// Copyright (C) 2018-2019, Cloudflare, Inc.
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright notice,
//       this list of conditions and the following disclaimer.
//
//     * Redistributions in binary form must reproduce the above copyright
//       notice, this list of conditions and the following disclaimer in the
//       documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
// IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
// THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
// PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
// CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
// EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

use std::cmp;

use std::str::FromStr;

use std::time::Duration;
use std::time::Instant;

use std::collections::VecDeque;

use crate::Config;
use crate::Error;
use crate::Result;

use crate::frame;
use crate::minmax;
use crate::packet;
use crate::ranges;

// Loss Recovery
const INITIAL_PACKET_THRESHOLD: u64 = 3;

const MAX_PACKET_THRESHOLD: u64 = 20;

const INITIAL_TIME_THRESHOLD: f64 = 9.0 / 8.0;

const GRANULARITY: Duration = Duration::from_millis(1);

const INITIAL_RTT: Duration = Duration::from_millis(333);

const PERSISTENT_CONGESTION_THRESHOLD: u32 = 3;

const RTT_WINDOW: Duration = Duration::from_secs(300);

const MAX_PTO_PROBES_COUNT: usize = 2;

// Congestion Control
const INITIAL_WINDOW_PACKETS: usize = 10;

const MINIMUM_WINDOW_PACKETS: usize = 2;

const LOSS_REDUCTION_FACTOR: f64 = 0.5;

pub struct Recovery {
    loss_detection_timer: Option<Instant>,

    pto_count: u32,

    time_of_last_sent_ack_eliciting_pkt: [Option<Instant>; packet::EPOCH_COUNT],

    largest_acked_pkt: [u64; packet::EPOCH_COUNT],

    largest_sent_pkt: [u64; packet::EPOCH_COUNT],

    latest_rtt: Duration,

    smoothed_rtt: Option<Duration>,

    rttvar: Duration,

    minmax_filter: minmax::Minmax<Duration>,

    min_rtt: Duration,

    pub max_ack_delay: Duration,

    loss_time: [Option<Instant>; packet::EPOCH_COUNT],

    sent: [VecDeque<Sent>; packet::EPOCH_COUNT],

    pub lost: [Vec<frame::Frame>; packet::EPOCH_COUNT],

    pub acked: [Vec<frame::Frame>; packet::EPOCH_COUNT],

    pub lost_count: usize,

    pub lost_spurious_count: usize,

    pub loss_probes: [usize; packet::EPOCH_COUNT],

    in_flight_count: [usize; packet::EPOCH_COUNT],

    app_limited: bool,

    delivery_rate: delivery_rate::Rate,

    pkt_thresh: u64,

    time_thresh: f64,

    // Congestion control.
    cc_ops: &'static CongestionControlOps,

    congestion_window: usize,

    bytes_in_flight: usize,

    ssthresh: usize,

    bytes_acked_sl: usize,

    bytes_acked_ca: usize,

    bytes_sent: usize,

    pub bytes_lost: u64,

    congestion_recovery_start_time: Option<Instant>,

    max_datagram_size: usize,

    cubic_state: cubic::State,

    // HyStart++.
    hystart: hystart::Hystart,

    // Pacing.
    pacing_rate: u64,

    last_packet_scheduled_time: Option<Instant>,

    // RFC6937 PRR.
    prr: prr::PRR,
}

impl Recovery {
    pub fn new(config: &Config) -> Self {
        Recovery {
            loss_detection_timer: None,

            pto_count: 0,

            time_of_last_sent_ack_eliciting_pkt: [None; packet::EPOCH_COUNT],

            largest_acked_pkt: [std::u64::MAX; packet::EPOCH_COUNT],

            largest_sent_pkt: [0; packet::EPOCH_COUNT],

            latest_rtt: Duration::new(0, 0),

            // This field should be initialized to `INITIAL_RTT` for the initial
            // PTO calculation, but it also needs to be an `Option` to track
            // whether any RTT sample was received, so the initial value is
            // handled by the `rtt()` method instead.
            smoothed_rtt: None,

            minmax_filter: minmax::Minmax::new(Duration::new(0, 0)),

            min_rtt: Duration::new(0, 0),

            rttvar: INITIAL_RTT / 2,

            max_ack_delay: Duration::new(0, 0),

            loss_time: [None; packet::EPOCH_COUNT],

            sent: [VecDeque::new(), VecDeque::new(), VecDeque::new()],

            lost: [Vec::new(), Vec::new(), Vec::new()],

            acked: [Vec::new(), Vec::new(), Vec::new()],

            lost_count: 0,
            lost_spurious_count: 0,

            loss_probes: [0; packet::EPOCH_COUNT],

            in_flight_count: [0; packet::EPOCH_COUNT],

            congestion_window: config.max_send_udp_payload_size *
                INITIAL_WINDOW_PACKETS,

            pkt_thresh: INITIAL_PACKET_THRESHOLD,

            time_thresh: INITIAL_TIME_THRESHOLD,

            bytes_in_flight: 0,

            ssthresh: std::usize::MAX,

            bytes_acked_sl: 0,

            bytes_acked_ca: 0,

            bytes_sent: 0,

            bytes_lost: 0,

            congestion_recovery_start_time: None,

            max_datagram_size: config.max_send_udp_payload_size,

            cc_ops: config.cc_algorithm.into(),

            delivery_rate: delivery_rate::Rate::default(),

            cubic_state: cubic::State::default(),

            app_limited: false,

            hystart: hystart::Hystart::new(config.hystart),

            pacing_rate: 0,

            last_packet_scheduled_time: None,

            prr: prr::PRR::default(),
        }
    }

    pub fn on_packet_sent(
        &mut self, mut pkt: Sent, epoch: packet::Epoch,
        handshake_status: HandshakeStatus, now: Instant, trace_id: &str,
    ) {
        let ack_eliciting = pkt.ack_eliciting;
        let in_flight = pkt.in_flight;
        let sent_bytes = pkt.size;
        let pkt_num = pkt.pkt_num;

