//! The dashboard (`--features dashboard`): a page showing the node's figures
//! (`/admin/stats`), its peers (`/peers`) and a map of the swarm, kept up to date
//! through Server-Sent Events.
//!
//! One producer per node serves every client: while at least one is connected (or
//! asked for the state within `LINGER`), it builds the snapshot every `TICK` and, if
//! something changed, publishes it, and a JSON merge patch (RFC 7386) from the one
//! before to every client connected. Nothing changed, nothing is sent (but SSE's
//! keep-alive comments). Ages (`…SecsAgo`, `uptimeSecs` and the like, `GROWING`) grow by
//! themselves: they alone changing is no change, and the page counts them on.
//!
//! The map: the graph the member list makes (`topology::intended_graph`, as this node
//! sees it), and the links every member measures, asked for with LINKS every
//! `LINKS_EVERY` while anyone looks (`swarm::links`), the same answers for all clients.

use std::collections::{BTreeMap, BTreeSet};
use std::convert::Infallible;
use std::sync::{Arc, Mutex};
use std::time::Duration;

use axum::http::{HeaderValue, header::CONTENT_TYPE};
use axum::response::sse::{Event, KeepAlive, Sse};
use axum::response::{Html, IntoResponse, Response};
use axum::routing::get;
use axum::{Extension, Router};
use serde_json::{Map, Value as JsonValue, json};
use tokio::sync::{broadcast, watch};
use tokio::time::Instant;

use crate::locks::MutexExt;
use crate::state::AppState;
use crate::swarm::{links, members, topology};
use crate::transport::Link;

/// How often the snapshot is built and compared, while anyone looks.
const TICK: Duration = Duration::from_secs(2);
/// How often members are asked for their links, and the graph recomputed.
const LINKS_EVERY: Duration = Duration::from_secs(10);
/// A member's links older than this are left out (it didn't answer since).
const LINKS_KEPT_FOR: Duration = Duration::from_secs(30);
/// The producer runs on this long after the last request for the state.
const LINGER: Duration = Duration::from_secs(60);
/// Members asked for their links at most, each round.
const MAX_ASKED: usize = 512;

/// A snapshot or a patch as published: its number, and its JSON.
struct Published {
    seq: u64,
    json: String,
}

/// The producer and what it published, shared by all clients.
struct Hub {
    state: AppState,
    /// The latest snapshot; None while the producer hasn't made one (since it started).
    latest: watch::Sender<Option<Arc<Published>>>,
    /// Each patch, to the clients connected.
    patches: broadcast::Sender<Arc<Published>>,
    control: Mutex<Control>,
}

struct Control {
    running: bool,
    /// The producer stops once no client is connected and this is past.
    wanted_until: Instant,
}

impl Hub {
    fn new(state: AppState) -> Arc<Self> {
        let (latest, _) = watch::channel(None);
        let (patches, _) = broadcast::channel(64);
        Arc::new(Self {
            state,
            latest,
            patches,
            control: Mutex::new(Control {
                running: false,
                wanted_until: Instant::now(),
            }),
        })
    }

    /// Someone looks: the producer runs (started if it wasn't), for `LINGER` at least.
    fn want(self: &Arc<Self>) {
        let mut control = self.control.lock_or_recover();
        control.wanted_until = Instant::now() + LINGER;
        if !control.running {
            control.running = true;
            // What it published before it stopped is stale.
            self.latest.send_replace(None);
            tokio::spawn(Arc::clone(self).run());
        }
    }

    /// The latest snapshot, once there is one.
    async fn current(&self) -> Arc<Published> {
        let mut latest = self.latest.subscribe();
        loop {
            // The borrow (a lock guard, not Send) ends here, before any later await.
            let published = latest
                .wait_for(Option::is_some)
                .await
                .ok()
                .and_then(|current| current.clone());
            if let Some(published) = published {
                return published;
            }
            // The sender lives as long as the hub: not reached.
            tokio::time::sleep(TICK).await;
        }
    }

    async fn run(self: Arc<Self>) {
        let mut last: Option<JsonValue> = None;
        let mut seq = 0u64;
        let mut graph = Graph::default();
        let mut graph_due = Instant::now();
        loop {
            if Instant::now() >= graph_due {
                graph.refresh(&self.state).await;
                graph_due = Instant::now() + LINKS_EVERY;
            }
            let snapshot = build(&self.state, &graph).await;
            let changed = last
                .as_ref()
                .is_none_or(|old| without_ages(old) != without_ages(&snapshot));
            if changed {
                seq += 1;
                if let Some(old) = &last
                    && let Some(patch) = merge_patch(old, &snapshot)
                {
                    let _ = self.patches.send(Arc::new(Published {
                        seq,
                        json: patch.to_string(),
                    }));
                }
                self.latest.send_replace(Some(Arc::new(Published {
                    seq,
                    json: snapshot.to_string(),
                })));
                last = Some(snapshot);
            }
            tokio::time::sleep(TICK).await;
            {
                let mut control = self.control.lock_or_recover();
                if self.patches.receiver_count() == 0 && Instant::now() >= control.wanted_until {
                    control.running = false;
                    return;
                }
            }
        }
    }
}

