use futures::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};

pub const VERSION: u8 = 1;
pub const MAX_FRAME: usize = 65535;

pub type Result<T> = std::result::Result<T, Error>;

#[derive(Debug)]
pub struct Error(pub String);

impl std::fmt::Display for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.0.fmt(f)
    }
}

impl std::error::Error for Error {}

pub fn encode(body: &[u8]) -> Result<Vec<u8>> {
    if body.len() > MAX_FRAME {
        return Err(Error("frame too large".into()));
    }
    let mut out = Vec::with_capacity(5 + body.len());
    out.push(VERSION);
    out.extend_from_slice(&(body.len() as u32).to_be_bytes());
    out.extend_from_slice(body);
    Ok(out)
}

pub fn decode(data: &[u8]) -> Result<Vec<u8>> {
    if data.len() < 5 {
        return Err(Error("short frame".into()));
    }
    if data[0] != VERSION {
        return Err(Error("bad version".into()));
    }
    let len = u32::from_be_bytes(data[1..5].try_into().expect("4 bytes")) as usize;
    if data.len() != 5 + len {
        return Err(Error("length mismatch".into()));
    }
    if len > MAX_FRAME {
        return Err(Error("frame too large".into()));
    }
    Ok(data[5..].to_vec())
}

pub async fn write_frame<W: AsyncWrite + Unpin>(w: &mut W, body: &[u8]) -> Result<()> {
    let framed = encode(body)?;
    w.write_all(&framed)
        .await
        .map_err(|e| Error(e.to_string()))?;
    w.flush().await.map_err(|e| Error(e.to_string()))
}

pub async fn read_frame<R: AsyncRead + Unpin>(r: &mut R) -> Result<Vec<u8>> {
    let mut hdr = [0u8; 5];
    r.read_exact(&mut hdr)
        .await
        .map_err(|e| Error(e.to_string()))?;
    if hdr[0] != VERSION {
        return Err(Error("bad version".into()));
    }
    let len = u32::from_be_bytes(hdr[1..5].try_into().expect("4 bytes")) as usize;
    if len > MAX_FRAME {
        return Err(Error("frame too large".into()));
    }
    let mut body = vec![0u8; len];
    r.read_exact(&mut body)
        .await
        .map_err(|e| Error(e.to_string()))?;
    Ok(body)
}