//! The matrix agreement: where an account's credential lives in the store, and
//! what the object at that path holds.
//!
//! Both halves are one agreement and neither end of it is senior, so they are
//! stated together here rather than split between the path module and the
//! client. Nothing in [`crate::client`] knows this shape — it moves whatever
//! type a caller names — so a second kind of swarm secret gets its own module
//! beside this one instead of another field on a shared struct.

use serde::{Deserialize, Serialize};

use crate::{
    Error,
    path::{AGENT_PREFIX, checked_segment},
};

/// The path holding `agent`'s token for the external matrix account `account`.
///
/// # Errors
/// [`Error::PathSegment`] when either name contains anything but
/// `[A-Za-z0-9_-]`, which is what keeps one agent's name from addressing
/// another agent's secret.
pub fn account_path(agent: &str, account: &str) -> Result<String, Error> {
    checked_segment("agent", agent)?;
    checked_segment("account", account)?;
    Ok(format!("{AGENT_PREFIX}/{agent}/matrix/{account}"))
}

/// What an account's path holds: the token, plus the homeserver it belongs to.
///
/// The homeserver rides with the token rather than on the queue notice that
/// triggers a delivery, because a notice is not persistence — re-delivering a
/// credential has to reconstruct it, and the store is the only thing that keeps
/// it.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Credential {
    /// `glue-matrix-bao-token.nix` reads the store with
    /// `bao kv get -field=value`, so this name is load-bearing for a reader
    /// this crate does not control. Renaming it silently breaks that unit.
    pub value: String,

    /// Absent on every object written before this field existed, and KV2 keeps
    /// those versions forever. **`Option` is what tolerates that** — serde
    /// decodes a missing field to `None` for an optional type, so the type is
    /// the compatibility guarantee and changing it to a bare `String` is what
    /// would break every stored credential at once.
    ///
    /// `skip_serializing_if` is doing separate work: without it a token-only
    /// credential serialises `"homeserver":null`, and this object is read by
    /// `glue-matrix-bao-token.nix` with `bao kv get -field=value`.
    #[serde(skip_serializing_if = "Option::is_none")]
    pub homeserver: Option<String>,
}

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

    #[test]
    fn a_well_formed_pair_lands_under_the_agent_prefix() {
        let p = account_path("atlas", "ops-relay").expect("both segments are legal");
        assert_eq!(p, "swarm/agents/atlas/matrix/ops-relay");
        assert!(p.starts_with(AGENT_PREFIX));
    }

    #[test]
    fn a_segment_cannot_escape_its_own_directory() {
        // Each of these is a *different* way to address another agent's tree,
        // and the last two are the ones a charset check catches but a
        // `contains("..")` check does not.
        for bad in [
            "../argus",
            "atlas/../argus",
            "atlas/matrix",
            "a b",
            "a.b",
            "",
        ] {
            assert!(
                account_path(bad, "ops-relay").is_err(),
                "agent segment {bad:?} must be refused"
            );
            assert!(
                account_path("atlas", bad).is_err(),
                "account segment {bad:?} must be refused"
            );
        }
    }

    #[test]
    fn the_legal_charset_is_actually_reachable() {
        // The control for the test above: if `checked_segment` rejected
        // everything, the escape cases would pass for the wrong reason.
        assert!(account_path("a-b_C9", "d-e_F0").is_ok());
    }

    /// KV2 keeps every prior version, so objects written before `homeserver`
    /// existed are still readable and still get decoded by this type. What
    /// tolerates the absence is the field being `Option`, not any attribute:
    /// making it a bare `String` would not surface as a migration, it would
    /// surface as every previously-stored credential becoming unreadable at
    /// once.
    #[test]
    fn a_credential_stored_before_the_homeserver_field_still_decodes() {
        let old: Credential = serde_json::from_str(r#"{"value":"t"}"#)
            .expect("an object written by the previous version must still decode");
        assert_eq!(old.value, "t");
        assert_eq!(old.homeserver, None);

        // Control: the field is genuinely read when present, so the arm above
        // is about absence being tolerated rather than the field being ignored.
        let new: Credential = serde_json::from_str(r#"{"value":"t","homeserver":"https://hs"}"#)
            .expect("an object with the field decodes too");
        assert_eq!(new.homeserver.as_deref(), Some("https://hs"));
    }

    #[test]
    fn the_value_field_matches_what_the_nix_reader_asks_for() {
        // The literal is the point: `bao kv get -field=value` is the other end
        // of this agreement and lives in a file no Rust test can reach, so the
        // name is pinned here rather than derived from the struct.
        //
        // It doubles as the compatibility control for `homeserver`: a
        // token-only credential must still serialise to exactly these bytes,
        // with no `homeserver` key at all, so adding the field cannot change
        // what that unit reads.
        let json = serde_json::to_string(&Credential {
            value: "t".to_owned(),
            homeserver: None,
        })
        .expect("a struct of one String serialises");
        assert_eq!(json, r#"{"value":"t"}"#);
    }
}