//! 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: subscribed to with LINKS while anyone
//! looks (renewed every `LINKS_EVERY`, ended when nobody does; `swarm::links`), so a
//! member tells of a change as it happens; the same answers for all clients. Or, if
//! asked (`POST /admin/dashboard/links?live=false`, for every client, until asked
//! otherwise or the node restarts), only asked every `LINKS_EVERY`, without
//! subscribing: fewer messages in the swarm, a map up to `LINKS_EVERY` behind. With
//! the clocks the links carry, every member's clock against the swarm's (`clocks`).

mod clocks;

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

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

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

/// How often the snapshot is built and compared, while anyone looks.
const TICK: Duration = Duration::from_secs(2);
/// How often the subscriptions to the members' links are renewed (each renewal is
/// answered: a reply at least this often), and the graph recomputed; changes come in
/// between, as they happen (`swarm::links`).
const LINKS_EVERY: Duration = Duration::from_secs(30);
/// How long a subscription lasts unless renewed: a node that is gone stops it.
const LINKS_LEASE: Duration = Duration::from_secs(90);
/// A member's links older than this are left out (it didn't answer since).
const LINKS_KEPT_FOR: Duration = Duration::from_secs(90);
/// 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>>,
    /// Where the document changed since the last tick, to the clients connected:
    /// `{"paths": [...]}`, or `{"all": true}` if too many or not known.
    documents: broadcast::Sender<Arc<String>>,
    /// The messages exchanged since the last tick, one by one (`traffic::MessageLog`),
    /// to the clients connected: a JSON list.
    messages: broadcast::Sender<Arc<String>>,
    control: Mutex<Control>,
    /// Someone asked for it now (`refresh`): a tick at once, members asked again.
    poke: Notify,
    /// The members' links subscribed to (true, at first), or only asked for every
    /// `LINKS_EVERY` (`set_links`).
    live: AtomicBool,
    /// True once the node stops: every client gets an `offline` event, and its stream
    /// ends (so the HTTP server needn't wait for clients to go).
    closing: watch::Receiver<bool>,
}

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, closing: watch::Receiver<bool>) -> Arc<Self> {
        let (latest, _) = watch::channel(None);
        let (patches, _) = broadcast::channel(64);
        let (documents, _) = broadcast::channel(64);
        let (messages, _) = broadcast::channel(64);
        Arc::new(Self {
            state,
            latest,
            patches,
            documents,
            messages,
            poke: Notify::new(),
            live: AtomicBool::new(true),
            control: Mutex::new(Control {
                running: false,
                wanted_until: Instant::now(),
            }),
            closing,
        })
    }

    /// 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 && !*self.closing.borrow() {
            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; None if the node stops first.
    async fn current(&self) -> Option<Arc<Published>> {
        let mut latest = self.latest.subscribe();
        let mut closing = self.closing.clone();
        // The borrows (lock guards, not Send) end within the arms.
        tokio::select! {
            published = latest.wait_for(Option::is_some) => {
                published.ok().and_then(|current| current.clone())
            }
            _ = closing.wait_for(|closing| *closing) => None,
        }
    }

    /// Resolves once the node stops (or its signal is gone).
    async fn closed(&self) {
        let mut closing = self.closing.clone();
        let _ = closing.wait_for(|closing| *closing).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();
        let mut changes = self.state.change_events.subscribe();
        // The messages one by one, from now on, while it runs.
        let log = &self.state.traffic.log;
        log.watch(true);
        let mut logged = 0u64;
        loop {
            let live = self.live.load(Ordering::Relaxed);
            if Instant::now() >= graph_due {
                graph.refresh(&self.state, live).await;
                graph_due = Instant::now() + LINKS_EVERY;
            }
            let snapshot = build(&self.state, &graph, live).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);
            }
            let poked = tokio::select! {
                () = tokio::time::sleep(TICK) => false,
                () = self.poke.notified() => true,
            };
            if poked {
                graph_due = Instant::now();
            }
            if let Some(changed) = document_changes(&mut changes) {
                let _ = self.documents.send(Arc::new(changed.to_string()));
            }
            let mut messages = log.after(logged, MESSAGES_AT_MOST);
            by_node(&self.state, &mut messages).await;
            if let Some(last) = messages.last() {
                logged = last.seq;
                if let Ok(json) = serde_json::to_string(&messages) {
                    let _ = self.messages.send(Arc::new(json));
                }
            }
            let unwanted = || {
                self.patches.receiver_count() == 0
                    && Instant::now() >= self.control.lock_or_recover().wanted_until
            };
            if *self.closing.borrow() {
                log.watch(false);
                self.control.lock_or_recover().running = false;
                return;
            }
            if unwanted() {
                // Nobody looks: the members needn't tell us any more. Unless someone
                // came meanwhile, then it goes on (and subscribes again).
                for member in self.state.links.subscribed() {
                    links::unsubscribe(&self.state, &member).await;
                }
                let mut control = self.control.lock_or_recover();
                if self.patches.receiver_count() == 0 && Instant::now() >= control.wanted_until {
                    log.watch(false);
                    control.running = false;
                    return;
                }
                drop(control);
                graph_due = Instant::now();
            }
        }
    }
}

