offline_protocol_reliability/

ack_manager.rs

//! ACK (acknowledgment) management for tracking pending acknowledgments.

use crate::constants::{DEFAULT_ACK_TIMEOUT_MS, DEFAULT_MAX_PENDING_ACKS};
use chrono::{DateTime, Utc};
use offline_protocol_core::{MessageId, MessagePriority};
use std::collections::HashMap;
use std::time::Duration;

/// Information about an ACK that was evicted due to capacity pressure.
#[derive(Debug, Clone)]
#[non_exhaustive]
pub struct AckEvictionInfo {
    /// ID of the message whose ACK tracking was evicted.
    pub message_id: MessageId,
    /// Priority of the evicted message.
    pub priority: MessagePriority,
}

/// Status of an ACK.
///
/// There is no `Received` state: an acknowledged message is removed from
/// tracking entirely via [`AckManager::remove_ack`].
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AckStatus {
    /// Waiting for ACK.
    Pending,
    /// ACK timed out.
    TimedOut,
}

/// Information about a pending ACK.
#[derive(Debug, Clone)]
pub struct PendingAck {
    /// Message ID waiting for ACK.
    pub message_id: MessageId,

    /// When the message was sent.
    pub sent_at: DateTime<Utc>,

    /// Timeout for this ACK (in milliseconds).
    pub timeout_ms: u64,

    /// Current status.
    pub status: AckStatus,

    /// Number of times the message has been retried.
    pub retry_count: u32,

    /// Priority of the message (for eviction decisions).
    pub priority: MessagePriority,
}

impl PendingAck {
    /// Checks if this ACK has timed out.
    pub fn is_timed_out(&self) -> bool {
        if self.status != AckStatus::Pending {
            return false;
        }

        let elapsed = Utc::now().signed_duration_since(self.sent_at);
        elapsed.num_milliseconds() >= self.timeout_ms as i64
    }
}

/// Configuration for ACK management.
#[derive(Debug, Clone)]
pub struct AckConfig {
    /// Default ACK timeout in milliseconds.
    pub default_timeout_ms: u64,

    /// Maximum number of pending ACKs to track.
    pub max_pending_acks: usize,
}

impl Default for AckConfig {
    fn default() -> Self {
        Self {
            default_timeout_ms: DEFAULT_ACK_TIMEOUT_MS,
            max_pending_acks: DEFAULT_MAX_PENDING_ACKS,
        }
    }
}

/// ACK manager for tracking pending acknowledgments.
pub struct AckManager {
    config: AckConfig,
    pending_acks: HashMap<MessageId, PendingAck>,
}

impl AckManager {
    /// Creates a new ACK manager with default configuration.
    pub fn new() -> Self {
        Self::with_config(AckConfig::default())
    }

    /// Creates a new ACK manager with custom configuration.
    pub fn with_config(config: AckConfig) -> Self {
        Self {
            config,
            pending_acks: HashMap::new(),
        }
    }

    /// Registers a message as waiting for ACK.
    ///
    /// # Arguments
    ///
    /// * `message_id` - ID of the message waiting for ACK
    /// * `timeout_ms` - Optional custom timeout (uses default if None)
    ///
    /// # Returns
    ///
    /// Returns `Ok(Some(info))` if a lower-priority ACK was evicted to make room,
    /// `Ok(None)` if no eviction was needed, or `Err` if capacity is full and no
    /// lower-priority ACK exists to evict.
    pub fn register_pending_ack(
        &mut self,
        message_id: MessageId,
        timeout_ms: Option<u64>,
    ) -> crate::Result<Option<AckEvictionInfo>> {
        self.register_pending_ack_with_priority(message_id, timeout_ms, MessagePriority::Medium)
    }

