use read::{FrameReader, ReceivedFrame};
use rtt::RttState;
use std::{collections::VecDeque, sync::Arc};
use tokio::io::AsyncReadExt;
use tokio::{
    io::{AsyncBufRead, AsyncWrite, AsyncWriteExt},
    pin, select, spawn,
    sync::mpsc::{unbounded_channel, UnboundedReceiver, UnboundedSender},
    task::JoinHandle,
    time::{sleep_until, Instant},
};
use tokio_util::{bytes::BytesMut, sync::CancellationToken};

use crate::encoding::crc16;

mod read;
mod rtt;

pub(crate) const MESSAGE_HEADER_SIZE: usize = 2;
pub(crate) const MESSAGE_TRAILER_SIZE: usize = 3;
pub(crate) const MESSAGE_LENGTH_MIN: usize = MESSAGE_HEADER_SIZE + MESSAGE_TRAILER_SIZE;
pub(crate) const MESSAGE_LENGTH_MAX: usize = 64;
pub(crate) const MESSAGE_LENGTH_PAYLOAD_MAX: usize = MESSAGE_LENGTH_MAX - MESSAGE_LENGTH_MIN;
pub(crate) const MESSAGE_POSITION_SEQ: usize = 1;
pub(crate) const MESSAGE_TRAILER_CRC: usize = 3;
pub(crate) const MESSAGE_VALUE_SYNC: u8 = 0x7E;
pub(crate) const MESSAGE_DEST: u8 = 0x10;
pub(crate) const MESSAGE_SEQ_MASK: u8 = 0x0F;

/// Wrapper around a connection to a klipper firmware MCU, which deals with
/// retransmission, flow control, etc.
///
/// Internally, this holds a handle to an async task and a channel to communicate with it,
/// meaning operations don't necessarily block the current task.
#[derive(Debug)]
pub struct Transport {
    // Handles to the associated async task
    task_inner: JoinHandle<()>,

    /// Queue for outbound messages
    cmd_send: UnboundedSender<TransportCommand>,
}

/// A message sent to the transport task
#[derive(Debug)]
enum TransportCommand {
    SendMessage(Vec<u8>),
    Exit,
}

pub(crate) type TransportReceiver = UnboundedReceiver<Result<Vec<u8>, TransportError>>;

impl Transport {
    pub(crate) async fn connect(
        rdr: impl AsyncBufRead + Unpin + Send + 'static,
        wr: impl AsyncWrite + Unpin + Send + 'static,
    ) -> (Transport, TransportReceiver) {
        let (data_send, data_recv) = unbounded_channel();
        let (cmd_send, cmd_recv) = unbounded_channel();

        let cancel_token = CancellationToken::new();
        let task_inner = spawn(async move {
            let mut ts = TransportState::new(rdr, wr, data_send, cmd_recv, cancel_token);
            if let Err(e) = ts.run().await {
                let _ = ts.data_send.send(Err(e));
            }
        });

        (
            Transport {
                task_inner,
                cmd_send,
            },
            data_recv,
        )
    }

    pub(crate) fn send(&self, msg: &[u8]) -> Result<(), TransmitterError> {
        self.cmd_send
            .send(TransportCommand::SendMessage(msg.into()))
            .map_err(|_| TransmitterError::ConnectionClosed)
    }

    pub(crate) async fn close(self) {
        let _ = self.cmd_send.send(TransportCommand::Exit);
        let _ = self.task_inner.await;
    }
}

#[derive(thiserror::Error, Debug)]
pub enum TransportError {
    #[error("message encoding failed: {0}")]
    MessageEncode(#[from] MessageEncodeError),

    #[error("transmitter error: {0}")]
    Transmitter(#[from] TransmitterError),

    #[error("io error: {0}")]
    IOError(#[from] std::io::Error),
}

/// State for the task which deals with transport state
#[derive(Debug)]
struct TransportState<R, W> {
    frdr: FrameReader,
    rdr: R,
    wr: W,

    data_send: UnboundedSender<Result<Vec<u8>, TransportError>>,
    cmd_recv: UnboundedReceiver<TransportCommand>,

    cancel: CancellationToken,

    is_synchronized: bool,
    rtt_state: RttState,
    receive_sequence: u64,
    send_sequence: u64,
    last_ack_sequence: u64,
    ignore_nak_seq: u64,
    retransmit_seq: u64,
    retransmit_now: bool,

