/*
 * Copyright (c) 2018 ETH Zürich, Educational Development and Technology (LET)
 * 
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */

using System;
using System.ServiceModel;
using System.Timers;
using SafeExamBrowser.Contracts.Communication;
using SafeExamBrowser.Contracts.Communication.Messages;
using SafeExamBrowser.Contracts.Communication.Responses;
using SafeExamBrowser.Contracts.Logging;

namespace SafeExamBrowser.Core.Communication
{
	/// <summary>
	/// Base implementation of an <see cref="ICommunicationProxy"/>.
	/// </summary>
	public abstract class BaseProxy : ICommunicationProxy
	{
		private const int ONE_MINUTE = 60000;
		private static readonly object @lock = new object();

		private string address;
		private ICommunication proxy;
		private IProxyObjectFactory factory;
		private Guid? communicationToken;
		private Timer timer;

		protected ILogger Logger { get; private set; }

		public event CommunicationEventHandler ConnectionLost;

		public BaseProxy(string address, IProxyObjectFactory factory, ILogger logger)
		{
			this.address = address;
			this.factory = factory;
			this.Logger = logger;
		}

		public virtual bool Connect(Guid? token = null, bool autoPing = true)
		{
			proxy = factory.CreateObject(address);

			if (proxy is ICommunicationObject communicationObject)
			{
				communicationObject.Closed += BaseProxy_Closed;
				communicationObject.Closing += BaseProxy_Closing;
				communicationObject.Faulted += BaseProxy_Faulted;
				communicationObject.Opened += BaseProxy_Opened;
				communicationObject.Opening += BaseProxy_Opening;
			}

			Logger.Debug($"Trying to connect to endpoint '{address}'{(token.HasValue ? $" with authentication token '{token}'" : string.Empty)}...");

			var response = proxy.Connect(token);

			communicationToken = response.CommunicationToken;
			Logger.Debug($"Connection was {(response.ConnectionEstablished ? "established" : "refused")}.");

			if (response.ConnectionEstablished && autoPing)
			{
				StartAutoPing();
			}

			return response.ConnectionEstablished;
		}

		public virtual bool Disconnect()
		{
			FailIfNotConnected(nameof(Disconnect));
			StopAutoPing();

			var message = new DisconnectionMessage { CommunicationToken = communicationToken.Value };
			var response = proxy.Disconnect(message);

			Logger.Debug($"{(response.ConnectionTerminated ? "Disconnected" : "Failed to disconnect")} from {address}.");

			return response.ConnectionTerminated;
		}

		/// <summary>
		/// Sends the given message, optionally returning a response. If no response is expected, <c>null</c> will be returned.
		/// </summary>
		/// <exception cref="ArgumentNullException">If the given message is <c>null</c>.</exception>
		/// <exception cref="InvalidOperationException">If no connection has been established yet.</exception>
		/// <exception cref="System.ServiceModel.*">If the communication failed.</exception>
		protected virtual Response Send(Message message)
		{
			if (message is null)
			{
				throw new ArgumentNullException(nameof(message));
			}

			FailIfNotConnected(nameof(Send));

			message.CommunicationToken = communicationToken.Value;

			var response = proxy.Send(message);

			Logger.Debug($"Sent message '{ToString(message)}', got response '{ToString(response)}'.");

			return response;
		}

		/// <summary>
		/// Sends the given purport as <see cref="SimpleMessage"/>.
		/// </summary>
		protected Response Send(SimpleMessagePurport purport)
		{
			return Send(new SimpleMessage(purport));
		}

		/// <summary>
		/// Determines whether the given response is a <see cref="SimpleResponse"/> with purport <see cref="SimpleResponsePurport.Acknowledged"/>.
		/// </summary>
		protected bool IsAcknowledged(Response response)
		{
			return response is SimpleResponse simpleResponse && simpleResponse.Purport == SimpleResponsePurport.Acknowledged;
		}

		/// <summary>
		/// Retrieves the string representation of the given <see cref="Message"/>, or indicates that a message is <c>null</c>.
		/// </summary>
		protected string ToString(Message message)
		{
			return message != null ? message.ToString() : "<null>";
		}

		/// <summary>
		/// etrieves the string representation of the given <see cref="Response"/>, or indicates that a response is <c>null</c>.
		/// </summary>
		protected string ToString(Response response)
		{
			return response != null ? response.ToString() : "<null>";
		}

		private void BaseProxy_Closed(object sender, EventArgs e)
		{
			Logger.Debug("Communication channel has been closed.");
		}

		private void BaseProxy_Closing(object sender, EventArgs e)
		{
			Logger.Debug("Communication channel is closing...");
		}

		private void BaseProxy_Faulted(object sender, EventArgs e)
		{
			Logger.Error("Communication channel has faulted!");
		}

		private void BaseProxy_Opened(object sender, EventArgs e)
		{
			Logger.Debug("Communication channel has been opened.");
		}

		private void BaseProxy_Opening(object sender, EventArgs e)
		{
			Logger.Debug("Communication channel is opening...");
		}

		private void FailIfNotConnected(string operationName)
		{
			if (!communicationToken.HasValue)
			{
				throw new InvalidOperationException($"Cannot perform '{operationName}' before being connected to endpoint!");
			}

			if (proxy == null || (proxy as ICommunicationObject)?.State != CommunicationState.Opened)
			{
				throw new CommunicationException($"Tried to perform {operationName}, but channel was {GetChannelState()}!");
			}
		}

		private string GetChannelState()
		{
			return proxy == null ? "null" : $"in state '{(proxy as ICommunicationObject)?.State}'";
		}

		private void StartAutoPing()
		{
			lock (@lock)
			{
				timer = new Timer(ONE_MINUTE) { AutoReset = true };
				timer.Elapsed += Timer_Elapsed;
				timer.Start();
			}
		}

		private void StopAutoPing()
		{
			lock (@lock)
			{
				timer?.Stop();
			}
		}

		private void Timer_Elapsed(object sender, ElapsedEventArgs args)
		{
			lock (@lock)
			{
				if (timer.Enabled)
				{
					try
					{
						var response = Send(SimpleMessagePurport.Ping);

						if (IsAcknowledged(response))
						{
							Logger.Info("Pinged host, connection is alive.");
						}
						else
						{
							Logger.Error($"Host did not acknowledge ping message! Received: {ToString(response)}.");
							timer.Stop();
							ConnectionLost?.Invoke();
						}
					}
					catch (Exception e)
					{
						Logger.Error("Failed to ping host!", e);
						timer.Stop();
						ConnectionLost?.Invoke();
					}
				}
			}
		}
	}
}