Improve LRU performance

1. ``` class Pair``` replaced with ```struct Pair```
2. `ValueRef` used instead of `Value` in the `LinkedListNode`.
The performance gain is up to 5% (on small numbers no gain or a minor loss), memory gain is up to 23%.

Author: quzary

bench/algorithm/lru/2.cs

@@ -32,65 +32,67 @@ public static void Main(string[] args)
     }
 }
 
-public class Pair<TK, TV>
+public struct Pair<TK, TV>
 {
-    public TK Key;
+	public TK Key;
 
-    public TV Value;
+	public TV Value;
 }
 
 public class LRU<TK, TV>
+	where TK : notnull
+	where TV : notnull
 {
-    public int Size { get; private set; }
+	public int Size { get; private set; }
 
-    private readonly Dictionary<TK, LinkedListNode<Pair<TK, TV>>> _key_lookup;
+	private readonly Dictionary<TK, LinkedListNode<Pair<TK, TV>>> _key_lookup;
 
-    private readonly LinkedList<Pair<TK, TV>> _entries;
+	private readonly LinkedList<Pair<TK, TV>> _entries;
 
-    public LRU(int size)
-    {
-        Size = size;
-        _key_lookup = new Dictionary<TK, LinkedListNode<Pair<TK, TV>>>(size);
-        _entries = new LinkedList<Pair<TK, TV>>();
-    }
+	public LRU(int size)
+	{
+		Size = size;
+		_key_lookup = new Dictionary<TK, LinkedListNode<Pair<TK, TV>>>(size);
+		_entries = new LinkedList<Pair<TK, TV>>();
+	}
 
-    public bool TryGet(TK key, out TV value)
-    {
-        if (_key_lookup.TryGetValue(key, out var node))
-        {
-            value = node.Value.Value;
-            _entries.Remove(node);
-            _entries.AddLast(node);
-            return true;
-        }
-        value = default;
-        return false;
-    }
+	public bool TryGet(TK key, out TV value)
+	{
+		if (_key_lookup.TryGetValue(key, out var node))
+		{
+			value = node.ValueRef.Value;
+			_entries.Remove(node);
+			_entries.AddLast(node);
+			return true;
+		}
+		value = default;
+		return false;
+	}
 
-    public void Put(TK key, TV value)
-    {
-        if (_key_lookup.TryGetValue(key, out var node))
-        {
-            node.Value.Value = value;
-            _entries.Remove(node);
-            _entries.AddLast(node);
-        }
-        else if (_entries.Count == Size)
-        {
-            var first = _entries.First;
-            _key_lookup.Remove(first.Value.Key);
-            _entries.RemoveFirst();
-            first.Value.Key = key;
-            first.Value.Value = value;
-            _entries.AddLast(first);
-            _key_lookup[key] = first;
-        }
-        else
-        {
-            _entries.AddLast(new Pair<TK, TV> { Key = key, Value = value });
-            _key_lookup[key] = _entries.Last;
-        }
-    }
+	public void Put(TK key, TV value)
+	{
+		if (_key_lookup.TryGetValue(key, out var node))
+		{
+			node.ValueRef.Value = value;
+			_entries.Remove(node);
+			_entries.AddLast(node);
+		}
+		else if (_entries.Count == Size)
+		{
+			var first = _entries.First;
+			_key_lookup.Remove(first!.Value.Key);
+			_entries.RemoveFirst();
+			first.ValueRef.Key = key;
+			first.ValueRef.Value = value;
+			_entries.AddLast(first);
+			_key_lookup[key] = first;
+		}
+		else
+		{
+			_entries.AddLast(new Pair<TK, TV> { Key = key, Value = value });
+			_key_lookup[key] = _entries.Last;
+		}
+	}
 }
 
 public class LCG