From 1f0a170bfb05adf27c1ee92bdb0b7fb536712950 Mon Sep 17 00:00:00 2001
From: Andrei Mihu <andrei@heroiclabs.com>
Date: Thu, 17 Mar 2022 14:01:42 +0000
Subject: [PATCH] Ensure the matchmaker always correctly prefers matches closer
 to the maximum count. (#809)

---
 CHANGELOG.md                      |  2 ++
 server/match_registry_test.go     |  4 ++--
 server/matchmaker.go              | 23 ++++++++++++++++++++---
 server/runtime_javascript_test.go |  5 +++--
 4 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8ef4834f3..7e37e397b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -28,6 +28,8 @@ The format is based on [keep a changelog](http://keepachangelog.com) and this pr
 - Fix incorrect link device behaviour in JavaScript runtime.
 - Fix JS runtime multi-update execution consistency when part of the operation fails.
 - Fix handling of wallet ledger lookups with no limit during account exports.
+- Ensure maximum count is accounted for in matchmaker mutual match checks.
+- Ensure the matchmaker always correctly prefers matches closer to the maximum count.
 
 ## [3.10.0] - 2021-12-16
 ### Added
diff --git a/server/match_registry_test.go b/server/match_registry_test.go
index 27198bfd0..945ba46f7 100644
--- a/server/match_registry_test.go
+++ b/server/match_registry_test.go
@@ -19,14 +19,14 @@ import (
 	"context"
 	"encoding/gob"
 	"fmt"
-	"go.uber.org/atomic"
-	"go.uber.org/zap"
 	"strings"
 	"testing"
 
 	"github.com/blugelabs/bluge"
 	"github.com/gofrs/uuid"
 	"github.com/heroiclabs/nakama-common/runtime"
+	"go.uber.org/atomic"
+	"go.uber.org/zap"
 	"google.golang.org/protobuf/types/known/wrapperspb"
 )
 
diff --git a/server/matchmaker.go b/server/matchmaker.go
index 19a7cc1f3..a33e1a2bd 100644
--- a/server/matchmaker.go
+++ b/server/matchmaker.go
@@ -260,7 +260,7 @@ func (m *LocalMatchmaker) process(batch *index.Batch) {
 
 		// Form possible combinations, in case multiple matches might be suitable.
 		entryCombos := make([][]*MatchmakerEntry, 0, 5)
-		for _, hit := range blugeMatches.Hits {
+		for hitCounter, hit := range blugeMatches.Hits {
 			if hit.ID == ticket {
 				// Skip the current ticket.
 				continue
@@ -278,7 +278,7 @@ func (m *LocalMatchmaker) process(batch *index.Batch) {
 				m.logger.Error("error validating mutual match", zap.Error(err))
 				continue
 			} else if !outerMutualMatch {
-				// this search hit is not a mutual match with the outer ticket
+				// This search hit is not a mutual match with the outer ticket.
 				continue
 			}
 
@@ -365,7 +365,12 @@ func (m *LocalMatchmaker) process(batch *index.Batch) {
 				foundComboIdx = len(entryCombos) - 1
 			}
 
-			if l := len(foundCombo) + index.Count; l == index.MaxCount || (lastInterval && l >= index.MinCount && l <= index.MaxCount) {
+			// The combo is considered match-worthy if either the max count has been satisfied, or ALL of these conditions are met:
+			// * It is the last interval for this active index.
+			// * The combo at least satisfies the min count.
+			// * The combo does not exceed the max count.
+			// * There are no further hits that may further fill the found combo, so we get as close as possible to the max count.
+			if l := len(foundCombo) + index.Count; l == index.MaxCount || (lastInterval && l >= index.MinCount && l <= index.MaxCount && hitCounter >= len(blugeMatches.Hits)-1) {
 				// Check that the minimum count that satisfies the current index is also good enough for all matched entries.
 				var minCountFailed bool
 				for _, e := range foundCombo {
@@ -378,6 +383,18 @@ func (m *LocalMatchmaker) process(batch *index.Batch) {
 					continue
 				}
 
+				// Check that the maximum count that satisfies the current index is also good enough for all matched entries.
+				var maxCountFailed bool
+				for _, e := range foundCombo {
+					if foundIndex, ok := m.indexes[e.Ticket]; ok && foundIndex.MaxCount < l {
+						maxCountFailed = true
+						break
+					}
+				}
+				if maxCountFailed {
+					continue
+				}
+
 				// Found a suitable match.
 				entries, ok := m.entries[ticket]
 				if !ok {
diff --git a/server/runtime_javascript_test.go b/server/runtime_javascript_test.go
index 33b97a5e2..e50eab6eb 100644
--- a/server/runtime_javascript_test.go
+++ b/server/runtime_javascript_test.go
@@ -15,11 +15,12 @@
 package server
 
 import (
+	"strings"
+	"testing"
+
 	"github.com/dop251/goja"
 	"go.uber.org/zap"
 	"go.uber.org/zap/zaptest/observer"
-	"strings"
-	"testing"
 )
 
 func TestJsObjectFreeze(t *testing.T) {
-- 
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