#nullable enable
using Swan.Formatters;
using System;
using System.Collections.Generic;
using System.IO;
using System.Threading.Tasks;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Runtime.InteropServices;
using System.Text;

namespace Swan.Net.Dns {
  /// <summary>
  /// DnsClient Request inner class.
  /// </summary>
  internal partial class DnsClient {
    public class DnsClientRequest : IDnsRequest {
      private readonly IDnsRequestResolver _resolver;
      private readonly IDnsRequest _request;

      public DnsClientRequest(IPEndPoint dns, IDnsRequest? request = null, IDnsRequestResolver? resolver = null) {
        this.Dns = dns;
        this._request = request == null ? new DnsRequest() : new DnsRequest(request);
        this._resolver = resolver ?? new DnsUdpRequestResolver();
      }

      public Int32 Id {
        get => this._request.Id;
        set => this._request.Id = value;
      }

      public DnsOperationCode OperationCode {
        get => this._request.OperationCode;
        set => this._request.OperationCode = value;
      }

      public Boolean RecursionDesired {
        get => this._request.RecursionDesired;
        set => this._request.RecursionDesired = value;
      }

      public IList<DnsQuestion> Questions => this._request.Questions;

      public Int32 Size => this._request.Size;

      public IPEndPoint Dns {
        get; set;
      }

      public Byte[] ToArray() => this._request.ToArray();

      public override String ToString() => this._request.ToString()!;

      /// <summary>
      /// Resolves this request into a response using the provided DNS information. The given
      /// request strategy is used to retrieve the response.
      /// </summary>
      /// <exception cref="DnsQueryException">Throw if a malformed response is received from the server.</exception>
      /// <exception cref="IOException">Thrown if a IO error occurs.</exception>
      /// <exception cref="SocketException">Thrown if a the reading or writing to the socket fails.</exception>
      /// <returns>The response received from server.</returns>
      public async Task<DnsClientResponse> Resolve() {
        try {
          DnsClientResponse response = await this._resolver.Request(this).ConfigureAwait(false);

          if(response.Id != this.Id) {
            throw new DnsQueryException(response, "Mismatching request/response IDs");
          }

          if(response.ResponseCode != DnsResponseCode.NoError) {
            throw new DnsQueryException(response);
          }

          return response;
        } catch(Exception e) {
          if(e is ArgumentException || e is SocketException) {
            throw new DnsQueryException("Invalid response", e);
          }

          throw;
        }
      }
    }

    public class DnsRequest : IDnsRequest {
      private static readonly Random Random = new Random();

      private DnsHeader header;

      public DnsRequest() {
        this.Questions = new List<DnsQuestion>();
        this.header = new DnsHeader {
          OperationCode = DnsOperationCode.Query,
          Response = false,
          Id = Random.Next(UInt16.MaxValue),
        };
      }

      public DnsRequest(IDnsRequest request) {
        this.header = new DnsHeader();
        this.Questions = new List<DnsQuestion>(request.Questions);

        this.header.Response = false;

        this.Id = request.Id;
        this.OperationCode = request.OperationCode;
        this.RecursionDesired = request.RecursionDesired;
      }

      public IList<DnsQuestion> Questions {
        get;
      }

      public Int32 Size => this.header.Size + this.Questions.Sum(q => q.Size);

      public Int32 Id {
        get => this.header.Id;
        set => this.header.Id = value;
      }

      public DnsOperationCode OperationCode {
        get => this.header.OperationCode;
        set => this.header.OperationCode = value;
      }

      public Boolean RecursionDesired {
        get => this.header.RecursionDesired;
        set => this.header.RecursionDesired = value;
      }

      public Byte[] ToArray() {
        this.UpdateHeader();
        using MemoryStream result = new MemoryStream(this.Size);

        return result.Append(this.header.ToArray()).Append(this.Questions.Select(q => q.ToArray())).ToArray();
      }

      public override String ToString() {
        this.UpdateHeader();

        return Json.Serialize(this, true);
      }

      private void UpdateHeader() => this.header.QuestionCount = this.Questions.Count;
    }

    public class DnsTcpRequestResolver : IDnsRequestResolver {
      public async Task<DnsClientResponse> Request(DnsClientRequest request) {
        TcpClient tcp = new TcpClient();

        try {
          await tcp.Client.ConnectAsync(request.Dns).ConfigureAwait(false);

