//! MMP derived metrics.
//!
//! `MmpMetrics` processes incoming ReceiverReports (from our peer) and
//! maintains derived metrics: SRTT, loss rate, goodput, ETX, and dual
//! EWMA trend indicators. Updated by the sender side when it receives
//! a ReceiverReport about its own traffic.

use crate::mmp::algorithms::{DualEwma, SrttEstimator, compute_etx};
use crate::mmp::report::ReceiverReport;
use std::time::Instant;
use tracing::trace;

/// Derived MMP metrics, updated from incoming ReceiverReports.
///
/// This lives on the sender side: when we receive a ReceiverReport from
/// our peer describing what they observed about our traffic, we process
/// it here to compute RTT, loss, goodput, and trend indicators.
pub struct MmpMetrics {
    /// Smoothed RTT from timestamp echo.
    pub srtt: SrttEstimator,

    /// Dual EWMA trend detectors.
    pub rtt_trend: DualEwma,
    pub loss_trend: DualEwma,
    pub goodput_trend: DualEwma,
    pub jitter_trend: DualEwma,
    pub etx_trend: DualEwma,

    /// Forward delivery ratio (what fraction of our frames the peer received).
    pub delivery_ratio_forward: f64,
    /// Reverse delivery ratio (set when we compute from our own receiver state).
    pub delivery_ratio_reverse: f64,
    /// ETX computed from bidirectional delivery ratios.
    pub etx: f64,

    /// Smoothed goodput in bytes/sec (forward direction: what the peer received from us).
    pub goodput_bps: f64,

    // --- State for delta computation ---
    /// Previous ReceiverReport's cumulative counters (for computing interval deltas).
    prev_rr_cum_packets: u64,
    prev_rr_cum_bytes: u64,
    prev_rr_highest_counter: u64,
    prev_rr_ecn_ce: u32,
    prev_rr_reorder: u32,
    /// Time of previous ReceiverReport (for goodput rate computation).
    prev_rr_time: Option<Instant>,
    /// Time of the most recent accepted RTT sample.
    last_srtt_update: Option<Instant>,
    /// Whether we have a previous ReceiverReport for delta computation.
    has_prev_rr: bool,
    /// Counter span in the most recent ReceiverReport delta.
    last_forward_counter_span: u64,
    /// Loss rate in the most recent ReceiverReport delta.
    last_forward_loss_rate: Option<f64>,

    // --- State for reverse delivery ratio delta computation ---
    /// Previous reverse-side cumulative packets received (our receiver state).
    prev_reverse_packets: u64,
    /// Previous reverse-side highest counter (our receiver state).
    prev_reverse_highest: u64,
    /// Whether we have a previous reverse-side snapshot for delta computation.
    has_prev_reverse: bool,
}

impl MmpMetrics {
    /// Reset state derived from ReceiverReport counters for rekey cutover.
    ///
    /// The new session starts with counter 0, so the prev_rr deltas must
    /// be reset to avoid computing bogus loss/goodput from the counter
    /// discontinuity. RTT (SRTT) is preserved since it remains valid.
    pub fn reset_for_rekey(&mut self) {
        self.prev_rr_cum_packets = 0;
        self.prev_rr_cum_bytes = 0;
        self.prev_rr_highest_counter = 0;
        self.prev_rr_ecn_ce = 0;
        self.prev_rr_reorder = 0;
        self.prev_rr_time = None;
        self.has_prev_rr = false;
        self.last_forward_counter_span = 0;
        self.last_forward_loss_rate = None;
        self.delivery_ratio_forward = 1.0;
        self.prev_reverse_packets = 0;
        self.prev_reverse_highest = 0;
        self.has_prev_reverse = false;
        // Keep srtt, srtt freshness, etx, trends, goodput_bps — they'll refresh from data.
    }

    pub fn new() -> Self {
        Self {
            srtt: SrttEstimator::new(),
            rtt_trend: DualEwma::new(),
            loss_trend: DualEwma::new(),
            goodput_trend: DualEwma::new(),
            jitter_trend: DualEwma::new(),
            etx_trend: DualEwma::new(),
            delivery_ratio_forward: 1.0,
            delivery_ratio_reverse: 1.0,
            etx: 1.0,
            goodput_bps: 0.0,
            prev_rr_cum_packets: 0,
            prev_rr_cum_bytes: 0,
            prev_rr_highest_counter: 0,
            prev_rr_ecn_ce: 0,
            prev_rr_reorder: 0,
            prev_rr_time: None,
            last_srtt_update: None,
            has_prev_rr: false,
            last_forward_counter_span: 0,
            last_forward_loss_rate: None,
            prev_reverse_packets: 0,
            prev_reverse_highest: 0,
            has_prev_reverse: false,
        }
    }

