//! Observer visibility query system (#112)
//!
//! Replaces the unfiltered `generate_snapshot` with a visibility-aware version.
//! Combines shadowcasting + vision cone to determine what the observer can see,
//! then populates ObserverSnapshot v2 with only visible entities and tiles.

use bevy_ecs::prelude::*;
use std::collections::HashSet;

use crate::bridge::types::*;
use crate::knowledge::{EntityRegistry, KnowledgeGraph};
use crate::perception::shadowcast::compute_fov;
use crate::perception::vision_cone::{apply_vision_cone, Facing, VisionConeConfig};
use crate::simulation::movement::{PlayerCharacter, TilePosition, WalkabilityMap};
use crate::simulation::time::SimulationTime;

/// Compute observer snapshot with LOS filtering and vision cone.
///
/// System ordering: after validate_movement, before advance_tick.
/// Replaces bridge::generate_snapshot.
pub fn compute_observer_snapshot(
    time: Res<SimulationTime>,
    walkability: Res<WalkabilityMap>,
    registry: Res<EntityRegistry>,
    observer_query: Query<(&TilePosition, Option<&Facing>, &KnowledgeGraph), With<PlayerCharacter>>,
    all_entities: Query<(
        Entity,
        &TilePosition,
        Option<&PlayerCharacter>,
        Option<&crate::npc::Npc>,
    )>,
    mut buffer: ResMut<SnapshotBuffer>,
) {
    let Ok((observer_pos, facing_opt, observer_kg)) = observer_query.single() else {
        return;
    };

    let facing = facing_opt
        .map(|f| f.0)
        .unwrap_or(FacingDirection::default());

    let config = VisionConeConfig::default();
    let z = observer_pos.z;

    // Step 1: Compute raw FOV using symmetric shadowcasting
    let fov = compute_fov(
        |x, y| !walkability.can_move_to(&TilePosition::new(x, y, z)),
        observer_pos.x,
        observer_pos.y,
        config.forward_range,
        z,
    );

    // Step 2: Apply vision cone to get sector-tagged tiles
    let cone_tiles =
        apply_vision_cone(&fov, observer_pos.x, observer_pos.y, facing, &config);

    // Step 3: Build visible_tiles for the snapshot
    let visible_tiles: Vec<VisibleTile> = cone_tiles
        .iter()
        .map(|&(x, y, sector)| VisibleTile {
            x,
            y,
            z,
            visibility: sector,
        })
        .collect();

    // Step 4: Build lookup set for fast entity visibility check
    let visible_positions: HashSet<(i32, i32)> =
        cone_tiles.iter().map(|&(x, y, _)| (x, y)).collect();

    // Build sector lookup (position -> sector)
    let sector_lookup: std::collections::HashMap<(i32, i32), VisibilitySector> = cone_tiles
        .iter()
        .map(|&(x, y, sector)| ((x, y), sector))
        .collect();

    // Step 5: Filter entities by visibility, overlay knowledge
    let mut entities = Vec::new();
    let mut visible_entity_bits: HashSet<u64> = HashSet::new();
    for (entity, pos, is_player, is_npc) in all_entities.iter() {
        // Different z-level: not visible
        if pos.z != z {
            continue;
        }

        // Not in visible tile set: not visible
        if !visible_positions.contains(&(pos.x, pos.y)) {
            continue;
        }

        let (rx, ry, rz) = pos.to_render_coords();
        let kind = if is_player.is_some() {
            EntityKind::Player
        } else if is_npc.is_some() {
            EntityKind::Npc
        } else {
            EntityKind::Object
        };

        let sector = sector_lookup
            .get(&(pos.x, pos.y))
            .copied()
            .unwrap_or(VisibilitySector::Peripheral);

