//! Adaptive bitrate: the client-side AIMD controller behind the "Automatic" bitrate setting.
//!
//! Runs inside [`crate::client`]'s data-plane pump on the same 750 ms cadence as the adaptive-FEC
//! [`crate::quic::LossReport`], deciding when to ask the host for a different encoder bitrate via
//! [`crate::quic::SetBitrate`]. Division of labour with adaptive FEC: **FEC answers fast, random
//! loss** (Wi-Fi bursts, RF noise — recoverable redundancy is the right tool); **bitrate answers
//! persistent congestion** (the link simply can't carry the rate — more FEC only adds load). The
//! controller therefore reacts to *sustained* signals only:
//!
//! - **unrecoverable frames** — loss exceeded the FEC budget (the stream visibly froze/recovered);
//! - **heavy loss** — a window whose shard loss is beyond what FEC should be left to absorb alone;
//! - **one-way-delay rise** — capture→received latency (host-clock skew corrected) climbing above
//!   its rolling baseline: standing queue growth, the *pre-loss* signature of a saturated link
//!   (bufferbloat) — this is the early-warning signal loss-based control lacks;
//! - **a jump-to-live flush** — the pump discarded its backlog, the strongest "we were behind"
//!   evidence there is.
//!
//! AIMD shape: a SEVERE window (an unrecoverable frame, a flush, or ≥6 % loss) backs off ×0.7
//! immediately; ordinary congestion (heavy-but-recoverable loss, an OWD rise) needs two
//! consecutive bad windows. Recovery is two-mode: **slow start** — until the first congestion
//! signal the rate DOUBLES each clean window (cooldown-paced), which is how an Automatic session
//! climbs from the conservative start to the [`set_ceiling`](BitrateController::set_ceiling)
//! measured by the startup link-capacity probe in seconds instead of minutes — then classic
//! additive recovery (+~6 % after ~4.5 s clean, ceilinged). Changes are rate-limited (each one
//! costs the IDR the host's rebuilt encoder opens with) and the whole controller disables itself
//! against a host that never answers [`crate::quic::BitrateChanged`] (an older build that
//! ignores unknown control messages).

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

/// Never ask for less than this — below it the stream is unusable anyway and the floor keeps a
/// mis-measured window from cratering the session.
const FLOOR_KBPS: u32 = 5_000;
/// Consecutive bad windows before an ORDINARY decrease — one window can be a scheduler blip or a
/// single Wi-Fi scan; two in a row (1.5 s) is a condition. A SEVERE window skips the wait.
const BAD_WINDOWS_TO_DECREASE: u32 = 2;
/// Window shard loss at/above which ONE window is enough to back off — 6 % is past any
/// blip/retry tail, and every 750 ms spent there is visible damage. Unrecoverable frames and
/// jump-to-live flushes are severe for the same reason.
const SEVERE_LOSS_PPM: u32 = 60_000;
/// Consecutive clean windows before probing back up in congestion-avoidance mode (~4.5 s at the
/// 750 ms cadence): recovery stays slower than backoff, classic AIMD. (Slow start ignores this —
/// it doubles on every cooled clean window until the first congestion signal.)
const CLEAN_WINDOWS_TO_INCREASE: u32 = 6;
/// Minimum gap between requested changes — every accepted change costs an encoder rebuild + IDR
/// on the host today (in-place reconfigure is planned), and back-to-back steps would outrun the
/// ack/effect round trip.
const CHANGE_COOLDOWN: Duration = Duration::from_millis(1500);
/// Window shard loss beyond which the window counts bad even without an unrecoverable frame:
/// 2 % sustained is congestion territory, not the random tail FEC exists for.
const HEAVY_LOSS_PPM: u32 = 20_000;
/// How far the window's mean one-way delay may sit above the rolling baseline before it counts
/// as queue growth. 25 ms is far beyond jitter at any streamable frame rate.
const OWD_RISE_US: i64 = 25_000;
/// Rolling window (in 750 ms report windows, ~30 s) whose minimum mean is the OWD baseline.
/// Long enough to remember the uncongested floor, short enough to follow genuine path changes.
const BASELINE_WINDOWS: usize = 40;
/// Requests sent without a single [`crate::quic::BitrateChanged`] ack before concluding the host
/// predates bitrate renegotiation and going quiet for the rest of the session.
const MAX_UNACKED: u32 = 3;

