Add entity blocking and a wandering NPC

Occupied tiles now block movement through World::blocked, the one predicate
the sim and the pathfinder share. Entities carry an EntityKind so the sim
knows which ones to think for; Sim::think gives each NPC a random king move
one window in four. Intents execute in id order so contested tiles resolve
deterministically.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

sim/src/entity.rs

@@ -1,15 +1,25 @@
+/// Who decides an entity's actions. The sim only schedules intents for `Npc`s; a
+/// `Player`'s intents come from outside (input) through `Sim::set_action`.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum EntityKind {
+    Player,
+    Npc,
+}
+
 pub struct Entity {
     pub id:      u32,
-    pub type_id: u16,   // 0 = player
+    pub kind:    EntityKind,
+    pub type_id: u16,   // sprite index into the entity tileset
     pub pos:     (i16, i16),
     pub hp:      u16,
     pub hp_max:  u16,
 }
 
 impl Entity {
-    pub fn new(id: u32, type_id: u16, pos: (i16, i16), hp: u16) -> Self {
+    pub fn new(id: u32, kind: EntityKind, type_id: u16, pos: (i16, i16), hp: u16) -> Self {
         Self {
             id,
+            kind,
             type_id,
             pos,
             hp,

sim/src/sim.rs

@@ -1,7 +1,12 @@
 use std::collections::HashMap;
 use crate::TICKS_PER_MOVE;
+use crate::entity::EntityKind;
+use crate::player_action;
 use crate::world::World;
 
+/// How often a wandering NPC takes a step: one window in `WANDER_ODDS`.
+const WANDER_ODDS: u32 = 4;
+
 pub struct Sim {
     pub world: World,
     /// Base tick counter (24 Hz). Movement resolves every `TICKS_PER_MOVE`-th tick.
@@ -10,11 +15,26 @@ pub struct Sim {
     /// it, executing the window consumes it — so an actor moves at most one tile per
     /// window no matter how often it changes its mind in between.
     intents:   HashMap<u32, u16>,
+    /// xorshift state for NPC decisions. Seeded, so a run is reproducible.
+    rng:       u64,
 }
 
 impl Sim {
     pub fn new(world: World) -> Self {
-        Self { world, tick: 0, intents: HashMap::new() }
+        Self::with_seed(world, 0x9E37_79B9_7F4A_7C15)
+    }
+
+    pub fn with_seed(world: World, seed: u64) -> Self {
+        Self { world, tick: 0, intents: HashMap::new(), rng: seed.max(1) }
+    }
+
+    fn rand(&mut self) -> u32 {
+        let mut x = self.rng;
+        x ^= x << 13;
+        x ^= x >> 7;
+        x ^= x << 17;
+        self.rng = x;
+        (x >> 32) as u32
     }
 
     /// Schedule `action` for `entity_id`'s next movement window. NOOP clears the intent.
@@ -35,6 +55,7 @@ impl Sim {
     pub fn step(&mut self) -> bool {
         self.tick = self.tick.wrapping_add(1);
         if self.tick.is_multiple_of(TICKS_PER_MOVE) {
+            self.think();
             self.entity_tick();
             true
         } else {
@@ -42,8 +63,30 @@ impl Sim {
         }
     }
 
+    /// Schedule intents for every NPC. Wandering for now: occasionally a random king
+    /// move, otherwise stand still. Blocked directions are simply refused by
+    /// `entity_tick` — a wanderer bumping a wall just idles that window.
+    fn think(&mut self) {
+        let mut npcs: Vec<u32> = self.world.entities.values()
+            .filter(|e| e.kind == EntityKind::Npc)
+            .map(|e| e.id)
+            .collect();
+        npcs.sort_unstable();
+        for id in npcs {
+            if self.rand() % WANDER_ODDS != 0 {
+                continue;
+            }
+            let action = player_action::NORTH + (self.rand() % 8) as u16;
+            self.intents.insert(id, action);
+        }
+    }
+
     fn entity_tick(&mut self) {
-        for (entity_id, action) in self.intents.drain() {
+        // Lowest id first: deterministic, and the player (spawned first) wins a
+        // contested tile.
+        let mut intents: Vec<(u32, u16)> = self.intents.drain().collect();
+        intents.sort_unstable();
+        for (entity_id, action) in intents {
             let delta = match crate::action_delta(action) {
                 Some(d) => d,
                 None => continue,
@@ -56,10 +99,7 @@ impl Sim {
             let to = (from.0 + delta.0, from.1 + delta.1);
             // `step_allowed` is the shared movement rule (chessboard geometry, no corner
             // cutting) — the pathfinder plans with the exact same function.
-            let allowed = crate::step_allowed(from, to, |x, y| {
-                self.world.tile_flags(x as i16, y as i16).map_or(true, |f| f.collidable())
-            });
-            if allowed {
+            if crate::step_allowed(from, to, |x, y| self.world.blocked(x, y)) {
                 self.world.move_entity(entity_id, (to.0 as i16, to.1 as i16));
             }
         }
@@ -70,7 +110,6 @@ impl Sim {
 mod tests {
     use super::*;
     use crate::chunk::Chunk;
-    use crate::player_action;
 
