From 5adf4be0ebe5f740313ebcdc6af2fa7ab3eec746 Mon Sep 17 00:00:00 2001
From: Kalle <38327916+Sendouc@users.noreply.github.com>
Date: Tue, 4 Aug 2026 21:52:31 +0300
Subject: [PATCH] Fix Swiss algorithm not always finding the correct team to
 give bye to

---
 .../tournament-bracket/core/Swiss.test.ts     | 18 +++++++++++++++++
 .../core/engine/swiss/pairing.ts              | 20 ++++++++++++++-----
 2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/app/features/tournament-bracket/core/Swiss.test.ts b/app/features/tournament-bracket/core/Swiss.test.ts
index 243d1f402..b8b171a17 100644
--- a/app/features/tournament-bracket/core/Swiss.test.ts
+++ b/app/features/tournament-bracket/core/Swiss.test.ts
@@ -358,6 +358,24 @@ describe("Swiss", () => {
 
 			expect(includesPair(result, 1, 2)).toBe(false);
 		});
+
+		it("prefers giving the bye to the lowest standing team without a previous bye", () => {
+			// five team swiss entering round 4: teams 3, 4 and 5 have already had a
+			// bye, team 3 in the round right before this one. Teams 1 and 2 have not,
+			// and a rematch free pairing where team 2 (the lowest standing team
+			// without a previous bye) gets the bye exists: 1-4, 3-5
+			const result = Swiss.pairUp([
+				{ id: 1, score: 3, avoid: [3, 5, 2] },
+				{ id: 2, score: 2, avoid: [4, 3, 1] },
+				{ id: 4, score: 2, avoid: [2, 5], receivedBye: true },
+				{ id: 3, score: 1, avoid: [1, 2], receivedBye: true },
+				{ id: 5, score: 1, avoid: [1, 4], receivedBye: true },
+			]);
+
+			const bye = result.find((match) => match.opponentTwo === null);
+
+			expect(bye?.opponentOne).toBe(2);
+		});
 	});
 
 	describe("calculateTeamStatus()", () => {
diff --git a/app/features/tournament-bracket/core/engine/swiss/pairing.ts b/app/features/tournament-bracket/core/engine/swiss/pairing.ts
index 0b7619e11..d9e18e268 100644
--- a/app/features/tournament-bracket/core/engine/swiss/pairing.ts
+++ b/app/features/tournament-bracket/core/engine/swiss/pairing.ts
@@ -18,6 +18,16 @@ import { calculateTeamStatus } from "./team-status";
  */
 const REMATCH_PENALTY = 1_000_000;
 
+/**
+ * Weight added for each team of a pair that has already received a bye, making
+ * previously-byed teams more attractive to pair up. Applied per team (not per pair)
+ * so that a matching leaving a bye-less team unpaired always beats one giving a team
+ * a second bye: with an odd team count exactly one team is left unpaired, so the
+ * matchings differ by exactly one bonus, which is set to dwarf every score-based
+ * weight while staying far below REMATCH_PENALTY.
+ */
+const PRIOR_BYE_PAIRING_BONUS = 10_000;
+
 interface GroupArgs {
 	groupId: number;
 	standings: SwissStanding[];
@@ -285,11 +295,11 @@ function generateWeightedPairs({
 				wt -= 10;
 			}
 
-			if (
-				(Object.hasOwn(curr, "receivedBye") && curr.receivedBye) ||
-				(Object.hasOwn(opp, "receivedBye") && opp.receivedBye)
-			) {
-				wt += 40;
+			if (curr.receivedBye) {
+				wt += PRIOR_BYE_PAIRING_BONUS;
+			}
+			if (opp.receivedBye) {
+				wt += PRIOR_BYE_PAIRING_BONUS;
 			}
 
 			wt -= timesPlayed(curr, opp) * REMATCH_PENALTY;