Conversion from the standard form to range form

[nguidotti]

Added routine for converting from the standard form (B&B, dual simplex)
$\min \mathbf{c}^T \mathbf{x}$
$\mathbf{A}\mathbf{x} = \mathbf{b}$
$\mathbf{l} \leq \mathbf{x} \leq \mathbf{u}$

to the range form (heuristics, presolve)

$\min \mathbf{c}^T \mathbf{x}$
$\mathbf{w} \leq \mathbf{A}\mathbf{x} \leq \mathbf{v}$
$\mathbf{l} \leq \mathbf{x} \leq \mathbf{u}$

This is needed for propagating the reduced problem via reduced cost fixing as well as the cuts from one restart to the next.

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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📥 Commits

Reviewing files that changed from the base of the PR and between 8c79892 and 465d6e2.

📒 Files selected for processing (3)

• cpp/src/dual_simplex/presolve.cpp
• cpp/src/dual_simplex/presolve.hpp
• cpp/tests/dual_simplex/unit_tests/solve.cpp

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📝 Walkthrough

Walkthrough

Adds convert_simplex_problem, an inverse of the existing convert_user_problem routine, which reconstructs a user_problem_t (range-row form) from a simplex-standard-form lp_problem_t. The implementation maps slack/artificial column bounds back to row senses and RHS values, rebuilds the constraint matrix from structural columns only, copies objective and variable data, and clears cone fields. A new unit test verifies a full round-trip for a mixed-integer problem covering all row sense types.

Changes

Inverse simplex-to-user problem conversion

Layer / File(s)	Summary
Header declaration of convert_simplex_problem cpp/src/dual_simplex/presolve.hpp	Adds the templated convert_simplex_problem declaration with documentation describing new_slacks-based slack/artificial filtering and var_types coverage over the full simplex problem.
Implementation and explicit instantiation cpp/src/dual_simplex/presolve.cpp	Implements convert_simplex_problem: resets user_problem metadata, derives each row's sense and RHS/range from its slack column's bounds and single nonzero, rebuilds the constraint matrix dropping slack/artificial columns, copies structural variable bounds and objective, filters var_types, clears cone fields, and asserts the primal path. Adds the <int, double> explicit instantiation in DUAL_SIMPLEX_INSTANTIATE_DOUBLE.
Round-trip MIP unit test cpp/tests/dual_simplex/unit_tests/solve.cpp	Adds convert_simplex_problem_mip_round_trip which converts a mixed-integer problem with all row senses to simplex form, recovers it via convert_simplex_problem, asserts recovered fields, then re-converts and asserts exact equality of the standard-form representation.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Conversion from the standard form to range form' directly and clearly describes the main change in the changeset: a new conversion routine that transforms LP problems from standard form to range form.
Description check	✅ Passed	The description is directly related to the changeset, providing mathematical context for the standard and range forms, explaining the conversion routine's purpose, and referencing the specific use cases (reduced cost fixing and cut propagation).
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.

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