/Microsoft.Scripting/Utils/CacheDict.cs

/* ****************************************************************************

 *

 * Copyright (c) Microsoft Corporation. 

 *

 * This source code is subject to terms and conditions of the Microsoft Public License. A 

 * copy of the license can be found in the License.html file at the root of this distribution. If 

 * you cannot locate the  Microsoft Public License, please send an email to 

 * dlr@microsoft.com. By using this source code in any fashion, you are agreeing to be bound 

 * by the terms of the Microsoft Public License.

 *

 * You must not remove this notice, or any other, from this software.

 *

 *

 * ***************************************************************************/



#if CODEPLEX_40

using System;

#else

using System; using Microsoft;

#endif

using System.Collections.Generic;

using System.Text;

using System.Diagnostics;



namespace Microsoft.Scripting.Utils {

    /// <summary>

    /// Provides a dictionary-like object used for caches which holds onto a maximum

    /// number of elements specified at construction time.

    /// 

    /// This class is not thread safe.

    /// </summary>

    public class CacheDict<TKey, TValue> {

        private readonly Dictionary<TKey, KeyInfo> _dict = new Dictionary<TKey, KeyInfo>();

        private readonly LinkedList<TKey> _list = new LinkedList<TKey>();

        private readonly int _maxSize;



        /// <summary>

        /// Creates a dictionary-like object used for caches.

        /// </summary>

        /// <param name="maxSize">The maximum number of elements to store.</param>

        public CacheDict(int maxSize) {

            _maxSize = maxSize;

        }



        /// <summary>

        /// Tries to get the value associated with 'key', returning true if it's found and

        /// false if it's not present.

        /// </summary>

        public bool TryGetValue(TKey key, out TValue value) {

            KeyInfo storedValue;

            if (_dict.TryGetValue(key, out storedValue)) {

                LinkedListNode<TKey> node = storedValue.List;

                if (node.Previous != null) {

                    // move us to the head of the list...

                    _list.Remove(node);

                    _list.AddFirst(node);

                }



                value = storedValue.Value;

                return true;

            }



            value = default(TValue);

            return false;

        }



        /// <summary>

        /// Adds a new element to the cache, replacing and moving it to the front if the

        /// element is already present.

        /// </summary>

        public void Add(TKey key, TValue value) {

            KeyInfo keyInfo;

            if (_dict.TryGetValue(key, out keyInfo)) {

                // remove original entry from the linked list

                _list.Remove(keyInfo.List);

            } else if (_list.Count == _maxSize) {

                // we've reached capacity, remove the last used element...

                LinkedListNode<TKey> node = _list.Last;

                _list.RemoveLast();

                bool res = _dict.Remove(node.Value);

                Debug.Assert(res);

            }

            

            // add the new entry to the head of the list and into the dictionary

            LinkedListNode<TKey> listNode = new LinkedListNode<TKey>(key);

            _list.AddFirst(listNode);

            _dict[key] = new CacheDict<TKey, TValue>.KeyInfo(value, listNode);

        }



        /// <summary>

        /// Returns the value associated with the given key, or throws KeyNotFoundException

        /// if the key is not present.

        /// </summary>

        public TValue this[TKey key] {

            get {

                TValue res;

                if (TryGetValue(key, out res)) {

                    return res;

                }

                throw new KeyNotFoundException();

            }

            set {

                Add(key, value);

            }

        }



        private struct KeyInfo {

            internal readonly TValue Value;

            internal readonly LinkedListNode<TKey> List;



            internal KeyInfo(TValue value, LinkedListNode<TKey> list) {

                Value = value;

                List = list;

            }

        }

    }

}