        self.delivery_rate.on_packet_sent(&mut pkt, now);

        self.largest_sent_pkt[epoch] =
            cmp::max(self.largest_sent_pkt[epoch], pkt_num);

        self.sent[epoch].push_back(pkt);

        if in_flight {
            if ack_eliciting {
                self.time_of_last_sent_ack_eliciting_pkt[epoch] = Some(now);
            }

            self.in_flight_count[epoch] += 1;

            self.app_limited =
                (self.bytes_in_flight + sent_bytes) < self.congestion_window;

            self.on_packet_sent_cc(sent_bytes, now);

            self.prr.on_packet_sent(sent_bytes);

            self.set_loss_detection_timer(handshake_status, now);
        }

        // HyStart++: Start of the round in a slow start.
        if self.hystart.enabled() &&
            epoch == packet::EPOCH_APPLICATION &&
            self.congestion_window < self.ssthresh
        {
            self.hystart.start_round(pkt_num);
        }

        // Pacing: Set the pacing rate if CC doesn't do its own.
        if !(self.cc_ops.has_custom_pacing)() {
            if let Some(srtt) = self.smoothed_rtt {
                let rate = (self.congestion_window as u64 * 1000000) /
                    srtt.as_micros() as u64;
                self.set_pacing_rate(rate);
            }
        }

        self.schedule_next_packet(epoch, now, sent_bytes);

        self.bytes_sent += sent_bytes;
        trace!("{} {:?}", trace_id, self);
    }

    fn on_packet_sent_cc(&mut self, sent_bytes: usize, now: Instant) {
        (self.cc_ops.on_packet_sent)(self, sent_bytes, now);
    }

    pub fn set_pacing_rate(&mut self, rate: u64) {
        if rate != 0 {
            self.pacing_rate = rate;
        }
    }

    pub fn get_packet_send_time(&self) -> Option<Instant> {
        self.last_packet_scheduled_time
    }

    fn schedule_next_packet(
        &mut self, epoch: packet::Epoch, now: Instant, packet_size: usize,
    ) {
        // Don't pace in any of these cases:
        //   * Packet epoch is not EPOCH_APPLICATION.
        //   * Packet contains only ACK frames.
        //   * The start of the connection.
        if epoch != packet::EPOCH_APPLICATION ||
            packet_size == 0 ||
            self.bytes_sent <= self.congestion_window ||
            self.pacing_rate == 0
        {
            self.last_packet_scheduled_time = Some(now);
            return;
        }

        self.last_packet_scheduled_time = match self.last_packet_scheduled_time {
            Some(last_scheduled_time) => {
                let interval: u64 =
                    (packet_size as u64 * 1000000) / self.pacing_rate;
                let interval = Duration::from_micros(interval);
                let next_schedule_time = last_scheduled_time + interval;
                Some(cmp::max(now, next_schedule_time))
            },

            None => Some(now),
        };
    }

    pub fn on_ack_received(
        &mut self, ranges: &ranges::RangeSet, ack_delay: u64,
        epoch: packet::Epoch, handshake_status: HandshakeStatus, now: Instant,
        trace_id: &str,
    ) -> Result<()> {
        let largest_acked = ranges.last().unwrap();

        // If the largest packet number acked exceeds any packet number we have
        // sent, then the ACK is obviously invalid, so there's no need to
        // continue further.
        if largest_acked > self.largest_sent_pkt[epoch] {
            if cfg!(feature = "fuzzing") {
                return Ok(());
            }

            return Err(Error::InvalidPacket);
        }

        if self.largest_acked_pkt[epoch] == std::u64::MAX {
            self.largest_acked_pkt[epoch] = largest_acked;
        } else {
            self.largest_acked_pkt[epoch] =
                cmp::max(self.largest_acked_pkt[epoch], largest_acked);
        }

        let mut has_ack_eliciting = false;

        let mut largest_newly_acked_pkt_num = 0;
        let mut largest_newly_acked_sent_time = now;

        let mut newly_acked = Vec::new();

        let mut undo_cwnd = false;

        let max_rtt = cmp::max(self.latest_rtt, self.rtt());

        // Detect and mark acked packets, without removing them from the sent
        // packets list.
        for r in ranges.iter() {
            let lowest_acked_in_block = r.start;
            let largest_acked_in_block = r.end - 1;

            let unacked_iter = self.sent[epoch]
                .iter_mut()
                // Skip packets that precede the lowest acked packet in the block.
                .skip_while(|p| p.pkt_num < lowest_acked_in_block)
                // Skip packets that follow the largest acked packet in the block.
                .take_while(|p| p.pkt_num <= largest_acked_in_block)
                // Skip packets that have already been acked or lost.
                .filter(|p| p.time_acked.is_none());

            for unacked in unacked_iter {
                unacked.time_acked = Some(now);

