stateset_sync/

engine.rs

use std::collections::HashSet;

use chrono::Utc;

use crate::buffer::EventBuffer;
use crate::config::SyncConfig;
use crate::conflict::{ConflictResolver, ConflictStrategy, Resolution};
use crate::error::SyncError;
use crate::event::SyncEvent;
use crate::outbox::Outbox;
use crate::state::{SyncState, SyncStatus};
use crate::transport::{PullPage, PullResult, PushResult, Transport, derive_next_cursor};

/// Safety stop for paginated pull loops in `full_sync`.
const MAX_PULL_PAGES: usize = 10_000;

/// The sync engine orchestrates synchronization between local state and
/// a remote sequencer.
///
/// This is the Rust equivalent of the JS `SyncEngine` class, providing:
/// - Event recording to the outbox
/// - Push (outbox -> remote) via a [`Transport`]
/// - Pull (remote -> buffer) via a [`Transport`]
/// - Conflict resolution during pull
/// - Status reporting
///
/// # Examples
///
/// ```
/// use stateset_sync::{SyncEngine, SyncConfig, SyncEvent};
/// use serde_json::json;
///
/// let config = SyncConfig::new("agent-1", "tenant-1", "store-1");
/// let mut engine = SyncEngine::new(config).expect("valid sync config");
///
/// let seq = engine.record(SyncEvent::new("order.created", "order", "ORD-1", json!({"total": 99})));
/// assert!(seq.is_ok());
/// assert_eq!(engine.pending_count(), 1);
/// ```
#[derive(Debug)]
pub struct SyncEngine {
    config: SyncConfig,
    state: SyncState,
    outbox: Outbox,
    buffer: EventBuffer,
    resolver: ConflictResolver,
    last_pulled_sequence: u64,
    next_pull_cursor: Option<u64>,
    initialized: bool,
}

impl SyncEngine {
    /// Create a new `SyncEngine` with the given configuration.
    ///
    /// # Errors
    ///
    /// Returns [`SyncError::InvalidConfig`] for invalid settings or
    /// [`SyncError::Storage`] when durable outbox initialization fails.
    pub fn new(config: SyncConfig) -> Result<Self, SyncError> {
        Self::try_new(config)
    }

    /// Create a `SyncEngine` with a custom conflict resolution strategy.
    ///
    /// # Errors
    ///
    /// Returns [`SyncError::InvalidConfig`] for invalid settings or
    /// [`SyncError::Storage`] when durable outbox initialization fails.
    pub fn with_strategy(
        config: SyncConfig,
        strategy: ConflictStrategy,
    ) -> Result<Self, SyncError> {
        Self::try_with_strategy(config, strategy)
    }

    /// Fallible constructor that validates config and initializes persistence.
    ///
    /// # Errors
    ///
    /// Returns [`SyncError::InvalidConfig`] for invalid settings or
    /// [`SyncError::Storage`] when durable outbox initialization fails.
    pub fn try_new(config: SyncConfig) -> Result<Self, SyncError> {
        Self::try_with_strategy(config, ConflictStrategy::default())
    }

    /// Fallible constructor with explicit conflict strategy.
    ///
    /// # Errors
    ///
    /// Returns [`SyncError::InvalidConfig`] for invalid settings or
    /// [`SyncError::Storage`] when durable outbox initialization fails.
    pub fn try_with_strategy(
        config: SyncConfig,
        strategy: ConflictStrategy,
    ) -> Result<Self, SyncError> {
        config.validate()?;
        let buffer_capacity = config.resolved_buffer_capacity();
        let outbox = if let Some(path) = config.outbox_path.as_deref() {
            Outbox::with_persistence(config.resolved_outbox_capacity(), path)?
        } else {
            Outbox::new(config.resolved_outbox_capacity())
        };

        Ok(Self {
            config,
            state: SyncState::default(),
            outbox,
            buffer: EventBuffer::new(buffer_capacity),
            resolver: ConflictResolver::new(strategy),
            last_pulled_sequence: 0,
            next_pull_cursor: None,
            initialized: true,
        })
    }

