perf: reduce interval and certified reduction overhead

[acgetchell]

The public interval and certified-reduction APIs passed correctness checks but materially slowed Delaunay's predicate and simplex-intersection adoption candidates. This change removes redundant arithmetic and certificate work while preserving checked inputs, finite outward endpoints, typed failures, const evaluation, and exact fallback.

Implementation

• Reuse singleton interval results, round only the needed sum endpoints, and size determinant workspaces to the compile-time dimension without changing expansion order.
• Use a proved FMA residual fast path for product rounding, retaining integer comparison wherever residual underflow could lose information.
• Store scalar estimates and error bounds compactly; streamline magnitude accumulation and omit only provably bit-identical zero-product FMAs. Single normal unit products receive an exact zero bound, including at f64::MAX.
• Expand the upstream harness to 183 cases and retain independent exact-rational checks, mathematical derivations, downstream adoption patches, raw measurements, and unsuccessful intermediate candidates.
• Protect zero/signed-zero subtraction, tight square bounds, D=0/1 determinants, and persistent certified proof loss with regression tests. Validate scalar benchmark endpoint bits before timing, and keep every reproduction invocation's raw data and logs in a separate directory.

Performance evidence

Same-machine Rust 1.99 / LLVM 23 comparisons use identical fixtures, features, profiles, and sampling settings for each pair, with downstream runs repeated in reverse order:

• In-sphere workloads: 8–21% faster across D=2–5.
• Single shared vertex in 2D: 9–13% faster; whole 2D validation changes by −0.3% / +1.3%.
• Other whole-validation controls show small slowdowns. The whole-3D reverse run's 95% interval narrowly crosses the study's 5% materiality band; an extended 200-sample control measured −1.32%, so that material slowdown did not repeat.
• Three scalar shortcut microbenchmarks regress by 0.15–0.92 ns, with final costs below 2 ns. These tradeoffs remain documented.

Downstream gains include the retained caller adapter, including projection reuse and direct per-vertex loops; they are not library-only speed claims. Vector construction is measured separately and unchanged.

See the complete study and reproducible evidence for every case, confidence intervals, provenance, and limitations. Historical measurements retain their original source, harness, and dependency locks in the evidence archive; the follow-up changes preserve arithmetic and timed closures.

Validation

• At dd909a7, just check and just ci passed: 904 Rust tests, 499 Python tests, both doctest configurations, Clippy/rustdoc, static analysis, benchmark compilation, and examples.
• 54 exact-arithmetic properties passed with PROPTEST_CASES=512 and PROPTEST_RNG_SEED=248.
• The isolated downstream adapter passed Clippy and 97 selected predicate/intersection tests, including D=6 agreement and shared-face/overlap cases.
• Final Markdown, spelling, shell, and documentation checks passed. Saved downstream patches apply cleanly in a dry run.
• At 1858b5e, just coverage-ci passed all 830 tests. Local patch line coverage is 100% (309/309), up from the initial Codecov report's 97.64%; policy-filtered project coverage is 98.13%. Targeted default/exact tests, Rust core checks, and spelling passed.
• At 0b97b4b, just check and the full just ci passed: 913 Rust tests, 499 Python tests, both doctest configurations, Clippy/rustdoc, static analysis, benchmark compilation, and examples. GitHub CI also passed on Linux, macOS, and Windows. Context-manager generator annotations and explicit optional-value checks resolve all four diagnostics from ty 0.0.85 while preserving support-script behavior. Coverage thresholds and exclusions are unchanged.
• At 73dd02f, focused tests cover absent and empty retained-output mappings, unchanged report files, and the complete retained-path collision error before mutation. just check and just python-ci passed, including all 502 Python tests; coderabbit review --agent --uncommitted completed with zero findings for the test follow-up. No additional production-code change was needed.

