send fewer crowd websocket events in very large rooms

we have broadcasts with 10k+ members.
the most frequent message sent to all members is `crowd`, once per
second.
if the room contains more than 100 members,
we only send `crowd` 10% of the time.
if the room has 1000 members, we send 1% of the time.

importantly, if the named users of the crowd change, then we always
send the message. That's necessary to know which contributors and
streamers are present.

src/main/scala/actor/RoomActor.scala

@@ -51,12 +51,16 @@ object RoomActor:
       None -> None
 
     case crowd: ClientIn.Crowd =>
-      if crowd == state.lastCrowd then None -> None
-      else
-        Some {
-          deps.clientIn(crowd)
-          state.copy(lastCrowd = crowd)
-        } -> None
+      val shouldSend =
+        if crowd == state.lastCrowd then false
+        else if crowd.users != state.lastCrowd.users then true
+        else if crowd.members > 1000 && crowd.members % 100 != 0 then false
+        else if crowd.members > 100 && crowd.members % 10 != 0 then false
+        else true
+      if shouldSend then
+        deps.clientIn(crowd)
+        Some(state.copy(lastCrowd = crowd)) -> None
+      else None -> None
 
     case SetTroll(v) =>
       Some(state.copy(isTroll = v)) -> None

src/main/scala/ipc/ClientIn.scala

@@ -68,9 +68,9 @@ object ClientIn:
   def payload(js: JsValue)                 = Payload(JsonString(Json.stringify(js)))
   def payload(tpe: String, js: JsonString) = Payload(JsonString(cliMsg(tpe, js)))
 
-  case class Crowd(doc: JsObject) extends ClientIn:
+  case class Crowd(doc: JsObject, members: Int, users: Set[User.Id]) extends ClientIn:
     lazy val write = cliMsg("crowd", doc)
-  val emptyCrowd = Crowd(Json.obj())
+  val emptyCrowd = Crowd(Json.obj(), 0, Set.empty)
 
   case class LobbyPairing(fullId: Game.FullId) extends ClientIn:
     def write =

src/main/scala/ipc/CrowdJson.scala

@@ -11,7 +11,7 @@ final class CrowdJson(inquirers: Inquirers, mongo: Mongo, lightUserApi: LightUse
       keepOnlyStudyMembers(crowd).map: users =>
         crowd.copy(users = users, anons = 0)
     else Future.successful(crowd)
-  }.flatMap(spectatorsOf(_, crowd.users)).map(ClientIn.Crowd.apply)
+  }.flatMap(spectatorsOf(_, crowd.users)).map(ClientIn.Crowd(_, crowd.members, crowd.users.toSet))
 
   def round(crowd: RoundCrowd.Output): Future[ClientIn.Crowd] =
     spectatorsOf(
@@ -26,7 +26,9 @@ final class CrowdJson(inquirers: Inquirers, mongo: Mongo, lightUserApi: LightUse
             "white"    -> (crowd.players.white > 0),
             "black"    -> (crowd.players.black > 0),
             "watchers" -> spectators
-          )
+          ),
+        crowd.room.members,
+        Set.empty
       )
     }