/// One decision per report window; `Some(kbps)` = send a [`crate::quic::SetBitrate`].
pub(crate) struct BitrateController {
    /// `false` = permanently off (explicit user bitrate, an old host, or ack silence).
    enabled: bool,
    /// The rate we believe the host encodes at (updated by acks; requests are not assumed).
    current_kbps: u32,
    /// The climb ceiling: the negotiated start rate until the startup link-capacity probe
    /// raises it via [`set_ceiling`](Self::set_ceiling) — that measurement is what lets an
    /// Automatic session scale past its conservative start.
    ceiling_kbps: u32,
    floor_kbps: u32,
    /// Slow start: true until the first congestion signal — clean windows DOUBLE the rate
    /// (cooldown-paced) instead of the +6 % additive step.
    probing: bool,
    /// Recent window mean OWDs (µs); the rolling min is the uncongested baseline.
    owd_means: VecDeque<i64>,
    bad_windows: u32,
    clean_windows: u32,
    last_change: Option<Instant>,
    /// Requests since the last ack — reaching [`MAX_UNACKED`] disables the controller.
    unacked: u32,
}

impl BitrateController {
    /// `start_kbps` is the Welcome-resolved session bitrate when the user chose Automatic, or `0`
    /// to build a permanently-disabled controller (explicit bitrate / an old host that didn't
    /// echo one — no known ceiling to work against).
    pub(crate) fn new(start_kbps: u32) -> Self {
        BitrateController {
            enabled: start_kbps > 0,
            current_kbps: start_kbps,
            ceiling_kbps: start_kbps,
            floor_kbps: FLOOR_KBPS.min(start_kbps.max(1)),
            probing: true,
            owd_means: VecDeque::with_capacity(BASELINE_WINDOWS),
            bad_windows: 0,
            clean_windows: 0,
            last_change: None,
            unacked: 0,
        }
    }

    /// Raise the climb ceiling to a measured link capacity (the startup speed-test probe's
    /// delivered throughput with headroom already subtracted by the caller). Without this call
    /// the ceiling stays the negotiated start rate — exactly the old behavior. Never lowers:
    /// a congested-moment measurement must not shrink authority below what was negotiated
    /// (descent is the congestion signals' job).
    pub(crate) fn set_ceiling(&mut self, kbps: u32) {
        if self.enabled && kbps > self.ceiling_kbps {
            self.ceiling_kbps = kbps;
        }
    }

    /// The host's [`crate::quic::BitrateChanged`] ack: its clamp is authoritative for what the
    /// encoder now targets, and any ack proves the host renegotiates (resets the silence counter).
    pub(crate) fn on_ack(&mut self, kbps: u32) {
        if kbps > 0 {
            self.current_kbps = kbps;
        }
        self.unacked = 0;
    }

    /// Feed one report window; returns the rate to request now, if any. `dropped` = frames that
    /// went FEC-unrecoverable in the window, `loss_ppm` the window's [`crate::quic::LossReport`]
    /// figure, `owd_mean_us` the window's mean skew-corrected capture→received latency (`None`
    /// without a clock handshake), `flushed` = the pump's jump-to-live fired in the window.
    pub(crate) fn on_window(
        &mut self,
        now: Instant,
        dropped: u64,
        loss_ppm: u32,
        owd_mean_us: Option<i64>,
        flushed: bool,
    ) -> Option<u32> {
        if !self.enabled {
            return None;
        }
        if self.unacked >= MAX_UNACKED {
            // The host never answered: an older build. Go quiet instead of spamming a message it
            // logs as unknown every few seconds.
            self.enabled = false;
            tracing::info!("adaptive bitrate off — host never acked a SetBitrate (older host)");
            return None;
        }
        // OWD: compare against the rolling-min baseline of PRIOR windows (so a rising window
        // doesn't drag its own baseline up), then record it.
        let owd_bad = match owd_mean_us {
            Some(mean) => {
                let bad = self
                    .owd_means
                    .iter()
                    .min()
                    .is_some_and(|&base| mean > base + OWD_RISE_US);
                if self.owd_means.len() == BASELINE_WINDOWS {
                    self.owd_means.pop_front();
                }
                self.owd_means.push_back(mean);
                bad
            }
            None => false,
        };
        // SEVERE = the user already saw damage (an unrecoverable frame, a jump-to-live flush) or
        // loss far past any blip — one window is enough. Ordinary congestion (heavy-but-
        // recoverable loss, an OWD rise) still needs two consecutive windows.
        let severe = dropped > 0 || flushed || loss_ppm >= SEVERE_LOSS_PPM;
        let bad = severe || loss_ppm >= HEAVY_LOSS_PPM || owd_bad;
        if bad {
            self.bad_windows += 1;
            self.clean_windows = 0;
            // Any congestion signal ends slow start for good — from here on, climbs are additive.
            self.probing = false;
        } else {
            self.clean_windows += 1;
            self.bad_windows = 0;
        }
        let cooled = self
            .last_change
            .is_none_or(|t| now.duration_since(t) >= CHANGE_COOLDOWN);
        if !cooled {
            return None;
        }
        if (self.bad_windows >= BAD_WINDOWS_TO_DECREASE || (severe && self.bad_windows >= 1))
            && self.current_kbps > self.floor_kbps
        {
            let next = ((self.current_kbps as u64 * 7 / 10) as u32).max(self.floor_kbps);
            self.bad_windows = 0;
            return self.request(next, now);
        }
        if self.current_kbps < self.ceiling_kbps {
            // Slow start: double on every cooled clean window until the first congestion signal
            // (this is how an Automatic session reaches a probe-measured ceiling in seconds).
            // Congestion avoidance: +~6 % after a sustained clean run.
            if self.probing && self.clean_windows >= 1 {
                let next = self.current_kbps.saturating_mul(2).min(self.ceiling_kbps);
                self.clean_windows = 0;
                return self.request(next, now);
            }
            if self.clean_windows >= CLEAN_WINDOWS_TO_INCREASE {
                let next = (self.current_kbps + self.current_kbps / 16 + 1).min(self.ceiling_kbps);
                self.clean_windows = 0;
                return self.request(next, now);
            }
        }
        None
    }