Completes #247 and #248. This PR supplies the upstream implementation and acceptance evidence; publication in v0.4.7 does not gate closure of those implementation issues. Downstream adoption and its correctness/performance acceptance are tracked separately in acgetchell/delaunay#577 (interval determinant signs) and acgetchell/delaunay#579 (certified linear-form bounds). Publication remains separate release work. The la-stack package version and generated changelog are unchanged.

Summary by CodeRabbit

• Improvements

  • Interval arithmetic and certified dot products provide tighter, more reliable results across edge cases, including underflow, overflow, and signed zero.
  • Determinant calculations use dimension-specific workspace sizing.

• Compatibility

  • ScalarWithErrorBound no longer exposes lower_bound and upper_bound fields; finite endpoints are computed when requested.

• Documentation and Benchmarks

  • Added performance study documentation and expanded benchmarks for interval operations, certified reductions, and projection batches. Benchmarks validate expected results before timing.

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📝 Walkthrough

Walkthrough

Interval arithmetic and certified reductions now use updated rounding and error-bound calculations. The pull request adds interval and linear-form benchmarks and archives measurement results, provenance, and downstream adapter patches. Supporting updates change benchmark-report handling, validation, type annotations, and development-tool versions.

Changes

Interval arithmetic and certified reductions

Layer / File(s)	Summary
Interval arithmetic and product rounding src/interval.rs, src/rounding.rs, tests/proptest_interval.rs, docs/mathematical_basis.md, REFERENCES.md	Interval endpoint calculations and determinant workspace sizing change. Product comparison uses an FMA residual at or above 2^-968 and exact comparison below that threshold. Tests and references describe the arithmetic and threshold.
Certified reduction bounds and endpoints src/vector.rs, tests/proptest_exact.rs, docs/mathematical_basis.md	ScalarWithErrorBound stores the estimate and error bound, then computes endpoints on demand. Certified reductions change magnitude accumulation, exact-product handling, and zero-product behavior. Tests cover proof loss, endpoint limits, and signed zero.
Interval and linear-form benchmarks benches/interval.rs, benches/linear_form.rs, docs/BENCHMARKING.md	Benchmarks cover scalar interval operations, lifted matrices, certified dot products and dot differences, and projection batches. Fixture validation remains outside timed closures.
Archived study and downstream adapter evidence docs/archive/performance/studies/*, docs/code_organization.md	The archived study records numerical contracts, measurement results, and validation. It includes downstream adapter patch snapshots, benchmark provenance, a measurement script, and study links.
Benchmark and archive tooling checks scripts/archive_performance.py, scripts/bench_compare.py, scripts/tests/test_archive_performance.py, pyproject.toml	Archive validation checks retained-output paths when the mapping is present. Comparison-report handling distinguishes nonempty baseline names. Tests cover empty outputs and report-path aliases. Development-tool pins are updated.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Refactor

Merge Risk: ⚪ Minimal · up to 73dd0

The reviewed changes add checks for preserving existing reports and rejecting a report-path collision before mutation. No actionable merge blocker is identified.

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✅ Passed checks (4 passed)

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Title check	✅ Passed	The title clearly and concisely describes the pull request’s primary changes to reduce interval arithmetic and certified-reduction overhead.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.13%. Comparing base (2095d18) to head (73dd02f).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #272      +/-   ##
==========================================
+ Coverage   98.02%   98.13%   +0.11%     
==========================================
  Files          13       13              
  Lines        6724     6966     +242     
==========================================
+ Hits         6591     6836     +245     
+ Misses        133      130       -3     

Flag	Coverage Δ
unittests	98.13% <100.00%> (+0.11%)	⬆️

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Actionable comments posted: 1

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Inline comments:
Review comments at @scripts/archive_performance.py:
- Line 1492: Add focused pytest coverage for `_validate_promotion_paths` with
`retained_outputs=None` and with an empty mapping, asserting their distinct
behavior and preserving an assertion for the relevant retained-path collision
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