NadekoBot.Extensions should now be fully annotated with nullable reference types as well as many classes from NadekoBot.Common

src/NadekoBot/Modules/Administration/GreetBye/GreetGrouper.cs

@@ -0,0 +1,72 @@
+namespace NadekoBot.Services;
+
+public class GreetGrouper<T>
+{
+    private readonly Dictionary<ulong, HashSet<T>> _group;
+    private readonly object _locker = new();
+
+    public GreetGrouper()
+        => _group = new();
+
+
+    /// <summary>
+    /// Creates a group, if group already exists, adds the specified user
+    /// </summary>
+    /// <param name="guildId">Id of the server for which to create group for</param>
+    /// <param name="toAddIfExists">User to add if group already exists</param>
+    /// <returns></returns>
+    public bool CreateOrAdd(ulong guildId, T toAddIfExists)
+    {
+        lock (_locker)
+        {
+            if (_group.TryGetValue(guildId, out var list))
+            {
+                list.Add(toAddIfExists);
+                return false;
+            }
+
+            _group[guildId] = new();
+            return true;
+        }
+    }
+
+    /// <summary>
+    /// Remove the specified amount of items from the group. If all items are removed, group will be removed.
+    /// </summary>
+    /// <param name="guildId">Id of the group</param>
+    /// <param name="count">Maximum number of items to retrieve</param>
+    /// <param name="items">Items retrieved</param>
+    /// <returns>Whether the group has no more items left and is deleted</returns>
+    public bool ClearGroup(ulong guildId, int count, out IReadOnlyCollection<T> items)
+    {
+        lock (_locker)
+        {
+            if (_group.TryGetValue(guildId, out var set))
+            {
+                // if we want more than there are, return everything
+                if (count >= set.Count)
+                {
+                    items = set;
+                    _group.Remove(guildId);
+                    return true;
+                }
+
+                // if there are more in the group than what's needed
+                // take the requested number, remove them from the set
+                // and return them
+                var toReturn = set.TakeWhile(_ => count-- != 0)
+                    .ToList();
+                foreach (var item in toReturn)
+                    set.Remove(item);
+
+                items = toReturn;
+                // returning falsemeans group is not yet deleted
+                // because there are items left
+                return false;
+            }
+
+            items = Array.Empty<T>();
+            return true;
+        }
+    }
+}