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const WINDOW: usize = 64;
/// Rolling packet-loss estimator over the last 64 resolved packets.
/// Tracks outcomes (acked vs lost) for `PacketType::Data` packets only;
/// heartbeats and other non-data packets are excluded by the caller.
///
/// O(1) record and O(1) query. Memory cost: 64 bytes + 3 words.
pub struct LossMonitor {
outcomes: [bool; WINDOW], // true = acked, false = lost
write_pos: usize,
total: usize, // number of valid entries, capped at WINDOW
acked_count: usize, // number of acked entries in the valid window
}
impl Default for LossMonitor {
fn default() -> Self {
Self::new()
}
}
impl LossMonitor {
/// Creates a new `LossMonitor` with a zeroed sliding window.
pub fn new() -> Self {
Self {
outcomes: [false; WINDOW],
write_pos: 0,
total: 0,
acked_count: 0,
}
}
/// Records that a packet was acknowledged (not lost).
pub fn record_acked(&mut self) {
self.record(true);
}
/// Records that a packet was lost (not acknowledged).
pub fn record_lost(&mut self) {
self.record(false);
}
fn record(&mut self, acked: bool) {
if self.total == WINDOW {
// Evict oldest outcome from running tally before overwriting.
if self.outcomes[self.write_pos] {
self.acked_count -= 1;
}
} else {
self.total += 1;
}
self.outcomes[self.write_pos] = acked;
if acked {
self.acked_count += 1;
}
self.write_pos = (self.write_pos + 1) % WINDOW;
}
/// Fraction of tracked packets that were lost (0.0–1.0).
/// Returns 0.0 if no packets have been tracked yet.
pub fn packet_loss_pct(&self) -> f32 {
if self.total == 0 {
return 0.0;
}
1.0 - (self.acked_count as f32 / self.total as f32)
}
}
#[cfg(test)]
mod tests {
use super::LossMonitor;
#[test]
fn empty_returns_zero() {
let m = LossMonitor::new();
assert_eq!(m.packet_loss_pct(), 0.0);
}
#[test]
fn all_acked_returns_zero_loss() {
let mut m = LossMonitor::new();
for _ in 0..32 {
m.record_acked();
}
assert_eq!(m.packet_loss_pct(), 0.0);
}
#[test]
fn all_lost_returns_one() {
let mut m = LossMonitor::new();
for _ in 0..64 {
m.record_lost();
}
assert_eq!(m.packet_loss_pct(), 1.0);
}
#[test]
fn fifty_percent_loss() {
let mut m = LossMonitor::new();
for i in 0..64 {
if i % 2 == 0 { m.record_acked(); } else { m.record_lost(); }
}
assert!((m.packet_loss_pct() - 0.5).abs() < 1e-6);
}
#[test]
fn window_evicts_oldest_entries() {
let mut m = LossMonitor::new();
// Fill with 64 losses, then add 64 acks — loss should drop to 0.
for _ in 0..64 { m.record_lost(); }
assert_eq!(m.packet_loss_pct(), 1.0);
for _ in 0..64 { m.record_acked(); }
assert_eq!(m.packet_loss_pct(), 0.0);
}
}