//! AZ-675 — multi-topic, per-client lossy publisher.
//!
//! The publisher owns one `tokio::sync::broadcast` channel per topic.
//! Each subscribed client gets per-topic receivers; falling behind
//! more than the channel capacity causes `tokio::sync::broadcast` to
//! return `RecvError::Lagged(n)` which the server-side stream
//! handler turns into a `drops_total{client_id, topic} += n`
//! increment. Slow clients never block the publisher, and a slow
//! client on one topic does not affect any other client or topic.

use std::collections::HashMap;
use std::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
use std::sync::Arc;

use parking_lot::Mutex;
use serde::Serialize;
use tokio::sync::broadcast;
use tracing::warn;

use crate::internal::proto::{TelemetryMessage, Topic};

/// Per-topic broadcast capacity. A client falling more than this many
/// messages behind on a topic experiences drops on that topic. The
/// value is chosen empirically: 10 Hz telemetry → 256 messages ≈ 25
/// seconds of buffering, which is more than enough for transient
/// modem stalls but bounded for memory.
pub const DEFAULT_TOPIC_CAPACITY: usize = 256;

/// All known topics, in the same order as the proto enum (skipping
/// UNSPECIFIED).
pub const ALL_TOPICS: &[Topic] = &[
    Topic::TelemetrySample,
    Topic::GimbalState,
    Topic::DetectionEvent,
    Topic::MovementCandidate,
    Topic::MapObjectsBundle,
];

/// Errors returned by [`TelemetryPublisher::publish`]. Publish never
/// blocks on slow clients; the only producer-side failure is
/// serialization of the payload, which is a programmer error caught
/// at compile time everywhere we control. We surface it explicitly
/// rather than swallow.
#[derive(Debug, thiserror::Error)]
pub enum PublishError {
    #[error("serialize topic={topic:?}: {source}")]
    Serialize {
        topic: Topic,
        #[source]
        source: serde_json::Error,
    },
}

#[derive(Debug, Clone)]
pub struct PerTopicCounters {
    pub published: u64,
    pub bytes_out: u64,
}

/// Snapshot of publisher health for the [`crate::TelemetryStreamHandle::health`]
/// surface and the AZ-675 NFR observability hooks.
#[derive(Debug, Clone, Default)]
pub struct PublisherSnapshot {
    pub subscribed_clients: usize,
    pub published_total: u64,
    pub per_topic: HashMap<Topic, PerTopicCounters>,
    pub drops_total: HashMap<(String, Topic), u64>,
}

struct TopicChannel {
    tx: broadcast::Sender<TelemetryMessage>,
    seq: AtomicU64,
    published: AtomicU64,
    bytes_out: AtomicU64,
}

impl TopicChannel {
    fn new(capacity: usize) -> Self {
        let (tx, _) = broadcast::channel(capacity);
        Self {
            tx,
            seq: AtomicU64::new(0),
            published: AtomicU64::new(0),
            bytes_out: AtomicU64::new(0),
        }
    }
}

/// Drop counter map. Keyed by `(client_id, topic)`. We isolate it
/// behind a `parking_lot::Mutex` because writes happen from
/// per-client tonic stream tasks; reads happen from the health
/// surface — both are infrequent compared to the broadcast hot path.
type DropMap = Mutex<HashMap<(String, Topic), AtomicU64>>;

pub struct TelemetryPublisher {
    topics: HashMap<Topic, TopicChannel>,
    drops: DropMap,
    subscribed_clients: AtomicUsize,
}

impl TelemetryPublisher {
    /// Build a publisher with the same per-topic capacity for every
    /// topic. Use [`new_with_capacities`] if a single topic needs a
    /// different buffer (e.g. MapObjectsBundle which is bursty).
    pub fn new(capacity: usize) -> Arc<Self> {
        let mut topics = HashMap::with_capacity(ALL_TOPICS.len());
        for &t in ALL_TOPICS {
            topics.insert(t, TopicChannel::new(capacity));
        }
        Arc::new(Self {
            topics,
            drops: Mutex::new(HashMap::new()),
            subscribed_clients: AtomicUsize::new(0),
        })
    }

    pub fn default_capacity() -> Arc<Self> {
        Self::new(DEFAULT_TOPIC_CAPACITY)
    }

    /// Serialise `payload` and fan out to every subscriber on `topic`.
    ///
    /// Never blocks. `broadcast::Sender::send` returns the number of
    /// receivers it queued for; a return of 0 means no current
    /// subscribers, which is fine — we still bump the published
    /// counter so the operator can confirm the producer is alive.
    pub fn publish<T: Serialize>(&self, topic: Topic, payload: &T) -> Result<(), PublishError> {
        let channel = match self.topics.get(&topic) {
            Some(c) => c,
            None => {
                warn!(?topic, "unknown topic; dropping publish");
                return Ok(());
            }
        };

        let payload_json = serde_json::to_vec(payload)
            .map_err(|e| PublishError::Serialize { topic, source: e })?;
        let bytes_len = payload_json.len() as u64;

        let seq = channel.seq.fetch_add(1, Ordering::Relaxed) + 1;
        let msg = TelemetryMessage {
            topic: topic as i32,
            monotonic_ts_ns: shared::clock::MonoClock::new().elapsed_ns(),
            sequence: seq,
            payload_json,
        };

        let _ = channel.tx.send(msg);
        channel.published.fetch_add(1, Ordering::Relaxed);
        channel.bytes_out.fetch_add(bytes_len, Ordering::Relaxed);
        Ok(())
    }