                // Check if acked packet was already declared lost.
                if unacked.time_lost.is_some() {
                    // Calculate new packet reordering threshold.
                    let pkt_thresh =
                        self.largest_acked_pkt[epoch] - unacked.pkt_num + 1;
                    let pkt_thresh = cmp::min(MAX_PACKET_THRESHOLD, pkt_thresh);

                    self.pkt_thresh = cmp::max(self.pkt_thresh, pkt_thresh);

                    // Calculate new time reordering threshold.
                    let loss_delay = max_rtt.mul_f64(self.time_thresh);
                    if now.duration_since(unacked.time_sent) > loss_delay {
                        // TODO: do time threshold update
                        self.time_thresh = 5_f64 / 4_f64;
                    }

                    undo_cwnd = true;

                    self.lost_spurious_count += 1;
                    continue;
                }

                if unacked.ack_eliciting {
                    has_ack_eliciting = true;
                }

                largest_newly_acked_pkt_num = unacked.pkt_num;
                largest_newly_acked_sent_time = unacked.time_sent;

                self.acked[epoch].append(&mut unacked.frames);

                if unacked.in_flight {
                    self.in_flight_count[epoch] =
                        self.in_flight_count[epoch].saturating_sub(1);

                    self.delivery_rate.on_packet_acked(&unacked, now);
                }

                newly_acked.push(Acked {
                    pkt_num: unacked.pkt_num,

                    time_sent: unacked.time_sent,

                    size: unacked.size,
                });

                trace!("{} packet newly acked {}", trace_id, unacked.pkt_num);
            }
        }

        // Undo congestion window update.
        if undo_cwnd {
            (self.cc_ops.rollback)(self);
        }

        self.delivery_rate.estimate();

        if newly_acked.is_empty() {
            return Ok(());
        }

        if largest_newly_acked_pkt_num == largest_acked && has_ack_eliciting {
            let latest_rtt = now - largest_newly_acked_sent_time;

            let ack_delay = if epoch == packet::EPOCH_APPLICATION {
                Duration::from_micros(ack_delay)
            } else {
                Duration::from_micros(0)
            };

            self.update_rtt(latest_rtt, ack_delay, now);
        }

        // Detect and mark lost packets without removing them from the sent
        // packets list.
        self.detect_lost_packets(epoch, now, trace_id);

        self.on_packets_acked(newly_acked, epoch, now);

        self.pto_count = 0;

        self.set_loss_detection_timer(handshake_status, now);

        self.drain_packets(epoch, now);

        Ok(())
    }

    pub fn on_loss_detection_timeout(
        &mut self, handshake_status: HandshakeStatus, now: Instant,
        trace_id: &str,
    ) {
        let (earliest_loss_time, epoch) = self.loss_time_and_space();

        if earliest_loss_time.is_some() {
            // Time threshold loss detection.
            self.detect_lost_packets(epoch, now, trace_id);

            self.set_loss_detection_timer(handshake_status, now);

            trace!("{} {:?}", trace_id, self);
            return;
        }

        let epoch = if self.bytes_in_flight > 0 {
            // Send new data if available, else retransmit old data. If neither
            // is available, send a single PING frame.
            let (_, e) = self.pto_time_and_space(handshake_status, now);

            e
        } else {
            // Client sends an anti-deadlock packet: Initial is padded to earn
            // more anti-amplification credit, a Handshake packet proves address
            // ownership.
            if handshake_status.has_handshake_keys {
                packet::EPOCH_HANDSHAKE
            } else {
                packet::EPOCH_INITIAL
            }
        };

        self.pto_count += 1;

        self.loss_probes[epoch] =
            cmp::min(self.pto_count as usize, MAX_PTO_PROBES_COUNT);

        let unacked_iter = self.sent[epoch]
            .iter_mut()
            // Skip packets that have already been acked or lost, and packets
            // that don't contain either CRYPTO or STREAM frames.
            .filter(|p| p.has_data && p.time_acked.is_none() && p.time_lost.is_none())
            // Only return as many packets as the number of probe packets that
            // will be sent.
            .take(self.loss_probes[epoch]);

        // Retransmit the frames from the oldest sent packets on PTO. However
        // the packets are not actually declared lost (so there is no effect to
        // congestion control), we just reschedule the data they carried.
        //
        // This will also trigger sending an ACK and retransmitting frames like
        // HANDSHAKE_DONE and MAX_DATA / MAX_STREAM_DATA as well, in addition
        // to CRYPTO and STREAM, if the original packet carried them.
        for unacked in unacked_iter {
            self.lost[epoch].extend_from_slice(&unacked.frames);
        }

        self.set_loss_detection_timer(handshake_status, now);

        trace!("{} {:?}", trace_id, self);
    }

    pub fn on_pkt_num_space_discarded(
        &mut self, epoch: packet::Epoch, handshake_status: HandshakeStatus,
        now: Instant,
    ) {
        let unacked_bytes = self.sent[epoch]
            .iter()
            .filter(|p| {
                p.in_flight && p.time_acked.is_none() && p.time_lost.is_none()
            })
            .fold(0, |acc, p| acc + p.size);

        self.bytes_in_flight = self.bytes_in_flight.saturating_sub(unacked_bytes);

        self.sent[epoch].clear();
        self.lost[epoch].clear();
        self.acked[epoch].clear();

        self.time_of_last_sent_ack_eliciting_pkt[epoch] = None;
        self.loss_time[epoch] = None;
        self.loss_probes[epoch] = 0;
        self.in_flight_count[epoch] = 0;

        self.set_loss_detection_timer(handshake_status, now);
    }

    pub fn loss_detection_timer(&self) -> Option<Instant> {
        self.loss_detection_timer
    }

    pub fn cwnd(&self) -> usize {
        self.congestion_window
    }

    pub fn cwnd_available(&self) -> usize {
        // Ignore cwnd when sending probe packets.
        if self.loss_probes.iter().any(|&x| x > 0) {
            return std::usize::MAX;
        }

