//! Whom each neighbour reaches: a node's 2-hop neighbourhood, as measured, not as the
//! member list says.
//!
//! A node knows its own links, and from them whom it reaches: the members it has an
//! active session with (handshake done, neither dormant nor unreachable) that answer
//! (`PeerState::answer_overdue`: none left a keepalive or sync message unanswered for
//! `Timing::unreached_after`, asked again halfway, with nothing else heard from it; a
//! member gone shows within seconds, not after `dormant_after`). It tells its
//! neighbours so in its STATUS (`Status::reaches`, each member by its pair key,
//! `topology::pair_key`, 8 bytes instead of a node ID): in every keepalive STATUS and
//! STATUS_REPLY, and soon to every neighbour when it changed (the peer worker looks
//! every 5 s, and at once when a member we reach stops answering), but not in
//! announcements of heads, which go often. So every node knows whom its neighbours
//! reach, as of their latest STATUS, for as long as the session lasts (a
//! neighbour that goes dormant, or starts anew, says it again). Not for a set time:
//! while a pair exchanges changes it sends no keepalives, and so no STATUS, for as
//! long as the writes go on. Nothing goes into the document, and nobody knows more than
//! two hops out.
//!
//! What it is for: passing changes on around links that don't work, and to exactly the
//! members the sender doesn't reach; a member a neighbour reaches is one worth trying,
//! and a way to it. Not for choosing multipoint relays (as OLSR does): over the member
//! list's graph, which is close to random, neighbourhoods hardly overlap, and nearly
//! every neighbour would be one (13.7 of 13.7 at 250 members and more), saving next to
//! nothing but in a full mesh.

use std::collections::HashMap;

use tokio::time::Instant;
use tracing::debug;

use crate::locks::MutexExt;
use crate::state::{AppState, HelloState, Standing};
use crate::swarm::peer::announce_heads;
use crate::swarm::{members, topology};

/// Whom we reach, as last told our neighbours.
#[derive(Default)]
pub(crate) struct Reach {
    published: Option<Vec<u64>>,
}

/// The pair keys of the members we reach now, sorted: those with an active session
/// that answer.
async fn measure(state: &AppState) -> Vec<u64> {
    let now = Instant::now();
    let mut reached = Vec::new();
    for (peer, peer_state) in state.peers_snapshot().await {
        let peer_state = peer_state.lock().await;
        if peer_state.hello_state == HelloState::Established
            && !peer_state.liveness.is_dormant()
            && !peer_state.is_unreachable()
            && !peer_state.answer_overdue(now)
            && matches!(peer_state.standing, Standing::Member { .. })
        {
            reached.push(topology::pair_key(&peer));
        }
    }
    reached.sort_unstable();
    reached
}

/// Whom we reach, as we tell it (`Status::reaches`); empty until first measured.
pub(crate) fn published(state: &AppState) -> Vec<u64> {
    state
        .reach
        .lock_or_recover()
        .published
        .clone()
        .unwrap_or_default()
}

/// With the peer worker's periodic pass (every 5 s), and at once when a member we said
/// we reach stops answering (`reached_stops_answering`): whom we reach now; if it changed,
/// our neighbours are told at once (a STATUS each, unanswered unless they lack some of
/// our heads). A change that is undone within a pass isn't told.
pub(crate) async fn check(state: &AppState) {
    let now = measure(state).await;
    {
        let mut reach = state.reach.lock_or_recover();
        if reach.published.as_ref() == Some(&now) {
            return;
        }
        reach.published = Some(now.clone());
    }
    debug!(
        reached = now.len(),
        "Whom we reach changed; telling our neighbours"
    );
    for (peer, peer_state) in state.peers_snapshot().await {
        let established = peer_state.lock().await.hello_state == HelloState::Established;
        if !established || !topology::is_neighbour(state, &peer) {
            continue;
        }
        if let Err(error) = tell(state, &peer).await {
            debug!(peer = %peer, error = %format!("{error:#}"), "Telling whom we reach failed");
        }
    }
}

/// Whether `peer`, whose answer is overdue (`PeerState::answer_overdue`), is one we
/// said we reach: then the peer worker checks at once (`check`).
pub(crate) fn reached_stops_answering(state: &AppState, peer: &str) -> bool {
    state
        .reach
        .lock_or_recover()
        .published
        .as_ref()
        .is_some_and(|keys| keys.contains(&topology::pair_key(peer)))
}

/// A STATUS to `peer` with whom we reach: as an announcement (unanswered unless it lacks
/// some of our heads), but with `reaches`.
async fn tell(state: &AppState, peer: &str) -> anyhow::Result<()> {
    announce_heads(state, peer, true).await
}

/// The members a peer reaches, as it last said in this session (`said`, kept with its
/// state, cleared when the session ends: `PeerState::reset_sync`); by pair key.
pub(crate) fn of(said: Option<&(Instant, Vec<u64>)>) -> Option<Vec<u64>> {
    said.map(|(_, reaches)| reaches.clone())
}

/// The members of our member list by pair key, to name what neighbours say they reach.
pub(crate) async fn names(state: &AppState) -> HashMap<u64, String> {
    let replica = state.doc.read().await;
    members::members(&replica.doc)
        .into_iter()
        .map(|member| (topology::pair_key(&member), member))
        .collect()
}