feat(multiplayer): rotate ranked arenas deterministically

server/domain/ranked_test.go

@@ -1,6 +1,9 @@
 package domain
 
-import "testing"
+import (
+	"fmt"
+	"testing"
+)
 
 func rankedParticipants() []RankedParticipant {
 	result := make([]RankedParticipant, 6)
@@ -44,3 +47,21 @@ func TestRankedArenaRegistryExcludesElevatedVariants(t *testing.T) {
 		}
 	}
 }
+
+func TestRankedArenaForProposalIsStableAndRotatesEligibleRegistry(t *testing.T) {
+	first := RankedArenaForProposal("proposal-stable")
+	if first != RankedArenaForProposal("proposal-stable") {
+		t.Fatal("same proposal selected different arenas")
+	}
+	seen := map[string]bool{}
+	for index := 0; index < 128; index++ {
+		arena := RankedArenaForProposal(fmt.Sprintf("proposal-%d", index))
+		if !validRankedArena(arena) {
+			t.Fatalf("proposal selected ineligible arena %+v", arena)
+		}
+		seen[arena.ID] = true
+	}
+	if len(seen) != len(rankedArenaOrder) {
+		t.Fatalf("selected arenas = %v, want every eligible arena", seen)
+	}
+}