Try to fix reordering of sets due to network lag

src/game.js

@@ -401,7 +401,7 @@ function processEvent(internalGameState, event) {
   updateBoard(internalGameState, event, cards);
 }
 
-export function computeState(gameData, gameMode) {
+export function computeState(gameData, gameMode, newEvents = null) {
   if (!modes.hasOwnProperty(gameMode)) {
     throw new Error(`invalid gameMode: ${gameMode}`);
   }
@@ -440,10 +440,21 @@ export function computeState(gameData, gameMode) {
   };
 
   if (gameData.events) {
-    // Array.sort() is guaranteed to be stable in since around 2018
-    const events = Object.values(gameData.events).sort(
-      (e1, e2) => e1.time - e2.time
-    );
+    let events;
+    // Array.sort() is guaranteed to be stable in since around 2018.
+    if (newEvents?.size) {
+      // Always sort new events (with approximate times) after old events.
+      events = Object.entries(gameData.events)
+        .sort(
+          ([k1, e1], [k2, e2]) =>
+            newEvents.has(k1) - newEvents.has(k2) || e1.time - e2.time
+        )
+        .map(([k, e]) => e);
+    } else {
+      events = Object.values(gameData.events).sort(
+        (e1, e2) => e1.time - e2.time
+      );
+    }
     for (const event of events) {
       processEvent(internalGameState, event);
     }

src/pages/GamePage.js

@@ -124,6 +124,7 @@ function GamePage({ match }) {
 
   const [game, loadingGame] = useFirebaseRef(`games/${gameId}`);
   const [gameData, loadingGameData] = useFirebaseRef(`gameData/${gameId}`);
+  const newEvents = useRef(new Set());
   const [hasNextGame] = useFirebaseRef(
     game?.status === "done" && (!game.users || !(user.id in game.users))
       ? `games/${nextGameId}/status`
@@ -201,7 +202,11 @@ function GamePage({ match }) {
     lastKeptSet,
   } = useMemo(() => {
     if (!gameData) return {};
-    const state = computeState({ ...gameData, random, deck }, gameMode);
+    const state = computeState(
+      { ...gameData, random, deck },
+      gameMode,
+      newEvents.current
+    );
     const { current, boardSize, findState, history } = state;
     const board = current.slice(0, boardSize);
     const answer = findSet(board, gameMode, findState);
@@ -251,14 +256,29 @@ function GamePage({ match }) {
     if (numHints) {
       firebase.database().ref(`gameData/${gameId}/hints`).remove();
     }
-    firebase
+    const eventRef = firebase
       .database()
       .ref(`gameData/${gameId}/events`)
       .push({
         ...event,
         user: user.id,
         time: firebase.database.ServerValue.TIMESTAMP,
       });
+    // Track "new" events that have approximate times. An event is new since
+    // the time it was created until its time is updated from the server.
+    // This happens when our callback is called the second time (the first
+    // time it is called with the approximate time).
+    const eventKey = eventRef.key;
+    const timeRef = eventRef.child("time");
+    newEvents.current.add(eventKey);
+    let updateCount = 0;
+    const timeUpdated = () => {
+      if (++updateCount === 2) {
+        newEvents.current.delete(eventKey);
+        timeRef.off("value", timeUpdated);
+      }
+    };
+    timeRef.on("value", timeUpdated);
   }
 
   const hint = game.enableHint && answer ? answer.slice(0, numHints) : null;