use std::collections::{HashMap, VecDeque};
use std::pin::Pin;
use std::sync::Arc;
use std::task::{Context, Poll};

use log::{trace, warn};

use futures::channel::mpsc::{unbounded, UnboundedReceiver, UnboundedSender};
use futures::Stream;
use futures::{AsyncReadExt, AsyncWriteExt, FutureExt, StreamExt};
use kuska_handshake::async_std::BoxStreamWrite;

use tokio::sync::mpsc;

use async_trait::async_trait;

use crate::error::*;
use crate::util::AssociatedStream;

/// Priority of a request (click to read more about priorities).
///
/// This priority value is used to priorize messages
/// in the send queue of the client, and their responses in the send queue of the
/// server. Lower values mean higher priority.
///
/// This mechanism is usefull for messages bigger than the maximum chunk size
/// (set at `0x4000` bytes), such as large file transfers.
/// In such case, all of the messages in the send queue with the highest priority
/// will take turns to send individual chunks, in a round-robin fashion.
/// Once all highest priority messages are sent successfully, the messages with
/// the next highest priority will begin being sent in the same way.
///
/// The same priority value is given to a request and to its associated response.
pub type RequestPriority = u8;

/// Priority class: high
pub const PRIO_HIGH: RequestPriority = 0x20;
/// Priority class: normal
pub const PRIO_NORMAL: RequestPriority = 0x40;
/// Priority class: background
pub const PRIO_BACKGROUND: RequestPriority = 0x80;
/// Priority: primary among given class
pub const PRIO_PRIMARY: RequestPriority = 0x00;
/// Priority: secondary among given class (ex: `PRIO_HIGH | PRIO_SECONDARY`)
pub const PRIO_SECONDARY: RequestPriority = 0x01;

// Messages are sent by chunks
// Chunk format:
// - u32 BE: request id (same for request and response)
// - u16 BE: chunk length, possibly with CHUNK_HAS_CONTINUATION flag
//					when this is not the last chunk of the message
// - [u8; chunk_length] chunk data

pub(crate) type RequestID = u32;
type ChunkLength = u16;
pub(crate) const MAX_CHUNK_LENGTH: ChunkLength = 0x4000;
const CHUNK_HAS_CONTINUATION: ChunkLength = 0x8000;

struct SendQueueItem {
	id: RequestID,
	prio: RequestPriority,
	data: DataReader,
}

pub(crate) enum Data {
	Full(Vec<u8>),
	Streaming(AssociatedStream),
}

#[pin_project::pin_project(project = DataReaderProj)]
enum DataReader {
	Full {
		#[pin]
		data: Vec<u8>,
		pos: usize,
	},
	Streaming {
		#[pin]
		reader: AssociatedStream,
	},
}

impl From<Data> for DataReader {
	fn from(data: Data) -> DataReader {
		match data {
			Data::Full(data) => DataReader::Full { data, pos: 0 },
			Data::Streaming(reader) => DataReader::Streaming { reader },
		}
	}
}

impl Stream for DataReader {
	type Item = ([u8; MAX_CHUNK_LENGTH as usize], usize);

	fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
		match self.project() {
			DataReaderProj::Full { data, pos } => {
				let len = std::cmp::min(MAX_CHUNK_LENGTH as usize, data.len() - *pos);
				let end = *pos + len;

				if len == 0 {
					Poll::Ready(None)
				} else {
					let mut body = [0; MAX_CHUNK_LENGTH as usize];
					body[..len].copy_from_slice(&data[*pos..end]);
					*pos = end;
					Poll::Ready(Some((body, len)))
				}
			}
			DataReaderProj::Streaming { reader } => {
				reader.poll_next(cx).map(|opt| {
					opt.map(|v| {
						let mut body = [0; MAX_CHUNK_LENGTH as usize];
						let len = std::cmp::min(MAX_CHUNK_LENGTH as usize, v.len());
						// TODO this can throw away long vec, they should be splited instead
						body[..len].copy_from_slice(&v[..len]);
						(body, len)
					})
				})
			}
		}
	}
}

