use std::{net::SocketAddr, time::Duration};

use anyhow::Context;
use log::{debug, trace};
use tokio::{io::AsyncReadExt, time::timeout};

use crate::{
    buffers::{ByteBuf, ByteString},
    clone_to_owned::CloneToOwned,
    lengths::ChunkInfo,
    peer_binary_protocol::{
        serialize_piece_preamble, ExtendedHandshake, ExtendedMessage, Handshake, Message,
        MessageBorrowed, MessageDeserializeError, MessageOwned, PIECE_MESSAGE_DEFAULT_LEN,
    },
    peer_id::try_decode_peer_id,
};

pub trait PeerConnectionHandler {
    fn get_have_bytes(&self) -> u64;
    fn serialize_bitfield_message_to_buf(&self, buf: &mut Vec<u8>) -> Option<usize>;
    fn on_handshake(&self, handshake: Handshake);
    fn on_extended_handshake(&self, extended_handshake: &ExtendedHandshake<ByteBuf>);
    fn on_received_message(&self, msg: Message<ByteBuf<'_>>) -> anyhow::Result<()>;
    fn on_uploaded_bytes(&self, bytes: u32);
    fn read_chunk(&self, chunk: &ChunkInfo, buf: &mut [u8]) -> anyhow::Result<()>;
}

#[derive(Debug)]
pub enum WriterRequest {
    Message(MessageOwned),
    ReadChunkRequest(ChunkInfo),
}

pub struct PeerConnection<H> {
    handler: H,
    addr: SocketAddr,
    info_hash: [u8; 20],
    peer_id: [u8; 20],
}

// async fn read_one<'a, R: AsyncReadExt + Unpin>(
//     mut reader: R,
//     read_buf: &'a mut Vec<u8>,
//     read_so_far: &mut usize,
// ) -> anyhow::Result<(MessageBorrowed<'a>, usize)> {
//     loop {
//         match MessageBorrowed::deserialize(&read_buf[..*read_so_far]) {
//             Ok((msg, size)) => return Ok((msg, size)),
//             Err(MessageDeserializeError::NotEnoughData(d, _)) => {
//                 if read_buf.len() < *read_so_far + d {
//                     read_buf.reserve(d);
//                     read_buf.resize(read_buf.capacity(), 0);
//                 }

//                 let size = reader
//                     .read(&mut read_buf[*read_so_far..])
//                     .await
//                     .context("error reading from peer")?;
//                 if size == 0 {
//                     anyhow::bail!("disconnected while reading, read so far: {}", *read_so_far)
//                 }
//                 *read_so_far += size;
//             }
//             Err(e) => return Err(e.into()),
//         }
//     }
// }

impl<H: PeerConnectionHandler> PeerConnection<H> {
    pub fn new(addr: SocketAddr, info_hash: [u8; 20], peer_id: [u8; 20], handler: H) -> Self {
        PeerConnection {
            handler,
            addr,
            info_hash,
            peer_id,
        }
    }
    pub fn into_handler(self) -> H {
        self.handler
    }
    pub async fn manage_peer(
        &self,
        mut outgoing_chan: tokio::sync::mpsc::UnboundedReceiver<WriterRequest>,
    ) -> anyhow::Result<()> {
        use tokio::io::AsyncReadExt;
        use tokio::io::AsyncWriteExt;
        let mut conn = tokio::net::TcpStream::connect(self.addr)
            .await
            .context("error connecting")?;
        let handshake = Handshake::new(self.info_hash, self.peer_id);
        conn.write_all(&handshake.serialize())
            .await
            .context("error writing handshake")?;
        let mut read_buf = vec![0u8; PIECE_MESSAGE_DEFAULT_LEN * 2];
        let mut read_so_far = conn
            .read(&mut read_buf)
            .await
            .context("error reading handshake")?;
        if read_so_far == 0 {
            anyhow::bail!("bad handshake");
        }
        let (h, size) = Handshake::deserialize(&read_buf[..read_so_far])
            .map_err(|e| anyhow::anyhow!("error deserializing handshake: {:?}", e))?;

        debug!(
            "connected peer {}: {:?}",
            self.addr,
            try_decode_peer_id(h.peer_id)
        );
        if h.info_hash != self.info_hash {
            anyhow::bail!("info hash does not match");
        }

        let mut extended_handshake: Option<ExtendedHandshake<ByteString>> = None;
        let supports_extended = h.supports_extended();

        self.handler.on_handshake(h);
        if read_so_far > size {
            read_buf.copy_within(size..read_so_far, 0);
        }
        read_so_far -= size;

        if supports_extended {
            // Read extended handshake
            // I wasn't able to extract that into a function.
            // TODO: extract into a macro.
            let (extended, size) = loop {
                match MessageBorrowed::deserialize(&read_buf[..read_so_far]) {
                    Ok((msg, size)) => break (msg, size),
                    Err(MessageDeserializeError::NotEnoughData(d, _)) => {
                        if read_buf.len() < read_so_far + d {
                            read_buf.reserve(d);
                            read_buf.resize(read_buf.capacity(), 0);
                        }

