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

use log::trace;

use futures::channel::mpsc::{unbounded, UnboundedSender};
use futures::{AsyncReadExt, AsyncWriteExt};
use futures::{Stream, 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;
const MAX_CHUNK_LENGTH: ChunkLength = 0x3FF0;
const ERROR_MARKER: ChunkLength = 0x4000;
const CHUNK_HAS_CONTINUATION: ChunkLength = 0x8000;

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

#[pin_project::pin_project]
struct DataReader {
	#[pin]
	reader: AssociatedStream,
	packet: Result<Vec<u8>, u8>,
	pos: usize,
	buf: Vec<u8>,
	eos: bool,
}

impl From<AssociatedStream> for DataReader {
	fn from(data: AssociatedStream) -> DataReader {
		DataReader {
			reader: data,
			packet: Ok(Vec::new()),
			pos: 0,
			buf: Vec::with_capacity(MAX_CHUNK_LENGTH as usize),
			eos: false,
		}
	}
}

enum DataFrame {
	Data {
		/// a fixed size buffer containing some data, possibly padded with 0s
		data: [u8; MAX_CHUNK_LENGTH as usize],
		/// actual lenght of data
		len: usize,
	},
	Error(u8),
}

struct DataReaderItem {
	data: DataFrame,
	/// whethere there may be more data comming from this stream. Can be used for some
	/// optimization. It's an error to set it to false if there is more data, but it is correct
	/// (albeit sub-optimal) to set it to true if there is nothing coming after
	may_have_more: bool,
}

impl DataReaderItem {
	fn empty_last() -> Self {
		DataReaderItem {
			data: DataFrame::Data {
				data: [0; MAX_CHUNK_LENGTH as usize],
				len: 0,
			},
			may_have_more: false,
		}
	}

	fn header(&self) -> [u8; 2] {
		let continuation = if self.may_have_more {
			CHUNK_HAS_CONTINUATION
		} else {
			0
		};
		let len = match self.data {
			DataFrame::Data { len, .. } => len as u16,
			DataFrame::Error(e) => e as u16 | ERROR_MARKER,
		};

		ChunkLength::to_be_bytes(len | continuation)
	}

	fn data(&self) -> &[u8] {
		match self.data {
			DataFrame::Data { ref data, len } => &data[..len],
			DataFrame::Error(_) => &[],
		}
	}
}

impl Stream for DataReader {
	type Item = DataReaderItem;

	fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
		let mut this = self.project();

		if *this.eos {
			// eos was reached at previous call to poll_next, where a partial packet
			// was returned. Now return None
			return Poll::Ready(None);
		}

		loop {
			let packet = match this.packet {
				Ok(v) => v,
				Err(e) => {
					let e = *e;
					*this.packet = Ok(Vec::new());
					return Poll::Ready(Some(DataReaderItem {
						data: DataFrame::Error(e),
						may_have_more: true,
					}));
				}
			};
			let packet_left = packet.len() - *this.pos;
			let buf_left = MAX_CHUNK_LENGTH as usize - this.buf.len();
			let to_read = std::cmp::min(buf_left, packet_left);
			this.buf
				.extend_from_slice(&packet[*this.pos..*this.pos + to_read]);
			*this.pos += to_read;
			if this.buf.len() == MAX_CHUNK_LENGTH as usize {
				// we have a full buf, ready to send
				break;
			}

			// we don't have a full buf, packet is empty; try receive more
			if let Some(p) = futures::ready!(this.reader.as_mut().poll_next(cx)) {
				*this.packet = p;
				*this.pos = 0;
				// if buf is empty, we will loop and return the error directly. If buf
				// isn't empty, send it before by breaking.
				if this.packet.is_err() && !this.buf.is_empty() {
					break;
				}
			} else {
				*this.eos = true;
				break;
			}
		}

		let mut body = [0; MAX_CHUNK_LENGTH as usize];
		let len = this.buf.len();
		body[..len].copy_from_slice(this.buf);
		this.buf.clear();
		Poll::Ready(Some(DataReaderItem {
			data: DataFrame::Data { data: body, len },
			may_have_more: !*this.eos,
		}))
	}
}

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);
	}
	// used only in tests. They should probably be rewriten
	#[allow(dead_code)]
	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())
	}

	// this is like an async fn, but hand implemented
	fn next_ready(&mut self) -> SendQueuePollNextReady<'_> {
		SendQueuePollNextReady { queue: self }
	}
}

struct SendQueuePollNextReady<'a> {
	queue: &'a mut SendQueue,
}

impl<'a> futures::Future for SendQueuePollNextReady<'a> {
	type Output = (RequestID, DataReaderItem);

	fn poll(mut self: Pin<&mut Self>, ctx: &mut Context<'_>) -> Poll<Self::Output> {
		for i in 0..self.queue.items.len() {
			let (_prio, items_at_prio) = &mut self.queue.items[i];

			for _ in 0..items_at_prio.len() {
				let mut item = items_at_prio.pop_front().unwrap();

				match Pin::new(&mut item.data).poll_next(ctx) {
					Poll::Pending => items_at_prio.push_back(item),
					Poll::Ready(Some(data)) => {
						let id = item.id;
						if data.may_have_more {
							self.queue.push(item);
						} else {
							if items_at_prio.is_empty() {
								// this priority level is empty, remove it
								self.queue.items.remove(i);
							}
						}
						return Poll::Ready((id, data));
					}
					Poll::Ready(None) => {
						if items_at_prio.is_empty() {
							// this priority level is empty, remove it
							self.queue.items.remove(i);
						}
						return Poll::Ready((item.id, DataReaderItem::empty_last()));
					}
				}
			}
		}
		// TODO what do we do if self.queue is empty? We won't get scheduled again.
		Poll::Pending
	}
}

