PaperMC/Remapped-Spigot-Server-Patches/0440-Reduce-memory-footprint-of-NBTTagCompound.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <spottedleaf@spottedleaf.dev>
Date: Mon, 6 Apr 2020 17:39:25 -0700
Subject: [PATCH] Reduce memory footprint of NBTTagCompound

Fastutil maps are going to have a lower memory footprint - which
is important because we clone chunk data after reading it for safety.
So, reduce the impact of the clone on GC.

diff --git a/src/main/java/net/minecraft/nbt/CompoundTag.java b/src/main/java/net/minecraft/nbt/CompoundTag.java
index a91bf94ed9f2f353a685194fc91c4b101ccc1232..c856ca720a9329a94bb07eaa3060c034f95718b3 100644
--- a/src/main/java/net/minecraft/nbt/CompoundTag.java
+++ b/src/main/java/net/minecraft/nbt/CompoundTag.java
@@ -26,6 +26,7 @@ import net.minecraft.ReportedException;
 import net.minecraft.network.chat.Component;
 import net.minecraft.network.chat.MutableComponent;
 import net.minecraft.network.chat.TextComponent;
+import it.unimi.dsi.fastutil.objects.Object2ObjectOpenHashMap; // Paper
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
@@ -47,7 +48,7 @@ public class CompoundTag implements Tag {
             if (i > 512) {
                 throw new RuntimeException("Tried to read NBT tag with too high complexity, depth > 512");
             } else {
-                HashMap hashmap = Maps.newHashMap();
+                Object2ObjectOpenHashMap<String, Tag> hashmap = new Object2ObjectOpenHashMap<>(8, 0.8f); // Paper - reduce memory footprint of NBTTagCompound
 
                 byte b0;
 
@@ -83,7 +84,7 @@ public class CompoundTag implements Tag {
     }
 
     public CompoundTag() {
-        this(Maps.newHashMap());
+        this(new Object2ObjectOpenHashMap<>(8, 0.8f)); // Paper - reduce memory footprint of NBTTagCompound
     }
 
     @Override
@@ -417,9 +418,17 @@ public class CompoundTag implements Tag {
 
     @Override
     public CompoundTag copy() {
-        Map<String, Tag> map = Maps.newHashMap(Maps.transformValues(this.tags, Tag::copy));
+        // Paper start - reduce memory footprint of NBTTagCompound
+        Object2ObjectOpenHashMap<String, Tag> ret = new Object2ObjectOpenHashMap<>(this.tags.size(), 0.8f);
 
-        return new CompoundTag(map);
+        Iterator<Map.Entry<String, Tag>> iterator = (this.tags instanceof Object2ObjectOpenHashMap) ? ((Object2ObjectOpenHashMap)this.tags).object2ObjectEntrySet().fastIterator() : this.tags.entrySet().iterator();
+        while (iterator.hasNext()) {
+            Map.Entry<String, Tag> entry = iterator.next();
+            ret.put(entry.getKey(), entry.getValue().copy());
+        }
+
+        return new CompoundTag(ret);
+        // Paper end - reduce memory footprint of NBTTagCompound
     }
 
     public boolean equals(Object object) {