                        let size = conn
                            .read(&mut read_buf[read_so_far..])
                            .await
                            .context("error reading from peer")?;
                        if size == 0 {
                            anyhow::bail!(
                                "disconnected while reading, read so far: {}",
                                read_so_far
                            )
                        }
                        read_so_far += size;
                    }
                    Err(e) => return Err(e.into()),
                }
            };

            match extended {
                Message::Extended(ExtendedMessage::Handshake(h)) => {
                    trace!("received from {}: {:?}", self.addr, &h);
                    self.handler.on_extended_handshake(&h);
                    extended_handshake = Some(h.clone_to_owned())
                }
                other => anyhow::bail!("expected extended handshake, but got {:?}", other),
            };

            if read_so_far > size {
                read_buf.copy_within(size..read_so_far, 0);
            }
            read_so_far -= size;
        }

        let (mut read_half, mut write_half) = tokio::io::split(conn);

        let writer = async move {
            let mut buf = Vec::<u8>::with_capacity(PIECE_MESSAGE_DEFAULT_LEN);
            let keep_alive_interval = Duration::from_secs(120);

            if self.handler.get_have_bytes() > 0 {
                if let Some(len) = self.handler.serialize_bitfield_message_to_buf(&mut buf) {
                    write_half
                        .write_all(&buf[..len])
                        .await
                        .context("error writing bitfield to peer")?;
                    debug!("sent bitfield to {}", self.addr);
                }
            }

            loop {
                let req = match timeout(keep_alive_interval, outgoing_chan.recv()).await {
                    Ok(Some(msg)) => msg,
                    Ok(None) => {
                        anyhow::bail!("closing writer, channel closed")
                    }
                    Err(_) => WriterRequest::Message(MessageOwned::KeepAlive),
                };

                let mut uploaded_add = None;

                let len = match &req {
                    WriterRequest::Message(msg) => {
                        msg.serialize(&mut buf, extended_handshake.as_ref())?
                    }
                    WriterRequest::ReadChunkRequest(chunk) => {
                        // this whole section is an optimization
                        buf.resize(PIECE_MESSAGE_DEFAULT_LEN, 0);
                        let preamble_len = serialize_piece_preamble(&chunk, &mut buf);
                        let full_len = preamble_len + chunk.size as usize;
                        buf.resize(full_len, 0);
                        self.handler
                            .read_chunk(chunk, &mut buf[preamble_len..])
                            .with_context(|| format!("error reading chunk {:?}", chunk))?;
                        uploaded_add = Some(chunk.size);
                        full_len
                    }
                };

                debug!("sending to {}: {:?}, length={}", self.addr, &req, len);

                write_half
                    .write_all(&buf[..len])
                    .await
                    .context("error writing the message to peer")?;

                if let Some(uploaded_add) = uploaded_add {
                    self.handler.on_uploaded_bytes(uploaded_add)
                }
            }

            // For type inference.
            #[allow(unreachable_code)]
            Ok::<_, anyhow::Error>(())
        };

        let reader = async move {
            loop {
                let (message, size) = loop {
                    match MessageBorrowed::deserialize(&read_buf[..read_so_far]) {
                        Ok((msg, size)) => {
                            break (msg, size);
                        }
                        Err(MessageDeserializeError::NotEnoughData(d, _)) => {
                            if read_buf.len() < read_so_far + d {
                                read_buf.reserve(d);
                                read_buf.resize(read_buf.capacity(), 0);
                            }

                            let size = read_half
                                .read(&mut read_buf[read_so_far..])
                                .await
                                .context("error reading from peer")?;
                            if size == 0 {
                                anyhow::bail!(
                                    "disconnected while reading, read so far: {}",
                                    read_so_far
                                )
                            }
                            read_so_far += size;
                        }
                        Err(e) => return Err(e.into()),
                    }
                };

                trace!("received from {}: {:?}", self.addr, &message);

                self.handler
                    .on_received_message(message)
                    .context("error in handler.on_received_message()")?;

                if read_so_far > size {
                    read_buf.copy_within(size..read_so_far, 0);
                }
                read_so_far -= size;
            }

            // For type inference.
            #[allow(unreachable_code)]
            Ok::<_, anyhow::Error>(())
        };

        let r = tokio::select! {
            r = reader => {r}
            r = writer => {r}
        };
        debug!("{}: either reader or writer are done, exiting", self.addr);
        r
    }
}