using System.Collections.Concurrent;
using System.Diagnostics;
using System.Globalization;
using System.Net;
using System.Net.Sockets;
using System.Security.Cryptography;
using System.Text;
using System.Text.RegularExpressions;

using MeWeb.Shared;

using Microsoft.Extensions.Options;

namespace MeWeb.Server;

public class ProbeOptions
{
  public List<string> Refuse { get; set; } = [];
  public bool AllowPrivate { get; set; }
  public int PerHour { get; set; } = 6;
  public int PerDay { get; set; } = 30;
}

public partial class ProbeService(IOptionsMonitor<ProbeOptions> options, ILogger<ProbeService> log)
{
  static readonly TimeSpan TicketLife = TimeSpan.FromSeconds(60);
  readonly ConcurrentDictionary<string, Ticket> tickets = new();
  readonly ConcurrentDictionary<string, List<DateTimeOffset>> issued = new();
  readonly ConcurrentDictionary<string, byte> running = new();
  readonly SemaphoreSlim outbound = new(32, 32);
  readonly SemaphoreSlim tracing = new(4, 4);

  public (ViewProbeTicket Ticket, ViewRefusal Refusal, int Status) Issue(IPAddress caller)
  {
    var refusal = Refuse(caller);
    if (refusal is not null) return (null, new ViewRefusal { Reason = refusal }, StatusCodes.Status403Forbidden);
    var now = DateTimeOffset.UtcNow;
    foreach (var (id, old) in tickets)
      if (old.Expires < now) tickets.TryRemove(id, out _);

    var network = NetworkOf(caller);
    var times = issued.GetOrAdd(network, _ => []);
    lock (times)
    {
      times.RemoveAll(t => now - t > TimeSpan.FromDays(1));
      var lastHour = times.Count(t => now - t < TimeSpan.FromHours(1));
      if (lastHour >= options.CurrentValue.PerHour || times.Count >= options.CurrentValue.PerDay)
      {
        var retry = lastHour >= options.CurrentValue.PerHour ? times.Where(t => now - t < TimeSpan.FromHours(1)).Min().AddHours(1) : times.Min().AddDays(1);
        return (null, new ViewRefusal { Reason = "too-many", RetryAfterSeconds = (int)Math.Ceiling((retry - now).TotalSeconds) }, StatusCodes.Status429TooManyRequests);
      }
      times.Add(now);
    }

    var ticket = new Ticket
    {
      Id = Convert.ToHexStringLower(RandomNumberGenerator.GetBytes(12)),
      Nonce = Convert.ToHexStringLower(RandomNumberGenerator.GetBytes(12)),
      Key = RandomNumberGenerator.GetBytes(32),
      Address = caller,
      Expires = now + TicketLife,
    };
    tickets[ticket.Id] = ticket;
    return (new ViewProbeTicket { Id = ticket.Id, Nonce = ticket.Nonce, Key = Convert.ToBase64String(ticket.Key), Address = caller.ToString(), Seconds = (int)TicketLife.TotalSeconds }, null, StatusCodes.Status200OK);
  }

  public async Task<(ViewProbe Probe, ViewRefusal Refusal, int Status)> Run(string id, IPAddress caller, ProbeForm form, CancellationToken ct)
  {
    if (id is null || !tickets.TryRemove(id, out var ticket) || ticket.Expires < DateTimeOffset.UtcNow)
      return (null, new ViewRefusal { Reason = "unknown-ticket" }, StatusCodes.Status404NotFound);
    if (!ticket.Address.Equals(caller)) return (null, new ViewRefusal { Reason = "other-address" }, StatusCodes.Status403Forbidden);
    var ports = form?.Ports.Distinct().ToList() ?? [];
    if (ports.Count is 0 or > 3 || ports.Any(p => p is not (80 or 443) and not (>= 49152 and <= 65535)))
      return (null, new ViewRefusal { Reason = "bad-ports" }, StatusCodes.Status400BadRequest);
    var network = NetworkOf(caller);
    if (!running.TryAdd(network, 0)) return (null, new ViewRefusal { Reason = "busy" }, StatusCodes.Status409Conflict);
    try
    {
      var result = new ViewProbe { Address = caller.ToString() };
      var route = Trace(caller, ct);
      foreach (var port in ports) result.Ports.Add(new ViewProbePort { Port = port, Outcome = await Knock(ticket, port, ct) });
      result.Route = await route;
      log.LogInformation("{Service} ran {Count} port(s)", nameof(ProbeService), ports.Count);
      return (result, null, StatusCodes.Status200OK);
    }
    finally
    {
      running.TryRemove(network, out _);
    }
  }

