Add Brain enum with hunting foxes; move the A* into the sim

NPC strategy is now a Brain field on the entity (Idle, Wander, Hunt(Prey))
that Sim::think matches on, so behavior is data independent of species.
Hunt follows the first step of an A* route to the nearest prey in sight,
replanned every window; the pathfinder moves from game/ to sim/ for that.
A greedy step-toward-target was tried first and oscillated at walls. Bumps
between the same species no longer fight, and six foxes join the hens.

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

game/src/game.rs

@@ -1,11 +1,10 @@
-mod pathfind;
 mod pixelhelper;
 
 use std::collections::{HashMap, VecDeque};
 
 use crate::assets::{Image, Tile};
 use crate::input::{GameAction, InputState};
-use sim::entity::{Entity, EntityKind};
+use sim::entity::{Brain, Entity, EntityKind, Prey};
 use sim::map::TileMap;
 use sim::{delta_action, load_world, step_allowed, Event, Sim, TICK_HZ, TICKS_PER_MOVE};
 
@@ -44,6 +43,18 @@ const CAM_SMOOTH_TAU_MS: f32 = 120.0;
 /// A position correction farther than this is a teleport — snap instead of easing.
 const CAM_SNAP_PX: f32 = 96.0;
 
+/// Animals scattered over the map at start, one per free tile picked at random.
+const HEN_COUNT: usize = 150;
+const FOX_COUNT: usize = 6;
+
+/// Sprite indices in `entities.png`.
+const SPRITE_PLAYER: u16 = 0;
+const SPRITE_HEN:    u16 = 1;
+const SPRITE_FOX:    u16 = 2;
+
+/// Interval of the population report on stdout.
+const REPORT_MS: usize = 5000;
+
 /// Render-side smoothing state for one entity: the previous tile and how long ago the
 /// current one was taken. Positions are tiles; rendering lerps between them in pixels
 /// over one movement interval.
@@ -74,6 +85,10 @@ pub struct Game {
     /// Per-entity interpolation state, keyed by entity id. Purely cosmetic — all game
     /// logic keeps using the sim's tile positions.
     lerp:           HashMap<u32, EntityLerp>,
+    /// Running totals for the population report.
+    attacks:        u32,
+    deaths:         u32,
+    report_ms:      usize,
 }
 
 impl Game {
@@ -87,8 +102,25 @@ impl Game {
         println!("loaded {map_path}: {}×{} tiles in {:.1} ms",
                  map.width, map.height, t0.elapsed().as_secs_f64() * 1e3);
         let mut sim = Sim::new(world);
-        let player_id = sim.world.spawn_entity(EntityKind::Player, 0, (0, 0), 100, 3);
-        sim.world.spawn_entity(EntityKind::Npc, 1, (6, 6), 5, 1);
+        let player_id = sim.world.spawn_entity(
+            EntityKind::Player, SPRITE_PLAYER, Brain::Idle, (0, 0), 100, 3);
+
+        // Scatter animals over free tiles. A cheap LCG is plenty for placement.
+        let mut seed: u32 = 12345;
+        let mut scatter = |sim: &mut Sim, count: usize, sprite: u16, brain: Brain, hp: u16, attack: u16| {
+            let mut placed = 0;
+            while placed < count {
+                seed = seed.wrapping_mul(1_664_525).wrapping_add(1_013_904_223);
+                let x = ((seed >> 8) % map.width as u32) as i32;
+                let y = ((seed >> 20) % map.height as u32) as i32;
+                if !sim.world.blocked(x, y) {
+                    sim.world.spawn_entity(EntityKind::Npc, sprite, brain, (x as i16, y as i16), hp, attack);
+                    placed += 1;
+                }
+            }
+        };
+        scatter(&mut sim, HEN_COUNT, SPRITE_HEN, Brain::Wander, 5, 1);
+        scatter(&mut sim, FOX_COUNT, SPRITE_FOX, Brain::Hunt(Prey::Kind(SPRITE_HEN)), 8, 2);
 