    corked_until: Option<Instant>,

    inflight_messages: VecDeque<SentFrame>,
    pending_messages: VecDeque<Vec<u8>>,
}

impl<R: AsyncBufRead + Unpin, W: AsyncWrite + Unpin> TransportState<R, W> {
    fn new(
        rdr: R,
        wr: W,
        data_send: UnboundedSender<Result<Vec<u8>, TransportError>>,
        cmd_recv: UnboundedReceiver<TransportCommand>,
        cancel: CancellationToken,
    ) -> Self {
        Self {
            frdr: FrameReader::default(),
            rdr,
            wr,
            data_send,
            cmd_recv,
            cancel,

            is_synchronized: false,
            rtt_state: RttState::default(),
            receive_sequence: 1,
            send_sequence: 1,
            last_ack_sequence: 0,
            ignore_nak_seq: 0,
            retransmit_seq: 0,
            retransmit_now: false,

            corked_until: None,

            inflight_messages: VecDeque::new(),
            pending_messages: VecDeque::new(),
        }
    }

    async fn run(&mut self) -> Result<(), TransportError> {
        let mut buf = BytesMut::with_capacity(MESSAGE_LENGTH_MAX);
        loop {
            if self.retransmit_now {
                self.retransmit_pending().await?;
            }

            if !self.pending_messages.is_empty() && self.can_send() {
                self.send_more_frames().await?;
            }

            while let Some(frame) = self.frdr.read_frame() {
                self.handle_frame(frame);
            }

            let retransmit_deadline = self
                .inflight_messages
                .front()
                .map(|msg| msg.sent_at + self.rtt_state.rto());
            let retransmit_timeout: futures::future::OptionFuture<_> =
                retransmit_deadline.map(sleep_until).into();
            pin!(retransmit_timeout);

            let corked_timeout: futures::future::OptionFuture<_> =
                self.corked_until.map(sleep_until).into();
            pin!(corked_timeout);

            select! {
                _ = self.rdr.read_buf(&mut buf) => {
                    self.frdr.receive_data(&buf);
                    buf.clear();
                },

                msg = self.cmd_recv.recv() => {
                    match msg {
                        Some(TransportCommand::SendMessage(msg)) => {
                            self.pending_messages.push_back(msg);
                        },
                        Some(TransportCommand::Exit) => {
                            self.cancel.cancel();
                        }
                        None => break,
                    };
                },

                _ = &mut retransmit_timeout, if retransmit_deadline.is_some() => {
                    self.retransmit_now = true;
                },

                _ = &mut corked_timeout, if self.corked_until.is_some() => {
                    // Timeout for when we are able to send again
                }

                _ = self.cancel.cancelled() => {
                    break;
                },
            }
        }
        Ok(())
    }

    /// Handle an incoming frame, by updating sequence numbers and sending the data upwards if needed
    fn handle_frame(&mut self, frame: ReceivedFrame) {
        let rseq = self.receive_sequence;

        // wrap-around logic(?)
        let mut sequence = (rseq & !(MESSAGE_SEQ_MASK as u64)) | (frame.sequence as u64);
        if sequence < rseq {
            sequence += (MESSAGE_SEQ_MASK as u64) + 1;
        }

        // Frame acknowledges some messages
        if !self.is_synchronized || sequence != rseq {
            if sequence > self.send_sequence && self.is_synchronized {
                // Ack for unsent message - weird, but ignore and try to continue
                return;
            }

            self.update_receive_seq(frame.receive_time, sequence);
        }

        if !frame.payload.is_empty() {
            // Data message, we deliver this directly to the application as the MCU can't actually
            // retransmit anyway.
            // TODO: Maybe check the CRC anyway so we can discard it here
            let _ = self.data_send.send(Ok(frame.payload));
        } else if sequence > self.last_ack_sequence {
            // ACK
            self.last_ack_sequence = sequence;
        } else if sequence > self.ignore_nak_seq && !self.inflight_messages.is_empty() {
            // NAK
            self.retransmit_now = true;
        }
    }

    /// Update the last received sequence number, removing acknowledged messages from `self.inflight_messages`
    fn update_receive_seq(&mut self, receive_time: Instant, sequence: u64) {
        let mut sent_seq = self.receive_sequence;

