using System;
using System.Buffers;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

class Program
{
    static string USER = "user";
    static string PASS = "pass";
    static long _connectionSeq;

    static async Task Main()
    {
        int port = 8888;
        RegisterGlobalExceptionHandlers();

        Log.Info("=== LOCAL PROXY ===");
        Log.Info($"http://127.0.0.1:{port}");
        Log.Info($"login: {USER}");
        Log.Info($"pass : {PASS}");

        var listener = new TcpListener(IPAddress.Any, port);
        listener.Start();
        Log.Info($"Listening on {listener.LocalEndpoint}");

        while (true)
        {
            try
            {
                var client = await listener.AcceptTcpClientAsync();
                var connectionId = Interlocked.Increment(ref _connectionSeq);
                Log.Info($"[conn:{connectionId}] accepted from {client.Client.RemoteEndPoint}");
                _ = Task.Run(() => HandleClient(client, connectionId));
            }
            catch (Exception ex)
            {
                Log.Error("AcceptTcpClientAsync failed", ex);
            }
        }
    }

    static void RegisterGlobalExceptionHandlers()
    {
        AppDomain.CurrentDomain.UnhandledException += (_, args) =>
            Log.Error("Unhandled exception", args.ExceptionObject as Exception);

        TaskScheduler.UnobservedTaskException += (_, args) =>
        {
            Log.Error("Unobserved task exception", args.Exception);
            args.SetObserved();
        };
    }

    static async Task HandleClient(TcpClient client, long connectionId)
    {
        using (client)
        {
            try
            {
                var stream = client.GetStream();
                var request = await ReadHeaders(stream, $"conn:{connectionId} client-request");

                if (request == null)
                {
                    Log.Warn($"[conn:{connectionId}] empty or invalid request");
                    return;
                }

                Log.Info($"[conn:{connectionId}] {request.Method} {request.Target}");

                if (!CheckAuth(request.Raw))
                {
                    Log.Warn($"[conn:{connectionId}] proxy auth failed");
                    await Write407(stream);
                    return;
                }

                if (request.Method == "CONNECT")
                    await HandleConnect(stream, request.Target, connectionId);
                else
                    await HandleHttp(stream, request, connectionId);
            }
            catch (Exception ex)
            {
                Log.Error($"[conn:{connectionId}] client handling failed", ex);
            }
            finally
            {
                Log.Info($"[conn:{connectionId}] closed");
            }
        }
    }

    // ================= HTTP =================

    static async Task HandleHttp(NetworkStream clientStream, HttpRequest req, long connectionId)
    {
        var proxyUri = WinHttpHelper.GetProxyForUrl(new Uri(req.Url));
        Log.Info($"[conn:{connectionId}] HTTP upstream {proxyUri.Host}:{proxyUri.Port} for {req.Url}");

        using var upstream = new TcpClient();
        await upstream.ConnectAsync(proxyUri.Host, proxyUri.Port);

        var upstreamStream = upstream.GetStream();

        await upstreamStream.WriteAsync(req.RawBytes);

        await Pump(upstreamStream, clientStream, $"conn:{connectionId} HTTP upstream->client");
    }

    // ================= CONNECT =================

    static async Task HandleConnect(NetworkStream clientStream, string target, long connectionId)
    {
        var parts = target.Split(':');
        if (parts.Length != 2 || !int.TryParse(parts[1], out int port))
        {
            Log.Error($"[conn:{connectionId}] invalid CONNECT target: {target}");
            await clientStream.WriteAsync(Encoding.ASCII.GetBytes("HTTP/1.1 400 Bad Request\r\n\r\n"));
            return;
        }

        string host = parts[0];

        var proxyUri = WinHttpHelper.GetProxyForUrl(new Uri($"https://{host}:{port}"));
        Log.Info($"[conn:{connectionId}] CONNECT {host}:{port} via {proxyUri.Host}:{proxyUri.Port}");

        using var upstream = new TcpClient();
        await upstream.ConnectAsync(proxyUri.Host, proxyUri.Port);

        var upstreamStream = upstream.GetStream();

        string connectReq =
            $"CONNECT {host}:{port} HTTP/1.1\r\nHost: {host}:{port}\r\n\r\n";

        await upstreamStream.WriteAsync(Encoding.ASCII.GetBytes(connectReq));

        var resp = await ReadHeaders(upstreamStream, $"conn:{connectionId} upstream-connect-response");

        if (resp == null || !resp.Raw.Contains("200"))
        {
            Log.Error($"[conn:{connectionId}] upstream CONNECT rejected");
            await clientStream.WriteAsync(Encoding.ASCII.GetBytes("HTTP/1.1 502 Bad Gateway\r\n\r\n"));
            return;
        }

        await clientStream.WriteAsync(Encoding.ASCII.GetBytes("HTTP/1.1 200 Connection Established\r\n\r\n"));

        var t1 = Pump(upstreamStream, clientStream, $"conn:{connectionId} tunnel upstream->client");
        var t2 = Pump(clientStream, upstreamStream, $"conn:{connectionId} tunnel client->upstream");

        await Task.WhenAny(t1, t2);
    }

    // ================= AUTH =================

    static bool CheckAuth(string raw)
    {
        string expected = Convert.ToBase64String(Encoding.ASCII.GetBytes($"{USER}:{PASS}"));
        return raw.Contains($"Proxy-Authorization: Basic {expected}");
    }