    /// Record an event into the outbox for later push.
    ///
    /// Returns the assigned local sequence number.
    ///
    /// # Errors
    ///
    /// Returns [`SyncError::OutboxFull`] if the outbox is at capacity.
    pub fn record(&mut self, event: SyncEvent) -> Result<u64, SyncError> {
        let seq = self.outbox.append(event)?;
        self.state.local_head = seq;
        self.state.pending_count = self.outbox.count();
        Ok(seq)
    }

    /// Push pending events from the outbox to the remote via the given transport.
    ///
    /// Drains up to `batch_size` events from the outbox.
    ///
    /// # Errors
    ///
    /// Returns [`SyncError::Transport`] if the transport operation fails.
    pub async fn push(&mut self, transport: &dyn Transport) -> Result<PushResult, SyncError> {
        let batch_size = self.config.resolved_batch_size();
        let events: Vec<SyncEvent> = self.outbox.peek(batch_size).into_iter().cloned().collect();

        if events.is_empty() {
            return Ok(PushResult { accepted: 0, remote_head: self.state.remote_head });
        }

        let result = transport.push_events(&events).await?;
        let accepted = result.accepted.min(events.len());
        if accepted > 0 {
            let _ = self.outbox.drain(accepted);
        }

        self.state.remote_head = result.remote_head;
        self.state.last_push = Some(Utc::now());
        self.state.pending_count = self.outbox.count();

        Ok(result)
    }

    /// Pull events from the remote sequencer into the local buffer.
    ///
    /// Pulled events are added to the event buffer. If conflicts are
    /// detected (same `entity_type` + `entity_id` in both outbox and pulled),
    /// they are resolved using the configured strategy.
    ///
    /// # Errors
    ///
    /// Returns [`SyncError::Transport`] if the transport operation fails.
    pub async fn pull(&mut self, transport: &dyn Transport) -> Result<PullResult, SyncError> {
        let since = self.next_pull_cursor.unwrap_or(self.last_pulled_sequence);
        let (result, next_cursor) = self.pull_since(transport, since).await?;
        self.next_pull_cursor = next_cursor;
        Ok(result)
    }

    async fn pull_since(
        &mut self,
        transport: &dyn Transport,
        since: u64,
    ) -> Result<(PullResult, Option<u64>), SyncError> {
        let limit = self.config.resolved_batch_size();
        let PullPage { result, next_cursor: transport_next_cursor } =
            transport.pull_events_page(since, limit).await?;

        // Detect and resolve conflicts between pending outbox events and pulled events
        let pending: Vec<SyncEvent> =
            self.outbox.peek(self.outbox.count()).into_iter().cloned().collect();
        let mut drop_local_ids = HashSet::new();
        let mut events_to_buffer = Vec::with_capacity(result.events.len());

        for pulled_event in &result.events {
            let mut keep_remote = true;

            for local_event in &pending {
                if local_event.entity_type == pulled_event.entity_type
                    && local_event.entity_id == pulled_event.entity_id
                {
                    match self.resolver.resolve(local_event, pulled_event) {
                        Resolution::KeepLocal => {
                            keep_remote = false;
                            break;
                        }
                        Resolution::KeepRemote => {
                            drop_local_ids.insert(local_event.id);
                        }
                        Resolution::Merge(merged) => {
                            drop_local_ids.insert(local_event.id);
                            events_to_buffer.push(merged);
                            keep_remote = false;
                            break;
                        }
                    }
                }
            }

            if keep_remote {
                events_to_buffer.push(pulled_event.clone());
            }
        }

        if !drop_local_ids.is_empty() {
            self.outbox.retain(|event| !drop_local_ids.contains(&event.id));
            self.state.pending_count = self.outbox.count();
        }