        // Look up relationship from knowledge graph (D-033 entity color)
        let relationship = if is_player.is_some() {
            RelationshipState::Known // Self
        } else if let Some(stable_id) = registry.to_stable(entity) {
            observer_kg.relationship_with(&stable_id)
        } else {
            RelationshipState::Unknown
        };

        visible_entity_bits.insert(entity.to_bits());
        entities.push(VisibleEntity {
            entity_id: entity.to_bits(),
            x: rx,
            y: ry,
            z: rz,
            kind,
            visibility: sector,
            relationship,
            observation: EntityVisibility::Visible,
        });
    }

    // Step 6: Add remembered entities from knowledge graph (#366)
    // Entities the observer knows about but can't currently see.
    for (stable_id, knowledge) in observer_kg.known_entities_iter() {
        // Skip if currently visible (already in the entity list)
        if let Some(entity) = registry.to_entity(stable_id) {
            if visible_entity_bits.contains(&entity.to_bits()) {
                continue;
            }
        }

        // Skip if no known position (never directly observed)
        let Some(position) = knowledge.last_known_position else {
            continue;
        };

        // Skip if remembered position is on a different z-level than the observer
        if position.z != z {
            continue;
        }

        // Skip if the remembered tile is currently visible — if the player
        // can see the tile and the entity isn't there, don't show a ghost.
        if visible_positions.contains(&(position.x, position.y)) {
            continue;
        }

        // Direct confidence means the entity should be in LOS — if it isn't,
        // that's a transient data inconsistency. Skip rather than show a ghost.
        if knowledge.confidence == KnowledgeConfidence::Direct {
            continue;
        }

        let (rx, ry, rz) = position.to_render_coords();
        let age_ticks = time.tick.saturating_sub(knowledge.last_observed_tick);
        let entity_id = registry
            .to_entity(stable_id)
            .map(|e| e.to_bits())
            .unwrap_or(stable_id.0);

        entities.push(VisibleEntity {
            entity_id,
            x: rx,
            y: ry,
            z: rz,
            kind: EntityKind::Npc, // Remembered entities are NPCs (only NPCs are tracked)
            visibility: VisibilitySector::Forward, // Not meaningful for remembered entities
            relationship: knowledge.relationship,
            observation: EntityVisibility::Remembered {
                confidence: knowledge.confidence,
                age_ticks,
            },
        });
    }

    // Step 7: Build GameTime from SimulationTime
    let game_time = GameTime {
        day: time.day(),
        time_of_day: time.time_of_day_minutes(),
        day_phase: time.day_phase(),
        paused: time.paused,
    };

    tracing::trace!(
        "compute_observer_snapshot: tick={}, visible={}, remembered={}, tiles={}",
        time.tick,
        visible_entity_bits.len(),
        entities.len() - visible_entity_bits.len(),
        visible_tiles.len(),
    );

    // Step 8: Assemble snapshot (v3: added relationship + observation fields)
    buffer.snapshot = Some(ObserverSnapshot {
        version: 3,
        tick: time.tick,
        game_time,
        player_facing: facing,
        entities,
        visible_tiles,
    });
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::knowledge::{EntityRegistry, KnowledgeGraph};
    use crate::perception::vision_cone::Facing;
    use bevy_ecs::world::World;

    /// Helper: set up a test world with player, walkability map, and knowledge resources
    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::<EntityRegistry>();
        world
    }

    #[test]
    fn player_always_visible_in_snapshot() {
        let mut world = setup_world(32, 32);
        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing::default(),
            KnowledgeGraph::new(),
        ));

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().expect("snapshot should exist");
        assert_eq!(snapshot.version, 3);
        assert_eq!(snapshot.entities.len(), 1);
        assert!(matches!(snapshot.entities[0].kind, EntityKind::Player));
        assert_eq!(snapshot.entities[0].observation, EntityVisibility::Visible);
    }

    #[test]
    fn npc_in_los_visible() {
        let mut world = setup_world(32, 32);
        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing(FacingDirection::North),
            KnowledgeGraph::new(),
        ));
        // NPC directly north of player (in forward cone)
        world.spawn((crate::npc::Npc, TilePosition::new(16, 14, 0)));

