use tokio::io::{AsyncRead, AsyncReadExt};

// Frame type constants
pub const FRAME_OPEN: u8 = 0x01;
pub const FRAME_DATA: u8 = 0x02;
pub const FRAME_CLOSE: u8 = 0x03;
pub const FRAME_DATA_BACK: u8 = 0x04;
pub const FRAME_CLOSE_BACK: u8 = 0x05;
pub const FRAME_CONFIG: u8 = 0x06;     // Hub -> Edge: configuration update
pub const FRAME_PING: u8 = 0x07;       // Hub -> Edge: heartbeat probe
pub const FRAME_PONG: u8 = 0x08;       // Edge -> Hub: heartbeat response
pub const FRAME_WINDOW_UPDATE: u8 = 0x09;      // Edge -> Hub: per-stream flow control
pub const FRAME_WINDOW_UPDATE_BACK: u8 = 0x0A;  // Hub -> Edge: per-stream flow control

// Frame header size: 4 (stream_id) + 1 (type) + 4 (length) = 9 bytes
pub const FRAME_HEADER_SIZE: usize = 9;

// Maximum payload size (16 MB)
pub const MAX_PAYLOAD_SIZE: u32 = 16 * 1024 * 1024;

// Per-stream flow control constants
/// Initial per-stream window size (4 MB). Sized for full throughput at high RTT:
/// at 100ms RTT, this sustains ~40 MB/s per stream.
pub const INITIAL_STREAM_WINDOW: u32 = 4 * 1024 * 1024;
/// Send WINDOW_UPDATE after consuming this many bytes (half the initial window).
pub const WINDOW_UPDATE_THRESHOLD: u32 = INITIAL_STREAM_WINDOW / 2;
/// Maximum window size to prevent overflow.
pub const MAX_WINDOW_SIZE: u32 = 16 * 1024 * 1024;

/// Encode a WINDOW_UPDATE frame for a specific stream.
pub fn encode_window_update(stream_id: u32, frame_type: u8, increment: u32) -> Vec<u8> {
    encode_frame(stream_id, frame_type, &increment.to_be_bytes())
}

/// Compute the target per-stream window size based on the number of active streams.
/// Total memory budget is ~32MB shared across all streams. As more streams are active,
/// each gets a smaller window. This adapts to current demand — few streams get high
/// throughput, many streams save memory and reduce control frame pressure.
pub fn compute_window_for_stream_count(active: u32) -> u32 {
    let per_stream = (32 * 1024 * 1024u64) / (active.max(1) as u64);
    per_stream.clamp(64 * 1024, INITIAL_STREAM_WINDOW as u64) as u32
}

/// Decode a WINDOW_UPDATE payload into a byte increment. Returns None if payload is malformed.
pub fn decode_window_update(payload: &[u8]) -> Option<u32> {
    if payload.len() != 4 {
        return None;
    }
    Some(u32::from_be_bytes([payload[0], payload[1], payload[2], payload[3]]))
}

/// A single multiplexed frame.
#[derive(Debug, Clone)]
pub struct Frame {
    pub stream_id: u32,
    pub frame_type: u8,
    pub payload: Vec<u8>,
}

/// Encode a frame into bytes: [stream_id:4][type:1][length:4][payload]
pub fn encode_frame(stream_id: u32, frame_type: u8, payload: &[u8]) -> Vec<u8> {
    let len = payload.len() as u32;
    let mut buf = Vec::with_capacity(FRAME_HEADER_SIZE + payload.len());
    buf.extend_from_slice(&stream_id.to_be_bytes());
    buf.push(frame_type);
    buf.extend_from_slice(&len.to_be_bytes());
    buf.extend_from_slice(payload);
    buf
}

/// Build a PROXY protocol v1 header line.
/// Format: `PROXY TCP4 <client_ip> <edge_ip> <client_port> <dest_port>\r\n`
pub fn build_proxy_v1_header(
    client_ip: &str,
    edge_ip: &str,
    client_port: u16,
    dest_port: u16,
) -> String {
    format!(
        "PROXY TCP4 {} {} {} {}\r\n",
        client_ip, edge_ip, client_port, dest_port
    )
}

/// Stateful async frame reader that yields `Frame` values from an `AsyncRead`.
pub struct FrameReader<R> {
    reader: R,
    header_buf: [u8; FRAME_HEADER_SIZE],
}

impl<R: AsyncRead + Unpin> FrameReader<R> {
    pub fn new(reader: R) -> Self {
        Self {
            reader,
            header_buf: [0u8; FRAME_HEADER_SIZE],
        }
    }