    /// Process an incoming ReceiverReport (from the peer about our traffic).
    ///
    /// `our_timestamp_ms` is the current session-relative time in ms (for RTT).
    /// `now` is the current monotonic time (for goodput rate computation).
    ///
    /// Returns `true` if this report produced the first SRTT measurement
    /// (transition from uninitialized to initialized).
    pub fn process_receiver_report(
        &mut self,
        rr: &ReceiverReport,
        our_timestamp_ms: u32,
        now: Instant,
    ) -> bool {
        let had_srtt = self.srtt.initialized();

        if self.has_prev_rr {
            let counters_regressed = rr.highest_counter < self.prev_rr_highest_counter
                || rr.cumulative_packets_recv < self.prev_rr_cum_packets
                || rr.cumulative_bytes_recv < self.prev_rr_cum_bytes
                || rr.ecn_ce_count < self.prev_rr_ecn_ce
                || rr.cumulative_reorder_count < self.prev_rr_reorder;
            let duplicate_counters = rr.highest_counter == self.prev_rr_highest_counter
                && rr.cumulative_packets_recv == self.prev_rr_cum_packets
                && rr.cumulative_bytes_recv == self.prev_rr_cum_bytes
                && rr.ecn_ce_count == self.prev_rr_ecn_ce
                && rr.cumulative_reorder_count == self.prev_rr_reorder;
            if counters_regressed || duplicate_counters {
                trace!(
                    highest_counter = rr.highest_counter,
                    prev_highest_counter = self.prev_rr_highest_counter,
                    cumulative_packets_recv = rr.cumulative_packets_recv,
                    prev_cumulative_packets_recv = self.prev_rr_cum_packets,
                    cumulative_bytes_recv = rr.cumulative_bytes_recv,
                    prev_cumulative_bytes_recv = self.prev_rr_cum_bytes,
                    "Ignoring stale MMP ReceiverReport"
                );
                return false;
            }
        }

        // --- RTT from timestamp echo ---
        // RTT = now - echoed_timestamp - dwell_time
        if rr.timestamp_echo > 0 {
            let echo_ms = rr.timestamp_echo;
            let dwell_ms = u32::from(rr.dwell_time);
            let rtt_sample_ms = echo_ms
                .checked_add(dwell_ms)
                .and_then(|send_done_ms| our_timestamp_ms.checked_sub(send_done_ms));

            match rtt_sample_ms {
                Some(rtt_ms) if rtt_ms > 0 => {
                    let rtt_us = (rtt_ms as i64) * 1000;
                    trace!(
                        our_ts = our_timestamp_ms,
                        echo = echo_ms,
                        dwell = dwell_ms,
                        rtt_ms = rtt_ms,
                        srtt_ms = self.srtt.srtt_us() as f64 / 1000.0,
                        "RTT sample from timestamp echo"
                    );
                    self.srtt.update(rtt_us);
                    self.last_srtt_update = Some(now);
                    self.rtt_trend.update(rtt_us as f64);
                }
                _ => {
                    trace!(
                        our_ts = our_timestamp_ms,
                        echo = echo_ms,
                        dwell = dwell_ms,
                        "Ignoring invalid MMP RTT sample"
                    );
                }
            }
        }

        // --- Loss rate from cumulative counters ---
        // Delta: frames the peer should have received vs. actually received
        self.last_forward_counter_span = 0;
        self.last_forward_loss_rate = None;
        if self.has_prev_rr {
            let counter_span = rr
                .highest_counter
                .saturating_sub(self.prev_rr_highest_counter);
            let packets_delta = rr
                .cumulative_packets_recv
                .saturating_sub(self.prev_rr_cum_packets);

            if counter_span > 0 {
                let delivery = (packets_delta as f64) / (counter_span as f64);
                self.delivery_ratio_forward = delivery.clamp(0.0, 1.0);
                let loss_rate = 1.0 - self.delivery_ratio_forward;
                self.last_forward_counter_span = counter_span;
                self.last_forward_loss_rate = Some(loss_rate);
                self.loss_trend.update(loss_rate);
                self.etx = compute_etx(self.delivery_ratio_forward, self.delivery_ratio_reverse);
                self.etx_trend.update(self.etx);
            }
        }