/// The intended graph, recomputed every `LINKS_EVERY` (it costs about `members²`), when
/// members are asked for their links too.
#[derive(Default)]
struct Graph {
    intended: BTreeMap<String, BTreeSet<String>>,
}

impl Graph {
    async fn refresh(&mut self, state: &AppState) {
        self.intended = topology::intended_graph(state).unwrap_or_default();
        let members: Vec<String> = {
            let replica = state.doc.read().await;
            members::members(&replica.doc)
        };
        for member in members
            .into_iter()
            .filter(|member| member != state.local_id())
            .take(MAX_ASKED)
        {
            let state = state.clone();
            tokio::spawn(async move { links::ask(&state, &member).await });
        }
    }
}

/// The snapshot: the node, its figures, its peers, and the map.
async fn build(state: &AppState, graph: &Graph) -> JsonValue {
    let stats = serde_json::to_value(super::stats::collect(state).await).unwrap_or_default();
    let peers = serde_json::to_value(super::peers::peer_list(state).await).unwrap_or_default();
    let aliases = {
        let replica = state.doc.read().await;
        members::aliases(&replica.doc)
    };
    let local = state.local_id();
    let intended: Map<String, JsonValue> = graph
        .intended
        .iter()
        .map(|(member, neighbours)| (member.clone(), json!(neighbours)))
        .collect();
    let mut measured = Map::new();
    measured.insert(
        local.to_owned(),
        json!({ "secsAgo": 0, "links": links_json(&links::own_links(state).await) }),
    );
    for (member, age, member_links) in state.links.replies() {
        if age <= LINKS_KEPT_FOR {
            measured.insert(
                member,
                json!({ "secsAgo": age.as_secs(), "links": links_json(&member_links) }),
            );
        }
    }
    // Everyone on the map: in the member list's graph, or measured.
    let mut members_json = Map::new();
    for member in graph.intended.keys().chain(measured.keys()) {
        members_json.insert(
            member.clone(),
            json!({ "alias": aliases.get(member), "self": member == local }),
        );
    }
    json!({
        "node": { "id": local, "alias": state.alias },
        "stats": stats,
        "peers": peers,
        "graph": { "members": members_json, "intended": intended, "measured": measured },
    })
}

fn links_json(links: &[Link]) -> JsonValue {
    links
        .iter()
        .map(|link| {
            json!({
                "peer": link.peer,
                "transport": link.transport,
                "relayed": link.relayed,
                "rttMs": link.rtt_us.map(|us| f64::from(us) / 1000.0),
                "dormant": link.dormant,
                "others": link.others.iter().map(|other| json!({
                    "transport": other.transport,
                    "rttMs": other.rtt_us.map(|us| f64::from(us) / 1000.0),
                })).collect::<Vec<_>>(),
            })
        })
        .collect()
}

/// Fields that grow by themselves besides those named `…SecsAgo` (the page knows
/// them too).
const GROWING: &[&str] = &[
    "uptimeSecs",
    "dormantForSecs",
    "observedSecs",
    "lastFromNeighbourSecs",
    "oldestQuarantineSecs",
];

/// Whether the field `key` is an age, which grows by itself.
fn is_age(key: &str) -> bool {
    key == "secsAgo" || key.ends_with("SecsAgo") || GROWING.contains(&key)
}

/// `value` without its ages, to tell whether anything else changed.
fn without_ages(value: &JsonValue) -> JsonValue {
    match value {
        JsonValue::Object(object) => JsonValue::Object(
            object
                .iter()
                .filter(|(key, _)| !is_age(key))
                .map(|(key, value)| (key.clone(), without_ages(value)))
                .collect(),
        ),
        JsonValue::Array(items) => JsonValue::Array(items.iter().map(without_ages).collect()),
        other => other.clone(),
    }
}

