gossan-engine 0.3.1

//! Token-bucket rate limiter with sub-microsecond precision.
//!
//! Controls packet transmission rate to avoid overwhelming the NIC or
//! triggering upstream rate limits. Uses `Instant` for high-resolution
//! timing without syscall overhead.

use std::time::Instant;

/// Token-bucket rate limiter.
///
/// Refills at `rate_pps` tokens per second. Each `consume()` call
/// blocks until a token is available, providing smooth rate control.
pub struct RateLimiter {
    /// Tokens available (scaled by 1000 for sub-token precision).
    tokens_x1000: i64,
    /// Maximum tokens (burst capacity).
    max_tokens_x1000: i64,
    /// Tokens added per microsecond (scaled by 1000).
    refill_per_us_x1000: i64,
    /// Last refill time.
    last_refill: Instant,
    /// Target rate in packets per second.
    rate_pps: u64,
}

impl RateLimiter {
    /// Create a new rate limiter.
    ///
    /// - `rate_pps`: target packets per second (0 = unlimited)
    /// - `burst`: maximum burst size in packets
    #[must_use]
    pub fn new(rate_pps: u64, burst: u64) -> Self {
        let burst = burst.max(1).min(i64::MAX as u64 / 1000);
        let refill_per_us_x1000 = if rate_pps == 0 {
            i64::MAX / 2 // Effectively unlimited
        } else {
            // rate_pps tokens/sec = rate_pps/1_000_000 tokens/μs
            // Scaled by 1000: rate_pps * 1000 / 1_000_000 = rate_pps / 1000
            (rate_pps as i64).max(1) // At least 1 token per 1000μs
        };

        Self {
            tokens_x1000: (burst as i64) * 1000,
            max_tokens_x1000: (burst as i64) * 1000,
            refill_per_us_x1000,
            last_refill: Instant::now(),
            rate_pps,
        }
    }

    /// Create an unlimited rate limiter (no throttling).
    #[must_use]
    pub fn unlimited() -> Self {
        Self::new(0, u64::MAX / 2)
    }

    /// Try to consume one token. Returns `true` if the token was available.
    /// Does NOT block.
    pub fn try_consume(&mut self) -> bool {
        self.refill();
        if self.tokens_x1000 >= 1000 {
            self.tokens_x1000 -= 1000;
            true
        } else {
            false
        }
    }

    /// Try to consume `n` tokens. Returns the number actually consumed.
    pub fn try_consume_batch(&mut self, n: u64) -> u64 {
        self.refill();
        let available = (self.tokens_x1000 / 1000).max(0) as u64;
        let consumed = available.min(n);
        self.tokens_x1000 -= (consumed as i64) * 1000;
        consumed
    }

    /// Block until a token is available, then consume it.
    ///
    /// Uses spin-wait for sub-microsecond precision when the wait is short,
    /// and `thread::yield_now` for longer waits.
    pub fn consume_blocking(&mut self) {
        loop {
            self.refill();
            if self.tokens_x1000 >= 1000 {
                self.tokens_x1000 -= 1000;
                return;
            }
            // Estimate wait time
            let deficit = 1000 - self.tokens_x1000;
            if self.refill_per_us_x1000 > 0 {
                let wait_us = deficit / self.refill_per_us_x1000.max(1);
                if wait_us > 100 {
                    std::thread::yield_now();
                } else {
                    std::hint::spin_loop();
                }
            } else {
                std::hint::spin_loop();
            }
        }
    }

    /// Current target rate.
    #[must_use]
    pub fn rate_pps(&self) -> u64 {
        self.rate_pps
    }

    /// Whether this limiter is unlimited.
    #[must_use]
    pub fn is_unlimited(&self) -> bool {
        self.rate_pps == 0
    }

    /// Re-target the rate at runtime. Used by `AdaptiveLoop` to react
    /// to TX drops / ICMP-unreachable bursts without rebuilding the
    /// limiter (which would lose the bucket fill state and stutter).
    pub fn set_rate_pps(&mut self, rate_pps: u64) {
        self.rate_pps = rate_pps;
        self.refill_per_us_x1000 = if rate_pps == 0 {
            i64::MAX / 2
        } else {
            (rate_pps as i64).max(1)
        };
    }

    fn refill(&mut self) {
        let now = Instant::now();
        let elapsed_us = now.duration_since(self.last_refill).as_micros() as i64;
        if elapsed_us > 0 {
            let new_tokens = elapsed_us.saturating_mul(self.refill_per_us_x1000);
            self.tokens_x1000 = self
                .tokens_x1000
                .saturating_add(new_tokens)
                .min(self.max_tokens_x1000);
            self.last_refill = now;
        }
    }
}

