attune-core 0.1.0

use arc_swap::ArcSwap;
use crossbeam_channel::{Receiver, Sender, select, unbounded};
use std::collections::HashMap;
use std::sync::{Arc, Mutex, Weak};
use std::thread::{self, JoinHandle};
use std::time::SystemTime;

use crate::{ChangeEvent, ChangeSource, SettingsError, StorageBackend, StoredValue};

pub type ExternalApplier<T> =
    Box<dyn Fn(&mut T, &str, &StoredValue) -> ApplyResult + Send + Sync + 'static>;

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ApplyResult {
    Applied,
    AppliedWithValue { value: StoredValue },
    Ignored,
    DeserializeFailure { raw: String, error: String },
}

pub struct SettingsHandle<T> {
    inner: Arc<SettingsInner<T>>,
}

struct SettingsInner<T> {
    current: ArcSwap<T>,
    write_lock: Mutex<()>,
    backend: Mutex<Box<dyn StorageBackend>>,
    external_applier: ExternalApplier<T>,
    subscribers: Mutex<Vec<Sender<ChangeEvent>>>,
    // Last-known DB state. Updated by `write_field` on local writes (so the diff
    // loop doesn't re-emit them as External) and by the diff loop after each tick.
    last_seen: Mutex<HashMap<String, StoredValue>>,
    diff_shutdown_tx: Sender<()>,
    diff_thread: Option<JoinHandle<()>>,
}

impl<T> SettingsHandle<T>
where
    T: Clone + Send + Sync + 'static,
{
    /// Creates a settings handle and initializes diff state from the backend.
    pub fn new(initial: T, backend: Box<dyn StorageBackend>) -> Self {
        let initial_last_seen = backend.load_all().unwrap_or_default();
        Self::new_with_stored(initial, backend, initial_last_seen)
    }

    /// Creates a settings handle with caller-provided persisted values.
    ///
    /// This constructor is used when startup code has already loaded persisted
    /// values to resolve the initial settings snapshot. Passing the same map
    /// into the handle keeps the cross-process diff state aligned with that
    /// snapshot without reading the backend a second time.
    pub fn new_with_stored(
        initial: T,
        backend: Box<dyn StorageBackend>,
        stored: HashMap<String, StoredValue>,
    ) -> Self {
        Self::new_with_stored_and_applier(initial, backend, stored, noop_external_applier())
    }

    /// Creates a settings handle with caller-provided persisted values and an external applier.
    ///
    /// This constructor is used by generated settings code that has already
    /// loaded stored values and can also map persisted storage keys back onto
    /// fields in `T`. The applier is called when the cross-process diff loop
    /// observes an external stored-value change; successful applications update
    /// the in-memory snapshot before the corresponding change event is
    /// broadcast.
    pub fn new_with_stored_and_applier(
        initial: T,
        backend: Box<dyn StorageBackend>,
        stored: HashMap<String, StoredValue>,
        external_applier: ExternalApplier<T>,
    ) -> Self {
        let (diff_shutdown_tx, diff_shutdown_rx) = unbounded::<()>();
        let commits_rx = backend.watch_changes();

        let inner = Arc::new_cyclic(move |weak: &Weak<SettingsInner<T>>| {
            let diff_thread = if let Some(commits_rx) = commits_rx {
                let weak = weak.clone();
                Some(thread::spawn(move || {
                    diff_loop(weak, commits_rx, diff_shutdown_rx);
                }))
            } else {
                None
            };

            SettingsInner {
                current: ArcSwap::from_pointee(initial),
                write_lock: Mutex::new(()),
                backend: Mutex::new(backend),
                external_applier,
                subscribers: Mutex::new(Vec::new()),
                last_seen: Mutex::new(stored),
                diff_shutdown_tx,
                diff_thread,
            }
        });