          NetworkStream stream = tcp.GetStream();
          Byte[] buffer = request.ToArray();
          Byte[] length = BitConverter.GetBytes((UInt16)buffer.Length);

          if(BitConverter.IsLittleEndian) {
            Array.Reverse(length);
          }

          await stream.WriteAsync(length, 0, length.Length).ConfigureAwait(false);
          await stream.WriteAsync(buffer, 0, buffer.Length).ConfigureAwait(false);

          buffer = new Byte[2];
          await Read(stream, buffer).ConfigureAwait(false);

          if(BitConverter.IsLittleEndian) {
            Array.Reverse(buffer);
          }

          buffer = new Byte[BitConverter.ToUInt16(buffer, 0)];
          await Read(stream, buffer).ConfigureAwait(false);

          DnsResponse response = DnsResponse.FromArray(buffer);

          return new DnsClientResponse(request, response, buffer);
        } finally {
          tcp.Dispose();
        }
      }

      private static async Task Read(Stream stream, Byte[] buffer) {
        Int32 length = buffer.Length;
        Int32 offset = 0;
        Int32 size;

        while(length > 0 && (size = await stream.ReadAsync(buffer, offset, length).ConfigureAwait(false)) > 0) {
          offset += size;
          length -= size;
        }

        if(length > 0) {
          throw new IOException("Unexpected end of stream");
        }
      }
    }

    public class DnsUdpRequestResolver : IDnsRequestResolver {
      private readonly IDnsRequestResolver _fallback;

      public DnsUdpRequestResolver(IDnsRequestResolver fallback) => this._fallback = fallback;

      public DnsUdpRequestResolver() => this._fallback = new DnsNullRequestResolver();

      public async Task<DnsClientResponse> Request(DnsClientRequest request) {
        UdpClient udp = new UdpClient();
        IPEndPoint dns = request.Dns;

        try {
          udp.Client.SendTimeout = 7000;
          udp.Client.ReceiveTimeout = 7000;

          await udp.Client.ConnectAsync(dns).ConfigureAwait(false);


          _ = await udp.SendAsync(request.ToArray(), request.Size).ConfigureAwait(false);

          List<Byte> bufferList = new List<Byte>();

          do {
            Byte[] tempBuffer = new Byte[1024];
            Int32 receiveCount = udp.Client.Receive(tempBuffer);
            bufferList.AddRange(tempBuffer.Skip(0).Take(receiveCount));
          }
          while(udp.Client.Available > 0 || bufferList.Count == 0);

          Byte[] buffer = bufferList.ToArray();
          DnsResponse response = DnsResponse.FromArray(buffer);

          return response.IsTruncated
              ? await this._fallback.Request(request).ConfigureAwait(false)
              : new DnsClientResponse(request, response, buffer);
        } finally {
          udp.Dispose();
        }
      }
    }

    public class DnsNullRequestResolver : IDnsRequestResolver {
      public Task<DnsClientResponse> Request(DnsClientRequest request) => throw new DnsQueryException("Request failed");
    }

    // 12 bytes message header
    [StructEndianness(Endianness.Big)]
    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public struct DnsHeader {
      public const Int32 SIZE = 12;

      private UInt16 id;

      // Question count: number of questions in the Question section
      private UInt16 questionCount;

      // Answer record count: number of records in the Answer section
      private UInt16 answerCount;

      // Authority record count: number of records in the Authority section
      private UInt16 authorityCount;

      // Additional record count: number of records in the Additional section
      private UInt16 addtionalCount;

      public Int32 Id {
        get => this.id;
        set => this.id = (UInt16)value;
      }

      public Int32 QuestionCount {
        get => this.questionCount;
        set => this.questionCount = (UInt16)value;
      }

      public Int32 AnswerRecordCount {
        get => this.answerCount;
        set => this.answerCount = (UInt16)value;
      }

      public Int32 AuthorityRecordCount {
        get => this.authorityCount;
        set => this.authorityCount = (UInt16)value;
      }

      public Int32 AdditionalRecordCount {
        get => this.addtionalCount;
        set => this.addtionalCount = (UInt16)value;
      }

