Natural spawning: test a position's placement before its spawn list

Every position a spawn group tries away from where its entry was picked
asked mobsAt again (structures reaching it, the biome, a string-keyed
list) before isSpawnPositionOk, which turns most positions away with a
few block reads. Both only read the level, so the placement now goes
first. isSpawnPositionOk also looked its placement up by an id built with
format! after a linear search for the species' definition: MobData keeps
a table by species.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-client/src/game/perspective.rs

@@ -1192,6 +1192,7 @@ mod preview_tests {
             entity_tags: Default::default(),
             villager_hats: Default::default(),
             pois: Default::default(),
+            species_placements: Default::default(),
         };
         let draw = |body: bool| {
             let mut out = Vec::new();

crates/horizon-client/src/mobs/data.rs

@@ -33,7 +33,7 @@ pub struct StateData {
     /// Collision shape top (0 when the shape is empty).
     pub top: f32,
 }
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Placement {
     OnGround,
     InWater,
@@ -55,6 +55,9 @@ pub struct MobData {
     /// The villager POI type of every block state (`PoiTypes.forState`),
     /// worked out the first time a villager looks for one.
     pub pois: std::sync::OnceLock<Vec<Option<super::villager_brain::Poi>>>,
+    /// [`MobData::placement`] of each species' entity type (by `Species as
+    /// usize`), worked out the first time a spawn asks.
+    pub species_placements: std::sync::OnceLock<Vec<Placement>>,
 }
 impl MobData {
     pub fn load(root: &Path, facts: &BlockFacts) -> Self {
@@ -145,6 +148,7 @@ impl MobData {
             biome_tags: HashMap::new(),
             entity_tags: HashMap::new(),
             pois: Default::default(),
+            species_placements: Default::default(),
         }
     }
     /// Without the export, approximate path types from the gameplay facts.
@@ -201,6 +205,7 @@ impl MobData {
             biome_tags: HashMap::new(),
             entity_tags: HashMap::new(),
             pois: Default::default(),
+            species_placements: Default::default(),
         }
     }
     pub fn state(&self, id: horizon_world::registry::BlockStateId) -> StateData {
@@ -222,7 +227,28 @@ impl MobData {
     pub fn placement(&self, entity: &str) -> Placement {
         self.placements
             .get(entity)
-            .cloned()
+            .copied()
             .unwrap_or(Placement::OnGround)
     }
+    /// [`MobData::placement`] of the species' entity type, without building
+    /// its id: natural spawning asks it for every position it tries.
+    pub fn species_placement(&self, s: super::Species) -> Placement {
+        let table = self.species_placements.get_or_init(|| {
+            let defs = super::species::ALL;
+            let len = defs
+                .iter()
+                .map(|d| d.species as usize + 1)
+                .max()
+                .unwrap_or(0);
+            let mut table = vec![Placement::OnGround; len];
+            // `species::def` takes a species' first definition: so does this.
+            for d in defs.iter().rev() {
+                table[d.species as usize] = self.placement(&format!("minecraft:{}", d.name));
+            }
+            table
+        });
+        table.get(s as usize).copied().unwrap_or_else(|| {
+            self.placement(&format!("minecraft:{}", super::species::def(s).name))
+        })
+    }
 }

crates/horizon-client/src/mobs/render.rs

@@ -1887,6 +1887,7 @@ mod tests {
             entity_tags: Default::default(),
             villager_hats: Default::default(),
             pois: Default::default(),
+            species_placements: Default::default(),
         };
         let atlas = |name: &str| {
             Some(if name.contains("_inner") {
@@ -1975,6 +1976,7 @@ mod tests {
             entity_tags: Default::default(),
             villager_hats: Default::default(),
             pois: Default::default(),
+            species_placements: Default::default(),
         };
         let atlas = |name: &str| {
             Some(if name.contains("copper_golem_eyes") {
@@ -2027,6 +2029,7 @@ mod tests {
             entity_tags: Default::default(),
             villager_hats: Default::default(),
             pois: Default::default(),
+            species_placements: Default::default(),
         };
         for left in [false, true] {
             let t = held_item_transform(&mob, 0.5, glam::DVec3::ZERO, &data, left).unwrap();
@@ -2112,6 +2115,7 @@ mod tests {
             entity_tags: Default::default(),
             villager_hats: Default::default(),
             pois: Default::default(),
+            species_placements: Default::default(),
         };
         let atlas = |_: &str| Some([0.0, 0.0, 0.25, 0.25]);
         let part = &kind.model;

crates/horizon-client/src/mobs/spawn_rules.rs

@@ -244,8 +244,7 @@ pub fn is_valid_empty_spawn<L: SpawnLevel>(s: Species, l: &L, x: i32, y: i32, z:
 }
 /// `SpawnPlacements.isSpawnPositionOk`.
 pub fn is_spawn_position_ok<L: SpawnLevel>(s: Species, l: &L, x: i32, y: i32, z: i32) -> bool {
-    let id = format!("minecraft:{}", super::species::def(s).name);
-    match l.mob_data().placement(&id) {
+    match l.mob_data().species_placement(s) {
         data::Placement::OnGround => {
             is_valid_spawn(s, l, x, y - 1, z)
                 && is_valid_empty_spawn(s, l, x, y, z)

crates/horizon-client/src/mobs/spawn_selection.rs

@@ -1209,6 +1209,13 @@ fn valid_position_for_type<L: NaturalLevel, R: RandomSource>(
         return false;
     }
     let [x, y, z] = pos;
+    // `canSpawnMobAt` comes first in vanilla, but it and `isSpawnPositionOk`
+    // only read the level, so their order cannot change the answer: the few
+    // block reads of the placement turn most positions away before the spawn
+    // list (structures, biome) is looked up again.
+    if !spawn_rules::is_spawn_position_ok(species, l, x, y, z) {
+        return false;
+    }
     if !listed
         && !mobs_at(l, category, x, y, z)
             .iter()
@@ -1216,9 +1223,6 @@ fn valid_position_for_type<L: NaturalLevel, R: RandomSource>(
     {
         return false;
     }
-    if !spawn_rules::is_spawn_position_ok(species, l, x, y, z) {
-        return false;
-    }
     if !spawn_rules::check_spawn_rules(species, l, Reason::Natural, random, x, y, z) {
         return false;
     }