/// 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, AssociatedStream)>,
		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() {
			let recv_fut = msg_recv.recv();
			futures::pin_mut!(recv_fut);
			let send_fut = sending.next_ready();

			// recv_fut is cancellation-safe according to tokio doc,
			// send_fut is cancellation-safe as implemented above?
			use futures::future::Either;
			match futures::future::select(recv_fut, send_fut).await {
				Either::Left((sth, _send_fut)) => {
					if let Some((id, prio, data)) = sth {
						sending.push(SendQueueItem {
							id,
							prio,
							data: data.into(),
						});
					} else {
						should_exit = true;
					};
				}
				Either::Right(((id, data), _recv_fut)) => {
					trace!("send_loop: sending bytes for {}", id);

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

					write.write_all(&data.header()).await?;
					write.write_all(data.data()).await?;
					write.flush().await?;
				}
			}
		}

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

pub(crate) struct Framing {
	direct: Vec<u8>,
	stream: Option<AssociatedStream>,
}

impl Framing {
	pub fn new(direct: Vec<u8>, stream: Option<AssociatedStream>) -> Self {
		assert!(direct.len() <= u32::MAX as usize);
		Framing { direct, stream }
	}

	pub fn into_stream(self) -> AssociatedStream {
		use futures::stream;
		let len = self.direct.len() as u32;
		// required because otherwise the borrow-checker complains
		let Framing { direct, stream } = self;

		let res = stream::once(async move { Ok(u32::to_be_bytes(len).to_vec()) })
			.chain(stream::once(async move { Ok(direct) }));

		if let Some(stream) = stream {
			Box::pin(res.chain(stream))
		} else {
			Box::pin(res)
		}
	}

	pub async fn from_stream<S: Stream<Item = Result<Vec<u8>, u8>> + Unpin + Send + 'static>(
		mut stream: S,
	) -> Result<Self, Error> {
		let mut packet = stream
			.next()
			.await
			.ok_or(Error::Framing)?
			.map_err(|_| Error::Framing)?;
		if packet.len() < 4 {
			return Err(Error::Framing);
		}

		let mut len = [0; 4];
		len.copy_from_slice(&packet[..4]);
		let len = u32::from_be_bytes(len);
		packet.drain(..4);

		let mut buffer = Vec::new();
		let len = len as usize;
		loop {
			let max_cp = std::cmp::min(len - buffer.len(), packet.len());

			buffer.extend_from_slice(&packet[..max_cp]);
			if buffer.len() == len {
				packet.drain(..max_cp);
				break;
			}
			packet = stream
				.next()
				.await
				.ok_or(Error::Framing)?
				.map_err(|_| Error::Framing)?;
		}

		let stream: AssociatedStream = if packet.is_empty() {
			Box::pin(stream)
		} else {
			Box::pin(futures::stream::once(async move { Ok(packet) }).chain(stream))
		};

		Ok(Framing {
			direct: buffer,
			stream: Some(stream),
		})
	}

	pub fn into_parts(self) -> (Vec<u8>, AssociatedStream) {
		let Framing { direct, stream } = self;
		(direct, stream.unwrap_or(Box::pin(futures::stream::empty())))
	}
}

/// 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, stream: AssociatedStream);

	async fn recv_loop<R>(self: Arc<Self>, mut read: R) -> Result<(), Error>
	where
		R: AsyncReadExt + Unpin + Send + Sync,
	{
		let mut streams: HashMap<RequestID, UnboundedSender<Result<Vec<u8>, u8>>> = 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 is_error = (size & ERROR_MARKER) != 0;
			let packet = if is_error {
				Err(size as u8)
			} else {
				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());
				Ok(next_slice)
			};

			let sender = if let Some(send) = streams.remove(&(id)) {
				send
			} else {
				let (send, recv) = unbounded();
				self.recv_handler(id, Box::pin(recv));
				send
			};

			// if we get an error, the receiving end is disconnected. We still need to
			// reach eos before dropping this sender
			let _ = sender.unbounded_send(packet);

			if has_cont {
				streams.insert(id, sender);
			}
		}
		Ok(())
	}
}

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

	fn empty_data() -> DataReader {
		type Item = Result<Vec<u8>, u8>;
		let stream: Pin<Box<dyn futures::Stream<Item = Item> + Send + 'static>> =
			Box::pin(futures::stream::empty::<Result<Vec<u8>, u8>>());
		stream.into()
	}

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

		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());
	}
}