perf: SmallVec inline storage, TOML drop tables, seeded deterministic RNG

- SmallVec<[DropEntry; 2]> replaces Vec in DropTable: zero heap allocation for
  all current tiles (0 or 1 entries), spills to heap only at 3+
- DropEntry fields packed to u8 (chance_pct, min_count, max_count): ~12 bytes ->
  4 bytes; derives Copy so no extra clones
- Replaced SystemTime pseudo-RNG with dig_rng(pos): PCG-style hash of world SEED
  + tile position. Deterministic per world, same dig = same roll every time
- TOML-driven drop tables: assets/drop_tables.toml (grass=5%/1-2, rock=10%/1,
  dirt/air=none). TOML parsed at startup into DropTableRegistry resource
- OnceLock global map for async terrain generation tasks to access drop tables
  without Bevy resource borrowing
- terrain.rs: DropTableRegistry::global_get("tile") replaces hardcoded drop_table_for

Cargo.lock

@@ -2444,6 +2444,7 @@ dependencies = [
  "rayon",
  "rustc-hash 2.1.1",
  "serde",
+ "smallvec",
  "toml",
 ]
 

Cargo.toml

@@ -14,6 +14,7 @@ serde = { version = "1.0", features = ["derive"] }
 toml = "0.9.8"
 rayon = "1.11.0"
 rustc-hash = "2.1.1"
+smallvec = { version = "1", features = ["union"] }
 ahash = "0.8.12"
 nohash-hasher = "0.2.0"
 futures-lite = "2.6.1"

assets/drop_tables.toml

@@ -0,0 +1,15 @@
+[grass]
+drops = [
+    { prefab = "Coin", chance_pct = 5, min_count = 1, max_count = 2 },
+]
+
+[dirt]
+drops = []
+
+[rock]
+drops = [
+    { prefab = "Coin", chance_pct = 10, min_count = 1, max_max = 1 },
+]
+
+[air]
+drops = []

src/entities/item/drop_table.rs

@@ -1,46 +1,50 @@
+use crate::constants::SEED;
 use crate::entities::item::prefabs::misc::misc_prefabs::MiscPrefab;
 use bevy::prelude::*;
+use serde::Deserialize;
+use smallvec::SmallVec;
+use std::collections::HashMap;
+use std::sync::OnceLock;
 
-#[derive(Clone, Debug)]
+static DROP_TABLE_GLOBAL: OnceLock<HashMap<String, DropTable>> = OnceLock::new();
+
+#[derive(Clone, Copy, Debug)]
 pub struct DropEntry {
     pub prefab: MiscPrefab,
-    pub chance: f32,
+    pub chance_pct: u8,
-    pub min_count: u32,
+    pub min_count: u8,
-    pub max_count: u32,
+    pub max_count: u8,
 }
 
 impl DropEntry {
     pub fn always(prefab: MiscPrefab) -> Self {
         Self {
             prefab,
-            chance: 1.0,
+            chance_pct: 100,
             min_count: 1,
             max_count: 1,
         }
     }
 
-    pub fn chance(prefab: MiscPrefab, chance: f32, min_count: u32, max_count: u32) -> Self {
+    pub fn chance(prefab: MiscPrefab, chance_pct: u8, min_count: u8, max_count: u8) -> Self {
         Self {
             prefab,
-            chance,
+            chance_pct,
             min_count,
             max_count,
         }
     }
 
-    pub fn roll(&self, pos: IVec3) -> u32 {
+    pub fn roll(&self, rng_val: u32) -> u8 {
-        if self.chance < 1.0 {
+        if self.chance_pct < 100 && ((rng_val % 100) as u8) >= self.chance_pct {
-            let r = pseudo_rand_f32(pos);
-            if r >= self.chance {
             return 0;
         }
-        }
+        self.min_count + (((rng_val >> 8) % ((self.max_count - self.min_count + 1) as u32)) as u8)
-        pseudo_rand_u32(pos) % (self.max_count - self.min_count + 1) + self.min_count
     }
 }
 
 #[derive(Clone, Debug, Default)]
-pub struct DropTable(pub Vec<DropEntry>);
+pub struct DropTable(pub SmallVec<[DropEntry; 2]>);
 
 impl DropTable {
     pub fn is_empty(&self) -> bool {
@@ -48,26 +52,79 @@ impl DropTable {
     }
 }
 
