GF2 presolve rank-deficiency fix

[aliceb-nv]

The GF(2) presolver treated a missing pivot column as proof of infeasibility, so rank-deficient but consistent GF2 blocks were wrongly reported infeasible. It now runs a full RREF that separates inconsistency from underdetermination and fixes only variables whose value is identical in every solution.

The GF2 presolver also now supports rectangular matrices, and test coverage has been expanded.

Description

Issue

Checklist

• I am familiar with the Contributing Guidelines.
• Testing
  • New or existing tests cover these changes
  • Added tests
  • Created an issue to follow-up
  • NA
• Documentation
  • The documentation is up to date with these changes
  • Added new documentation
  • NA

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[aliceb-nv]

/ok to test 1e6415d

[coderabbitai]

📝 Walkthrough

Walkthrough

The PR replaces the square GF(2) solver with a rectangular rank-aware implementation, adds validated presolve fixings and extensive coverage, and introduces optional Papilo reduction-method allowlists for main and simplex-subproblem presolve paths.

Changes

GF(2) presolve

Layer / File(s)	Summary
Rectangular GF(2) solver contract cpp/src/mip_heuristics/presolve/gf2_presolve.hpp, cpp/src/mip_heuristics/presolve/gf2_presolve.cpp	Adds gf2_status_t and a packed-bit RREF solver with infeasibility and variable-determination results.
Validated GF(2) presolve fixings cpp/src/mip_heuristics/presolve/gf2_presolve.cpp	Validates key mappings, supports differing matrix dimensions, applies uniquely determined binary fixings, and checks residuals, conflicts, and bounds.
GF(2) solver and presolve validation cpp/tests/mip/gf2_presolve_test.cpp, cpp/tests/internal/CMakeLists.txt, cpp/tests/mip/presolve_test.cu	Adds exhaustive, randomized, permutation, rectangular, singular, conflict, and bounds tests. Removes the obsolete compact-index test.

Papilo reduction allowlist

Layer / File(s)	Summary
Papilo allowlist configuration cpp/src/mip_heuristics/presolve/third_party_presolve.hpp, cpp/src/mip_heuristics/presolve/third_party_presolve.cpp	Adds optional allowlist storage and filters Papilo methods and method-specific parameters.
Papilo presolve call-site wiring cpp/src/mip_heuristics/presolve/third_party_presolve.cpp	Passes reduction_allowlist_ through the main and simplex-subproblem presolve paths.

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

Possibly related PRs

• NVIDIA/cuopt#1657: Implements the same GF(2) solver and presolve changes, Papilo allowlist, and test updates.

Suggested labels: non-breaking

Suggested reviewers: tmckayus, chris-maes, mlubin

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main GF(2) presolve rank-deficiency fix.
Description check	✅ Passed	The description accurately explains the GF(2) presolver fix, rectangular support, and expanded test coverage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

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• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (1)

cpp/tests/mip/gf2_presolve_test.cpp (1)

523-542: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

Align the test name and comment with the asserted status.

The test is named more_rows_than_bins_reduces and the comment says the third row is redundant, but the assertion expects OPTIMAL, not REDUCED. The presolver fully solves this instance, so OPTIMAL is the real outcome. Rename the test to reflect that, or state in the comment why presolve reaches OPTIMAL here. This keeps a future reader from treating an OPTIMAL-to-REDUCED change as a test bug.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@cpp/tests/mip/gf2_presolve_test.cpp` around lines 523 - 542, Rename the test
`more_rows_than_bins_reduces` to reflect that presolve fully solves the instance
with `OPTIMAL`, and update the comment so it no longer describes only a
redundant row without explaining the optimal outcome.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@cpp/tests/mip/gf2_presolve_test.cpp`:
- Around line 523-542: Rename the test `more_rows_than_bins_reduces` to reflect
that presolve fully solves the instance with `OPTIMAL`, and update the comment
so it no longer describes only a redundant row without explaining the optimal
outcome.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 0a583e30-9d20-488e-b0ca-9142b3365d5c

📥 Commits

Reviewing files that changed from the base of the PR and between 2f035ba and 1e6415d.

📒 Files selected for processing (7)

• cpp/src/mip_heuristics/presolve/gf2_presolve.cpp
• cpp/src/mip_heuristics/presolve/gf2_presolve.hpp
• cpp/src/mip_heuristics/presolve/third_party_presolve.cpp
• cpp/src/mip_heuristics/presolve/third_party_presolve.hpp
• cpp/tests/internal/CMakeLists.txt
• cpp/tests/mip/gf2_presolve_test.cpp
• cpp/tests/mip/presolve_test.cu

💤 Files with no reviewable changes (1)

• cpp/tests/mip/presolve_test.cu

[github-actions]

CI Test Summary

✅ All 31 test job(s) passed.