        // Open more space (snd_cnt) for PRR when allowed.
        self.congestion_window.saturating_sub(self.bytes_in_flight) +
            self.prr.snd_cnt
    }

    pub fn rtt(&self) -> Duration {
        self.smoothed_rtt.unwrap_or(INITIAL_RTT)
    }

    pub fn pto(&self) -> Duration {
        self.rtt() + cmp::max(self.rttvar * 4, GRANULARITY)
    }

    pub fn delivery_rate(&self) -> u64 {
        self.delivery_rate.delivery_rate()
    }

    pub fn max_datagram_size(&self) -> usize {
        self.max_datagram_size
    }

    pub fn update_max_datagram_size(&mut self, new_max_datagram_size: usize) {
        let max_datagram_size =
            cmp::min(self.max_datagram_size, new_max_datagram_size);

        // Congestion Window is updated only when it's not updated already.
        if self.congestion_window ==
            self.max_datagram_size * INITIAL_WINDOW_PACKETS
        {
            self.congestion_window = max_datagram_size * INITIAL_WINDOW_PACKETS;
        }

        self.max_datagram_size = max_datagram_size;
    }

    fn update_rtt(
        &mut self, latest_rtt: Duration, ack_delay: Duration, now: Instant,
    ) {
        self.latest_rtt = latest_rtt;

        match self.smoothed_rtt {
            // First RTT sample.
            None => {
                self.min_rtt = self.minmax_filter.reset(now, latest_rtt);

                self.smoothed_rtt = Some(latest_rtt);

                self.rttvar = latest_rtt / 2;
            },

            Some(srtt) => {
                self.min_rtt =
                    self.minmax_filter.running_min(RTT_WINDOW, now, latest_rtt);

                let ack_delay = cmp::min(self.max_ack_delay, ack_delay);

                // Adjust for ack delay if plausible.
                let adjusted_rtt = if latest_rtt > self.min_rtt + ack_delay {
                    latest_rtt - ack_delay
                } else {
                    latest_rtt
                };

                self.rttvar = self.rttvar.mul_f64(3.0 / 4.0) +
                    sub_abs(srtt, adjusted_rtt).mul_f64(1.0 / 4.0);

                self.smoothed_rtt = Some(
                    srtt.mul_f64(7.0 / 8.0) + adjusted_rtt.mul_f64(1.0 / 8.0),
                );
            },
        }
    }

    fn loss_time_and_space(&self) -> (Option<Instant>, packet::Epoch) {
        let mut epoch = packet::EPOCH_INITIAL;
        let mut time = self.loss_time[epoch];

        // Iterate over all packet number spaces starting from Handshake.
        #[allow(clippy::needless_range_loop)]
        for e in packet::EPOCH_HANDSHAKE..packet::EPOCH_COUNT {
            let new_time = self.loss_time[e];

            if time.is_none() || new_time < time {
                time = new_time;
                epoch = e;
            }
        }

        (time, epoch)
    }

    fn pto_time_and_space(
        &self, handshake_status: HandshakeStatus, now: Instant,
    ) -> (Option<Instant>, packet::Epoch) {
        let mut duration = self.pto() * 2_u32.pow(self.pto_count);

        // Arm PTO from now when there are no inflight packets.
        if self.bytes_in_flight == 0 {
            if handshake_status.has_handshake_keys {
                return (Some(now + duration), packet::EPOCH_HANDSHAKE);
            } else {
                return (Some(now + duration), packet::EPOCH_INITIAL);
            }
        }

        let mut pto_timeout = None;
        let mut pto_space = packet::EPOCH_INITIAL;

        // Iterate over all packet number spaces.
        for e in packet::EPOCH_INITIAL..packet::EPOCH_COUNT {
            if self.in_flight_count[e] == 0 {
                continue;
            }

            if e == packet::EPOCH_APPLICATION {
                // Skip Application Data until handshake completes.
                if !handshake_status.completed {
                    return (pto_timeout, pto_space);
                }

                // Include max_ack_delay and backoff for Application Data.
                duration += self.max_ack_delay * 2_u32.pow(self.pto_count);
            }

            let new_time =
                self.time_of_last_sent_ack_eliciting_pkt[e].map(|t| t + duration);

            if pto_timeout.is_none() || new_time < pto_timeout {
                pto_timeout = new_time;
                pto_space = e;
            }
        }

        (pto_timeout, pto_space)
    }

    fn set_loss_detection_timer(
        &mut self, handshake_status: HandshakeStatus, now: Instant,
    ) {
        let (earliest_loss_time, _) = self.loss_time_and_space();

        if earliest_loss_time.is_some() {
            // Time threshold loss detection.
            self.loss_detection_timer = earliest_loss_time;
            return;
        }

        if self.bytes_in_flight == 0 && handshake_status.peer_verified_address {
            self.loss_detection_timer = None;
            return;
        }

