// Copyright 2020 Parity Technologies (UK) Ltd.
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

mod protocol;

use crate::{EMPTY_QUEUE_SHRINK_THRESHOLD, RequestId};
use crate::codec::RequestResponseCodec;

pub use protocol::{RequestProtocol, ResponseProtocol, ProtocolSupport};

use futures::{
    channel::oneshot,
    future::BoxFuture,
    prelude::*,
    stream::FuturesUnordered
};
use tetsy_libp2p_core::{
    upgrade::{UpgradeError, NegotiationError},
};
use tetsy_libp2p_swarm::{
    SubstreamProtocol,
    protocols_handler::{
        KeepAlive,
        ProtocolsHandler,
        ProtocolsHandlerEvent,
        ProtocolsHandlerUpgrErr,
    }
};
use smallvec::SmallVec;
use std::{
    collections::VecDeque,
    io,
    sync::{atomic::{AtomicU64, Ordering}, Arc},
    time::Duration,
    task::{Context, Poll}
};
use wasm_timer::Instant;

/// A connection handler of a `RequestResponse` protocol.
#[doc(hidden)]
pub struct RequestResponseHandler<TCodec>
where
    TCodec: RequestResponseCodec,
{
    /// The supported inbound protocols.
    inbound_protocols: SmallVec<[TCodec::Protocol; 2]>,
    /// The request/response message codec.
    codec: TCodec,
    /// The keep-alive timeout of idle connections. A connection is considered
    /// idle if there are no outbound substreams.
    keep_alive_timeout: Duration,
    /// The timeout for inbound and outbound substreams (i.e. request
    /// and response processing).
    substream_timeout: Duration,
    /// The current connection keep-alive.
    keep_alive: KeepAlive,
    /// A pending fatal error that results in the connection being closed.
    pending_error: Option<ProtocolsHandlerUpgrErr<io::Error>>,
    /// Queue of events to emit in `poll()`.
    pending_events: VecDeque<RequestResponseHandlerEvent<TCodec>>,
    /// Outbound upgrades waiting to be emitted as an `OutboundSubstreamRequest`.
    outbound: VecDeque<RequestProtocol<TCodec>>,
    /// Inbound upgrades waiting for the incoming request.
    inbound: FuturesUnordered<BoxFuture<'static,
        Result<
            ((RequestId, TCodec::Request), oneshot::Sender<TCodec::Response>),
            oneshot::Canceled
        >>>,
    inbound_request_id: Arc<AtomicU64>
}

impl<TCodec> RequestResponseHandler<TCodec>
where
    TCodec: RequestResponseCodec,
{
    pub(super) fn new(
        inbound_protocols: SmallVec<[TCodec::Protocol; 2]>,
        codec: TCodec,
        keep_alive_timeout: Duration,
        substream_timeout: Duration,
        inbound_request_id: Arc<AtomicU64>
    ) -> Self {
        Self {
            inbound_protocols,
            codec,
            keep_alive: KeepAlive::Yes,
            keep_alive_timeout,
            substream_timeout,
            outbound: VecDeque::new(),
            inbound: FuturesUnordered::new(),
            pending_events: VecDeque::new(),
            pending_error: None,
            inbound_request_id
        }
    }
}

/// The events emitted by the [`RequestResponseHandler`].
#[doc(hidden)]
#[derive(Debug)]
pub enum RequestResponseHandlerEvent<TCodec>
where
    TCodec: RequestResponseCodec
{
    /// A request has been received.
    Request {
        request_id: RequestId,
        request: TCodec::Request,
        sender: oneshot::Sender<TCodec::Response>
    },
    /// A response has been received.
    Response {
        request_id: RequestId,
        response: TCodec::Response
    },
    /// A response to an inbound request has been sent.
    ResponseSent(RequestId),
    /// A response to an inbound request was omitted as a result
    /// of dropping the response `sender` of an inbound `Request`.
    ResponseOmission(RequestId),
    /// An outbound request timed out while sending the request
    /// or waiting for the response.
    OutboundTimeout(RequestId),
    /// An outbound request failed to negotiate a mutually supported protocol.
    OutboundUnsupportedProtocols(RequestId),
    /// An inbound request timed out while waiting for the request
    /// or sending the response.
    InboundTimeout(RequestId),
    /// An inbound request failed to negotiate a mutually supported protocol.
    InboundUnsupportedProtocols(RequestId),
}

