namespace gatuna_client;

using System.Text;

static class Proto
{
    public const ushort EtherType = 0x6969;
    public const int EthHeaderLen = 14;
    public const byte Version = 1;
    public const int MaxPayload = 1480;

    public const byte TypeDiscover = 0x01;
    public const byte TypeManifest = 0x02;
    public const byte TypeOpen = 0x03;
    public const byte TypeOpenAck = 0x04;
    public const byte TypeOpenNak = 0x05;
    public const byte TypeData = 0x06;
    public const byte TypeClose = 0x07;

    public const byte ProtoTcp = 1;
    public const byte ProtoUdp = 2;

    public const byte ReasonUnspecified = 0;
    public const byte ReasonUnknownUpstream = 1;
    public const byte ReasonConnectFailed = 2;
    public const byte ReasonOversize = 3;
    public const byte ReasonUnknownSession = 4;
}

readonly record struct UpstreamEntry(
    byte Id,
    byte Protocol,
    ushort Port,
    string? Label)
{
    public string ProtoName => Protocol == Proto.ProtoTcp ? "tcp" : "udp";
}

abstract record Frame
{
    internal record Discover : Frame;
    internal record Manifest(UpstreamEntry[] Entries) : Frame;
    internal record Open(byte UpstreamId) : Frame;
    internal record OpenAck(uint SessionId, byte UpstreamId) : Frame;
    internal record OpenNak(byte UpstreamId, byte Reason) : Frame;
    internal record Data(uint SessionId, byte[] Payload) : Frame;
    internal record Close(uint SessionId, byte? Reason) : Frame;
}

static class FrameCodec
{
    public static byte[] Encode(Frame frame)
    {
        return frame switch
        {
            Frame.Discover =>
                Header(Proto.TypeDiscover, 0),
            Frame.Manifest manifest =>
                BuildManifest(manifest.Entries),
            Frame.Open open =>
                [.. Header(Proto.TypeOpen, 0), open.UpstreamId],
            Frame.OpenAck ack =>
                [.. Header(Proto.TypeOpenAck, ack.SessionId), ack.UpstreamId],
            Frame.OpenNak nak =>
                [.. Header(Proto.TypeOpenNak, 0), nak.UpstreamId, nak.Reason],
            Frame.Data data =>
                [.. Header(Proto.TypeData, data.SessionId), .. data.Payload],
            Frame.Close close =>
                close.Reason.HasValue
                    ? [.. Header(Proto.TypeClose, close.SessionId), close.Reason.Value]
                    : Header(Proto.TypeClose, close.SessionId),
            _ => throw new InvalidOperationException($"unknown frame type: {frame.GetType()}"),
        };
    }

    static byte[] BuildManifest(UpstreamEntry[] entries)
    {
        using var ms = new MemoryStream();
        ms.Write(Header(Proto.TypeManifest, 0));
        foreach (var e in entries)
        {
            var labelBytes = Encoding.UTF8.GetBytes(e.Label ?? "");
            var labelLen = (byte)Math.Min(labelBytes.Length, 255);
            ms.WriteByte(e.Id);
            ms.WriteByte(e.Protocol);
            ms.WriteByte((byte)(e.Port >> 8));
            ms.WriteByte((byte)(e.Port & 0xFF));
            ms.WriteByte(labelLen);
            if (labelLen > 0)
                ms.Write(labelBytes, 0, labelLen);
        }
        return ms.ToArray();
    }

    static byte[] Header(byte type, uint sessionId)
    {
        return [
            Proto.Version, type,
            (byte)(sessionId >> 24),
            (byte)(sessionId >> 16),
            (byte)(sessionId >> 8),
            (byte)(sessionId & 0xFF),
        ];
    }

    public static Frame? Parse(ReadOnlySpan<byte> buf)
    {
        if (buf.Length < 6)
            return null;
        if (buf[0] != Proto.Version)
            return null;
        var type = buf[1];
        var sessionId = (uint)(buf[2] << 24 | buf[3] << 16 | buf[4] << 8 | buf[5]);
        var payload = buf[6..];

        return type switch
        {
            Proto.TypeManifest => ParseManifest(payload),
            Proto.TypeOpenAck when payload.Length == 1 =>
                new Frame.OpenAck(sessionId, payload[0]),
            Proto.TypeOpenNak when payload.Length == 2 =>
                new Frame.OpenNak(payload[0], payload[1]),
            Proto.TypeData when payload.Length <= Proto.MaxPayload =>
                new Frame.Data(sessionId, payload.ToArray()),
            Proto.TypeClose when payload.Length is 0 or 1 =>
                new Frame.Close(sessionId, payload.Length == 1 ? payload[0] : null),
            Proto.TypeDiscover when payload.Length == 0 =>
                new Frame.Discover(),
            _ => null,
        };
    }

    static Frame.Manifest? ParseManifest(ReadOnlySpan<byte> payload)
    {
        var entries = new List<UpstreamEntry>();
        int i = 0;
        while (i < payload.Length)
        {
            if (i + 5 > payload.Length)
                return null;
            var id = payload[i];
            var proto = payload[i + 1];
            var port = (ushort)(payload[i + 2] << 8 | payload[i + 3]);
            var labelLen = payload[i + 4];
            i += 5;
            if (i + labelLen > payload.Length)
                return null;
            string? label = labelLen == 0
                ? null
                : Encoding.UTF8.GetString(payload[i..(i + labelLen)]);
            i += labelLen;
            entries.Add(new UpstreamEntry(id, proto, port, label));
        }
        return new Frame.Manifest(entries.ToArray());
    }
}