use std::path::PathBuf;
use std::sync::Arc;
use anyhow::Result;
use tokio::sync::mpsc;
use tokio::time::{Duration, Instant};
use crate::event_log::EventLogState;

pub trait EventStore: Send + Sync {
    fn load(&self) -> Result<EventLogState>;
    fn save(&self, state: &EventLogState) -> Result<()>;
}

/// JSON file store with atomic write (temp + fsync + rename) and corrupt backup.
pub struct JsonEventStore {
    path: PathBuf,
}

impl JsonEventStore {
    pub fn new(path: PathBuf) -> Self {
        Self { path }
    }

    fn backup_corrupt(&self, ts: u128) {
        let bak = self.path.with_extension(format!("corrupt-{ts}"));
        let _ = std::fs::rename(&self.path, bak);
    }
}

impl EventStore for JsonEventStore {
    fn load(&self) -> Result<EventLogState> {
        if !self.path.exists() {
            return Ok(EventLogState { version: 1, next_id: 1, records: vec![] });
        }
        let bytes = std::fs::read(&self.path)?;
        match serde_json::from_slice::<EventLogState>(&bytes) {
            Ok(state) => Ok(state),
            Err(_) => {
                let ts = std::time::SystemTime::now()
                    .duration_since(std::time::UNIX_EPOCH)
                    .map(|d| d.as_nanos())
                    .unwrap_or(0);
                self.backup_corrupt(ts);
                Ok(EventLogState { version: 1, next_id: 1, records: vec![] })
            }
        }
    }

    fn save(&self, state: &EventLogState) -> Result<()> {
        if let Some(parent) = self.path.parent() {
            std::fs::create_dir_all(parent)?;
        }
        let tmp = self.path.with_extension("json.tmp");
        let bytes = serde_json::to_vec_pretty(state)?;
        std::fs::write(&tmp, &bytes)?;
        let f = std::fs::File::open(&tmp)?;
        // fsync the temp file before rename for durability.
        f.sync_all()?;
        std::fs::rename(&tmp, &self.path)?;
        Ok(())
    }
}

/// Handle the recorder uses to request a debounced persist.
#[derive(Clone)]
pub struct EventPersister {
    tx: mpsc::Sender<EventLogState>,
}

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

/// Spawn the debounce task; coalesces a burst into one save.
pub fn spawn(store: Arc<dyn EventStore>, debounce: Duration) -> EventPersister {
    let (tx, mut rx) = mpsc::channel::<EventLogState>(64);
    tokio::spawn(async move {
        let mut latest: Option<EventLogState> = 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;
                }
            }
        }
    });
    EventPersister { tx }
}

#[cfg(test)]
mod tests {
    use super::*;
    use spacesh_proto::event::{EventKind, EventRecord};
    use spacesh_proto::ids::{SurfaceId, WorkspaceId};

    fn tmp_file(name: &str) -> PathBuf {
        let mut p = std::env::temp_dir();
        let n = std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH).unwrap().as_nanos();
        p.push(format!("spacesh-events-{name}-{n}.json"));
        p
    }

    fn sample() -> EventLogState {
        EventLogState {
            version: 1,
            next_id: 2,
            records: vec![EventRecord {
                id: 1,
                surface_id: SurfaceId("s_1".into()),
                workspace_id: WorkspaceId("w_1".into()),
                workspace_name: "infra".into(),
                agent_label: Some("claude".into()),
                kind: EventKind::Done,
                ts: 1,
                read: false,
            }],
        }
    }

    #[test]
    fn save_then_load_round_trips() {
        let path = tmp_file("roundtrip");
        let store = JsonEventStore::new(path.clone());
        store.save(&sample()).unwrap();
        assert_eq!(store.load().unwrap(), sample());
        let _ = std::fs::remove_file(path);
    }

    #[test]
    fn missing_file_loads_empty() {
        let store = JsonEventStore::new(tmp_file("missing"));
        let s = store.load().unwrap();
        assert_eq!(s.next_id, 1);
        assert!(s.records.is_empty());
    }

    #[test]
    fn corrupt_file_is_backed_up_and_load_returns_empty() {
        let path = tmp_file("corrupt");
        std::fs::write(&path, b"{ not valid json").unwrap();
        let store = JsonEventStore::new(path.clone());
        let s = store.load().unwrap();
        assert!(s.records.is_empty());
        assert!(!path.exists());
        let _ = std::fs::remove_file(path);
    }

    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
    async fn burst_coalesces_to_one_save() {
        struct Counting {
            saves: std::sync::atomic::AtomicUsize,
            last: std::sync::Mutex<Option<EventLogState>>,
        }
        impl EventStore for Counting {
            fn load(&self) -> Result<EventLogState> { Ok(EventLogState::default()) }
            fn save(&self, s: &EventLogState) -> Result<()> {
                self.saves.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
                *self.last.lock().unwrap() = Some(s.clone());
                Ok(())
            }
        }
        let store = Arc::new(Counting {
            saves: std::sync::atomic::AtomicUsize::new(0),
            last: std::sync::Mutex::new(None),
        });
        let p = spawn(store.clone(), Duration::from_millis(80));
        for v in 1..=5u64 {
            let mut s = EventLogState::default();
            s.next_id = v;
            p.mark_dirty(s);
            tokio::time::sleep(Duration::from_millis(10)).await;
        }
        tokio::time::sleep(Duration::from_millis(200)).await;
        assert_eq!(store.saves.load(std::sync::atomic::Ordering::SeqCst), 1, "burst should coalesce to one save");
        assert_eq!(store.last.lock().unwrap().as_ref().unwrap().next_id, 5, "save uses the latest snapshot");
    }
}