impl<TCodec> ProtocolsHandler for RequestResponseHandler<TCodec>
where
    TCodec: RequestResponseCodec + Send + Clone + 'static,
{
    type InEvent = RequestProtocol<TCodec>;
    type OutEvent = RequestResponseHandlerEvent<TCodec>;
    type Error = ProtocolsHandlerUpgrErr<io::Error>;
    type InboundProtocol = ResponseProtocol<TCodec>;
    type OutboundProtocol = RequestProtocol<TCodec>;
    type OutboundOpenInfo = RequestId;
    type InboundOpenInfo = RequestId;

    fn listen_protocol(&self) -> SubstreamProtocol<Self::InboundProtocol, Self::InboundOpenInfo> {
        // A channel for notifying the handler when the inbound
        // upgrade received the request.
        let (rq_send, rq_recv) = oneshot::channel();

        // A channel for notifying the inbound upgrade when the
        // response is sent.
        let (rs_send, rs_recv) = oneshot::channel();

        let request_id = RequestId(self.inbound_request_id.fetch_add(1, Ordering::Relaxed));

        // By keeping all I/O inside the `ResponseProtocol` and thus the
        // inbound substream upgrade via above channels, we ensure that it
        // is all subject to the configured timeout without extra bookkeeping
        // for inbound substreams as well as their timeouts and also make the
        // implementation of inbound and outbound upgrades symmetric in
        // this sense.
        let proto = ResponseProtocol {
            protocols: self.inbound_protocols.clone(),
            codec: self.codec.clone(),
            request_sender: rq_send,
            response_receiver: rs_recv,
            request_id
        };

        // The handler waits for the request to come in. It then emits
        // `RequestResponseHandlerEvent::Request` together with a
        // `ResponseChannel`.
        self.inbound.push(rq_recv.map_ok(move |rq| (rq, rs_send)).boxed());

        SubstreamProtocol::new(proto, request_id).with_timeout(self.substream_timeout)
    }

    fn inject_fully_negotiated_inbound(
        &mut self,
        sent: bool,
        request_id: RequestId
    ) {
        if sent {
            self.pending_events.push_back(
                RequestResponseHandlerEvent::ResponseSent(request_id))
        } else {
            self.pending_events.push_back(
                RequestResponseHandlerEvent::ResponseOmission(request_id))
        }
    }

    fn inject_fully_negotiated_outbound(
        &mut self,
        response: TCodec::Response,
        request_id: RequestId,
    ) {
        self.pending_events.push_back(
            RequestResponseHandlerEvent::Response {
                request_id, response
            });
    }

    fn inject_event(&mut self, request: Self::InEvent) {
        self.keep_alive = KeepAlive::Yes;
        self.outbound.push_back(request);
    }

    fn inject_dial_upgrade_error(
        &mut self,
        info: RequestId,
        error: ProtocolsHandlerUpgrErr<io::Error>,
    ) {
        match error {
            ProtocolsHandlerUpgrErr::Timeout => {
                self.pending_events.push_back(
                    RequestResponseHandlerEvent::OutboundTimeout(info));
            }
            ProtocolsHandlerUpgrErr::Upgrade(UpgradeError::Select(NegotiationError::Failed)) => {
                // The remote merely doesn't support the protocol(s) we requested.
                // This is no reason to close the connection, which may
                // successfully communicate with other protocols already.
                // An event is reported to permit user code to react to the fact that
                // the remote peer does not support the requested protocol(s).
                self.pending_events.push_back(
                    RequestResponseHandlerEvent::OutboundUnsupportedProtocols(info));
            }
            _ => {
                // Anything else is considered a fatal error or misbehaviour of
                // the remote peer and results in closing the connection.
                self.pending_error = Some(error);
            }
        }
    }

