//! The per-agent schedule KV bucket: one key per agent, holding the scheduled
//! prompts that fire in that agent.
//!
//! **swarm-controller is the only writer**, and creates the bucket on its
//! first write. It keeps each prompt once, with its name and every agent it
//! goes to, and writes each agent's key as the prompts naming that agent,
//! without the name or the other agents. Each agent's harness reads its own
//! key and fires the schedules itself; the fire times are a pure function of
//! each schedule ([`Schedule::latest_slot`]), so rewriting the list never moves
//! them.
//!
//! **An agent reads with one `DIRECT.GET` on its own key, not a watch.** A KV
//! watch is an unnamed consumer, whose create subject carries no filter, so
//! granting it would grant every agent's key. The direct get carries the key
//! in its subject ([`direct_get_subject`]), which is what lets the queue grant
//! scope the read to one agent. It is also the agent's whole `JetStream` grant:
//! `swarm_queue_client::agent_schedules::read_own` builds the stream handle
//! without a `STREAM.INFO`.
//!
//! Keyed by the agent alone, like [`crate::agent_icon`], and for the same
//! reason: an agent can move between hives.

/// The KV bucket schedules are published into, keyed by agent name.
pub const BUCKET: &str = "agent-schedules";

/// The stream backing [`BUCKET`]. `create_key_value` prefixes `KV_`.
pub const STREAM: &str = "KV_agent-schedules";

/// The subject `agent`'s entry is stored under.
#[must_use]
pub fn key_subject(agent: &str) -> String {
    format!("$KV.{BUCKET}.{agent}")
}

/// The request subject that reads `agent`'s current entry, and so the one
/// subject the queue grants that agent in this bucket.
#[must_use]
pub fn direct_get_subject(agent: &str) -> String {
    format!("$JS.API.DIRECT.GET.{STREAM}.{}", key_subject(agent))
}

/// The prompts that fire in one agent: the value under `BUCKET`/`<agent>`.
#[derive(Debug, Clone, Default, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
pub struct AgentSchedules {
    #[serde(default)]
    pub schedules: Vec<Schedule>,
}

/// One scheduled prompt.
///
/// Its fire times are `anchor_unix + k * interval_seconds` for `k >= 0`, or
/// `anchor_unix` alone when there is no interval.
#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
pub struct Schedule {
    /// Minted by the controller, the same in every agent the prompt names,
    /// and never reused, so an agent's record of what it last fired for an
    /// id cannot apply to a different schedule.
    pub id: String,
    /// The prompt, delivered verbatim as the todo's summary.
    pub body: String,
    /// The first fire time, unix seconds.
    pub anchor_unix: i64,
    /// Seconds between fires; `None` is a one-shot.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub interval_seconds: Option<u64>,
    /// A paused schedule does not fire, and the slots that pass while it is
    /// paused are not fired when it resumes.
    #[serde(default)]
    pub paused: bool,
}

impl Schedule {
    /// The latest fire time at or before `now`, or `None` while the first is
    /// still ahead.
    #[must_use]
    pub fn latest_slot(&self, now: i64) -> Option<i64> {
        if now < self.anchor_unix {
            return None;
        }
        let Some(interval) = self
            .interval_seconds
            .filter(|i| *i > 0)
            .and_then(|i| i64::try_from(i).ok())
        else {
            return Some(self.anchor_unix);
        };
        let elapsed = now - self.anchor_unix;
        Some(self.anchor_unix + elapsed - elapsed % interval)
    }
}

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

    fn every(anchor_unix: i64, interval: Option<u64>) -> Schedule {
        Schedule {
            id: "abc".to_owned(),
            body: "hi".to_owned(),
            anchor_unix,
            interval_seconds: interval,
            paused: false,
        }
    }

    /// The names the grant in `swarm-nats.nix` spells with `{agent}`.
    #[test]
    fn names_match_the_queue_grant() {
        assert_eq!(STREAM, format!("KV_{BUCKET}"));
        assert_eq!(key_subject("iris"), "$KV.agent-schedules.iris");
        assert_eq!(
            direct_get_subject("iris"),
            "$JS.API.DIRECT.GET.KV_agent-schedules.$KV.agent-schedules.iris"
        );
    }

    #[test]
    fn slots_sit_on_the_anchor_grid() {
        let s = every(1000, Some(60));
        assert_eq!(s.latest_slot(999), None);
        assert_eq!(s.latest_slot(1000), Some(1000));
        assert_eq!(s.latest_slot(1059), Some(1000));
        assert_eq!(s.latest_slot(1060), Some(1060));
        assert_eq!(s.latest_slot(1000 + 60 * 7 + 5), Some(1000 + 60 * 7));
    }

    #[test]
    fn a_one_shot_has_one_slot() {
        let s = every(1000, None);
        assert_eq!(s.latest_slot(999), None);
        assert_eq!(s.latest_slot(1000), Some(1000));
        assert_eq!(s.latest_slot(1_000_000), Some(1000));
    }

    /// Fields a newer controller adds must not cost an older agent its list,
    /// and a list written without the optional fields decodes to a recurring
    /// or one-shot schedule that is not paused.
    #[test]
    fn decoding_tolerates_missing_and_unknown_fields() {
        let doc = r#"{"schedules":[{"id":"a","body":"b","anchor_unix":5,"note":"x"}],"rev":2}"#;
        let decoded: AgentSchedules = serde_json::from_str(doc).expect("decodes");
        let mut want = every(5, None);
        "a".clone_into(&mut want.id);
        "b".clone_into(&mut want.body);
        assert_eq!(decoded.schedules, vec![want]);
    }
}