        // Buffer resolved events
        for event in events_to_buffer {
            self.buffer.push(event);
        }

        self.state.remote_head = result.remote_head;
        self.state.last_pull = Some(Utc::now());
        let observed_cursor = transport_next_cursor
            .filter(|cursor| *cursor > since)
            .or_else(|| derive_next_cursor(since, &result.events))
            .unwrap_or(since);
        self.last_pulled_sequence = self.last_pulled_sequence.max(observed_cursor);

        let next_cursor = Self::resolve_next_cursor(
            since,
            &result.events,
            result.has_more,
            transport_next_cursor,
        )?;

        Ok((result, next_cursor))
    }

    fn resolve_next_cursor(
        since: u64,
        events: &[SyncEvent],
        has_more: bool,
        transport_next_cursor: Option<u64>,
    ) -> Result<Option<u64>, SyncError> {
        if !has_more {
            return Ok(None);
        }

        let next_cursor = transport_next_cursor.or_else(|| derive_next_cursor(since, events));
        let Some(next_cursor) = next_cursor else {
            return Err(SyncError::Transport(
                "pull pagination stalled: has_more=true but no advancing cursor available"
                    .to_string(),
            ));
        };
        if next_cursor <= since {
            return Err(SyncError::Transport(format!(
                "pull pagination cursor did not advance (since={since}, next_cursor={next_cursor})"
            )));
        }
        Ok(Some(next_cursor))
    }

    /// Get the current sync status.
    #[must_use]
    pub fn status(&self) -> SyncStatus {
        SyncStatus {
            initialized: self.initialized,
            local_head: self.state.local_head,
            remote_head: self.state.remote_head,
            pending: self.outbox.count(),
            lag: self.state.lag(),
            last_push: self.state.last_push,
            last_pull: self.state.last_pull,
            buffered_events: self.buffer.len(),
        }
    }

    /// Return the number of events pending in the outbox.
    #[must_use]
    pub fn pending_count(&self) -> usize {
        self.outbox.count()
    }

    /// Return the number of events currently in the pull buffer.
    #[must_use]
    pub fn buffered_count(&self) -> usize {
        self.buffer.len()
    }

    /// Drain all events from the pull buffer.
    pub fn drain_buffer(&mut self) -> Vec<SyncEvent> {
        self.buffer.drain_all()
    }

    /// Return a reference to the current sync state.
    #[must_use]
    pub const fn state(&self) -> &SyncState {
        &self.state
    }

    /// Return a reference to the sync configuration.
    #[must_use]
    pub const fn config(&self) -> &SyncConfig {
        &self.config
    }

    /// Return a reference to the conflict resolver.
    #[must_use]
    pub const fn resolver(&self) -> &ConflictResolver {
        &self.resolver
    }

    /// Perform a full sync: push first, then pull.
    ///
    /// # Errors
    ///
    /// Returns the first error encountered during push or pull.
    pub async fn full_sync(
        &mut self,
        transport: &dyn Transport,
    ) -> Result<(PushResult, PullResult), SyncError> {
        let push_result = self.push(transport).await?;
        let mut since = self.next_pull_cursor.unwrap_or(self.last_pulled_sequence);
        let (mut pull_result, next_cursor) = self.pull_since(transport, since).await?;
        self.next_pull_cursor = next_cursor;
        let mut pull_pages = 1;

        while pull_result.has_more {
            if pull_pages >= MAX_PULL_PAGES {
                return Err(SyncError::Transport(
                    "pull pagination exceeded safety limit".to_string(),
                ));
            }

            since = self.next_pull_cursor.ok_or_else(|| {
                SyncError::Transport(
                    "pull pagination stalled: has_more=true but no continuation cursor".to_string(),
                )
            })?;

            let (next_page, page_cursor) = self.pull_since(transport, since).await?;
            self.next_pull_cursor = page_cursor;

            pull_result.events.extend(next_page.events);
            pull_result.remote_head = next_page.remote_head;
            pull_result.has_more = next_page.has_more;
            pull_pages += 1;
        }

        self.next_pull_cursor = None;
        Ok((push_result, pull_result))
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::transport::NullTransport;
    use proptest::prelude::*;
    use serde_json::json;
    use std::sync::Arc;
    use std::sync::Mutex;
    use std::sync::atomic::{AtomicU64, Ordering};
    use tempfile::tempdir;

    fn make_config() -> SyncConfig {
        SyncConfig::new("agent-1", "tenant-1", "store-1")
    }

    fn make_event(event_type: &str) -> SyncEvent {
        SyncEvent::new(event_type, "order", "ORD-1", json!({}))
    }