    /// Read the next frame. Returns `None` on EOF, `Err` on protocol violation.
    pub async fn next_frame(&mut self) -> Result<Option<Frame>, std::io::Error> {
        // Read header
        match self.reader.read_exact(&mut self.header_buf).await {
            Ok(_) => {}
            Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => return Ok(None),
            Err(e) => return Err(e),
        }

        let stream_id = u32::from_be_bytes([
            self.header_buf[0],
            self.header_buf[1],
            self.header_buf[2],
            self.header_buf[3],
        ]);
        let frame_type = self.header_buf[4];
        let length = u32::from_be_bytes([
            self.header_buf[5],
            self.header_buf[6],
            self.header_buf[7],
            self.header_buf[8],
        ]);

        if length > MAX_PAYLOAD_SIZE {
            return Err(std::io::Error::new(
                std::io::ErrorKind::InvalidData,
                format!("frame payload too large: {} bytes", length),
            ));
        }

        let mut payload = vec![0u8; length as usize];
        if length > 0 {
            self.reader.read_exact(&mut payload).await?;
        }

        Ok(Some(Frame {
            stream_id,
            frame_type,
            payload,
        }))
    }

    /// Consume the reader and return the inner stream.
    pub fn into_inner(self) -> R {
        self.reader
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_encode_frame() {
        let data = b"hello";
        let encoded = encode_frame(42, FRAME_DATA, data);
        assert_eq!(encoded.len(), FRAME_HEADER_SIZE + data.len());
        // stream_id = 42 in BE
        assert_eq!(&encoded[0..4], &42u32.to_be_bytes());
        // frame type
        assert_eq!(encoded[4], FRAME_DATA);
        // length
        assert_eq!(&encoded[5..9], &5u32.to_be_bytes());
        // payload
        assert_eq!(&encoded[9..], b"hello");
    }

    #[test]
    fn test_encode_empty_frame() {
        let encoded = encode_frame(1, FRAME_CLOSE, &[]);
        assert_eq!(encoded.len(), FRAME_HEADER_SIZE);
        assert_eq!(&encoded[5..9], &0u32.to_be_bytes());
    }

    #[test]
    fn test_proxy_v1_header() {
        let header = build_proxy_v1_header("1.2.3.4", "5.6.7.8", 12345, 443);
        assert_eq!(header, "PROXY TCP4 1.2.3.4 5.6.7.8 12345 443\r\n");
    }

    #[tokio::test]
    async fn test_frame_reader() {
        let frame1 = encode_frame(1, FRAME_OPEN, b"PROXY TCP4 1.2.3.4 5.6.7.8 1234 443\r\n");
        let frame2 = encode_frame(1, FRAME_DATA, b"GET / HTTP/1.1\r\n");
        let frame3 = encode_frame(1, FRAME_CLOSE, &[]);

        let mut data = Vec::new();
        data.extend_from_slice(&frame1);
        data.extend_from_slice(&frame2);
        data.extend_from_slice(&frame3);

        let cursor = std::io::Cursor::new(data);
        let mut reader = FrameReader::new(cursor);

        let f1 = reader.next_frame().await.unwrap().unwrap();
        assert_eq!(f1.stream_id, 1);
        assert_eq!(f1.frame_type, FRAME_OPEN);
        assert!(f1.payload.starts_with(b"PROXY"));

        let f2 = reader.next_frame().await.unwrap().unwrap();
        assert_eq!(f2.frame_type, FRAME_DATA);

        let f3 = reader.next_frame().await.unwrap().unwrap();
        assert_eq!(f3.frame_type, FRAME_CLOSE);
        assert!(f3.payload.is_empty());

        // EOF
        assert!(reader.next_frame().await.unwrap().is_none());
    }

    #[test]
    fn test_encode_frame_config_type() {
        let payload = b"{\"listenPorts\":[443]}";
        let encoded = encode_frame(0, FRAME_CONFIG, payload);
        assert_eq!(encoded[4], FRAME_CONFIG);
        assert_eq!(&encoded[0..4], &0u32.to_be_bytes());
        assert_eq!(&encoded[9..], payload.as_slice());
    }

    #[test]
    fn test_encode_frame_data_back_type() {
        let payload = b"response data";
        let encoded = encode_frame(7, FRAME_DATA_BACK, payload);
        assert_eq!(encoded[4], FRAME_DATA_BACK);
        assert_eq!(&encoded[0..4], &7u32.to_be_bytes());
        assert_eq!(&encoded[5..9], &(payload.len() as u32).to_be_bytes());
        assert_eq!(&encoded[9..], payload.as_slice());
    }