        Self { inner }
    }

    pub fn snapshot(&self) -> Arc<T> {
        self.inner.current.load_full()
    }

    pub fn on_change(&self) -> Receiver<ChangeEvent> {
        let (tx, rx) = unbounded();
        self.inner.subscribers.lock().unwrap().push(tx);
        rx
    }

    fn broadcast(&self, event: ChangeEvent) {
        inner_broadcast(&self.inner, event);
    }

    pub fn write_field(
        &self,
        key: &str,
        old_value: Option<StoredValue>,
        new_value: StoredValue,
        mutator: impl FnOnce(&mut T),
    ) -> Result<(), SettingsError> {
        // 1. Serialize writers.
        let _writer = self.inner.write_lock.lock().unwrap();

        // 2. Lock the backend.
        let backend = self.inner.backend.lock().unwrap();

        // 3. Persist before memory update.
        backend.set(key, &new_value)?;

        // 4. Update last_seen so the diff loop won't re-emit this change as External.
        self.inner
            .last_seen
            .lock()
            .unwrap()
            .insert(key.to_string(), new_value.clone());

        // 5. Clone, mutate, store via ArcSwap.
        let prev = self.inner.current.load_full();
        let mut next = (*prev).clone();
        mutator(&mut next);
        self.inner.current.store(Arc::new(next));

        // 6. Release locks before broadcasting.
        drop(backend);
        drop(_writer);

        // 7. Broadcast a Local-source Set event.
        self.broadcast(ChangeEvent::Set {
            key: key.into(),
            old_value,
            new_value,
            source: ChangeSource::Local,
            timestamp: SystemTime::now(),
        });

        Ok(())
    }
}

fn noop_external_applier<T>() -> ExternalApplier<T> {
    Box::new(|_, _, _| ApplyResult::Ignored)
}

impl<T> Clone for SettingsHandle<T> {
    fn clone(&self) -> Self {
        Self {
            inner: Arc::clone(&self.inner),
        }
    }
}

impl<T> Drop for SettingsInner<T> {
    fn drop(&mut self) {
        // Wake the diff thread so it exits immediately rather than waiting on a tick.
        let _ = self.diff_shutdown_tx.send(());
        // Join the thread. Errors are swallowed — a panicked thread shouldn't
        // propagate during Drop.
        if let Some(handle) = self.diff_thread.take() {
            let _ = handle.join();
        }
    }
}

fn inner_broadcast<T>(inner: &SettingsInner<T>, event: ChangeEvent) {
    let mut subs = inner.subscribers.lock().unwrap();
    subs.retain(|tx| tx.send(event.clone()).is_ok());
}

fn diff_loop<T>(weak: Weak<SettingsInner<T>>, commits_rx: Receiver<()>, shutdown_rx: Receiver<()>)
where
    T: Clone + Send + Sync + 'static,
{
    loop {
        select! {
            recv(shutdown_rx) -> _ => return,
            recv(commits_rx) -> msg => {
                if msg.is_err() {
                    return;
                }
                let Some(inner) = weak.upgrade() else { return };

                // Serialize behind in-flight local writers. By the time we get the
                // write_lock, any local writer has already updated last_seen, so
                // its change won't appear as a delta below.
                let _writer = inner.write_lock.lock().unwrap();
                let fresh = {
                    let backend = inner.backend.lock().unwrap();
                    match backend.load_all() {
                        Ok(map) => map,
                        Err(_) => continue,
                    }
                };

                let mut last_seen = inner.last_seen.lock().unwrap();
                let current = inner.current.load_full();
                let mut next = (*current).clone();
                let mut should_store_next = false;
                let mut events = Vec::new();