    #[test]
    fn new_engine() {
        let engine = SyncEngine::new(make_config()).unwrap();
        assert_eq!(engine.pending_count(), 0);
        assert_eq!(engine.buffered_count(), 0);
        assert!(engine.status().initialized);
    }

    #[test]
    fn record_event() {
        let mut engine = SyncEngine::new(make_config()).unwrap();
        let seq = engine.record(make_event("order.created")).unwrap();
        assert_eq!(seq, 1);
        assert_eq!(engine.pending_count(), 1);
        assert_eq!(engine.state().local_head, 1);
    }

    #[test]
    fn record_multiple_events() {
        let mut engine = SyncEngine::new(make_config()).unwrap();
        engine.record(make_event("a")).unwrap();
        engine.record(make_event("b")).unwrap();
        engine.record(make_event("c")).unwrap();
        assert_eq!(engine.pending_count(), 3);
        assert_eq!(engine.state().local_head, 3);
    }

    #[tokio::test]
    async fn push_with_null_transport() {
        let mut engine = SyncEngine::new(make_config()).unwrap();
        engine.record(make_event("a")).unwrap();
        engine.record(make_event("b")).unwrap();

        let transport = NullTransport::new();
        let result = engine.push(&transport).await.unwrap();
        assert_eq!(result.accepted, 2);
        assert_eq!(engine.pending_count(), 0);
        assert!(engine.state().last_push.is_some());
    }

    #[tokio::test]
    async fn push_empty_outbox() {
        let mut engine = SyncEngine::new(make_config()).unwrap();
        let transport = NullTransport::new();
        let result = engine.push(&transport).await.unwrap();
        assert_eq!(result.accepted, 0);
    }

    #[tokio::test]
    async fn pull_with_null_transport() {
        let mut engine = SyncEngine::new(make_config()).unwrap();
        let transport = NullTransport::new();
        let result = engine.pull(&transport).await.unwrap();
        assert!(result.events.is_empty());
        assert!(!result.has_more);
        assert!(engine.state().last_pull.is_some());
    }

    #[tokio::test]
    async fn pull_buffers_events() {
        /// Mock transport that returns predefined events on pull.
        #[derive(Debug)]
        struct MockPullTransport {
            events: Vec<SyncEvent>,
            head: u64,
        }

        #[async_trait::async_trait]
        impl Transport for MockPullTransport {
            async fn push_events(&self, events: &[SyncEvent]) -> Result<PushResult, SyncError> {
                Ok(PushResult { accepted: events.len(), remote_head: self.head })
            }
            async fn pull_events(
                &self,
                _since: u64,
                _limit: usize,
            ) -> Result<PullResult, SyncError> {
                Ok(PullResult {
                    events: self.events.clone(),
                    remote_head: self.head,
                    has_more: false,
                })
            }
        }

        let transport = MockPullTransport {
            events: vec![
                make_event("pulled-1").with_sequence(1),
                make_event("pulled-2").with_sequence(2),
            ],
            head: 2,
        };

        let mut engine = SyncEngine::new(make_config()).unwrap();
        let result = engine.pull(&transport).await.unwrap();
        assert_eq!(result.events.len(), 2);
        assert_eq!(engine.buffered_count(), 2);
        assert_eq!(engine.state().remote_head, 2);
    }

    #[tokio::test]
    async fn full_sync() {
        let mut engine = SyncEngine::new(make_config()).unwrap();
        engine.record(make_event("local")).unwrap();

        let transport = NullTransport::new();
        let (push_result, pull_result) = engine.full_sync(&transport).await.unwrap();
        assert_eq!(push_result.accepted, 1);
        assert!(pull_result.events.is_empty());
        assert_eq!(engine.pending_count(), 0);
    }

