//! Knowledge event queue and processing system (#363).
//!
//! Event-driven knowledge updates. Perception and other systems push
//! KnowledgeEvents; the processing system drains them per tick.

use bevy_ecs::prelude::*;

use crate::simulation::movement::TilePosition;

use super::graph::KnowledgeGraph;
use super::registry::EntityRegistry;
use super::types::*;

/// Events that modify knowledge graphs. Produced by perception and
/// other systems. Consumed by the knowledge update system.
#[derive(Debug, Clone)]
pub struct KnowledgeEvent {
    pub observer: Entity,
    pub tick: u64,
    pub event_type: KnowledgeEventType,
}

#[derive(Debug, Clone)]
pub enum KnowledgeEventType {
    /// Observer saw entity at position (sets Direct confidence).
    DirectObservation {
        target: Entity,
        position: TilePosition,
    },
    /// Entity left observer's LOS (downgrades from Direct).
    LeftLOS { target: Entity },
}

/// Resource: queue of pending knowledge events.
/// Drained once per tick by the knowledge update system.
#[derive(Resource, Default)]
pub struct KnowledgeEventQueue {
    pub(crate) events: Vec<KnowledgeEvent>,
}

impl KnowledgeEventQueue {
    /// Push a knowledge event into the queue.
    pub fn push(&mut self, event: KnowledgeEvent) {
        self.events.push(event);
    }

    /// Drain all pending events.
    pub fn drain(&mut self) -> Vec<KnowledgeEvent> {
        std::mem::take(&mut self.events)
    }

    /// Number of pending events.
    pub fn len(&self) -> usize {
        self.events.len()
    }

    /// Whether the queue is empty.
    pub fn is_empty(&self) -> bool {
        self.events.is_empty()
    }
}

/// System: process pending knowledge events.
/// Runs once per tick, drains KnowledgeEventQueue and applies updates
/// to the relevant KnowledgeGraph components.
pub fn process_knowledge_events(
    mut queue: ResMut<KnowledgeEventQueue>,
    registry: Res<EntityRegistry>,
    mut knowledge_query: Query<&mut KnowledgeGraph>,
) {
    let events = queue.drain();
    for event in events {
        let Ok(mut observer_kg) = knowledge_query.get_mut(event.observer) else {
            continue;
        };

        match event.event_type {
            KnowledgeEventType::DirectObservation { target, position } => {
                if let Some(stable_id) = registry.to_stable(target) {
                    observer_kg.observe_entity(stable_id, position, event.tick);
                } else {
                    debug_assert!(false, "DirectObservation target {:?} not in EntityRegistry", target);
                    tracing::error!("DirectObservation target {:?} not in EntityRegistry", target);
                }
            }
            KnowledgeEventType::LeftLOS { target } => {
                if let Some(stable_id) = registry.to_stable(target) {
                    observer_kg.observe_entity_leaving_los(&stable_id, event.tick);
                } else {
                    debug_assert!(false, "LeftLOS target {:?} not in EntityRegistry", target);
                    tracing::error!("LeftLOS target {:?} not in EntityRegistry", target);
                }
            }
        }
    }
}

/// System: run knowledge decay once per game-minute (every 10 ticks per D-031).
pub fn decay_knowledge(
    time: Res<crate::simulation::time::SimulationTime>,
    thresholds: Res<DecayThresholds>,
    mut knowledge_query: Query<&mut KnowledgeGraph>,
) {
    // Decay runs every 10 ticks (1 game-minute per D-031)
    if !time.tick.is_multiple_of(10) {
        return;
    }
    for mut kg in knowledge_query.iter_mut() {
        kg.decay(time.tick, &thresholds);
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use bevy_ecs::world::World;

    #[test]
    fn queue_push_and_drain() {
        let mut queue = KnowledgeEventQueue::default();
        assert!(queue.is_empty());

        let mut world = World::new();
        let e1 = world.spawn_empty().id();
        let e2 = world.spawn_empty().id();

        queue.push(KnowledgeEvent {
            observer: e1,
            tick: 100,
            event_type: KnowledgeEventType::DirectObservation {
                target: e2,
                position: TilePosition::new(5, 5, 0),
            },
        });
        assert_eq!(queue.len(), 1);

        let events = queue.drain();
        assert_eq!(events.len(), 1);
        assert!(queue.is_empty());
    }

