#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.Runtime.InteropServices;

using log4net.Core;
using log4net.Util;

namespace log4net.Appender;

/// <summary>
/// Logs events to a local syslog service.
/// </summary>
/// <remarks>
/// <note>
/// This appender uses the POSIX libc library functions <c>openlog</c>, <c>syslog</c>, and <c>closelog</c>.
/// If these functions are not available on the local system then this appender will not work!
/// </note>
/// <para>
/// The functions <c>openlog</c>, <c>syslog</c>, and <c>closelog</c> are specified in SUSv2 and 
/// POSIX 1003.1-2001 standards. These are used to log messages to the local syslog service.
/// </para>
/// <para>
/// This appender talks to a local syslog service. If you need to log to a remote syslog
/// daemon and you cannot configure your local syslog service to do this you may be
/// able to use the <see cref="RemoteSyslogAppender"/> to log via UDP.
/// </para>
/// <para>
/// Syslog messages must have a facility and and a severity. The severity
/// is derived from the Level of the logging event.
/// The facility must be chosen from the set of defined syslog 
/// <see cref="SyslogFacility"/> values. The facilities list is predefined
/// and cannot be extended.
/// </para>
/// <para>
/// An identifier is specified with each log message. This can be specified
/// by setting the <see cref="Identity"/> property. The identity (also know 
/// as the tag) must not contain white space. The default value for the
/// identity is the application name (from <see cref="SystemInfo.ApplicationFriendlyName"/>).
/// </para>
/// </remarks>
/// <author>Rob Lyon</author>
/// <author>Nicko Cadell</author>
public class LocalSyslogAppender : AppenderSkeleton
{
  /// <summary>
  /// syslog severities
  /// </summary>
  /// <remarks>
  /// <para>
  /// The log4net Level maps to a syslog severity using the
  /// <see cref="AddMapping"/> method and the <see cref="LevelSeverity"/>
  /// class. The severity is set on <see cref="LevelSeverity.Severity"/>.
  /// </para>
  /// </remarks>
  public enum SyslogSeverity
  {
    /// <summary>
    /// system is unusable
    /// </summary>
    Emergency = 0,

    /// <summary>
    /// action must be taken immediately
    /// </summary>
    Alert = 1,

    /// <summary>
    /// critical conditions
    /// </summary>
    Critical = 2,

    /// <summary>
    /// error conditions
    /// </summary>
    Error = 3,

    /// <summary>
    /// warning conditions
    /// </summary>
    Warning = 4,

    /// <summary>
    /// normal but significant condition
    /// </summary>
    Notice = 5,

    /// <summary>
    /// informational
    /// </summary>
    Informational = 6,

    /// <summary>
    /// debug-level messages
    /// </summary>
    Debug = 7
  };

  /// <summary>
  /// syslog facilities
  /// </summary>
  /// <remarks>
  /// <para>
  /// The syslog facility defines which subsystem the logging comes from.
  /// This is set on the <see cref="Facility"/> property.
  /// </para>
  /// </remarks>
  public enum SyslogFacility
  {
    /// <summary>
    /// kernel messages
    /// </summary>
    Kernel = 0,

    /// <summary>
    /// random user-level messages
    /// </summary>
    User = 1,

    /// <summary>
    /// mail system
    /// </summary>
    Mail = 2,

    /// <summary>
    /// system daemons
    /// </summary>
    Daemons = 3,

    /// <summary>
    /// security/authorization messages
    /// </summary>
    Authorization = 4,

    /// <summary>
    /// messages generated internally by syslogd
    /// </summary>
    Syslog = 5,

    /// <summary>
    /// line printer subsystem
    /// </summary>
    Printer = 6,

    /// <summary>
    /// network news subsystem
    /// </summary>
    News = 7,

    /// <summary>
    /// UUCP subsystem
    /// </summary>
    Uucp = 8,

    /// <summary>
    /// clock (cron/at) daemon
    /// </summary>
    Clock = 9,

    /// <summary>
    /// security/authorization  messages (private)
    /// </summary>
    Authorization2 = 10,

    /// <summary>
    /// ftp daemon
    /// </summary>
    Ftp = 11,

    /// <summary>
    /// NTP subsystem
    /// </summary>
    Ntp = 12,

    /// <summary>
    /// log audit
    /// </summary>
    Audit = 13,

    /// <summary>
    /// log alert
    /// </summary>
    Alert = 14,

    /// <summary>
    /// clock daemon
    /// </summary>
    Clock2 = 15,

    /// <summary>
    /// reserved for local use
    /// </summary>
    Local0 = 16,

    /// <summary>
    /// reserved for local use
    /// </summary>
    Local1 = 17,

    /// <summary>
    /// reserved for local use
    /// </summary>
    Local2 = 18,

    /// <summary>
    /// reserved for local use
    /// </summary>
    Local3 = 19,

    /// <summary>
    /// reserved for local use
    /// </summary>
    Local4 = 20,

    /// <summary>
    /// reserved for local use
    /// </summary>
    Local5 = 21,

    /// <summary>
    /// reserved for local use
    /// </summary>
    Local6 = 22,

    /// <summary>
    /// reserved for local use
    /// </summary>
    Local7 = 23
  }

  /// <summary>
  /// Message identity
  /// </summary>
  /// <remarks>
  /// <para>
  /// An identifier is specified with each log message. This can be specified
  /// by setting the <see cref="Identity"/> property. The identity (also known 
  /// as the tag) must not contain white space. The default value for the
  /// identity is the application name (from <see cref="SystemInfo.ApplicationFriendlyName"/>).
  /// </para>
  /// </remarks>
  public string? Identity { get; set; }

