use actual client buzzing time to determine winner

By waiting and collecting buzzes over a short period of time we can try
to compensate for network latency.  This opens up a way to cheat for the
player by forging timestamps, but normally it should makes things more
fair.

public/monitor.js

@@ -4,7 +4,16 @@
 const location = document.location
 const performance = window.performance
 
-import { Connection, ms_ns, node, q, s_ns, session_id, session_url } from "./shared.js"
+import {
+  Connection,
+  isEmpty,
+  ms_ns,
+  node,
+  q,
+  s_ns,
+  session_id,
+  session_url,
+} from "./shared.js"
 
 let conn
 
@@ -126,8 +135,9 @@ function highlight(client_id, until_ns) {
 function setup_ws() {
   let clients = {},
     me,
-    monitored = [],
-    sounds = {}
+    monitored = [], // list of client IDs
+    sounds = {}, // mapping of client ID to Audio instance
+    buzzes = {} // mapping of client ID to buzz time
   const sid = session_id()
   const overlay = q("#text-overlay")
   conn = new Connection()
@@ -138,12 +148,24 @@ function setup_ws() {
     me = value
   })
   conn.on("buzz", ({ value }) => {
+    const collect_duration_ms = 500
+    const highlight_duration_ns = 12 * s_ns
+    // Collect buzzes to find the true winner.
+    if (isEmpty(buzzes)) {
+      setTimeout(() => {
+        // Show winner in UI.
+        const winner_id = Object.entries(buzzes).sort(([x, a], [y, b]) => a - b)[0][0]
+        buzzes = {}
+        const until_ns = buzztime_ns + highlight_duration_ns
+        const is_new_buzz = highlight(winner_id, until_ns)
+        if (is_new_buzz && sounds[winner_id]) {
+          sounds[winner_id].play()
+        }
+      }, collect_duration_ms)
+    }
     const { time: buzztime_ns, client: client_id } = value
-    const duration_ns = 12 * s_ns
-    const until_ns = buzztime_ns + duration_ns
-    const is_new_buzz = highlight(client_id, until_ns)
-    if (is_new_buzz && sounds[client_id]) {
-      sounds[client_id].play()
+    if (!(client_id in buzzes)) {
+      buzzes[client_id] = buzztime_ns
     }
   })
   conn.on("clients", ({ value }) => {