         let mut game = Game {
             tileset,
@@ -100,6 +132,9 @@ impl Game {
             goal:          None,
             cam:           (0.0, 0.0),
             lerp:          HashMap::new(),
+            attacks:       0,
+            deaths:        0,
+            report_ms:     0,
         };
         game.cam = game.cam_target();
         game.track_lerp();
@@ -144,21 +179,41 @@ impl Game {
         self.step_lerp(dt);
         self.step_camera(dt);
         self.render_viewport(render_frame);
+
+        self.report_ms += dt;
+        if self.report_ms >= REPORT_MS {
+            self.report_ms = 0;
+            self.print_report();
+        }
         None
     }
 
+    fn print_report(&self) {
+        let count = |sprite| self.sim.world.entities.values().filter(|e| e.type_id == sprite).count();
+        println!("hens: {:3}  foxes: {:2}  attacks: {:4}  deaths: {:3}",
+                 count(SPRITE_HEN), count(SPRITE_FOX), self.attacks, self.deaths);
+    }
+
     /// React to what the sim reported for the last window. Returns `true` once the
     /// player is dead — the one event the game cannot continue past.
     fn handle_events(&mut self) -> bool {
         let mut dead = false;
+        // Only the player's own fights are logged; the rest is tallied for the report.
         for ev in self.sim.take_events() {
             match ev {
                 Event::Attacked { by, target, damage } => {
-                    println!("#{by} hits #{target} for {damage}");
+                    self.attacks += 1;
+                    if by == self.player_id || target == self.player_id {
+                        println!("#{by} hits #{target} for {damage}");
+                    }
                 }
                 Event::Died { id, at } => {
-                    println!("#{id} dies at {at:?}");
-                    dead |= id == self.player_id;
+                    self.deaths += 1;
+                    if id == self.player_id {
+                        dead = true;
+                    } else {
+                        println!("#{id} dies at {at:?}");
+                    }
                 }
             }
         }
@@ -316,7 +371,7 @@ impl Game {
         if self.sim.world.solid(goal.0, goal.1) {
             return None;
         }
-        pathfind::find_path(self.player_pos(), goal, |x, y| {
+        sim::pathfind::find_path(self.player_pos(), goal, |x, y| {
             (x, y) != goal && self.sim.world.blocked(x, y)
         })
     }
@@ -433,6 +488,23 @@ impl Game {
 
 #[cfg(test)]
 mod snapshot {
+    /// Runs the world headlessly for `$SECONDS` (default 60) of sim time and prints the
+    /// population report — to watch the hens without a window:
+    /// `cargo test -p game population -- --ignored --nocapture`
+    #[test]
+    #[ignore]
+    fn population() {
+        std::env::set_current_dir("..").unwrap();
+        let secs: usize = std::env::var("SECONDS").ok().and_then(|s| s.parse().ok()).unwrap_or(60);
+        let mut g = super::Game::start("assets/map_test");
+        let input = crate::input::InputState::new();
+        let mut frame = vec![0u16; 320 * 240];
+        for _ in 0..secs * 60 {
+            g.update(&mut frame, 16, &input);
+        }
+        g.print_report();
+    }
+
     /// Renders the first frame headlessly to the PNG named by `$SNAPSHOT` — a way to
     /// eyeball the renderer without a window. `$SNAPSHOT_MAP` picks the map:
     /// `SNAPSHOT=/tmp/frame.png cargo test -p game snapshot -- --ignored`