-fn hash3(pos: IVec3) -> u32 {
+pub fn dig_rng(pos: IVec3) -> u32 {
-    let mut h: u32 = 0;
+    let mut h = (SEED as u64)
-    h = h.wrapping_mul(374761393).wrapping_add(pos.x as u32);
+        .wrapping_add(pos.x as u64)
-    h = h.wrapping_mul(374761393).wrapping_add(pos.y as u32);
+        .wrapping_mul(0x9e3779b97f4a7c15)
-    h = h.wrapping_mul(374761393).wrapping_add(pos.z as u32);
+        ^ (pos.y as u64).wrapping_mul(0x6c62272e07bb0142)
-    h ^= h >> 13;
+        ^ (pos.z as u64).wrapping_mul(0x94d049bb133111eb);
-    h = h.wrapping_mul(1274126177);
+    let h32 = (h ^ (h >> 32)) as u32;
-    h ^= h >> 16;
+    h32 ^ (h32 >> 16)
-    h
 }
 
-fn pseudo_rand_u32(pos: IVec3) -> u32 {
+#[derive(Deserialize)]
-    let timestamp = std::time::SystemTime::now()
+struct DropEntryToml {
-        .duration_since(std::time::UNIX_EPOCH)
+    pub prefab: String,
-        .unwrap()
+    pub chance_pct: u8,
-        .as_nanos() as u32;
+    pub min_count: u8,
-    hash3(pos) ^ timestamp
+    #[serde(rename = "max_max")]
+    pub max_count: u8,
 }
 
-fn pseudo_rand_f32(pos: IVec3) -> f32 {
+#[derive(Deserialize)]
-    let val = pseudo_rand_u32(pos);
+struct DropTableToml {
-    (val as f32) / (u32::MAX as f32)
+    #[serde(default)]
+    pub drops: Vec<DropEntryToml>,
+}
+
+#[derive(Deserialize)]
+struct DropTablesFile {
+    #[serde(flatten)]
+    pub tiles: HashMap<String, DropTableToml>,
+}
+
+fn prefab_from_str(s: &str) -> MiscPrefab {
+    match s {
+        "Coin" => MiscPrefab::Coin,
+        "RawMeat" => MiscPrefab::RawMeat,
+        other => panic!("Unknown prefab in drop_tables.toml: \"{}\"", other),
+    }
+}
+
+#[derive(Resource)]
+pub struct DropTableRegistry(pub HashMap<String, DropTable>);
+
+impl DropTableRegistry {
+    pub fn load() -> Self {
+        let src = std::fs::read_to_string("assets/drop_tables.toml")
+            .expect("assets/drop_tables.toml not found");
+        let file: DropTablesFile =
+            toml::from_str(&src).expect("Failed to parse assets/drop_tables.toml");
+        let mut map = HashMap::new();
+        for (tile_name, table_toml) in file.tiles {
+            let entries: SmallVec<[DropEntry; 2]> = table_toml
+                .drops
+                .iter()
+                .map(|e| DropEntry {
+                    prefab: prefab_from_str(&e.prefab),
+                    chance_pct: e.chance_pct,
+                    min_count: e.min_count,
+                    max_count: e.max_count,
+                })
+                .collect();
+            map.insert(tile_name, DropTable(entries));
+        }
+        Self(map)
+    }
+
+    pub fn init_global(registry: &DropTableRegistry) {
+        DROP_TABLE_GLOBAL.get_or_init(|| registry.0.clone());
+    }
+
+    pub fn global_get(tile_name: &str) -> DropTable {
+        DROP_TABLE_GLOBAL
+            .get()
+            .and_then(|m| m.get(tile_name).cloned())
+            .unwrap_or_default()
+    }
 }

src/entities/shared_systems/digging.rs

@@ -1,11 +1,11 @@
 use crate::constants::ITILE_SIZE;
+use crate::entities::item::drop_table::dig_rng;
 use crate::entities::item::prefabs::misc::misc_prefabs::spawn_prefab;
 use crate::world::chunks::{Z_ABOVE, Z_BELOW};
 use crate::world::tiles::tile_changed::TileChangedEvent;
 use crate::world::tiles::visibility::TileOcclusionEvent;
 use crate::world::tiles::TileMap;
 use bevy::prelude::*;
-use rand::RngExt;
 
 #[derive(Component)]
 pub struct Digger {
@@ -52,6 +52,8 @@ pub fn dig_system(
             continue;
         }
 