struct SendQueue {
	items: VecDeque<(u8, VecDeque<SendQueueItem>)>,
}

impl SendQueue {
	fn new() -> Self {
		Self {
			items: VecDeque::with_capacity(64),
		}
	}
	fn push(&mut self, item: SendQueueItem) {
		let prio = item.prio;
		let pos_prio = match self.items.binary_search_by(|(p, _)| p.cmp(&prio)) {
			Ok(i) => i,
			Err(i) => {
				self.items.insert(i, (prio, VecDeque::new()));
				i
			}
		};
		self.items[pos_prio].1.push_back(item);
	}
	fn pop(&mut self) -> Option<SendQueueItem> {
		match self.items.pop_front() {
			None => None,
			Some((prio, mut items_at_prio)) => {
				let ret = items_at_prio.pop_front();
				if !items_at_prio.is_empty() {
					self.items.push_front((prio, items_at_prio));
				}
				ret.or_else(|| self.pop())
			}
		}
	}
	fn is_empty(&self) -> bool {
		self.items.iter().all(|(_k, v)| v.is_empty())
	}
}

/// The SendLoop trait, which is implemented both by the client and the server
/// connection objects (ServerConna and ClientConn) adds a method `.send_loop()`
/// that takes a channel of messages to send and an asynchronous writer,
/// and sends messages from the channel to the async writer, putting them in a queue
/// before being sent and doing the round-robin sending strategy.
///
/// The `.send_loop()` exits when the sending end of the channel is closed,
/// or if there is an error at any time writing to the async writer.
#[async_trait]
pub(crate) trait SendLoop: Sync {
	async fn send_loop<W>(
		self: Arc<Self>,
		mut msg_recv: mpsc::UnboundedReceiver<(RequestID, RequestPriority, Data)>,
		mut write: BoxStreamWrite<W>,
	) -> Result<(), Error>
	where
		W: AsyncWriteExt + Unpin + Send + Sync,
	{
		let mut sending = SendQueue::new();
		let mut should_exit = false;
		while !should_exit || !sending.is_empty() {
			if let Ok((id, prio, data)) = msg_recv.try_recv() {
				match &data {
					Data::Full(data) => {
						trace!("send_loop: got {}, {} bytes", id, data.len());
					}
					Data::Streaming(_) => {
						trace!("send_loop: got {}, unknown size", id);
					}
				}
				sending.push(SendQueueItem {
					id,
					prio,
					data: data.into(),
				});
			} else if let Some(mut item) = sending.pop() {
				trace!(
					"send_loop: sending bytes for {}",
                    item.id,
				);

				let data = futures::select! {
					data = item.data.next().fuse() => data,
					default => {
						// nothing to send yet; re-schedule and find something else to do
						sending.push(item);
						continue;

						// TODO if every SendQueueItem is waiting on data, use select_all to await
						// something to do
						// TODO find some way to not require sending empty last chunk
					}
				};

				let header_id = RequestID::to_be_bytes(item.id);
				write.write_all(&header_id[..]).await?;

				let data = match data.as_ref() {
					Some((data, len)) => &data[..*len],
					None => &[],
				};

				if !data.is_empty() {
					let size_header =
						ChunkLength::to_be_bytes(data.len() as u16 | CHUNK_HAS_CONTINUATION);
					write.write_all(&size_header[..]).await?;

					write.write_all(data).await?;

					sending.push(item);
				} else {
					// this is always zero for now, but may be more when above TODO get fixed
					let size_header = ChunkLength::to_be_bytes(data.len() as u16);
					write.write_all(&size_header[..]).await?;

					write.write_all(data).await?;
				}

				write.flush().await?;
			} else {
				let sth = msg_recv.recv().await;
				if let Some((id, prio, data)) = sth {
					match &data {
						Data::Full(data) => {
							trace!("send_loop: got {}, {} bytes", id, data.len());
						}
						Data::Streaming(_) => {
							trace!("send_loop: got {}, unknown size", id);
						}
					}
					sending.push(SendQueueItem {
						id,
						prio,
						data: data.into(),
					});
				} else {
					should_exit = true;
				}
			}
		}

