new: Add patch to optimize mob spawning

Retrieving the entity spawn list for a given entity category is a
rather hot activity in mob spawning. This replaces the EntityCategory->List map
with a specialized type for enum keys, which reduces a hash lookup to an array index.

src/main/java/me/jellysquid/mods/lithium/mixin/world/mob_spawning/MixinBiome.java

@@ -0,0 +1,35 @@
+package me.jellysquid.mods.lithium.mixin.world.mob_spawning;
+
+import com.google.common.collect.Maps;
+import net.minecraft.entity.EntityCategory;
+import net.minecraft.world.biome.Biome;
+import org.spongepowered.asm.mixin.Final;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Mutable;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+import java.util.List;
+import java.util.Map;
+
+@Mixin(Biome.class)
+public class MixinBiome {
+    @Mutable
+    @Shadow
+    @Final
+    private Map<EntityCategory, List<Biome.SpawnEntry>> spawns;
+
+    /**
+     * Re-initialize the spawn category lists with a much faster backing collection type for enum keys. This provides
+     * a modest speed-up for mob spawning as {@link Biome#getEntitySpawnList(EntityCategory)} is a rather hot method.
+     */
+    @Inject(method = "<init>", at = @At("RETURN"))
+    private void reinit(Biome.Settings settings, CallbackInfo ci) {
+        Map<EntityCategory, List<Biome.SpawnEntry>> spawns = Maps.newEnumMap(EntityCategory.class);
+        spawns.putAll(this.spawns);
+
+        this.spawns = spawns;
+    }
+}

src/main/resources/lithium.mixins.json

@@ -77,6 +77,7 @@
         "world.chunk_ticking.MixinThreadedAnvilChunkStorage",
         "world.explosions.MixinExplosion",
         "world.fire_checks.MixinWorld",
+        "world.mob_spawning.MixinBiome",
         "world.tick_scheduler.MixinServerWorld"
     ]
 }