using System.Buffers;
using System.IO.Pipelines;
using Microsoft.AspNetCore.Connections;
using System.Net;
using System.Security.Cryptography.X509Certificates;
using DnsClient;
using System.Text;

#region INIT

using Microsoft.Extensions.Logging.Console;
using System.Net.Security;
using System.Net.Sockets;
using System.Security.Cryptography;

var builder = WebApplication.CreateBuilder(args);

//控制台日志格式
builder.Services.AddLogging(opt =>
{
    opt.AddSimpleConsole(p =>
    {
        p.TimestampFormat = "[dd HH:mm:ss] ";
        p.SingleLine = true;
        p.ColorBehavior = LoggerColorBehavior.Enabled;
    });
});

builder.WebHost.UseKestrel(opt =>
{
    opt.Listen(IPAddress.Any, 0, lisOpt =>
    {
        lisOpt.UseConnectionHandler<NoSniProxyHandler>();
    });
});

await using var host = builder.Build();

var logger = host.Services.GetRequiredService<ILogger<Program>>();
logger.LogInformation("Hello, World!");
await host.RunAsync();

#endregion INIT

public class NoSniProxyHandler : ConnectionHandler
{
    private const string dnsServerName = "reliable-dns-server-in-hosts";
    private static readonly IPAddress? dnsServerIp = Dns.GetHostEntry(dnsServerName).AddressList.FirstOrDefault();
    private static readonly LookupClient lookup = new(dnsServerIp);

    public override async Task OnConnectedAsync(ConnectionContext connection)
    {
        var requestStream = connection.Transport.Input;
        var responseStream = connection.Transport.Output;

        var firstLine = await ReadLineAsync(requestStream);
        if (firstLine == null)
        {
            connection.Abort(new ConnectionAbortedException("Canceled: First line rad fail"));
            return;
        }

        var firstLineParts = firstLine.Split(' ', 3);
        if (firstLineParts.Length < 3)
        {
            connection.Abort(new ConnectionAbortedException("Canceled: First line bad"));
            return;
        }

        var method = firstLineParts[0];
        var url = firstLineParts[1];
        var ver = firstLineParts[2];

        var uri = new Uri(url);
        var targetHost = uri.Host;

        var result = await lookup.QueryAsync(targetHost, QueryType.A);
        var record = result.Answers.ARecords().FirstOrDefault();
        var ip = record?.Address;

        var tcpClient = new TcpClient();
        await tcpClient.ConnectAsync(new IPEndPoint(ip, 443));
        var ssl = new SslStream(tcpClient.GetStream());
        var sslOptions = new SslClientAuthenticationOptions
        {
            TargetHost = string.Empty, // Leave this empty to avoid sending SNI
            RemoteCertificateValidationCallback = (o, certificate, chain, errors) => VerifyServerCert(targetHost, certificate, chain, errors),
        };

        await ssl.AuthenticateAsClientAsync(sslOptions, new CancellationTokenSource(TimeSpan.FromSeconds(10)).Token);
        ssl.Write(Encoding.ASCII.GetBytes($"{method} {uri.PathAndQuery} {ver}\r\n"));

        var outgoing = requestStream.CopyToAsync(ssl);
        var inbound = ssl.CopyToAsync(responseStream);

        await Task.WhenAll(outgoing, inbound);
    }

    private async Task<string?> ReadLineAsync(PipeReader requestStream)
    {
        while (true)
        {
            var readResult = await requestStream.ReadAsync();
            if (readResult.IsCanceled) return null;

            var (seq, exm) = ExtractLine(readResult.Buffer, out var line);
            requestStream.AdvanceTo(seq, exm);
            if (line != null) return line;
        }
    }

    private (SequencePosition pos, SequencePosition exm) ExtractLine(ReadOnlySequence<byte> buffer, out string? line)
    {
        var reader = new SequenceReader<byte>(buffer);
        if (reader.TryReadTo(out ReadOnlySpan<byte> span, "\r\n"u8))
        {
            if (span.Length > 4096) throw new InvalidDataException("Too long for line");
            line = Encoding.ASCII.GetString(span);
            return (reader.Position, reader.Position);
        }

        line = null;
        return (buffer.Start, buffer.End);
    }

    private bool VerifyServerCert(string targetHost, X509Certificate certificate, X509Chain? chain, SslPolicyErrors errs)
    {
        if (errs == SslPolicyErrors.None) return true;

        if (errs != SslPolicyErrors.RemoteCertificateNameMismatch) return false;

        if (certificate is not X509Certificate2 cert2) return false;

        // 验证证书的有效期
        if (DateTime.Now < cert2.NotBefore || DateTime.Now > cert2.NotAfter) return false;

        // 然后比较证书名称和主机名称 (任意一个匹配)
        var names = GetAllSubjectAlternativeNames(cert2);
        var flagNameMatched = false;
        foreach (var certName in names)
        {
            if (certName.StartsWith("*."))
            {
                if (!targetHost.EndsWith(certName[2..], StringComparison.OrdinalIgnoreCase)) continue;
                flagNameMatched = true;
                break;
            }

            if (certName.Equals(targetHost, StringComparison.OrdinalIgnoreCase))
            {
                flagNameMatched = true;
                break;
            }
        }

        if (flagNameMatched == false) return false;

        // 构建证书链
        if (chain == null) return false;

        chain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck; // 不检查吊销，太耗时了
        chain.ChainPolicy.RevocationFlag = X509RevocationFlag.EntireChain;
        chain.ChainPolicy.UrlRetrievalTimeout = new TimeSpan(0, 0, 10);
        chain.ChainPolicy.VerificationFlags = X509VerificationFlags.NoFlag;

        var isValidChain = chain.Build(cert2);
        if (isValidChain) return true;

        foreach (X509ChainStatus chainStatus in chain.ChainStatus)
        {
            // 仅处理会影响安全性的错误状态
            if (chainStatus.Status == X509ChainStatusFlags.RevocationStatusUnknown ||
                chainStatus.Status == X509ChainStatusFlags.OfflineRevocation ||
                chainStatus.Status == X509ChainStatusFlags.NoError)
            {
                continue;
            }

            // 其他任何错误状态都认为证书无效
            return false;
        }

        return true;
    }

    IReadOnlyList<string> GetAllSubjectAlternativeNames(X509Certificate2 cert)
    {
        var names = new HashSet<string>();

        foreach (var extension in cert.Extensions)
        {
            if (extension is X509SubjectAlternativeNameExtension sanExtension)
            {
                foreach (var name in sanExtension.EnumerateDnsNames()) names.Add(name);
            }
            else if (extension.Oid?.Value == "2.5.29.17") // Subject Alternative Name OID
            {
                var asnData = new AsnEncodedData(extension.Oid, extension.RawData);
                var sanString = asnData.Format(true);

                var sanParts = sanString.Split(new[] { ", ", "DNS Name=", " " }, StringSplitOptions.RemoveEmptyEntries);
                foreach (var part in sanParts)
                {
                    if (!string.IsNullOrEmpty(part) && !part.StartsWith("IPAddress") && !part.StartsWith("Uri"))
                    {
                        names.Add(part);
                    }
                }
            }
        }

        // 添加证书的CN（通用名称）
        var certName = cert.GetNameInfo(X509NameType.DnsName, false);
        if (!string.IsNullOrEmpty(certName)) names.Add(certName);

        return [.. names];
    }
}