        // PTO timer.
        let (timeout, _) = self.pto_time_and_space(handshake_status, now);
        self.loss_detection_timer = timeout;
    }

    fn detect_lost_packets(
        &mut self, epoch: packet::Epoch, now: Instant, trace_id: &str,
    ) {
        let largest_acked = self.largest_acked_pkt[epoch];

        self.loss_time[epoch] = None;

        let loss_delay =
            cmp::max(self.latest_rtt, self.rtt()).mul_f64(self.time_thresh);

        // Minimum time of kGranularity before packets are deemed lost.
        let loss_delay = cmp::max(loss_delay, GRANULARITY);

        // Packets sent before this time are deemed lost.
        let lost_send_time = now - loss_delay;

        let mut lost_bytes = 0;

        let mut largest_lost_pkt = None;

        let unacked_iter = self.sent[epoch]
            .iter_mut()
            // Skip packets that follow the largest acked packet.
            .take_while(|p| p.pkt_num <= largest_acked)
            // Skip packets that have already been acked or lost.
            .filter(|p| p.time_acked.is_none() && p.time_lost.is_none());

        for unacked in unacked_iter {
            // Mark packet as lost, or set time when it should be marked.
            if unacked.time_sent <= lost_send_time ||
                largest_acked >= unacked.pkt_num + self.pkt_thresh
            {
                self.lost[epoch].append(&mut unacked.frames);

                unacked.time_lost = Some(now);

                if unacked.in_flight {
                    lost_bytes += unacked.size;

                    // Frames have already been removed from the packet, so
                    // cloning the whole packet should be relatively cheap.
                    largest_lost_pkt = Some(unacked.clone());

                    self.in_flight_count[epoch] =
                        self.in_flight_count[epoch].saturating_sub(1);

                    trace!(
                        "{} packet {} lost on epoch {}",
                        trace_id,
                        unacked.pkt_num,
                        epoch
                    );
                }

                self.lost_count += 1;
            } else {
                let loss_time = match self.loss_time[epoch] {
                    None => unacked.time_sent + loss_delay,

                    Some(loss_time) =>
                        cmp::min(loss_time, unacked.time_sent + loss_delay),
                };

                self.loss_time[epoch] = Some(loss_time);
            }
        }

        self.bytes_lost += lost_bytes as u64;

        if let Some(pkt) = largest_lost_pkt {
            self.on_packets_lost(lost_bytes, &pkt, epoch, now);
        }

        self.drain_packets(epoch, now);
    }

    fn drain_packets(&mut self, epoch: packet::Epoch, now: Instant) {
        let mut lowest_non_expired_pkt_index = self.sent[epoch].len();

        // In order to avoid removing elements from the middle of the list
        // (which would require copying other elements to compact the list),
        // we only remove a contiguous range of elements from the start of the
        // list.
        //
        // This means that acked or lost elements coming after this will not
        // be removed at this point, but their removal is delayed for a later
        // time, once the gaps have been filled.

        // First, find the first element that is neither acked nor lost.
        for (i, pkt) in self.sent[epoch].iter().enumerate() {
            if let Some(time_lost) = pkt.time_lost {
                if time_lost + self.rtt() > now {
                    lowest_non_expired_pkt_index = i;
                    break;
                }
            }

            if pkt.time_acked.is_none() && pkt.time_lost.is_none() {
                lowest_non_expired_pkt_index = i;
                break;
            }
        }

        // Then remove elements up to the previously found index.
        self.sent[epoch].drain(..lowest_non_expired_pkt_index);
    }

    fn on_packets_acked(
        &mut self, acked: Vec<Acked>, epoch: packet::Epoch, now: Instant,
    ) {
        for pkt in acked {
            (self.cc_ops.on_packet_acked)(self, &pkt, epoch, now);
        }
    }

    fn in_congestion_recovery(&self, sent_time: Instant) -> bool {
        match self.congestion_recovery_start_time {
            Some(congestion_recovery_start_time) =>
                sent_time <= congestion_recovery_start_time,

            None => false,
        }
    }

    fn in_persistent_congestion(&mut self, _largest_lost_pkt_num: u64) -> bool {
        let _congestion_period = self.pto() * PERSISTENT_CONGESTION_THRESHOLD;

        // TODO: properly detect persistent congestion
        false
    }

    fn on_packets_lost(
        &mut self, lost_bytes: usize, largest_lost_pkt: &Sent,
        epoch: packet::Epoch, now: Instant,
    ) {
        self.bytes_in_flight = self.bytes_in_flight.saturating_sub(lost_bytes);

        self.congestion_event(largest_lost_pkt.time_sent, epoch, now);

        if self.in_persistent_congestion(largest_lost_pkt.pkt_num) {
            self.collapse_cwnd();
        }
    }

    fn congestion_event(
        &mut self, time_sent: Instant, epoch: packet::Epoch, now: Instant,
    ) {
        if !self.in_congestion_recovery(time_sent) {
            (self.cc_ops.checkpoint)(self);
        }

        (self.cc_ops.congestion_event)(self, time_sent, epoch, now);
    }

    fn collapse_cwnd(&mut self) {
        (self.cc_ops.collapse_cwnd)(self);
    }

    pub fn rate_check_app_limited(&mut self) {
        if self.app_limited {
            self.delivery_rate.check_app_limited(self.bytes_in_flight)
        }
    }

    pub fn update_app_limited(&mut self, v: bool) {
        self.app_limited = v;
    }

    pub fn app_limited(&mut self) -> bool {
        self.app_limited
    }

    #[cfg(feature = "qlog")]
    pub fn to_qlog(&self) -> qlog::event::Event {
        // QVis can't use all these fields and they can be large.
        qlog::event::Event::metrics_updated(
            Some(self.min_rtt.as_millis() as u64),
            Some(self.rtt().as_millis() as u64),
            Some(self.latest_rtt.as_millis() as u64),
            Some(self.rttvar.as_millis() as u64),
            None, // delay
            None, // probe_count
            Some(self.cwnd() as u64),
            Some(self.bytes_in_flight as u64),
            Some(self.ssthresh as u64),
            None, // packets_in_flight
            None, // in_recovery
            None, // pacing_rate
        )
    }
}

