addOperatorSparsity: Add unit tests

This function will add non-zeros to a Jacobian matrix, based on
a submatrix from a PetscCellOperator.

Dummy implementation so unit tests fail.

Author: bendudson

CMakeLists.txt

@@ -178,6 +178,7 @@ set(BOUT_SOURCES
     ./include/bout/paralleltransform.hxx
     ./include/bout/petsc_interface.hxx
     ./include/bout/petsc_operators.hxx
+    ./include/bout/petsc_jacobian.hxx
     ./include/bout/petsclib.hxx
     ./include/bout/physicsmodel.hxx
     ./include/bout/rajalib.hxx
@@ -310,6 +311,7 @@ set(BOUT_SOURCES
     ./src/mesh/parallel_boundary_op.cxx
     ./src/mesh/parallel_boundary_region.cxx
     ./src/mesh/petsc_operators.cxx
+    ./src/mesh/petsc_jacobian.cxx
     ./src/mesh/surfaceiter.cxx
     ./src/physics/gyro_average.cxx
     ./src/physics/physicsmodel.cxx

include/bout/petsc_jacobian.hxx

@@ -0,0 +1,32 @@
+#pragma once
+
+#ifndef BOUT_PETSC_JACOBIAN_H
+#define BOUT_PETSC_JACOBIAN_H
+
+#include "bout/build_defines.hxx"
+
+#if BOUT_HAS_PETSC
+
+#include <petscmat.h>
+
+/// Insert the nonzero pattern of @p sub into the variable block
+/// (@p out_var, @p in_var) of the Jacobian @p Jfd.
+///
+/// @p Jfd is a square matrix of size (n_evolving * nvars) where nvars is
+/// inferred as Jfd_global_size / sub_global_size.  Each nonzero (r, c) in
+/// @p sub produces an entry at (r * nvars + out_var, c * nvars + in_var)
+/// in @p Jfd.
+///
+/// @p Jfd must already be preallocated.  Entries are inserted with
+/// INSERT_VALUES; MatAssemblyBegin/End must be called by the caller after
+/// all insertions are complete.
+///
+/// @param Jfd     The Jacobian matrix to populate. Must be preallocated.
+/// @param sub     Evolving-cell submatrix providing the nonzero pattern.
+/// @param out_var Row variable index in [0, nvars).
+/// @param in_var  Column variable index in [0, nvars).
+void addOperatorSparsity(Mat Jfd, Mat sub, int out_var, int in_var);
+
+#endif // BOUT_HAS_PETSC
+
+#endif // BOUT_PETSC_JACOBIAN_H

src/mesh/petsc_jacobian.cxx

@@ -0,0 +1,3 @@
+#include "bout/petsc_jacobian.hxx"
+
+void addOperatorSparsity(Mat Jfd, Mat sub, int out_var, int in_var) {}

tests/unit/CMakeLists.txt

@@ -100,6 +100,7 @@ set(serial_tests_source
     ./mesh/test_petsc_operators.cxx
     ./mesh/test_petsc_operators_make_evolving_is.cxx
     ./mesh/test_petsc_operators_extract_evolving_submatrix.cxx
+    ./mesh/test_petsc_jacobian.cxx
     ./solver/test_fakesolver.cxx
     ./solver/test_fakesolver.hxx
     ./solver/test_solver.cxx