+        let rng_val = dig_rng(below_pos);
+
         let drop_table = tilemap
             .floor_tiles
             .get(&below_pos)
@@ -60,12 +62,12 @@ pub fn dig_system(
         if let Some(_removed) = tilemap.dig_floor(&below_pos) {
             if let Some(table) = drop_table {
                 for entry in table.0 {
-                    let count = entry.roll(below_pos);
+                    let count = entry.roll(rng_val);
-                    for _ in 0..count {
+                    for _ in 0..u32::from(count) {
                         spawn_prefab(
                             &mut commands,
                             &asset_server,
-                            entry.prefab.clone(),
+                            entry.prefab,
                             below_pos.as_vec3(),
                             &mut tilemap,
                         );

src/world/generation/terrain.rs

@@ -4,8 +4,7 @@ use bevy_platform::time::Instant;
 use noise::{NoiseFn, Perlin};
 use std::sync::{Arc, Mutex};
 
-use crate::entities::item::drop_table::{DropEntry, DropTable};
+use crate::entities::item::drop_table::DropTableRegistry;
-use crate::entities::item::prefabs::misc::misc_prefabs::MiscPrefab;
 
 use crate::{
     config::TileRegistry,
@@ -17,19 +16,6 @@ use crate::{
     },
 };
 
-fn drop_table_for(tile_name: &str) -> DropTable {
-    match tile_name {
-        "grass" => DropTable(vec![
-            DropEntry::chance(MiscPrefab::Coin, 0.05, 1, 2),
-        ]),
-        "dirt" => DropTable::default(),
-        "rock" => DropTable(vec![
-            DropEntry::chance(MiscPrefab::Coin, 0.1, 1, 1),
-        ]),
-        _ => DropTable::default(),
-    }
-}
-
 /// Thread-safe storage for completed terrain blobs.
 /// Uses type erasure to avoid Debug bounds on TerrainBlob.
 type BlobStorage = Arc<Mutex<Box<dyn Send + Sync>>>;
@@ -130,7 +116,7 @@ fn generate_terrain_blob(chunk_pos: IVec2) -> TerrainBlob {
                                 tile.transparent,
                                 tile.astar_weight,
                                 [0; 8],
-                                drop_table_for("air"),
+                                DropTableRegistry::global_get("air"),
                             ),
                         ));
                         chunk_data.set_floor_tile(
@@ -153,7 +139,7 @@ fn generate_terrain_blob(chunk_pos: IVec2) -> TerrainBlob {
                                 tile.transparent,
                                 tile.astar_weight,
                                 [0; 8],
-                                drop_table_for("rock"),
+                                DropTableRegistry::global_get("rock"),
                             ),
                         ));
                         chunk_data.set_floor_tile(
@@ -176,7 +162,7 @@ fn generate_terrain_blob(chunk_pos: IVec2) -> TerrainBlob {
                             tile.transparent,
                             tile.astar_weight,
                             [0; 8],
-                            drop_table_for("dirt"),
+                            DropTableRegistry::global_get("dirt"),
                         ),
                     ));
                         chunk_data.set_floor_tile(
@@ -201,7 +187,7 @@ fn generate_terrain_blob(chunk_pos: IVec2) -> TerrainBlob {
                                 tile.transparent,
                                 tile.astar_weight,
                                 [0; 8],
-                                drop_table_for("grass"),
+                                DropTableRegistry::global_get("grass"),
                             ),
                         ));
                         chunk_data.set_floor_tile(
@@ -225,7 +211,7 @@ fn generate_terrain_blob(chunk_pos: IVec2) -> TerrainBlob {
                                 tile.transparent,
                                 tile.astar_weight,
                                 [0; 8],
-                                drop_table_for("dirt"),
+                                DropTableRegistry::global_get("dirt"),
                             ),
                         ));
                         chunk_data.set_floor_tile(
@@ -249,7 +235,7 @@ fn generate_terrain_blob(chunk_pos: IVec2) -> TerrainBlob {
                             tile.transparent,
                             tile.astar_weight,
                             [0; 8],
-                            drop_table_for("air"),
+                            DropTableRegistry::global_get("air"),
                         ),
                     ));
                     chunk_data.set_floor_tile(

src/world/mod.rs

@@ -34,7 +34,10 @@ pub struct WorldPlugin;
 
 impl Plugin for WorldPlugin {
     fn build(&self, app: &mut App) {
-        app.init_resource::<ChunkMap>()
+        let drop_registry = crate::entities::item::drop_table::DropTableRegistry::load();
+        crate::entities::item::drop_table::DropTableRegistry::init_global(&drop_registry);
+        app.insert_resource(drop_registry)
+            .init_resource::<ChunkMap>()
             .init_resource::<TerrainBlobStorage>()
             .init_resource::<tiles::TilemapBenchmark>()
             .init_resource::<tiles::TilemapChunkRegistry>()