// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Caching.Distributed;
using Microsoft.Extensions.Logging;

namespace Microsoft.AspNetCore.Session
{
    public class DistributedSession : ISession
    {
        private static readonly RandomNumberGenerator CryptoRandom = RandomNumberGenerator.Create();
        private const int IdByteCount = 16;

        private const byte SerializationRevision = 2;
        private const int KeyLengthLimit = ushort.MaxValue;

        private readonly IDistributedCache _cache;
        private readonly string _sessionKey;
        private readonly TimeSpan _idleTimeout;
        private readonly Func<bool> _tryEstablishSession;
        private readonly ILogger _logger;
        private IDictionary<EncodedKey, byte[]> _store;
        private bool _isModified;
        private bool _loaded;
        private bool _isAvailable;
        private bool _isNewSessionKey;
        private string _sessionId;
        private byte[] _sessionIdBytes;

        public DistributedSession(
            IDistributedCache cache,
            string sessionKey,
            TimeSpan idleTimeout,
            Func<bool> tryEstablishSession,
            ILoggerFactory loggerFactory,
            bool isNewSessionKey)
        {
            if (cache == null)
            {
                throw new ArgumentNullException(nameof(cache));
            }

            if (string.IsNullOrEmpty(sessionKey))
            {
                throw new ArgumentException(Resources.ArgumentCannotBeNullOrEmpty, nameof(sessionKey));
            }

            if (tryEstablishSession == null)
            {
                throw new ArgumentNullException(nameof(tryEstablishSession));
            }

            if (loggerFactory == null)
            {
                throw new ArgumentNullException(nameof(loggerFactory));
            }

            _cache = cache;
            _sessionKey = sessionKey;
            _idleTimeout = idleTimeout;
            _tryEstablishSession = tryEstablishSession;
            _store = new Dictionary<EncodedKey, byte[]>();
            _logger = loggerFactory.CreateLogger<DistributedSession>();
            _isNewSessionKey = isNewSessionKey;
        }

        public bool IsAvailable
        {
            get
            {
                Load();
                return _isAvailable;
            }
        }

        public string Id
        {
            get
            {
                Load();
                if (_sessionId == null)
                {
                    _sessionId = new Guid(IdBytes).ToString();
                }
                return _sessionId;
            }
        }

        private byte[] IdBytes
        {
            get
            {
                if (IsAvailable && _sessionIdBytes == null)
                {
                    _sessionIdBytes = new byte[IdByteCount];
                    CryptoRandom.GetBytes(_sessionIdBytes);
                }
                return _sessionIdBytes;
            }
        }

        public IEnumerable<string> Keys
        {
            get
            {
                Load();
                return _store.Keys.Select(key => key.KeyString);
            }
        }

        public bool TryGetValue(string key, out byte[] value)
        {
            Load();
            return _store.TryGetValue(new EncodedKey(key), out value);
        }

        public void Set(string key, byte[] value)
        {
            if (value == null)
            {
                throw new ArgumentNullException(nameof(value));
            }

            if (IsAvailable)
            {
                var encodedKey = new EncodedKey(key);
                if (encodedKey.KeyBytes.Length > KeyLengthLimit)
                {
                    throw new ArgumentOutOfRangeException(nameof(key),
                        Resources.FormatException_KeyLengthIsExceeded(KeyLengthLimit));
                }

                if (!_tryEstablishSession())
                {
                    throw new InvalidOperationException(Resources.Exception_InvalidSessionEstablishment);
                }
                _isModified = true;
                byte[] copy = new byte[value.Length];
                Buffer.BlockCopy(src: value, srcOffset: 0, dst: copy, dstOffset: 0, count: value.Length);
                _store[encodedKey] = copy;
            }
        }

        public void Remove(string key)
        {
            Load();
            _isModified |= _store.Remove(new EncodedKey(key));
        }

        public void Clear()
        {
            Load();
            _isModified |= _store.Count > 0;
            _store.Clear();
        }

        private void Load()
        {
            if (!_loaded)
            {
                try
                {
                    var data = _cache.Get(_sessionKey);
                    if (data != null)
                    {
                        Deserialize(new MemoryStream(data));
                    }
                    else if (!_isNewSessionKey)
                    {
                        _logger.AccessingExpiredSession(_sessionKey);
                    }
                    _isAvailable = true;
                }
                catch (Exception exception)
                {
                    _logger.SessionCacheReadException(_sessionKey, exception);
                    _isAvailable = false;
                    _sessionId = string.Empty;
                    _sessionIdBytes = null;
                    _store = new NoOpSessionStore();
                }
                finally
                {
                    _loaded = true;
                }
            }
        }