notes/roadmap.md

@@ -47,7 +47,7 @@ crate's `tick`/`set_action` interface is the seam a server could be wrapped arou
       (trees/rocks) and id 0 (empty / the invisible world border). A proper tile-data file
       format replaces this table later.
 - [x] Click-to-move: framebuffer click → world tile, A* over the world
-      (`game/src/game/pathfind.rs`, 8-connected, Chebyshev heuristic), route fed into the
+      (`sim/src/pathfind.rs`, 8-connected, Chebyshev heuristic), route fed into the
       sim one step per movement window. Keyboard input cancels the route; each step is
       re-validated at scheduling time and a blocked step replans toward the persistent
       goal. Click-and-hold steers continuously: while the button is held the route keeps
@@ -74,7 +74,17 @@ crate's `tick`/`set_action` interface is the seam a server could be wrapped arou
       drained by the game with `Sim::take_events` after every window — which is the
       seam for sound, message log, camera shake, spawns. Player death quits for now.
       Click-to-move treats an occupied goal as a target (route ends in a bump) but
-      walks around entities in the way; keyboard steps always bump.
+      walks around entities in the way; keyboard steps always bump. A bump into the
+      same `type_id` (species) is a refused move, not a fight.
+- [x] NPC brains: `Brain { Idle, Wander, Hunt(Prey) }` as a field on `Entity` — strategy
+      is data, independent of species and swappable at runtime. `Sim::think` matches on
+      it. `Hunt` takes the first step of an A* route (the pathfinder moved into `sim/`
+      for this) to the nearest prey within `SIGHT_RANGE` (8), replanned every window;
+      out of sight or unreachable it wanders. A greedy step-toward-target was tried
+      first and oscillated at any wall between hunter and prey. Six foxes
+      (`Hunt(Kind(hen))`, 8 hp / 2 dmg) clear 150 wandering hens off `map_test` in
+      about a minute; `cargo test -p game population -- --ignored --nocapture` watches
+      it headlessly.
 - [x] Entity interpolation: per-id lerp table (`EntityLerp` in `game.rs`) — previous/current
       tile plus a clock, rendered as a pixel lerp over one movement interval. Jumps of
       more than one tile (Chebyshev) snap. Purely cosmetic; game logic keeps using the
@@ -117,6 +127,12 @@ budget model goes once there are enough NPCs to need it. The action-point model
 
 - [x] `EntityKind::Npc`, `Sim::think` scheduling intents through the same intent table
 - [x] Bump-to-attack, death, `Event` queue
+- [x] `Brain` enum, `Hunt` via A*
+- A* per hunter per window is fine for six foxes; the think budget below becomes
+  necessary once hunters number in the dozens (or `hunt` caches its route and replans
+  only when the target moved).
+- Hunters don't defend themselves: a fox bitten by a hen does not retaliate unless
+  the hen happens to be its nearest prey. `Brain::Hunt` could switch target on `Attacked`.
 - Stats beyond `hp`/`attack` (defense, speed) as plain fields; a `Stats` substruct once
   they crowd `Entity`. No ECS until entities actually differ in *which* components they
   carry — see the discussion of 2026-09-19.

sim/src/entity.rs

@@ -6,10 +6,31 @@ pub enum EntityKind {
     Npc,
 }
 
+/// What a hunter goes after.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum Prey {
+    Player,
+    Kind(u16),
+}
+
+/// The decision strategy `Sim::think` runs for an NPC. Data, not code: it can be
+/// swapped at runtime (a hit hen may start fleeing) and serializes with the entity.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum Brain {
+    /// Never schedules anything — the player, furniture.
+    Idle,
+    /// Occasionally a random king move.
+    Wander,
+    /// Step toward the nearest prey in sight, bumping into it on contact. Wanders
+    /// when nothing is in sight.
+    Hunt(Prey),
+}
+
 pub struct Entity {
     pub id:      u32,
     pub kind:    EntityKind,
-    pub type_id: u16,   // sprite index into the entity tileset
+    pub type_id: u16,   // sprite index into the entity tileset; also the species
+    pub brain:   Brain,
     pub pos:     (i16, i16),
     pub hp:      u16,
     pub hp_max:  u16,
@@ -18,11 +39,12 @@ pub struct Entity {
 }
 
