//! Creating an agent's subagent stream. The names, subjects, grants and the
//! subagent list are `swarm_wire::subagent_term`.
//!
//! `swarm-controller` creates one stream per agent with the fixed config
//! [`crate::subagent_term::open_or_create`] spells.

use swarm_wire::subagent_term::{MAX_AGE, stream_name, stream_subjects};

/// How large one agent's stream may grow before the discard policy reclaims
/// space. 64 MiB is enough rows from a day of ordinary subagent chatter to
/// stay well inside it while still bounding the disk one compromised or
/// runaway agent can claim for 24h.
const MAX_BYTES: i64 = 64 * 1024 * 1024;

/// The config [`open_or_create`] creates `agent`'s stream with.
///
/// `max_message_size` is taken as a parameter rather than read from a
/// constant here: it tracks the queue's live `max_payload`
/// ([`crate::max_payload`]), the same value the publisher already sizes its
/// rows against, so the stream never rejects a row the queue itself
/// accepted.
pub(crate) fn stream_config(
    agent: &str,
    max_message_size: i32,
) -> async_nats::jetstream::stream::Config {
    async_nats::jetstream::stream::Config {
        name: stream_name(agent),
        description: Some(format!("Terminal rows of {agent}'s subagents")),
        subjects: vec![stream_subjects(agent)],
        max_age: MAX_AGE,
        max_bytes: MAX_BYTES,
        discard: async_nats::jetstream::stream::DiscardPolicy::Old,
        max_message_size,
        ..Default::default()
    }
}

/// Open `agent`'s subagent stream, creating it if it does not exist yet. An
/// existing stream is opened as it is.
pub async fn open_or_create(
    client: &async_nats::Client,
    agent: &str,
) -> Result<async_nats::jetstream::stream::Stream, crate::Error> {
    let js = async_nats::jetstream::new(client.clone());
    let name = stream_name(agent);
    match js.get_stream(&name).await {
        Ok(stream) => Ok(stream),
        Err(e) => {
            tracing::info!(stream = %name, reason = %e, "subagent terminal stream not available, creating it");
            let max_message_size = i32::try_from(crate::max_payload(client)).unwrap_or(i32::MAX);
            js.create_stream(stream_config(agent, max_message_size))
                .await
                .map_err(|source| crate::Error::CreateAgentStream {
                    stream: name,
                    source,
                })
        }
    }
}

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

    /// The config the controller creates caps disk per agent and never
    /// fails a publish because of that cap — `discard: Old` reclaims the
    /// oldest rows instead of rejecting the newest.
    #[test]
    fn stream_config_caps_bytes_and_discards_the_oldest() {
        let cfg = stream_config("iris", 8_388_608);
        assert_eq!(cfg.max_bytes, MAX_BYTES);
        assert_eq!(
            cfg.discard,
            async_nats::jetstream::stream::DiscardPolicy::Old
        );
        assert_eq!(cfg.max_message_size, 8_388_608);
    }
}