    #[test]
    fn test_encode_frame_close_back_type() {
        let encoded = encode_frame(99, FRAME_CLOSE_BACK, &[]);
        assert_eq!(encoded[4], FRAME_CLOSE_BACK);
        assert_eq!(&encoded[0..4], &99u32.to_be_bytes());
        assert_eq!(&encoded[5..9], &0u32.to_be_bytes());
        assert_eq!(encoded.len(), FRAME_HEADER_SIZE);
    }

    #[test]
    fn test_encode_frame_large_stream_id() {
        let encoded = encode_frame(u32::MAX, FRAME_DATA, b"x");
        assert_eq!(&encoded[0..4], &u32::MAX.to_be_bytes());
        assert_eq!(encoded[4], FRAME_DATA);
        assert_eq!(&encoded[5..9], &1u32.to_be_bytes());
        assert_eq!(encoded[9], b'x');
    }

    #[tokio::test]
    async fn test_frame_reader_max_payload_rejection() {
        let mut data = Vec::new();
        data.extend_from_slice(&1u32.to_be_bytes());
        data.push(FRAME_DATA);
        data.extend_from_slice(&(MAX_PAYLOAD_SIZE + 1).to_be_bytes());

        let cursor = std::io::Cursor::new(data);
        let mut reader = FrameReader::new(cursor);

        let result = reader.next_frame().await;
        assert!(result.is_err());
        let err = result.unwrap_err();
        assert_eq!(err.kind(), std::io::ErrorKind::InvalidData);
    }

    #[tokio::test]
    async fn test_frame_reader_eof_mid_header() {
        // Only 5 bytes — not enough for a 9-byte header
        let data = vec![0u8; 5];
        let cursor = std::io::Cursor::new(data);
        let mut reader = FrameReader::new(cursor);

        // Should return Ok(None) on partial header EOF
        let result = reader.next_frame().await;
        assert!(result.unwrap().is_none());
    }

    #[tokio::test]
    async fn test_frame_reader_eof_mid_payload() {
        // Full header claiming 100 bytes of payload, but only 10 bytes present
        let mut data = Vec::new();
        data.extend_from_slice(&1u32.to_be_bytes());
        data.push(FRAME_DATA);
        data.extend_from_slice(&100u32.to_be_bytes());
        data.extend_from_slice(&[0xAB; 10]);

        let cursor = std::io::Cursor::new(data);
        let mut reader = FrameReader::new(cursor);

        let result = reader.next_frame().await;
        assert!(result.is_err());
    }

    #[tokio::test]
    async fn test_frame_reader_all_frame_types() {
        let types = [
            FRAME_OPEN,
            FRAME_DATA,
            FRAME_CLOSE,
            FRAME_DATA_BACK,
            FRAME_CLOSE_BACK,
            FRAME_CONFIG,
            FRAME_PING,
            FRAME_PONG,
        ];

        let mut data = Vec::new();
        for (i, &ft) in types.iter().enumerate() {
            let payload = format!("payload_{}", i);
            data.extend_from_slice(&encode_frame(i as u32, ft, payload.as_bytes()));
        }

        let cursor = std::io::Cursor::new(data);
        let mut reader = FrameReader::new(cursor);

        for (i, &ft) in types.iter().enumerate() {
            let frame = reader.next_frame().await.unwrap().unwrap();
            assert_eq!(frame.stream_id, i as u32);
            assert_eq!(frame.frame_type, ft);
            assert_eq!(frame.payload, format!("payload_{}", i).as_bytes());
        }

        assert!(reader.next_frame().await.unwrap().is_none());
    }

    #[tokio::test]
    async fn test_frame_reader_zero_length_payload() {
        let data = encode_frame(42, FRAME_CLOSE, &[]);
        let cursor = std::io::Cursor::new(data);
        let mut reader = FrameReader::new(cursor);

        let frame = reader.next_frame().await.unwrap().unwrap();
        assert_eq!(frame.stream_id, 42);
        assert_eq!(frame.frame_type, FRAME_CLOSE);
        assert!(frame.payload.is_empty());
    }

    #[test]
    fn test_encode_frame_ping_pong() {
        // PING: stream_id=0, empty payload (control frame)
        let ping = encode_frame(0, FRAME_PING, &[]);
        assert_eq!(ping[4], FRAME_PING);
        assert_eq!(&ping[0..4], &0u32.to_be_bytes());
        assert_eq!(ping.len(), FRAME_HEADER_SIZE);

        // PONG: stream_id=0, empty payload (control frame)
        let pong = encode_frame(0, FRAME_PONG, &[]);
        assert_eq!(pong[4], FRAME_PONG);
        assert_eq!(&pong[0..4], &0u32.to_be_bytes());
        assert_eq!(pong.len(), FRAME_HEADER_SIZE);
    }
}