fix(draft): back the Swiss pairing search out of avoidable rematches - #7938
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Fixes phase-rs#7937. generate_swiss_pairings paired greedily: pool head, first non-rematch partner, and on failure the first partner regardless (unwrap_or(0)) — pairs already made were never revisited. A 4-player pod's round 3 then produced two rematches although the round-robin completion always exists. The greedy is replaced by a backtracking search over the standings- ordered pool (bracket order preserved, partners tried in standings order, depth bounded by the pod size <= 8): a dead end backtracks; only if no rematch-free perfect matching exists (e.g. a 2-player pod from round 2 on) are rematches admitted. Odd pods pick the bye bottom-up so the paired remainder still admits a rematch-free matching. New regression: a 4-pod driven through two recorded rounds must complete the round robin in round 3 (every unordered pair exactly once). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Included review availability: Your plan provides up to 2 included reviews per hour; 0 remain after this review. 📝 WalkthroughWalkthroughSwiss pairing generation now uses bounded recursive backtracking over standings order. It selects rematch-free pairings when possible, chooses an eligible bye for odd pods, and permits rematches only when no valid alternative exists. Regression tests cover these cases. ChangesSwiss Pairing Generation
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: ⚪ Minimal · up to This PR replaces greedy Swiss pairing with bounded backtracking to avoid rematches while preserving existing bye-credit behavior; no actionable merge-blocking risk remains. Sequence Diagram(s)sequenceDiagram
participant SwissPairing
participant StandingsPool
participant OpponentHistory
participant BacktrackingSearch
SwissPairing->>StandingsPool: flatten standings-ordered pod
SwissPairing->>BacktrackingSearch: search candidate pairings
BacktrackingSearch->>OpponentHistory: check prior opponents
OpponentHistory-->>BacktrackingSearch: accept or reject candidates
BacktrackingSearch-->>SwissPairing: return pairings or rematch fallback
Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
Full details: Linked Issues checkExplanation The changes address issue ✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@crates/draft-core/src/session.rs`:
- Around line 1467-1525: Expand the pairing regression tests around
swiss_four_pod_round_three_completes_the_round_robin to cover a genuine
backtracking case where the first legal partner dead-ends but a later choice
succeeds, a two-player round-two rematch fallback, and an odd-pod bye choice
where giving the bye to the lowest player would force a rematch. Keep the setups
deterministic and assert the exact expected pairings or bye so backtracking,
fallback, and bye-selection behavior are each exercised.
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crates/draft-core/src/session.rs
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Review follow-up (phase-rs#7938): the round-robin regression never popped a backtrack frame (the leader had exactly one legal partner), so a first-legal-partner greedy would also pass it. Three function-level cases on crafted singleton brackets (shuffle-proof): - first legal partner dead-ends, the search must back out (A-C/B-D), - two-player round two admits the unavoidable rematch, - the odd-pod bye walks past the bottom seat when that bye would force a rematch. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Approved: bounded Swiss matching uses the draft-core authority, preserves deterministic standings ordering, and has production-path regression coverage for backtracking, rematch fallback, and odd-pod bye selection.
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Fixes #7937.
What: Swiss pairing paired greedily — pool head, first non-rematch partner, and on failure the first partner regardless (
unwrap_or(0)); pairs already made were never revisited. A 4-player pod's round 3 then produced two rematches although the round-robin completion always exists (observed live, see the issue).How: The greedy is replaced by
pair_pool_avoiding_rematches, a backtracking search over the standings-ordered pool (bracket order preserved, partners tried in standings order, recursion depth bounded by the pod size ≤ 8): a dead end backtracks; only when no rematch-free perfect matching exists (e.g. a 2-player pod from round 2 on) are rematches admitted via an always-succeeding first-fit. Odd pods pick the bye bottom-up so the paired remainder still admits a rematch-free matching; the bye keeps being reported to the caller for the match-win credit (#6351 semantics unchanged).Evidence
swiss_four_pod_round_three_completes_the_round_robin: the two-round history is CRAFTED (A–B, C–D, then A–C, B–D; records 2/1/1/0), so the forcing shape holds in every shuffle order, and round 3 must be exactly A–D / B–C. A first driven-through-the-rng version of this test passed even against the greedy (lucky shuffle) and was discarded for this deterministic one.Known gap: the bye chooser does not yet avoid giving the same player a second bye across rounds (byes are not recorded as pairings); unchanged from before.
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