      public Boolean Response {
        get => this.Qr == 1;
        set => this.Qr = Convert.ToByte(value);
      }

      public DnsOperationCode OperationCode {
        get => (DnsOperationCode)this.Opcode;
        set => this.Opcode = (Byte)value;
      }

      public Boolean AuthorativeServer {
        get => this.Aa == 1;
        set => this.Aa = Convert.ToByte(value);
      }

      public Boolean Truncated {
        get => this.Tc == 1;
        set => this.Tc = Convert.ToByte(value);
      }

      public Boolean RecursionDesired {
        get => this.Rd == 1;
        set => this.Rd = Convert.ToByte(value);
      }

      public Boolean RecursionAvailable {
        get => this.Ra == 1;
        set => this.Ra = Convert.ToByte(value);
      }

      public DnsResponseCode ResponseCode {
        get => (DnsResponseCode)this.RCode;
        set => this.RCode = (Byte)value;
      }

      public Int32 Size => SIZE;

      // Query/Response Flag
      private Byte Qr {
        get => this.Flag0.GetBitValueAt(7);
        set => this.Flag0 = this.Flag0.SetBitValueAt(7, 1, value);
      }

      // Operation Code
      private Byte Opcode {
        get => this.Flag0.GetBitValueAt(3, 4);
        set => this.Flag0 = this.Flag0.SetBitValueAt(3, 4, value);
      }

      // Authorative Answer Flag
      private Byte Aa {
        get => this.Flag0.GetBitValueAt(2);
        set => this.Flag0 = this.Flag0.SetBitValueAt(2, 1, value);
      }

      // Truncation Flag
      private Byte Tc {
        get => this.Flag0.GetBitValueAt(1);
        set => this.Flag0 = this.Flag0.SetBitValueAt(1, 1, value);
      }

      // Recursion Desired
      private Byte Rd {
        get => this.Flag0.GetBitValueAt(0);
        set => this.Flag0 = this.Flag0.SetBitValueAt(0, 1, value);
      }

      // Recursion Available
      private Byte Ra {
        get => this.Flag1.GetBitValueAt(7);
        set => this.Flag1 = this.Flag1.SetBitValueAt(7, 1, value);
      }

      // Zero (Reserved)
      private Byte Z {
        get => this.Flag1.GetBitValueAt(4, 3);
        set {
        }
      }

      // Response Code
      private Byte RCode {
        get => this.Flag1.GetBitValueAt(0, 4);
        set => this.Flag1 = this.Flag1.SetBitValueAt(0, 4, value);
      }

      private Byte Flag0 {
        get;
        set;
      }

      private Byte Flag1 {
        get;
        set;
      }

      public static DnsHeader FromArray(Byte[] header) => header.Length < SIZE ? throw new ArgumentException("Header length too small") : header.ToStruct<DnsHeader>(0, SIZE);

      public Byte[] ToArray() => this.ToBytes();

      public override String ToString() => Json.SerializeExcluding(this, true, nameof(this.Size));
    }

    public class DnsDomain : IComparable<DnsDomain> {
      private readonly String[] _labels;

      public DnsDomain(String domain) : this(domain.Split('.')) {
      }

      public DnsDomain(String[] labels) => this._labels = labels;

      public Int32 Size => this._labels.Sum(l => l.Length) + this._labels.Length + 1;

      public static DnsDomain FromArray(Byte[] message, Int32 offset) => FromArray(message, offset, out _);

      public static DnsDomain FromArray(Byte[] message, Int32 offset, out Int32 endOffset) {
        List<Byte[]> labels = new List<Byte[]>();
        Boolean endOffsetAssigned = false;
        endOffset = 0;
        Byte lengthOrPointer;

        while((lengthOrPointer = message[offset++]) > 0) {
          // Two heighest bits are set (pointer)
          if(lengthOrPointer.GetBitValueAt(6, 2) == 3) {
            if(!endOffsetAssigned) {
              endOffsetAssigned = true;
              endOffset = offset + 1;
            }

            UInt16 pointer = lengthOrPointer.GetBitValueAt(0, 6);
            offset = (pointer << 8) | message[offset];

            continue;
          }

          if(lengthOrPointer.GetBitValueAt(6, 2) != 0) {
            throw new ArgumentException("Unexpected bit pattern in label length");
          }