/// Adaptive rate controller that adjusts pps based on observed packet loss.
pub struct AdaptiveRate {
    /// Current target rate.
    current_pps: u64,
    /// Maximum configured rate.
    max_pps: u64,
    /// Minimum rate floor.
    min_pps: u64,
    /// Consecutive successful batches.
    success_streak: u32,
    /// Consecutive failed/dropped batches.
    drop_streak: u32,
}

impl AdaptiveRate {
    /// Create a new adaptive rate controller.
    #[must_use]
    pub fn new(max_pps: u64) -> Self {
        Self {
            current_pps: max_pps / 2, // Start at half rate
            max_pps,
            min_pps: 1000, // Never go below 1K pps
            success_streak: 0,
            drop_streak: 0,
        }
    }

    /// Report a successful batch (all packets sent).
    pub fn report_success(&mut self) {
        self.drop_streak = 0;
        self.success_streak += 1;

        // Additive increase after 10 consecutive successes
        if self.success_streak >= 10 {
            self.current_pps = (self.current_pps + self.max_pps / 20).min(self.max_pps);
            self.success_streak = 0;
        }
    }

    /// Report packet drops.
    pub fn report_drops(&mut self, _drop_count: u64) {
        self.success_streak = 0;
        self.drop_streak += 1;

        // Multiplicative decrease
        self.current_pps = (self.current_pps * 3 / 4).max(self.min_pps);
    }

    /// Current recommended rate.
    #[must_use]
    pub fn current_pps(&self) -> u64 {
        self.current_pps
    }
}

/// Closed-loop wrapper: drives a [`RateLimiter`] from netforge
/// `EngineStats` deltas. Call [`AdaptiveLoop::tick`] every batch.
///
/// Decision rules:
///
/// * `tx_drops` increased since the last tick → packets are being lost
///   on the way out (NIC ring full, kernel back-pressure). Halve the
///   target rate via [`AdaptiveRate::report_drops`].
/// * `tx_drops` flat AND `tx_packets` increased → call
///   [`AdaptiveRate::report_success`]; after 10 clean ticks the rate
///   creeps back up by 5% of the configured ceiling.
/// * The applied rate is propagated to the wrapped [`RateLimiter`] so
///   the per-batch consume path actually slows down.
///
/// This is the interlock between the *observed* loss signal and the
/// *enforced* token bucket. Without it the bucket would stay pegged
/// at the configured ceiling regardless of what the wire is doing.
pub struct AdaptiveLoop {
    rate: AdaptiveRate,
    last_tx_packets: u64,
    last_tx_drops: u64,
    initialized: bool,
}

impl AdaptiveLoop {
    /// Construct with `max_pps` as the ceiling. Initial enforced rate
    /// starts at half (`max_pps / 2`) per [`AdaptiveRate::new`].
    #[must_use]
    pub fn new(max_pps: u64) -> Self {
        Self {
            rate: AdaptiveRate::new(max_pps),
            last_tx_packets: 0,
            last_tx_drops: 0,
            initialized: false,
        }
    }

    /// Process a fresh stats snapshot and return the enforced rate.
    ///
    /// First call seeds the baseline and returns the initial rate
    /// without classifying anything.
    pub fn tick(&mut self, tx_packets: u64, tx_drops: u64) -> u64 {
        if !self.initialized {
            self.last_tx_packets = tx_packets;
            self.last_tx_drops = tx_drops;
            self.initialized = true;
            return self.rate.current_pps();
        }
        let drop_delta = tx_drops.saturating_sub(self.last_tx_drops);
        let packet_delta = tx_packets.saturating_sub(self.last_tx_packets);
        if drop_delta > 0 {
            self.rate.report_drops(drop_delta);
        } else if packet_delta > 0 {
            self.rate.report_success();
        }
        self.last_tx_packets = tx_packets;
        self.last_tx_drops = tx_drops;
        self.rate.current_pps()
    }

    /// Apply the loop's current target to a [`RateLimiter`]. Call
    /// after every tick — cheap (one assignment) and safe.
    pub fn apply(&self, limiter: &mut RateLimiter) {
        limiter.set_rate_pps(self.rate.current_pps());
    }

    /// Current enforced rate.
    #[must_use]
    pub fn current_pps(&self) -> u64 {
        self.rate.current_pps()
    }
}

