Provisional: non-mutating solution comparison for Gecode 7 (breaking changes)

[zayenz]

Provisional draft — future breaking-change release only

This PR is a provisional design and implementation for Gecode 7, or another future release that explicitly permits breaking changes. It is not ready for merge and must not be included in Gecode 6.x or any compatibility-preserving release. The comparison contract, search policy for incomparability, and migration requirements remain subject to review and testing.

Adding Space::compare() changes the C++ ABI of Space. Optimization models with custom objectives must implement comparison as well as constrain(). Release version and ABI-number changes are intentionally left to release preparation.

Changes

• Add a read-only, receiver-relative Space::compare() operation with better, equivalent, worse, and incomparable results. Models decide when two spaces contain enough objective information to compare; the spaces need not otherwise be fully assigned.
• Implement comparison for the integer and floating-point optimization convenience classes and FlatZinc. Integer objectives compare assigned values. Floating-point objectives rank their documented interval endpoints independently of the improvement step, while requiring compatible directions and steps.
• Use comparison for incumbent arbitration in sequential and parallel BAB, RBS, and portfolio search. constrain() remains responsible for pruning working spaces and is no longer used to rank independently produced solutions.
• Preserve pending-result correctness for external incumbent updates and return comparison failures from parallel workers to the controlling caller after workers are quiescent.
• Add focused coverage for equivalent and worse candidates, external bounds, float improvements smaller than the cut step, mixed PBS/RBS/BAB portfolios, comparison errors, reset/reuse, and satisfaction portfolios.

Policy requiring review

The current exact best-solution engines reject an incomparable incoming solution. This makes policy failures explicit while the interface and use cases are evaluated.

A possible future best-effort policy is that an incomparable incoming solution becomes the current best. The comparison interface already supports that policy, so testing it would not require another API change. Such a policy would not by itself guarantee completeness, Pareto maximality, or preservation of previously incomparable solutions. Pareto archives and diverse-solution search remain outside this PR.

Verification

Local verification on arm64 macOS with Apple Clang 21:

• Threaded gecode-test build and the complete Search::PBS:: matrix.
• Focused sequential and parallel BAB/RBS/PBS comparison, external-incumbent, nested-failure, reset/reuse, and float-objective checks.
• Relevant FlatZinc comparison and search checks.
• Existing parallel BAB stop/resume and recomputation coverage under bounded execution.
• Fresh GECODE_ENABLE_THREAD=OFF build with applicable comparison and PBS checks.

The full combinatorial Search:: matrix was sampled under a time bound; all executed cases passed. Other platforms and configurations have not been verified locally. Passing these checks does not change the provisional draft status.