        // This will throw if called directly and a failure occurs. The user is expected to handle the failures.
        public async Task LoadAsync()
        {
            if (!_loaded)
            {
                var data = await _cache.GetAsync(_sessionKey);
                if (data != null)
                {
                    Deserialize(new MemoryStream(data));
                }
                else if (!_isNewSessionKey)
                {
                    _logger.AccessingExpiredSession(_sessionKey);
                }
                _isAvailable = true;
                _loaded = true;
            }
        }

        public async Task CommitAsync()
        {
            if (_isModified)
            {
                if (_logger.IsEnabled(LogLevel.Information))
                {
                    try
                    {
                        var data = await _cache.GetAsync(_sessionKey);
                        if (data == null)
                        {
                            _logger.SessionStarted(_sessionKey, Id);
                        }
                    }
                    catch (Exception exception)
                    {
                        _logger.SessionCacheReadException(_sessionKey, exception);
                    }
                }

                var stream = new MemoryStream();
                Serialize(stream);
                
                await _cache.SetAsync(
                    _sessionKey,
                    stream.ToArray(),
                    new DistributedCacheEntryOptions().SetSlidingExpiration(_idleTimeout));

                _isModified = false;
                _logger.SessionStored(_sessionKey, Id, _store.Count);
            }
            else
            {
                await _cache.RefreshAsync(_sessionKey);
            }
        }

        // Format:
        // Serialization revision: 1 byte, range 0-255
        // Entry count: 3 bytes, range 0-16,777,215
        // SessionId: IdByteCount bytes (16)
        // foreach entry:
        //   key name byte length: 2 bytes, range 0-65,535
        //   UTF-8 encoded key name byte[]
        //   data byte length: 4 bytes, range 0-2,147,483,647
        //   data byte[]
        private void Serialize(Stream output)
        {
            output.WriteByte(SerializationRevision);
            SerializeNumAs3Bytes(output, _store.Count);
            output.Write(IdBytes, 0, IdByteCount);

            foreach (var entry in _store)
            {
                var keyBytes = entry.Key.KeyBytes;
                SerializeNumAs2Bytes(output, keyBytes.Length);
                output.Write(keyBytes, 0, keyBytes.Length);
                SerializeNumAs4Bytes(output, entry.Value.Length);
                output.Write(entry.Value, 0, entry.Value.Length);
            }
        }

        private void Deserialize(Stream content)
        {
            if (content == null || content.ReadByte() != SerializationRevision)
            {
                // Replace the un-readable format.
                _isModified = true;
                return;
            }

            int expectedEntries = DeserializeNumFrom3Bytes(content);
            _sessionIdBytes = ReadBytes(content, IdByteCount);

            for (int i = 0; i < expectedEntries; i++)
            {
                int keyLength = DeserializeNumFrom2Bytes(content);
                var key = new EncodedKey(ReadBytes(content, keyLength));
                int dataLength = DeserializeNumFrom4Bytes(content);
                _store[key] = ReadBytes(content, dataLength);
            }

            if (_logger.IsEnabled(LogLevel.Debug))
            {
                _sessionId = new Guid(_sessionIdBytes).ToString();
                _logger.SessionLoaded(_sessionKey, _sessionId, expectedEntries);
            }
        }

        private void SerializeNumAs2Bytes(Stream output, int num)
        {
            if (num < 0 || ushort.MaxValue < num)
            {
                throw new ArgumentOutOfRangeException(nameof(num), Resources.Exception_InvalidToSerializeIn2Bytes);
            }
            output.WriteByte((byte)(num >> 8));
            output.WriteByte((byte)(0xFF & num));
        }

        private int DeserializeNumFrom2Bytes(Stream content)
        {
            return content.ReadByte() << 8 | content.ReadByte();
        }

        private void SerializeNumAs3Bytes(Stream output, int num)
        {
            if (num < 0 || 0xFFFFFF < num)
            {
                throw new ArgumentOutOfRangeException(nameof(num), Resources.Exception_InvalidToSerializeIn3Bytes);
            }
            output.WriteByte((byte)(num >> 16));
            output.WriteByte((byte)(0xFF & (num >> 8)));
            output.WriteByte((byte)(0xFF & num));
        }

        private int DeserializeNumFrom3Bytes(Stream content)
        {
            return content.ReadByte() << 16 | content.ReadByte() << 8 | content.ReadByte();
        }

        private void SerializeNumAs4Bytes(Stream output, int num)
        {
            if (num < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(num), Resources.Exception_NumberShouldNotBeNegative);
            }
            output.WriteByte((byte)(num >> 24));
            output.WriteByte((byte)(0xFF & (num >> 16)));
            output.WriteByte((byte)(0xFF & (num >> 8)));
            output.WriteByte((byte)(0xFF & num));
        }

        private int DeserializeNumFrom4Bytes(Stream content)
        {
            return content.ReadByte() << 24 | content.ReadByte() << 16 | content.ReadByte() << 8 | content.ReadByte();
        }

        private byte[] ReadBytes(Stream stream, int count)
        {
            var output = new byte[count];
            int total = 0;
            while (total < count)
            {
                var read = stream.Read(output, total, count - total);
                if (read == 0)
                {
                    throw new EndOfStreamException();
                }
                total += read;
            }
            return output;
        }

    }
}