/// Available congestion control algorithms.
///
/// This enum provides currently available list of congestion control
/// algorithms.
#[derive(Debug, Copy, Clone, PartialEq)]
#[repr(C)]
pub enum CongestionControlAlgorithm {
    /// Reno congestion control algorithm. `reno` in a string form.
    Reno  = 0,
    /// CUBIC congestion control algorithm (default). `cubic` in a string form.
    CUBIC = 1,
}

impl FromStr for CongestionControlAlgorithm {
    type Err = crate::Error;

    /// Converts a string to `CongestionControlAlgorithm`.
    ///
    /// If `name` is not valid, `Error::CongestionControl` is returned.
    fn from_str(name: &str) -> std::result::Result<Self, Self::Err> {
        match name {
            "reno" => Ok(CongestionControlAlgorithm::Reno),
            "cubic" => Ok(CongestionControlAlgorithm::CUBIC),

            _ => Err(crate::Error::CongestionControl),
        }
    }
}

pub struct CongestionControlOps {
    pub on_packet_sent: fn(r: &mut Recovery, sent_bytes: usize, now: Instant),

    pub on_packet_acked:
        fn(r: &mut Recovery, packet: &Acked, epoch: packet::Epoch, now: Instant),

    pub congestion_event: fn(
        r: &mut Recovery,
        time_sent: Instant,
        epoch: packet::Epoch,
        now: Instant,
    ),

    pub collapse_cwnd: fn(r: &mut Recovery),

    pub checkpoint: fn(r: &mut Recovery),

    pub rollback: fn(r: &mut Recovery),

    pub has_custom_pacing: fn() -> bool,
}

impl From<CongestionControlAlgorithm> for &'static CongestionControlOps {
    fn from(algo: CongestionControlAlgorithm) -> Self {
        match algo {
            CongestionControlAlgorithm::Reno => &reno::RENO,
            CongestionControlAlgorithm::CUBIC => &cubic::CUBIC,
        }
    }
}

impl std::fmt::Debug for Recovery {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        match self.loss_detection_timer {
            Some(v) => {
                let now = Instant::now();

                if v > now {
                    let d = v.duration_since(now);
                    write!(f, "timer={:?} ", d)?;
                } else {
                    write!(f, "timer=exp ")?;
                }
            },

            None => {
                write!(f, "timer=none ")?;
            },
        };

        write!(f, "latest_rtt={:?} ", self.latest_rtt)?;
        write!(f, "srtt={:?} ", self.smoothed_rtt)?;
        write!(f, "min_rtt={:?} ", self.min_rtt)?;
        write!(f, "rttvar={:?} ", self.rttvar)?;
        write!(f, "loss_time={:?} ", self.loss_time)?;
        write!(f, "loss_probes={:?} ", self.loss_probes)?;
        write!(f, "cwnd={} ", self.congestion_window)?;
        write!(f, "ssthresh={} ", self.ssthresh)?;
        write!(f, "bytes_in_flight={} ", self.bytes_in_flight)?;
        write!(f, "app_limited={} ", self.app_limited)?;
        write!(
            f,
            "congestion_recovery_start_time={:?} ",
            self.congestion_recovery_start_time
        )?;
        write!(f, "{:?} ", self.delivery_rate)?;
        write!(f, "pacing_rate={:?} ", self.pacing_rate)?;
        write!(
            f,
            "last_packet_scheduled_time={:?} ",
            self.last_packet_scheduled_time
        )?;

        if self.hystart.enabled() {
            write!(f, "hystart={:?} ", self.hystart)?;
        }

        Ok(())
    }
}

#[derive(Clone)]
pub struct Sent {
    pub pkt_num: u64,

    pub frames: Vec<frame::Frame>,

    pub time_sent: Instant,

    pub time_acked: Option<Instant>,

    pub time_lost: Option<Instant>,

    pub size: usize,

    pub ack_eliciting: bool,

    pub in_flight: bool,

    pub delivered: usize,

    pub delivered_time: Instant,

    pub recent_delivered_packet_sent_time: Instant,

    pub is_app_limited: bool,

    pub has_data: bool,
}

impl std::fmt::Debug for Sent {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "pkt_num={:?} ", self.pkt_num)?;
        write!(f, "pkt_sent_time={:?} ", self.time_sent.elapsed())?;
        write!(f, "pkt_size={:?} ", self.size)?;
        write!(f, "delivered={:?} ", self.delivered)?;
        write!(f, "delivered_time={:?} ", self.delivered_time.elapsed())?;
        write!(
            f,
            "recent_delivered_packet_sent_time={:?} ",
            self.recent_delivered_packet_sent_time.elapsed()
        )?;
        write!(f, "is_app_limited={} ", self.is_app_limited)?;
        write!(f, "has_data={} ", self.has_data)?;

