use crate::message::Envelope;
use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};

/// Maximum frame size we will accept (16 MiB). Guards against a corrupt length prefix.
pub const MAX_FRAME: u32 = 16 * 1024 * 1024;

#[derive(Debug, thiserror::Error)]
pub enum CodecError {
    #[error("io: {0}")]
    Io(#[from] std::io::Error),
    #[error("json: {0}")]
    Json(#[from] serde_json::Error),
    #[error("frame too large: {0} bytes")]
    FrameTooLarge(u32),
}

/// Write one envelope as `u32` BE length prefix + JSON payload.
pub async fn write_frame<W: AsyncWrite + Unpin>(w: &mut W, env: &Envelope) -> Result<(), CodecError> {
    let payload = serde_json::to_vec(env)?;
    let len = payload.len() as u32;
    if len > MAX_FRAME {
        return Err(CodecError::FrameTooLarge(len));
    }
    w.write_all(&len.to_be_bytes()).await?;
    w.write_all(&payload).await?;
    w.flush().await?;
    Ok(())
}

/// Read one length-prefixed envelope. Returns `Ok(None)` on clean EOF.
pub async fn read_frame<R: AsyncRead + Unpin>(r: &mut R) -> Result<Option<Envelope>, CodecError> {
    let mut len_buf = [0u8; 4];
    match r.read_exact(&mut len_buf).await {
        Ok(_) => {}
        Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => return Ok(None),
        Err(e) => return Err(e.into()),
    }
    let len = u32::from_be_bytes(len_buf);
    if len > MAX_FRAME {
        return Err(CodecError::FrameTooLarge(len));
    }
    let mut payload = vec![0u8; len as usize];
    r.read_exact(&mut payload).await?;
    let env: Envelope = serde_json::from_slice(&payload)?;
    Ok(Some(env))
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::ids::SurfaceId;
    use crate::message::{Cmd, Envelope};

    #[tokio::test]
    async fn frame_round_trips_over_a_pipe() {
        let (mut client, mut server) = tokio::io::duplex(1024);
        let env = Envelope::Req { id: 9, cmd: Cmd::Status };
        write_frame(&mut client, &env).await.unwrap();
        let got = read_frame(&mut server).await.unwrap().unwrap();
        assert_eq!(got, env);
    }

    #[tokio::test]
    async fn two_frames_are_decoded_independently() {
        let (mut client, mut server) = tokio::io::duplex(4096);
        let a = Envelope::Req { id: 1, cmd: Cmd::Status };
        let b = Envelope::Req { id: 2, cmd: Cmd::Close { surface_id: SurfaceId("s_1".into()) } };
        write_frame(&mut client, &a).await.unwrap();
        write_frame(&mut client, &b).await.unwrap();
        assert_eq!(read_frame(&mut server).await.unwrap().unwrap(), a);
        assert_eq!(read_frame(&mut server).await.unwrap().unwrap(), b);
    }

    #[tokio::test]
    async fn clean_eof_returns_none() {
        let (client, mut server) = tokio::io::duplex(16);
        drop(client);
        assert!(read_frame(&mut server).await.unwrap().is_none());
    }
}