use std::sync::Arc;
use tokio::sync::mpsc;
use tokio::time::{Duration, Instant};
use crate::state_store::{PersistState, StateStore};

/// A handle the registry uses to request a persist. `mark_dirty(state)` records
/// the latest snapshot and (re)arms the debounce timer.
#[derive(Clone)]
pub struct Persister {
    tx: mpsc::Sender<PersistState>,
}

impl Persister {
    pub fn mark_dirty(&self, state: PersistState) {
        // Best-effort; dropping a snapshot is fine because a newer one will arrive.
        let _ = self.tx.try_send(state);
    }
}

/// Spawn the debounce task. Returns the `Persister` handle.
/// `debounce` is configurable so tests can use a short window.
pub fn spawn(store: Arc<dyn StateStore>, debounce: Duration) -> Persister {
    let (tx, mut rx) = mpsc::channel::<PersistState>(64);
    tokio::spawn(async move {
        let mut latest: Option<PersistState> = None;
        let mut deadline: Option<Instant> = None;
        loop {
            let timer = async {
                match deadline {
                    Some(d) => tokio::time::sleep_until(d).await,
                    None => std::future::pending::<()>().await,
                }
            };
            tokio::select! {
                msg = rx.recv() => {
                    match msg {
                        Some(state) => {
                            latest = Some(state);
                            deadline = Some(Instant::now() + debounce);
                        }
                        None => {
                            // channel closed: final flush then exit
                            if let Some(s) = latest.take() { let _ = store.save(&s); }
                            break;
                        }
                    }
                }
                _ = timer => {
                    if let Some(s) = latest.take() { let _ = store.save(&s); }
                    deadline = None;
                }
            }
        }
    });
    Persister { tx }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::sync::atomic::{AtomicUsize, Ordering};
    use std::sync::Mutex;

    struct CountingStore {
        saves: AtomicUsize,
        last: Mutex<Option<PersistState>>,
    }
    impl StateStore for CountingStore {
        fn load(&self) -> anyhow::Result<PersistState> { Ok(PersistState::default()) }
        fn save(&self, state: &PersistState) -> anyhow::Result<()> {
            self.saves.fetch_add(1, Ordering::SeqCst);
            *self.last.lock().unwrap() = Some(state.clone());
            Ok(())
        }
    }

    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
    async fn burst_coalesces_to_one_save() {
        let store = Arc::new(CountingStore { saves: AtomicUsize::new(0), last: Mutex::new(None) });
        let p = spawn(store.clone(), Duration::from_millis(80));
        // Fire several dirty signals rapidly.
        for v in 1..=5u32 {
            let mut s = PersistState::default();
            s.version = v;
            p.mark_dirty(s);
            tokio::time::sleep(Duration::from_millis(10)).await;
        }
        // Wait past the debounce window.
        tokio::time::sleep(Duration::from_millis(200)).await;
        assert_eq!(store.saves.load(Ordering::SeqCst), 1, "burst should coalesce to one save");
        assert_eq!(store.last.lock().unwrap().as_ref().unwrap().version, 5, "save uses the latest snapshot");
    }
}