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();
        assert_eq!(snapshot.entities.len(), 2);
        let npc = snapshot
            .entities
            .iter()
            .find(|e| matches!(e.kind, EntityKind::Npc))
            .expect("NPC should be visible");
        assert_eq!(npc.visibility, VisibilitySector::Forward);
        assert_eq!(npc.observation, EntityVisibility::Visible);
    }

    #[test]
    fn npc_behind_wall_not_visible() {
        let mut world = setup_world(32, 32);
        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing(FacingDirection::North),
            KnowledgeGraph::new(),
        ));
        // Wall between player and NPC
        let mut walkability = world.resource_mut::<WalkabilityMap>();
        walkability.set_walkable(&TilePosition::new(16, 14, 0), false);
        // NPC behind the wall
        world.spawn((crate::npc::Npc, TilePosition::new(16, 12, 0)));

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();
        // Only player should be visible, not the NPC behind the wall
        let npcs: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.kind, EntityKind::Npc))
            .collect();
        assert!(npcs.is_empty(), "NPC behind wall should not be visible");
    }

    #[test]
    fn npc_behind_player_not_visible() {
        let mut world = setup_world(32, 32);
        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing(FacingDirection::North),
            KnowledgeGraph::new(),
        ));
        // NPC far behind player (south, in blind spot)
        world.spawn((crate::npc::Npc, TilePosition::new(16, 26, 0)));

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();
        let npcs: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.kind, EntityKind::Npc))
            .collect();
        assert!(npcs.is_empty(), "NPC in blind spot should not be visible");
    }

    #[test]
    fn different_z_level_not_visible() {
        let mut world = setup_world(32, 32);
        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing::default(),
            KnowledgeGraph::new(),
        ));
        // NPC on different z-level
        world.spawn((crate::npc::Npc, TilePosition::new(16, 14, 1)));

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();
        let npcs: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.kind, EntityKind::Npc))
            .collect();
        assert!(npcs.is_empty(), "NPC on different z should not be visible");
    }

    #[test]
    fn game_time_populated() {
        let mut world = setup_world(32, 32);
        world.insert_resource(SimulationTime {
            tick: 7200, // 720 minutes = Evening
            paused: true,
        });
        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing::default(),
            KnowledgeGraph::new(),
        ));

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();
        assert_eq!(snapshot.game_time.time_of_day, 720);
        assert_eq!(
            snapshot.game_time.day_phase,
            crate::simulation::time::DayPhase::Evening
        );
        assert!(snapshot.game_time.paused);
    }

    #[test]
    fn visible_tiles_populated() {
        let mut world = setup_world(32, 32);
        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing::default(),
            KnowledgeGraph::new(),
        ));

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();
        assert!(
            !snapshot.visible_tiles.is_empty(),
            "should have visible tiles"
        );
        // Observer's tile should be in the list
        let has_observer_tile = snapshot
            .visible_tiles
            .iter()
            .any(|t| t.x == 16 && t.y == 16 && t.z == 0);
        assert!(has_observer_tile, "observer tile should be visible");
    }

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

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

        // Player knows NPC is hostile
        let mut kg = KnowledgeGraph::new();
        kg.observe_entity(npc_sid, TilePosition::new(16, 14, 0), 50);
        kg.set_relationship(&npc_sid, RelationshipState::Hostile);

        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing(FacingDirection::North),
            kg,
        ));

        world.insert_resource(registry);

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();
        let npc_entity = snapshot
            .entities
            .iter()
            .find(|e| matches!(e.kind, EntityKind::Npc))
            .expect("NPC should be visible");
        assert_eq!(npc_entity.relationship, RelationshipState::Hostile);
        assert_eq!(npc_entity.observation, EntityVisibility::Visible);
    }

