Attach absent tile entity and entity lists to the chunk tag

src/main/java/com/gtnewhorizons/postea/api/ChunkTransformContext.java

@@ -1,7 +1,5 @@
 package com.gtnewhorizons.postea.api;
 
-import static net.minecraftforge.common.util.Constants.NBT.TAG_COMPOUND;
-
 import java.util.function.Consumer;
 
 import net.minecraft.nbt.NBTTagCompound;
@@ -13,14 +11,15 @@ import net.minecraft.world.chunk.storage.ExtendedBlockStorage;
 import com.gtnewhorizons.postea.compat.Compat;
 import com.gtnewhorizons.postea.compat.SubChunkAccess;
 import com.gtnewhorizons.postea.utility.NbtItemStacks;
+import com.gtnewhorizons.postea.utility.NbtLists;
 import com.gtnewhorizons.postea.utility.VersionStamps;
 
 /**
  * A chunk as handed to {@link IVersionedTransformer#transformChunk}: its block storage and the raw {@code Level}
  * tag as read from disk, before tile entities and entities are parsed. Coordinates are chunk-local
  * ({@code 0..15}, {@code y 0..255}); block ids and metadata are the world's numeric values at full EndlessIDs
- * width. The world is not loaded yet: block access through {@link #world()} is not possible. Changing a block id
- * does not recompute lighting or the height map; Postea flags the chunk for a relight when an id changed.
+ * width. The world is not loaded yet: block access through {@link #world()} is not possible. When a transformer
+ * changed a block id, Postea recomputes the height map and flags the chunk for a relight once all transformers ran.
  */
 public final class ChunkTransformContext {
 
@@ -68,11 +67,11 @@ public final class ChunkTransformContext {
     }
 
     public NBTTagList tileEntityTags() {
-        return levelTag.getTagList("TileEntities", TAG_COMPOUND);
+        return NbtLists.compoundList(levelTag, "TileEntities");
     }
 
     public NBTTagList entityTags() {
-        return levelTag.getTagList("Entities", TAG_COMPOUND);
+        return NbtLists.compoundList(levelTag, "Entities");
     }
 
     /**

src/main/java/com/gtnewhorizons/postea/utility/NbtLists.java

@@ -0,0 +1,21 @@
+package com.gtnewhorizons.postea.utility;
+
+import static net.minecraftforge.common.util.Constants.NBT.TAG_COMPOUND;
+import static net.minecraftforge.common.util.Constants.NBT.TAG_LIST;
+
+import net.minecraft.nbt.NBTTagCompound;
+import net.minecraft.nbt.NBTTagList;
+
+public final class NbtLists {
+
+    private NbtLists() {}
+
+    /**
+     * The list of compounds stored under {@code key}, attached to {@code parent} first when absent so that tags
+     * appended to it are part of {@code parent}.
+     */
+    public static NBTTagList compoundList(NBTTagCompound parent, String key) {
+        if (!parent.hasKey(key, TAG_LIST)) parent.setTag(key, new NBTTagList());
+        return parent.getTagList(key, TAG_COMPOUND);
+    }
+}

src/test/java/com/gtnewhorizons/postea/utility/NbtListsTest.java

@@ -0,0 +1,35 @@
+package com.gtnewhorizons.postea.utility;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import net.minecraft.nbt.NBTTagCompound;
+import net.minecraft.nbt.NBTTagList;
+
+import org.junit.jupiter.api.Test;
+
+class NbtListsTest {
+
+    @Test
+    void anAbsentListIsCreatedAndAttachedSoAppendedTagsAreKept() {
+        NBTTagCompound level = new NBTTagCompound();
+
+        NbtLists.compoundList(level, "TileEntities")
+            .appendTag(new NBTTagCompound());
+
+        assertEquals(
+            1,
+            level.getTagList("TileEntities", 10)
+                .tagCount());
+    }
+
+    @Test
+    void anExistingListIsReturnedAsIs() {
+        NBTTagCompound level = new NBTTagCompound();
+        NBTTagList existing = new NBTTagList();
+        existing.appendTag(new NBTTagCompound());
+        level.setTag("Entities", existing);
+
+        assertSame(existing, NbtLists.compoundList(level, "Entities"));
+    }
+}