Compact NBT used for Flattening world upgrades

Author: embeddedt

common/src/main/java/org/embeddedt/modernfix/common/mixin/perf/compact_mojang_registries/BlockStateDataMixin.java

@@ -0,0 +1,50 @@
+package org.embeddedt.modernfix.common.mixin.perf.compact_mojang_registries;
+
+import com.llamalad7.mixinextras.injector.ModifyExpressionValue;
+import it.unimi.dsi.fastutil.objects.ObjectOpenHashSet;
+import net.minecraft.nbt.CompoundTag;
+import net.minecraft.nbt.Tag;
+import net.minecraft.util.datafix.fixes.BlockStateData;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Unique;
+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.Map;
+
+@Mixin(value = BlockStateData.class, priority = 2000)
+public class BlockStateDataMixin {
+    @Unique
+    private static ObjectOpenHashSet<Tag> TAG_INTERNER;
+
+    /**
+     * @author embeddedt
+     * @reason Reduce memory use of these constant CompoundTags via aggressive interning.
+     */
+    @ModifyExpressionValue(method = "parse", at = @At(value = "INVOKE", target = "Lnet/minecraft/nbt/TagParser;parseTag(Ljava/lang/String;)Lnet/minecraft/nbt/CompoundTag;"))
+    private static CompoundTag compactTag(CompoundTag tag) {
+        if (TAG_INTERNER == null) {
+            TAG_INTERNER = new ObjectOpenHashSet<>();
+        }
+        Map.Entry<String, Tag>[] entries = new Map.Entry[tag.size()];
+        int i = 0;
+        for (var key : tag.getAllKeys()) {
+            Tag t = tag.get(key);
+            if (t instanceof CompoundTag ct) {
+                t = compactTag(ct);
+            }
+            t = TAG_INTERNER.addOrGet(t);
+            entries[i++] = Map.entry(key, t);
+        }
+        return new CompoundTag(Map.ofEntries(entries));
+    }
+
+    @Inject(method = "<clinit>", at = @At("RETURN"))
+    private static void clearInterner(CallbackInfo ci) {
+        if (TAG_INTERNER != null) {
+            TAG_INTERNER.clear();
+            TAG_INTERNER.trim();
+        }
+    }
+}