISs(Mat A, IS rows[], IS cols[])
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, MAT_CLASSID, 1);
  PetscUseMethod(A, "MatNestGetISs_C", (Mat, IS[], IS[]), (A, rows, cols));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestGetLocalISs_Nest(Mat A, IS rows[], IS cols[])
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  i;

  PetscFunctionBegin;
  if (rows)
    for (i = 0; i < vs->nr; i++) rows[i] = vs->islocal.row[i];
  if (cols)
    for (i = 0; i < vs->nc; i++) cols[i] = vs->islocal.col[i];
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  MatNestGetLocalISs - Returns the index sets partitioning the row and column spaces of a `MATNEST`

  Not Collective

  Input Parameter:
. A - `MATNEST` matrix

  Output Parameters:
+ rows - array of row index sets (pass `NULL` to ignore)
- cols - array of column index sets (pass `NULL` to ignore)

  Level: advanced

  Note:
  The user must have allocated arrays of the correct size. The reference count is not increased on the returned `IS`s.

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatNestGetSubMat()`, `MatNestGetSubMats()`, `MatNestGetSize()`, `MatNestGetISs()`, `MatCreateNest()`,
          `MatNestSetSubMats()`, `MatNestSetSubMat()`
@*/
PetscErrorCode MatNestGetLocalISs(Mat A, IS rows[], IS cols[])
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, MAT_CLASSID, 1);
  PetscUseMethod(A, "MatNestGetLocalISs_C", (Mat, IS[], IS[]), (A, rows, cols));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestSetVecType_Nest(Mat A, VecType vtype)
{
  PetscBool flg;

  PetscFunctionBegin;
  PetscCall(PetscStrcmp(vtype, VECNEST, &flg));
  /* In reality, this only distinguishes VECNEST and "other" */
  if (flg) A->ops->getvecs = MatCreateVecs_Nest;
  else A->ops->getvecs = NULL;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  MatNestSetVecType - Sets the type of `Vec` returned by `MatCreateVecs()`

  Not Collective

  Input Parameters:
+ A     - `MATNEST` matrix
- vtype - `VecType` to use for creating vectors

  Level: developer

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatCreateVecs()`, `MatCreateNest()`, `VecType`
@*/
PetscErrorCode MatNestSetVecType(Mat A, VecType vtype)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, MAT_CLASSID, 1);
  PetscTryMethod(A, "MatNestSetVecType_C", (Mat, VecType), (A, vtype));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestSetSubMats_Nest(Mat A, PetscInt nr, const IS is_row[], PetscInt nc, const IS is_col[], const Mat a[])
{
  Mat_Nest *s = (Mat_Nest *)A->data;
  PetscInt  i, j, m, n, M, N;
  PetscBool cong, isstd, sametype = PETSC_FALSE;
  VecType   vtype, type;

  PetscFunctionBegin;
  PetscCall(MatReset_Nest(A));

  s->nr = nr;
  s->nc = nc;

  /* Create space for submatrices */
  PetscCall(PetscMalloc1(nr, &s->m));
  PetscCall(PetscMalloc1(nr * nc, &s->m[0]));
  for (i = 0; i < nr; i++) {
    s->m[i] = s->m[0] + i * nc;
    for (j = 0; j < nc; j++) {
      s->m[i][j] = a ? a[i * nc + j] : NULL;
      PetscCall(PetscObjectReference((PetscObject)s->m[i][j]));
    }
  }
  PetscCall(MatGetVecType(A, &vtype));
  PetscCall(PetscStrcmp(vtype, VECSTANDARD, &isstd));
  if (isstd) {
    /* check if all blocks have the same vectype */
    vtype = NULL;
    for (i = 0; i < nr; i++) {
      for (j = 0; j < nc; j++) {
        if (s->m[i][j]) {
          if (!vtype) { /* first visited block */
            PetscCall(MatGetVecType(s->m[i][j], &vtype));
            sametype = PETSC_TRUE;
          } else if (sametype) {
            PetscCall(MatGetVecType(s->m[i][j], &type));
            PetscCall(PetscStrcmp(vtype, type, &sametype));
          }
        }
      }
    }
    if (sametype) { /* propagate vectype */
      PetscCall(MatSetVecType(A, vtype));
    }
  }

  PetscCall(MatSetUp_NestIS_Private(A, nr, is_row, nc, is_col));

  PetscCall(PetscMalloc1(nr, &s->row_len));
  PetscCall(PetscMalloc1(nc, &s->col_len));
  for (i = 0; i < nr; i++) s->row_len[i] = -1;
  for (j = 0; j < nc; j++) s->col_len[j] = -1;

  PetscCall(PetscCalloc1(nr * nc, &s->nnzstate));
  for (i = 0; i < nr; i++) {
    for (j = 0; j < nc; j++) {
      if (s->m[i][j]) PetscCall(MatGetNonzeroState(s->m[i][j], &s->nnzstate[i * nc + j]));
    }
  }

  PetscCall(MatNestGetSizes_Private(A, &m, &n, &M, &N));

  PetscCall(PetscLayoutSetSize(A->rmap, M));
  PetscCall(PetscLayoutSetLocalSize(A->rmap, m));
  PetscCall(PetscLayoutSetSize(A->cmap, N));
  PetscCall(PetscLayoutSetLocalSize(A->cmap, n));

  PetscCall(PetscLayoutSetUp(A->rmap));
  PetscCall(PetscLayoutSetUp(A->cmap));

  /* disable operations that are not supported for non-square matrices,
     or matrices for which is_row != is_col  */
  PetscCall(MatHasCongruentLayouts(A, &cong));
  if (cong && nr != nc) cong = PETSC_FALSE;
  if (cong) {
    for (i = 0; cong && i < nr; i++) PetscCall(ISEqualUnsorted(s->isglobal.row[i], s->isglobal.col[i], &cong));
  }
  if (!cong) {
    A->ops->missingdiagonal = NULL;
    A->ops->getdiagonal     = NULL;
    A->ops->shift           = NULL;
    A->ops->diagonalset     = NULL;
  }

  PetscCall(PetscCalloc2(nr, &s->left, nc, &s->right));
  PetscCall(PetscObjectStateIncrease((PetscObject)A));
  A->nonzerostate++;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  MatNestSetSubMats - Sets the nested submatrices in a `MATNEST`

  Collective

  Input Parameters:
+ A      - `MATNEST` matrix
. nr     - number of nested row blocks
. is_row - index sets for each nested row block, or `NULL` to make contiguous
. nc     - number of nested column blocks
. is_col - index sets for each nested column block, or `NULL` to make contiguous
- a      - array of $ nr \times nc$ submatrices, or `NULL`

  Level: advanced

  Notes:
  This always resets any block matrix information previously set.

