#region Apache License
//
// 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.
//
#endregion

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using log4net.Appender.Internal;
using log4net.Core;
using log4net.Util;

namespace log4net.Appender;

/// <summary>
/// Sends logging events as connectionless UDP datagrams to a remote host or a 
/// multicast group using an <see cref="UdpClient" />.
/// </summary>
/// <remarks>
/// <para>
/// UDP guarantees neither that messages arrive, nor that they arrive in the correct order.
/// </para>
/// <para>
/// To view the logging results, a custom application can be developed that listens for logging 
/// events.
/// </para>
/// <para>
/// When decoding events send via this appender remember to use the same encoding
/// to decode the events as was used to send the events. See the <see cref="Encoding"/>
/// property to specify the encoding to use.
/// </para>
/// </remarks>
/// <example>
/// This example shows how to log receive logging events that are sent 
/// on IP address 244.0.0.1 and port 8080 to the console. The event is 
/// encoded in the packet as a unicode string and it is decoded as such. 
/// <code lang="C#">
/// IPEndPoint remoteEndPoint = new IPEndPoint(IPAddress.Any, 0);
/// UdpClient udpClient;
/// byte[] buffer;
/// string loggingEvent;
/// 
/// try 
/// {
///     udpClient = new UdpClient(8080);
///     
///     while(true) 
///     {
///         buffer = udpClient.Receive(ref remoteEndPoint);
///         loggingEvent = System.Text.Encoding.Unicode.GetString(buffer);
///         Console.WriteLine(loggingEvent);
///     }
/// } 
/// catch(Exception e) 
/// {
///     Console.WriteLine(e.ToString());
/// }
/// </code>
/// <code lang="Visual Basic">
/// Dim remoteEndPoint as IPEndPoint
/// Dim udpClient as UdpClient
/// Dim buffer as Byte()
/// Dim loggingEvent as String
/// 
/// Try 
///     remoteEndPoint = new IPEndPoint(IPAddress.Any, 0)
///     udpClient = new UdpClient(8080)
///
///     While True
///         buffer = udpClient.Receive(ByRef remoteEndPoint)
///         loggingEvent = System.Text.Encoding.Unicode.GetString(buffer)
///         Console.WriteLine(loggingEvent)
///     Wend
/// Catch e As Exception
///     Console.WriteLine(e.ToString())
/// End Try
/// </code>
/// <para>
/// An example configuration section to log information using this appender to the 
/// IP 224.0.0.1 on port 8080:
/// </para>
/// <code lang="XML" escaped="true">
/// <appender name="UdpAppender" type="log4net.Appender.UdpAppender">
///     <remoteAddress value="224.0.0.1" />
///     <remotePort value="8080" />
///     <layout type="log4net.Layout.PatternLayout" value="%-5level %logger [%ndc] - %message%newline" />
/// </appender>
/// </code>
/// </example>
/// <author>Gert Driesen</author>
/// <author>Nicko Cadell</author>
public class UdpAppender : AppenderSkeleton
{
  /// <summary>
  /// Initializes a new instance of the <see cref="UdpAppender" /> class.
  /// </summary>
  /// <remarks>
  /// The default constructor initializes all fields to their default values.
  /// </remarks>
  public UdpAppender()
  {
  }

  /// <summary>
  /// Gets or sets the IP address of the remote host or multicast group to which
  /// the underlying <see cref="UdpClient" /> should sent the logging event.
  /// </summary>
  /// <value>
  /// The IP address of the remote host or multicast group to which the logging event 
  /// will be sent.
  /// </value>
  /// <remarks>
  /// <para>
  /// Multicast addresses are identified by IP class <b>D</b> addresses (in the range 224.0.0.0 to
  /// 239.255.255.255).  Multicast packets can pass across different networks through routers, so
  /// it is possible to use multicasts in an Internet scenario as long as your network provider 
  /// supports multicasting.
  /// </para>
  /// <para>
  /// Hosts that want to receive particular multicast messages must register their interest by joining
  /// the multicast group.  Multicast messages are not sent to networks where no host has joined
  /// the multicast group.  Class <b>D</b> IP addresses are used for multicast groups, to differentiate
  /// them from normal host addresses, allowing nodes to easily detect if a message is of interest.
  /// </para>
  /// <para>
  /// Static multicast addresses that are needed globally are assigned by IANA.  A few examples are listed in the table below:
  /// </para>
  /// <para>
  /// <list type="table">
  ///     <listheader>
  ///         <term>IP Address</term>
  ///         <description>Description</description>
  ///     </listheader>
  ///     <item>
  ///         <term>224.0.0.1</term>
  ///         <description>
  ///             <para>
  ///             Sends a message to all system on the subnet.
  ///             </para>
  ///         </description>
  ///     </item>
  ///     <item>
  ///         <term>224.0.0.2</term>
  ///         <description>
  ///             <para>
  ///             Sends a message to all routers on the subnet.
  ///             </para>
  ///         </description>
  ///     </item>
  ///     <item>
  ///         <term>224.0.0.12</term>
  ///         <description>
  ///             <para>
  ///             The DHCP server answers messages on the IP address 224.0.0.12, but only on a subnet.
  ///             </para>
  ///         </description>
  ///     </item>
  /// </list>
  /// </para>
  /// <para>
  /// A complete list of actually reserved multicast addresses and their owners in the ranges
  /// defined by RFC 3171 can be found at the <A href="http://www.iana.org/assignments/multicast-addresses">IANA web site</A>. 
  /// </para>
  /// <para>
  /// The address range 239.0.0.0 to 239.255.255.255 is reserved for administrative scope-relative 
  /// addresses.  These addresses can be reused with other local groups.  Routers are typically 
  /// configured with filters to prevent multicast traffic in this range from flowing outside
  /// of the local network.
  /// </para>
  /// </remarks>
  public IPAddress? RemoteAddress { get; set; }

