Combinatorial: fix GA crossover identity bug, 2-opt move, ACO-MST tail, CH retry guard - #115
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…H retry Correctness fixes in the TSP/combinatorial solvers (autonomous review 2026-08-23). - GA crossover (ga.py): np.unique(return_index=True) returns indices sorted by value, so `child[idx]` rebuilt the tour as [0,1,..,N-1] EVERY time -- crossover discarded both parents and injected the identity tour, making the GA effectively mutation-only. Sort the indices to preserve first-occurrence order. Verified: now 0/200 identity tours, distinct valid children. - GA _2opt_refine (ga.py): the accepted move swapped only the two segment endpoints, but d1/d2 measure a full segment REVERSAL, so an "improving" test could lengthen the tour. Reverse new_route[ij+1 .. jk]. Verified: tour length now decreases on an accepted move. - ACO-MST (aco_mst.py): inverse-CDF sampling `argmin(new_p > cum_p)` returns index 0 when new_p exceeds every cumulative bin (float round-off) -> silently picks the first city-pair. Use searchsorted with a clip, like aco.py. - ConvexHullTSP (strategy.py): the "retry once" guard `restarted` was never set True, so the else/break was unreachable. Set it in the retry branch. Deferred: StepWiseOptimizer's optimize_whole_solution_deck=True branch is broken (crashes on entry) but the flag defaults False and is never set True anywhere -- a dead, unexercised path; fixing it safely needs a test of that path (separate issue). Tests: tests/test_combinatorics.py green (10). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01LxgEQmoqtAj589pvbwuGo4
This was referenced Aug 23, 2026
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Correctness fixes in the TSP/combinatorial solvers, from the 2026-08-23 review.
np.unique(return_index=True)sorts by value, sochild[idx]rebuilt[0..N-1], discarding both parents (GA was mutation-only). Sort the indices._2opt_refineapplied an endpoint swap butd1/d2measure a segment reversal → "improving" moves could lengthen the tour. Reverse the segment.argmin(new_p>cum_p)picks index 0 on tail float-overflow. Usesearchsorted+ clip.ConvexHullTSP"retry once" guardrestartednever set True.Verified
_crossover: 0/200 identity tours (was always identity), all valid perms._2opt_refine: accepted move now shortens the tour.tests/test_combinatorics.pygreen (10). Full suite + Cython via CI.Deferred
StepWiseOptimizer.optimize_whole_solution_deck=True(O3) crashes on entry but is a dead, never-enabled branch. Separate follow-ups: O2 (thread-unsafe legacy np.random under joblib), O4/O6/O9 (determinism), O10 (early-stop window logic).🤖 Generated with Claude Code
https://claude.ai/code/session_01LxgEQmoqtAj589pvbwuGo4