    #[test]
    fn status_reporting() {
        let mut engine = SyncEngine::new(make_config()).unwrap();
        engine.record(make_event("a")).unwrap();
        engine.record(make_event("b")).unwrap();

        let status = engine.status();
        assert!(status.initialized);
        assert_eq!(status.pending, 2);
        assert_eq!(status.local_head, 2);
        assert_eq!(status.remote_head, 0);
        assert_eq!(status.lag, 0);
        assert!(status.last_push.is_none());
    }

    #[test]
    fn drain_buffer() {
        let mut engine = SyncEngine::new(make_config()).unwrap();
        // Manually push to buffer via engine internals
        engine.buffer.push(make_event("buffered"));
        assert_eq!(engine.buffered_count(), 1);

        let drained = engine.drain_buffer();
        assert_eq!(drained.len(), 1);
        assert_eq!(engine.buffered_count(), 0);
    }

    #[test]
    fn engine_with_strategy() {
        let engine = SyncEngine::with_strategy(make_config(), ConflictStrategy::LocalWins).unwrap();
        assert_eq!(engine.resolver().strategy(), ConflictStrategy::LocalWins);
    }

    #[test]
    fn config_accessor() {
        let config = make_config();
        let engine = SyncEngine::new(config).unwrap();
        assert_eq!(engine.config().agent_id, "agent-1");
    }

    #[test]
    fn try_new_rejects_invalid_config() {
        let bad = SyncConfig::new("", "tenant", "store");
        assert!(SyncEngine::try_new(bad).is_err());
    }

    #[test]
    fn try_new_with_persistent_outbox_restores_pending_events() {
        let dir = tempdir().unwrap();
        let outbox_path = dir.path().join("sync-outbox.json");
        let path_str = outbox_path.to_string_lossy().to_string();

        {
            let mut engine = SyncEngine::try_new(
                SyncConfig::new("agent-1", "tenant-1", "store-1")
                    .with_outbox_path(path_str.clone()),
            )
            .unwrap();
            engine.record(make_event("persisted-a")).unwrap();
            engine.record(make_event("persisted-b")).unwrap();
            assert_eq!(engine.pending_count(), 2);
        }

        let engine = SyncEngine::try_new(
            SyncConfig::new("agent-1", "tenant-1", "store-1").with_outbox_path(path_str),
        )
        .unwrap();
        assert_eq!(engine.pending_count(), 2);
    }

    #[tokio::test]
    async fn push_respects_batch_size() {
        let config = SyncConfig::new("agent-1", "tenant-1", "store-1").with_batch_size(2);
        let mut engine = SyncEngine::new(config).unwrap();
        engine.record(make_event("a")).unwrap();
        engine.record(make_event("b")).unwrap();
        engine.record(make_event("c")).unwrap();

        let transport = NullTransport::new();
        let result = engine.push(&transport).await.unwrap();
        // Should only push 2 due to batch_size
        assert_eq!(result.accepted, 2);
        assert_eq!(engine.pending_count(), 1);
    }

    #[tokio::test]
    async fn push_updates_state() {
        /// Mock transport that returns an increasing remote head.
        #[derive(Debug)]
        struct MockHeadTransport {
            head: Arc<AtomicU64>,
        }

        #[async_trait::async_trait]
        impl Transport for MockHeadTransport {
            async fn push_events(&self, events: &[SyncEvent]) -> Result<PushResult, SyncError> {
                let new_head = self.head.fetch_add(events.len() as u64, Ordering::SeqCst)
                    + events.len() as u64;
                Ok(PushResult { accepted: events.len(), remote_head: new_head })
            }
            async fn pull_events(
                &self,
                _since: u64,
                _limit: usize,
            ) -> Result<PullResult, SyncError> {
                Ok(PullResult {
                    events: vec![],
                    remote_head: self.head.load(Ordering::SeqCst),
                    has_more: false,
                })
            }
        }

        let transport = MockHeadTransport { head: Arc::new(AtomicU64::new(0)) };

        let mut engine = SyncEngine::new(make_config()).unwrap();
        engine.record(make_event("a")).unwrap();
        engine.record(make_event("b")).unwrap();

        let result = engine.push(&transport).await.unwrap();
        assert_eq!(result.remote_head, 2);
        assert_eq!(engine.state().remote_head, 2);
    }