        // --- Goodput from cumulative bytes + time delta ---
        if self.has_prev_rr {
            let bytes_delta = rr
                .cumulative_bytes_recv
                .saturating_sub(self.prev_rr_cum_bytes);
            self.goodput_trend.update(bytes_delta as f64);

            // Compute bytes/sec if we have a time reference
            if let Some(prev_time) = self.prev_rr_time {
                let elapsed = now.duration_since(prev_time);
                let secs = elapsed.as_secs_f64();
                if secs > 0.0 {
                    let bps = bytes_delta as f64 / secs;
                    // EWMA smoothing: α = 1/4
                    if self.goodput_bps == 0.0 {
                        self.goodput_bps = bps;
                    } else {
                        self.goodput_bps += (bps - self.goodput_bps) * 0.25;
                    }
                }
            }
        }

        // --- Jitter trend ---
        self.jitter_trend.update(rr.jitter as f64);

        // --- Save for next delta ---
        self.prev_rr_cum_packets = rr.cumulative_packets_recv;
        self.prev_rr_cum_bytes = rr.cumulative_bytes_recv;
        self.prev_rr_highest_counter = rr.highest_counter;
        self.prev_rr_ecn_ce = rr.ecn_ce_count;
        self.prev_rr_reorder = rr.cumulative_reorder_count;
        self.prev_rr_time = Some(now);
        self.has_prev_rr = true;

        !had_srtt && self.srtt.initialized()
    }

    /// Update the reverse delivery ratio from our own receiver state.
    ///
    /// Computes a per-interval delta (same as forward ratio) rather than
    /// a lifetime cumulative ratio, so ETX responds to recent conditions.
    pub fn update_reverse_delivery(&mut self, our_recv_packets: u64, peer_highest: u64) {
        if self.has_prev_reverse {
            let counter_span = peer_highest.saturating_sub(self.prev_reverse_highest);
            let packets_delta = our_recv_packets.saturating_sub(self.prev_reverse_packets);

            if counter_span > 0 {
                let delivery = (packets_delta as f64) / (counter_span as f64);
                self.delivery_ratio_reverse = delivery.clamp(0.0, 1.0);
                self.etx = compute_etx(self.delivery_ratio_forward, self.delivery_ratio_reverse);
                self.etx_trend.update(self.etx);
            }
        }

        self.prev_reverse_packets = our_recv_packets;
        self.prev_reverse_highest = peer_highest;
        self.has_prev_reverse = true;
    }

    /// Current smoothed RTT in milliseconds, or `None` if not yet measured.
    pub fn srtt_ms(&self) -> Option<f64> {
        if self.srtt.initialized() {
            Some(self.srtt.srtt_us() as f64 / 1000.0)
        } else {
            None
        }
    }

    /// Age of the current SRTT sample in milliseconds.
    pub fn srtt_age_ms(&self, now: Instant) -> Option<u64> {
        self.last_srtt_update.map(|updated_at| {
            now.saturating_duration_since(updated_at)
                .as_millis()
                .min(u128::from(u64::MAX)) as u64
        })
    }

    /// Current loss rate (0.0 = no loss, 1.0 = total loss).
    pub fn loss_rate(&self) -> f64 {
        1.0 - self.delivery_ratio_forward
    }

    /// Smoothed loss rate (long-term EWMA), or `None` if not yet initialized.
    pub fn smoothed_loss(&self) -> Option<f64> {
        if self.loss_trend.initialized() {
            Some(self.loss_trend.long())
        } else {
            None
        }
    }

    /// Most recent forward-loss sample from a ReceiverReport delta.
    pub fn last_forward_loss_sample(&self) -> Option<(u64, f64)> {
        self.last_forward_loss_rate
            .map(|loss| (self.last_forward_counter_span, loss))
    }

    /// Smoothed ETX (long-term EWMA), or `None` if not yet initialized.
    pub fn smoothed_etx(&self) -> Option<f64> {
        if self.etx_trend.initialized() {
            Some(self.etx_trend.long())
        } else {
            None
        }
    }

    /// Current smoothed goodput in bytes/sec, or 0 if not yet measured.
    pub fn goodput_bps(&self) -> f64 {
        self.goodput_bps
    }

