Transferred LRU cache code.

ExtraCollectionSharp/ExtraCollectionSharp.csproj

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net5.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>

ExtraCollectionSharp/LRUCache.cs

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+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace ExtraCollectionSharp
+{
+public class LRUCache<K, V> : IEnumerable<V>
+    {
+        private class Node
+        {
+            public K key;
+            public V value;
+            public Node front;
+            public Node back;
+        }
+
+        public int Size {get { return size; } }
+        public int Count {get { return valueDictionary.Count; } }
+        private readonly int size;
+        private Node top;
+        private Node bottom;
+        private Dictionary<K, Node> valueDictionary;
+        private Action<V> cleanupAction;
+
+        public LRUCache(int size = 100, Action<V> cleanup = null)
+        {
+            this.size = size;
+            valueDictionary = new Dictionary<K, Node>(size);
+            this.cleanupAction = cleanup;
+        }
+
+        private void MoveToTop(K key) {
+            Node node = valueDictionary[key];
+            if (node != null && top != node) {
+                node.front.back = node.back;
+                node.back = top;
+                node.front = null;
+                top = node;
+            }
+        }
+
+        private Node AddToTop(K key, V value) {
+            Node node = new Node();
+            node.front = null;
+            node.back = top;
+            node.value = value;
+            node.key = key;
+            top = node;
+            if (bottom == null) {
+                bottom = node;
+            } else if (valueDictionary.Count == Size) {
+                valueDictionary.Remove(bottom.key);
+                cleanupAction?.Invoke(bottom.value);
+                bottom = bottom.front;
+            }
+            valueDictionary[key] = node;
+            return node;
+        }
+
+        public V Use(K key, Func<V> generate) {
+            if (generate == null) throw new ArgumentNullException("generate");
+            Node value = null;
+            if (valueDictionary.ContainsKey(key)) {
+                value = valueDictionary[key];
+                MoveToTop(key);
+            } else {
+                value = AddToTop(key, generate());
+            }
+            return value.value;
+        }
+
+        public bool IsCached(K key) {
+            return valueDictionary.ContainsKey(key);
+        }
+
+        public void Clear() {
+            top = null;
+            bottom = null;
+            valueDictionary.Clear();
+        }
+
+        public IEnumerator<V> GetEnumerator()
+        {
+            foreach (Node node in valueDictionary.Values)
+            {
+                yield return node.value;
+            }
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
+    }
+}