// Copyright ?2016, 2018, Oracle and/or its affiliates. All rights reserved.
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License, version 2.0, as
// published by the Free Software Foundation.
//
// This program is also distributed with certain software (including
// but not limited to OpenSSL) that is licensed under separate terms,
// as designated in a particular file or component or in included license
// documentation.  The authors of MySQL hereby grant you an
// additional permission to link the program and your derivative works
// with the separately licensed software that they have included with
// MySQL.
//
// Without limiting anything contained in the foregoing, this file,
// which is part of MySQL Connector/NET, is also subject to the
// Universal FOSS Exception, version 1.0, a copy of which can be found at
// http://oss.oracle.com/licenses/universal-foss-exception.
//
// This program is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
// See the GNU General Public License, version 2.0, for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software Foundation, Inc.,
// 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA

using System;
using System.Collections.Generic;
using System.Linq;

namespace MySql.Data.MySqlClient
{
    internal class TableCache
    {
        private static readonly BaseTableCache cache;

        static TableCache()
        {
            cache = new BaseTableCache(480 /* 8 hour max by default */);
        }

        public static void AddToCache(string commandText, ResultSet resultSet)
        {
            cache.AddToCache(commandText, resultSet);
        }

        public static ResultSet RetrieveFromCache(string commandText, int cacheAge)
        {
            return (ResultSet)cache.RetrieveFromCache(commandText, cacheAge);
        }

        public static void RemoveFromCache(string commandText)
        {
            cache.RemoveFromCache(commandText);
        }

        public static void DumpCache()
        {
            cache.Dump();
        }
    }

    /// <summary>
    /// Defines the basic operations to be performed on the table cache.
    /// </summary>
    public class BaseTableCache
    {
        /// <summary>
        /// The maximum age allowed for cache entries.
        /// </summary>
        protected int MaxCacheAge;

        private Dictionary<string, CacheEntry> cache = new Dictionary<string, CacheEntry>();

        public BaseTableCache(int maxCacheAge)
        {
            this.MaxCacheAge = maxCacheAge;
        }

        /// <summary>
        /// Adds the given command and result set to the cache.
        /// </summary>
        /// <param name="commandText">The command to store in the cache.</param>
        /// <param name="resultSet">The resultset associated to the stored command.</param>
        public virtual void AddToCache(string commandText, object resultSet)
        {
            this.CleanCache();
            CacheEntry entry = new CacheEntry();
            entry.CacheTime = DateTime.Now;
            entry.CacheElement = resultSet;
            lock (this.cache)
            {
                if (this.cache.ContainsKey(commandText))
                {
                    return;
                }

                this.cache.Add(commandText, entry);
            }
        }

        /// <summary>
        /// Retrieves the specified command from the cache.
        /// </summary>
        /// <param name="commandText">The command to retrieve.</param>
        /// <param name="cacheAge">The allowed age for the cache entry.</param>
        /// <returns></returns>
        public virtual object RetrieveFromCache(string commandText, int cacheAge)
        {
            this.CleanCache();
            lock (this.cache)
            {
                if (!this.cache.ContainsKey(commandText))
                {
                    return null;
                }

                CacheEntry entry = this.cache[commandText];
                if (DateTime.Now.Subtract(entry.CacheTime).TotalSeconds > cacheAge)
                {
                    return null;
                }

                return entry.CacheElement;
            }
        }

        /// <summary>
        /// Removes the specified command from the cache.
        /// </summary>
        /// <param name="commandText">The command to remove from the cache.</param>
        public void RemoveFromCache(string commandText)
        {
            lock (this.cache)
            {
                if (!this.cache.ContainsKey(commandText))
                {
                    return;
                }

                this.cache.Remove(commandText);
            }
        }

        /// <summary>
        /// Clears the cache.
        /// </summary>
        public virtual void Dump()
        {
            lock (this.cache)
            {
                this.cache.Clear();
            }
        }

        /// <summary>
        /// Removes cache entries older than the value defined by <see cref="MaxCacheAge"/>.
        /// </summary>
        protected virtual void CleanCache()
        {
            DateTime now = DateTime.Now;
            List<string> keysToRemove = new List<string>();

            lock (this.cache)
            {
                keysToRemove.AddRange(from key in this.cache.Keys let diff = now.Subtract(this.cache[key].CacheTime) where diff.TotalSeconds > this.MaxCacheAge select key);

                foreach (string key in keysToRemove)
                {
                    this.cache.Remove(key);
                }
            }
        }

        private struct CacheEntry
        {
            public DateTime CacheTime;
            public object CacheElement;
        }
    }

}