From 14c41940712dcd717c453ad0e15be3be42154755 Mon Sep 17 00:00:00 2001
From: embeddedt <42941056+embeddedt@users.noreply.github.com>
Date: Sun, 24 Mar 2024 14:19:44 -0400
Subject: [PATCH] Minimize allocations and avoid rebuilding weighted list when
 retrieving potential spawns (#732)

---
 .../neoforged/neoforge/event/EventHooks.java  |  6 ++++-
 .../neoforge/event/level/LevelEvent.java      | 23 ++++++++++++-------
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/src/main/java/net/neoforged/neoforge/event/EventHooks.java b/src/main/java/net/neoforged/neoforge/event/EventHooks.java
index ffbab83a9f..618d88c50d 100644
--- a/src/main/java/net/neoforged/neoforge/event/EventHooks.java
+++ b/src/main/java/net/neoforged/neoforge/event/EventHooks.java
@@ -850,10 +850,14 @@ public static void onPostServerTick(BooleanSupplier haveTime, MinecraftServer se
         NeoForge.EVENT_BUS.post(new TickEvent.ServerTickEvent(TickEvent.Phase.END, haveTime, server));
     }
 
+    private static final WeightedRandomList<MobSpawnSettings.SpawnerData> NO_SPAWNS = WeightedRandomList.create();
+
     public static WeightedRandomList<MobSpawnSettings.SpawnerData> getPotentialSpawns(LevelAccessor level, MobCategory category, BlockPos pos, WeightedRandomList<MobSpawnSettings.SpawnerData> oldList) {
         LevelEvent.PotentialSpawns event = new LevelEvent.PotentialSpawns(level, category, pos, oldList);
         if (NeoForge.EVENT_BUS.post(event).isCanceled())
-            return WeightedRandomList.create();
+            return NO_SPAWNS;
+        else if (event.getSpawnerDataList() == oldList.unwrap())
+            return oldList;
         return WeightedRandomList.create(event.getSpawnerDataList());
     }
 
diff --git a/src/main/java/net/neoforged/neoforge/event/level/LevelEvent.java b/src/main/java/net/neoforged/neoforge/event/level/LevelEvent.java
index a293b94e35..4e8aaa3a51 100644
--- a/src/main/java/net/neoforged/neoforge/event/level/LevelEvent.java
+++ b/src/main/java/net/neoforged/neoforge/event/level/LevelEvent.java
@@ -24,6 +24,7 @@
 import net.neoforged.bus.api.ICancellableEvent;
 import net.neoforged.fml.LogicalSide;
 import net.neoforged.neoforge.common.NeoForge;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * This event is fired whenever an event involving a {@link LevelAccessor} occurs.
@@ -133,19 +134,16 @@ public ServerLevelData getSettings() {
     public static class PotentialSpawns extends LevelEvent implements ICancellableEvent {
         private final MobCategory mobcategory;
         private final BlockPos pos;
-        private final List<MobSpawnSettings.SpawnerData> list;
-        private final List<MobSpawnSettings.SpawnerData> view;
+        @Nullable
+        private List<MobSpawnSettings.SpawnerData> list;
+        private List<MobSpawnSettings.SpawnerData> view;
 
         public PotentialSpawns(LevelAccessor level, MobCategory category, BlockPos pos, WeightedRandomList<MobSpawnSettings.SpawnerData> oldList) {
             super(level);
             this.pos = pos;
             this.mobcategory = category;
-            if (!oldList.isEmpty())
-                this.list = new ArrayList<>(oldList.unwrap());
-            else
-                this.list = new ArrayList<>();
-
-            this.view = Collections.unmodifiableList(list);
+            this.list = null;
+            this.view = oldList.unwrap();
         }
 
         /**
@@ -169,12 +167,20 @@ public List<MobSpawnSettings.SpawnerData> getSpawnerDataList() {
             return view;
         }
 
+        private void makeList() {
+            if (list == null) {
+                list = new ArrayList<>(view);
+                view = Collections.unmodifiableList(list);
+            }
+        }
+
         /**
          * Appends a SpawnerData entry to the spawn list.
          *
          * @param data SpawnerData entry to be appended to the spawn list.
          */
         public void addSpawnerData(MobSpawnSettings.SpawnerData data) {
+            makeList();
             list.add(data);
         }
 
@@ -186,6 +192,7 @@ public void addSpawnerData(MobSpawnSettings.SpawnerData data) {
          *             {@return {@code true} if the spawn list contained the specified element.}
          */
         public boolean removeSpawnerData(MobSpawnSettings.SpawnerData data) {
+            makeList();
             return list.remove(data);
         }
     }