    #[test]
    fn process_direct_observation_event() {
        let mut world = World::new();
        let mut registry = EntityRegistry::new(0);

        let observer = world.spawn(KnowledgeGraph::new()).id();
        let target = world.spawn_empty().id();

        let observer_sid = registry.register(observer);
        let target_sid = registry.register(target);
        let _ = observer_sid; // registered for completeness

        world.insert_resource(registry);

        let mut queue = KnowledgeEventQueue::default();
        queue.push(KnowledgeEvent {
            observer,
            tick: 50,
            event_type: KnowledgeEventType::DirectObservation {
                target,
                position: TilePosition::new(10, 10, 0),
            },
        });
        world.insert_resource(queue);

        let mut schedule = bevy_ecs::schedule::Schedule::default();
        schedule.add_systems(process_knowledge_events);
        schedule.run(&mut world);

        let kg = world.entity(observer).get::<KnowledgeGraph>().unwrap();
        assert!(kg.knows_entity(&target_sid));
        assert_eq!(
            kg.confidence_of(&target_sid),
            Some(KnowledgeConfidence::Direct)
        );
    }

    #[test]
    fn process_left_los_event() {
        let mut world = World::new();
        let mut registry = EntityRegistry::new(0);

        let target = world.spawn_empty().id();
        let target_sid = registry.register(target);

        // Pre-populate observer with Direct knowledge
        let mut kg = KnowledgeGraph::new();
        kg.observe_entity(target_sid, TilePosition::new(10, 10, 0), 50);

        let observer = world.spawn(kg).id();
        registry.register(observer);

        world.insert_resource(registry);

        let mut queue = KnowledgeEventQueue::default();
        queue.push(KnowledgeEvent {
            observer,
            tick: 60,
            event_type: KnowledgeEventType::LeftLOS { target },
        });
        world.insert_resource(queue);

        let mut schedule = bevy_ecs::schedule::Schedule::default();
        schedule.add_systems(process_knowledge_events);
        schedule.run(&mut world);

        let kg = world.entity(observer).get::<KnowledgeGraph>().unwrap();
        assert_eq!(
            kg.confidence_of(&target_sid),
            Some(KnowledgeConfidence::KnowsDetails)
        );
    }

    #[test]
    fn process_event_for_missing_observer_is_no_op() {
        let mut world = World::new();
        let registry = EntityRegistry::new(0);
        world.insert_resource(registry);

        let fake_observer = world.spawn_empty().id(); // no KnowledgeGraph
        let fake_target = world.spawn_empty().id();

        let mut queue = KnowledgeEventQueue::default();
        queue.push(KnowledgeEvent {
            observer: fake_observer,
            tick: 100,
            event_type: KnowledgeEventType::DirectObservation {
                target: fake_target,
                position: TilePosition::new(5, 5, 0),
            },
        });
        world.insert_resource(queue);

        let mut schedule = bevy_ecs::schedule::Schedule::default();
        schedule.add_systems(process_knowledge_events);
        schedule.run(&mut world); // Should not panic
    }

    #[test]
    fn decay_skips_non_minute_ticks() {
        use crate::simulation::time::SimulationTime;

        let mut world = World::new();
        let mut registry = EntityRegistry::new(0);

        let target_entity = world.spawn_empty().id();
        let target_sid = registry.register(target_entity);

        let mut kg = KnowledgeGraph::new();
        kg.observe_entity(target_sid, TilePosition::new(5, 5, 0), 0);
        kg.observe_entity_leaving_los(&target_sid, 1); // KnowsDetails

        let observer = world.spawn(kg).id();
        registry.register(observer);
        world.insert_resource(registry);

        let thresholds = DecayThresholds {
            decay_after: 5,
            stale_after: 100,
        };
        world.insert_resource(thresholds);

        // Tick 7: not a multiple of 10, decay should NOT run
        world.insert_resource({ let mut t = SimulationTime::default(); t.tick = 7; t });
        let mut schedule = bevy_ecs::schedule::Schedule::default();
        schedule.add_systems(decay_knowledge);
        schedule.run(&mut world);

        let kg = world.entity(observer).get::<KnowledgeGraph>().unwrap();
        assert_eq!(
            kg.confidence_of(&target_sid),
            Some(KnowledgeConfidence::KnowsDetails),
            "decay should not run on tick 7 (not a game-minute boundary)"
        );

        // Tick 10: multiple of 10, decay SHOULD run (age = 10 > decay_after = 5)
        world.insert_resource({ let mut t = SimulationTime::default(); t.tick = 10; t });
        let mut schedule2 = bevy_ecs::schedule::Schedule::default();
        schedule2.add_systems(decay_knowledge);
        schedule2.run(&mut world);

        let kg = world.entity(observer).get::<KnowledgeGraph>().unwrap();
        assert_eq!(
            kg.confidence_of(&target_sid),
            Some(KnowledgeConfidence::KnowsOf),
            "decay should run on tick 10 and downgrade confidence"
        );
    }
}