PetscCellMapping::extractEvolvingSubmatrix unit tests

Method that will extract the part of a PetscCellOperator that
couples evolving cells to other evolving cells.

Dummy implementation with failing tests.

Author: bendudson

include/bout/petsc_operators.hxx

@@ -44,6 +44,10 @@ struct ForwardLegSpaceTag {};
 /// leg space. See @ref ForwardLegSpaceTag and @ref CellSpaceTag.
 struct BackwardLegSpaceTag {};
 
+// Forward declare
+template <typename OutSpaceTag, typename InSpaceTag>
+class PetscOperator;
+
 /// @brief Bidirectional index mapping between mesh-file stored numbering and PETSc
 ///        row ownership.
 ///
@@ -290,6 +294,21 @@ public:
   ///          evolving cells, holding global PETSc row indices.
   IS makeEvolvingIS() const;
 
+  /// @brief Extract the evolving-cell submatrix from a cell-to-cell operator.
+  ///
+  /// Restricts @p op to the rows and columns that correspond to evolving
+  /// interior cells, discarding any rows or columns that belong to inner/outer
+  /// X-boundary cells or forward/backward parallel boundary virtual cells.
+  ///
+  /// The returned Mat is an independent copy (MAT_INITIAL_MATRIX): subsequent
+  /// modifications to @p op do not affect it.  The caller owns the returned
+  /// Mat and must call MatDestroy when finished.
+  ///
+  /// @param op A cell-to-cell operator whose row and column space is the full
+  ///           cell space C managed by this mapping.
+  /// @returns  A square Mat of global size n_evolving × n_evolving.
+  Mat extractEvolvingSubmatrix(const PetscOperator<CellSpaceTag, CellSpaceTag>& op) const;
+
 private:
   Field3D cell_number; ///< Stored cell numbers for interior/X-boundary cells.
   Field3D

src/mesh/petsc_operators.cxx

@@ -142,6 +142,14 @@ IS PetscCellMapping::makeEvolvingIS() const {
   return is;
 }
 
+// Dummy implementation. Note: Result is owned by the caller.
+Mat PetscCellMapping::extractEvolvingSubmatrix(
+    const PetscOperator<CellSpaceTag, CellSpaceTag>& op) const {
+  Mat new_mat;
+  BOUT_DO_PETSC(MatDuplicate(op.raw(), MAT_COPY_VALUES, &new_mat));
+  return new_mat;
+}
+
 PetscLegMapping::PetscLegMapping(int total_legs, std::vector<int> local_leg_indices) {
   std::sort(local_leg_indices.begin(), local_leg_indices.end());
   local_leg_indices.erase(std::unique(local_leg_indices.begin(), local_leg_indices.end()),

tests/unit/CMakeLists.txt

@@ -99,6 +99,7 @@ set(serial_tests_source
     ./mesh/test_paralleltransform.cxx
     ./mesh/test_petsc_operators.cxx
     ./mesh/test_petsc_operators_make_evolving_is.cxx
+    ./mesh/test_petsc_operators_extract_evolving_submatrix.cxx
     ./solver/test_fakesolver.cxx
     ./solver/test_fakesolver.hxx
     ./solver/test_solver.cxx