From 6779b4931e6c2ffe05b438b574b3413dc19e11af Mon Sep 17 00:00:00 2001
From: ze97286 <zohar.etzioni@gmail.com>
Date: Thu, 8 Aug 2024 11:50:13 +0100
Subject: [PATCH] fix: non determinism in lottery ranking fixed

---
 CHANGELOG.md                         |  1 +
 core/rewards/lottery_ranking_test.go | 24 ++++++++++++-----------
 core/rewards/reward_distribution.go  | 29 ++++++++++++++++++++++++----
 3 files changed, 39 insertions(+), 15 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index dcef12293a..44ed6fc774 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@
 - [11521](https://github.com/vegaprotocol/vega/issues/11521) - Restore `AMM` position factor when loading from a snapshot.
 - [11526](https://github.com/vegaprotocol/vega/issues/11526) - `EstimateAMMBounds` now respects the market's decimal places.
 - [11540](https://github.com/vegaprotocol/vega/issues/11540) - Fix spam check for spots to use not double count quantum.
+- [11542](https://github.com/vegaprotocol/vega/issues/11542) - Fix non determinism in lottery ranking. 
 
 ## 0.77.5
 
diff --git a/core/rewards/lottery_ranking_test.go b/core/rewards/lottery_ranking_test.go
index 87197bc89a..d5ce8a6e46 100644
--- a/core/rewards/lottery_ranking_test.go
+++ b/core/rewards/lottery_ranking_test.go
@@ -26,16 +26,18 @@ import (
 )
 
 func TestLotteryRewardScoreSorting(t *testing.T) {
-	adjustedRewardScores := []*types.PartyContributionScore{
-		{Party: "p1", Score: num.DecimalOne()},
-		{Party: "p2", Score: num.DecimalTwo()},
-		{Party: "p3", Score: num.DecimalFromFloat(0.01)},
-	}
-	const layout = "Jan 2, 2006 at 3:04pm"
+	for i := 0; i < 100; i++ {
+		adjustedRewardScores := []*types.PartyContributionScore{
+			{Party: "p1", Score: num.DecimalOne()},
+			{Party: "p2", Score: num.DecimalTwo()},
+			{Party: "p3", Score: num.DecimalFromFloat(0.01)},
+		}
+		const layout = "Jan 2, 2006 at 3:04pm"
 
-	timestamp, _ := time.Parse(layout, "Aug 7, 2024 at 12:00pm")
-	lottery := lotteryRewardScoreSorting(adjustedRewardScores, timestamp)
-	require.Equal(t, "p2", lottery[0].Party)
-	require.Equal(t, "p1", lottery[1].Party)
-	require.Equal(t, "p3", lottery[2].Party)
+		timestamp, _ := time.Parse(layout, "Aug 7, 2024 at 12:00pm")
+		lottery := lotteryRewardScoreSorting(adjustedRewardScores, timestamp)
+		require.Equal(t, "p2", lottery[0].Party)
+		require.Equal(t, "p1", lottery[1].Party)
+		require.Equal(t, "p3", lottery[2].Party)
+	}
 }
diff --git a/core/rewards/reward_distribution.go b/core/rewards/reward_distribution.go
index 187f0c8fdd..7d15e5e777 100644
--- a/core/rewards/reward_distribution.go
+++ b/core/rewards/reward_distribution.go
@@ -128,6 +128,11 @@ func selectIndex(rng *rand.Rand, probabilities []num.Decimal) int {
 	return len(probabilities) - 1
 }
 
+type PartyProbability struct {
+	Probability num.Decimal
+	Party       string
+}
+
 func lotteryRewardScoreSorting(adjustedPartyScores []*types.PartyContributionScore, timestamp time.Time) []*types.PartyContributionScore {
 	source := rand.NewSource(uint64(timestamp.UnixNano()))
 	rng := rand.New(source)
@@ -148,12 +153,28 @@ func lotteryRewardScoreSorting(adjustedPartyScores []*types.PartyContributionSco
 		if len(unselectedParties) < 1 {
 			break
 		}
-		probabilities := make([]num.Decimal, 0, len(unselectedParties))
-		parties := make([]string, 0, len(unselectedParties))
+		pp := make([]PartyProbability, 0, len(unselectedParties))
 		for _, ps := range unselectedParties {
-			probabilities = append(probabilities, ps.Score.Div(totalScores))
-			parties = append(parties, ps.Party)
+			pp = append(pp, PartyProbability{
+				Probability: ps.Score.Div(totalScores),
+				Party:       ps.Party,
+			})
+		}
+		sort.Slice(pp, func(i, j int) bool {
+			if pp[i].Probability.Equal(pp[j].Probability) {
+				return pp[i].Party < pp[j].Party
+			}
+			return pp[i].Probability.LessThan(pp[j].Probability)
+		})
+
+		probabilities := make([]num.Decimal, len(pp))
+		parties := make([]string, len(pp))
+
+		for i, partyProb := range pp {
+			probabilities[i] = partyProb.Probability
+			parties[i] = partyProb.Party
 		}
+
 		selected := selectIndex(rng, probabilities)
 		selectedParty := unselectedParties[parties[selected]]
 		delete(unselectedParties, selectedParty.Party)