Main merge release/26.02 6

[rgsl888prabhu]

For #838

Summary by CodeRabbit

• New Features

  • Added PSLP presolver for LP problems (PSLP enabled by default for LP; Papilo for MIP)
  • New presolver selection parameter with modes: disabled, Papilo, or PSLP
  • Configurable PDLP tolerance settings for finer solver control

• Improvements

  • Enhanced solver robustness with better breakdown detection and error handling
  • Improved iterative refinement convergence checks

• Documentation

  • Updated documentation clarifying presolve defaults and configuration options

[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

This PR integrates PSLP (Simplex Linear Programming) presolver library into cuOpt and replaces boolean presolve flags with an enum-based presolver selection system. Changes include CMake build integration, public API updates introducing presolver_t enum, solver refactoring across LP/MIP/barrier paths, third-party presolve orchestration, and comprehensive test/documentation updates.

Changes

Cohort / File(s)	Summary
Build System & Dependencies cpp/CMakeLists.txt, cpp/tests/CMakeLists.txt, cpp/tests/linear_programming/CMakeLists.txt	Adds PSLP as FetchContent dependency with static linking to cuopt; extends include paths for PSLP and papilo headers; updates test target includes.
Public API Constants & Types cpp/include/cuopt/linear_programming/constants.h, cpp/include/cuopt/linear_programming/utilities/internals.hpp	Introduces four presolve configuration constants (DEFAULT, OFF, PAPILO, PSLP) and new public enum presolver_t mapping to these constants.
Solver Settings API cpp/include/cuopt/linear_programming/mip/solver_settings.hpp, cpp/include/cuopt/linear_programming/pdlp/solver_settings.hpp	Replaces bool presolve member with presolver_t presolver in both MIP and PDLP solver settings structures.
Core LP Solver Implementation cpp/src/dual_simplex/solve.cpp, cpp/src/dual_simplex/solve.hpp, cpp/src/linear_programming/solve.cu	Adds solver overloads accepting optional start_time parameter; refactors presolve handling to use presolver_t enum; adds fast-path for completely presolved problems; enhances timing and residual logging.
Barrier & Iterative Refinement cpp/src/dual_simplex/barrier.cu, cpp/src/dual_simplex/iterative_refinement.hpp	Fixes histogram indexing; adds NaN checks and breakdown validation in Givens refinement to prevent invalid result propagation.
Folding Algorithm Refinement cpp/src/dual_simplex/folding.cpp	Implements Paige-Tarjan optimization in color splitting; changes map types from unordered_map to map; adds extensive DEBUG verification; updates function signatures for error handling (returns int status codes).
Third-Party Presolve Integration cpp/src/mip/presolve/third_party_presolve.hpp, cpp/src/mip/presolve/third_party_presolve.cpp	Adds PSLP presolver path with builder/adapter functions; extends apply/undo signatures to support presolver selection; deletes copy/move constructors; introduces PSLP-specific postsolve storage and cleanup logic.
MIP Solver Integration cpp/src/mip/solve.cu, cpp/src/mip/solver.cu, cpp/src/mip/relaxed_lp/relaxed_lp.cu, cpp/src/mip/diversity/diversity_manager.cu	Refactors presolve gating to use presolver_t != None; forces PSLP→Papilo redirection for MIP; propagates presolver parameter through presolve apply calls; disables presolver for relaxed LP subproblems.
Parameter Configuration cpp/src/math_optimization/solver_settings.cu	Updates int_parameters mapping for CUOPT_PRESOLVE to target presolver fields (via reinterpret_cast) instead of boolean presolve; removes presolve from bool_parameters.
LP/MIP Unit Tests cpp/tests/linear_programming/pdlp_test.cu, cpp/tests/linear_programming/unit_tests/presolve_test.cu, cpp/tests/mip/cuts_test.cu, cpp/tests/mip/.../*_test.cu	Replaces presolve boolean assignments with presolver_t::None/Papilo/Default values; adds comprehensive PSLP presolve validation tests (postsolve accuracy, dual consistency, reduced costs); updates test helper signatures.
Python Bindings & Documentation python/cuopt/cuopt/tests/linear_programming/test_lp_solver.py, python/cuopt/cuopt/tests/linear_programming/test_python_API.py, python/cuopt_server/cuopt_server/.../*, docs/cuopt/source/.../*.rst	Exports CUOPT_PRESOLVE constant; changes presolve parameter type from bool to int (0/1/2 modes); updates warm-start examples and FAQ/feature docs to describe PSLP/Papilo selection; clarifies default presolvers per problem type.
Benchmark & License benchmarks/linear_programming/run_mps_files.sh, thirdparty/THIRD_PARTY_LICENSES	Adds --pdlp-tolerances script option with PDLP tolerance flags; updates license declarations (PSLP Apache-2.0).

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Possibly related PRs

• NVIDIA/cuopt#816: Introduces PSLP presolver integration surfaces and presolver enum refactoring affecting the same public API, settings, and third-party presolve orchestration.

Suggested labels

feature request, breaking

Suggested reviewers

• Iroy30
• gforsyth
• akifcorduk
• chris-maes

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🧪 Generate unit tests (beta)

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