use std::collections::{HashMap, VecDeque};
use std::fmt::Write;
use std::sync::Arc;

use log::trace;

use futures::{AsyncReadExt, AsyncWriteExt};
use kuska_handshake::async_std::BoxStreamWrite;

use tokio::sync::mpsc;

use async_trait::async_trait;

use crate::error::*;

/// 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 = 0x4000;
const CHUNK_HAS_CONTINUATION: ChunkLength = 0x8000;

struct SendQueueItem {
	id: RequestID,
	prio: RequestPriority,
	data: Vec<u8>,
	cursor: usize,
}

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())
	}
	fn dump(&self) -> String {
		let mut ret = String::new();
		for (prio, q) in self.items.iter() {
			for item in q.iter() {
				write!(
					&mut ret,
					" [{} {} ({})]",
					prio,
					item.data.len() - item.cursor,
					item.id
				)
				.unwrap();
			}
		}
		ret
	}
}

/// 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, Vec<u8>)>,
		mut write: BoxStreamWrite<W>,
		debug_name: String,
	) -> Result<(), Error>
	where
		W: AsyncWriteExt + Unpin + Send + Sync,
	{
		let mut sending = SendQueue::new();
		let mut should_exit = false;
		while !should_exit || !sending.is_empty() {
			trace!("send_loop({}): queue = {}", debug_name, sending.dump());
			if let Ok((id, prio, data)) = msg_recv.try_recv() {
				trace!(
					"send_loop({}): new message to send, id = {}, prio = {}, {} bytes",
					debug_name,
					id,
					prio,
					data.len()
				);
				sending.push(SendQueueItem {
					id,
					prio,
					data,
					cursor: 0,
				});
			} else if let Some(mut item) = sending.pop() {
				trace!(
					"send_loop({}): sending bytes for {} ({} bytes, {} already sent)",
					debug_name,
					item.id,
					item.data.len(),
					item.cursor
				);
				let header_id = RequestID::to_be_bytes(item.id);
				write.write_all(&header_id[..]).await?;

				if item.data.len() - item.cursor > MAX_CHUNK_LENGTH as usize {
					let size_header =
						ChunkLength::to_be_bytes(MAX_CHUNK_LENGTH | CHUNK_HAS_CONTINUATION);
					write.write_all(&size_header[..]).await?;

					let new_cursor = item.cursor + MAX_CHUNK_LENGTH as usize;
					write.write_all(&item.data[item.cursor..new_cursor]).await?;
					item.cursor = new_cursor;

					sending.push(item);
				} else {
					let send_len = (item.data.len() - item.cursor) as ChunkLength;

					let size_header = ChunkLength::to_be_bytes(send_len);
					write.write_all(&size_header[..]).await?;

					write.write_all(&item.data[item.cursor..]).await?;
				}
				write.flush().await?;
			} else {
				let sth = msg_recv.recv().await;
				if let Some((id, prio, data)) = sth {
					trace!(
						"send_loop({}): new message to send, id = {}, prio = {}, {} bytes",
						debug_name,
						id,
						prio,
						data.len()
					);
					sending.push(SendQueueItem {
						id,
						prio,
						data,
						cursor: 0,
					});
				} else {
					should_exit = true;
				}
			}
		}

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

/// 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>);

	async fn recv_loop<R>(self: Arc<Self>, mut read: R, debug_name: String) -> Result<(), Error>
	where
		R: AsyncReadExt + Unpin + Send + Sync,
	{
		let mut receiving = HashMap::new();
		loop {
			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);

			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 id = {}, size = 0x{:04x} ({} bytes)",
				debug_name,
				id,
				size,
				size & !CHUNK_HAS_CONTINUATION
			);

			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", debug_name, next_slice.len());

			let mut msg_bytes: Vec<_> = receiving.remove(&id).unwrap_or_default();
			msg_bytes.extend_from_slice(&next_slice[..]);

			if has_cont {
				receiving.insert(id, msg_bytes);
			} else {
				self.recv_handler(id, msg_bytes);
			}
		}
		Ok(())
	}
}

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

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