LRUCache.cs

/**
 * This code is based on
 * https://github.com/migueldeicaza/MonoTouch.Dialog/blob/e69b806efee4ead3c39926d5164c15ae90a92b7a/MonoTouch.Dialog/Utilities/LRUCache.cs
 * 
 * Minor adaptations were made for compatibility with structs and classes which don't implement IDisposable
 */
//
// A simple LRU cache used for tracking the images
//
// Authors:
//   Miguel de Icaza (miguel@gnome.org)
//
// Copyright 2010 Miguel de Icaza
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
// 
using System;
using System.Collections.Generic;

namespace MonoTouch.Dialog.Utilities
{

    public class LRUCache<TKey, TValue>
    {
        Dictionary<TKey, LinkedListNode<TValue>> dict;
        Dictionary<LinkedListNode<TValue>, TKey> revdict;
        LinkedList<TValue> list;
        int entryLimit, sizeLimit, currentSize;
        Func<TValue, int> slotSizeFunc;

        public LRUCache(int entryLimit) : this(entryLimit, 0, null)
        {
        }

        public LRUCache(int entryLimit, int sizeLimit, Func<TValue, int> slotSizer)
        {
            list = new LinkedList<TValue>();
            dict = new Dictionary<TKey, LinkedListNode<TValue>>();
            revdict = new Dictionary<LinkedListNode<TValue>, TKey>();

            if (sizeLimit != 0 && slotSizer == null)
                throw new ArgumentNullException("If sizeLimit is set, the slotSizer must be provided");

            this.entryLimit = entryLimit;
            this.sizeLimit = sizeLimit;
            slotSizeFunc = slotSizer;
        }

        void Evict()
        {
            var last = list.Last;
            var key = revdict[last];

            if (sizeLimit > 0)
            {
                int size = slotSizeFunc(last.Value);
                currentSize -= size;
            }

            dict.Remove(key);
            revdict.Remove(last);
            list.RemoveLast();
            (last.Value as IDisposable)?.Dispose();
        }

        public void Purge()
        {
            foreach (var element in list)
                (element as IDisposable)?.Dispose();

            dict.Clear();
            revdict.Clear();
            list.Clear();
            currentSize = 0;
        }

        public TValue this[TKey key]
        {
            get
            {

                if (dict.TryGetValue(key, out LinkedListNode<TValue> node))
                {
                    list.Remove(node);
                    list.AddFirst(node);

                    return node.Value;
                }
                return default;
            }

            set
            {
                int size = sizeLimit > 0 ? slotSizeFunc(value) : 0;

                if (dict.TryGetValue(key, out LinkedListNode<TValue> node))
                {
                    if (sizeLimit > 0 && node.Value != null)
                    {
                        int repSize = slotSizeFunc(node.Value);
                        currentSize -= repSize;
                        currentSize += size;
                    }

                    // If we already have a key, move it to the front
                    list.Remove(node);
                    list.AddFirst(node);

                    // Remove the old value
                    if (node.Value != null)
                        (node.Value as IDisposable)?.Dispose();
                    node.Value = value;
                    while (sizeLimit > 0 && currentSize > sizeLimit && list.Count > 1)
                        Evict();
                    return;
                }
                if (sizeLimit > 0)
                {
                    while (sizeLimit > 0 && currentSize + size > sizeLimit && list.Count > 0)
                        Evict();
                }
                if (dict.Count >= entryLimit)
                    Evict();
                // Adding new node
                node = new LinkedListNode<TValue>(value);
                list.AddFirst(node);
                dict[key] = node;
                revdict[node] = key;
                currentSize += size;
            }
        }

        public override string ToString()
        {
            return "LRUCache dict={0} revdict={1} list={2}";
        }
    }
}