fix: darwin calculate correct tunIndex

https://github.com/MetaCubeX/mihomo/pull/1285

listener/sing_tun/server.go

@@ -59,15 +59,23 @@ func CalculateInterfaceName(name string) (tunName string) {
 	if err != nil {
 		return
 	}
-	var tunIndex int
+	tunIndex := 0
+	indexSet := make(map[int]struct{})
 	for _, netInterface := range interfaces {
 		if strings.HasPrefix(netInterface.Name, tunName) {
 			index, parseErr := strconv.ParseInt(netInterface.Name[len(tunName):], 10, 16)
 			if parseErr == nil {
-				tunIndex = int(index) + 1
+				indexSet[int(index)] = struct{}{}
 			}
 		}
 	}
+	for index := range indexSet {
+		if index == tunIndex {
+			tunIndex += 1
+		} else { // indexSet already sorted and distinct, so this tunIndex nobody used
+			break
+		}
+	}
 	tunName = F.ToString(tunName, tunIndex)
 	return
 }