pounce-feral

POUNCE's default sparse symmetric LDLᵀ backend, built on the pure-Rust FERAL library. Implements SparseSymLinearSolverInterface so the upstream TSymLinearSolver wrapper requires no changes versus the MA57 path.

This crate is what makes the default POUNCE build pure Rust — no Fortran, no HSL, no system BLAS required.

Lifecycle

The wrapper mirrors pounce-hsl::Ma57SolverInterface:

1. matrix_format() returns EMatrixFormat::TripletFormat (1-based, lower-triangle COO).
2. initialize_structure caches 0-based row/col arrays and allocates the values buffer; the upper-triangle entries that POUNCE's KKT assembly emits are canonicalized to the lower triangle here, because FERAL's CscMatrix::from_triplets silently drops upper-triangle entries during LDLᵀ.
3. multi_solve rebuilds the CSC matrix from the cached pattern + caller-filled values, then dispatches to feral::Solver::factor / solve_many. FERAL's pattern-fingerprint cache reuses the symbolic factorization across iterates with identical structure — the IPM common case.
4. increase_quality delegates to feral::Solver::increase_quality and uses MA57's pivtol_changed / CallAgain protocol so the upper-layer reload-and-retry semantics line up.

When to use FERAL vs MA57

FERAL is the default. MA57 is generally faster for very large sparse KKT systems (≳ 50k variables) and remains the option of choice when libcoinhsl is available; enable it with --features ma57 on pounce-cli. See benchmarks/BENCHMARK_REPORT.md for a head-to-head comparison across the .nl benchmark suites.

License

EPL-2.0.