ripopt v0.6.2

[0.6.2] - 2026-04-12

Added

• Reorganized benchmarks under benchmarks/ with one self-contained
  subdirectory per suite: hs/, cutest/, electrolyte/, grid/ (renamed
  from opf/), cho/, large_scale/, gas/, and the new water/. Each
  suite has its own README and per-suite report. The composite report at
  benchmarks/BENCHMARK_REPORT.md aggregates HS + CUTEst + electrolyte +
  grid + CHO + large-scale; the gas and water suites are standalone (AMPL
  .nl interface, per-problem .sol output).
• benchmarks/water/: 6 water distribution network design NLPs from
  MINLPLib (Hazen-Williams head-loss formulation). Solved via the AMPL
  interface; ripopt matches the best-known primal bound for water.nl
  (963.13) where Ipopt converges to a different local minimum (1001.16).
• benchmarks/gas/: 4 gas pipeline NLPs from PDE-discretized Euler
  equations on pipe networks (gaslib11/40, steady/dynamic). Solved via the
  AMPL interface.
• Loqo mu oracle (mu_oracle = "loqo") enabled by default, matching Ipopt's
  mu_oracle=quality-function strategy. Uses centrality measure
  xi = min(z*s)/avg_compl to set centering parameter sigma, preventing
  premature mu decrease from highly infeasible starting points. On gaslib11_steady:
  164→134 iters, NLP restorations 1→0, mode switches 24→8.
• Sparse Gauss-Newton restoration: sparse J*J^T factorization for GN restoration
  when m > 500, removing the dense Bunch-Kaufman bottleneck (6s/step → 0.02s for
  gas pipeline NLPs). Sparse LS multiplier estimates for post-restoration y
  initialization.
• Ruiz equilibration KKT scaling matching MUMPS SimScale schedule
  (KEEP(52)=7 for SYM=2): 1 inf-norm iteration + 3 one-norm iterations. Activated
  on demand when backward error is poor. Added row_abs_sum() to
  DenseSymmetric, SparseSymmetric, and KktMatrix.
• Pretend-singular fallback using Ipopt's normwise residual ratio (threshold
  1e-5). When iterative refinement cannot reach target accuracy, tries delta_c
  first (PerturbForSingularity), then delta_w.
• Adaptive mu dual infeasibility safeguard: prevents mu from collapsing to
  1e-11 while dual infeasibility remains large. Adds du_floor to barrier error
  and dual-infeasibility stagnation detection.
• Structural degeneracy detection: after 3 consecutive iterations requiring
  delta_w > 0, skips the unperturbed factorization trial on subsequent iterations.
• Dense BK increase_quality(): configurable pivot threshold with escalation
  0.64 → 0.8 → 0.95 → 1.0.
• KKT factorization diagnostics (dim, nnz, wall time) at print_level ≥ 5.
  Line-search rejection details at print_level ≥ 7.

Changed

• PretendSingular chain now matches Ipopt's PDFullSpaceSolver:
  solve → refine(fail) → IncreaseQuality → re-solve → perturbation.
  Previously perturbation was applied before IncreaseQuality.
• Iterative refinement: max steps raised from 3–5 to 5–10 (matching Ipopt
  default). Stagnation detection extended to the non-IC path with a relaxed
  factor (1−1e−6 vs Ipopt's 1−1e−9).
• Default sparse threshold unchanged (n+m ≥ 110) but sparse GN restoration
  now activates at m > 500 even when the outer IPM is dense.
• Workspace version bumped — ripopt → 0.6.2, rmumps → 0.1.1.
• ref/ directory and pyomo-ripopt/build/ now ignored in git.

Fixed

• Fallback-result regression (is_strictly_better): when the main IPM
  reports NumericalError at a feasible iterate with a meaningful objective,
  a fallback solver that converges to a worse local minimum no longer
  silently replaces the main-IPM result. The comparator now requires either
  strict objective improvement (with primal feasibility ≤ 1e-4) or that the
  current result has no usable objective. This was the root cause of the
  c_api_hs071_basic, c_api_hs071_multiplier_extraction,
  c_api_null_output_params, test_hs071_sensitivity_vs_finite_differences,
  and test_sensitivity_linear_prediction_accuracy regressions introduced by
  the recent Loqo oracle / KKT quality-chain work.
• Phosphoric-acid electrolyte test (electrolyte_05_phosphoric_acid)
  pinned to mu_oracle_quality_function=false. The Loqo oracle steers the
  solver into a chemically-wrong local minimum (pH ≈ 11.84) of the Gibbs
  free-energy surface; the pre-Loqo default converges to the correct basin
  (pH ≈ 2.25). Documented in the test comment.
• All compiler warnings cleaned up in src/ipm.rs, rmumps/src/frontal.rs,
  and the adversary example suite.
• 13 adversary example files updated to the current NlpProblem trait
  signature (removed _new_x: bool parameters and -> bool return types).
• tests/large_scale_benchmark.rs unused import cleaned up.

Performance

• Fresh benchmark results (2026-04-11 on Apple Mac Mini, aarch64-apple-darwin):
  • HS suite: ripopt 115/120 (95.8%), Ipopt 116/120 (96.7%) — nearly tied.
    14.0× geometric mean speedup on 113 commonly solved, median 15.1×,
    ripopt faster on 111/113 (98%).
  • CUTEst suite: ripopt 553/727 (76.1%), Ipopt 561/727 (77.2%). 8.0× geometric
    mean speedup on 513 commonly solved, median 18.8×, ripopt faster on 415/513 (81%).
    ripopt-only solves: 40; Ipopt-only solves: 48.
  • Electrolyte thermodynamics: ripopt 13/13 (100%), Ipopt 12/13, 20.8× geo mean.
  • Grid (AC OPF): 4/4 for both, Ipopt faster (0.2× geo mean).
• gas pipeline NLPs: sparse GN restoration reduces per-step cost 300× on m > 500.

Notes

• The fresh benchmark shows a small shift in both solvers' solve counts vs. the
  0.6.1 numbers reported in the prior CHANGELOG. This reflects run-to-run
  floating-point sensitivity on borderline problems combined with the quality-chain
  changes; the dominant failure modes (NumericalError, LocalInfeasibility) are
  unchanged.