//! Round Trip Time measurement (§ 2.1)

use std::cmp::{max, min};

use std::collections::VecDeque;

use std::sync::atomic::{AtomicBool, Ordering};

use std::time::{Duration, Instant};

use super::params::RoundTripEstimatorParams;

use super::{CongestionWindow, State};

use thiserror::Error;

use tor_error::{ErrorKind, HasKind};

/// An error originating from the tor-congestion crate.

#[derive(Error, Debug, Clone)]

#[non_exhaustive]

pub(crate) enum Error {

    /// A call to `RoundtripTimeEstimator::sendme_received` was made without calling

    /// `RoundtripTimeEstimator::expect_sendme` first.

    #[error("Informed of a SENDME we weren't expecting")]

    MismatchedEstimationCall,

}

impl HasKind for Error {

        use Error as E;

        }

}

/// Provides an estimate of the round-trip time (RTT) of a Tor circuit.

#[derive(Debug)]

#[allow(dead_code)]

pub(crate) struct RoundtripTimeEstimator {

    /// A queue of times we sent a cell that we'd expect a SENDME for.

    ///

    /// When a data cell is sent and for which we expect a SENDME next, the timestamp at the send

    /// is kept in this queue so we can use it to measure the RTT when the SENDME is received.

    ///

    /// A queue is used here because the protocol allows to send all pending SENDMEs at once as

    /// long as it is within one congestion window.

    sendme_expected_from: VecDeque<Instant>,

    /// The last *measured* round-trip time.

    ///

    /// This is `None` iff we have not managed to get any estimate yet.

    last_rtt: Option<Duration>,

    /// The current smoothed *estimate* of what the round-trip time is.

    ///

    /// This is `None` iff we have not managed to get any estimate yet.

    ewma_rtt: Option<Duration>,

    /// The minimum observed value of `last_rtt`.

    ///

    /// This is `None` iff we have not managed to get any estimate yet.

    min_rtt: Option<Duration>,

    /// The maximum observed value of `last_rtt`.

    ///

    /// This is `None` iff we have not managed to get any estimate yet.

    max_rtt: Option<Duration>,

    /// The network parameters we're using.

    params: RoundTripEstimatorParams,

    /// A reference to a shared boolean for storing if the clock is stalled or not.

    /// Spec: CLOCK_HEURISTICS from prop324. See is_clock_stalled() for the implementation.

    clock_stalled: AtomicBool,

}

#[allow(dead_code)]

impl RoundtripTimeEstimator {

    /// Create a new `RoundtripTimeEstimator`, using a set of `NetParameters` and a shared boolean

    /// to cache clock stalled state in.

    /// Return true iff the estimator is ready to be used or read.

    /// Return the state of the clock stalled indicator.

    /// Return the EWMA RTT in usec or `None` if we don't have an estimate yet.

    /// Return the Minimum RTT in usec or `None` if we don't have an estimate yet.

    /// Return the maximum observed RTT in usec or `None` if we don't have an estimate yet.

    /// Inform the estimator that we did (at time `now`) something that we'll expect a SENDME to

    /// be received for.

    /// Return whether we can use heuristics to sanity-check RTT values against our EWMA value.

    /// Spec: 2.1.1. Clock Jump Heuristics CLOCK_HEURISTICS

    ///

    /// Used in [`is_clock_stalled`](RoundtripTimeEstimator::is_clock_stalled), to check the sanity of

    /// a newly measured RTT value.

        // If we're in slow start, we don't perform any sanity checks, as per spec. If we don't

        // have a current estimate, we can't use it for sanity checking, because it doesn't

        // exist.

    /// Given a raw RTT value we just observed, compute whether or not we think the clock has

    /// stalled or jumped, and we should throw it out as a result.

            // Clock is stalled.

            /// Discrepancy ratio of a new RTT value that we allow against the current RTT in order

            /// to declare if the clock has stalled or not. This value is taken from proposal 324

            /// section 2.1.1 CLOCK_HEURISTICS and has the same name as in C-tor.

            const DELTA_DISCREPANCY_RATIO_MAX: u32 = 5000;

            // If we have enough data, check the sanity of our measurement against our EWMA value.

                // The clock significantly jumped forward.

                //

                // Don't update the global cache, though, since this is triggerable over the

                // network.

                //

                // FIXME(eta): We should probably log something here?

                // The clock might have stalled. We can't really make a decision just off this

                // one measurement, though, so we'll use the stored stall value.

            } else {

                // If we got here, we're not stalled.

            }

        } else {

            // If we don't have enough measurements to sanity check, assume it's okay.

        }

    /// Update the estimator on time `now` and at the congestion window `cwnd`.