    /// Registers a message as waiting for ACK with specified priority.
    ///
    /// # Arguments
    ///
    /// * `message_id` - ID of the message waiting for ACK
    /// * `timeout_ms` - Optional custom timeout (uses default if None)
    /// * `priority` - Message priority for eviction decisions
    ///
    /// # Returns
    ///
    /// Returns `Ok(Some(info))` if a lower-priority ACK was evicted to make room,
    /// `Ok(None)` if no eviction was needed, or `Err` if capacity is full and no
    /// lower-priority ACK exists to evict.
    pub fn register_pending_ack_with_priority(
        &mut self,
        message_id: MessageId,
        timeout_ms: Option<u64>,
        priority: MessagePriority,
    ) -> crate::Result<Option<AckEvictionInfo>> {
        // Check if we've hit the limit
        let mut eviction = None;
        if self.pending_acks.len() >= self.config.max_pending_acks {
            // Clean up timed out ACKs first
            self.cleanup_timed_out();

            // If still at limit, try priority-based eviction
            if self.pending_acks.len() >= self.config.max_pending_acks {
                match self.evict_lowest_priority(priority) {
                    Some(info) => eviction = Some(info),
                    None => {
                        return Err(crate::Error::Other(
                            "Maximum pending ACKs limit reached (no lower priority ACKs to evict)"
                                .to_string(),
                        ));
                    }
                }
            }
        }

        let pending = PendingAck {
            message_id: message_id.clone(),
            sent_at: Utc::now(),
            timeout_ms: timeout_ms.unwrap_or(self.config.default_timeout_ms),
            status: AckStatus::Pending,
            retry_count: 0,
            priority,
        };

        self.pending_acks.insert(message_id, pending);
        Ok(eviction)
    }

    /// Attempts to evict the lowest priority ACK to make room for a higher priority one.
    /// Returns `Some(info)` if an ACK was evicted, `None` if no lower priority ACK exists.
    fn evict_lowest_priority(&mut self, new_priority: MessagePriority) -> Option<AckEvictionInfo> {
        // Find the lowest priority pending ACK
        let evict_candidate = self
            .pending_acks
            .iter()
            .filter(|(_, ack)| ack.status == AckStatus::Pending)
            .min_by(|(_, a), (_, b)| {
                // Compare by priority first (lower priority = evict first)
                let priority_cmp =
                    Self::priority_rank(a.priority).cmp(&Self::priority_rank(b.priority));
                if priority_cmp != std::cmp::Ordering::Equal {
                    return priority_cmp;
                }
                // Within same priority, evict oldest first
                a.sent_at.cmp(&b.sent_at)
            })
            .map(|(id, ack)| (id.clone(), ack.priority));

        if let Some((evict_id, evict_priority)) = evict_candidate {
            // Only evict if the candidate's priority is strictly lower
            if Self::priority_rank(evict_priority) < Self::priority_rank(new_priority) {
                self.pending_acks.remove(&evict_id);
                return Some(AckEvictionInfo {
                    message_id: evict_id,
                    priority: evict_priority,
                });
            }
        }

        None
    }

    /// Returns a numeric rank for priority (higher = more important).
    fn priority_rank(priority: MessagePriority) -> u8 {
        match priority {
            MessagePriority::Low => 0,
            MessagePriority::Medium => 1,
            MessagePriority::High => 2,
            MessagePriority::Critical => 3,
        }
    }

    /// Returns all ACKs that have timed out and marks them as `TimedOut`.
    ///
    /// Note: This does NOT remove entries from the map. Entries are kept so that:
    /// - `increment_retry_count()` can reset them when the message is retried
    /// - Late ACKs can still be matched via `remove_ack()`
    ///
    /// Entries are eventually removed by:
    /// - `remove_ack()` when ACK is received or max retries exceeded
    /// - `prune_old_timeouts()` for stale entries
    pub fn drain_timed_out(&mut self) -> Vec<PendingAck> {
        let mut timed_out = Vec::new();
        for pending in self.pending_acks.values_mut() {
            if pending.is_timed_out() {
                pending.status = AckStatus::TimedOut;
                timed_out.push(pending.clone());
            }
        }
        timed_out
    }

    /// Removes an ACK from tracking (e.g., after successful delivery).
    pub fn remove_ack(&mut self, message_id: &MessageId) -> Option<PendingAck> {
        self.pending_acks.remove(message_id)
    }

    /// Gets all timed out ACKs.
    ///
    /// # Returns
    ///
    /// Returns a vector of message IDs that have timed out.
    pub fn get_timed_out_acks(&self) -> Vec<MessageId> {
        self.pending_acks
            .values()
            .filter(|ack| ack.is_timed_out())
            .map(|ack| ack.message_id.clone())
            .collect()
    }