 impl Entity {
-    pub fn new(id: u32, kind: EntityKind, type_id: u16, pos: (i16, i16), hp: u16, attack: u16) -> Self {
+    pub fn new(id: u32, kind: EntityKind, type_id: u16, brain: Brain, pos: (i16, i16), hp: u16, attack: u16) -> Self {
         Self {
             id,
             kind,
             type_id,
+            brain,
             pos,
             hp,
             hp_max: hp,

sim/src/lib.rs

@@ -4,6 +4,7 @@
 pub mod chunk;
 pub mod entity;
 pub mod map;
+pub mod pathfind;
 pub mod world;
 mod sim;
 

sim/src/pathfind.rs

@@ -1,10 +1,10 @@
-//! Grid A* over the client's chunk cache. The client only *plans* here — every step
-//! still goes to the server as a cardinal action, and the server stays authoritative.
+//! Grid A* over the world. Planning only: a route is a suggestion, every step of it
+//! is re-validated by `entity_tick` when it is actually taken.
 
 use std::cmp::Reverse;
 use std::collections::{BinaryHeap, HashMap};
 
-/// Hard cap on expanded nodes, so a click on an unreachable tile can't stall the frame.
+/// Hard cap on expanded nodes, so a search for an unreachable tile can't stall the tick.
 const MAX_EXPANSIONS: usize = 4096;
 
 /// Longest route we will plan. Keeps the reply small and bounds replan cost.
@@ -22,7 +22,7 @@ const DIAGONAL_COST: i32 = 101;
 /// every step is one game turn, so routes are minimal in step count; the tiny diagonal
 /// surcharge (see `DIAGONAL_COST`) only breaks ties among equally short paths. Each
 /// expansion goes through the shared `step_allowed` rule — so the plan can never contain
-/// a step the server would refuse (including corner cutting). Returns the tiles to walk,
+/// a step the sim would refuse (including corner cutting). Returns the tiles to walk,
 /// excluding `start`, ending on `goal`, or `None` if the goal is unreachable within the
 /// search budget.
 pub fn find_path(
@@ -75,7 +75,7 @@ pub fn find_path(
             (1, -1), (1, 1), (-1, 1), (-1, -1),
         ] {
             let next = (cur.0 + dx, cur.1 + dy);
-            if !sim::step_allowed(cur, next, &blocked) {
+            if !crate::step_allowed(cur, next, &blocked) {
                 continue;
             }
             let ng = g + if dx != 0 && dy != 0 { DIAGONAL_COST } else { CARDINAL_COST };
@@ -96,7 +96,7 @@ mod tests {
     fn assert_valid(start: (i32, i32), path: &[(i32, i32)], blocked: impl Fn(i32, i32) -> bool) {
         let mut from = start;
         for &to in path {
-            assert!(sim::step_allowed(from, to, &blocked), "illegal step {from:?} → {to:?}");
+            assert!(crate::step_allowed(from, to, &blocked), "illegal step {from:?} → {to:?}");
             from = to;
         }
     }

sim/src/sim.rs

@@ -1,12 +1,15 @@
 use std::collections::HashMap;
 use crate::TICKS_PER_MOVE;
-use crate::entity::EntityKind;
+use crate::entity::{Brain, EntityKind, Prey};
 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;
 
+/// How far (Chebyshev) a hunter notices prey.
+const SIGHT_RANGE: i32 = 8;
+
 /// Something that happened during a tick and that the outside (rendering, sound,
 /// message log, spawning) may want to react to. The sim itself never reads these
 /// back — they are an output channel, drained with `Sim::take_events`.
@@ -81,9 +84,9 @@ 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.
+    /// Schedule intents for every NPC according to its `Brain`. 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)
@@ -91,14 +94,51 @@ impl Sim {
             .collect();
         npcs.sort_unstable();
         for id in npcs {
-            if self.rand() % WANDER_ODDS != 0 {
-                continue;
+            let action = match self.world.entities[&id].brain {
+                Brain::Idle       => None,
+                Brain::Wander     => self.wander(),
+                Brain::Hunt(prey) => self.hunt(id, prey).or_else(|| self.wander()),
+            };
+            if let Some(action) = action {
+                self.intents.insert(id, action);
             }
-            let action = player_action::NORTH + (self.rand() % 8) as u16;
-            self.intents.insert(id, action);
         }
     }
 
