using System;
using System.Collections.Generic;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading.Tasks;

namespace Unosquare.Swan.Networking {
  /// <summary>
  /// Represents a little SNMP client based on http://www.java2s.com/Code/CSharp/Network/SimpleSNMP.htm.
  /// </summary>
  public static class SnmpClient {
    private static readonly Byte[] DiscoverMessage =
    {
            48, 41, 2, 1, 1, 4, 6, 112, 117, 98, 108, 105, 99, 160, 28, 2, 4, 111, 81, 45, 144, 2, 1, 0, 2, 1, 0, 48,
            14, 48, 12, 6, 8, 43, 6, 1, 2, 1, 1, 1, 0, 5, 0,
        };

    /// <summary>
    /// Discovers the specified SNMP time out.
    /// </summary>
    /// <param name="snmpTimeOut">The SNMP time out.</param>
    /// <returns>An array of network endpoint as an IP address and a port number.</returns>
    public static IPEndPoint[] Discover(Int32 snmpTimeOut = 6000) {
      List<IPEndPoint> endpoints = new List<IPEndPoint>();

      Task[] tasks =
      {
                Task.Run(async () =>
                {
                    using (UdpClient udp = new UdpClient(IPAddress.Broadcast.AddressFamily))
                    {
                        udp.EnableBroadcast = true;
                        await udp.SendAsync(
                            DiscoverMessage,
                            DiscoverMessage.Length,
                            new IPEndPoint(IPAddress.Broadcast, 161));

                        while (true)
                        {
                            try
                            {
                                Byte[] buffer = new Byte[udp.Client.ReceiveBufferSize];
                                EndPoint remote = new IPEndPoint(IPAddress.Any, 0);
                                udp.Client.ReceiveFrom(buffer, ref remote);
                                endpoints.Add(remote as IPEndPoint);
                            }
                            catch
                            {
                                break;
                            }
                        }
#if NET452
                        udp.Close();
#endif
                    }
                }),
                Task.Delay(snmpTimeOut),
            };

      Task.WaitAny(tasks);

      return endpoints.ToArray();
    }

    /// <summary>
    /// Gets the name of the public.
    /// </summary>
    /// <param name="host">The host.</param>
    /// <returns>
    /// A string that contains the results of decoding the specified sequence 
    /// of bytes ref=GetString".
    /// </returns>
    public static String GetPublicName(IPEndPoint host) => GetString(host, "1.3.6.1.2.1.1.5.0");

    /// <summary>
    /// Gets the up-time.
    /// </summary>
    /// <param name="host">The host.</param>
    /// <param name="mibString">The mibString.</param>
    /// <returns>
    ///  A time interval that represents a specified number of seconds, 
    ///  where the specification is accurate to the nearest millisecond.
    ///  </returns>
    public static TimeSpan GetUptime(IPEndPoint host, String mibString = "1.3.6.1.2.1.1.3.0") {
      Byte[] response = Get(host, mibString);
      if(response[0] == 0xff) {
        return TimeSpan.Zero;
      }

      // If response, get the community name and MIB lengths
      Int16 commlength = Convert.ToInt16(response[6]);
      Int16 miblength = Convert.ToInt16(response[23 + commlength]);

      // Extract the MIB data from the SNMP response
      Int16 datalength = Convert.ToInt16(response[25 + commlength + miblength]);
      Int32 datastart = 26 + commlength + miblength;

      Int32 uptime = 0;

      while(datalength > 0) {
        uptime = (uptime << 8) + response[datastart++];
        datalength--;
      }

      return TimeSpan.FromSeconds(uptime);
    }

    /// <summary>
    /// Gets the string.
    /// </summary>
    /// <param name="host">The host.</param>
    /// <param name="mibString">The mibString.</param>
    /// <returns>A <see cref="System.String" /> that contains the results of decoding the specified sequence of bytes.</returns>
    public static String GetString(IPEndPoint host, String mibString) {
      Byte[] response = Get(host, mibString);
      if(response[0] == 0xff) {
        return String.Empty;
      }

      // If response, get the community name and MIB lengths
      Int16 commlength = Convert.ToInt16(response[6]);
      Int16 miblength = Convert.ToInt16(response[23 + commlength]);

      // Extract the MIB data from the SNMP response
      Int16 datalength = Convert.ToInt16(response[25 + commlength + miblength]);
      Int32 datastart = 26 + commlength + miblength;

      return Encoding.ASCII.GetString(response, datastart, datalength);
    }

    /// <summary>
    /// Gets the specified host.
    /// </summary>
    /// <param name="host">The host.</param>
    /// <param name="mibString">The mibString.</param>
    /// <returns>A byte array containing the results of encoding the specified set of characters.</returns>
    public static Byte[] Get(IPEndPoint host, String mibString) => Get("get", host, "public", mibString);