  Pass `NULL` in the corresponding entry of `a` for an empty block.

  In both C and Fortran, `a` must be a one-dimensional array representing a two-dimensional row-major order array containing the matrices. See
  `MatCreateNest()` for an example.

  Fortran Note:
  Pass `PETSC_NULL_MAT` in the corresponding entry of `a` for an empty block

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatCreateNest()`, `MatNestSetSubMat()`, `MatNestGetSubMat()`, `MatNestGetSubMats()`
@*/
PetscErrorCode MatNestSetSubMats(Mat A, PetscInt nr, const IS is_row[], PetscInt nc, const IS is_col[], const Mat a[]) PeNSS
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, MAT_CLASSID, 1);
  PetscValidLogicalCollectiveInt(A, nr, 2);
  PetscCheck(nr >= 0, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_OUTOFRANGE, "Number of rows cannot be negative");
  if (nr && is_row) {
    PetscAssertPointer(is_row, 3);
    for (PetscInt i = 0; i < nr; i++) PetscValidHeaderSpecific(is_row[i], IS_CLASSID, 3);
  }
  PetscValidLogicalCollectiveInt(A, nc, 4);
  PetscCheck(nc >= 0, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_OUTOFRANGE, "Number of columns cannot be negative");
  if (nc && is_col) {
    PetscAssertPointer(is_col, 5);
    for (PetscInt i = 0; i < nc; i++) PetscValidHeaderSpecific(is_col[i], IS_CLASSID, 5);
  }
  PetscTryMethod(A, "MatNestSetSubMats_C", (Mat, PetscInt, const IS[], PetscInt, const IS[], const Mat[]), (A, nr, is_row, nc, is_col, a));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatNestCreateAggregateL2G_Private(Mat A, PetscInt n, const IS islocal[], const IS isglobal[], PetscBool colflg, ISLocalToGlobalMapping *ltog)
{
  PetscBool flg;
  PetscInt  i, j, m, mi, *ix;

  PetscFunctionBegin;
  *ltog = NULL;
  for (i = 0, m = 0, flg = PETSC_FALSE; i < n; i++) {
    if (islocal[i]) {
      PetscCall(ISGetLocalSize(islocal[i], &mi));
      flg = PETSC_TRUE; /* We found a non-trivial entry */
    } else {
      PetscCall(ISGetLocalSize(isglobal[i], &mi));
    }
    m += mi;
  }
  if (!flg) PetscFunctionReturn(PETSC_SUCCESS);

  PetscCall(PetscMalloc1(m, &ix));
  for (i = 0, m = 0; i < n; i++) {
    ISLocalToGlobalMapping smap = NULL;
    Mat                    sub  = NULL;
    PetscSF                sf;
    PetscLayout            map;
    const PetscInt        *ix2;

    if (!colflg) {
      PetscCall(MatNestFindNonzeroSubMatRow(A, i, &sub));
    } else {
      PetscCall(MatNestFindNonzeroSubMatCol(A, i, &sub));
    }
    if (sub) {
      if (!colflg) {
        PetscCall(MatGetLocalToGlobalMapping(sub, &smap, NULL));
      } else {
        PetscCall(MatGetLocalToGlobalMapping(sub, NULL, &smap));
      }
    }
    /*
       Now we need to extract the monolithic global indices that correspond to the given split global indices.
       In many/most cases, we only want MatGetLocalSubMatrix() to work, in which case we only need to know the size of the local spaces.
    */
    PetscCall(ISGetIndices(isglobal[i], &ix2));
    if (islocal[i]) {
      PetscInt *ilocal, *iremote;
      PetscInt  mil, nleaves;

      PetscCall(ISGetLocalSize(islocal[i], &mi));
      PetscCheck(smap, PetscObjectComm((PetscObject)A), PETSC_ERR_PLIB, "Missing local to global map");
      for (j = 0; j < mi; j++) ix[m + j] = j;
      PetscCall(ISLocalToGlobalMappingApply(smap, mi, ix + m, ix + m));

      /* PetscSFSetGraphLayout does not like negative indices */
      PetscCall(PetscMalloc2(mi, &ilocal, mi, &iremote));
      for (j = 0, nleaves = 0; j < mi; j++) {
        if (ix[m + j] < 0) continue;
        ilocal[nleaves]  = j;
        iremote[nleaves] = ix[m + j];
        nleaves++;
      }
      PetscCall(ISGetLocalSize(isglobal[i], &mil));
      PetscCall(PetscSFCreate(PetscObjectComm((PetscObject)A), &sf));
      PetscCall(PetscLayoutCreate(PetscObjectComm((PetscObject)A), &map));
      PetscCall(PetscLayoutSetLocalSize(map, mil));
      PetscCall(PetscLayoutSetUp(map));
      PetscCall(PetscSFSetGraphLayout(sf, map, nleaves, ilocal, PETSC_USE_POINTER, iremote));
      PetscCall(PetscLayoutDestroy(&map));
      PetscCall(PetscSFBcastBegin(sf, MPIU_INT, ix2, ix + m, MPI_REPLACE));
      PetscCall(PetscSFBcastEnd(sf, MPIU_INT, ix2, ix + m, MPI_REPLACE));
      PetscCall(PetscSFDestroy(&sf));
      PetscCall(PetscFree2(ilocal, iremote));
    } else {
      PetscCall(ISGetLocalSize(isglobal[i], &mi));
      for (j = 0; j < mi; j++) ix[m + j] = ix2[i];
    }
    PetscCall(ISRestoreIndices(isglobal[i], &ix2));
    m += mi;
  }
  PetscCall(ISLocalToGlobalMappingCreate(PetscObjectComm((PetscObject)A), 1, m, ix, PETSC_OWN_POINTER, ltog));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* If an IS was provided, there is nothing Nest needs to do, otherwise Nest will build a strided IS */
/*
  nprocessors = NP
  Nest x^T = ((g_0,g_1,...g_nprocs-1), (h_0,h_1,...h_NP-1))
       proc 0: => (g_0,h_0,)
       proc 1: => (g_1,h_1,)
       ...
       proc nprocs-1: => (g_NP-1,h_NP-1,)

            proc 0:                      proc 1:                    proc nprocs-1:
    is[0] = (0,1,2,...,nlocal(g_0)-1)  (0,1,...,nlocal(g_1)-1)  (0,1,...,nlocal(g_NP-1))

            proc 0:
    is[1] = (nlocal(g_0),nlocal(g_0)+1,...,nlocal(g_0)+nlocal(h_0)-1)
            proc 1:
    is[1] = (nlocal(g_1),nlocal(g_1)+1,...,nlocal(g_1)+nlocal(h_1)-1)

            proc NP-1:
    is[1] = (nlocal(g_NP-1),nlocal(g_NP-1)+1,...,nlocal(g_NP-1)+nlocal(h_NP-1)-1)
*/
static PetscErrorCode MatSetUp_NestIS_Private(Mat A, PetscInt nr, const IS is_row[], PetscInt nc, const IS is_col[])
{
  Mat_Nest *vs = (Mat_Nest *)A->data;
  PetscInt  i, j, offset, n, nsum, bs;
  Mat       sub = NULL;