+    /// One window in `WANDER_ODDS`: a random king move.
+    fn wander(&mut self) -> Option<u16> {
+        if self.rand() % WANDER_ODDS != 0 {
+            return None;
+        }
+        Some(player_action::NORTH + (self.rand() % 8) as u16)
+    }
+
+    /// Pursuit: the first step of a route to the nearest prey within `SIGHT_RANGE`.
+    /// Stepping onto the prey is the bump that attacks it. Replanned every window, so
+    /// a moving target and other entities in the way are handled by construction;
+    /// unreachable prey (walled off, search budget exceeded) is ignored.
+    fn hunt(&mut self, id: u32, prey: Prey) -> Option<u16> {
+        let me = &self.world.entities[&id];
+        let from = (me.pos.0 as i32, me.pos.1 as i32);
+        let dist = |p: (i16, i16)| (p.0 as i32 - from.0).abs().max((p.1 as i32 - from.1).abs());
+
+        let (_, _, goal) = self.world.entities.values()
+            .filter(|e| e.id != id && match prey {
+                Prey::Player  => e.kind == EntityKind::Player,
+                Prey::Kind(t) => e.type_id == t,
+            })
+            .map(|e| (dist(e.pos), e.id, (e.pos.0 as i32, e.pos.1 as i32)))
+            .filter(|&(d, ..)| d <= SIGHT_RANGE)
+            .min()?;
+
+        let w = &self.world;
+        let route = crate::pathfind::find_path(from, goal, |x, y| {
+            (x, y) != goal && w.blocked(x, y)
+        })?;
+        let step = route[0];
+        Some(crate::delta_action(step.0 - from.0, step.1 - from.1))
+    }
+
     fn entity_tick(&mut self) {
         // Lowest id first: deterministic, and the player (spawned first) wins a
         // contested tile.
@@ -121,7 +161,10 @@ impl Sim {
                 continue;
             }
             match self.world.entity_at(to.0, to.1) {
-                None         => self.world.move_entity(entity_id, (to.0 as i16, to.1 as i16)),
+                None => self.world.move_entity(entity_id, (to.0 as i16, to.1 as i16)),
+                // Bumping into one's own species is a refused move, not a fight.
+                Some(target) if self.world.entities[&target].type_id
+                             == self.world.entities[&entity_id].type_id => {}
                 Some(target) => self.attack(entity_id, target),
             }
         }
@@ -162,7 +205,7 @@ mod tests {
     #[test]
     fn one_step_per_window_and_replacement() {
         let mut sim = Sim::new(open_world());
-        let id = sim.world.spawn_entity(EntityKind::Player, 0, (4, 4), 1, 1);
+        let id = sim.world.spawn_entity(EntityKind::Player, 0, Brain::Idle, (4, 4), 1, 1);
         sim.set_action(id, player_action::SOUTH);
         sim.set_action(id, player_action::NORTH); // replaces
         sim.step(); // no window yet
@@ -176,12 +219,12 @@ 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(EntityKind::Player, 0, (4, 4), 1, 1);
+        let id = sim.world.spawn_entity(EntityKind::Player, 0, Brain::Idle, (4, 4), 1, 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(EntityKind::Player, 0, (0, 0), 1, 1);
+        let edge = sim.world.spawn_entity(EntityKind::Player, 0, Brain::Idle, (0, 0), 1, 1);
         sim.set_action(edge, player_action::WEST); // no chunk there
         step_window(&mut sim);
         assert_eq!(sim.world.entities[&edge].pos, (0, 0));
@@ -190,7 +233,7 @@ mod tests {
     #[test]
     fn noop_clears_intent() {
         let mut sim = Sim::new(open_world());
-        let id = sim.world.spawn_entity(EntityKind::Player, 0, (4, 4), 1, 1);
+        let id = sim.world.spawn_entity(EntityKind::Player, 0, Brain::Idle, (4, 4), 1, 1);
         sim.set_action(id, player_action::SOUTH);
         sim.set_action(id, player_action::NOOP);
         step_window(&mut sim);
@@ -200,8 +243,8 @@ mod tests {
     #[test]
     fn entities_block_each_other() {
         let mut sim = Sim::new(open_world());
-        let a = sim.world.spawn_entity(EntityKind::Player, 0, (4, 4), 5, 0);
-        let b = sim.world.spawn_entity(EntityKind::Player, 0, (4, 5), 5, 0);
+        let a = sim.world.spawn_entity(EntityKind::Player, 0, Brain::Idle, (4, 4), 5, 0);
+        let b = sim.world.spawn_entity(EntityKind::Player, 0, Brain::Idle, (4, 5), 5, 0);
         sim.set_action(a, player_action::SOUTH); // onto b: a bump, not a move
         step_window(&mut sim);
         assert_eq!(sim.world.entities[&a].pos, (4, 4));
@@ -222,7 +265,7 @@ mod tests {
     #[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, 1);
+        let npc = sim.world.spawn_entity(EntityKind::Npc, 1, Brain::Wander, (10, 10), 1, 1);
         let mut prev = (10, 10);
         let mut moved = false;
         for _ in 0..200 {
@@ -241,8 +284,8 @@ mod tests {
     #[test]
     fn bump_attacks_and_kills() {
         let mut sim = Sim::new(open_world());
-        let p = sim.world.spawn_entity(EntityKind::Player, 0, (4, 4), 10, 3);
-        let n = sim.world.spawn_entity(EntityKind::Player, 1, (4, 5), 5, 0);
+        let p = sim.world.spawn_entity(EntityKind::Player, 0, Brain::Idle, (4, 4), 10, 3);
+        let n = sim.world.spawn_entity(EntityKind::Player, 1, Brain::Idle, (4, 5), 5, 0);
 
