Prune inactive users from the activity tracker

src/UserActivityTracker.cs

@@ -1,18 +1,22 @@
 using System;
 using System.Collections.Concurrent;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace NotifierRedirecter;
 
 public sealed class UserActivityTracker
 {
     private static readonly TimeSpan _activityTimeout = TimeSpan.FromSeconds(5);
+    private static readonly TimeSpan _cleanupInterval = TimeSpan.FromMinutes(5);
 
     /// <summary>
     /// Tracks the user's last activity within a channel.
     /// </summary>
     private readonly ConcurrentDictionary<ulong, (ulong ChannelId, DateTimeOffset LastActivity)> _tracker = new();
 
+    public UserActivityTracker() => _ = CleanupInactiveUsersAsync();
+
     public void UpdateUser(ulong userId, ulong channelId) => this._tracker.AddOrUpdate(userId, (channelId, DateTimeOffset.UtcNow), (_, _) => (channelId, DateTimeOffset.UtcNow));
     public async ValueTask<bool> IsActiveAsync(ulong userId, ulong channelId)
     {
@@ -21,4 +25,20 @@ public async ValueTask<bool> IsActiveAsync(ulong userId, ulong channelId)
         await Task.Delay(_activityTimeout);
         return this._tracker.TryGetValue(userId, out (ulong ChannelId, DateTimeOffset LastActivity) value) && value.ChannelId == channelId && value.LastActivity > DateTimeOffset.UtcNow.AddSeconds(-15);
     }
+
+    public async Task CleanupInactiveUsersAsync()
+    {
+        PeriodicTimer timer = new(_cleanupInterval);
+        while (await timer.WaitForNextTickAsync())
+        {
+            foreach ((ulong userId, (ulong _, DateTimeOffset LastActivity)) in this._tracker)
+            {
+                // If the user hasn't done anything since the last cleanup interval, remove them from the tracker.
+                if (LastActivity < DateTimeOffset.UtcNow.Add(-_cleanupInterval))
+                {
+                    this._tracker.TryRemove(userId, out _);
+                }
+            }
+        }
+    }
 }