Add a rolling queue

Common/Collections/RollingQueue.cs

@@ -0,0 +1,46 @@
+using System.Collections;
+
+namespace Voxel.Common.Collections;
+
+public class RollingQueue<T> : IEnumerable<HashSet<T>>, IEnumerable {
+    public HashSet<T> this[int index] {
+        get => CreateIfEmpty(index);
+        set => AppendEach(index, value);
+    }
+
+    private Dictionary<int, HashSet<T>> internalHolder = [];
+
+    public HashSet<T> Shift() {
+        var output = this[0];
+
+        var next = new Dictionary<int, HashSet<T>>();
+
+        foreach (var elem in internalHolder) {
+            var newKey = elem.Key-1;
+            if (newKey >= 0)
+                next[newKey] = elem.Value;
+        }
+
+        internalHolder = next;
+
+        return output;
+    }
+
+    public void Append(int index, T value)
+        => this[index].Add(value);
+
+    public void AppendEach(int index, HashSet<T> value)
+        => this[index].UnionWith(value);
+
+    public IEnumerator<HashSet<T>> GetEnumerator()
+        => throw new NotImplementedException();
+
+    IEnumerator IEnumerable.GetEnumerator()
+        => GetEnumerator();
+
+    private HashSet<T> CreateIfEmpty(int index) {
+        if (!internalHolder.ContainsKey(index))
+            internalHolder[index] = [];
+        return internalHolder[index];
+    }
+}