        Ok(())
    }
}

#[derive(Clone)]
pub struct Acked {
    pub pkt_num: u64,

    pub time_sent: Instant,

    pub size: usize,
}

#[derive(Clone, Copy, Debug)]
pub struct HandshakeStatus {
    pub has_handshake_keys: bool,

    pub peer_verified_address: bool,

    pub completed: bool,
}

#[cfg(test)]
impl Default for HandshakeStatus {
    fn default() -> HandshakeStatus {
        HandshakeStatus {
            has_handshake_keys: true,

            peer_verified_address: true,

            completed: true,
        }
    }
}

fn sub_abs(lhs: Duration, rhs: Duration) -> Duration {
    if lhs > rhs {
        lhs - rhs
    } else {
        rhs - lhs
    }
}

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

    #[test]
    fn lookup_cc_algo_ok() {
        let algo = CongestionControlAlgorithm::from_str("reno").unwrap();
        assert_eq!(algo, CongestionControlAlgorithm::Reno);
    }

    #[test]
    fn lookup_cc_algo_bad() {
        assert_eq!(
            CongestionControlAlgorithm::from_str("???"),
            Err(Error::CongestionControl)
        );
    }

    #[test]
    fn collapse_cwnd() {
        let mut cfg = crate::Config::new(crate::PROTOCOL_VERSION).unwrap();
        cfg.set_cc_algorithm(CongestionControlAlgorithm::Reno);

        let mut r = Recovery::new(&cfg);

        // cwnd will be reset.
        r.collapse_cwnd();
        assert_eq!(r.cwnd(), r.max_datagram_size * MINIMUM_WINDOW_PACKETS);
    }

    #[test]
    fn loss_on_pto() {
        let mut cfg = crate::Config::new(crate::PROTOCOL_VERSION).unwrap();
        cfg.set_cc_algorithm(CongestionControlAlgorithm::Reno);

        let mut r = Recovery::new(&cfg);

        let mut now = Instant::now();

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 0);

        // Start by sending a few packets.
        let p = Sent {
            pkt_num: 0,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 1);
        assert_eq!(r.bytes_in_flight, 1000);

        let p = Sent {
            pkt_num: 1,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 2);
        assert_eq!(r.bytes_in_flight, 2000);

        let p = Sent {
            pkt_num: 2,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 3);
        assert_eq!(r.bytes_in_flight, 3000);

        let p = Sent {
            pkt_num: 3,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 4);
        assert_eq!(r.bytes_in_flight, 4000);

        // Wait for 10ms.
        now += Duration::from_millis(10);

        // Only the first 2 packets are acked.
        let mut acked = ranges::RangeSet::default();
        acked.insert(0..2);

        assert_eq!(
            r.on_ack_received(
                &acked,
                25,
                packet::EPOCH_APPLICATION,
                HandshakeStatus::default(),
                now,
                ""
            ),
            Ok(())
        );

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 2);
        assert_eq!(r.bytes_in_flight, 2000);
        assert_eq!(r.lost_count, 0);

        // Wait until loss detection timer expires.
        now = r.loss_detection_timer().unwrap();

        // PTO.
        r.on_loss_detection_timeout(HandshakeStatus::default(), now, "");
        assert_eq!(r.loss_probes[packet::EPOCH_APPLICATION], 1);
        assert_eq!(r.lost_count, 0);
        assert_eq!(r.pto_count, 1);

        let p = Sent {
            pkt_num: 4,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 3);
        assert_eq!(r.bytes_in_flight, 3000);

        let p = Sent {
            pkt_num: 5,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 4);
        assert_eq!(r.bytes_in_flight, 4000);
        assert_eq!(r.lost_count, 0);

        // Wait for 10ms.
        now += Duration::from_millis(10);

        // PTO packets are acked.
        let mut acked = ranges::RangeSet::default();
        acked.insert(4..6);

        assert_eq!(
            r.on_ack_received(
                &acked,
                25,
                packet::EPOCH_APPLICATION,
                HandshakeStatus::default(),
                now,
                ""
            ),
            Ok(())
        );

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 4);
        assert_eq!(r.bytes_in_flight, 0);

        assert_eq!(r.lost_count, 2);

        // Wait 1 RTT.
        now += r.rtt();

        r.detect_lost_packets(packet::EPOCH_APPLICATION, now, "");

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 0);
    }

    #[test]
    fn loss_on_timer() {
        let mut cfg = crate::Config::new(crate::PROTOCOL_VERSION).unwrap();
        cfg.set_cc_algorithm(CongestionControlAlgorithm::Reno);

        let mut r = Recovery::new(&cfg);

        let mut now = Instant::now();

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 0);

        // Start by sending a few packets.
        let p = Sent {
            pkt_num: 0,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 1);
        assert_eq!(r.bytes_in_flight, 1000);

        let p = Sent {
            pkt_num: 1,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 2);
        assert_eq!(r.bytes_in_flight, 2000);

        let p = Sent {
            pkt_num: 2,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 3);
        assert_eq!(r.bytes_in_flight, 3000);

        let p = Sent {
            pkt_num: 3,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 4);
        assert_eq!(r.bytes_in_flight, 4000);

        // Wait for 10ms.
        now += Duration::from_millis(10);

        // Only the first 2 packets and the last one are acked.
        let mut acked = ranges::RangeSet::default();
        acked.insert(0..2);
        acked.insert(3..4);

        assert_eq!(
            r.on_ack_received(
                &acked,
                25,
                packet::EPOCH_APPLICATION,
                HandshakeStatus::default(),
                now,
                ""
            ),
            Ok(())
        );