    static async Task Write407(NetworkStream stream)
    {
        string resp =
            "HTTP/1.1 407 Proxy Authentication Required\r\n" +
            "Proxy-Authenticate: Basic realm=\"proxy\"\r\n\r\n";

        await stream.WriteAsync(Encoding.ASCII.GetBytes(resp));
    }

    // ================= PARSER =================

    class HttpRequest
    {
        public string Method;
        public string Target;
        public string Url;
        public string Raw;
        public byte[] RawBytes;
    }

    static async Task<HttpRequest> ReadHeaders(Stream stream, string context)
    {
        try
        {
            var buffer = new byte[8192];
            int read = await stream.ReadAsync(buffer);

            if (read <= 0) return null;

            string raw = Encoding.ASCII.GetString(buffer, 0, read);

            var lines = raw.Split("\r\n");
            var first = lines[0].Split(' ', StringSplitOptions.RemoveEmptyEntries);
            if (first.Length < 2)
            {
                Log.Error($"[{context}] invalid request line: {lines[0]}");
                return null;
            }

            var req = new HttpRequest
            {
                Method = first[0],
                Target = first[1],
                Raw = raw,
                RawBytes = buffer.Take(read).ToArray()
            };

            if (req.Method != "CONNECT")
            {
                string host = lines
                    .FirstOrDefault(l => l.StartsWith("Host:", StringComparison.OrdinalIgnoreCase))
                    ?.Split(':', 2)[1]
                    .Trim();
                req.Url = req.Target.StartsWith("http", StringComparison.OrdinalIgnoreCase)
                    ? req.Target
                    : $"http://{host}{req.Target}";
            }

            return req;
        }
        catch (Exception ex)
        {
            Log.Error($"[{context}] failed to read/parse headers", ex);
            return null;
        }
    }

    // ================= STREAM =================

    static async Task Pump(Stream from, Stream to, string context)
    {
        var buffer = ArrayPool<byte>.Shared.Rent(8192);

        try
        {
            while (true)
            {
                int read = await from.ReadAsync(buffer);
                if (read <= 0) break;

                await to.WriteAsync(buffer.AsMemory(0, read));
            }
        }
        catch (Exception ex)
        {
            Log.Error($"[{context}] stream pump failed", ex);
        }
        finally
        {
            ArrayPool<byte>.Shared.Return(buffer);
        }
    }
}

// ================= WINHTTP =================

static class WinHttpHelper
{
    [DllImport("winhttp.dll", SetLastError = true)]
    static extern IntPtr WinHttpOpen(string agent, int accessType, string proxy, string bypass, int flags);

    [DllImport("winhttp.dll", SetLastError = true)]
    static extern bool WinHttpGetProxyForUrl(
        IntPtr hSession,
        string url,
        ref WINHTTP_AUTOPROXY_OPTIONS options,
        out WINHTTP_PROXY_INFO proxyInfo);

    [DllImport("winhttp.dll")]
    static extern bool WinHttpCloseHandle(IntPtr handle);

    public static Uri GetProxyForUrl(Uri url)
    {
        IntPtr session = WinHttpOpen("proxy", 0, null, null, 0);
        if (session == IntPtr.Zero)
        {
            Log.Error("WinHttpOpen failed");
            return url;
        }

        var options = new WINHTTP_AUTOPROXY_OPTIONS
        {
            dwFlags = 0x00000001 | 0x00000002,
            dwAutoDetectFlags = 0x00000001 | 0x00000002
        };

        if (WinHttpGetProxyForUrl(session, url.ToString(), ref options, out var info))
        {
            string proxy = PtrToString(info.lpszProxy);

            if (!string.IsNullOrEmpty(proxy))
            {
                var first = proxy.Split(';')[0];
                WinHttpCloseHandle(session);
                return new Uri(Normalize(first));
            }
        }
        else
        {
            Log.Error($"WinHttpGetProxyForUrl failed for {url} (Win32={Marshal.GetLastWin32Error()})");
        }

        WinHttpCloseHandle(session);
        return url;
    }

    static string PtrToString(IntPtr ptr)
    {
        if (ptr == IntPtr.Zero) return null;
        return Marshal.PtrToStringUni(ptr);
    }

    static string Normalize(string proxy)
    {
        if (!proxy.StartsWith("http"))
            return "http://" + proxy;
        return proxy;
    }

    struct WINHTTP_AUTOPROXY_OPTIONS
    {
        public int dwFlags;
        public int dwAutoDetectFlags;
        public IntPtr lpszAutoConfigUrl;
        public IntPtr lpvReserved;
        public int dwReserved;
        public bool fAutoLogonIfChallenged;
    }

    struct WINHTTP_PROXY_INFO
    {
        public int dwAccessType;
        public IntPtr lpszProxy;
        public IntPtr lpszProxyBypass;
    }
}

static class Log
{
    static readonly object _sync = new();

    public static void Info(string message) => Write("INF", message);
    public static void Warn(string message) => Write("WRN", message);
    public static void Error(string message, Exception ex = null)
    {
        Write("ERR", ex == null ? message : $"{message}{Environment.NewLine}{ex}");
    }

    static void Write(string level, string message)
    {
        lock (_sync)
        {
            Console.WriteLine($"[{DateTime.Now:yyyy-MM-dd HH:mm:ss.fff}] [{level}] {message}");
        }
    }
}