    ///

    /// # Errors

    ///

    /// Each call to this function removes an entry from `sendme_expected_from` (the entries are

    /// added using [`sendme_expected_from`](Self::sendme_expected_from)).

    ///

    /// Returns an error if are not expecting any SENDMEs at this time (if `expect_sendme` was

    /// never called, or if we have exhausted all `sendme_expected_from` added by previous

    /// `expect_sendme` calls).

    ///

    /// Spec: prop324 section 2.1 C-tor: congestion_control_update_circuit_rtt() in

    /// congestion_control_common.c

        // This is the "N" for N-EWMA.

        } else {

            )

        });

        // Get the USEC values.

        // This is the actual EWMA calculation.

        // C-tor simplifies this as follows for rounding error reasons:

        //

        // EWMA = value*2/(N+1) + EMA_prev*(N-1)/(N+1)

        //      = (value*2 + EWMA_prev*(N-1))/(N+1)

        //

        // Spec: prop324 section 2.1.2 (N_EWMA_SMOOTHING)

            }

        };

        };

}

#[cfg(test)]

mod test {

    // @@ begin test lint list maintained by maint/add_warning @@

    #![allow(clippy::bool_assert_comparison)]

    #![allow(clippy::clone_on_copy)]

    #![allow(clippy::dbg_macro)]

    #![allow(clippy::mixed_attributes_style)]

    #![allow(clippy::print_stderr)]

    #![allow(clippy::print_stdout)]

    #![allow(clippy::single_char_pattern)]

    #![allow(clippy::unwrap_used)]

    #![allow(clippy::unchecked_duration_subtraction)]

    #![allow(clippy::useless_vec)]

    #![allow(clippy::needless_pass_by_value)]

    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->

    use std::time::{Duration, Instant};

    use crate::congestion::test_utils::{new_cwnd, new_rtt_estimator};

    use super::*;

    #[derive(Debug)]

    struct RttTestSample {

        sent_usec_in: u64,

        sendme_received_usec_in: u64,

        cwnd_in: u32,

        ss_in: bool,

        last_rtt_usec_out: u64,

        ewma_rtt_usec_out: u64,

        min_rtt_usec_out: u64,

    }

    impl From<[u64; 7]> for RttTestSample {

        fn from(arr: [u64; 7]) -> Self {

            Self {

                sent_usec_in: arr[0],

                sendme_received_usec_in: arr[1],

                cwnd_in: arr[2] as u32,

                ss_in: arr[3] == 1,

                last_rtt_usec_out: arr[4],

                ewma_rtt_usec_out: arr[5],

                min_rtt_usec_out: arr[6],

            }

        }

    }

    impl RttTestSample {

        fn test(&self, estimator: &mut RoundtripTimeEstimator, start: Instant) {

            let state = if self.ss_in {

                State::SlowStart

            } else {

                State::Steady

            };

            let mut cwnd = new_cwnd();

            cwnd.set(self.cwnd_in);

            let sent = start + Duration::from_micros(self.sent_usec_in);

            let sendme_received = start + Duration::from_micros(self.sendme_received_usec_in);

            estimator.expect_sendme(sent);

            estimator

                .update(sendme_received, &state, &cwnd)

                .expect("Error on RTT update");

            assert_eq!(

                estimator.last_rtt,

                Some(Duration::from_micros(self.last_rtt_usec_out))

            );

            assert_eq!(

                estimator.ewma_rtt,

                Some(Duration::from_micros(self.ewma_rtt_usec_out))

            );

            assert_eq!(

                estimator.min_rtt,

                Some(Duration::from_micros(self.min_rtt_usec_out))

            );

        }

    }

    #[test]

    fn test_vectors() {

        let mut rtt = new_rtt_estimator();

        let now = Instant::now();

        // from C-tor src/test/test_congestion_control.c

        let vectors = [

            [100000, 200000, 124, 1, 100000, 100000, 100000],

            [200000, 300000, 124, 1, 100000, 100000, 100000],

            [350000, 500000, 124, 1, 150000, 133333, 100000],

            [500000, 550000, 124, 1, 50000, 77777, 77777],

            [600000, 700000, 124, 1, 100000, 92592, 77777],

            [700000, 750000, 124, 1, 50000, 64197, 64197],

            [750000, 875000, 124, 0, 125000, 104732, 104732],

            [875000, 900000, 124, 0, 25000, 51577, 104732],

            [900000, 950000, 200, 0, 50000, 50525, 50525],

        ];

        for vect in vectors {

            let vect = RttTestSample::from(vect);

            eprintln!("Testing vector: {:?}", vect);

            vect.test(&mut rtt, now);

        }

    }

}