/*
 * Greenshot - a free and open source screenshot tool
 * Copyright (C) 2007-2021 Thomas Braun, Jens Klingen, Robin Krom
 * 
 * For more information see: https://getgreenshot.org/
 * The Greenshot project is hosted on GitHub https://github.com/greenshot/greenshot
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 1 of the License, or
 * (at your option) any later version.
 * 
 * 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 for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

using System;
using System.Collections.Generic;
using System.Timers;
using log4net;

namespace Greenshot.Base.Core
{
    /// <summary>
    /// Cache class 
    /// </summary>
    /// <typeparam name="TK">Type of key</typeparam>
    /// <typeparam name="TV">Type of value</typeparam>
    public class Cache<TK, TV>
    {
        private static readonly ILog Log = LogManager.GetLogger(typeof(Cache<TK, TV>));
        private readonly IDictionary<TK, TV> _internalCache = new Dictionary<TK, TV>();
        private readonly object _lockObject = new object();
        private readonly int _secondsToExpire = 10;
        private readonly CacheObjectExpired _expiredCallback;

        public delegate void CacheObjectExpired(TK key, TV cacheValue);

        /// <summary>
        /// Initialize the cache
        /// </summary>
        public Cache()
        {
        }

        /// <summary>
        /// Initialize the cache
        /// </summary>
        /// <param name="expiredCallback"></param>
        public Cache(CacheObjectExpired expiredCallback) : this()
        {
            _expiredCallback = expiredCallback;
        }

        /// <summary>
        /// Initialize the cache with a expire setting
        /// </summary>
        /// <param name="secondsToExpire"></param>
        public Cache(int secondsToExpire) : this()
        {
            _secondsToExpire = secondsToExpire;
        }

        /// <summary>
        /// Initialize the cache with a expire setting
        /// </summary>
        /// <param name="secondsToExpire"></param>
        /// <param name="expiredCallback"></param>
        public Cache(int secondsToExpire, CacheObjectExpired expiredCallback) : this(expiredCallback)
        {
            _secondsToExpire = secondsToExpire;
        }

        /// <summary>
        /// Enumerable for the values in the cache
        /// </summary>
        public IEnumerable<TV> Elements
        {
            get
            {
                List<TV> elements = new List<TV>();

                lock (_lockObject)
                {
                    foreach (TV element in _internalCache.Values)
                    {
                        elements.Add(element);
                    }
                }

                foreach (TV element in elements)
                {
                    yield return element;
                }
            }
        }

        /// <summary>
        /// Get the value by key from the cache
        /// </summary>
        /// <param name="key"></param>
        /// <returns></returns>
        public TV this[TK key]
        {
            get
            {
                TV result = default;
                lock (_lockObject)
                {
                    if (_internalCache.ContainsKey(key))
                    {
                        result = _internalCache[key];
                    }
                }

                return result;
            }
        }

        /// <summary>
        /// Contains
        /// </summary>
        /// <param name="key"></param>
        /// <returns>true if the cache contains the key</returns>
        public bool Contains(TK key)
        {
            lock (_lockObject)
            {
                return _internalCache.ContainsKey(key);
            }
        }

        /// <summary>
        /// Add a value to the cache
        /// </summary>
        /// <param name="key"></param>
        /// <param name="value"></param>
        public void Add(TK key, TV value)
        {
            Add(key, value, null);
        }

        /// <summary>
        /// Add a value to the cache
        /// </summary>
        /// <param name="key"></param>
        /// <param name="value"></param>
        /// <param name="secondsToExpire">optional value for the seconds to expire</param>
        public void Add(TK key, TV value, int? secondsToExpire)
        {
            lock (_lockObject)
            {
                var cachedItem = new CachedItem(key, value, secondsToExpire ?? _secondsToExpire);
                cachedItem.Expired += delegate(TK cacheKey, TV cacheValue)
                {
                    if (_internalCache.ContainsKey(cacheKey))
                    {
                        Log.DebugFormat("Expiring object with Key: {0}", cacheKey);
                        _expiredCallback?.Invoke(cacheKey, cacheValue);
                        Remove(cacheKey);
                    }
                    else
                    {
                        Log.DebugFormat("Expired old object with Key: {0}", cacheKey);
                    }
                };

                if (_internalCache.ContainsKey(key))
                {
                    _internalCache[key] = value;
                    Log.DebugFormat("Updated item with Key: {0}", key);
                }
                else
                {
                    _internalCache.Add(key, cachedItem);
                    Log.DebugFormat("Added item with Key: {0}", key);
                }
            }
        }

        /// <summary>
        /// Remove item from cache
        /// </summary>
        /// <param name="key"></param>
        public void Remove(TK key)
        {
            lock (_lockObject)
            {
                if (!_internalCache.ContainsKey(key))
                {
                    throw new ApplicationException($"An object with key ‘{key}’ does not exists in cache");
                }

                _internalCache.Remove(key);
                Log.DebugFormat("Removed item with Key: {0}", key);
            }
        }

        /// <summary>
        /// A cache item
        /// </summary>
        private class CachedItem
        {
            public event CacheObjectExpired Expired;
            private readonly int _secondsToExpire;
            private readonly Timer _timerEvent;

            public CachedItem(TK key, TV item, int secondsToExpire)
            {
                if (key == null)
                {
                    throw new ArgumentNullException(nameof(key));
                }

                Key = key;
                Item = item;
                _secondsToExpire = secondsToExpire;
                if (secondsToExpire <= 0)
                {
                    return;
                }

                _timerEvent = new Timer(secondsToExpire * 1000)
                {
                    AutoReset = false
                };
                _timerEvent.Elapsed += timerEvent_Elapsed;
                _timerEvent.Start();
            }

            private void ExpireNow()
            {
                _timerEvent.Stop();
                if (_secondsToExpire > 0)
                {
                    Expired?.Invoke(Key, Item);
                }
            }

            private void timerEvent_Elapsed(object sender, ElapsedEventArgs e)
            {
                ExpireNow();
            }

            public TK Key { get; private set; }
            public TV Item { get; private set; }

            public static implicit operator TV(CachedItem a)
            {
                return a.Item;
            }
        }
    }
}