//! Direct observation knowledge flow (#364).
//!
//! Emits knowledge events when entities enter or leave the observer's LOS.
//! Runs after compute_observer_snapshot, before process_knowledge_events.

use bevy_ecs::prelude::*;

use crate::bridge::types::*;
use crate::knowledge::{
    EntityRegistry, KnowledgeEvent, KnowledgeEventQueue, KnowledgeEventType, KnowledgeGraph,
};
use crate::simulation::movement::{PlayerCharacter, TilePosition};
use crate::simulation::time::SimulationTime;

/// Emit knowledge events for entities entering/leaving the observer's LOS.
///
/// Compares the current snapshot's visible entities against the observer's
/// knowledge graph. Emits DirectObservation for newly visible entities and
/// LeftLOS for entities that were Direct but are no longer in the snapshot.
pub fn emit_observation_events(
    time: Res<SimulationTime>,
    buffer: Res<SnapshotBuffer>,
    registry: Res<EntityRegistry>,
    observer_query: Query<(Entity, &KnowledgeGraph), With<PlayerCharacter>>,
    mut event_queue: ResMut<KnowledgeEventQueue>,
    entity_positions: Query<&TilePosition>,
) {
    let Some(snapshot) = &buffer.snapshot else {
        return;
    };

    let Ok((observer_entity, observer_kg)) = observer_query.single() else {
        return;
    };

    // Collect visible entity IDs (Vec — linear search is faster at 5-20 entities)
    let visible_entity_ids: Vec<u64> = snapshot
        .entities
        .iter()
        .filter(|e| !matches!(e.kind, EntityKind::Player))
        .map(|e| e.entity_id)
        .collect();

    // Emit DirectObservation for each visible non-player entity
    for visible in &snapshot.entities {
        if matches!(visible.kind, EntityKind::Player) {
            continue;
        }

        // Look up the bevy Entity from the entity_id (which is Entity::to_bits())
        let entity = Entity::from_bits(visible.entity_id);

        // Get tile position for knowledge tracking
        if let Ok(pos) = entity_positions.get(entity) {
            event_queue.push(KnowledgeEvent {
                observer: observer_entity,
                tick: time.tick,
                event_type: KnowledgeEventType::DirectObservation {
                    target: entity,
                    position: *pos,
                },
            });
        }
    }

    // Emit LeftLOS for entities that were Direct but are no longer visible
    for (stable_id, knowledge) in observer_kg.known_entities_iter() {
        if knowledge.confidence != crate::knowledge::KnowledgeConfidence::Direct {
            continue;
        }

        // Check if this entity is still visible in the current snapshot
        if let Some(entity) = registry.to_entity(stable_id) {
            let entity_bits = entity.to_bits();
            if !visible_entity_ids.contains(&entity_bits) {
                event_queue.push(KnowledgeEvent {
                    observer: observer_entity,
                    tick: time.tick,
                    event_type: KnowledgeEventType::LeftLOS { target: entity },
                });
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::knowledge::{EntityRegistry, KnowledgeGraph};
    use crate::npc::Npc;
    use crate::perception::observer::compute_observer_snapshot;
    use crate::perception::vision_cone::Facing;
    use crate::simulation::movement::WalkabilityMap;

    fn setup_world(width: i32, height: i32) -> World {
        let mut world = World::new();
        world.insert_resource(SimulationTime::default());
        world.insert_resource(WalkabilityMap::new(width, height, 1));
        world.init_resource::<SnapshotBuffer>();
        world.init_resource::<KnowledgeEventQueue>();
        world.init_resource::<EntityRegistry>();
        world.init_resource::<crate::simulation::interaction::NearbyInteractionBuffer>();
        world
    }

    #[test]
    fn visible_npc_emits_direct_observation() {
        let mut world = setup_world(32, 32);
        let mut registry = EntityRegistry::new(0);

        let player = world
            .spawn((
                PlayerCharacter,
                TilePosition::new(16, 16, 0),
                Facing(FacingDirection::North),
                KnowledgeGraph::new(),
            ))
            .id();
        registry.register(player);

        let npc = world
            .spawn((Npc, TilePosition::new(16, 14, 0)))
            .id();
        registry.register(npc);

        world.insert_resource(registry);

        // First: compute snapshot so NPC is visible
        let mut schedule = bevy_ecs::schedule::Schedule::default();
        schedule.add_systems((compute_observer_snapshot, emit_observation_events).chain());
        schedule.run(&mut world);

        let queue = world.resource::<KnowledgeEventQueue>();
        assert!(
            !queue.is_empty(),
            "should emit DirectObservation for visible NPC"
        );
    }

    #[test]
    fn npc_leaving_los_emits_left_los() {
        let mut world = setup_world(32, 32);
        let mut registry = EntityRegistry::new(0);

        let npc = world
            .spawn((Npc, TilePosition::new(16, 14, 0)))
            .id();
        let npc_sid = registry.register(npc);

        // Player with pre-existing Direct knowledge of NPC
        let mut kg = KnowledgeGraph::new();
        kg.observe_entity(npc_sid, TilePosition::new(16, 14, 0), 50);

        let player = world
            .spawn((
                PlayerCharacter,
                TilePosition::new(16, 16, 0),
                Facing(FacingDirection::North),
                kg,
            ))
            .id();
        registry.register(player);

        world.insert_resource(registry);

        // Now block the NPC behind a wall so it's NOT visible
        let mut walkability = world.resource_mut::<WalkabilityMap>();
        walkability.set_walkable(&TilePosition::new(16, 15, 0), false);

        let mut schedule = bevy_ecs::schedule::Schedule::default();
        schedule.add_systems((compute_observer_snapshot, emit_observation_events).chain());
        schedule.run(&mut world);

        let queue = world.resource::<KnowledgeEventQueue>();
        let events = &queue.events;
        let has_left_los = events.iter().any(|e| {
            matches!(
                e.event_type,
                KnowledgeEventType::LeftLOS { target } if target == npc
            )
        });
        assert!(has_left_los, "should emit LeftLOS when NPC is no longer visible");
    }

    #[test]
    fn no_events_for_player_entity() {
        let mut world = setup_world(32, 32);
        let mut registry = EntityRegistry::new(0);

        let player = world
            .spawn((
                PlayerCharacter,
                TilePosition::new(16, 16, 0),
                Facing::default(),
                KnowledgeGraph::new(),
            ))
            .id();
        registry.register(player);
        world.insert_resource(registry);

        let mut schedule = bevy_ecs::schedule::Schedule::default();
        schedule.add_systems((compute_observer_snapshot, emit_observation_events).chain());
        schedule.run(&mut world);

        let queue = world.resource::<KnowledgeEventQueue>();
        assert!(
            queue.is_empty(),
            "should not emit observation events for the player entity itself"
        );
    }
}