
ID	0051
Class	TEST
Severity	1
Status	ready

Models: —

Found: 2026-10-20 architecture review (a406e955); owner ruled "default ok" on the whole plan 2026-10-20

Source plan: plans/arch-2026-10-20/not-implemented/f9-sundials-solver-plan.md (workspace fhsim_workspace)

Kind: Bit for bit refactor. Take when passing; rides with: next feature in src/engine/integrator/.

Evidence

src/engine/integrator/sundials/FhIntegratorSundials.cpp:352-402, FhIntegratorSundials::JacobianCallback. The block :368-399 copies the dense buffer localJ into a dense (:369), sparse (:375) or band (SUNMATRIX_BAND) SUNMatrix inline inside the callback, so each copy can only be reached by running CVODE/ARKODE.

The sparse branch may be unreachable: a sparse matrix is only accepted with SPGMR (SolverPlan.cpp:153-163 refuses it for DENSE and BAND), and CVODE/ARKODE may not call JacFn for a matrix-free SPGMR. Check that first.

Effect

None for users. The three copies have no direct unit test.

Possible fix

Free function CopyIntoSunMatrix(localJ, n, J, sparsity) in the same file's anonymous namespace or a small header, with one unit test per matrix type. Worth doing only when someone touches this code for another reason; if the sparse branch is unreachable, say so there instead of testing it.

Test that would prove it

Unit tests that build a SUNMatrix of each type, call CopyIntoSunMatrix and compare entries.

Risk

Very low; the loops move unchanged. Keep the rule that the SUNDIALS callbacks do not call OdeFcn (settled-decisions.md).