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

        // NPC exists far behind the player (not visible)
        let npc = world
            .spawn((crate::npc::Npc, TilePosition::new(16, 30, 0)))
            .id();
        let npc_sid = registry.register(npc);

        // Player previously saw NPC at (16, 28) — behind the player (south),
        // well beyond peripheral range. The tile is NOT in the player's FOV.
        let mut kg = KnowledgeGraph::new();
        kg.observe_entity(npc_sid, TilePosition::new(16, 28, 0), 50);
        kg.observe_entity_leaving_los(&npc_sid, 60);
        kg.set_relationship(&npc_sid, RelationshipState::PersonOfInterest);

        let player = world
            .spawn((
                PlayerCharacter,
                TilePosition::new(16, 16, 0),
                Facing(FacingDirection::North),
                kg,
            ))
            .id();
        registry.register(player);
        world.insert_resource(registry);
        world.insert_resource(SimulationTime { tick: 100, paused: false });

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();

        // Should have player (visible) + NPC (remembered)
        let remembered: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.observation, EntityVisibility::Remembered { .. }))
            .collect();
        assert_eq!(remembered.len(), 1, "should have one remembered entity");
        assert_eq!(remembered[0].relationship, RelationshipState::PersonOfInterest);

        // Remembered entity at last_known_position (16, 28), not actual (16, 30)
        assert_eq!(remembered[0].x, 16.5);
        assert_eq!(remembered[0].y, 28.5);

        if let EntityVisibility::Remembered { confidence, age_ticks } = &remembered[0].observation {
            assert_eq!(*confidence, KnowledgeConfidence::KnowsDetails);
            assert_eq!(*age_ticks, 50); // tick 100 - last_observed 50
        }
    }

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

        // NPC exists but not in LOS
        let npc = world
            .spawn((crate::npc::Npc, TilePosition::new(16, 10, 0)))
            .id();
        let npc_sid = registry.register(npc);

        // Knowledge still shows Direct (transient inconsistency)
        let mut kg = KnowledgeGraph::new();
        kg.observe_entity(npc_sid, TilePosition::new(16, 14, 0), 50);
        // Still Direct — don't show as ghost

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

        // Wall blocks actual NPC position
        let mut walkability = world.resource_mut::<WalkabilityMap>();
        walkability.set_walkable(&TilePosition::new(16, 12, 0), false);

        world.insert_resource(registry);

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();

        let remembered: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.observation, EntityVisibility::Remembered { .. }))
            .collect();
        assert!(
            remembered.is_empty(),
            "Direct-confidence entities should not appear as remembered ghosts"
        );
    }

    #[test]
    fn remembered_entity_on_visible_tile_not_shown() {
        // If the player can see a tile and the entity isn't there,
        // don't show a ghost — the player knows it moved.
        let mut world = setup_world(32, 32);
        let mut registry = EntityRegistry::new(0);

        let npc = world
            .spawn((crate::npc::Npc, TilePosition::new(30, 30, 0)))
            .id();
        let npc_sid = registry.register(npc);

        // Player remembers NPC at (16, 15) — a tile the player can currently see
        let mut kg = KnowledgeGraph::new();
        kg.observe_entity(npc_sid, TilePosition::new(16, 15, 0), 50);
        kg.observe_entity_leaving_los(&npc_sid, 60);

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

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();

        let remembered: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.observation, EntityVisibility::Remembered { .. }))
            .collect();
        assert!(
            remembered.is_empty(),
            "ghost should not appear on a tile the player can currently see"
        );
    }

    #[test]
    fn remembered_entity_different_z_not_shown() {
        // Remembered entity on a different z-level should not appear
        let mut world = setup_world(32, 32);
        let mut registry = EntityRegistry::new(0);

        let npc = world
            .spawn((crate::npc::Npc, TilePosition::new(5, 5, 1)))
            .id();
        let npc_sid = registry.register(npc);