    fn request(&mut self, kbps: u32, now: Instant) -> Option<u32> {
        self.last_change = Some(now);
        self.unacked += 1;
        // `current_kbps` is NOT updated here — the host's ack is authoritative. A lost/ignored
        // request just recomputes from the same base next time (and counts toward MAX_UNACKED).
        Some(kbps)
    }
}

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

    /// A window cadence matching the pump's 750 ms tick, safely past the change cooldown when
    /// stepped 5× between decisions.
    const TICK: Duration = Duration::from_millis(750);

    fn ticks(start: Instant, n: u32) -> Instant {
        start + TICK * n
    }

    /// Drive `n` clean windows, asserting no decision fires before the clean threshold.
    fn run_clean(c: &mut BitrateController, start: Instant, from: u32, n: u32) -> Option<u32> {
        let mut out = None;
        for i in from..from + n {
            out = c.on_window(ticks(start, i), 0, 0, Some(10_000), false);
            if out.is_some() {
                return out;
            }
        }
        out
    }

    #[test]
    fn disabled_when_not_automatic_or_old_host() {
        // start 0 = explicit user bitrate or a host that didn't echo one → permanently off.
        let mut c = BitrateController::new(0);
        let now = Instant::now();
        assert_eq!(c.on_window(now, 5, 900_000, Some(500_000), true), None);
    }

    #[test]
    fn two_ordinary_bad_windows_step_down_multiplicatively() {
        let mut c = BitrateController::new(20_000);
        let start = Instant::now();
        // Heavy-but-recoverable loss (2–6 %) is ORDINARY: one window is a blip — no reaction.
        assert_eq!(c.on_window(ticks(start, 0), 0, 25_000, None, false), None);
        // The second consecutive bad window backs off ×0.7.
        assert_eq!(
            c.on_window(ticks(start, 1), 0, 25_000, None, false),
            Some(14_000)
        );
        c.on_ack(14_000);
        // Still bad after the cooldown → another ×0.7 step from the ACKED rate.
        assert_eq!(c.on_window(ticks(start, 6), 0, 25_000, None, false), None); // bad #1 again
        assert_eq!(
            c.on_window(ticks(start, 7), 0, 25_000, None, false),
            Some(9_800)
        );
    }

    #[test]
    fn severe_window_backs_off_immediately() {
        // An unrecoverable frame (the user SAW a freeze) skips the two-window wait…
        let mut c = BitrateController::new(20_000);
        let start = Instant::now();
        assert_eq!(
            c.on_window(ticks(start, 0), 1, 0, None, false),
            Some(14_000)
        );
        // …and so does a jump-to-live flush.
        let mut c = BitrateController::new(20_000);
        assert_eq!(c.on_window(ticks(start, 0), 0, 0, None, true), Some(14_000));
        // …and ≥6 % window loss.
        let mut c = BitrateController::new(20_000);
        assert_eq!(
            c.on_window(ticks(start, 0), 0, 80_000, None, false),
            Some(14_000)
        );
    }

    #[test]
    fn cooldown_blocks_back_to_back_steps() {
        let mut c = BitrateController::new(20_000);
        let start = Instant::now();
        assert_eq!(
            c.on_window(ticks(start, 0), 1, 0, None, false),
            Some(14_000)
        );
        c.on_ack(14_000);
        // A severe window INSIDE the 1.5 s cooldown (tick 1 = 750 ms) → held; at the cooldown
        // boundary (tick 2 = 1.5 s) it fires.
        assert_eq!(c.on_window(ticks(start, 1), 1, 0, None, false), None);
        assert_eq!(c.on_window(ticks(start, 2), 1, 0, None, false), Some(9_800));
    }