  async Task<List<ViewHop>> Trace(IPAddress target, CancellationToken ct)
  {
    var hops = new List<ViewHop>();
    await tracing.WaitAsync(ct);
    try
    {
      var info = new ProcessStartInfo("stdbuf") { RedirectStandardOutput = true, RedirectStandardError = true, UseShellExecute = false };
      foreach (var argument in new[] { "-oL", "tracepath", "-n", "-m", "16", target.ToString() }) info.ArgumentList.Add(argument);
      using var process = Process.Start(info);
      if (process is null) return hops;
      using var timeout = CancellationTokenSource.CreateLinkedTokenSource(ct);
      timeout.CancelAfter(TimeSpan.FromSeconds(25));
      try
      {
        var silent = 0;
        while (await process.StandardOutput.ReadLineAsync(timeout.Token) is { } line)
        {
          if (SilentLine().IsMatch(line) && ++silent >= 3) break;
          var match = HopLine().Match(line);
          if (!match.Success || !IPAddress.TryParse(match.Groups[2].Value, out var address)) continue;
          silent = 0;
          var distance = int.Parse(match.Groups[1].Value, CultureInfo.InvariantCulture);
          if (hops.Any(h => h.Distance == distance)) continue;
          if (!IsPublic(address) || Refuse(address) is not null) continue;
          hops.Add(new ViewHop { Distance = distance, Address = address.ToString(), Milliseconds = double.Parse(match.Groups[3].Value, CultureInfo.InvariantCulture) });
        }
      }
      catch (OperationCanceledException) when (!ct.IsCancellationRequested)
      {
      }
      finally
      {
        if (!process.HasExited) process.Kill();
      }
      await Task.WhenAll(hops.Select(async hop =>
      {
        try
        {
          using var lookup = CancellationTokenSource.CreateLinkedTokenSource(ct);
          lookup.CancelAfter(TimeSpan.FromSeconds(2));
          var entry = await Dns.GetHostEntryAsync(hop.Address, lookup.Token);
          if (!string.IsNullOrWhiteSpace(entry.HostName) && entry.HostName != hop.Address) hop.Name = entry.HostName;
        }
        catch (Exception) when (!ct.IsCancellationRequested)
        {
        }
      }));
    }
    catch (Exception ex) when (ex is System.ComponentModel.Win32Exception or InvalidOperationException)
    {
      log.LogWarning("{Service} could not trace: {Error}", nameof(ProbeService), ex.Message);
    }
    finally
    {
      tracing.Release();
    }
    return hops;
  }

  [GeneratedRegex(@"^\s*(\d+):\s+([0-9a-fA-F.:]+)\s+([\d.]+)ms")]
  internal static partial Regex HopLine();

  [GeneratedRegex(@"^\s*\d+:\s+no reply")]
  internal static partial Regex SilentLine();

  async Task<ProbeOutcome> Knock(Ticket ticket, int port, CancellationToken ct)
  {
    await outbound.WaitAsync(ct);
    try
    {
      using var client = new TcpClient(ticket.Address.AddressFamily);
      using (var connect = CancellationTokenSource.CreateLinkedTokenSource(ct))
      {
        connect.CancelAfter(TimeSpan.FromSeconds(3));
        try
        {
          await client.ConnectAsync(ticket.Address, port, connect.Token);
        }
        catch (OperationCanceledException) when (!ct.IsCancellationRequested)
        {
          return ProbeOutcome.TimedOut;
        }
        catch (SocketException ex) when (ex.SocketErrorCode is SocketError.ConnectionRefused)
        {
          return ProbeOutcome.Refused;
        }
        catch (SocketException)
        {
          return ProbeOutcome.TimedOut;
        }
      }
      using var answer = CancellationTokenSource.CreateLinkedTokenSource(ct);
      answer.CancelAfter(TimeSpan.FromSeconds(2));
      try
      {
        var stream = client.GetStream();
        await stream.WriteAsync(Encoding.ASCII.GetBytes($"{ProbeProof.Greeting} {ticket.Nonce} {port}\n"), answer.Token);
        var buffer = new byte[96];
        var length = 0;
        while (length < buffer.Length && Array.IndexOf(buffer, (byte)'\n', 0, length) < 0)
        {
          var read = await stream.ReadAsync(buffer.AsMemory(length), answer.Token);
          if (read == 0) break;
          length += read;
        }
        var reply = Encoding.ASCII.GetString(buffer, 0, length).Trim();
        var expected = ProbeProof.For(ticket.Key, ticket.Nonce, port);
        return CryptographicOperations.FixedTimeEquals(Encoding.ASCII.GetBytes(reply), Encoding.ASCII.GetBytes(expected)) ? ProbeOutcome.Reached : ProbeOutcome.OpenNotYou;
      }
      catch (Exception ex) when (ex is IOException or SocketException or OperationCanceledException && !ct.IsCancellationRequested)
      {
        return ProbeOutcome.OpenNotYou;
      }
    }
    finally
    {
      outbound.Release();
    }
  }

  string Refuse(IPAddress caller)
  {
    var settings = options.CurrentValue;
    if (!settings.AllowPrivate && !IsPublic(caller)) return "not-public";
    foreach (var text in settings.Refuse)
      if (IPNetwork.TryParse(text, out var network) && network.Contains(caller)) return "own-network";
    return null;
  }

  static bool IsPublic(IPAddress address)
  {
    if (IPAddress.IsLoopback(address) || address.IsIPv6LinkLocal || address.IsIPv6SiteLocal || address.IsIPv6UniqueLocal || address.IsIPv6Multicast) return false;
    if (address.AddressFamily == AddressFamily.InterNetworkV6) return (address.GetAddressBytes()[0] & 0xe0) == 0x20;
    var b = address.GetAddressBytes();
    return b switch
    {
      [0 or 10 or 127, ..] => false,
      [100, >= 64 and <= 127, ..] => false,
      [169, 254, ..] or [192, 168, ..] or [172, >= 16 and <= 31, ..] => false,
      [>= 224, ..] => false,
      _ => true,
    };
  }

  public static string NetworkOf(IPAddress address)
  {
    if (address.AddressFamily != AddressFamily.InterNetworkV6) return address.ToString();
    var bytes = address.GetAddressBytes();
    Array.Clear(bytes, 8, 8);
    return new IPAddress(bytes) + "/64";
  }

  sealed class Ticket
  {
    public string Id { get; set; }
    public string Nonce { get; set; }
    public byte[] Key { get; set; }
    public IPAddress Address { get; set; }
    public DateTimeOffset Expires { get; set; }
  }
}