                // Emit events for changed/new keys.
                for (key, new_value) in &fresh {
                    let old_value = last_seen.get(key).cloned();
                    if old_value.as_ref() != Some(new_value) {
                        match (inner.external_applier)(&mut next, key, new_value) {
                            ApplyResult::Applied => {
                                should_store_next = true;
                                events.push(ChangeEvent::Set {
                                    key: key.clone(),
                                    old_value,
                                    new_value: new_value.clone(),
                                    source: ChangeSource::External,
                                    timestamp: SystemTime::now(),
                                });
                            }
                            ApplyResult::AppliedWithValue { value } => {
                                should_store_next = true;
                                events.push(ChangeEvent::Set {
                                    key: key.clone(),
                                    old_value,
                                    new_value: value,
                                    source: ChangeSource::External,
                                    timestamp: SystemTime::now(),
                                });
                            }
                            ApplyResult::Ignored => {
                                events.push(ChangeEvent::Set {
                                    key: key.clone(),
                                    old_value,
                                    new_value: new_value.clone(),
                                    source: ChangeSource::External,
                                    timestamp: SystemTime::now(),
                                });
                            }
                            ApplyResult::DeserializeFailure { raw, error } => {
                                events.push(ChangeEvent::DeserializeFailure {
                                    key: key.clone(),
                                    raw,
                                    error,
                                    source: ChangeSource::External,
                                    timestamp: SystemTime::now(),
                                });
                            }
                        }
                    }
                }
                // Emit events for deleted keys.
                for (key, old_value) in last_seen.iter() {
                    if !fresh.contains_key(key) {
                        events.push(ChangeEvent::Deleted {
                                key: key.clone(),
                                old_value: old_value.clone(),
                                source: ChangeSource::External,
                                timestamp: SystemTime::now(),
                        });
                    }
                }

                if should_store_next {
                    inner.current.store(Arc::new(next));
                }
                *last_seen = fresh;
                drop(last_seen);
                drop(_writer);

                for event in events {
                    inner_broadcast(&inner, event);
                }
            }
        }
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use std::time::{Duration, SystemTime};

    use crate::{BackendError, ChangeSource, StoredValue};

    struct MockBackend {
        data: Arc<Mutex<HashMap<String, StoredValue>>>,
        commits_tx: Sender<()>,
        commits_rx: Receiver<()>,
    }

    struct CountingBackend {
        load_count: Arc<Mutex<usize>>,
    }

    impl MockBackend {
        fn new() -> Self {
            let (commits_tx, commits_rx) = unbounded();
            Self {
                data: Arc::new(Mutex::new(HashMap::new())),
                commits_tx,
                commits_rx,
            }
        }

        fn data(&self) -> Arc<Mutex<HashMap<String, StoredValue>>> {
            Arc::clone(&self.data)
        }

        fn commit_signal(&self) -> Sender<()> {
            self.commits_tx.clone()
        }
    }

    impl StorageBackend for MockBackend {
        fn load_all(&self) -> Result<HashMap<String, StoredValue>, BackendError> {
            Ok(self.data.lock().unwrap().clone())
        }

        fn set(&self, key: &str, value: &StoredValue) -> Result<(), BackendError> {
            self.data
                .lock()
                .unwrap()
                .insert(key.to_string(), value.clone());
            Ok(())
        }

        fn delete(&self, key: &str) -> Result<(), BackendError> {
            self.data.lock().unwrap().remove(key);
            Ok(())
        }

        fn watch_changes(&self) -> Option<Receiver<()>> {
            Some(self.commits_rx.clone())
        }
    }

    impl CountingBackend {
        fn new(load_count: Arc<Mutex<usize>>) -> Self {
            Self { load_count }
        }
    }

    impl StorageBackend for CountingBackend {
        fn load_all(&self) -> Result<HashMap<String, StoredValue>, BackendError> {
            *self.load_count.lock().unwrap() += 1;
            Ok(HashMap::new())
        }

        fn set(&self, _key: &str, _value: &StoredValue) -> Result<(), BackendError> {
            Ok(())
        }

        fn delete(&self, _key: &str) -> Result<(), BackendError> {
            Ok(())
        }

        fn watch_changes(&self) -> Option<Receiver<()>> {
            None
        }
    }

    fn sample_event() -> ChangeEvent {
        ChangeEvent::Set {
            key: "theme".into(),
            old_value: None,
            new_value: StoredValue::encode(&"dark").unwrap(),
            source: ChangeSource::Local,
            timestamp: SystemTime::now(),
        }
    }

    #[test]
    fn test_snapshot_returns_initial_value() {
        let backend = Box::new(MockBackend::new());
        let handle = SettingsHandle::new(42, backend);
        let snap = handle.snapshot();
        assert_eq!(*snap, 42)
    }

