using System.Net;
using System.Net.Http.Headers;
using System.Net.Sockets;
using System.Text;
using System.Xml;
using System.Xml.Linq;

using MeWeb.Core.Net;

namespace MeWeb.Core.Router;

public static class Gateway
{
  const int PortMappingPort = 5351;
  static readonly IPEndPoint Ssdp = new(IPAddress.Parse("239.255.255.250"), 1900);
  static readonly string[] SearchTargets = ["urn:schemas-upnp-org:device:InternetGatewayDevice:2", "urn:schemas-upnp-org:device:InternetGatewayDevice:1"];
  static readonly HttpClient Lan = new(new SocketsHttpHandler { UseProxy = false, AllowAutoRedirect = false, ConnectTimeout = TimeSpan.FromSeconds(2) })
  {
    Timeout = TimeSpan.FromSeconds(4),
  };

  public static async Task<RouterReport> Inspect(IPlatform platform, string gateway, CancellationToken ct)
  {
    var report = new RouterReport { Gateway = gateway };
    var address = Addresses.Parse(gateway);
    if (address is null || address.AddressFamily != AddressFamily.InterNetwork) return report;

    var upnp = Upnp(platform, address, report, ct);
    var natPmp = NatPmp(address, report, ct);
    var pcp = Pcp(address, report, ct);
    await Task.WhenAll(upnp, natPmp, pcp);
    return report;
  }

  static async Task Upnp(IPlatform platform, IPAddress gateway, RouterReport report, CancellationToken ct)
  {
    var location = await Discover(platform, gateway, ct);
    if (location is null) return;
    try
    {
      var description = XDocument.Parse(await Lan.GetStringAsync(location, ct));
      var root = description.Root?.Elements().FirstOrDefault(e => e.Name.LocalName == "device");
      if (root is null) return;
      report.Upnp = Answer.Answered;
      report.UpnpVersion = Text(root, "deviceType")?.EndsWith(":2", StringComparison.Ordinal) == true ? 2 : 1;
      report.Name = Text(root, "friendlyName");
      report.Make = Text(root, "manufacturer");
      report.Model = Text(root, "modelName");
      var service = root.Descendants().Where(e => e.Name.LocalName == "service")
        .Select(e => (Type: Text(e, "serviceType"), Control: Text(e, "controlURL")))
        .FirstOrDefault(s => s.Type is not null && (s.Type.Contains(":WANIPConnection:") || s.Type.Contains(":WANPPPConnection:")));
      if (service.Type is null || service.Control is null) return;
      var baseText = description.Root.Elements().FirstOrDefault(e => e.Name.LocalName == "URLBase")?.Value;
      var baseUri = Uri.TryCreate(baseText, UriKind.Absolute, out var parsed) && IsLocal(parsed) ? parsed : location;
      if (!Uri.TryCreate(baseUri, service.Control, out var control) || !IsLocal(control)) return;
      report.UpnpControl = control.ToString();
      report.UpnpService = service.Type;
      await ExternalAddress(control, service.Type, report, ct);
    }
    catch (Exception) when (!ct.IsCancellationRequested)
    {
    }
  }

  public static async Task<(XDocument Answer, int Status)> Soap(Uri control, string serviceType, string action, string arguments, CancellationToken ct)
  {
    var body = $"""<?xml version="1.0"?><s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/" s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"><s:Body><u:{action} xmlns:u="{serviceType}">{arguments}</u:{action}></s:Body></s:Envelope>""";
    using var request = new HttpRequestMessage(HttpMethod.Post, control) { Content = new StringContent(body, Encoding.UTF8) };
    request.Content.Headers.ContentType = MediaTypeHeaderValue.Parse("text/xml; charset=\"utf-8\"");
    request.Headers.TryAddWithoutValidation("SOAPAction", $"\"{serviceType}#{action}\"");
    using var response = await Lan.SendAsync(request, ct);
    return (XDocument.Parse(await response.Content.ReadAsStringAsync(ct)), (int)response.StatusCode);
  }

  public static int ErrorCode(XDocument answer, int status)
  {
    var code = answer.Descendants().FirstOrDefault(e => e.Name.LocalName == "errorCode")?.Value;
    return int.TryParse(code, out var number) ? number : status;
  }

  static async Task ExternalAddress(Uri control, string serviceType, RouterReport report, CancellationToken ct)
  {
    var (answer, status) = await Soap(control, serviceType, "GetExternalIPAddress", "", ct);
    var external = answer.Descendants().FirstOrDefault(e => e.Name.LocalName == "NewExternalIPAddress")?.Value?.Trim();
    if (Addresses.Parse(external) is not null)
    {
      report.WanAddress = external;
      return;
    }
    report.UpnpError = ErrorCode(answer, status);
    report.Upnp = Answer.Refused;
  }