    #[tokio::test]
    async fn transport_error_propagates() {
        /// Transport that always fails.
        #[derive(Debug)]
        struct FailTransport;

        #[async_trait::async_trait]
        impl Transport for FailTransport {
            async fn push_events(&self, _events: &[SyncEvent]) -> Result<PushResult, SyncError> {
                Err(SyncError::Transport("network down".into()))
            }
            async fn pull_events(
                &self,
                _since: u64,
                _limit: usize,
            ) -> Result<PullResult, SyncError> {
                Err(SyncError::Transport("network down".into()))
            }
        }

        let mut engine = SyncEngine::new(make_config()).unwrap();
        engine.record(make_event("a")).unwrap();

        let transport = FailTransport;
        let result = engine.push(&transport).await;
        assert!(result.is_err());
        assert!(matches!(result.unwrap_err(), SyncError::Transport(_)));
        // Failed push must not drop local events.
        assert_eq!(engine.pending_count(), 1);

        let pull_result = engine.pull(&transport).await;
        assert!(pull_result.is_err());
    }

    #[tokio::test]
    async fn push_only_drains_accepted_events() {
        /// Transport that only accepts one event from each batch.
        #[derive(Debug)]
        struct PartialAcceptTransport;

        #[async_trait::async_trait]
        impl Transport for PartialAcceptTransport {
            async fn push_events(&self, _events: &[SyncEvent]) -> Result<PushResult, SyncError> {
                Ok(PushResult { accepted: 1, remote_head: 1 })
            }

            async fn pull_events(
                &self,
                _since: u64,
                _limit: usize,
            ) -> Result<PullResult, SyncError> {
                Ok(PullResult { events: vec![], remote_head: 1, has_more: false })
            }
        }

        let mut engine = SyncEngine::new(make_config()).unwrap();
        engine.record(make_event("a")).unwrap();
        engine.record(make_event("b")).unwrap();
        engine.record(make_event("c")).unwrap();

        let result = engine.push(&PartialAcceptTransport).await.unwrap();
        assert_eq!(result.accepted, 1);
        assert_eq!(engine.pending_count(), 2);
    }

    #[tokio::test]
    async fn pull_conflict_resolution() {
        /// Transport that returns events conflicting with local outbox.
        #[derive(Debug)]
        struct ConflictTransport;

        #[async_trait::async_trait]
        impl Transport for ConflictTransport {
            async fn push_events(&self, events: &[SyncEvent]) -> Result<PushResult, SyncError> {
                Ok(PushResult { accepted: events.len(), remote_head: 10 })
            }
            async fn pull_events(
                &self,
                _since: u64,
                _limit: usize,
            ) -> Result<PullResult, SyncError> {
                // Return an event for the same entity as the pending local event
                let remote_event =
                    SyncEvent::new("order.updated", "order", "ORD-1", json!({"status": "remote"}))
                        .with_sequence(5);
                Ok(PullResult { events: vec![remote_event], remote_head: 5, has_more: false })
            }
        }

        let mut engine =
            SyncEngine::with_strategy(make_config(), ConflictStrategy::RemoteWins).unwrap();
        engine
            .record(SyncEvent::new("order.updated", "order", "ORD-1", json!({"status": "local"})))
            .unwrap();

        let transport = ConflictTransport;
        let result = engine.pull(&transport).await.unwrap();
        assert_eq!(result.events.len(), 1);
        // RemoteWins removes conflicting local outbox events and keeps pulled event.
        assert_eq!(engine.buffered_count(), 1);
        assert_eq!(engine.pending_count(), 0);
    }

