//! Loaded states cached by their heads (hub spec §16.3). A state is a pure
//! function of its heads, so entries never go stale; the cache is only
//! bounded. Shared across threads because `oxy_core::state::State` is
//! `Send + Sync`.

use oxy_core::hash::ChangeHash;
use oxy_core::repo::Repo;
use oxy_core::state::{Snapshot, State};
use std::collections::HashMap;
use std::sync::{Arc, Mutex};

/// A state and its materialised snapshot.
pub struct Loaded {
    pub heads: Vec<ChangeHash>,
    pub state: State,
    pub snapshot: Snapshot,
}

#[derive(Clone, Debug, PartialEq, Eq, Hash)]
struct Key {
    repo: [u8; 16],
    heads: Vec<ChangeHash>,
}

struct Entry {
    loaded: Arc<Loaded>,
    last_used: u64,
}

/// A bounded least-recently-used cache of loaded states.
pub struct StateCache {
    capacity: usize,
    inner: Mutex<(HashMap<Key, Entry>, u64)>,
}

impl StateCache {
    pub fn new(capacity: usize) -> StateCache {
        StateCache { capacity: capacity.max(1), inner: Mutex::new((HashMap::new(), 0)) }
    }

    /// The state with these heads, loaded once per hub process.
    pub fn load(&self, repo: &Repo, heads: &[ChangeHash]) -> Result<Arc<Loaded>, oxy_core::repo::RepoError> {
        let mut sorted = heads.to_vec();
        sorted.sort();
        sorted.dedup();
        let key = Key { repo: repo.id()?, heads: sorted.clone() };
        {
            let mut guard = self.inner.lock().unwrap();
            let (map, tick) = &mut *guard;
            *tick += 1;
            if let Some(e) = map.get_mut(&key) {
                e.last_used = *tick;
                return Ok(e.loaded.clone());
            }
        }
        let state = repo.load_state(&sorted)?;
        let snapshot = state.materialize();
        let loaded = Arc::new(Loaded { heads: sorted, state, snapshot });
        let mut guard = self.inner.lock().unwrap();
        let (map, tick) = &mut *guard;
        if map.len() >= self.capacity {
            if let Some(oldest) = map.iter().min_by_key(|(_, e)| e.last_used).map(|(k, _)| k.clone()) {
                map.remove(&oldest);
            }
        }
        map.insert(key, Entry { loaded: loaded.clone(), last_used: *tick });
        Ok(loaded)
    }

    pub fn len(&self) -> usize {
        self.inner.lock().unwrap().0.len()
    }

    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }
}

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

    #[test]
    fn caches_by_heads_and_evicts_the_oldest() {
        let dir = tempfile::tempdir().unwrap();
        let repo = Repo::init_bare(dir.path()).unwrap();
        let cache = StateCache::new(2);
        let a = cache.load(&repo, &[]).unwrap();
        let b = cache.load(&repo, &[]).unwrap();
        assert!(Arc::ptr_eq(&a, &b), "same heads, same entry");
        assert_eq!(cache.len(), 1);
        let h1 = oxy_core::hash::ObjectRef([1; 32]);
        let h2 = oxy_core::hash::ObjectRef([2; 32]);
        // Unknown heads fail to load and are not cached.
        assert!(cache.load(&repo, &[h1]).is_err());
        assert_eq!(cache.len(), 1);
        let _ = (h2, dir);
    }
}