  PetscFunctionBegin;
  PetscCall(PetscMalloc1(nr, &vs->isglobal.row));
  PetscCall(PetscMalloc1(nc, &vs->isglobal.col));
  if (is_row) { /* valid IS is passed in */
    /* refs on is[] are incremented */
    for (i = 0; i < vs->nr; i++) {
      PetscCall(PetscObjectReference((PetscObject)is_row[i]));
      vs->isglobal.row[i] = is_row[i];
    }
  } else { /* Create the ISs by inspecting sizes of a submatrix in each row */
    nsum = 0;
    for (i = 0; i < vs->nr; i++) { /* Add up the local sizes to compute the aggregate offset */
      PetscCall(MatNestFindNonzeroSubMatRow(A, i, &sub));
      PetscCheck(sub, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_WRONG, "No nonzero submatrix in row %" PetscInt_FMT, i);
      PetscCall(MatGetLocalSize(sub, &n, NULL));
      PetscCheck(n >= 0, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_WRONGSTATE, "Sizes have not yet been set for submatrix");
      nsum += n;
    }
    PetscCallMPI(MPI_Scan(&nsum, &offset, 1, MPIU_INT, MPI_SUM, PetscObjectComm((PetscObject)A)));
    offset -= nsum;
    for (i = 0; i < vs->nr; i++) {
      PetscCall(MatNestFindNonzeroSubMatRow(A, i, &sub));
      PetscCall(MatGetLocalSize(sub, &n, NULL));
      PetscCall(MatGetBlockSizes(sub, &bs, NULL));
      PetscCall(ISCreateStride(PetscObjectComm((PetscObject)sub), n, offset, 1, &vs->isglobal.row[i]));
      PetscCall(ISSetBlockSize(vs->isglobal.row[i], bs));
      offset += n;
    }
  }

  if (is_col) { /* valid IS is passed in */
    /* refs on is[] are incremented */
    for (j = 0; j < vs->nc; j++) {
      PetscCall(PetscObjectReference((PetscObject)is_col[j]));
      vs->isglobal.col[j] = is_col[j];
    }
  } else { /* Create the ISs by inspecting sizes of a submatrix in each column */
    offset = A->cmap->rstart;
    nsum   = 0;
    for (j = 0; j < vs->nc; j++) {
      PetscCall(MatNestFindNonzeroSubMatCol(A, j, &sub));
      PetscCheck(sub, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_WRONG, "No nonzero submatrix in column %" PetscInt_FMT, i);
      PetscCall(MatGetLocalSize(sub, NULL, &n));
      PetscCheck(n >= 0, PetscObjectComm((PetscObject)A), PETSC_ERR_ARG_WRONGSTATE, "Sizes have not yet been set for submatrix");
      nsum += n;
    }
    PetscCallMPI(MPI_Scan(&nsum, &offset, 1, MPIU_INT, MPI_SUM, PetscObjectComm((PetscObject)A)));
    offset -= nsum;
    for (j = 0; j < vs->nc; j++) {
      PetscCall(MatNestFindNonzeroSubMatCol(A, j, &sub));
      PetscCall(MatGetLocalSize(sub, NULL, &n));
      PetscCall(MatGetBlockSizes(sub, NULL, &bs));
      PetscCall(ISCreateStride(PetscObjectComm((PetscObject)sub), n, offset, 1, &vs->isglobal.col[j]));
      PetscCall(ISSetBlockSize(vs->isglobal.col[j], bs));
      offset += n;
    }
  }

  /* Set up the local ISs */
  PetscCall(PetscMalloc1(vs->nr, &vs->islocal.row));
  PetscCall(PetscMalloc1(vs->nc, &vs->islocal.col));
  for (i = 0, offset = 0; i < vs->nr; i++) {
    IS                     isloc;
    ISLocalToGlobalMapping rmap = NULL;
    PetscInt               nlocal, bs;
    PetscCall(MatNestFindNonzeroSubMatRow(A, i, &sub));
    if (sub) PetscCall(MatGetLocalToGlobalMapping(sub, &rmap, NULL));
    if (rmap) {
      PetscCall(MatGetBlockSizes(sub, &bs, NULL));
      PetscCall(ISLocalToGlobalMappingGetSize(rmap, &nlocal));
      PetscCall(ISCreateStride(PETSC_COMM_SELF, nlocal, offset, 1, &isloc));
      PetscCall(ISSetBlockSize(isloc, bs));
    } else {
      nlocal = 0;
      isloc  = NULL;
    }
    vs->islocal.row[i] = isloc;
    offset += nlocal;
  }
  for (i = 0, offset = 0; i < vs->nc; i++) {
    IS                     isloc;
    ISLocalToGlobalMapping cmap = NULL;
    PetscInt               nlocal, bs;
    PetscCall(MatNestFindNonzeroSubMatCol(A, i, &sub));
    if (sub) PetscCall(MatGetLocalToGlobalMapping(sub, NULL, &cmap));
    if (cmap) {
      PetscCall(MatGetBlockSizes(sub, NULL, &bs));
      PetscCall(ISLocalToGlobalMappingGetSize(cmap, &nlocal));
      PetscCall(ISCreateStride(PETSC_COMM_SELF, nlocal, offset, 1, &isloc));
      PetscCall(ISSetBlockSize(isloc, bs));
    } else {
      nlocal = 0;
      isloc  = NULL;
    }
    vs->islocal.col[i] = isloc;
    offset += nlocal;
  }

