hop delays

1 files changed, 102 insertions, 0 deletions

diff --git a/src/swarm/hop.rs b/src/swarm/hop.rs
new file mode 100644
index 0000000..4cd413f
--- /dev/null
+++ b/src/swarm/hop.rs
@@ -0,0 +1,102 @@
+//! How long sync messages take from a peer to us, per hop: from the peer's clock when it
+//! generated one (`SyncEnvelope::sent`) to ours when we start handling it, the peer's
+//! clock offset taken off (`Transport::clock_offset`, from PING and PONG). So the
+//! transport, the network and the queues at both ends, which a PING's round trip leaves
+//! out (PINGs don't wait behind sync messages).
+//!
+//! The offset is known to within half a round trip only, so a single figure is too;
+//! but that error is the same for every message from a peer while the offset stands, so
+//! how the delay changes over time is exact: a link getting worse shows whatever the
+//! offset. Hence the means of the last `MINUTES` minutes besides all since start, and
+//! half the round trip to compare with (`api::peers`).
+
+use std::collections::VecDeque;
+
+use tokio::time::{Duration, Instant};
+
+use crate::swarm::stats::Propagation;
+
+/// The minutes whose means are kept.
+pub(crate) const MINUTES: usize = 15;
+const MINUTE: Duration = Duration::from_secs(60);
+
+/// A peer's hop delays (`PeerState::hop_delays`).
+#[derive(Debug, Default, Clone)]
+pub(crate) struct HopDelays {
+    /// All since we started, in buckets.
+    pub(crate) all: Propagation,
+    /// The minutes with messages, oldest first: when each began, how many, their sum.
+    minutes: VecDeque<(Instant, u64, i64)>,
+}
+
+impl HopDelays {
+    /// A message that took `delay_ms` (negative where the offset's error exceeds it).
+    pub(crate) fn record(&mut self, delay_ms: i64, now: Instant) {
+        self.all.record(delay_ms);
+        match self.minutes.back_mut() {
+            Some((began, count, sum)) if now.saturating_duration_since(*began) < MINUTE => {
+                *count += 1;
+                *sum += delay_ms;
+            }
+            _ => self.minutes.push_back((now, 1, delay_ms)),
+        }
+        while self.minutes.front().is_some_and(|(began, _, _)| {
+            now.saturating_duration_since(*began) >= MINUTE * MINUTES as u32
+        }) {
+            self.minutes.pop_front();
+        }
+    }
+
+    /// The mean of each of the last `MINUTES` minutes, oldest first, None for one without
+    /// messages; the last is the current one.
+    pub(crate) fn by_minute(&self, now: Instant) -> Vec<Option<i64>> {
+        let mut means = vec![None; MINUTES];
+        for (began, count, sum) in &self.minutes {
+            let ago = (now.saturating_duration_since(*began).as_secs() / 60) as usize;
+            if ago < MINUTES {
+                means[MINUTES - 1 - ago] = Some(sum / (*count).max(1) as i64);
+            }
+        }
+        means
+    }
+}
+
+/// How long a sync message `sent` by a peer (its clock) took to come, as `arrived` (ours),
+/// with the peer's clock `offset_ms` ahead of ours.
+pub(crate) fn delay_ms(
+    sent: crate::clock::UnixMillis,
+    arrived: crate::clock::UnixMillis,
+    offset_ms: i64,
+) -> i64 {
+    arrived.as_u64() as i64 - sent.as_u64() as i64 + offset_ms
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::clock::UnixMillis;
+
+    /// Sent at 1000 by a clock 300 ahead (700 by ours), arrived at 750: 50 ms.
+    #[test]
+    fn the_offset_is_taken_off() {
+        assert_eq!(delay_ms(UnixMillis(1000), UnixMillis(750), 300), 50);
+    }
+
+    /// Each minute's mean, oldest first; minutes past `MINUTES` forgotten.
+    #[test]
+    fn means_by_minute() {
+        let start = Instant::now();
+        let mut delays = HopDelays::default();
+        delays.record(10, start);
+        delays.record(30, start + Duration::from_secs(10));
+        delays.record(100, start + MINUTE * 2);
+        let now = start + MINUTE * 2 + Duration::from_secs(1);
+        let means = delays.by_minute(now);
+        assert_eq!(means[MINUTES - 1], Some(100));
+        assert_eq!(means[MINUTES - 2], None);
+        assert_eq!(means[MINUTES - 3], Some(20));
+        assert_eq!(delays.all.count, 3);
+        delays.record(5, start + MINUTE * (MINUTES as u32 + 1));
+        assert_eq!(delays.minutes.len(), 2, "the first minute is forgotten");
+    }
+}