         sim.set_action(p, player_action::SOUTH);
         step_window(&mut sim);
@@ -271,11 +314,70 @@ mod tests {
         let mut sim = Sim::new(open_world());
         // Wall at (5,4), n at (4,5): the diagonal (4,4) → (5,5) is sealed on one side
         // by terrain — by the corner rule, a bump there is as illegal as a move.
-        let p = sim.world.spawn_entity(EntityKind::Player, 0, (4, 4), 10, 3);
-        let n = sim.world.spawn_entity(EntityKind::Player, 1, (5, 5), 5, 0);
+        let p = sim.world.spawn_entity(EntityKind::Player, 0, Brain::Idle, (4, 4), 10, 3);
+        let n = sim.world.spawn_entity(EntityKind::Player, 1, Brain::Idle, (5, 5), 5, 0);
         sim.set_action(p, player_action::SOUTH_EAST);
         step_window(&mut sim);
         assert_eq!(sim.world.entities[&n].hp, 5);
         assert!(sim.take_events().is_empty());
     }
+
+    #[test]
+    fn same_species_bump_is_not_a_fight() {
+        let mut sim = Sim::new(open_world());
+        let a = sim.world.spawn_entity(EntityKind::Player, 1, Brain::Idle, (4, 4), 5, 3);
+        let b = sim.world.spawn_entity(EntityKind::Player, 1, Brain::Idle, (4, 5), 5, 3);
+        sim.set_action(a, player_action::SOUTH);
+        step_window(&mut sim);
+        assert_eq!(sim.world.entities[&b].hp, 5);
+        assert_eq!(sim.world.entities[&a].pos, (4, 4));
+        assert!(sim.take_events().is_empty());
+    }
+
+    #[test]
+    fn hunter_closes_in_and_bites() {
+        let mut sim = Sim::new(open_world());
+        let hen = sim.world.spawn_entity(EntityKind::Player, 1, Brain::Idle, (10, 10), 5, 1);
+        let fox = sim.world.spawn_entity(EntityKind::Npc, 2, Brain::Hunt(Prey::Kind(1)), (14, 7), 8, 2);
+        // Chebyshev distance 4: three steps to get adjacent, the fourth is the bite.
+        for _ in 0..3 { step_window(&mut sim); }
+        let p = sim.world.entities[&fox].pos;
+        assert_eq!((p.0 - 10).abs().max((p.1 - 10).abs()), 1, "{p:?}");
+        assert_eq!(sim.world.entities[&hen].hp, 5);
+        step_window(&mut sim);
+        assert_eq!(sim.world.entities[&hen].hp, 3);
+        assert_eq!(sim.take_events(), vec![Event::Attacked { by: fox, target: hen, damage: 2 }]);
+    }
+
+    #[test]
+    fn hunter_sidesteps_an_obstacle() {
+        let mut sim = Sim::new(open_world());
+        // Wall at (5,4); fox at (6,4) wants (3,4) straight west. The diagonals past