    /// Updates the retry count for a message.
    pub fn increment_retry_count(&mut self, message_id: &MessageId) {
        if let Some(pending) = self.pending_acks.get_mut(message_id) {
            pending.retry_count += 1;
            pending.sent_at = Utc::now(); // Reset timer for retry
            pending.status = AckStatus::Pending;
        }
    }

    /// Gets information about a pending ACK.
    pub fn get_pending_ack(&self, message_id: &MessageId) -> Option<&PendingAck> {
        self.pending_acks.get(message_id)
    }

    /// Gets the number of pending ACKs.
    pub fn pending_count(&self) -> usize {
        self.pending_acks.len()
    }

    /// Checks if a message is waiting for ACK.
    pub fn is_waiting_for_ack(&self, message_id: &MessageId) -> bool {
        self.pending_acks.contains_key(message_id)
    }

    /// Cleans up timed out ACKs by marking them as timed out.
    fn cleanup_timed_out(&mut self) {
        self.drain_timed_out();
    }

    /// Removes all ACKs with TimedOut status that are older than a given duration.
    ///
    /// This helps prevent unbounded memory growth.
    pub fn prune_old_timeouts(&mut self, older_than: Duration) {
        let now = Utc::now();
        let cutoff = now
            - chrono::Duration::from_std(older_than).unwrap_or_else(|_| chrono::Duration::zero());

        self.pending_acks
            .retain(|_, ack| ack.status != AckStatus::TimedOut || ack.sent_at > cutoff);
    }
}

impl Default for AckManager {
    fn default() -> Self {
        Self::new()
    }
}

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

    #[test]
    fn test_register_and_receive_ack() {
        let mut manager = AckManager::new();
        let msg_id = MessageId::new();

        // Register pending ACK
        manager.register_pending_ack(msg_id.clone(), None).unwrap();
        assert!(manager.is_waiting_for_ack(&msg_id));
        assert_eq!(manager.pending_count(), 1);

        // Receive ACK: the entry is removed from tracking entirely
        let removed = manager.remove_ack(&msg_id).unwrap();
        assert_eq!(removed.message_id, msg_id);
        assert!(!manager.is_waiting_for_ack(&msg_id));
        assert_eq!(manager.pending_count(), 0);
    }

    #[test]
    fn test_ack_timeout() {
        let config = AckConfig {
            default_timeout_ms: 50, // 50ms for fast test
            max_pending_acks: 100,
        };
        let mut manager = AckManager::with_config(config);
        let msg_id = MessageId::new();

        manager
            .register_pending_ack(msg_id.clone(), Some(50))
            .unwrap();

        // Not timed out immediately
        assert!(manager.drain_timed_out().is_empty());

        // Wait for timeout
        thread::sleep(Duration::from_millis(60));

        // Should be timed out now
        let timed_out = manager.drain_timed_out();
        assert_eq!(timed_out.len(), 1);
        assert_eq!(timed_out[0].message_id, msg_id);
    }

    #[test]
    fn test_retry_count_increment() {
        let mut manager = AckManager::new();
        let msg_id = MessageId::new();

        manager.register_pending_ack(msg_id.clone(), None).unwrap();

        let pending = manager.get_pending_ack(&msg_id).unwrap();
        assert_eq!(pending.retry_count, 0);

        manager.increment_retry_count(&msg_id);

        let pending = manager.get_pending_ack(&msg_id).unwrap();
        assert_eq!(pending.retry_count, 1);
    }

    #[test]
    fn test_remove_ack() {
        let mut manager = AckManager::new();
        let msg_id = MessageId::new();

        manager.register_pending_ack(msg_id.clone(), None).unwrap();
        assert!(manager.is_waiting_for_ack(&msg_id));

        manager.remove_ack(&msg_id);
        assert!(!manager.is_waiting_for_ack(&msg_id));
    }

    #[test]
    fn test_max_pending_limit() {
        let config = AckConfig {
            default_timeout_ms: 5000,
            max_pending_acks: 3,
        };
        let mut manager = AckManager::with_config(config);