     /// Open floor (id 1) with a single wall tile at (5, 4) — id 146 is collidable in
     /// the tile vocabulary.
@@ -89,7 +128,7 @@ mod tests {
     #[test]
     fn one_step_per_window_and_replacement() {
         let mut sim = Sim::new(open_world());
-        let id = sim.world.spawn_entity(0, (4, 4), 1);
+        let id = sim.world.spawn_entity(EntityKind::Player, 0, (4, 4), 1);
         sim.set_action(id, player_action::SOUTH);
         sim.set_action(id, player_action::NORTH); // replaces
         sim.step(); // no window yet
@@ -103,12 +142,12 @@ mod tests {
     #[test]
     fn blocked_and_out_of_world_steps_are_rejected() {
         let mut sim = Sim::new(open_world());
-        let id = sim.world.spawn_entity(0, (4, 4), 1);
+        let id = sim.world.spawn_entity(EntityKind::Player, 0, (4, 4), 1);
         sim.set_action(id, player_action::EAST); // into the wall at (5, 4)
         step_window(&mut sim);
         assert_eq!(sim.world.entities[&id].pos, (4, 4));
 
-        let edge = sim.world.spawn_entity(0, (0, 0), 1);
+        let edge = sim.world.spawn_entity(EntityKind::Player, 0, (0, 0), 1);
         sim.set_action(edge, player_action::WEST); // no chunk there
         step_window(&mut sim);
         assert_eq!(sim.world.entities[&edge].pos, (0, 0));
@@ -117,10 +156,52 @@ mod tests {
     #[test]
     fn noop_clears_intent() {
         let mut sim = Sim::new(open_world());
-        let id = sim.world.spawn_entity(0, (4, 4), 1);
+        let id = sim.world.spawn_entity(EntityKind::Player, 0, (4, 4), 1);
         sim.set_action(id, player_action::SOUTH);
         sim.set_action(id, player_action::NOOP);
         step_window(&mut sim);
         assert_eq!(sim.world.entities[&id].pos, (4, 4));
     }
+
+    #[test]
+    fn entities_block_each_other() {
+        let mut sim = Sim::new(open_world());
+        let a = sim.world.spawn_entity(EntityKind::Player, 0, (4, 4), 1);
+        let b = sim.world.spawn_entity(EntityKind::Player, 0, (4, 5), 1);
+        sim.set_action(a, player_action::SOUTH); // onto b
+        step_window(&mut sim);
+        assert_eq!(sim.world.entities[&a].pos, (4, 4));
+
+        // Corner rule applies to entities too: b at (4,5) and a blocker at (5,4) seal
+        // the diagonal (4,4) → (5,5).
+        sim.set_action(a, player_action::SOUTH_EAST);
+        step_window(&mut sim);
+        assert_eq!(sim.world.entities[&a].pos, (4, 4));
+
+        // Contested tile: lower id (a) wins, b is refused in the same window.
+        sim.set_action(a, player_action::WEST);
+        sim.set_action(b, player_action::NORTH_WEST); // both → (3,4)
+        step_window(&mut sim);
+        assert_eq!(sim.world.entities[&a].pos, (3, 4));
+        assert_eq!(sim.world.entities[&b].pos, (4, 5));
+    }
+
+    #[test]
+    fn npc_wanders_but_never_walks_through_walls() {
+        let mut sim = Sim::new(open_world());
+        let npc = sim.world.spawn_entity(EntityKind::Npc, 1, (10, 10), 1);
+        let mut prev = (10, 10);
+        let mut moved = false;
+        for _ in 0..200 {
+            step_window(&mut sim);
+            let pos = sim.world.entities[&npc].pos;
+            let d = ((pos.0 - prev.0).abs()).max((pos.1 - prev.1).abs());
+            assert!(d <= 1, "more than one tile per window: {prev:?} → {pos:?}");
+            assert_ne!(pos, (5, 4), "walked into the wall");
+            assert!(sim.world.tile_flags(pos.0, pos.1).is_some(), "left the world: {pos:?}");
+            moved |= pos != prev;
+            prev = pos;
+        }
+        assert!(moved);
+    }
 }

sim/src/world.rs

@@ -1,6 +1,6 @@
 use std::collections::HashMap;
 use crate::chunk::{Chunk, TileFlags};
-use crate::entity::Entity;
+use crate::entity::{Entity, EntityKind};
 
 pub fn tile_to_chunk(tx: i16, ty: i16) -> (i16, i16) {
     (tx >> 5, ty >> 5)
@@ -52,14 +52,30 @@ impl World {
         chunk.set_tile(lx, ly, id);
     }
 
-    pub fn spawn_entity(&mut self, type_id: u16, pos: (i16, i16), hp: u16) -> u32 {
+    pub fn spawn_entity(&mut self, kind: EntityKind, type_id: u16, pos: (i16, i16), hp: u16) -> u32 {
         let id = self.next_entity_id;
         self.next_entity_id += 1;
-        let entity = Entity::new(id, type_id, pos, hp);
+        let entity = Entity::new(id, kind, type_id, pos, hp);
         self.add_entity(entity);
         id
     }
 
+    /// True if any entity stands on `(wx, wy)`. Goes through the chunk index, so it is
+    /// cheap enough to call per pathfinder expansion.
+    pub fn occupied(&self, wx: i16, wy: i16) -> bool {
+        let (cx, cy) = tile_to_chunk(wx, wy);
+        self.entities_in_chunk(cx, cy).iter()
+            .any(|id| self.entities[id].pos == (wx, wy))
+    }
+
+    /// The movement blocker predicate: a tile blocks if it is solid, outside the loaded
+    /// world, or occupied by an entity. Both the sim and the pathfinder plan with this.
+    pub fn blocked(&self, wx: i32, wy: i32) -> bool {
+        let Ok(wx) = i16::try_from(wx) else { return true };
+        let Ok(wy) = i16::try_from(wy) else { return true };
+        self.tile_flags(wx, wy).map_or(true, |f| f.collidable()) || self.occupied(wx, wy)
+    }
+
     pub fn add_entity(&mut self, entity: Entity) {
         let chunk_key = tile_to_chunk(entity.pos.0, entity.pos.1);
         let id = entity.id;