+        // the wall are illegal (corner rule), so it slides sideways, walks along the
+        // wall and bites diagonally once it is past.
+        let hen = sim.world.spawn_entity(EntityKind::Player, 1, Brain::Idle, (3, 4), 5, 1);
+        let fox = sim.world.spawn_entity(EntityKind::Npc, 2, Brain::Hunt(Prey::Kind(1)), (6, 4), 8, 2);
+        step_window(&mut sim);
+        let p = sim.world.entities[&fox].pos;
+        assert!(p == (6, 3) || p == (6, 5), "{p:?}");
+        step_window(&mut sim);
+        step_window(&mut sim);
+        let p = sim.world.entities[&fox].pos;
+        assert!(p == (4, 3) || p == (4, 5), "{p:?}");
+        step_window(&mut sim);
+        assert_eq!(sim.world.entities[&hen].hp, 3);
+    }
+
+    #[test]
+    fn hunter_ignores_prey_out_of_sight() {
+        let mut sim = Sim::with_seed(open_world(), 7);
+        let _hen = sim.world.spawn_entity(EntityKind::Player, 1, Brain::Idle, (2, 2), 5, 1);
+        let fox = sim.world.spawn_entity(EntityKind::Npc, 2, Brain::Hunt(Prey::Kind(1)), (25, 25), 8, 2);
+        // Out of sight it wanders: over a few windows it must not walk a straight line
+        // to the hen. Weak check, but it pins down that hunting is range-limited.
+        let start = sim.world.entities[&fox].pos;
+        for _ in 0..8 { step_window(&mut sim); }
+        let p = sim.world.entities[&fox].pos;
+        assert!((p.0 - start.0).abs().max((p.1 - start.1).abs()) < 8, "{p:?}");
+    }
 }

sim/src/world.rs

@@ -1,6 +1,6 @@
 use std::collections::HashMap;
 use crate::chunk::{Chunk, TileFlags};
-use crate::entity::{Entity, EntityKind};
+use crate::entity::{Brain, Entity, EntityKind};
 
 pub fn tile_to_chunk(tx: i16, ty: i16) -> (i16, i16) {
     (tx >> 5, ty >> 5)
@@ -52,10 +52,10 @@ impl World {
         chunk.set_tile(lx, ly, id);
     }
 
-    pub fn spawn_entity(&mut self, kind: EntityKind, type_id: u16, pos: (i16, i16), hp: u16, attack: u16) -> u32 {
+    pub fn spawn_entity(&mut self, kind: EntityKind, type_id: u16, brain: Brain, pos: (i16, i16), hp: u16, attack: u16) -> u32 {
         let id = self.next_entity_id;
         self.next_entity_id += 1;
-        let entity = Entity::new(id, kind, type_id, pos, hp, attack);
+        let entity = Entity::new(id, kind, type_id, brain, pos, hp, attack);
         self.add_entity(entity);
         id
     }