  /* Set up the aggregate ISLocalToGlobalMapping */
  {
    ISLocalToGlobalMapping rmap, cmap;
    PetscCall(MatNestCreateAggregateL2G_Private(A, vs->nr, vs->islocal.row, vs->isglobal.row, PETSC_FALSE, &rmap));
    PetscCall(MatNestCreateAggregateL2G_Private(A, vs->nc, vs->islocal.col, vs->isglobal.col, PETSC_TRUE, &cmap));
    if (rmap && cmap) PetscCall(MatSetLocalToGlobalMapping(A, rmap, cmap));
    PetscCall(ISLocalToGlobalMappingDestroy(&rmap));
    PetscCall(ISLocalToGlobalMappingDestroy(&cmap));
  }

  if (PetscDefined(USE_DEBUG)) {
    for (i = 0; i < vs->nr; i++) {
      for (j = 0; j < vs->nc; j++) {
        PetscInt m, n, M, N, mi, ni, Mi, Ni;
        Mat      B = vs->m[i][j];
        if (!B) continue;
        PetscCall(MatGetSize(B, &M, &N));
        PetscCall(MatGetLocalSize(B, &m, &n));
        PetscCall(ISGetSize(vs->isglobal.row[i], &Mi));
        PetscCall(ISGetSize(vs->isglobal.col[j], &Ni));
        PetscCall(ISGetLocalSize(vs->isglobal.row[i], &mi));
        PetscCall(ISGetLocalSize(vs->isglobal.col[j], &ni));
        PetscCheck(M == Mi && N == Ni, PetscObjectComm((PetscObject)sub), PETSC_ERR_ARG_INCOMP, "Global sizes (%" PetscInt_FMT ",%" PetscInt_FMT ") of nested submatrix (%" PetscInt_FMT ",%" PetscInt_FMT ") do not agree with space defined by index sets (%" PetscInt_FMT ",%" PetscInt_FMT ")", M, N, i, j, Mi, Ni);
        PetscCheck(m == mi && n == ni, PetscObjectComm((PetscObject)sub), PETSC_ERR_ARG_INCOMP, "Local sizes (%" PetscInt_FMT ",%" PetscInt_FMT ") of nested submatrix (%" PetscInt_FMT ",%" PetscInt_FMT ") do not agree with space defined by index sets (%" PetscInt_FMT ",%" PetscInt_FMT ")", m, n, i, j, mi, ni);
      }
    }
  }

  /* Set A->assembled if all non-null blocks are currently assembled */
  for (i = 0; i < vs->nr; i++) {
    for (j = 0; j < vs->nc; j++) {
      if (vs->m[i][j] && !vs->m[i][j]->assembled) PetscFunctionReturn(PETSC_SUCCESS);
    }
  }
  A->assembled = PETSC_TRUE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  MatCreateNest - Creates a new `MATNEST` matrix containing several nested submatrices, each stored separately

  Collective

  Input Parameters:
+ comm   - Communicator for the new `MATNEST`
. nr     - number of nested row blocks
. is_row - index sets for each nested row block, or `NULL` to make contiguous
. nc     - number of nested column blocks
. is_col - index sets for each nested column block, or `NULL` to make contiguous
- a      - array of $nr \times nc$ submatrices, empty submatrices can be passed using `NULL`

  Output Parameter:
. B - new matrix

  Level: advanced

  Note:
  In both C and Fortran, `a` must be a one-dimensional array representing a two-dimensional row-major order array holding references to the matrices.
  For instance, to represent the matrix
  $\begin{bmatrix} A_{11} & A_{12} \\ A_{21} & A_{22}\end{bmatrix}$
  one should use `Mat a[4]={A11,A12,A21,A22}`.

  Fortran Note:
  Pass `PETSC_NULL_MAT` in the corresponding entry of `a` for an empty block

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatCreate()`, `VecCreateNest()`, `DMCreateMatrix()`, `MatNestSetSubMat()`,
          `MatNestGetSubMat()`, `MatNestGetLocalISs()`, `MatNestGetSize()`,
          `MatNestGetISs()`, `MatNestSetSubMats()`, `MatNestGetSubMats()`
@*/
PetscErrorCode MatCreateNest(MPI_Comm comm, PetscInt nr, const IS is_row[], PetscInt nc, const IS is_col[], const Mat a[], Mat *B) PeNSS
{
  PetscFunctionBegin;
  PetscCall(MatCreate(comm, B));
  PetscCall(MatSetType(*B, MATNEST));
  (*B)->preallocated = PETSC_TRUE;
  PetscCall(MatNestSetSubMats(*B, nr, is_row, nc, is_col, a));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatConvert_Nest_SeqAIJ_fast(Mat A, MatType newtype, MatReuse reuse, Mat *newmat)
{
  Mat_Nest     *nest = (Mat_Nest *)A->data;
  Mat          *trans;
  PetscScalar **avv;
  PetscScalar  *vv;
  PetscInt    **aii, **ajj;
  PetscInt     *ii, *jj, *ci;
  PetscInt      nr, nc, nnz, i, j;
  PetscBool     done;

  PetscFunctionBegin;
  PetscCall(MatGetSize(A, &nr, &nc));
  if (reuse == MAT_REUSE_MATRIX) {
    PetscInt rnr;

    PetscCall(MatGetRowIJ(*newmat, 0, PETSC_FALSE, PETSC_FALSE, &rnr, (const PetscInt **)&ii, (const PetscInt **)&jj, &done));
    PetscCheck(done, PetscObjectComm((PetscObject)A), PETSC_ERR_PLIB, "MatGetRowIJ");
    PetscCheck(rnr == nr, PetscObjectComm((PetscObject)A), PETSC_ERR_USER, "Cannot reuse matrix, wrong number of rows");
    PetscCall(MatSeqAIJGetArray(*newmat, &vv));
  }
  /* extract CSR for nested SeqAIJ matrices */
  nnz = 0;
  PetscCall(PetscCalloc4(nest->nr * nest->nc, &aii, nest->nr * nest->nc, &ajj, nest->nr * nest->nc, &avv, nest->nr * nest->nc, &trans));
  for (i = 0; i < nest->nr; ++i) {
    for (j = 0; j < nest->nc; ++j) {
      Mat B = nest->m[i][j];
      if (B) {
        PetscScalar *naa;
        PetscInt    *nii, *njj, nnr;
        PetscBool    istrans;

        PetscCall(PetscObjectTypeCompare((PetscObject)B, MATTRANSPOSEVIRTUAL, &istrans));
        if (istrans) {
          Mat Bt;

          PetscCall(MatTransposeGetMat(B, &Bt));
          PetscCall(MatTranspose(Bt, MAT_INITIAL_MATRIX, &trans[i * nest->nc + j]));
          B = trans[i * nest->nc + j];
        } else {
          PetscCall(PetscObjectTypeCompare((PetscObject)B, MATHERMITIANTRANSPOSEVIRTUAL, &istrans));
          if (istrans) {
            Mat Bt;

