feat: add deterministic matchmaking team partitioning

server/domain/teams_test.go

@@ -0,0 +1,36 @@
+package domain
+
+import "testing"
+
+func TestPartitionTeamsBalancesMeanRatingBeforeOpposingSpread(t *testing.T) {
+	players := []Candidate{
+		{PlayerID: "a", Rating: 1000}, {PlayerID: "b", Rating: 1100},
+		{PlayerID: "c", Rating: 1900}, {PlayerID: "d", Rating: 2000},
+	}
+	teams, err := PartitionTeams(players)
+	if err != nil { t.Fatal(err) }
+	if playerIDs(teams.Team0) != "a\x00d\x00" || playerIDs(teams.Team1) != "b\x00c\x00" {
+		t.Fatalf("unexpected balanced partition: team0=%q team1=%q", playerIDs(teams.Team0), playerIDs(teams.Team1))
+	}
+}
+
+func TestPartitionTeamsIsIndependentOfInputOrder(t *testing.T) {
+	players := []Candidate{
+		{PlayerID: "d", Rating: 1500}, {PlayerID: "b", Rating: 1500},
+		{PlayerID: "c", Rating: 1500}, {PlayerID: "a", Rating: 1500},
+	}
+	first, err := PartitionTeams(players)
+	if err != nil { t.Fatal(err) }
+	second, err := PartitionTeams([]Candidate{players[2], players[0], players[3], players[1]})
+	if err != nil { t.Fatal(err) }
+	if playerIDs(first.Team0) != playerIDs(second.Team0) || playerIDs(first.Team1) != playerIDs(second.Team1) {
+		t.Fatalf("input order changed partition: first=%q/%q second=%q/%q", playerIDs(first.Team0), playerIDs(first.Team1), playerIDs(second.Team0), playerIDs(second.Team1))
+	}
+}
+
+func TestPartitionTeamsRejectsUnsupportedShapes(t *testing.T) {
+	for _, count := range []int{0, 1, 3, 7} {
+		players := make([]Candidate, count)
+		if _, err := PartitionTeams(players); err == nil { t.Fatalf("accepted %d players", count) }
+	}
+}