  /// <summary>
  /// Syslog facility
  /// </summary>
  /// <remarks>
  /// Set to one of the <see cref="SyslogFacility"/> values. The list of
  /// facilities is predefined and cannot be extended. The default value
  /// is <see cref="SyslogFacility.User"/>.
  /// </remarks>
  public SyslogFacility Facility { get; set; } = SyslogFacility.User;

  /// <summary>
  /// Add a mapping of level to severity
  /// </summary>
  /// <param name="mapping">The mapping to add</param>
  /// <remarks>
  /// <para>
  /// Adds a <see cref="LevelSeverity"/> to this appender.
  /// </para>
  /// </remarks>
  public void AddMapping(LevelSeverity mapping) => _levelMapping.Add(mapping);

  /// <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>
  /// </remarks>
  [System.Security.SecuritySafeCritical]
  public override void ActivateOptions()
  {
    base.ActivateOptions();

    _levelMapping.ActivateOptions();

    // Set to app name by default
    string? identString = Identity ?? SystemInfo.ApplicationFriendlyName;

    // create the native heap ansi string. Note this is a copy of our string
    // so we do not need to hold on to the string itself, holding on to the
    // handle will keep the heap ansi string alive.
    _handleToIdentity = Marshal.StringToHGlobalAnsi(identString);

    // open syslog
    NativeMethods.openlog(_handleToIdentity, 1, Facility);
  }

  /// <summary>
  /// This method is called by the <see cref="AppenderSkeleton.DoAppend(LoggingEvent)"/> method.
  /// </summary>
  /// <param name="loggingEvent">The event to log.</param>
  /// <remarks>
  /// <para>
  /// Writes the event to a remote syslog daemon.
  /// </para>
  /// <para>
  /// The format of the output will depend on the appender's layout.
  /// </para>
  /// </remarks>
  [System.Security.SecuritySafeCritical]
  [System.Security.Permissions.SecurityPermission(System.Security.Permissions.SecurityAction.Demand, UnmanagedCode = true)]
  protected override void Append(LoggingEvent loggingEvent)
  {
    int priority = GeneratePriority(Facility, GetSeverity(loggingEvent.EnsureNotNull().Level));
    string message = RenderLoggingEvent(loggingEvent);

    // Call the local libc syslog method
    // The second argument is a printf style format string
    NativeMethods.syslog(priority, "%s", message);
  }

  /// <summary>
  /// Close the syslog when the appender is closed
  /// </summary>
  /// <remarks>
  /// <para>
  /// Close the syslog when the appender is closed
  /// </para>
  /// </remarks>
  [System.Security.SecuritySafeCritical]
  protected override void OnClose()
  {
    base.OnClose();

    try
    {
      // close syslog
      NativeMethods.closelog();
    }
    catch (DllNotFoundException)
    {
      // Ignore dll not found at this point
    }

    if (_handleToIdentity != IntPtr.Zero)
    {
      // free global ident
      Marshal.FreeHGlobal(_handleToIdentity);
    }
  }

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

  /// <summary>
  /// Translates a log4net level to a syslog severity.
  /// </summary>
  /// <param name="level">A log4net level.</param>
  /// <returns>A syslog severity.</returns>
  protected virtual SyslogSeverity GetSeverity(Level? level)
  {
    if (_levelMapping.Lookup(level) is LevelSeverity levelSeverity)
    {
      return levelSeverity.Severity;
    }

    //
    // Fall back to sensible default values
    //

    if (level is null)
    {
      // Default setting
      return SyslogSeverity.Debug;
    }
    if (level >= Level.Alert)
    {
      return SyslogSeverity.Alert;
    }
    else if (level >= Level.Critical)
    {
      return SyslogSeverity.Critical;
    }
    else if (level >= Level.Error)
    {
      return SyslogSeverity.Error;
    }
    else if (level >= Level.Warn)
    {
      return SyslogSeverity.Warning;
    }
    else if (level >= Level.Notice)
    {
      return SyslogSeverity.Notice;
    }
    else if (level >= Level.Info)
    {
      return SyslogSeverity.Informational;
    }
    // Default setting
    return SyslogSeverity.Debug;
  }

  /// <summary>
  /// Generate a syslog priority.
  /// </summary>
  /// <param name="facility">The syslog facility.</param>
  /// <param name="severity">The syslog severity.</param>
  /// <returns>A syslog priority.</returns>
  private static int GeneratePriority(SyslogFacility facility, SyslogSeverity severity)
    => ((int)facility * 8) + (int)severity;

  /// <summary>
  /// Marshaled handle to the identity string. We have to hold on to the
  /// string as the <c>openlog</c> and <c>syslog</c> APIs just hold the
  /// pointer to the ident and dereference it for each log message.
  /// </summary>
  private IntPtr _handleToIdentity = IntPtr.Zero;

  /// <summary>
  /// Mapping from level object to syslog severity
  /// </summary>
  private readonly LevelMapping _levelMapping = new();

  /// <summary>
  /// A class to act as a mapping between the level that a logging call is made at and
  /// the syslog severity that is should be logged at.
  /// </summary>
  public class LevelSeverity : LevelMappingEntry
  {
    /// <summary>
    /// The mapped syslog severity for the specified level
    /// </summary>
    public SyslogSeverity Severity { get; set; }
  }
}