    #[tokio::test]
    async fn pull_conflict_local_wins_keeps_pending_and_skips_remote() {
        #[derive(Debug)]
        struct ConflictTransport;

        #[async_trait::async_trait]
        impl Transport for ConflictTransport {
            async fn push_events(&self, events: &[SyncEvent]) -> Result<PushResult, SyncError> {
                Ok(PushResult { accepted: events.len(), remote_head: 10 })
            }
            async fn pull_events(
                &self,
                _since: u64,
                _limit: usize,
            ) -> Result<PullResult, SyncError> {
                let remote_event =
                    SyncEvent::new("order.updated", "order", "ORD-1", json!({"status": "remote"}))
                        .with_sequence(5);
                Ok(PullResult { events: vec![remote_event], remote_head: 5, has_more: false })
            }
        }

        let mut engine =
            SyncEngine::with_strategy(make_config(), ConflictStrategy::LocalWins).unwrap();
        engine
            .record(SyncEvent::new("order.updated", "order", "ORD-1", json!({"status": "local"})))
            .unwrap();

        let result = engine.pull(&ConflictTransport).await.unwrap();
        assert_eq!(result.events.len(), 1);
        assert_eq!(engine.pending_count(), 1);
        assert_eq!(engine.buffered_count(), 0);
    }

    #[tokio::test]
    async fn full_sync_paginates_pull_until_complete() {
        #[derive(Debug)]
        struct PagingTransport {
            pulls: Arc<AtomicU64>,
            since_args: Arc<Mutex<Vec<u64>>>,
        }

        #[async_trait::async_trait]
        impl Transport for PagingTransport {
            async fn push_events(&self, events: &[SyncEvent]) -> Result<PushResult, SyncError> {
                Ok(PushResult { accepted: events.len(), remote_head: 0 })
            }

            async fn pull_events(
                &self,
                since: u64,
                _limit: usize,
            ) -> Result<PullResult, SyncError> {
                self.since_args.lock().unwrap().push(since);
                let call = self.pulls.fetch_add(1, Ordering::SeqCst);
                if call == 0 {
                    Ok(PullResult {
                        events: vec![
                            SyncEvent::new("order.updated", "order", "ORD-1", json!({}))
                                .with_sequence(1),
                        ],
                        // Simulate remote_head as global head watermark, not page cursor.
                        remote_head: 999,
                        has_more: true,
                    })
                } else {
                    Ok(PullResult {
                        events: vec![
                            SyncEvent::new("order.updated", "order", "ORD-2", json!({}))
                                .with_sequence(2),
                        ],
                        remote_head: 999,
                        has_more: false,
                    })
                }
            }
        }

        let since_args = Arc::new(Mutex::new(Vec::new()));
        let transport = PagingTransport {
            pulls: Arc::new(AtomicU64::new(0)),
            since_args: Arc::clone(&since_args),
        };
        let mut engine = SyncEngine::new(make_config()).unwrap();
        let (_push_result, pull_result) = engine.full_sync(&transport).await.unwrap();

        assert_eq!(pull_result.events.len(), 2);
        assert!(!pull_result.has_more);
        assert_eq!(engine.buffered_count(), 2);
        assert_eq!(engine.state().remote_head, 999);
        assert_eq!(transport.pulls.load(Ordering::SeqCst), 2);
        assert_eq!(&*since_args.lock().unwrap(), &[0, 1]);
    }

    #[tokio::test]
    async fn pull_cursor_does_not_advance_from_push_remote_head() {
        #[derive(Debug)]
        struct HeadSkewTransport {
            since_args: Arc<Mutex<Vec<u64>>>,
        }

