//! The one spelling of the token an agent presents its own queue secret in,
//! shared by the agent that formats it and the auth-callout responder that
//! parses it back:
//! [`AGENT_TOKEN_PREFIX`](crate::agent_token::AGENT_TOKEN_PREFIX), the agent's
//! name, `.`, the secret.
//!
//! The secret itself lives at `swarm/agents/<agent>/queue` in the swarm's
//! secret store (`swarm_secret_client::queue`). This module knows nothing of
//! the store, so an agent formats its token without linking a store client.

/// Marks an `auth_token` as an agent's own credential.
///
/// An OIDC access token may itself contain `.`, so the `<agent>.<secret>`
/// shape alone does not tell the two apart. Authelia's access tokens start
/// `authelia_at_`.
pub const AGENT_TOKEN_PREFIX: &str = "swarm-agent.";

/// An agent's own credential as presented at the queue.
///
/// No `Debug`: `secret` is the credential itself.
pub struct AgentToken<'a> {
    /// The agent the presenter claims to be. Unproven until `secret` is
    /// checked against that agent's stored credential.
    pub agent: &'a str,
    /// The presented secret.
    pub secret: &'a str,
}

/// A token that is not `<agent>.<secret>` after its prefix, or parts that
/// would not make one. Carries the reason only: a token holds a secret.
#[derive(Debug, thiserror::Error)]
#[error("malformed agent token: {0}")]
pub struct Malformed(pub &'static str);

/// Spell `agent`'s credential as the token it presents at the queue.
///
/// # Errors
/// [`Malformed`] when `agent` or `secret` is empty or holds anything outside
/// `[A-Za-z0-9_-]`. Either would make [`parse_agent_token`] split the token
/// differently than it was joined.
pub fn format_agent_token(agent: &str, secret: &str) -> Result<String, Malformed> {
    check_agent(agent)?;
    check_secret(secret)?;
    Ok(format!("{AGENT_TOKEN_PREFIX}{agent}.{secret}"))
}

/// Read a presented token back into an [`AgentToken`].
///
/// `None` when `token` does not start with [`AGENT_TOKEN_PREFIX`]: it is some
/// other kind of token, not a malformed one of these. `Some(Err(_))` when it
/// does and is not exactly `<agent>.<secret>` after it.
#[must_use]
pub fn parse_agent_token(token: &str) -> Option<Result<AgentToken<'_>, Malformed>> {
    let rest = token.strip_prefix(AGENT_TOKEN_PREFIX)?;
    Some(
        rest.split_once('.')
            .ok_or(Malformed("no `.` between the agent and the secret"))
            .and_then(|(agent, secret)| {
                check_agent(agent)?;
                check_secret(secret)?;
                Ok(AgentToken { agent, secret })
            }),
    )
}

fn is_segment(s: &str) -> bool {
    !s.is_empty()
        && s.bytes()
            .all(|b| b.is_ascii_alphanumeric() || b == b'-' || b == b'_')
}

/// The alphabet a store path segment allows, so the name cannot widen a
/// subject with `.`, `*` or `>`, or address another agent's path.
fn check_agent(agent: &str) -> Result<(), Malformed> {
    if is_segment(agent) {
        Ok(())
    } else {
        Err(Malformed("the agent is not a single [A-Za-z0-9_-] segment"))
    }
}

/// The alphabet the controller mints secrets in, base64url without padding.
/// The error never carries the value.
fn check_secret(secret: &str) -> Result<(), Malformed> {
    if is_segment(secret) {
        Ok(())
    } else {
        Err(Malformed(
            "the secret is empty or holds a byte outside [A-Za-z0-9_-]",
        ))
    }
}

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

    #[test]
    fn an_agent_token_round_trips() {
        let token = format_agent_token("atlas", "Ab9_-z").expect("legal");
        assert_eq!(token, "swarm-agent.atlas.Ab9_-z");
        let parsed = parse_agent_token(&token)
            .expect("carries the prefix")
            .expect("well-formed");
        assert_eq!(parsed.agent, "atlas");
        assert_eq!(parsed.secret, "Ab9_-z");
    }

    /// Anything without the prefix belongs to the OIDC path, including a
    /// token that happens to look like `<agent>.<secret>`.
    #[test]
    fn a_token_without_the_prefix_is_not_an_agent_token() {
        for token in ["authelia_at_abc.def", "atlas.s3cr3t", "", "swarm-agent"] {
            assert!(parse_agent_token(token).is_none(), "{token:?}");
        }
    }

    #[test]
    fn a_malformed_agent_token_is_refused() {
        for token in [
            "swarm-agent.",
            "swarm-agent.atlas",
            "swarm-agent.atlas.",
            "swarm-agent..s3cr3t",
            "swarm-agent.atlas.s3.cr3t",
            "swarm-agent.at*las.s3cr3t",
            "swarm-agent.at>las.s3cr3t",
            "swarm-agent.at/las.s3cr3t",
            "swarm-agent.atlas.s3cr3t=",
            "swarm-agent.atlas.s3 cr3t",
        ] {
            assert!(
                matches!(parse_agent_token(token), Some(Err(_))),
                "{token:?} must be refused"
            );
        }
    }

    /// What formats always parses back into the same two parts.
    #[test]
    fn formatting_refuses_what_parsing_would_split_differently() {
        assert!(format_agent_token("at.las", "s3cr3t").is_err());
        assert!(format_agent_token("atlas", "s3.cr3t").is_err());
        assert!(format_agent_token("atlas", "").is_err());
        assert!(format_agent_token("", "s3cr3t").is_err());
    }

    #[test]
    fn a_malformed_secret_is_not_echoed_in_the_error() {
        let Some(Err(e)) = parse_agent_token("swarm-agent.atlas.hunter2!") else {
            panic!("must be refused");
        };
        assert!(!e.to_string().contains("hunter2"), "{e}");
    }
}