    /// Cumulative ECN CE count from the most recent ReceiverReport.
    pub fn last_ecn_ce_count(&self) -> u32 {
        self.prev_rr_ecn_ce
    }
}

impl Default for MmpMetrics {
    fn default() -> Self {
        Self::new()
    }
}

// ============================================================================
// Tests
// ============================================================================

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

    fn make_rr(
        highest_counter: u64,
        cum_packets: u64,
        cum_bytes: u64,
        timestamp_echo: u32,
        dwell: u16,
        jitter: u32,
    ) -> ReceiverReport {
        ReceiverReport {
            highest_counter,
            cumulative_packets_recv: cum_packets,
            cumulative_bytes_recv: cum_bytes,
            timestamp_echo,
            dwell_time: dwell,
            max_burst_loss: 0,
            mean_burst_loss: 0,
            jitter,
            ecn_ce_count: 0,
            owd_trend: 0,
            burst_loss_count: 0,
            cumulative_reorder_count: 0,
            interval_packets_recv: 0,
            interval_bytes_recv: 0,
        }
    }

    #[test]
    fn test_rtt_from_echo() {
        let mut m = MmpMetrics::new();
        let now = Instant::now();
        // Peer echoes timestamp 1000ms, dwell=5ms, our current time=1050ms
        let rr = make_rr(10, 10, 5000, 1000, 5, 0);
        m.process_receiver_report(&rr, 1050, now);

        assert!(m.srtt.initialized());
        // RTT = 1050 - 1000 - 5 = 45ms
        let srtt_ms = m.srtt_ms().unwrap();
        assert!((srtt_ms - 45.0).abs() < 1.0, "srtt={srtt_ms}, expected ~45");
    }

    #[test]
    fn test_ignores_duplicate_receiver_report_after_valid_sample() {
        let mut m = MmpMetrics::new();
        let now = Instant::now();

        let valid_rr = make_rr(10, 10, 5000, 1000, 5, 0);
        m.process_receiver_report(&valid_rr, 1050, now);
        let baseline_srtt_ms = m.srtt_ms().unwrap();
        assert_eq!(m.srtt_age_ms(now), Some(0));

        // A duplicate of the same counters arriving later would be a 5s RTT
        // sample if accepted. It is stale and must not move SRTT.
        m.process_receiver_report(&valid_rr, 6000, now + Duration::from_secs(5));

        let srtt_ms = m.srtt_ms().unwrap();
        assert_eq!(srtt_ms, baseline_srtt_ms);
        assert_eq!(m.srtt_age_ms(now + Duration::from_secs(5)), Some(5000));
    }

    #[test]
    fn test_ignores_out_of_order_receiver_report_after_valid_sample() {
        let mut m = MmpMetrics::new();
        let now = Instant::now();

        let valid_rr = make_rr(20, 20, 10000, 1000, 5, 0);
        m.process_receiver_report(&valid_rr, 1050, now);
        let baseline_srtt_ms = m.srtt_ms().unwrap();

        let old_rr = make_rr(10, 10, 5000, 1000, 0, 0);
        m.process_receiver_report(&old_rr, 6000, now + Duration::from_secs(5));

        let srtt_ms = m.srtt_ms().unwrap();
        assert_eq!(srtt_ms, baseline_srtt_ms);
    }

    #[test]
    fn test_ignores_wrapped_rtt_sample() {
        let mut m = MmpMetrics::new();
        let now = Instant::now();

        let wrapped_rr = make_rr(10, 10, 5000, u32::MAX - 10, 20, 0);
        m.process_receiver_report(&wrapped_rr, 15, now);

        assert!(m.srtt_ms().is_none());
    }

    #[test]
    fn test_loss_rate_computation() {
        let mut m = MmpMetrics::new();
        let t0 = Instant::now();

        // First report: baseline
        let rr1 = make_rr(100, 100, 50000, 0, 0, 0);
        m.process_receiver_report(&rr1, 0, t0);

        // Second report: 200 counters sent, 190 received (5% loss)
        let rr2 = make_rr(300, 290, 145000, 0, 0, 0);
        m.process_receiver_report(&rr2, 0, t0 + Duration::from_secs(1));

        let loss = m.loss_rate();
        assert!((loss - 0.05).abs() < 0.01, "loss={loss}, expected ~0.05");
        assert_eq!(m.last_forward_loss_sample(), Some((200, loss)));
    }

