#include <../src/mat/impls/aij/mpi/mpiaij.h>

PetscErrorCode MatGetMultiProcBlock_MPIAIJ(Mat mat, MPI_Comm subComm, MatReuse scall, Mat *subMat)
{
  Mat_MPIAIJ  *aij  = (Mat_MPIAIJ *)mat->data;
  Mat_SeqAIJ  *aijB = (Mat_SeqAIJ *)aij->B->data;
  PetscMPIInt  subCommSize, subCommRank;
  PetscMPIInt *commRankMap, subRank, rank, commRank;
  PetscInt    *garrayCMap, col, i, j, *nnz, newRow, newCol;

  PetscFunctionBegin;
  PetscCallMPI(MPI_Comm_size(subComm, &subCommSize));
  PetscCallMPI(MPI_Comm_rank(subComm, &subCommRank));
  PetscCallMPI(MPI_Comm_rank(PetscObjectComm((PetscObject)mat), &commRank));

  /* create subMat object with the relevant layout */
  if (scall == MAT_INITIAL_MATRIX) {
    PetscCall(MatCreate(subComm, subMat));
    PetscCall(MatSetType(*subMat, MATMPIAIJ));
    PetscCall(MatSetSizes(*subMat, mat->rmap->n, mat->cmap->n, PETSC_DECIDE, PETSC_DECIDE));
    PetscCall(MatSetBlockSizesFromMats(*subMat, mat, mat));

    /* need to setup rmap and cmap before Preallocation */
    PetscCall(PetscLayoutSetUp((*subMat)->rmap));
    PetscCall(PetscLayoutSetUp((*subMat)->cmap));
  }

  /* create a map of comm_rank from subComm to comm - should commRankMap and garrayCMap be kept for reused? */
  PetscCall(PetscMalloc1(subCommSize, &commRankMap));
  PetscCallMPI(MPI_Allgather(&commRank, 1, MPI_INT, commRankMap, 1, MPI_INT, subComm));

  /* Traverse garray and identify column indices [of offdiag mat] that
   should be discarded. For the ones not discarded, store the newCol+1
   value in garrayCMap */
  PetscCall(PetscCalloc1(aij->B->cmap->n, &garrayCMap));
  for (i = 0; i < aij->B->cmap->n; i++) {
    col = aij->garray[i];
    for (subRank = 0; subRank < subCommSize; subRank++) {
      rank = commRankMap[subRank];
      if ((col >= mat->cmap->range[rank]) && (col < mat->cmap->range[rank + 1])) {
        garrayCMap[i] = (*subMat)->cmap->range[subRank] + col - mat->cmap->range[rank] + 1;
        break;
      }
    }
  }

  if (scall == MAT_INITIAL_MATRIX) {
    /* Compute preallocation for the offdiag mat */
    PetscCall(PetscCalloc1(aij->B->rmap->n, &nnz));
    for (i = 0; i < aij->B->rmap->n; i++) {
      for (j = aijB->i[i]; j < aijB->i[i + 1]; j++) {
        if (garrayCMap[aijB->j[j]]) nnz[i]++;
      }
    }
    PetscCall(MatMPIAIJSetPreallocation(*(subMat), 0, NULL, 0, nnz));

    /* reuse diag block with the new submat */
    PetscCall(MatDestroy(&((Mat_MPIAIJ *)((*subMat)->data))->A));
    ((Mat_MPIAIJ *)((*subMat)->data))->A = aij->A;
    PetscCall(PetscObjectReference((PetscObject)aij->A));
  } else if (((Mat_MPIAIJ *)(*subMat)->data)->A != aij->A) {
    PetscObject obj = (PetscObject)((Mat_MPIAIJ *)((*subMat)->data))->A;
    PetscCall(PetscObjectReference(obj));
    ((Mat_MPIAIJ *)((*subMat)->data))->A = aij->A;
    PetscCall(PetscObjectReference((PetscObject)aij->A));
  }

  /* Traverse aij->B and insert values into subMat */
  if ((*subMat)->assembled) {
    (*subMat)->was_assembled = PETSC_TRUE;
    (*subMat)->assembled     = PETSC_FALSE;
  }
  for (i = 0; i < aij->B->rmap->n; i++) {
    newRow = (*subMat)->rmap->range[subCommRank] + i;
    for (j = aijB->i[i]; j < aijB->i[i + 1]; j++) {
      newCol = garrayCMap[aijB->j[j]];
      if (newCol) {
        newCol--; /* remove the increment */
        PetscCall(MatSetValues_MPIAIJ(*subMat, 1, &newRow, 1, &newCol, aijB->a + j, INSERT_VALUES));
      }
    }
  }
  PetscCall(MatAssemblyBegin(*subMat, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(*subMat, MAT_FINAL_ASSEMBLY));

  /* deallocate temporary data */
  PetscCall(PetscFree(commRankMap));
  PetscCall(PetscFree(garrayCMap));
  if (scall == MAT_INITIAL_MATRIX) PetscCall(PetscFree(nnz));
  PetscFunctionReturn(PETSC_SUCCESS);
}