    fn inject_listen_upgrade_error(
        &mut self,
        info: RequestId,
        error: ProtocolsHandlerUpgrErr<io::Error>
    ) {
        match error {
            ProtocolsHandlerUpgrErr::Timeout => {
                self.pending_events.push_back(RequestResponseHandlerEvent::InboundTimeout(info))
            }
            ProtocolsHandlerUpgrErr::Upgrade(UpgradeError::Select(NegotiationError::Failed)) => {
                // The local peer merely doesn't support the protocol(s) requested.
                // This is no reason to close the connection, which may
                // successfully communicate with other protocols already.
                // An event is reported to permit user code to react to the fact that
                // the local peer does not support the requested protocol(s).
                self.pending_events.push_back(
                    RequestResponseHandlerEvent::InboundUnsupportedProtocols(info));
            }
            _ => {
                // Anything else is considered a fatal error or misbehaviour of
                // the remote peer and results in closing the connection.
                self.pending_error = Some(error);
            }
        }
    }

    fn connection_keep_alive(&self) -> KeepAlive {
        self.keep_alive
    }

    fn poll(
        &mut self,
        cx: &mut Context<'_>,
    ) -> Poll<
        ProtocolsHandlerEvent<RequestProtocol<TCodec>, RequestId, Self::OutEvent, Self::Error>,
    > {
        // Check for a pending (fatal) error.
        if let Some(err) = self.pending_error.take() {
            // The handler will not be polled again by the `Swarm`.
            return Poll::Ready(ProtocolsHandlerEvent::Close(err))
        }

        // Drain pending events.
        if let Some(event) = self.pending_events.pop_front() {
            return Poll::Ready(ProtocolsHandlerEvent::Custom(event))
        } else if self.pending_events.capacity() > EMPTY_QUEUE_SHRINK_THRESHOLD {
            self.pending_events.shrink_to_fit();
        }

        // Check for inbound requests.
        while let Poll::Ready(Some(result)) = self.inbound.poll_next_unpin(cx) {
            match result {
                Ok(((id, rq), rs_sender)) => {
                    // We received an inbound request.
                    self.keep_alive = KeepAlive::Yes;
                    return Poll::Ready(ProtocolsHandlerEvent::Custom(
                        RequestResponseHandlerEvent::Request {
                            request_id: id, request: rq, sender: rs_sender
                        }))
                }
                Err(oneshot::Canceled) => {
                    // The inbound upgrade has errored or timed out reading
                    // or waiting for the request. The handler is informed
                    // via `inject_listen_upgrade_error`.
                }
            }
        }

        // Emit outbound requests.
        if let Some(request) = self.outbound.pop_front() {
            let info = request.request_id;
            return Poll::Ready(
                ProtocolsHandlerEvent::OutboundSubstreamRequest {
                    protocol: SubstreamProtocol::new(request, info)
                        .with_timeout(self.substream_timeout)
                },
            )
        }

        debug_assert!(self.outbound.is_empty());

        if self.outbound.capacity() > EMPTY_QUEUE_SHRINK_THRESHOLD {
            self.outbound.shrink_to_fit();
        }

        if self.inbound.is_empty() && self.keep_alive.is_yes() {
            // No new inbound or outbound requests. However, we may just have
            // started the latest inbound or outbound upgrade(s), so make sure
            // the keep-alive timeout is preceded by the substream timeout.
            let until = Instant::now() + self.substream_timeout + self.keep_alive_timeout;
            self.keep_alive = KeepAlive::Until(until);
        }

        Poll::Pending
    }
}