//! Bearers verified recently enough to be granted again without a store
//! lookup.
//!
//! Only grants are held. A denial is never remembered, so a credential that
//! starts verifying is accepted on its next request; a credential that stops
//! verifying (re-minted, or the agent destroyed) is still accepted until its
//! entry expires, at most the TTL after its last successful lookup.
//!
//! Entries are keyed by the SHA-256 of the bearer, so the process never holds
//! a presented secret past the request that carried it.

use std::collections::HashMap;
use std::time::{Duration, Instant};

use sha2::{Digest, Sha256};

type Key = [u8; 32];

fn key(bearer: &str) -> Key {
    Sha256::digest(bearer.as_bytes()).into()
}

/// Granted bearers, each until its expiry, at most `cap` of them.
pub struct Verdicts {
    ttl: Duration,
    cap: usize,
    expiries: HashMap<Key, Instant>,
}

impl Verdicts {
    /// A TTL of zero holds nothing, so every request reaches the store.
    pub fn new(ttl: Duration, cap: usize) -> Self {
        Self {
            ttl,
            cap,
            expiries: HashMap::new(),
        }
    }

    /// Whether `bearer` was granted less than the TTL before `now`.
    pub fn granted(&self, bearer: &str, now: Instant) -> bool {
        self.expiries
            .get(&key(bearer))
            .is_some_and(|expiry| now < *expiry)
    }

    /// Records that `bearer` verified at `now`. When full, the entry closest
    /// to expiry goes, which with one TTL for every entry is the oldest.
    pub fn grant(&mut self, bearer: &str, now: Instant) {
        if self.ttl.is_zero() {
            return;
        }
        let key = key(bearer);
        if !self.expiries.contains_key(&key)
            && self.expiries.len() >= self.cap
            && let Some(oldest) = self
                .expiries
                .iter()
                .min_by_key(|(_, expiry)| **expiry)
                .map(|(k, _)| *k)
        {
            self.expiries.remove(&oldest);
        }
        self.expiries.insert(key, now + self.ttl);
    }

    #[cfg(test)]
    fn len(&self) -> usize {
        self.expiries.len()
    }
}

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

    const TTL: Duration = Duration::from_mins(1);

    #[test]
    fn a_grant_holds_until_the_ttl() {
        let t0 = Instant::now();
        let mut v = Verdicts::new(TTL, 8);
        assert!(!v.granted("swarm-agent.atlas.s", t0));
        v.grant("swarm-agent.atlas.s", t0);
        assert!(v.granted("swarm-agent.atlas.s", t0));
        assert!(v.granted("swarm-agent.atlas.s", t0 + Duration::from_millis(59_999)));
        assert!(!v.granted("swarm-agent.atlas.s", t0 + TTL));
    }

    #[test]
    fn a_grant_covers_only_the_exact_bearer() {
        let t0 = Instant::now();
        let mut v = Verdicts::new(TTL, 8);
        v.grant("swarm-agent.atlas.s", t0);
        assert!(!v.granted("swarm-agent.atlas.t", t0));
        assert!(!v.granted("swarm-agent.argus.s", t0));
    }

    #[test]
    fn a_zero_ttl_holds_nothing() {
        let t0 = Instant::now();
        let mut v = Verdicts::new(Duration::ZERO, 8);
        v.grant("swarm-agent.atlas.s", t0);
        assert!(!v.granted("swarm-agent.atlas.s", t0));
        assert_eq!(v.len(), 0);
    }

    #[test]
    fn a_full_cache_evicts_the_oldest_grant() {
        let t0 = Instant::now();
        let mut v = Verdicts::new(TTL, 3);
        for (i, bearer) in ["a", "b", "c", "d"].into_iter().enumerate() {
            v.grant(bearer, t0 + Duration::from_secs(i as u64));
        }
        let now = t0 + Duration::from_secs(4);
        assert_eq!(v.len(), 3);
        assert!(!v.granted("a", now));
        assert!(v.granted("b", now) && v.granted("c", now) && v.granted("d", now));
    }

    #[test]
    fn regranting_a_held_bearer_in_a_full_cache_evicts_nothing() {
        let t0 = Instant::now();
        let mut v = Verdicts::new(TTL, 2);
        v.grant("a", t0);
        v.grant("b", t0);
        v.grant("a", t0 + Duration::from_secs(1));
        assert!(v.granted("a", t0 + Duration::from_secs(1)));
        assert!(v.granted("b", t0 + Duration::from_secs(1)));
    }
}