    #[test]
    fn floor_is_never_crossed() {
        let mut c = BitrateController::new(6_000);
        let start = Instant::now();
        // ×0.7 of 6000 = 4200 < floor → clamped to 5000.
        assert_eq!(c.on_window(ticks(start, 0), 1, 0, None, false), Some(5_000));
        c.on_ack(5_000);
        // At the floor, further bad windows request nothing.
        assert_eq!(c.on_window(ticks(start, 6), 1, 0, None, false), None);
        assert_eq!(c.on_window(ticks(start, 7), 1, 0, None, false), None);
    }

    #[test]
    fn sustained_clean_recovers_toward_ceiling_only() {
        let mut c = BitrateController::new(20_000);
        let start = Instant::now();
        assert_eq!(
            c.on_window(ticks(start, 0), 1, 0, None, false),
            Some(14_000)
        );
        c.on_ack(14_000);
        // The backoff ended slow start → additive recovery: 6 clean windows → one +~6 % step
        // (14000 + 14000/16 + 1 = 14876).
        let up = run_clean(&mut c, start, 2, 7);
        assert_eq!(up, Some(14_876));
        c.on_ack(14_876);
        // Fully recovered → clean windows at the ceiling stay quiet (never probe past it).
        c.on_ack(20_000);
        assert_eq!(run_clean(&mut c, start, 40, 20), None);
    }

    #[test]
    fn slow_start_doubles_to_a_probed_ceiling_then_stops() {
        let mut c = BitrateController::new(20_000);
        // The startup link-capacity probe measured ~430 Mbps delivered → ×0.7 ceiling.
        c.set_ceiling(300_000);
        let start = Instant::now();
        // Every cooled clean window doubles until the ceiling caps the climb, then quiet.
        let mut got = Vec::new();
        for i in 0..14 {
            if let Some(k) = c.on_window(ticks(start, i), 0, 0, Some(10_000), false) {
                c.on_ack(k);
                got.push(k);
            }
        }
        assert_eq!(got, vec![40_000, 80_000, 160_000, 300_000]);
    }

    #[test]
    fn first_congestion_ends_slow_start_for_good() {
        let mut c = BitrateController::new(20_000);
        c.set_ceiling(300_000);
        let start = Instant::now();
        assert_eq!(
            c.on_window(ticks(start, 0), 0, 0, Some(10_000), false),
            Some(40_000)
        );
        c.on_ack(40_000);
        // Severe window → immediate ×0.7, and slow start is over.
        assert_eq!(
            c.on_window(ticks(start, 2), 1, 0, Some(10_000), false),
            Some(28_000)
        );
        c.on_ack(28_000);
        // Clean again — but the next climb is additive, after the 6-window clean run.
        let mut next = None;
        for i in 3..12 {
            next = c.on_window(ticks(start, i), 0, 0, Some(10_000), false);
            if next.is_some() {
                assert!(i >= 8, "additive climb must wait for the clean run");
                break;
            }
        }
        assert_eq!(next, Some(29_751)); // 28000 + 28000/16 + 1
    }

    #[test]
    fn set_ceiling_is_ignored_when_disabled_and_never_lowers() {
        let mut c = BitrateController::new(0);
        c.set_ceiling(1_000_000);
        assert_eq!(c.on_window(Instant::now(), 0, 0, None, false), None);
        let mut c = BitrateController::new(20_000);
        c.set_ceiling(10_000); // below the negotiated start → ignored
        assert_eq!(c.ceiling_kbps, 20_000);
    }

    #[test]
    fn owd_rise_alone_is_a_congestion_signal() {
        let mut c = BitrateController::new(20_000);
        let start = Instant::now();
        // Establish a ~10 ms baseline over a few clean windows.
        for i in 0..4 {
            assert_eq!(
                c.on_window(ticks(start, i), 0, 0, Some(10_000), false),
                None
            );
        }
        // Delay climbs 40 ms above baseline with ZERO loss — bufferbloat. Two windows → back off.
        assert_eq!(
            c.on_window(ticks(start, 4), 0, 0, Some(50_000), false),
            None
        );
        assert_eq!(
            c.on_window(ticks(start, 5), 0, 0, Some(52_000), false),
            Some(14_000)
        );
    }

    #[test]
    fn ack_silence_disables_the_controller() {
        let mut c = BitrateController::new(20_000);
        let start = Instant::now();
        let mut sent = 0;
        let mut i = 0;
        // Keep every window bad and never ack: exactly MAX_UNACKED requests, then silence.
        while i < 60 {
            if c.on_window(ticks(start, i), 1, 0, None, false).is_some() {
                sent += 1;
            }
            i += 1;
        }
        assert_eq!(sent, MAX_UNACKED);
    }
}