    #[test]
    fn test_clone_shares_state() {
        let backend = Box::new(MockBackend::new());
        let handle = SettingsHandle::new(42, backend);
        let clone = handle.clone();
        let s1 = handle.snapshot();
        let s2 = clone.snapshot();
        // Both Arcs should point to the same allocation.
        assert!(Arc::ptr_eq(&s1, &s2));
        // Both Arcs should point to the same value.
        assert_eq!(*s1, *s2)
    }

    #[test]
    fn test_on_change_receives_broadcast_event() {
        let backend = Box::new(MockBackend::new());
        let handle = SettingsHandle::new(42, backend);

        let rx = handle.on_change();

        let event = sample_event();
        handle.broadcast(event.clone());

        let received = rx.recv().unwrap();
        assert_eq!(received, event)
    }

    #[test]
    fn test_multiple_subscribers_all_receive() {
        let backend = Box::new(MockBackend::new());
        let handle = SettingsHandle::new(42, backend);

        let rx1 = handle.on_change();
        let rx2 = handle.on_change();

        let event = sample_event();
        handle.broadcast(event.clone());

        assert!(rx1.try_recv().is_ok());
        assert!(rx2.try_recv().is_ok());
    }

    #[test]
    fn new_with_stored_uses_provided_last_seen_without_loading_backend() {
        let load_count = Arc::new(Mutex::new(0));
        let backend = Box::new(CountingBackend::new(Arc::clone(&load_count)));
        let mut stored = HashMap::new();
        stored.insert("theme".to_string(), StoredValue::encode(&"dark").unwrap());

        let _handle = SettingsHandle::new_with_stored(42, backend, stored);

        assert_eq!(*load_count.lock().unwrap(), 0);
    }

    #[test]
    fn test_subscriber_is_cleaned_up_on_next_broadcast() {
        let backend = Box::new(MockBackend::new());
        let handle = SettingsHandle::new(42, backend);

        {
            let _rx1 = handle.on_change();
        }
        let rx2 = handle.on_change();

        let event = sample_event();
        handle.broadcast(event.clone());

        assert!(rx2.try_recv().is_ok());
        assert_eq!(handle.inner.subscribers.lock().unwrap().len(), 1)
    }

    #[test]
    fn test_write_field_persists_and_broadcasts() {
        let mock = MockBackend::new();
        let backend_data = mock.data();
        let handle: SettingsHandle<u32> = SettingsHandle::new(0, Box::new(mock));
        let rx = handle.on_change();

        let new_value = StoredValue::encode(&42u32).unwrap();
        let old_value = Some(StoredValue::encode(&0u32).unwrap());

        handle
            .write_field("the_value", old_value.clone(), new_value.clone(), |state| {
                *state = 42u32
            })
            .unwrap();

        // 1. In-memory state updated.
        assert_eq!(*handle.snapshot(), 42);

        // 2. Backend received the write.
        let stored = backend_data.lock().unwrap();
        assert_eq!(stored.get("the_value"), Some(&new_value));
        drop(stored);

        // 3. Subscriber received the right event.
        let event = rx.try_recv().unwrap();
        match event {
            ChangeEvent::Set {
                key,
                old_value: old,
                new_value: new,
                source,
                ..
            } => {
                assert_eq!(key, "the_value");
                assert_eq!(old, old_value);
                assert_eq!(new, new_value);
                assert_eq!(source, ChangeSource::Local);
            }
            other => panic!("expected ChangeEvent::Set, got {:?}", other),
        }
    }

    #[test]
    fn test_external_change_emits_external_event() {
        let mock = MockBackend::new();
        let data = mock.data();
        let commit_signal = mock.commit_signal();
        let handle: SettingsHandle<u32> = SettingsHandle::new(0, Box::new(mock));
        let rx = handle.on_change();

        // Simulate an external write: insert directly into the backend's storage,
        // bypassing the handle. Then signal "a commit happened".
        let new_value = StoredValue::encode(&42u32).unwrap();
        data.lock()
            .unwrap()
            .insert("the_value".to_string(), new_value.clone());
        commit_signal.send(()).unwrap();

        let event = rx.recv_timeout(Duration::from_secs(2)).unwrap();
        match event {
            ChangeEvent::Set {
                key,
                old_value,
                new_value: new,
                source,
                ..
            } => {
                assert_eq!(key, "the_value");
                assert_eq!(old_value, None);
                assert_eq!(new, new_value);
                assert_eq!(source, ChangeSource::External);
            }
            other => panic!("expected ChangeEvent::Set, got {:?}", other),
        }
    }