    #[test]
    fn test_etx_updates() {
        let mut m = MmpMetrics::new();
        assert_eq!(m.etx, 1.0); // initial: perfect

        // Simulate some loss via forward ratio
        m.delivery_ratio_forward = 0.9;

        // First call establishes the baseline (no ETX update yet)
        m.update_reverse_delivery(100, 100);
        assert_eq!(m.etx, 1.0); // still perfect — baseline only

        // Second call: 190 of 200 frames received (5% loss)
        m.update_reverse_delivery(290, 300);
        assert!(m.etx > 1.0);
        assert!(m.etx < 2.0);
    }

    #[test]
    fn test_no_rtt_without_echo() {
        let mut m = MmpMetrics::new();
        let now = Instant::now();
        let rr = make_rr(10, 10, 5000, 0, 0, 0);
        m.process_receiver_report(&rr, 1000, now);
        assert!(m.srtt_ms().is_none());
    }

    #[test]
    fn test_jitter_trend() {
        let mut m = MmpMetrics::new();
        let t0 = Instant::now();
        let rr1 = make_rr(10, 10, 5000, 0, 0, 100);
        m.process_receiver_report(&rr1, 0, t0);

        let rr2 = make_rr(20, 20, 10000, 0, 0, 500);
        m.process_receiver_report(&rr2, 0, t0 + Duration::from_secs(1));

        assert!(m.jitter_trend.initialized());
        // Short-term should be closer to 500 than long-term
        assert!(m.jitter_trend.short() > m.jitter_trend.long());
    }

    #[test]
    fn test_goodput_bps() {
        let mut m = MmpMetrics::new();
        let t0 = Instant::now();

        // First report: baseline (50KB received)
        let rr1 = make_rr(100, 100, 50_000, 0, 0, 0);
        m.process_receiver_report(&rr1, 0, t0);
        assert_eq!(m.goodput_bps(), 0.0); // no rate yet (first report)

        // Second report 1s later: 150KB total (100KB delta in 1s = 100KB/s)
        let rr2 = make_rr(300, 290, 150_000, 0, 0, 0);
        m.process_receiver_report(&rr2, 0, t0 + Duration::from_secs(1));
        assert!(
            m.goodput_bps() > 90_000.0,
            "goodput={}, expected ~100000",
            m.goodput_bps()
        );
        assert!(
            m.goodput_bps() < 110_000.0,
            "goodput={}, expected ~100000",
            m.goodput_bps()
        );
    }

    #[test]
    fn test_reverse_delivery_delta() {
        let mut m = MmpMetrics::new();

        // First call: baseline only, no ratio update
        m.update_reverse_delivery(100, 100);
        assert_eq!(m.delivery_ratio_reverse, 1.0); // unchanged from default

        // Second call: perfect delivery (200 new frames, all received)
        m.update_reverse_delivery(300, 300);
        assert!((m.delivery_ratio_reverse - 1.0).abs() < 0.001);

        // Third call: 50% loss (100 frames sent, 50 received)
        m.update_reverse_delivery(350, 400);
        assert!(
            (m.delivery_ratio_reverse - 0.5).abs() < 0.001,
            "reverse={}, expected 0.5",
            m.delivery_ratio_reverse
        );
    }

    #[test]
    fn test_reverse_delivery_rekey_reset() {
        let mut m = MmpMetrics::new();

        // Establish baseline and one measurement
        m.update_reverse_delivery(100, 100);
        m.update_reverse_delivery(300, 300);
        assert!((m.delivery_ratio_reverse - 1.0).abs() < 0.001);

        // Rekey resets reverse state
        m.reset_for_rekey();

        // First call after rekey: baseline only
        m.update_reverse_delivery(50, 50);
        // delivery_ratio_reverse was reset to 1.0 by reset_for_rekey's
        // clearing of delivery_ratio_forward; reverse is not explicitly
        // reset — but the delta state is, so next call computes fresh.
        assert_eq!(m.delivery_ratio_reverse, 1.0);

        // Second call after rekey: 80% delivery
        m.update_reverse_delivery(90, 100);
        assert!(
            (m.delivery_ratio_reverse - 0.8).abs() < 0.001,
            "reverse={}, expected 0.8",
            m.delivery_ratio_reverse
        );
    }
}