    /// Open a per-client receiver for `topic`. The caller (the
    /// `Subscribe` RPC handler) is responsible for handling
    /// `RecvError::Lagged` by calling [`record_drops`].
    pub(crate) fn subscribe_topic(
        &self,
        topic: Topic,
    ) -> Option<broadcast::Receiver<TelemetryMessage>> {
        self.topics.get(&topic).map(|c| c.tx.subscribe())
    }

    pub(crate) fn register_client(&self) {
        self.subscribed_clients.fetch_add(1, Ordering::Relaxed);
    }

    pub(crate) fn deregister_client(&self) {
        self.subscribed_clients.fetch_sub(1, Ordering::Relaxed);
    }

    pub fn record_drops(&self, client_id: &str, topic: Topic, n: u64) {
        if n == 0 {
            return;
        }
        let mut map = self.drops.lock();
        let key = (client_id.to_string(), topic);
        map.entry(key)
            .or_insert_with(|| AtomicU64::new(0))
            .fetch_add(n, Ordering::Relaxed);
    }

    pub fn snapshot(&self) -> PublisherSnapshot {
        let mut per_topic = HashMap::with_capacity(self.topics.len());
        let mut total_published = 0u64;
        for (&t, c) in &self.topics {
            let published = c.published.load(Ordering::Relaxed);
            let bytes_out = c.bytes_out.load(Ordering::Relaxed);
            total_published = total_published.saturating_add(published);
            per_topic.insert(
                t,
                PerTopicCounters {
                    published,
                    bytes_out,
                },
            );
        }
        let drops_map = self.drops.lock();
        let drops_total: HashMap<(String, Topic), u64> = drops_map
            .iter()
            .map(|(k, v)| (k.clone(), v.load(Ordering::Relaxed)))
            .collect();
        PublisherSnapshot {
            subscribed_clients: self.subscribed_clients.load(Ordering::Relaxed),
            published_total: total_published,
            per_topic,
            drops_total,
        }
    }
}

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

    #[derive(Serialize, Deserialize, Debug, PartialEq)]
    struct Sample {
        v: u32,
    }

    #[test]
    fn publish_with_no_subscribers_is_no_op() {
        // Arrange
        let pub_ = TelemetryPublisher::new(8);

        // Act
        pub_.publish(Topic::TelemetrySample, &Sample { v: 1 })
            .unwrap();

        // Assert
        let snap = pub_.snapshot();
        assert_eq!(snap.subscribed_clients, 0);
        assert_eq!(snap.per_topic[&Topic::TelemetrySample].published, 1);
    }

    #[tokio::test]
    async fn slow_subscriber_lags_fast_subscriber_does_not() {
        // Arrange — capacity 4 so 100 publishes overflow trivially.
        let pub_ = TelemetryPublisher::new(4);
        let mut fast = pub_.subscribe_topic(Topic::TelemetrySample).unwrap();
        let mut slow = pub_.subscribe_topic(Topic::TelemetrySample).unwrap();

        // Act — burst 100 messages WITHOUT giving `slow` a chance to drain.
        for v in 0..100u32 {
            pub_.publish(Topic::TelemetrySample, &Sample { v }).unwrap();
            // fast drains immediately; slow does nothing.
            let _ = fast.recv().await.unwrap();
        }

        // Drain `slow` and count lag.
        let mut total_drops: u64 = 0;
        let mut delivered: u64 = 0;
        loop {
            match slow.try_recv() {
                Ok(_) => delivered += 1,
                Err(broadcast::error::TryRecvError::Lagged(n)) => total_drops += n,
                Err(_) => break,
            }
        }

        // Assert — fast got all 100; slow lost most but delivered ≤ capacity.
        assert!(delivered <= 4, "slow can hold at most capacity (4)");
        assert_eq!(
            total_drops + delivered,
            100,
            "every published msg accounted for"
        );
        assert!(total_drops > 0, "slow subscriber MUST have lagged");
    }

    #[test]
    fn record_drops_accumulates_per_client_topic() {
        // Arrange
        let pub_ = TelemetryPublisher::new(8);

        // Act
        pub_.record_drops("client_a", Topic::TelemetrySample, 5);
        pub_.record_drops("client_a", Topic::TelemetrySample, 3);
        pub_.record_drops("client_a", Topic::GimbalState, 2);
        pub_.record_drops("client_b", Topic::TelemetrySample, 1);

        // Assert
        let snap = pub_.snapshot();
        assert_eq!(
            snap.drops_total[&("client_a".to_string(), Topic::TelemetrySample)],
            8
        );
        assert_eq!(
            snap.drops_total[&("client_a".to_string(), Topic::GimbalState)],
            2
        );
        assert_eq!(
            snap.drops_total[&("client_b".to_string(), Topic::TelemetrySample)],
            1
        );
    }

    #[test]
    fn register_deregister_balance_tracks_subscribed_clients() {
        // Arrange
        let pub_ = TelemetryPublisher::new(8);
        assert_eq!(pub_.snapshot().subscribed_clients, 0);

        // Act
        pub_.register_client();
        pub_.register_client();
        assert_eq!(pub_.snapshot().subscribed_clients, 2);
        pub_.deregister_client();

        // Assert
        assert_eq!(pub_.snapshot().subscribed_clients, 1);
    }
}