using Lucene.Net.Support;
using System;
using System.Collections;
using System.Collections.Generic;

namespace Lucene.Net.Facet.Taxonomy
{
    /*
     * Licensed to the Apache Software Foundation (ASF) under one or more
     * contributor license agreements.  See the NOTICE file distributed with
     * this work for additional information regarding copyright ownership.
     * The ASF licenses this file to You under the Apache License, Version 2.0
     * (the "License"); you may not use this file except in compliance with
     * the License.  You may obtain a copy of the License at
     *
     *     http://www.apache.org/licenses/LICENSE-2.0
     *
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */

    /// <summary>
    /// <see cref="LRUHashMap{TKey, TValue}"/> is similar to of Java's HashMap, which has a bounded <see cref="Limit"/>;
    /// When it reaches that <see cref="Limit"/>, each time a new element is added, the least
    /// recently used (LRU) entry is removed.
    /// <para>
    /// Unlike the Java Lucene implementation, this one is thread safe because it is backed by the <see cref="LurchTable{TKey, TValue}"/>.
    /// Do note that every time an element is read from <see cref="LRUHashMap{TKey, TValue}"/>,
    /// a write operation also takes place to update the element's last access time.
    /// This is because the LRU order needs to be remembered to determine which element
    /// to evict when the <see cref="Limit"/> is exceeded. 
    /// </para>
    /// <para>
    /// 
    /// @lucene.experimental
    /// </para>
    /// </summary>
    public class LRUHashMap<TKey, TValue> : IDictionary<TKey, TValue>
    {
        private LurchTable<TKey, TValue> cache;

        /// <summary>
        /// Create a new hash map with a bounded size and with least recently
        /// used entries removed.
        /// </summary>
        /// <param name="limit">
        /// The maximum size (in number of entries) to which the map can grow
        /// before the least recently used entries start being removed.
        /// <para/>
        /// Setting maxSize to a very large value, like <see cref="int.MaxValue"/>
        /// is allowed, but is less efficient than
        /// using <see cref="Support.HashMap{TKey, TValue}"/> or 
        /// <see cref="Dictionary{TKey, TValue}"/> because our class needs
        /// to keep track of the use order (via an additional doubly-linked
        /// list) which is not used when the map's size is always below the
        /// maximum size.
        /// </param>
        public LRUHashMap(int limit)
        {
            cache = new LurchTable<TKey, TValue>(LurchTableOrder.Access, limit);
        }

        /// <summary>
        /// allows changing the map's maximal number of elements
        /// which was defined at construction time.
        /// <para>
        /// Note that if the map is already larger than <see cref="Limit"/>, the current 
        /// implementation does not shrink it (by removing the oldest elements);
        /// Rather, the map remains in its current size as new elements are
        /// added, and will only start shrinking (until settling again on the
        /// given <see cref="Limit"/>) if existing elements are explicitly deleted.
        /// </para>
        /// </summary>
        public virtual int Limit
        {
            get
            {
                return cache.Limit;
            }
            set
            {
                if (value < 1)
                {
                    throw new ArgumentOutOfRangeException("Limit must be at least 1");
                }
                cache.Limit = value;
            }
        }

        public TValue Put(TKey key, TValue value)
        {
            TValue oldValue = default(TValue);
            cache.AddOrUpdate(key, value, (k, v) =>
            {
                oldValue = cache[key];
                return value;
            });
            return oldValue;
        }

        public TValue Get(TKey key)
        {
            TValue result;
            if (!cache.TryGetValue(key, out result))
            {
                return default(TValue);
            }
            return result;
        }

        #region IDictionary<TKey, TValue> members

        public TValue this[TKey key]
        {
            get
            {
                return cache[key];
            }
            set
            {
                cache[key] = value;
            }
        }

        public int Count
        {
            get
            {
                return cache.Count;
            }
        }

        public bool IsReadOnly
        {
            get
            {
                return false;
            }
        }

        public ICollection<TKey> Keys
        {
            get
            {
                return cache.Keys;
            }
        }

        public ICollection<TValue> Values
        {
            get
            {
                return cache.Values;
            }
        }

        public void Add(KeyValuePair<TKey, TValue> item)
        {
            throw new NotSupportedException();
        }

        public void Add(TKey key, TValue value)
        {
            cache.Add(key, value);
        }

        public void Clear()
        {
            cache.Clear();
        }

        public bool Contains(KeyValuePair<TKey, TValue> item)
        {
            throw new NotSupportedException();
        }

        public bool ContainsKey(TKey key)
        {
            return cache.ContainsKey(key);
        }

        public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
        {
            throw new NotSupportedException();
        }

        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
        {
            return cache.GetEnumerator();
        }

        public bool Remove(KeyValuePair<TKey, TValue> item)
        {
            throw new NotSupportedException();
        }

        public bool Remove(TKey key)
        {
            return cache.Remove(key);
        }

        public bool TryGetValue(TKey key, out TValue value)
        {
            return cache.TryGetValue(key, out value);
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return cache.GetEnumerator();
        }

        #endregion
    }
}