/// Paths changed in a tick listed at most; more: `all`.
/// The messages the paths layer logged under an address (its own PINGs and PONGs) under
/// the node there instead, where one is known: the address it proved itself at, or is
/// said to be at (`Transport::identities_of`), named in `address`.
async fn by_node(state: &AppState, messages: &mut [crate::transport::traffic::Logged]) {
    let peers = state.peers_snapshot().await;
    let known = |peer: &str| peers.iter().any(|(known, _)| known == peer);
    if messages.iter().all(|message| known(&message.peer)) {
        return;
    }
    let mut nodes: std::collections::HashMap<String, String> = std::collections::HashMap::new();
    for (peer, _) in &peers {
        for address in state.transport.identities_of(peer) {
            nodes.entry(address).or_insert_with(|| peer.clone());
        }
    }
    for message in messages {
        if let Some(node) = nodes.get(&message.peer) {
            message.address = Some(std::mem::replace(&mut message.peer, node.clone()));
        }
    }
}

const MAX_CHANGED: usize = 256;
/// Messages sent to the clients per tick at most, the latest (the console keeps fewer).
const MESSAGES_AT_MOST: usize = 500;

/// Where the document changed since last asked (`Hub::documents`), if anywhere.
fn document_changes(
    changes: &mut broadcast::Receiver<Arc<crate::document::store::ChangeSet>>,
) -> Option<JsonValue> {
    let mut paths = BTreeSet::new();
    let mut all = false;
    loop {
        match changes.try_recv() {
            Ok(set) => {
                paths.extend(set.paths.iter().cloned());
                all |= paths.len() > MAX_CHANGED;
            }
            Err(broadcast::error::TryRecvError::Lagged(_)) => all = true,
            Err(_) => break,
        }
    }
    if all {
        Some(json!({ "all": true }))
    } else if paths.is_empty() {
        None
    } else {
        Some(json!({ "paths": paths }))
    }
}

/// What a client's stream waits for.
enum Next {
    Patch(Result<Arc<Published>, broadcast::error::RecvError>),
    Document(Result<Arc<String>, broadcast::error::RecvError>),
    Messages(Result<Arc<String>, broadcast::error::RecvError>),
    Closing,
}

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

impl Graph {
    async fn refresh(&mut self, state: &AppState, live: bool) {
        let lease = if live { LINKS_LEASE } else { Duration::ZERO };
        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, lease).await });
        }
    }
}

/// The snapshot: the node, its figures, its peers, and the map (`live`: its links
/// subscribed to).
async fn build(state: &AppState, graph: &Graph, live: bool) -> JsonValue {
    // The peers once, for their list and the figures' totals over them.
    let list = super::peers::peer_list(state).await;
    let stats = serde_json::to_value(super::stats::collect(state, &list).await).unwrap_or_default();
    let peers = serde_json::to_value(&list).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 own = links::own_links(state).await;
    let replies: Vec<_> = state
        .links
        .replies()
        .into_iter()
        .filter(|(_, age, _)| *age <= LINKS_KEPT_FOR)
        .collect();
    let mut measured = Map::new();
    measured.insert(
        local.to_owned(),
        json!({ "secsAgo": 0, "links": links_json(&own) }),
    );
    for (member, age, member_links) in &replies {
        measured.insert(
            member.clone(),
            json!({ "secsAgo": age.as_secs(), "links": links_json(member_links) }),
        );
    }
    let clocks = clocks::swarm(
        local,
        std::iter::once((local, &own[..])).chain(
            replies
                .iter()
                .map(|(member, _, member_links)| (member.as_str(), &member_links[..])),
        ),
        u64::try_from(state.clock_skew_warn.as_millis()).unwrap_or(u64::MAX),
    );
    // 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 },
        "page": page().0,
        "stats": stats,
        "peers": peers,
        "graph": {
            "members": members_json,
            "intended": intended,
            "measured": measured,
            "clocks": clocks,
            "live": live,
        },
    })
}

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

/// Fields that grow by themselves besides those named `…SecsAgo`; the page gets the
/// list when served (`GROWING_MARK`).
const GROWING: &[&str] = &[
    "uptimeSecs",
    "dormantForSecs",
    "divergedForSecs",
    "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> {
    // Its streams end when the node shuts down (`AppState::closing`).
    let closing = state.closing.subscribe();
    Router::new()
        .route("/admin/dashboard", get(page_html))
        .route("/admin/dashboard/state", get(current_state))
        .route("/admin/dashboard/events", get(events))
        .route("/admin/dashboard/refresh", post(refresh))
        .route("/admin/dashboard/links", post(set_links))
        .layer(Extension(Hub::new(state, closing)))
}

