Killed history

NadekoBot.Core/Common/DownloadTracker.cs

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+using NadekoBot.Core.Services;
+using System;
+using System.Collections.Concurrent;
+using System.Threading;
+using System.Threading.Tasks;
+using Discord;
+
+namespace NadekoBot.Core.Common
+{
+    public class DownloadTracker : INService
+    {
+        private ConcurrentDictionary<ulong, DateTime> LastDownloads { get; } = new ConcurrentDictionary<ulong, DateTime>();
+        private SemaphoreSlim downloadUsersSemaphore = new SemaphoreSlim(1, 1);
+
+        /// <summary>
+        /// Ensures all users on the specified guild were downloaded within the last hour. 
+        /// </summary>
+        /// <param name="guild">Guild to check and potentially download users from</param>
+        /// <returns>Task representing download state</returns>
+        public async Task EnsureUsersDownloadedAsync(IGuild guild)
+        {
+            await downloadUsersSemaphore.WaitAsync();
+            try
+            {
+                var now = DateTime.UtcNow;
+
+                // download once per hour at most
+                var added = LastDownloads.AddOrUpdate(
+                    guild.Id,
+                    now,
+                    (key, old) => (now - old) > TimeSpan.FromHours(1) ? now : old);
+
+                // means that this entry was just added - download the users
+                if (added == now)
+                    await guild.DownloadUsersAsync();
+            }
+            finally
+            {
+                downloadUsersSemaphore.Release();
+            }
+        }
+    }
+}