		let _ = write.goodbye().await;
		Ok(())
	}
}

struct ChannelPair {
        receiver: Option<UnboundedReceiver<Vec<u8>>>,
        sender: Option<UnboundedSender<Vec<u8>>>,
}

impl ChannelPair {
    fn take_receiver(&mut self) -> Option<UnboundedReceiver<Vec<u8>>> {
        self.receiver.take()
    }

    fn take_sender(&mut self) -> Option<UnboundedSender<Vec<u8>>> {
        self.sender.take()
    }

    fn ref_sender(&mut self) -> Option<&UnboundedSender<Vec<u8>>> {
        self.sender.as_ref().take()
    }

    fn insert_into(self, map: &mut HashMap<RequestID, ChannelPair>, index: RequestID) {
        if self.receiver.is_some() || self.sender.is_some() {
            map.insert(index, self);
        }
    }
}

impl Default for ChannelPair {
    fn default() -> Self {
        let (send, recv) = unbounded();
        ChannelPair {
            receiver: Some(recv),
            sender: Some(send),
        }
    }
}

/// The RecvLoop trait, which is implemented both by the client and the server
/// connection objects (ServerConn and ClientConn) adds a method `.recv_loop()`
/// and a prototype of a handler for received messages `.recv_handler()` that
/// must be filled by implementors. `.recv_loop()` receives messages in a loop
/// according to the protocol defined above: chunks of message in progress of being
/// received are stored in a buffer, and when the last chunk of a message is received,
/// the full message is passed to the receive handler.
#[async_trait]
pub(crate) trait RecvLoop: Sync + 'static {
	fn recv_handler(self: &Arc<Self>, id: RequestID, msg: Vec<u8>, stream: AssociatedStream);

	async fn recv_loop<R>(self: Arc<Self>, mut read: R) -> Result<(), Error>
	where
		R: AsyncReadExt + Unpin + Send + Sync,
	{
		let mut receiving: HashMap<RequestID, Vec<u8>> = HashMap::new();
		let mut streams: HashMap<
			RequestID,
            ChannelPair,
		> = HashMap::new();
		loop {
			trace!("recv_loop: reading packet");
			let mut header_id = [0u8; RequestID::BITS as usize / 8];
			match read.read_exact(&mut header_id[..]).await {
				Ok(_) => (),
				Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => break,
				Err(e) => return Err(e.into()),
			};
			let id = RequestID::from_be_bytes(header_id);
			trace!("recv_loop: got header id: {:04x}", id);

			let mut header_size = [0u8; ChunkLength::BITS as usize / 8];
			read.read_exact(&mut header_size[..]).await?;
			let size = ChunkLength::from_be_bytes(header_size);
			trace!("recv_loop: got header size: {:04x}", size);

			let has_cont = (size & CHUNK_HAS_CONTINUATION) != 0;
			let size = size & !CHUNK_HAS_CONTINUATION;

			let mut next_slice = vec![0; size as usize];
			read.read_exact(&mut next_slice[..]).await?;
			trace!("recv_loop: read {} bytes", next_slice.len());

			if id & 1 == 0 {
                // main stream
				let mut msg_bytes = receiving.remove(&id).unwrap_or_default();
				msg_bytes.extend_from_slice(&next_slice[..]);

				if has_cont {
					receiving.insert(id, msg_bytes);
				} else {
					let mut channel_pair = streams.remove(&(id | 1)).unwrap_or_default();

					if let Some(receiver) = channel_pair.take_receiver() {
						self.recv_handler(id, msg_bytes, Box::pin(receiver));
					} else {
                        warn!("Couldn't take receiver part of stream")
                    }

                    channel_pair.insert_into(&mut streams, id | 1);
				}
			} else {
                // associated stream
                let mut channel_pair = streams.remove(&(id)).unwrap_or_default();

