//! How long sync messages (and BLOB messages) take from a peer to us, per hop: from the
//! peer's clock when it generated one (`SyncEnvelope::sent`, `BlobMessage::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
    }
}

/// A message from `peer` that it `sent` (its clock) is here: how long it took, recorded
/// with the peer's hop delays (`PeerState::hop_delays`); None while its clock isn't
/// known.
pub(crate) async fn note(
    state: &crate::state::AppState,
    peer: &str,
    sent: crate::clock::UnixMillis,
) -> Option<i64> {
    let offset = state.transport.clock_offset(peer)?;
    let delay = delay_ms(sent, state.clock.now_millis(), offset.offset_ms);
    if let Some(peer_state) = state.get_peer_state(peer).await {
        peer_state
            .lock()
            .await
            .hop_delays
            .record(delay, Instant::now());
    }
    Some(delay)
}

/// 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");
    }
}