        // Should be able to register up to limit
        manager
            .register_pending_ack(MessageId::new(), None)
            .unwrap();
        manager
            .register_pending_ack(MessageId::new(), None)
            .unwrap();
        manager
            .register_pending_ack(MessageId::new(), None)
            .unwrap();

        // Fourth should fail (same priority can't evict)
        let result = manager.register_pending_ack(MessageId::new(), None);
        assert!(result.is_err());
    }

    #[test]
    fn test_priority_based_eviction() {
        let config = AckConfig {
            default_timeout_ms: 5000,
            max_pending_acks: 2,
        };
        let mut manager = AckManager::with_config(config);

        let low_id = MessageId::new();
        let medium_id = MessageId::new();

        // Register low priority ACK
        manager
            .register_pending_ack_with_priority(low_id.clone(), None, MessagePriority::Low)
            .unwrap();
        // Register medium priority ACK
        manager
            .register_pending_ack_with_priority(medium_id.clone(), None, MessagePriority::Medium)
            .unwrap();

        assert_eq!(manager.pending_count(), 2);

        // High priority should evict low priority
        let high_id = MessageId::new();
        manager
            .register_pending_ack_with_priority(high_id.clone(), None, MessagePriority::High)
            .unwrap();

        assert_eq!(manager.pending_count(), 2);
        assert!(!manager.is_waiting_for_ack(&low_id)); // Low was evicted
        assert!(manager.is_waiting_for_ack(&medium_id)); // Medium still present
        assert!(manager.is_waiting_for_ack(&high_id)); // High was added
    }

    #[test]
    fn test_critical_priority_evicts_all() {
        let config = AckConfig {
            default_timeout_ms: 5000,
            max_pending_acks: 1,
        };
        let mut manager = AckManager::with_config(config);

        let medium_id = MessageId::new();
        manager
            .register_pending_ack_with_priority(medium_id.clone(), None, MessagePriority::Medium)
            .unwrap();

        // Critical should evict medium
        let critical_id = MessageId::new();
        manager
            .register_pending_ack_with_priority(
                critical_id.clone(),
                None,
                MessagePriority::Critical,
            )
            .unwrap();

        assert_eq!(manager.pending_count(), 1);
        assert!(!manager.is_waiting_for_ack(&medium_id));
        assert!(manager.is_waiting_for_ack(&critical_id));
    }

    #[test]
    fn test_prune_old_timeouts() {
        let config = AckConfig {
            default_timeout_ms: 10,
            max_pending_acks: 100,
        };
        let mut manager = AckManager::with_config(config);

        let msg_id = MessageId::new();
        manager
            .register_pending_ack(msg_id.clone(), Some(10))
            .unwrap();

        // Wait for timeout
        thread::sleep(Duration::from_millis(20));
        manager.cleanup_timed_out();

        assert_eq!(manager.pending_count(), 1);

        // Prune old timeouts
        manager.prune_old_timeouts(Duration::from_millis(5));
        assert_eq!(manager.pending_count(), 0);
    }

    #[test]
    fn test_ack_not_found() {
        let mut manager = AckManager::new();
        let msg_id = MessageId::new();

        // Removing an ACK for a non-existent message should return None
        assert!(manager.remove_ack(&msg_id).is_none());
    }

    #[test]
    fn test_eviction_returned_from_register() {
        let config = AckConfig {
            default_timeout_ms: 5000,
            max_pending_acks: 2,
        };
        let mut manager = AckManager::with_config(config);

        let low_id = MessageId::new();
        // No eviction when registering into non-full capacity.
        assert!(manager
            .register_pending_ack_with_priority(low_id.clone(), None, MessagePriority::Low)
            .unwrap()
            .is_none());
        assert!(manager
            .register_pending_ack_with_priority(MessageId::new(), None, MessagePriority::Medium)
            .unwrap()
            .is_none());

        // High priority triggers eviction of low — returned in-band.
        let eviction = manager
            .register_pending_ack_with_priority(MessageId::new(), None, MessagePriority::High)
            .unwrap();
        let eviction = eviction.expect("should have evicted the low-priority ACK");
        assert_eq!(eviction.message_id, low_id);
        assert_eq!(eviction.priority, MessagePriority::Low);
    }
}