            PetscCall(MatHermitianTransposeGetMat(B, &Bt));
            PetscCall(MatHermitianTranspose(Bt, MAT_INITIAL_MATRIX, &trans[i * nest->nc + j]));
            B = trans[i * nest->nc + j];
          }
        }
        PetscCall(MatGetRowIJ(B, 0, PETSC_FALSE, PETSC_FALSE, &nnr, (const PetscInt **)&nii, (const PetscInt **)&njj, &done));
        PetscCheck(done, PetscObjectComm((PetscObject)B), PETSC_ERR_PLIB, "MatGetRowIJ");
        PetscCall(MatSeqAIJGetArray(B, &naa));
        nnz += nii[nnr];

        aii[i * nest->nc + j] = nii;
        ajj[i * nest->nc + j] = njj;
        avv[i * nest->nc + j] = naa;
      }
    }
  }
  if (reuse != MAT_REUSE_MATRIX) {
    PetscCall(PetscMalloc1(nr + 1, &ii));
    PetscCall(PetscMalloc1(nnz, &jj));
    PetscCall(PetscMalloc1(nnz, &vv));
  } else {
    PetscCheck(nnz == ii[nr], PetscObjectComm((PetscObject)A), PETSC_ERR_USER, "Cannot reuse matrix, wrong number of nonzeros");
  }

  /* new row pointer */
  PetscCall(PetscArrayzero(ii, nr + 1));
  for (i = 0; i < nest->nr; ++i) {
    PetscInt ncr, rst;

    PetscCall(ISStrideGetInfo(nest->isglobal.row[i], &rst, NULL));
    PetscCall(ISGetLocalSize(nest->isglobal.row[i], &ncr));
    for (j = 0; j < nest->nc; ++j) {
      if (aii[i * nest->nc + j]) {
        PetscInt *nii = aii[i * nest->nc + j];
        PetscInt  ir;

        for (ir = rst; ir < ncr + rst; ++ir) {
          ii[ir + 1] += nii[1] - nii[0];
          nii++;
        }
      }
    }
  }
  for (i = 0; i < nr; i++) ii[i + 1] += ii[i];

  /* construct CSR for the new matrix */
  PetscCall(PetscCalloc1(nr, &ci));
  for (i = 0; i < nest->nr; ++i) {
    PetscInt ncr, rst;

    PetscCall(ISStrideGetInfo(nest->isglobal.row[i], &rst, NULL));
    PetscCall(ISGetLocalSize(nest->isglobal.row[i], &ncr));
    for (j = 0; j < nest->nc; ++j) {
      if (aii[i * nest->nc + j]) {
        PetscScalar *nvv = avv[i * nest->nc + j], vscale = 1.0, vshift = 0.0;
        PetscInt    *nii = aii[i * nest->nc + j];
        PetscInt    *njj = ajj[i * nest->nc + j];
        PetscInt     ir, cst;

        if (trans[i * nest->nc + j]) {
          vscale = ((Mat_Shell *)nest->m[i][j]->data)->vscale;
          vshift = ((Mat_Shell *)nest->m[i][j]->data)->vshift;
        }
        PetscCall(ISStrideGetInfo(nest->isglobal.col[j], &cst, NULL));
        for (ir = rst; ir < ncr + rst; ++ir) {
          PetscInt ij, rsize = nii[1] - nii[0], ist = ii[ir] + ci[ir];

          for (ij = 0; ij < rsize; ij++) {
            jj[ist + ij] = *njj + cst;
            vv[ist + ij] = vscale * *nvv;
            if (PetscUnlikely(vshift != 0.0 && *njj == ir - rst)) vv[ist + ij] += vshift;
            njj++;
            nvv++;
          }
          ci[ir] += rsize;
          nii++;
        }
      }
    }
  }
  PetscCall(PetscFree(ci));

  /* restore info */
  for (i = 0; i < nest->nr; ++i) {
    for (j = 0; j < nest->nc; ++j) {
      Mat B = nest->m[i][j];
      if (B) {
        PetscInt nnr = 0, k = i * nest->nc + j;

        B = (trans[k] ? trans[k] : B);
        PetscCall(MatRestoreRowIJ(B, 0, PETSC_FALSE, PETSC_FALSE, &nnr, (const PetscInt **)&aii[k], (const PetscInt **)&ajj[k], &done));
        PetscCheck(done, PetscObjectComm((PetscObject)B), PETSC_ERR_PLIB, "MatRestoreRowIJ");
        PetscCall(MatSeqAIJRestoreArray(B, &avv[k]));
        PetscCall(MatDestroy(&trans[k]));
      }
    }
  }
  PetscCall(PetscFree4(aii, ajj, avv, trans));

  /* finalize newmat */
  if (reuse == MAT_INITIAL_MATRIX) {
    PetscCall(MatCreateSeqAIJWithArrays(PetscObjectComm((PetscObject)A), nr, nc, ii, jj, vv, newmat));
  } else if (reuse == MAT_INPLACE_MATRIX) {
    Mat B;

    PetscCall(MatCreateSeqAIJWithArrays(PetscObjectComm((PetscObject)A), nr, nc, ii, jj, vv, &B));
    PetscCall(MatHeaderReplace(A, &B));
  }
  PetscCall(MatAssemblyBegin(*newmat, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(*newmat, MAT_FINAL_ASSEMBLY));
  {
    Mat_SeqAIJ *a = (Mat_SeqAIJ *)((*newmat)->data);
    a->free_a     = PETSC_TRUE;
    a->free_ij    = PETSC_TRUE;
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

PETSC_INTERN PetscErrorCode MatAXPY_Dense_Nest(Mat Y, PetscScalar a, Mat X)
{
  Mat_Nest *nest = (Mat_Nest *)X->data;
  PetscInt  i, j, k, rstart;
  PetscBool flg;

