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

use log::trace;

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

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;

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_else(|| 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.or_else(|| self.pop())
			}
		}
	}
	fn is_empty(&self) -> bool {
		self.items.iter().all(|(_k, v)| v.is_empty())
	}
}

#[async_trait]
pub(crate) trait SendLoop: Sync {
	async fn send_loop<W>(
		self: Arc<Self>,
		mut msg_recv: mpsc::UnboundedReceiver<Option<(RequestID, RequestPriority, Vec<u8>)>>,
		mut write: W,
	) -> Result<(), Error>
	where
		W: WriteExt + Unpin + Send + Sync,
	{
		let mut sending = SendQueue::new();
		let mut should_exit = false;
		while !should_exit || !sending.is_empty() {
			if let Ok(sth) = msg_recv.try_recv() {
				if let Some((id, prio, data)) = sth {
					trace!("send_loop: got {}, {} bytes", id, data.len());
					sending.push(SendQueueItem {
						id,
						prio,
						data,
						cursor: 0,
					});
				} else {
					should_exit = true;
				}
			} 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);
				write.write_all(&header_id[..]).await?;

				if item.data.len() - item.cursor > MAX_CHUNK_SIZE {
					let header_size = u16::to_be_bytes(MAX_CHUNK_SIZE as u16 | 0x8000);
					write.write_all(&header_size[..]).await?;

					let new_cursor = item.cursor + MAX_CHUNK_SIZE 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 u16;

					let header_size = u16::to_be_bytes(send_len);
					write.write_all(&header_size[..]).await?;

					write.write_all(&item.data[item.cursor..]).await?;
				}
				write.flush().await?;
			} else {
				let sth = msg_recv
					.recv()
					.await
					.ok_or_else(|| Error::Message("Connection closed.".into()))?;
				if let Some((id, prio, data)) = sth {
					trace!("send_loop: got {}, {} bytes", id, data.len());
					sending.push(SendQueueItem {
						id,
						prio,
						data,
						cursor: 0,
					});
				} else {
					should_exit = true;
				}
			}
		}
		Ok(())
	}
}

#[async_trait]
pub(crate) trait RecvLoop: Sync + 'static {
	// Returns true if we should stop receiving after this
	async fn recv_handler(self: Arc<Self>, id: RequestID, msg: Vec<u8>);

	async fn recv_loop<R>(self: Arc<Self>, mut read: R) -> Result<(), Error>
	where
		R: ReadExt + Unpin + Send + Sync,
	{
		let mut receiving = HashMap::new();
		loop {
			trace!("recv_loop: reading packet");
			let mut header_id = [0u8; 2];
			read.read_exact(&mut header_id[..]).await?;
			let id = RequestID::from_be_bytes(header_id);
			trace!("recv_loop: got header id: {:04x}", id);

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

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

			let mut next_slice = vec![0; size as usize];
			read.read_exact(&mut next_slice[..]).await?;
			trace!("recv_loop: read {} bytes", 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 {
				tokio::spawn(self.clone().recv_handler(id, msg_bytes));
			}
		}
	}
}