  /// <summary>
  /// Gets or sets the TCP port number of the remote host or multicast group to which 
  /// the underlying <see cref="UdpClient" /> should sent the logging event.
  /// </summary>
  /// <value>
  /// An integer value in the range <see cref="IPEndPoint.MinPort" /> to <see cref="IPEndPoint.MaxPort" /> 
  /// indicating the TCP port number of the remote host or multicast group to which the logging event 
  /// will be sent.
  /// </value>
  /// <remarks>
  /// The underlying <see cref="UdpClient" /> will send messages to this TCP port number
  /// on the remote host or multicast group.
  /// </remarks>
  /// <exception cref="ArgumentOutOfRangeException">The value specified is less than <see cref="IPEndPoint.MinPort" /> or greater than <see cref="IPEndPoint.MaxPort" />.</exception>
  public int RemotePort
  {
    get => _remotePort;
    set
    {
      if (value is < IPEndPoint.MinPort or > IPEndPoint.MaxPort)
      {
        throw SystemInfo.CreateArgumentOutOfRangeException(nameof(value), value,
          $"The value specified is less than {IPEndPoint.MinPort} or greater than {IPEndPoint.MaxPort}.");
      }
      _remotePort = value;
    }
  }

  /// <summary>
  /// Gets or sets the TCP port number from which the underlying <see cref="UdpClient" /> will communicate.
  /// </summary>
  /// <value>
  /// An integer value in the range <see cref="IPEndPoint.MinPort" /> to <see cref="IPEndPoint.MaxPort" /> 
  /// indicating the TCP port number from which the underlying <see cref="UdpClient" /> will communicate.
  /// </value>
  /// <remarks>
  /// <para>
  /// The underlying <see cref="UdpClient" /> will bind to this port for sending messages.
  /// </para>
  /// <para>
  /// Setting the value to 0 (the default) will cause the udp client not to bind to
  /// a local port.
  /// </para>
  /// </remarks>
  /// <exception cref="ArgumentOutOfRangeException">The value specified is less than <see cref="IPEndPoint.MinPort" /> or greater than <see cref="IPEndPoint.MaxPort" />.</exception>
  public int LocalPort
  {
    get => _localPort;
    set
    {
      if (value is not 0 and (< IPEndPoint.MinPort or > IPEndPoint.MaxPort))
      {
        throw SystemInfo.CreateArgumentOutOfRangeException(nameof(value), value,
          $"The value specified is less than {IPEndPoint.MinPort} or greater than {IPEndPoint.MaxPort}.");
      }
      _localPort = value;
    }
  }

  /// <summary>
  /// Gets or sets <see cref="Encoding"/> used to write the packets.
  /// </summary>
  /// <value>
  /// The <see cref="Encoding"/> used to write the packets.
  /// </value>
  /// <remarks>
  /// <para>
  /// The <see cref="Encoding"/> used to write the packets.
  /// </para>
  /// </remarks>
  public Encoding Encoding { get; set; } = Encoding.Default;

  /// <summary>
  /// Gets or sets the underlying <see cref="UdpClient" />.
  /// </summary>
  /// <value>
  /// The underlying <see cref="UdpClient" />.
  /// </value>
  /// <remarks>
  /// <see cref="UdpAppender" /> creates a <see cref="UdpClient" /> to send logging events
  /// over a network.  Classes deriving from <see cref="UdpAppender" /> can use this
  /// property to get or set this <see cref="UdpClient" />.  Use the underlying <see cref="UdpClient" />
  /// returned from <see cref="Client" /> if you require access beyond that which 
  /// <see cref="UdpAppender" /> provides.
  /// </remarks>
  protected UdpClient? Client { get; set; }

  /// <summary>
  /// Gets or sets the cached remote endpoint to which the logging events should be sent.
  /// </summary>
  /// <remarks>
  /// The <see cref="ActivateOptions" /> method will initialize the remote endpoint
  /// with the values of the <see cref="RemoteAddress" /> and <see cref="RemotePort"/>
  /// properties.
  /// </remarks>
  protected IPEndPoint? RemoteEndPoint { get; set; }