  PetscFunctionBegin;
  /* Fill by row */
  for (j = 0; j < nest->nc; ++j) {
    /* Using global column indices and ISAllGather() is not scalable. */
    IS              bNis;
    PetscInt        bN;
    const PetscInt *bNindices;
    PetscCall(ISAllGather(nest->isglobal.col[j], &bNis));
    PetscCall(ISGetSize(bNis, &bN));
    PetscCall(ISGetIndices(bNis, &bNindices));
    for (i = 0; i < nest->nr; ++i) {
      Mat             B = nest->m[i][j], D = NULL;
      PetscInt        bm, br;
      const PetscInt *bmindices;
      if (!B) continue;
      PetscCall(PetscObjectTypeCompareAny((PetscObject)B, &flg, MATTRANSPOSEVIRTUAL, MATHERMITIANTRANSPOSEVIRTUAL, ""));
      if (flg) {
        PetscTryMethod(B, "MatTransposeGetMat_C", (Mat, Mat *), (B, &D));
        PetscTryMethod(B, "MatHermitianTransposeGetMat_C", (Mat, Mat *), (B, &D));
        PetscCall(MatConvert(B, ((PetscObject)D)->type_name, MAT_INITIAL_MATRIX, &D));
        B = D;
      }
      PetscCall(PetscObjectTypeCompareAny((PetscObject)B, &flg, MATSEQSBAIJ, MATMPISBAIJ, ""));
      if (flg) {
        if (D) PetscCall(MatConvert(D, MATBAIJ, MAT_INPLACE_MATRIX, &D));
        else PetscCall(MatConvert(B, MATBAIJ, MAT_INITIAL_MATRIX, &D));
        B = D;
      }
      PetscCall(ISGetLocalSize(nest->isglobal.row[i], &bm));
      PetscCall(ISGetIndices(nest->isglobal.row[i], &bmindices));
      PetscCall(MatGetOwnershipRange(B, &rstart, NULL));
      for (br = 0; br < bm; ++br) {
        PetscInt           row = bmindices[br], brncols, *cols;
        const PetscInt    *brcols;
        const PetscScalar *brcoldata;
        PetscScalar       *vals = NULL;
        PetscCall(MatGetRow(B, br + rstart, &brncols, &brcols, &brcoldata));
        PetscCall(PetscMalloc1(brncols, &cols));
        for (k = 0; k < brncols; k++) cols[k] = bNindices[brcols[k]];
        /*
          Nest blocks are required to be nonoverlapping -- otherwise nest and monolithic index layouts wouldn't match.
          Thus, we could use INSERT_VALUES, but I prefer ADD_VALUES.
         */
        if (a != 1.0) {
          PetscCall(PetscMalloc1(brncols, &vals));
          for (k = 0; k < brncols; k++) vals[k] = a * brcoldata[k];
          PetscCall(MatSetValues(Y, 1, &row, brncols, cols, vals, ADD_VALUES));
          PetscCall(PetscFree(vals));
        } else {
          PetscCall(MatSetValues(Y, 1, &row, brncols, cols, brcoldata, ADD_VALUES));
        }
        PetscCall(MatRestoreRow(B, br + rstart, &brncols, &brcols, &brcoldata));
        PetscCall(PetscFree(cols));
      }
      if (D) PetscCall(MatDestroy(&D));
      PetscCall(ISRestoreIndices(nest->isglobal.row[i], &bmindices));
    }
    PetscCall(ISRestoreIndices(bNis, &bNindices));
    PetscCall(ISDestroy(&bNis));
  }
  PetscCall(MatAssemblyBegin(Y, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(Y, MAT_FINAL_ASSEMBLY));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatConvert_Nest_AIJ(Mat A, MatType newtype, MatReuse reuse, Mat *newmat)
{
  Mat_Nest   *nest = (Mat_Nest *)A->data;
  PetscInt    m, n, M, N, i, j, k, *dnnz, *onnz = NULL, rstart, cstart, cend;
  PetscMPIInt size;
  Mat         C;

  PetscFunctionBegin;
  PetscCallMPI(MPI_Comm_size(PetscObjectComm((PetscObject)A), &size));
  if (size == 1) { /* look for a special case with SeqAIJ matrices and strided-1, contiguous, blocks */
    PetscInt  nf;
    PetscBool fast;

    PetscCall(PetscStrcmp(newtype, MATAIJ, &fast));
    if (!fast) PetscCall(PetscStrcmp(newtype, MATSEQAIJ, &fast));
    for (i = 0; i < nest->nr && fast; ++i) {
      for (j = 0; j < nest->nc && fast; ++j) {
        Mat B = nest->m[i][j];
        if (B) {
          PetscCall(PetscObjectTypeCompare((PetscObject)B, MATSEQAIJ, &fast));
          if (!fast) {
            PetscBool istrans;

            PetscCall(PetscObjectTypeCompare((PetscObject)B, MATTRANSPOSEVIRTUAL, &istrans));
            if (istrans) {
              Mat Bt;

              PetscCall(MatTransposeGetMat(B, &Bt));
              PetscCall(PetscObjectTypeCompare((PetscObject)Bt, MATSEQAIJ, &fast));
            } else {
              PetscCall(PetscObjectTypeCompare((PetscObject)B, MATHERMITIANTRANSPOSEVIRTUAL, &istrans));
              if (istrans) {
                Mat Bt;

                PetscCall(MatHermitianTransposeGetMat(B, &Bt));
                PetscCall(PetscObjectTypeCompare((PetscObject)Bt, MATSEQAIJ, &fast));
              }
            }
            if (fast) fast = (PetscBool)(!((Mat_Shell *)B->data)->zrows && !((Mat_Shell *)B->data)->zcols && !((Mat_Shell *)B->data)->axpy && !((Mat_Shell *)B->data)->left && !((Mat_Shell *)B->data)->right && !((Mat_Shell *)B->data)->dshift);
          }
        }
      }
    }
    for (i = 0, nf = 0; i < nest->nr && fast; ++i) {
      PetscCall(PetscObjectTypeCompare((PetscObject)nest->isglobal.row[i], ISSTRIDE, &fast));
      if (fast) {
        PetscInt f, s;

        PetscCall(ISStrideGetInfo(nest->isglobal.row[i], &f, &s));
        if (f != nf || s != 1) {
          fast = PETSC_FALSE;
        } else {
          PetscCall(ISGetSize(nest->isglobal.row[i], &f));
          nf += f;
        }
      }
    }
    for (i = 0, nf = 0; i < nest->nc && fast; ++i) {
      PetscCall(PetscObjectTypeCompare((PetscObject)nest->isglobal.col[i], ISSTRIDE, &fast));
      if (fast) {
        PetscInt f, s;

