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

use log::trace;

use async_trait::async_trait;

use async_std::io::prelude::WriteExt;
use async_std::io::ReadExt;

use tokio::io::{ReadHalf, WriteHalf};
use tokio::net::TcpStream;
use tokio::sync::{mpsc, watch};

use crate::error::*;

use kuska_handshake::async_std::{BoxStreamRead, BoxStreamWrite, TokioCompat};

/// 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;

const MAX_CHUNK_SIZE: usize = 0x4000;

pub(crate) type RequestID = u16;

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

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

impl SendQueue {
	fn new() -> Self {
		Self {
			items: BTreeMap::new(),
		}
	}
	fn push(&mut self, item: SendQueueItem) {
		let prio = item.prio;
		let mut items_at_prio = self
			.items
			.remove(&prio)
			.unwrap_or(VecDeque::with_capacity(4));
		items_at_prio.push_back(item);
		self.items.insert(prio, items_at_prio);
	}
	fn pop(&mut self) -> Option<SendQueueItem> {
		match self.items.pop_first() {
			None => None,
			Some((prio, mut items_at_prio)) => {
				let ret = items_at_prio.pop_front();
				if !items_at_prio.is_empty() {
					self.items.insert(prio, items_at_prio);
				}
				ret
			}
		}
	}
}

#[async_trait]
pub(crate) trait SendLoop: Sync {
	async fn send_loop(
		self: Arc<Self>,
		mut msg_recv: mpsc::UnboundedReceiver<(RequestID, RequestPriority, Vec<u8>)>,
		mut write: BoxStreamWrite<TokioCompat<WriteHalf<TcpStream>>>,
		mut must_exit: watch::Receiver<bool>,
	) -> Result<(), Error> {
		let mut sending = SendQueue::new();
		while !*must_exit.borrow() {
			if let Ok((id, prio, data)) = msg_recv.try_recv() {
				trace!("send_loop: got {}, {} bytes", id, 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)",
					item.id,
					item.data.len(),
					item.cursor
				);
				let header_id = u16::to_be_bytes(item.id);
				if write_all_or_exit(&header_id[..], &mut write, &mut must_exit)
					.await?
					.is_none()
				{
					break;
				}

				if item.data.len() - item.cursor > MAX_CHUNK_SIZE {
					let header_size = u16::to_be_bytes(MAX_CHUNK_SIZE as u16 | 0x8000);
					if write_all_or_exit(&header_size[..], &mut write, &mut must_exit)
						.await?
						.is_none()
					{
						break;
					}

					let new_cursor = item.cursor + MAX_CHUNK_SIZE as usize;
					if write_all_or_exit(
						&item.data[item.cursor..new_cursor],
						&mut write,
						&mut must_exit,
					)
					.await?
					.is_none()
					{
						break;
					}
					item.cursor = new_cursor;

					sending.push(item);
				} else {
					let send_len = (item.data.len() - item.cursor) as u16;

					let header_size = u16::to_be_bytes(send_len);
					if write_all_or_exit(&header_size[..], &mut write, &mut must_exit)
						.await?
						.is_none()
					{
						break;
					}

					if write_all_or_exit(&item.data[item.cursor..], &mut write, &mut must_exit)
						.await?
						.is_none()
					{
						break;
					}
				}
				write.flush().await.log_err("Could not flush in send_loop");
			} else {
				let (id, prio, data) = msg_recv
					.recv()
					.await
					.ok_or(Error::Message("Connection closed.".into()))?;
				trace!("send_loop: got {}, {} bytes", id, data.len());
				sending.push(SendQueueItem {
					id,
					prio,
					data,
					cursor: 0,
				});
			}
		}
		Ok(())
	}
}

#[async_trait]
pub(crate) trait RecvLoop: Sync + 'static {
	async fn recv_handler(self: Arc<Self>, id: RequestID, msg: Vec<u8>);

	async fn recv_loop(
		self: Arc<Self>,
		mut read: BoxStreamRead<TokioCompat<ReadHalf<TcpStream>>>,
		mut must_exit: watch::Receiver<bool>,
	) -> Result<(), Error> {
		let mut receiving = HashMap::new();
		while !*must_exit.borrow() {
			trace!("recv_loop: reading packet");
			let mut header_id = [0u8; 2];
			if read_exact_or_exit(&mut header_id[..], &mut read, &mut must_exit)
				.await?
				.is_none()
			{
				break;
			}
			let id = RequestID::from_be_bytes(header_id);
			trace!("recv_loop: got header id: {:04x}", id);

			let mut header_size = [0u8; 2];
			if read_exact_or_exit(&mut header_size[..], &mut read, &mut must_exit)
				.await?
				.is_none()
			{
				break;
			}
			let size = RequestID::from_be_bytes(header_size);
			trace!("recv_loop: got header size: {:04x}", id);

			let has_cont = (size & 0x8000) != 0;
			let size = size & !0x8000;

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

			let mut msg_bytes = receiving.remove(&id).unwrap_or(vec![]);
			msg_bytes.extend_from_slice(&next_slice[..]);

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

async fn read_exact_or_exit(
	buf: &mut [u8],
	read: &mut BoxStreamRead<TokioCompat<ReadHalf<TcpStream>>>,
	must_exit: &mut watch::Receiver<bool>,
) -> Result<Option<()>, Error> {
	tokio::select!(
		res = read.read_exact(buf) => Ok(Some(res?)),
		_ = await_exit(must_exit) => Ok(None),
	)
}

async fn write_all_or_exit(
	buf: &[u8],
	write: &mut BoxStreamWrite<TokioCompat<WriteHalf<TcpStream>>>,
	must_exit: &mut watch::Receiver<bool>,
) -> Result<Option<()>, Error> {
	tokio::select!(
		res = write.write_all(buf) => Ok(Some(res?)),
		_ = await_exit(must_exit) => Ok(None),
	)
}

async fn await_exit(must_exit: &mut watch::Receiver<bool>) {
	loop {
		if must_exit.recv().await == Some(true) {
			return;
		}
	}
}