  /// <summary>
  /// Initialize the appender based on the options set.
  /// </summary>
  /// <remarks>
  /// <para>
  /// This is part of the <see cref="IOptionHandler"/> delayed object
  /// activation scheme. The <see cref="ActivateOptions"/> method must
  /// be called on this object after the configuration properties have
  /// been set. Until <see cref="ActivateOptions"/> is called this
  /// object is in an undefined state and must not be used.
  /// </para>
  /// <para>
  /// If any of the configuration properties are modified then
  /// <see cref="ActivateOptions"/> must be called again.
  /// </para>
  /// <para>
  /// The appender will be ignored if no <see cref="RemoteAddress" /> was specified or 
  /// an invalid remote or local TCP port number was specified.
  /// </para>
  /// </remarks>
  /// <exception cref="ArgumentNullException">The required property <see cref="RemoteAddress" /> was not specified.</exception>
  /// <exception cref="ArgumentOutOfRangeException">The TCP port number assigned to <see cref="LocalPort" /> or <see cref="RemotePort" /> is less than <see cref="IPEndPoint.MinPort" /> or greater than <see cref="IPEndPoint.MaxPort" />.</exception>
  public override void ActivateOptions()
  {
    base.ActivateOptions();
    RemoteAddress.EnsureNotNull();

    if (RemotePort is < IPEndPoint.MinPort or > IPEndPoint.MaxPort)
    {
      throw SystemInfo.CreateArgumentOutOfRangeException(nameof(RemotePort), RemotePort,
        $"The RemotePort is less than {IPEndPoint.MinPort} or greater than {IPEndPoint.MaxPort}.");
    }
    if (LocalPort is not 0 and (< IPEndPoint.MinPort or > IPEndPoint.MaxPort))
    {
      throw SystemInfo.CreateArgumentOutOfRangeException(nameof(LocalPort), LocalPort,
        $"The LocalPort is less than {IPEndPoint.MinPort} or greater than {IPEndPoint.MaxPort}.");
    }
    RemoteEndPoint = new(RemoteAddress, RemotePort);
    InitializeClientConnection();
  }

  /// <summary>
  /// This method is called by the <see cref="AppenderSkeleton.DoAppend(LoggingEvent)"/> method.
  /// </summary>
  /// <param name="loggingEvent">The event to log.</param>
  /// <remarks>
  /// <para>
  /// Sends the event using an UDP datagram.
  /// </para>
  /// <para>
  /// Exceptions are passed to the <see cref="AppenderSkeleton.ErrorHandler"/>.
  /// </para>
  /// </remarks>
  protected override void Append(LoggingEvent loggingEvent)
  {
    if (Client is null)
    {
      ErrorHandler.Error(
        $"Unable to send logging event to remote host {RemoteAddress} on port {RemotePort}, no client created",
        e: null,
        ErrorCode.WriteFailure);
      return;
    }

    try
    {
      byte[] buffer = Encoding.GetBytes(RenderLoggingEvent(loggingEvent).ToCharArray());
      Client.Send(buffer, buffer.Length, RemoteEndPoint);
    }
    catch (Exception e) when (!e.IsFatal())
    {
      ErrorHandler.Error(
        $"Unable to send logging event to remote host {RemoteAddress} on port {RemotePort}.",
        e, ErrorCode.WriteFailure);
    }
  }

  /// <summary>
  /// This appender requires a <see cref="Layout"/> to be set.
  /// </summary>
  protected override bool RequiresLayout => true;

  /// <summary>
  /// Closes the UDP connection and releases all resources associated with
  /// this <see cref="UdpAppender" /> instance.
  /// </summary>
  /// <remarks>
  /// <para>
  /// Disables the underlying <see cref="UdpClient" /> and releases all managed 
  /// and unmanaged resources associated with the <see cref="UdpAppender" />.
  /// </para>
  /// </remarks>
  protected override void OnClose()
  {
    base.OnClose();

    Client?.Close();
  }

  /// <summary>
  /// Initializes the underlying  <see cref="UdpClient" /> connection.
  /// </summary>
  /// <remarks>
  /// <para>
  /// The underlying <see cref="UdpClient"/> is initialized and binds to the
  /// port number from which you intend to communicate.
  /// </para>
  /// <para>
  /// Exceptions are passed to the <see cref="AppenderSkeleton.ErrorHandler"/>.
  /// </para>
  /// </remarks>
  protected virtual void InitializeClientConnection()
  {
    try
    {
      Client = UdpConnection.CreateClient(LocalPort, RemoteAddress.EnsureNotNull());
    }
    catch (Exception e) when (!e.IsFatal())
    {
      ErrorHandler.Error(
        $"Could not initialize the UdpClient connection on port {LocalPort}.",
        e, ErrorCode.GenericFailure);

      Client = null;
    }
  }

  /// <summary>
  /// The TCP port number of the remote host or multicast group to
  /// which the logging event will be sent.
  /// </summary>
  private int _remotePort;

  /// <summary>
  /// The TCP port number from which the <see cref="UdpClient" /> will communicate.
  /// </summary>
  private int _localPort;
}