using System.Buffers;
using System.Buffers.Binary;
using System.Text;

namespace Uwaa.HTTP.Websockets;

/// <summary>
/// A websocket wrapper over a HTTP stream.
/// </summary>
public class Websocket
{
    /// <summary>
    /// The chosen sub-protocol negotiated with the remote endpoint.
    /// </summary>
    public readonly string? SubProtocol;

    internal readonly HttpStream Stream;

    readonly List<byte> finalPayload = new List<byte>();
    WSOpcode currentOpcode;

    internal Websocket(HttpStream stream, string? subProtocol)
    {
        Stream = stream;
        SubProtocol = subProtocol;
    }

    /// <summary>
    /// Reads a data frame from the websocket.
    /// </summary>
    /// <returns></returns>
    /// <exception cref="EndOfStreamException">Thrown when the stream stops unexpectedly.</exception>
    /// <exception cref="IOException">Thrown when the client declares a payload which is too large.</exception>
    public async Task<DataFrame> Read()
    {
        var pool = ArrayPool<byte>.Shared;
        byte[] recvBuffer = pool.Rent(10);
        try
        {
            while (true)
            {
                //First byte
                if (await Stream.Read(recvBuffer.AsMemory(0, 2)) < 2)
                    throw new EndOfStreamException();

                byte firstByte = recvBuffer[0];
                bool fin = (firstByte & 1) != 0;
                //bool rsv1 = (firstByte & 2) != 0;
                //bool rsv2 = (firstByte & 4) != 0;
                //bool rsv3 = (firstByte & 8) != 0;
                WSOpcode opcode = (WSOpcode)(firstByte & 0b00001111);

        
                //Second byte
                byte secondByte = recvBuffer[1];

                bool maskEnabled = (secondByte & 0b10000000) != 0;
                if (maskEnabled)
                    secondByte &= 0b01111111;

                //Payload length
                uint payloadLength;
                if (secondByte < 126)
                {
                    payloadLength = secondByte;
                }
                else
                {
                    if (secondByte == 126)
                    {
                        if (await Stream.Read(recvBuffer.AsMemory(0, 2)) < 2)
                            throw new EndOfStreamException();

                        payloadLength = BinaryPrimitives.ReadUInt16BigEndian(recvBuffer);
                    }
                    else if (secondByte == 127)
                    {
                        if (await Stream.Read(recvBuffer.AsMemory(0, 8)) < 8)
                            throw new EndOfStreamException();

                        payloadLength = BinaryPrimitives.ReadUInt32BigEndian(recvBuffer);
                    }
                    else
                    {
                        throw new Exception("This shouldn't happen");
                    }
                }

                if (finalPayload.Count + payloadLength > 100_000)
                    throw new IOException("Payload too large");

                //Mask
                byte maskKey1, maskKey2, maskKey3, maskKey4;
                if (maskEnabled)
                {
                    if (await Stream.Read(recvBuffer.AsMemory(0, 4)) < 4)
                        throw new EndOfStreamException();

                    maskKey1 = recvBuffer[0];
                    maskKey2 = recvBuffer[1];
                    maskKey3 = recvBuffer[2];
                    maskKey4 = recvBuffer[3];
                }
                else
                {
                    maskKey1 = 0;
                    maskKey2 = 0;
                    maskKey3 = 0;
                    maskKey4 = 0;
                }

                //Payload
                byte[] payloadBuffer = pool.Rent((int)payloadLength);
                try
                {
                    ArraySegment<byte> payload = new ArraySegment<byte>(payloadBuffer, 0, (int)payloadLength);
                    if (await Stream.Read(payload) < payloadLength)
                        throw new EndOfStreamException();

                    //Unmask payload
                    //TODO: Optimize using unsafe
                    if (maskEnabled)
                    {
                        int index = 0;
                        while (true)
                        {
                            if (index >= payloadLength)
                                break;

                            payload[index] = (byte)(payload[index] ^ maskKey1);
                            index++;

                            if (index >= payloadLength)
                                break;

                            payload[index] = (byte)(payload[index] ^ maskKey2);
                            index++;

                            if (index >= payloadLength)
                                break;