          Byte length = lengthOrPointer;
          Byte[] label = new Byte[length];
          Array.Copy(message, offset, label, 0, length);

          labels.Add(label);

          offset += length;
        }

        if(!endOffsetAssigned) {
          endOffset = offset;
        }

        return new DnsDomain(labels.Select(l => l.ToText(Encoding.ASCII)).ToArray());
      }

      public static DnsDomain PointerName(IPAddress ip) => new DnsDomain(FormatReverseIP(ip));

      public Byte[] ToArray() {
        Byte[] result = new Byte[this.Size];
        Int32 offset = 0;

        foreach(Byte[] l in this._labels.Select(label => Encoding.ASCII.GetBytes(label))) {
          result[offset++] = (Byte)l.Length;
          l.CopyTo(result, offset);

          offset += l.Length;
        }

        result[offset] = 0;

        return result;
      }

      public override String ToString() => String.Join(".", this._labels);

      public Int32 CompareTo(DnsDomain other) => String.Compare(this.ToString(), other.ToString(), StringComparison.Ordinal);

      public override Boolean Equals(Object? obj) => obj is DnsDomain domain && this.CompareTo(domain) == 0;

      public override Int32 GetHashCode() => this.ToString().GetHashCode();

      private static String FormatReverseIP(IPAddress ip) {
        Byte[] address = ip.GetAddressBytes();

        if(address.Length == 4) {
          return String.Join(".", address.Reverse().Select(b => b.ToString())) + ".in-addr.arpa";
        }

        Byte[] nibbles = new Byte[address.Length * 2];

        for(Int32 i = 0, j = 0; i < address.Length; i++, j = 2 * i) {
          Byte b = address[i];

          nibbles[j] = b.GetBitValueAt(4, 4);
          nibbles[j + 1] = b.GetBitValueAt(0, 4);
        }

        return String.Join(".", nibbles.Reverse().Select(b => b.ToString("x"))) + ".ip6.arpa";
      }
    }

    public class DnsQuestion : IDnsMessageEntry {
      public static IList<DnsQuestion> GetAllFromArray(Byte[] message, Int32 offset, Int32 questionCount) => GetAllFromArray(message, offset, questionCount, out _);

      public static IList<DnsQuestion> GetAllFromArray(Byte[] message, Int32 offset, Int32 questionCount, out Int32 endOffset) {
        IList<DnsQuestion> questions = new List<DnsQuestion>(questionCount);

        for(Int32 i = 0; i < questionCount; i++) {
          questions.Add(FromArray(message, offset, out offset));
        }

        endOffset = offset;
        return questions;
      }

      public static DnsQuestion FromArray(Byte[] message, Int32 offset, out Int32 endOffset) {
        DnsDomain domain = DnsDomain.FromArray(message, offset, out offset);
        Tail tail = message.ToStruct<Tail>(offset, Tail.SIZE);

        endOffset = offset + Tail.SIZE;

        return new DnsQuestion(domain, tail.Type, tail.Class);
      }

      public DnsQuestion(DnsDomain domain, DnsRecordType type = DnsRecordType.A, DnsRecordClass klass = DnsRecordClass.IN) {
        this.Name = domain;
        this.Type = type;
        this.Class = klass;
      }

      public DnsDomain Name {
        get;
      }

      public DnsRecordType Type {
        get;
      }

      public DnsRecordClass Class {
        get;
      }

      public Int32 Size => this.Name.Size + Tail.SIZE;

      public Byte[] ToArray() => new MemoryStream(this.Size).Append(this.Name.ToArray()).Append(new Tail { Type = Type, Class = Class }.ToBytes()).ToArray();

      public override String ToString() => Json.SerializeOnly(this, true, nameof(this.Name), nameof(this.Type), nameof(this.Class));

      [StructEndianness(Endianness.Big)]
      [StructLayout(LayoutKind.Sequential, Pack = 2)]
      private struct Tail {
        public const Int32 SIZE = 4;

        private UInt16 type;
        private UInt16 klass;

        public DnsRecordType Type {
          get => (DnsRecordType)this.type;
          set => this.type = (UInt16)value;
        }

        public DnsRecordClass Class {
          get => (DnsRecordClass)this.klass;
          set => this.klass = (UInt16)value;
        }
      }
    }
  }
}