/// The JSON merge patch (RFC 7386) from `old` to `new`, if they differ: objects field
/// by field (a field gone is null), anything else replaced whole. A null in `new` reads
/// as the field gone, as a merge patch can't tell them apart.
fn merge_patch(old: &JsonValue, new: &JsonValue) -> Option<JsonValue> {
    match (old, new) {
        (JsonValue::Object(old), JsonValue::Object(new)) => {
            let mut patch = Map::new();
            for key in old.keys() {
                if !new.contains_key(key) {
                    patch.insert(key.clone(), JsonValue::Null);
                }
            }
            for (key, value) in new {
                match old.get(key) {
                    Some(previous) => {
                        if let Some(change) = merge_patch(previous, value) {
                            patch.insert(key.clone(), change);
                        }
                    }
                    None => {
                        patch.insert(key.clone(), value.clone());
                    }
                }
            }
            (!patch.is_empty()).then_some(JsonValue::Object(patch))
        }
        (old, new) if old == new => None,
        (_, new) => Some(new.clone()),
    }
}

/// The dashboard's routes (authenticated, as the rest of `/admin`), with their hub.
pub(crate) fn router(state: AppState) -> Router<AppState> {
    Router::new()
        .route("/admin/dashboard", get(page))
        .route("/admin/dashboard/state", get(current_state))
        .route("/admin/dashboard/events", get(events))
        .layer(Extension(Hub::new(state)))
}

/// `GET /admin/dashboard`: the page.
async fn page() -> Html<&'static str> {
    Html(include_str!("dashboard.html"))
}

/// `GET /admin/dashboard/state`: the latest snapshot, for a client that wants it whole.
async fn current_state(Extension(hub): Extension<Arc<Hub>>) -> Response {
    hub.want();
    let current = hub.current().await;
    (
        [(CONTENT_TYPE, HeaderValue::from_static("application/json"))],
        current.json.clone(),
    )
        .into_response()
}

/// `GET /admin/dashboard/events`: Server-Sent Events: a `snapshot` first, then a
/// `patch` whenever something changed (each with its number as the event's ID; a
/// client that fell behind gets a `snapshot` again).
async fn events(Extension(hub): Extension<Arc<Hub>>) -> impl IntoResponse {
    // Subscribed before the snapshot is read: no patch after it is missed.
    let mut patches = hub.patches.subscribe();
    hub.want();
    let first = hub.current().await;
    let stream = async_stream::stream! {
        let mut seen = first.seq;
        yield Ok::<_, Infallible>(snapshot_event(&first));
        loop {
            match patches.recv().await {
                Ok(patch) if patch.seq <= seen => {}
                Ok(patch) => {
                    seen = patch.seq;
                    yield Ok(Event::default()
                        .event("patch")
                        .id(patch.seq.to_string())
                        .data(patch.json.clone()));
                }
                Err(broadcast::error::RecvError::Lagged(_)) => {
                    let current = hub.current().await;
                    seen = current.seq;
                    yield Ok(snapshot_event(&current));
                }
                Err(broadcast::error::RecvError::Closed) => break,
            }
        }
    };
    Sse::new(stream).keep_alive(KeepAlive::default())
}

fn snapshot_event(published: &Published) -> Event {
    Event::default()
        .event("snapshot")
        .id(published.seq.to_string())
        .data(published.json.clone())
}

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

    #[test]
    fn merge_patches() {
        let old = json!({ "a": 1, "b": { "c": 2, "d": [1, 2] }, "gone": true });
        let new = json!({ "a": 1, "b": { "c": 3, "d": [1, 2] }, "added": "x" });
        assert_eq!(
            merge_patch(&old, &new),
            Some(json!({ "b": { "c": 3 }, "gone": null, "added": "x" }))
        );
        assert_eq!(merge_patch(&old, &old), None);
        assert_eq!(
            merge_patch(&json!([1]), &json!([1, 2])),
            Some(json!([1, 2]))
        );
    }

    #[test]
    fn ages_alone_are_no_change() {
        let old =
            json!({ "uptimeSecs": 1, "peers": { "p": { "lastSeenSecsAgo": 3, "rttMs": 2 } } });
        let older =
            json!({ "uptimeSecs": 9, "peers": { "p": { "lastSeenSecsAgo": 7, "rttMs": 2 } } });
        assert_eq!(without_ages(&old), without_ages(&older));
        let moved =
            json!({ "uptimeSecs": 9, "peers": { "p": { "lastSeenSecsAgo": 7, "rttMs": 5 } } });
        assert_ne!(without_ages(&old), without_ages(&moved));
    }
}