                            payload[index] = (byte)(payload[index] ^ maskKey3);
                            index++;

                            if (index >= payloadLength)
                                break;

                            payload[index] = (byte)(payload[index] ^ maskKey4);
                            index++;
                        }
                    }

                    switch (opcode)
                    {
                        case WSOpcode.Close:
                                await Write(new DataFrame(WSOpcode.Close, true, Array.Empty<byte>()));
                                return new DataFrame(WSOpcode.Close, true, FlushPayload());

                        case WSOpcode.Continuation:
                        case WSOpcode.Text:
                        case WSOpcode.Binary:
                        {
                            if (opcode is WSOpcode.Text or WSOpcode.Binary)
                                currentOpcode = opcode;

                            finalPayload.AddRange(payload);

                            if (fin)
                                return new DataFrame(currentOpcode, fin, FlushPayload());
                            else
                                break;
                        }
                        
                        case WSOpcode.Ping:
                            await Write(new DataFrame(WSOpcode.Pong, true, payload));
                            break;
                    }
                }
                finally
                {
                    pool.Return(payloadBuffer);
                }
            }
        }
        finally
        {
            pool.Return(recvBuffer);
        }
    }

    byte[] FlushPayload()
    {
        byte[] final = finalPayload.ToArray();
        finalPayload.Clear();
        return final;
    }

    public Task Write(byte[] payload)
    {
        return Write(new DataFrame(WSOpcode.Binary, true, payload));
    }

    public Task Write(string payload)
    {
        return Write(new DataFrame(WSOpcode.Text, true, Encoding.UTF8.GetBytes(payload)));
    }

    public async Task Write(DataFrame frame)
    {
        var pool = ArrayPool<byte>.Shared;
        byte[] writeBuf = pool.Rent(10);
        try
        {
            byte firstByte = 0;
            if (frame.EndOfMessage)
                firstByte |= 0b10000000;

            firstByte |= (byte)((int)frame.Opcode & 0b00001111);

            writeBuf[0] = firstByte;
            await Stream.Write(writeBuf.AsMemory(0, 1));

            if (frame.Payload.Count < 126)
            {
                writeBuf[0] = (byte)frame.Payload.Count;
                await Stream.Write(writeBuf.AsMemory(0, 1));
            }
            else
            {
                if (frame.Payload.Count < ushort.MaxValue)
                {
                    writeBuf[0] = 126;
                    BinaryPrimitives.WriteUInt16BigEndian(writeBuf.AsSpan(1), (ushort)frame.Payload.Count);
                    await Stream.Write(writeBuf.AsMemory(0, 3));
                }
                else
                {
                    writeBuf[0] = 127;
                    BinaryPrimitives.WriteUInt64BigEndian(writeBuf.AsSpan(1), (ulong)frame.Payload.Count);
                    await Stream.Write(writeBuf.AsMemory(0, 9));
                }
            }

            await Stream.Write(frame.Payload);
            await Stream.Flush();
        }
        finally
        {
            pool.Return(writeBuf);
        }
    }

    internal async Task Close(CloseStatus status = CloseStatus.NormalClosure)
    {
        var pool = ArrayPool<byte>.Shared;
        byte[] closeBuf = pool.Rent(2);
        try
        {
            BinaryPrimitives.WriteUInt16BigEndian(closeBuf, (ushort)status);
            await Write(new DataFrame(WSOpcode.Close, true, new ArraySegment<byte>(closeBuf, 0, 2)));
        }
        finally
        {
            pool.Return(closeBuf);
        }
    }
}

/// <summary>
/// A remote websocket connected to a local HTTP server.
/// </summary>
public class WebsocketRemote : Websocket
{
    /// <summary>
    /// The HTTP request encompassing the websocket.
    /// </summary>
    public readonly HttpRequest Request;

    /// <summary>
    /// The HTTP request encompassing the websocket.
    /// </summary>
    public readonly HttpClientInfo ClientInfo;

    internal WebsocketRemote(HttpRequest request, HttpClientInfo clientInfo, HttpStream stream, string? subProtocol) : base(stream, subProtocol)
    {
        Request = request;
        ClientInfo = clientInfo;
    }
}