        // Player remembers NPC at z=1, but player is at z=0
        let mut kg = KnowledgeGraph::new();
        kg.observe_entity(npc_sid, TilePosition::new(5, 5, 1), 50);
        kg.observe_entity_leaving_los(&npc_sid, 60);

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

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();

        let remembered: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.observation, EntityVisibility::Remembered { .. }))
            .collect();
        assert!(
            remembered.is_empty(),
            "remembered entity on different z-level should not appear in snapshot"
        );
    }

    #[test]
    fn knowledge_without_position_not_shown() {
        // Entity known via gossip (no last_known_position) should not appear
        let mut world = setup_world(32, 32);
        let mut registry = EntityRegistry::new(0);

        let npc = world
            .spawn((crate::npc::Npc, TilePosition::new(5, 5, 0)))
            .id();
        let npc_sid = registry.register(npc);

        // Player knows about NPC but has never seen it (no position)
        let mut kg = KnowledgeGraph::new();
        // Insert knowledge manually without a position
        kg.entities.insert(npc_sid, crate::knowledge::EntityKnowledge {
            last_known_position: None,
            last_observed_tick: 0,
            last_updated_tick: 50,
            confidence: KnowledgeConfidence::KnowsOf,
            source: crate::knowledge::KnowledgeSource::Background,
            state: crate::knowledge::KnowledgeState::Active,
            relationship: RelationshipState::PersonOfInterest,
            known_attributes: std::collections::BTreeMap::new(),
        });

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

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();

        let remembered: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.observation, EntityVisibility::Remembered { .. }))
            .collect();
        assert!(
            remembered.is_empty(),
            "entity without last_known_position should not appear as ghost"
        );
    }

    #[test]
    fn multiple_npcs_in_los_all_visible() {
        let mut world = setup_world(32, 32);
        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing(FacingDirection::North),
            KnowledgeGraph::new(),
        ));
        // Three NPCs in front of player, no walls
        world.spawn((crate::npc::Npc, TilePosition::new(16, 14, 0)));
        world.spawn((crate::npc::Npc, TilePosition::new(14, 14, 0)));
        world.spawn((crate::npc::Npc, TilePosition::new(18, 14, 0)));

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();
        // Player + 3 NPCs = 4 entities
        assert_eq!(snapshot.entities.len(), 4);
        let npcs: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.kind, EntityKind::Npc))
            .collect();
        assert_eq!(npcs.len(), 3);
        assert!(npcs.iter().all(|n| n.observation == EntityVisibility::Visible));
    }

    #[test]
    fn npc_behind_wall_excluded_from_multi_entity_snapshot() {
        let mut world = setup_world(32, 32);
        // Wall at (16,14)
        world
            .resource_mut::<WalkabilityMap>()
            .set_walkable(&TilePosition::new(16, 14, 0), false);

        world.spawn((
            PlayerCharacter,
            TilePosition::new(16, 16, 0),
            Facing(FacingDirection::North),
            KnowledgeGraph::new(),
        ));
        // NPC 1: behind wall (should be hidden)
        world.spawn((crate::npc::Npc, TilePosition::new(16, 13, 0)));
        // NPC 2: to the side, no wall (should be visible)
        world.spawn((crate::npc::Npc, TilePosition::new(14, 14, 0)));
        // NPC 3: also visible
        world.spawn((crate::npc::Npc, TilePosition::new(18, 15, 0)));

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

        let buffer = world.resource::<SnapshotBuffer>();
        let snapshot = buffer.snapshot.as_ref().unwrap();
        // Player + 2 visible NPCs = 3 (NPC behind wall excluded)
        let npcs: Vec<_> = snapshot
            .entities
            .iter()
            .filter(|e| matches!(e.kind, EntityKind::Npc))
            .collect();
        assert_eq!(npcs.len(), 2, "NPC behind wall should be excluded");
    }
}