Pools renamed to FixedCapacityPools. Other fixed capacity collections added. Queues, concurrent queues and standard stacks replaced with fixed capacity stacks. MethodImplOptions.AggressiveInlining and .AggressiveOptimization added to simple methods and getters / setters. Variables optimized in few places.

PerfectNumbers.Core/FixedCapacityStack.cs

@@ -0,0 +1,46 @@
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core;
+
+public sealed class FixedCapacityStack<T>(int capacity)
+{
+	private static readonly FixedCapacityArrayPool<T> _arrayPool = FixedCapacityPools<T>.ExclusiveArray;
+
+	public int Count = 0;
+	private T[] _items = _arrayPool.Rent(capacity);
+
+	private int _capacity = capacity;
+	public int Capacity
+	{
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		get => _capacity;
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		set
+		{
+			if (value < _capacity)
+			{
+				_arrayPool.Return(_items);
+				_items = _arrayPool.Rent(value);
+				_capacity = value;
+			}
+		}
+	}
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public void Clear() => Count = 0;
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public T? Pop() => Count > 0 ? _items[Count--] : default;
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public void Push(T item)
+	{
+		int newCount = Count + 1;
+		if (newCount < _items.Length)
+		{
+			_items[newCount] = item;
+			Count = newCount;
+		}
+	}
+}