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 2);
        assert_eq!(r.bytes_in_flight, 1000);
        assert_eq!(r.lost_count, 0);

        // Wait until loss detection timer expires.
        now = r.loss_detection_timer().unwrap();

        // Packet is declared lost.
        r.on_loss_detection_timeout(HandshakeStatus::default(), now, "");
        assert_eq!(r.loss_probes[packet::EPOCH_APPLICATION], 0);

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 2);
        assert_eq!(r.bytes_in_flight, 0);

        assert_eq!(r.lost_count, 1);

        // Wait 1 RTT.
        now += r.rtt();

        r.detect_lost_packets(packet::EPOCH_APPLICATION, now, "");

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 0);
    }

    #[test]
    fn loss_on_reordering() {
        let mut cfg = crate::Config::new(crate::PROTOCOL_VERSION).unwrap();
        cfg.set_cc_algorithm(CongestionControlAlgorithm::Reno);

        let mut r = Recovery::new(&cfg);

        let mut now = Instant::now();

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 0);

        // Start by sending a few packets.
        let p = Sent {
            pkt_num: 0,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 1);
        assert_eq!(r.bytes_in_flight, 1000);

        let p = Sent {
            pkt_num: 1,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 2);
        assert_eq!(r.bytes_in_flight, 2000);

        let p = Sent {
            pkt_num: 2,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 3);
        assert_eq!(r.bytes_in_flight, 3000);

        let p = Sent {
            pkt_num: 3,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 1000,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );
        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 4);
        assert_eq!(r.bytes_in_flight, 4000);

        // Wait for 10ms.
        now += Duration::from_millis(10);

        // ACKs are reordered.
        let mut acked = ranges::RangeSet::default();
        acked.insert(2..4);

        assert_eq!(
            r.on_ack_received(
                &acked,
                25,
                packet::EPOCH_APPLICATION,
                HandshakeStatus::default(),
                now,
                ""
            ),
            Ok(())
        );

        now += Duration::from_millis(10);

        let mut acked = ranges::RangeSet::default();
        acked.insert(0..2);

        assert_eq!(r.pkt_thresh, INITIAL_PACKET_THRESHOLD);

        assert_eq!(
            r.on_ack_received(
                &acked,
                25,
                packet::EPOCH_APPLICATION,
                HandshakeStatus::default(),
                now,
                ""
            ),
            Ok(())
        );

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 4);
        assert_eq!(r.bytes_in_flight, 0);

        // Spurious loss.
        assert_eq!(r.lost_count, 1);
        assert_eq!(r.lost_spurious_count, 1);

        // Packet threshold was increased.
        assert_eq!(r.pkt_thresh, 4);

        // Wait 1 RTT.
        now += r.rtt();

        r.detect_lost_packets(packet::EPOCH_APPLICATION, now, "");

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 0);
    }

    #[test]
    fn pacing() {
        let mut cfg = crate::Config::new(crate::PROTOCOL_VERSION).unwrap();
        cfg.set_cc_algorithm(CongestionControlAlgorithm::CUBIC);

        let mut r = Recovery::new(&cfg);

        let mut now = Instant::now();

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 0);

        // send out first packet.
        let p = Sent {
            pkt_num: 0,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 6500,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 1);
        assert_eq!(r.bytes_in_flight, 6500);

        // First packet will be sent out immidiately.
        assert_eq!(r.pacing_rate, 0);
        assert_eq!(r.get_packet_send_time().unwrap(), now);

        // Wait 50ms for ACK.
        now += Duration::from_millis(50);

        let mut acked = ranges::RangeSet::default();
        acked.insert(0..1);

        assert_eq!(
            r.on_ack_received(
                &acked,
                10,
                packet::EPOCH_APPLICATION,
                HandshakeStatus::default(),
                now,
                ""
            ),
            Ok(())
        );

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 0);
        assert_eq!(r.bytes_in_flight, 0);
        assert_eq!(r.smoothed_rtt.unwrap(), Duration::from_millis(50));

        // Send out second packet.
        let p = Sent {
            pkt_num: 1,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 6500,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 1);
        assert_eq!(r.bytes_in_flight, 6500);

        // Pacing is not done during intial phase of connection.
        assert_eq!(r.get_packet_send_time().unwrap(), now);

        // Send the third packet out.
        let p = Sent {
            pkt_num: 2,
            frames: vec![],
            time_sent: now,
            time_acked: None,
            time_lost: None,
            size: 6500,
            ack_eliciting: true,
            in_flight: true,
            delivered: 0,
            delivered_time: now,
            recent_delivered_packet_sent_time: now,
            is_app_limited: false,
            has_data: false,
        };

        r.on_packet_sent(
            p,
            packet::EPOCH_APPLICATION,
            HandshakeStatus::default(),
            now,
            "",
        );

        assert_eq!(r.sent[packet::EPOCH_APPLICATION].len(), 2);
        assert_eq!(r.bytes_in_flight, 13000);
        assert_eq!(r.smoothed_rtt.unwrap(), Duration::from_millis(50));

        // We pace this outgoing packet. as all conditions for pacing
        // are passed.
        assert_eq!(r.pacing_rate, (12000.0 / 0.05) as u64);
        assert_eq!(
            r.get_packet_send_time().unwrap(),
            now + Duration::from_micros(
                (6500 * 1000000) / (12000.0 / 0.05) as u64
            )
        );
    }
}

mod cubic;
mod delivery_rate;
mod hystart;
mod prr;
mod reno;
