[BUG] set_cover Python: fix UB in vector transforms and swapped GuidedTabuSearch bindings

[jg-codes]

What

Fixes three bugs in the pybind11 wrapper (ortools/set_cover/python/set_cover.cc), and adds regression tests for each.

Disclosure: This PR was drafted with AI assistance (Claude). I reviewed and tested all changes.

Bug 1 & 2: undefined behavior in std::transform on empty vectors

VectorIntToVectorSubsetIndex and the all_subsets property both call std::transform writing to subs.begin() / subsets.begin() on a default-constructed (empty, capacity 0) vector. The output iterator is invalid from the start — this is undefined behavior per the C++ standard.

Impact: Every Python call that passes a focus list hits VectorIntToVectorSubsetIndex: next_solution(focus) on all heuristic classes, compute_coverage_in_focus, both clear_random_subsets overloads, and both clear_most_covered_elements overloads. Accessing model.all_subsets from Python triggers the second instance. In practice, the write corrupts heap metadata. I observed crashes on macOS (heap-buffer-overflow) when calling next_solution with a focus list.

Alternatives considered:

• subs.resize(n) + write to begin() — correct, but default-constructs every element before overwriting. For SubsetIndex the cost is negligible, but reserve + back_inserter avoids even that.
• Ranged constructor std::vector<SubsetIndex>(ints.begin(), ints.end()) — not viable because the element types differ (BaseInt → SubsetIndex), so a transform is needed.

Fix: reserve(n) + std::back_inserter(). This is the same pattern already used in the columns and rows properties in the same file.

Bug 3: swapped getter/setter for GuidedTabuSearch::lagrangian_factor

Lines 558–560 bind the Python name "set_lagrangian_factor" to the C++ method GetLagrangianFactor and vice versa. Calling gts.set_lagrangian_factor(0.5) from Python returns a value instead of setting one; calling gts.get_lagrangian_factor() requires an argument and mutates state.

The neighboring bindings (epsilon, penalty_factor, tabu_list_size) are wired correctly — only the lagrangian pair is transposed.

Fix: swap the two Python method name strings.

Tests added

Test	Exercises
test_all_subsets_property	all_subsets property (Bug 2)
test_focus_based_next_solution	next_solution(focus) path through VectorIntToVectorSubsetIndex (Bug 1)
test_guided_tabu_search_lagrangian_factor	set/get_lagrangian_factor round-trip (Bug 3)

Checklist

• Read CONTRIBUTING.md and PR template — targeting main as instructed
• CLA signed
• Minimal diff — only the bug fixes and their tests
• No formatting or refactoring changes
• AI-assisted: disclosed above

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[jg-codes]

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