        #[async_trait::async_trait]
        impl Transport for HeadSkewTransport {
            async fn push_events(&self, events: &[SyncEvent]) -> Result<PushResult, SyncError> {
                Ok(PushResult { accepted: events.len(), remote_head: 1000 })
            }

            async fn pull_events(
                &self,
                since: u64,
                _limit: usize,
            ) -> Result<PullResult, SyncError> {
                self.since_args.lock().unwrap().push(since);
                Ok(PullResult {
                    events: vec![
                        SyncEvent::new("order.updated", "order", "ORD-1", json!({}))
                            .with_sequence(7),
                    ],
                    remote_head: 1000,
                    has_more: false,
                })
            }
        }

        let since_args = Arc::new(Mutex::new(Vec::new()));
        let transport = HeadSkewTransport { since_args: Arc::clone(&since_args) };
        let mut engine = SyncEngine::new(make_config()).unwrap();

        engine.record(make_event("local.pending")).unwrap();
        let (_push, pull) = engine.full_sync(&transport).await.unwrap();

        assert_eq!(pull.events.len(), 1);
        assert_eq!(&*since_args.lock().unwrap(), &[0]);
    }

    #[tokio::test]
    async fn pull_errors_when_has_more_but_cursor_cannot_advance() {
        #[derive(Debug)]
        struct StalledPagingTransport;

        #[async_trait::async_trait]
        impl Transport for StalledPagingTransport {
            async fn push_events(&self, events: &[SyncEvent]) -> Result<PushResult, SyncError> {
                Ok(PushResult { accepted: events.len(), remote_head: 0 })
            }

            async fn pull_events(
                &self,
                _since: u64,
                _limit: usize,
            ) -> Result<PullResult, SyncError> {
                // has_more=true but no event sequence progress, so default cursor
                // derivation cannot safely continue.
                Ok(PullResult {
                    events: vec![
                        SyncEvent::new("order.updated", "order", "ORD-1", json!({}))
                            .with_sequence(0),
                    ],
                    remote_head: 100,
                    has_more: true,
                })
            }
        }

        let mut engine = SyncEngine::new(make_config()).unwrap();
        let err = engine.pull(&StalledPagingTransport).await.unwrap_err();
        assert!(matches!(err, SyncError::Transport(_)));
    }

    proptest! {
        #[test]
        fn resolve_next_cursor_enforces_monotonic_progress(
            since in 0u64..20_000,
            transport_cursor in prop::option::of(0u64..20_000),
            sequences in prop::collection::vec(0u64..20_000, 0..64),
        ) {
            let events: Vec<SyncEvent> = sequences
                .iter()
                .enumerate()
                .map(|(i, seq)| {
                    SyncEvent::new(
                        format!("evt-{i}"),
                        "entity",
                        format!("id-{i}"),
                        json!({ "s": seq }),
                    )
                    .with_sequence(*seq)
                })
                .collect();

            let result = SyncEngine::resolve_next_cursor(since, &events, true, transport_cursor);
            if let Some(cursor) = transport_cursor {
                if cursor > since {
                    prop_assert_eq!(result.unwrap(), Some(cursor));
                } else {
                    prop_assert!(matches!(result, Err(SyncError::Transport(_))));
                }
            } else if let Some(expected) = derive_next_cursor(since, &events) {
                prop_assert_eq!(result.unwrap(), Some(expected));
            } else {
                prop_assert!(matches!(result, Err(SyncError::Transport(_))));
            }
        }
    }

    proptest! {
        #[test]
        fn resolve_next_cursor_returns_none_when_transport_signals_no_more(
            since in 0u64..20_000,
            transport_cursor in prop::option::of(0u64..20_000),
            sequences in prop::collection::vec(0u64..20_000, 0..64),
        ) {
            let events: Vec<SyncEvent> = sequences
                .iter()
                .enumerate()
                .map(|(i, seq)| {
                    SyncEvent::new(
                        format!("evt-{i}"),
                        "entity",
                        format!("id-{i}"),
                        json!({}),
                    )
                    .with_sequence(*seq)
                })
                .collect();

            let result = SyncEngine::resolve_next_cursor(since, &events, false, transport_cursor)
                .unwrap();
            prop_assert_eq!(result, None);
        }
    }

    #[test]
    fn engine_debug() {
        let engine = SyncEngine::new(make_config()).unwrap();
        let debug = format!("{engine:?}");
        assert!(debug.contains("SyncEngine"));
    }
}