    /// <summary>
    /// Gets the specified request.
    /// </summary>
    /// <param name="request">The request.</param>
    /// <param name="host">The host.</param>
    /// <param name="community">The community.</param>
    /// <param name="mibString">The mibString.</param>
    /// <returns>A byte array containing the results of encoding the specified set of characters.</returns>
    public static Byte[] Get(String request, IPEndPoint host, String community, String mibString) {
      Byte[] packet = new Byte[1024];
      Byte[] mib = new Byte[1024];
      Int32 comlen = community.Length;
      String[] mibvals = mibString.Split('.');
      Int32 miblen = mibvals.Length;
      Int32 cnt = 0;
      Int32 orgmiblen = miblen;
      Int32 pos = 0;

      // Convert the string MIB into a byte array of integer values
      // Unfortunately, values over 128 require multiple bytes
      // which also increases the MIB length
      for(Int32 i = 0; i < orgmiblen; i++) {
        Int32 temp = Convert.ToInt16(mibvals[i]);
        if(temp > 127) {
          mib[cnt] = Convert.ToByte(128 + temp / 128);
          mib[cnt + 1] = Convert.ToByte(temp - temp / 128 * 128);
          cnt += 2;
          miblen++;
        } else {
          mib[cnt] = Convert.ToByte(temp);
          cnt++;
        }
      }

      Int32 snmplen = 29 + comlen + miblen - 1;

      // The SNMP sequence start
      packet[pos++] = 0x30; // Sequence start
      packet[pos++] = Convert.ToByte(snmplen - 2); // sequence size

      // SNMP version
      packet[pos++] = 0x02; // Integer type
      packet[pos++] = 0x01; // length
      packet[pos++] = 0x00; // SNMP version 1

      // Community name
      packet[pos++] = 0x04; // String type
      packet[pos++] = Convert.ToByte(comlen); // length

      // Convert community name to byte array
      Byte[] data = Encoding.ASCII.GetBytes(community);

      foreach(Byte t in data) {
        packet[pos++] = t;
      }

      // Add GetRequest or GetNextRequest value
      packet[pos++] = request == "get" ? (Byte)0xA0 : (Byte)0xA1;

      packet[pos++] = Convert.ToByte(20 + miblen - 1); // Size of total MIB

      // Request ID
      packet[pos++] = 0x02; // Integer type
      packet[pos++] = 0x04; // length
      packet[pos++] = 0x00; // SNMP request ID
      packet[pos++] = 0x00;
      packet[pos++] = 0x00;
      packet[pos++] = 0x01;

      // Error status
      packet[pos++] = 0x02; // Integer type
      packet[pos++] = 0x01; // length
      packet[pos++] = 0x00; // SNMP error status

      // Error index
      packet[pos++] = 0x02; // Integer type
      packet[pos++] = 0x01; // length
      packet[pos++] = 0x00; // SNMP error index

      // Start of variable bindings
      packet[pos++] = 0x30; // Start of variable bindings sequence

      packet[pos++] = Convert.ToByte(6 + miblen - 1); // Size of variable binding

      packet[pos++] = 0x30; // Start of first variable bindings sequence
      packet[pos++] = Convert.ToByte(6 + miblen - 1 - 2); // size
      packet[pos++] = 0x06; // Object type
      packet[pos++] = Convert.ToByte(miblen - 1); // length

      // Start of MIB
      packet[pos++] = 0x2b;

      // Place MIB array in packet
      for(Int32 i = 2; i < miblen; i++) {
        packet[pos++] = Convert.ToByte(mib[i]);
      }

      packet[pos++] = 0x05; // Null object value
      packet[pos] = 0x00; // Null

      // Send packet to destination
      SendPacket(host, packet, snmplen);

      return packet;
    }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Codequalität", "IDE0067:Objekte verwerfen, bevor Bereich verloren geht", Justification = "<Ausstehend>")]
    private static void SendPacket(IPEndPoint host, Byte[] packet, Int32 length) {
      Socket sock = new Socket(
          AddressFamily.InterNetwork,
          SocketType.Dgram,
          ProtocolType.Udp);
      sock.SetSocketOption(
          SocketOptionLevel.Socket,
          SocketOptionName.ReceiveTimeout,
          5000);
      EndPoint ep = (EndPoint)host;
      _ = sock.SendTo(packet, length, SocketFlags.None, host);

      // Receive response from packet
      try {
        _ = sock.ReceiveFrom(packet, ref ep);
      } catch(SocketException) {
        packet[0] = 0xff;
      }
    }
  }
}