/// The page as built into this binary.
const PAGE: &str = include_str!("dashboard.html");
/// Where the page says which version of it it is.
const PAGE_VERSION_MARK: &str = "__PAGE_VERSION__";
/// Where the page gets the age fields (`GROWING`), so the list is kept once.
const GROWING_MARK: &str = "__GROWING__";

/// The page's version (a hash of it), and the page saying so: a page whose version
/// differs from the snapshot's `page` was served by another build, and reloads.
fn page() -> &'static (String, String) {
    static PAGE_SERVED: OnceLock<(String, String)> = OnceLock::new();
    PAGE_SERVED.get_or_init(|| {
        use sha2::{Digest, Sha256};
        let growing = serde_json::to_string(GROWING).unwrap_or_else(|_| "[]".to_owned());
        let page = PAGE.replace(GROWING_MARK, &growing);
        // Of the page with its list: a new list is a new version too.
        let version = Hex(&Sha256::digest(page.as_bytes())[..8]).to_string();
        let served = page.replace(PAGE_VERSION_MARK, &version);
        (version, served)
    })
}

/// `GET /admin/dashboard`: the page.
async fn page_html() -> Html<&'static str> {
    Html(page().1.as_str())
}

/// `POST /admin/dashboard/refresh`: a tick now rather than at the next `TICK`, and the
/// members asked for their links again; what changed comes as events, as ever.
async fn refresh(Extension(hub): Extension<Arc<Hub>>) -> StatusCode {
    hub.want();
    hub.poke.notify_one();
    StatusCode::NO_CONTENT
}

#[derive(serde::Deserialize)]
struct LinksQuery {
    live: bool,
}

/// `POST /admin/dashboard/links?live=…`: the members' links subscribed to (`true`), or
/// only asked for every `LINKS_EVERY` (`false`), for every client; the subscriptions
/// ended at once, and the members asked again now.
async fn set_links(
    Extension(hub): Extension<Arc<Hub>>,
    Query(query): Query<LinksQuery>,
) -> StatusCode {
    hub.live.store(query.live, Ordering::Relaxed);
    if !query.live {
        for member in hub.state.links.subscribed() {
            links::unsubscribe(&hub.state, &member).await;
        }
    }
    hub.want();
    hub.poke.notify_one();
    StatusCode::NO_CONTENT
}

/// `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 Some(current) = hub.current().await else {
        return StatusCode::SERVICE_UNAVAILABLE.into_response();
    };
    (
        [(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), a `document` where the document
/// changed (`Hub::documents`, at most every tick), `messages` with the messages since
/// the last tick (`Hub::messages`), and `offline` when the node stops, which ends the
/// stream.
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();
    let mut documents = hub.documents.subscribe();
    let mut messages = hub.messages.subscribe();
    hub.want();
    let first = hub.current().await;
    let stream = async_stream::stream! {
        let Some(first) = first else {
            yield Ok::<_, Infallible>(offline_event());
            return;
        };
        let mut seen = first.seq;
        yield Ok(snapshot_event(&first));
        loop {
            let next = tokio::select! {
                received = patches.recv() => Next::Patch(received),
                changed = documents.recv() => Next::Document(changed),
                logged = messages.recv() => Next::Messages(logged),
                () = hub.closed() => Next::Closing,
            };
            let received = match next {
                Next::Patch(received) => received,
                Next::Document(Ok(changed)) => {
                    yield Ok(Event::default().event("document").data(changed.as_str()));
                    continue;
                }
                Next::Document(Err(broadcast::error::RecvError::Lagged(_))) => {
                    yield Ok(Event::default().event("document").data(r#"{"all":true}"#));
                    continue;
                }
                Next::Document(Err(broadcast::error::RecvError::Closed)) => break,
                Next::Messages(Ok(logged)) => {
                    yield Ok(Event::default().event("messages").data(logged.as_str()));
                    continue;
                }
                // Missed ones are only missed in the console.
                Next::Messages(Err(broadcast::error::RecvError::Lagged(_))) => continue,
                Next::Messages(Err(broadcast::error::RecvError::Closed)) => break,
                Next::Closing => {
                    yield Ok(offline_event());
                    break;
                }
            };
            match received {
                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 Some(current) = hub.current().await else {
                        yield Ok(offline_event());
                        break;
                    };
                    seen = current.seq;
                    yield Ok(snapshot_event(&current));
                }
                Err(broadcast::error::RecvError::Closed) => break,
            }
        }
    };
    Sse::new(stream).keep_alive(KeepAlive::default())
}

/// The node stops: the client may reconnect once it is back.
fn offline_event() -> Event {
    Event::default().event("offline").data("{}")
}

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));
    }
}