    #[test]
    fn test_external_change_updates_snapshot_when_applier_succeeds() {
        let mock = MockBackend::new();
        let data = mock.data();
        let commit_signal = mock.commit_signal();
        let handle: SettingsHandle<u32> = SettingsHandle::new_with_stored_and_applier(
            0,
            Box::new(mock),
            HashMap::new(),
            Box::new(|state, key, value| {
                if key != "the_value" {
                    return ApplyResult::Ignored;
                }

                match value.decode::<u32>() {
                    Ok(decoded) => {
                        *state = decoded;
                        ApplyResult::Applied
                    }
                    Err(error) => ApplyResult::DeserializeFailure {
                        raw: value.as_str().to_string(),
                        error: error.to_string(),
                    },
                }
            }),
        );
        let rx = handle.on_change();

        let new_value = StoredValue::encode(&42u32).unwrap();
        data.lock()
            .unwrap()
            .insert("the_value".to_string(), new_value);
        commit_signal.send(()).unwrap();

        let event = rx.recv_timeout(Duration::from_secs(2)).unwrap();
        assert!(matches!(
            event,
            ChangeEvent::Set {
                source: ChangeSource::External,
                ..
            }
        ));
        assert_eq!(*handle.snapshot(), 42);
    }

    #[test]
    fn test_external_change_emits_deserialize_failure_and_preserves_snapshot() {
        let mock = MockBackend::new();
        let data = mock.data();
        let commit_signal = mock.commit_signal();
        let handle: SettingsHandle<u32> = SettingsHandle::new_with_stored_and_applier(
            7,
            Box::new(mock),
            HashMap::new(),
            Box::new(|state, key, value| {
                if key != "the_value" {
                    return ApplyResult::Ignored;
                }

                match value.decode::<u32>() {
                    Ok(decoded) => {
                        *state = decoded;
                        ApplyResult::Applied
                    }
                    Err(error) => ApplyResult::DeserializeFailure {
                        raw: value.as_str().to_string(),
                        error: error.to_string(),
                    },
                }
            }),
        );
        let rx = handle.on_change();

        data.lock().unwrap().insert(
            "the_value".to_string(),
            StoredValue::from_raw("\"not-a-number\"".to_string()),
        );
        commit_signal.send(()).unwrap();

        let event = rx.recv_timeout(Duration::from_secs(2)).unwrap();
        match event {
            ChangeEvent::DeserializeFailure {
                key, raw, source, ..
            } => {
                assert_eq!(key, "the_value");
                assert_eq!(raw, "\"not-a-number\"");
                assert_eq!(source, ChangeSource::External);
            }
            event => panic!("unexpected event: {event:?}"),
        }
        assert_eq!(*handle.snapshot(), 7);
    }

    #[test]
    fn test_local_write_does_not_re_emit_as_external() {
        let mock = MockBackend::new();
        let commit_signal = mock.commit_signal();
        let handle: SettingsHandle<u32> = SettingsHandle::new(0, Box::new(mock));
        let rx = handle.on_change();

        let new_value = StoredValue::encode(&42u32).unwrap();
        handle
            .write_field("the_value", None, new_value.clone(), |state| *state = 42u32)
            .unwrap();

        // Drain the Local event from write_field.
        let first = rx.try_recv().unwrap();
        match first {
            ChangeEvent::Set { source, .. } => assert_eq!(source, ChangeSource::Local),
            other => panic!("expected Local Set, got {:?}", other),
        }

        // Now signal the diff loop. Since last_seen was updated by write_field,
        // the diff loop should find no delta and emit nothing.
        commit_signal.send(()).unwrap();

        // Give the diff loop a moment to run, then assert no further events.
        let result = rx.recv_timeout(Duration::from_millis(500));
        assert!(
            result.is_err(),
            "expected timeout (no External event), got {:?}",
            result
        );
    }
}