  static async Task<Uri> Discover(IPlatform platform, IPAddress gateway, CancellationToken ct)
  {
    using var hold = platform.AllowMulticast();
    using var udp = new UdpClient(AddressFamily.InterNetwork);
    try
    {
      udp.Client.Bind(new IPEndPoint(IPAddress.Any, 0));
      foreach (var target in SearchTargets)
      {
        var search = Encoding.ASCII.GetBytes($"M-SEARCH * HTTP/1.1\r\nHOST: 239.255.255.250:1900\r\nMAN: \"ssdp:discover\"\r\nMX: 2\r\nST: {target}\r\n\r\n");
        await udp.SendAsync(search, Ssdp, ct);
        await udp.SendAsync(search, Ssdp, ct);
      }
      var answers = new List<(IPAddress From, Uri Location)>();
      using var window = CancellationTokenSource.CreateLinkedTokenSource(ct);
      window.CancelAfter(TimeSpan.FromSeconds(2.5));
      while (true)
      {
        var received = default(UdpReceiveResult);
        try
        {
          received = await udp.ReceiveAsync(window.Token);
        }
        catch (OperationCanceledException) when (!ct.IsCancellationRequested)
        {
          break;
        }
        var headers = ParseHeaders(Encoding.ASCII.GetString(received.Buffer));
        if (!headers.TryGetValue("LOCATION", out var text) || !Uri.TryCreate(text, UriKind.Absolute, out var location) || !IsLocal(location)) continue;
        answers.Add((received.RemoteEndPoint.Address, location));
      }
      if (answers.Count == 0) return null;
      return answers.OrderByDescending(a => a.From.Equals(gateway)).First().Location;
    }
    catch (SocketException)
    {
      return null;
    }
  }

  static async Task NatPmp(IPAddress gateway, RouterReport report, CancellationToken ct)
  {
    var answer = await Exchange(gateway, [0, 0], ct);
    if (answer is null) return;
    if (answer.Length < 12 || answer[0] != 0 || answer[1] != 128) return;
    report.NatPmpResult = (answer[2] << 8) | answer[3];
    if (report.NatPmpResult != 0)
    {
      report.NatPmp = Answer.Refused;
      return;
    }
    report.NatPmp = Answer.Answered;
    var external = new IPAddress(answer.AsSpan(8, 4));
    if (report.WanAddress is null && !external.Equals(IPAddress.Any)) report.WanAddress = external.ToString();
  }

  static async Task Pcp(IPAddress gateway, RouterReport report, CancellationToken ct)
  {
    using var udp = new UdpClient(AddressFamily.InterNetwork);
    try
    {
      udp.Connect(gateway, PortMappingPort);
    }
    catch (SocketException)
    {
      return;
    }
    var local = ((IPEndPoint)udp.Client.LocalEndPoint)?.Address ?? IPAddress.Any;
    var request = new byte[24];
    request[0] = 2;
    local.MapToIPv6().GetAddressBytes().CopyTo(request, 8);
    var answer = await Exchange(udp, request, ct);
    if (answer is null || answer.Length < 24 || answer[0] != 2 || answer[1] != 0x80) return;
    report.PcpResult = answer[3];
    report.Pcp = answer[3] == 0 ? Answer.Answered : Answer.Refused;
  }

  public static async Task<byte[]> Exchange(IPAddress gateway, byte[] request, CancellationToken ct)
  {
    using var udp = new UdpClient(AddressFamily.InterNetwork);
    try
    {
      udp.Connect(gateway, PortMappingPort);
    }
    catch (SocketException)
    {
      return null;
    }
    return await Exchange(udp, request, ct);
  }

  public static async Task<byte[]> Exchange(UdpClient udp, byte[] request, CancellationToken ct)
  {
    foreach (var wait in new[] { 1000, 2000 })
    {
      try
      {
        await udp.SendAsync(request, ct);
        using var window = CancellationTokenSource.CreateLinkedTokenSource(ct);
        window.CancelAfter(wait);
        var received = await udp.ReceiveAsync(window.Token);
        return received.Buffer;
      }
      catch (OperationCanceledException) when (!ct.IsCancellationRequested)
      {
      }
      catch (SocketException)
      {
        return null;
      }
    }
    return null;
  }

  static Dictionary<string, string> ParseHeaders(string message)
  {
    var headers = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
    foreach (var line in message.Split("\r\n"))
    {
      var colon = line.IndexOf(':');
      if (colon > 0) headers[line[..colon].Trim()] = line[(colon + 1)..].Trim();
    }
    return headers;
  }

  static bool IsLocal(Uri uri) =>
      uri.Scheme == Uri.UriSchemeHttp && IPAddress.TryParse(uri.Host, out var host) &&
      Addresses.Classify(host) is AddressKind.Private or AddressKind.Shared or AddressKind.LinkLocal or AddressKind.UniqueLocal;

  static string Text(XElement parent, string name)
  {
    var value = parent.Elements().FirstOrDefault(e => e.Name.LocalName == name)?.Value?.Trim();
    return string.IsNullOrEmpty(value) ? null : value;
  }
}