        // Discard messages from inflight_messages up to sequence
        loop {
            if let Some(msg) = self.inflight_messages.pop_front() {
                sent_seq += 1;
                if sequence == sent_seq {
                    // Found the matching sent message
                    if !msg.is_retransmit {
                        let elapsed = receive_time.saturating_duration_since(msg.sent_at);
                        self.rtt_state.update(elapsed);
                    }
                    break;
                }
            } else {
                // Ack with no outstanding messages, happens during connection init
                self.send_sequence = sequence;
                break;
            }
        }

        self.receive_sequence = sequence;
        self.is_synchronized = true;
    }

    fn can_send(&self) -> bool {
        self.corked_until.is_none() && self.inflight_messages.len() < 12
    }

    /// Send as many more frames as possible from [`self.pending_messages`]
    async fn send_more_frames(&mut self) -> Result<(), TransportError> {
        while self.can_send() && !self.pending_messages.is_empty() {
            self.send_new_frame().await?;
        }

        Ok(())
    }

    /// Send a single new frame from [`self.pending_messages`]
    async fn send_new_frame(&mut self) -> Result<(), TransportError> {
        let mut buf = Vec::new();
        while let Some(next) = self.pending_messages.front() {
            if !buf.is_empty() && buf.len() + next.len() <= MESSAGE_LENGTH_PAYLOAD_MAX {
                // Add to the end of the frame. Unwrap is safe because we already peeked.
                let mut next = self.pending_messages.pop_front().unwrap();
                buf.append(&mut next);
            } else {
                break;
            }
        }

        let frame = Arc::new(encode_frame(self.send_sequence, &buf)?);
        self.send_sequence += 1;
        self.inflight_messages.push_back(SentFrame {
            sent_at: Instant::now(),
            sequence: self.send_sequence,
            payload: frame.clone(),
            is_retransmit: false,
        });
        self.wr.write_all(&frame).await?;

        Ok(())
    }

    /// Retransmit all inflight messages
    async fn retransmit_pending(&mut self) -> Result<(), TransportError> {
        let len: usize = self
            .inflight_messages
            .iter()
            .map(|msg| msg.payload.len())
            .sum();
        let mut buf = Vec::with_capacity(1 + len);
        buf.push(MESSAGE_VALUE_SYNC);
        let now = Instant::now();
        for msg in self.inflight_messages.iter_mut() {
            buf.extend_from_slice(&msg.payload);
            msg.is_retransmit = true;
            msg.sent_at = now;
        }
        self.wr.write_all(&buf).await?;

        if self.retransmit_now {
            self.ignore_nak_seq = self.receive_sequence;
            if self.receive_sequence < self.retransmit_seq {
                self.ignore_nak_seq = self.retransmit_seq;
            }
            self.retransmit_now = false;
        } else {
            self.rtt_state.double();
            self.ignore_nak_seq = self.send_sequence;
        }
        self.retransmit_seq = self.send_sequence;

        Ok(())
    }
}

/// An error encountered when transmitting a message
#[derive(thiserror::Error, Debug)]
pub enum TransmitterError {
    #[error("io error: {0}")]
    IoError(#[from] std::io::Error),

    #[error("connection closed")]
    ConnectionClosed,
}

#[derive(Debug, Clone)]
pub(crate) struct SentFrame {
    pub sent_at: Instant,
    #[allow(dead_code)]
    pub sequence: u64,
    pub payload: Arc<Vec<u8>>,
    pub is_retransmit: bool,
}

#[derive(thiserror::Error, Debug)]
pub enum MessageEncodeError {
    #[error("message would exceed the maximum packet length of {MESSAGE_LENGTH_MAX} bytes")]
    MessageTooLong,
}

fn encode_frame(sequence: u64, payload: &[u8]) -> Result<Vec<u8>, MessageEncodeError> {
    let len = MESSAGE_LENGTH_MIN + payload.len();
    if len > MESSAGE_LENGTH_MAX {
        return Err(MessageEncodeError::MessageTooLong);
    }
    let mut buf = Vec::with_capacity(len);
    buf.push(len as u8);
    buf.push(MESSAGE_DEST | ((sequence as u8) & MESSAGE_SEQ_MASK));
    buf.extend_from_slice(payload);
    let crc = crc16(&buf[0..len - MESSAGE_TRAILER_SIZE]);
    buf.push(((crc >> 8) & 0xFF) as u8);
    buf.push((crc & 0xFF) as u8);
    buf.push(MESSAGE_VALUE_SYNC);
    Ok(buf)
}