				// if we get an error, the receiving end is disconnected. We still need to
				// reach eos before dropping this sender
                if !next_slice.is_empty() {
                    if let Some(sender) = channel_pair.ref_sender() {
                        let _ = sender.unbounded_send(next_slice);
                    } else {
                        warn!("Couldn't take sending part of stream")
                    }
                }

				if !has_cont {
                    channel_pair.take_sender();
                }

                channel_pair.insert_into(&mut streams, id);
			}
		}
		Ok(())
	}
}

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

	#[test]
	fn test_priority_queue() {
		let i1 = SendQueueItem {
			id: 1,
			prio: PRIO_NORMAL,
			data: DataReader::Full {
				data: vec![],
				pos: 0,
			},
		};
		let i2 = SendQueueItem {
			id: 2,
			prio: PRIO_HIGH,
			data: DataReader::Full {
				data: vec![],
				pos: 0,
			},
		};
		let i2bis = SendQueueItem {
			id: 20,
			prio: PRIO_HIGH,
			data: DataReader::Full {
				data: vec![],
				pos: 0,
			},
		};
		let i3 = SendQueueItem {
			id: 3,
			prio: PRIO_HIGH | PRIO_SECONDARY,
			data: DataReader::Full {
				data: vec![],
				pos: 0,
			},
		};
		let i4 = SendQueueItem {
			id: 4,
			prio: PRIO_BACKGROUND | PRIO_SECONDARY,
			data: DataReader::Full {
				data: vec![],
				pos: 0,
			},
		};
		let i5 = SendQueueItem {
			id: 5,
			prio: PRIO_BACKGROUND | PRIO_PRIMARY,
			data: DataReader::Full {
				data: vec![],
				pos: 0,
			},
		};

		let mut q = SendQueue::new();

		q.push(i1); // 1
		let a = q.pop().unwrap(); // empty -> 1
		assert_eq!(a.id, 1);
		assert!(q.pop().is_none());

		q.push(a); // 1
		q.push(i2); // 2 1
		q.push(i2bis); // [2 20] 1
		let a = q.pop().unwrap(); // 20 1 -> 2
		assert_eq!(a.id, 2);
		let b = q.pop().unwrap(); // 1 -> 20
		assert_eq!(b.id, 20);
		let c = q.pop().unwrap(); // empty -> 1
		assert_eq!(c.id, 1);
		assert!(q.pop().is_none());

		q.push(a); // 2
		q.push(b); // [2 20]
		q.push(c); // [2 20] 1
		q.push(i3); // [2 20] 3 1
		q.push(i4); // [2 20] 3 1 4
		q.push(i5); // [2 20] 3 1 5 4

		let a = q.pop().unwrap(); // 20 3 1 5 4 -> 2
		assert_eq!(a.id, 2);
		q.push(a); // [20 2] 3 1 5 4

		let a = q.pop().unwrap(); // 2 3 1 5 4 -> 20
		assert_eq!(a.id, 20);
		let b = q.pop().unwrap(); // 3 1 5 4 -> 2
		assert_eq!(b.id, 2);
		q.push(b); // 2 3 1 5 4
		let b = q.pop().unwrap(); // 3 1 5 4 -> 2
		assert_eq!(b.id, 2);
		let c = q.pop().unwrap(); // 1 5 4 -> 3
		assert_eq!(c.id, 3);
		q.push(b); // 2 1 5 4
		let b = q.pop().unwrap(); // 1 5 4 -> 2
		assert_eq!(b.id, 2);
		let e = q.pop().unwrap(); // 5 4 -> 1
		assert_eq!(e.id, 1);
		let f = q.pop().unwrap(); // 4 -> 5
		assert_eq!(f.id, 5);
		let g = q.pop().unwrap(); // empty -> 4
		assert_eq!(g.id, 4);
		assert!(q.pop().is_none());
	}
}