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

    #[test]
    fn rate_limiter_unlimited_always_succeeds() {
        let mut rl = RateLimiter::unlimited();
        for _ in 0..10_000 {
            assert!(rl.try_consume());
        }
    }

    #[test]
    fn rate_limiter_respects_burst() {
        let mut rl = RateLimiter::new(1_000_000, 10);
        // Should be able to consume burst immediately
        for i in 0..10 {
            assert!(rl.try_consume(), "failed at burst token {i}");
        }
        // Next one might fail (no time for refill)
        // (depending on timing, this is non-deterministic, so we just check burst worked)
    }

    #[test]
    fn rate_limiter_batch_consume() {
        // Rate kept very low (10 pps) so refill between the two
        // synchronous calls is effectively zero; otherwise at 1M pps
        // the bucket refills to `burst` in microseconds and the second
        // batch happily consumes the full 100, not the 50 the test
        // claims it should.
        let mut rl = RateLimiter::new(10, 100);
        let consumed = rl.try_consume_batch(50);
        assert_eq!(consumed, 50);
        let consumed2 = rl.try_consume_batch(100);
        assert_eq!(consumed2, 50); // Only 50 remaining from burst
    }

    #[test]
    fn adaptive_rate_decreases_on_drops() {
        let mut ar = AdaptiveRate::new(1_000_000);
        let initial = ar.current_pps();
        ar.report_drops(100);
        assert!(ar.current_pps() < initial);
    }

    #[test]
    fn adaptive_rate_increases_on_success() {
        let mut ar = AdaptiveRate::new(1_000_000);
        let initial = ar.current_pps();
        for _ in 0..20 {
            ar.report_success();
        }
        assert!(ar.current_pps() > initial);
    }

    #[test]
    fn adaptive_rate_never_below_floor() {
        let mut ar = AdaptiveRate::new(1_000_000);
        for _ in 0..100 {
            ar.report_drops(1000);
        }
        assert!(ar.current_pps() >= 1000);
    }

    #[test]
    fn set_rate_pps_changes_refill() {
        let mut r = RateLimiter::new(1_000, 100);
        assert_eq!(r.rate_pps(), 1_000);
        r.set_rate_pps(500);
        assert_eq!(r.rate_pps(), 500);
        // zero must put the limiter into unlimited mode
        r.set_rate_pps(0);
        assert!(r.is_unlimited());
    }

    #[test]
    fn adaptive_loop_initial_tick_is_baseline_only() {
        let mut lo = AdaptiveLoop::new(1_000_000);
        let initial = lo.current_pps();
        // First tick: no classification, just baseline capture.
        let returned = lo.tick(0, 0);
        assert_eq!(returned, initial);
    }

    #[test]
    fn adaptive_loop_decreases_on_tx_drop_burst() {
        let mut lo = AdaptiveLoop::new(1_000_000);
        let before = lo.tick(0, 0); // baseline
                                    // 1000 packets sent, 50 dropped — classifier sees drops.
        let after = lo.tick(1000, 50);
        assert!(
            after < before,
            "expected rate to decrease: {after} < {before}"
        );
    }

    #[test]
    fn adaptive_loop_increases_after_clean_streak() {
        let mut lo = AdaptiveLoop::new(1_000_000);
        lo.tick(0, 0); // baseline
        let before = lo.current_pps();
        for i in 1..=15 {
            // 100 packets per tick, no drops.
            lo.tick(i * 100, 0);
        }
        assert!(
            lo.current_pps() > before,
            "expected rate to increase after streak: {} > {before}",
            lo.current_pps()
        );
    }

    #[test]
    fn adaptive_loop_apply_propagates_to_limiter() {
        let mut lo = AdaptiveLoop::new(1_000_000);
        let mut limiter = RateLimiter::new(1_000_000, 1000);
        lo.tick(0, 0);
        // Force a drop tick.
        lo.tick(1000, 100);
        lo.apply(&mut limiter);
        assert_eq!(limiter.rate_pps(), lo.current_pps());
    }

    #[test]
    fn adaptive_loop_converges_under_synthetic_loss_pattern() {
        // 50% packet loss → loop must clamp the rate hard.
        let mut lo = AdaptiveLoop::new(10_000_000);
        let start = lo.current_pps();
        for i in 1..=20 {
            lo.tick(i * 1000, i * 500);
        }
        let final_pps = lo.current_pps();
        assert!(
            final_pps < start / 4,
            "expected aggressive decrease: {final_pps} >= {} (start/4)",
            start / 4
        );
    }
}