Reduce overhead of the PotentialSpawns event

Author: embeddedt

forge/src/main/java/org/embeddedt/modernfix/forge/mixin/perf/potential_spawns_alloc/ForgeEventFactoryMixin.java

@@ -0,0 +1,25 @@
+package org.embeddedt.modernfix.forge.mixin.perf.potential_spawns_alloc;
+
+import net.minecraft.core.BlockPos;
+import net.minecraft.util.random.WeightedRandomList;
+import net.minecraft.world.entity.MobCategory;
+import net.minecraft.world.level.LevelAccessor;
+import net.minecraft.world.level.biome.MobSpawnSettings;
+import net.minecraftforge.event.ForgeEventFactory;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Redirect;
+
+import java.util.List;
+
+@Mixin(ForgeEventFactory.class)
+public class ForgeEventFactoryMixin {
+    @Redirect(method = "getPotentialSpawns", at = @At(value = "INVOKE", target = "Lnet/minecraft/util/random/WeightedRandomList;create(Ljava/util/List;)Lnet/minecraft/util/random/WeightedRandomList;"))
+    private static WeightedRandomList<MobSpawnSettings.SpawnerData> reuseOldList(List<MobSpawnSettings.SpawnerData> items, LevelAccessor level, MobCategory category, BlockPos pos, WeightedRandomList<MobSpawnSettings.SpawnerData> oldList) {
+        // Our patched version of PotentialSpawns will return the same list as unwrap() if no one mutated the list
+        if(items == oldList.unwrap()) {
+            return oldList;
+        }
+        return WeightedRandomList.create(items);
+    }
+}

forge/src/main/java/org/embeddedt/modernfix/forge/mixin/perf/potential_spawns_alloc/PotentialSpawnsMixin.java

@@ -0,0 +1,68 @@
+package org.embeddedt.modernfix.forge.mixin.perf.potential_spawns_alloc;
+
+import net.minecraft.core.BlockPos;
+import net.minecraft.util.random.WeightedRandomList;
+import net.minecraft.world.entity.MobCategory;
+import net.minecraft.world.level.LevelAccessor;
+import net.minecraft.world.level.biome.MobSpawnSettings;
+import net.minecraftforge.event.world.WorldEvent;
+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.Redirect;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Mixin(WorldEvent.PotentialSpawns.class)
+public class PotentialSpawnsMixin {
+    @Shadow(remap = false) @Final @Mutable private List<MobSpawnSettings.SpawnerData> view;
+    @Shadow(remap = false) @Final @Mutable private List<MobSpawnSettings.SpawnerData> list;
+
+    private static final ArrayList<MobSpawnSettings.SpawnerData> SENTINEL = new ArrayList<>();
+
+    @Redirect(method = "<init>", at = @At(value = "NEW", target = "java/util/ArrayList", ordinal = 1))
+    private ArrayList<?> avoidListAlloc1() {
+        return SENTINEL;
+    }
+
+    @Redirect(method = "<init>", at = @At(value = "NEW", target = "java/util/ArrayList", ordinal = 0))
+    private ArrayList<?> avoidListAlloc2(Collection c) {
+        return SENTINEL;
+    }
+
+    @Redirect(method = "<init>", at = @At(value = "INVOKE", target = "Ljava/util/Collections;unmodifiableList(Ljava/util/List;)Ljava/util/List;"))
+    private List<?> avoidListAlloc3(List<?> l) {
+        return null;
+    }
+
+    @Inject(method = "<init>", at = @At("RETURN"))
+    private void initializeSmartLists(LevelAccessor level, MobCategory category, BlockPos pos, WeightedRandomList<MobSpawnSettings.SpawnerData> oldList, CallbackInfo ci) {
+        this.view = oldList.unwrap();
+        this.list = null;
+    }
+
+    private void mfix$populateList() {
+        if(this.list == null) {
+            this.list = new ArrayList<>(this.view);
+            this.view = Collections.unmodifiableList(this.list);
+        }
+    }
+
+    @Inject(method = {"addSpawnerData" }, at = @At("HEAD"), remap = false)
+    private void populateList(MobSpawnSettings.SpawnerData data, CallbackInfo ci) {
+        mfix$populateList();
+    }
+
+    @Inject(method = {"removeSpawnerData" }, at = @At("HEAD"), remap = false)
+    private void populateList(MobSpawnSettings.SpawnerData data, CallbackInfoReturnable<Boolean> cir) {
+        mfix$populateList();
+    }
+}