        PetscCall(ISStrideGetInfo(nest->isglobal.col[i], &f, &s));
        if (f != nf || s != 1) {
          fast = PETSC_FALSE;
        } else {
          PetscCall(ISGetSize(nest->isglobal.col[i], &f));
          nf += f;
        }
      }
    }
    if (fast) {
      PetscCall(MatConvert_Nest_SeqAIJ_fast(A, newtype, reuse, newmat));
      PetscFunctionReturn(PETSC_SUCCESS);
    }
  }
  PetscCall(MatGetSize(A, &M, &N));
  PetscCall(MatGetLocalSize(A, &m, &n));
  PetscCall(MatGetOwnershipRangeColumn(A, &cstart, &cend));
  if (reuse == MAT_REUSE_MATRIX) C = *newmat;
  else {
    PetscCall(MatCreate(PetscObjectComm((PetscObject)A), &C));
    PetscCall(MatSetType(C, newtype));
    PetscCall(MatSetSizes(C, m, n, M, N));
  }
  PetscCall(PetscMalloc1(2 * m, &dnnz));
  if (m) {
    onnz = dnnz + m;
    for (k = 0; k < m; k++) {
      dnnz[k] = 0;
      onnz[k] = 0;
    }
  }
  for (j = 0; j < nest->nc; ++j) {
    IS              bNis;
    PetscInt        bN;
    const PetscInt *bNindices;
    PetscBool       flg;
    /* Using global column indices and ISAllGather() is not scalable. */
    PetscCall(ISAllGather(nest->isglobal.col[j], &bNis));
    PetscCall(ISGetSize(bNis, &bN));
    PetscCall(ISGetIndices(bNis, &bNindices));
    for (i = 0; i < nest->nr; ++i) {
      PetscSF         bmsf;
      PetscSFNode    *iremote;
      Mat             B = nest->m[i][j], D = NULL;
      PetscInt        bm, *sub_dnnz, *sub_onnz, br;
      const PetscInt *bmindices;
      if (!B) continue;
      PetscCall(ISGetLocalSize(nest->isglobal.row[i], &bm));
      PetscCall(ISGetIndices(nest->isglobal.row[i], &bmindices));
      PetscCall(PetscSFCreate(PetscObjectComm((PetscObject)A), &bmsf));
      PetscCall(PetscMalloc1(bm, &iremote));
      PetscCall(PetscMalloc1(bm, &sub_dnnz));
      PetscCall(PetscMalloc1(bm, &sub_onnz));
      for (k = 0; k < bm; ++k) {
        sub_dnnz[k] = 0;
        sub_onnz[k] = 0;
      }
      PetscCall(PetscObjectTypeCompareAny((PetscObject)B, &flg, MATTRANSPOSEVIRTUAL, MATHERMITIANTRANSPOSEVIRTUAL, ""));
      if (flg) {
        PetscTryMethod(B, "MatTransposeGetMat_C", (Mat, Mat *), (B, &D));
        PetscTryMethod(B, "MatHermitianTransposeGetMat_C", (Mat, Mat *), (B, &D));
        PetscCall(MatConvert(B, ((PetscObject)D)->type_name, MAT_INITIAL_MATRIX, &D));
        B = D;
      }
      PetscCall(PetscObjectTypeCompareAny((PetscObject)B, &flg, MATSEQSBAIJ, MATMPISBAIJ, ""));
      if (flg) {
        if (D) PetscCall(MatConvert(D, MATBAIJ, MAT_INPLACE_MATRIX, &D));
        else PetscCall(MatConvert(B, MATBAIJ, MAT_INITIAL_MATRIX, &D));
        B = D;
      }
      /*
       Locate the owners for all of the locally-owned global row indices for this row block.
       These determine the roots of PetscSF used to communicate preallocation data to row owners.
       The roots correspond to the dnnz and onnz entries; thus, there are two roots per row.
       */
      PetscCall(MatGetOwnershipRange(B, &rstart, NULL));
      for (br = 0; br < bm; ++br) {
        PetscInt        row = bmindices[br], brncols, col;
        const PetscInt *brcols;
        PetscInt        rowrel   = 0; /* row's relative index on its owner rank */
        PetscMPIInt     rowowner = 0;
        PetscCall(PetscLayoutFindOwnerIndex(A->rmap, row, &rowowner, &rowrel));
        /* how many roots  */
        iremote[br].rank  = rowowner;
        iremote[br].index = rowrel; /* edge from bmdnnz to dnnz */
        /* get nonzero pattern */
        PetscCall(MatGetRow(B, br + rstart, &brncols, &brcols, NULL));
        for (k = 0; k < brncols; k++) {
          col = bNindices[brcols[k]];
          if (col >= A->cmap->range[rowowner] && col < A->cmap->range[rowowner + 1]) {
            sub_dnnz[br]++;
          } else {
            sub_onnz[br]++;
          }
        }
        PetscCall(MatRestoreRow(B, br + rstart, &brncols, &brcols, NULL));
      }
      if (D) PetscCall(MatDestroy(&D));
      PetscCall(ISRestoreIndices(nest->isglobal.row[i], &bmindices));
      /* bsf will have to take care of disposing of bedges. */
      PetscCall(PetscSFSetGraph(bmsf, m, bm, NULL, PETSC_OWN_POINTER, iremote, PETSC_OWN_POINTER));
      PetscCall(PetscSFReduceBegin(bmsf, MPIU_INT, sub_dnnz, dnnz, MPI_SUM));
      PetscCall(PetscSFReduceEnd(bmsf, MPIU_INT, sub_dnnz, dnnz, MPI_SUM));
      PetscCall(PetscSFReduceBegin(bmsf, MPIU_INT, sub_onnz, onnz, MPI_SUM));
      PetscCall(PetscSFReduceEnd(bmsf, MPIU_INT, sub_onnz, onnz, MPI_SUM));
      PetscCall(PetscFree(sub_dnnz));
      PetscCall(PetscFree(sub_onnz));
      PetscCall(PetscSFDestroy(&bmsf));
    }
    PetscCall(ISRestoreIndices(bNis, &bNindices));
    PetscCall(ISDestroy(&bNis));
  }
  /* Resize preallocation if overestimated */
  for (i = 0; i < m; i++) {
    dnnz[i] = PetscMin(dnnz[i], A->cmap->n);
    onnz[i] = PetscMin(onnz[i], A->cmap->N - A->cmap->n);
  }
  PetscCall(MatSeqAIJSetPreallocation(C, 0, dnnz));
  PetscCall(MatMPIAIJSetPreallocation(C, 0, dnnz, 0, onnz));
  PetscCall(PetscFree(dnnz));
  PetscCall(MatAXPY_Dense_Nest(C, 1.0, A));
  if (reuse == MAT_INPLACE_MATRIX) PetscCall(MatHeaderReplace(A, &C));
  else *newmat = C;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatConvert_Nest_Dense(Mat A, MatType newtype, MatReuse reuse, Mat *newmat)
{
  Mat      B;
  PetscInt m, n, M, N;

  PetscFunctionBegin;
  PetscCall(MatGetSize(A, &M, &N));
  PetscCall(MatGetLocalSize(A, &m, &n));
  if (reuse == MAT_REUSE_MATRIX) {
    B = *newmat;
    PetscCall(MatZeroEntries(B));
  } else {
    PetscCall(MatCreateDense(PetscObjectComm((PetscObject)A), m, PETSC_DECIDE, M, N, NULL, &B));
  }
  PetscCall(MatAXPY_Dense_Nest(B, 1.0, A));
  if (reuse == MAT_INPLACE_MATRIX) PetscCall(MatHeaderReplace(A, &B));
  else if (reuse == MAT_INITIAL_MATRIX) *newmat = B;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatHasOperation_Nest(Mat mat, MatOperation op, PetscBool *has)
{
  Mat_Nest    *bA = (Mat_Nest *)mat->data;
  MatOperation opAdd;
  PetscInt     i, j, nr = bA->nr, nc = bA->nc;
  PetscBool    flg;

  PetscFunctionBegin;
  *has = PETSC_FALSE;
  if (op == MATOP_MULT || op == MATOP_MULT_ADD || op == MATOP_MULT_TRANSPOSE || op == MATOP_MULT_TRANSPOSE_ADD) {
    opAdd = (op == MATOP_MULT || op == MATOP_MULT_ADD ? MATOP_MULT_ADD : MATOP_MULT_TRANSPOSE_ADD);
    for (j = 0; j < nc; j++) {
      for (i = 0; i < nr; i++) {
        if (!bA->m[i][j]) continue;
        PetscCall(MatHasOperation(bA->m[i][j], opAdd, &flg));
        if (!flg) PetscFunctionReturn(PETSC_SUCCESS);
      }
    }
  }
  if (((void **)mat->ops)[op]) *has = PETSC_TRUE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*MC
  MATNEST -  "nest" - Matrix type consisting of nested submatrices, each stored separately.

  Level: intermediate

  Notes:
  This matrix type permits scalable use of `PCFIELDSPLIT` and avoids the large memory costs of extracting submatrices.
  It allows the use of symmetric and block formats for parts of multi-physics simulations.
  It is usually used with `DMCOMPOSITE` and `DMCreateMatrix()`

  Each of the submatrices lives on the same MPI communicator as the original nest matrix (though they can have zero
  rows/columns on some processes.) Thus this is not meant for cases where the submatrices live on far fewer processes
  than the nest matrix.

.seealso: [](ch_matrices), `Mat`, `MATNEST`, `MatCreate()`, `MatType`, `MatCreateNest()`, `MatNestSetSubMat()`, `MatNestGetSubMat()`,
          `VecCreateNest()`, `DMCreateMatrix()`, `DMCOMPOSITE`, `MatNestSetVecType()`, `MatNestGetLocalISs()`,
          `MatNestGetISs()`, `MatNestSetSubMats()`, `MatNestGetSubMats()`
M*/
PETSC_EXTERN PetscErrorCode MatCreate_Nest(Mat A)
{
  Mat_Nest *s;

  PetscFunctionBegin;
  PetscCall(PetscNew(&s));
  A->data = (void *)s;

  s->nr            = -1;
  s->nc            = -1;
  s->m             = NULL;
  s->splitassembly = PETSC_FALSE;

  PetscCall(PetscMemzero(A->ops, sizeof(*A->ops)));

  A->ops->mult                      = MatMult_Nest;
  A->ops->multadd                   = MatMultAdd_Nest;
  A->ops->multtranspose             = MatMultTranspose_Nest;
  A->ops->multtransposeadd          = MatMultTransposeAdd_Nest;
  A->ops->transpose                 = MatTranspose_Nest;
  A->ops->multhermitiantranspose    = MatMultHermitianTranspose_Nest;
  A->ops->multhermitiantransposeadd = MatMultHermitianTransposeAdd_Nest;
  A->ops->assemblybegin             = MatAssemblyBegin_Nest;
  A->ops->assemblyend               = MatAssemblyEnd_Nest;
  A->ops->zeroentries               = MatZeroEntries_Nest;
  A->ops->copy                      = MatCopy_Nest;
  A->ops->axpy                      = MatAXPY_Nest;
  A->ops->duplicate                 = MatDuplicate_Nest;
  A->ops->createsubmatrix           = MatCreateSubMatrix_Nest;
  A->ops->destroy                   = MatDestroy_Nest;
  A->ops->view                      = MatView_Nest;
  A->ops->getvecs                   = NULL; /* Use VECNEST by calling MatNestSetVecType(A,VECNEST) */
  A->ops->getlocalsubmatrix         = MatGetLocalSubMatrix_Nest;
  A->ops->restorelocalsubmatrix     = MatRestoreLocalSubMatrix_Nest;
  A->ops->getdiagonal               = MatGetDiagonal_Nest;
  A->ops->diagonalscale             = MatDiagonalScale_Nest;
  A->ops->scale                     = MatScale_Nest;
  A->ops->shift                     = MatShift_Nest;
  A->ops->diagonalset               = MatDiagonalSet_Nest;
  A->ops->setrandom                 = MatSetRandom_Nest;
  A->ops->hasoperation              = MatHasOperation_Nest;
  A->ops->missingdiagonal           = MatMissingDiagonal_Nest;

  A->spptr     = NULL;
  A->assembled = PETSC_FALSE;

  /* expose Nest api's */
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetSubMat_C", MatNestGetSubMat_Nest));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestSetSubMat_C", MatNestSetSubMat_Nest));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetSubMats_C", MatNestGetSubMats_Nest));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetSize_C", MatNestGetSize_Nest));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetISs_C", MatNestGetISs_Nest));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestGetLocalISs_C", MatNestGetLocalISs_Nest));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestSetVecType_C", MatNestSetVecType_Nest));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatNestSetSubMats_C", MatNestSetSubMats_Nest));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_mpiaij_C", MatConvert_Nest_AIJ));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_seqaij_C", MatConvert_Nest_AIJ));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_aij_C", MatConvert_Nest_AIJ));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_is_C", MatConvert_Nest_IS));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_mpidense_C", MatConvert_Nest_Dense));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatConvert_nest_seqdense_C", MatConvert_Nest_Dense));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatProductSetFromOptions_nest_seqdense_C", MatProductSetFromOptions_Nest_Dense));
  PetscCall(PetscObjectComposeFunction((PetscObject)A, "MatProductSetFromOptions_nest_mpidense_C", MatProductSetFromOptions_Nest_Dense));

  PetscCall(PetscObjectChangeTypeName((PetscObject)A, MATNEST));
  PetscFunctionReturn(PETSC_SUCCESS);
}