Plain chunk output buffer

- ChunkDataHunkHolder stores cells in a plain array and tracks each column's highest stored y
- natural cave decoration and perfection stop scanning above the column top

core/src/main/java/art/arcane/iris/generation/chunk/ChunkDataHunkHolder.java

@@ -23,19 +23,35 @@ import art.arcane.volmlib.nativelib.terrain.NativeBlockVolume;
 import art.arcane.volmlib.nativelib.terrain.NativeBlockState;
 import art.arcane.iris.generation.block.BoundBlockState;
 import art.arcane.iris.generation.block.IrisCustomData;
-import art.arcane.volmlib.util.hunk.storage.AtomicHunk;
+import art.arcane.volmlib.util.hunk.storage.StorageHunk;
 import org.bukkit.block.data.BlockData;
 import org.bukkit.generator.ChunkGenerator.ChunkData;
 
-@SuppressWarnings("ClassCanBeRecord")
-public class ChunkDataHunkHolder extends AtomicHunk<NativeBlockState> implements NativeBlockVolume {
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.VarHandle;
+import java.util.Arrays;
+
+/**
+ * Plain-array block buffer for one chunk. Stages write disjoint columns when they fan out and every hand-off
+ * between threads is a join, so the cells need no per-access fences. Each column tracks its highest stored y;
+ * a raise is a CAS so concurrent writers to one column can never lose the maximum.
+ */
+public class ChunkDataHunkHolder extends StorageHunk<NativeBlockState> implements NativeBlockVolume, ColumnExtent {
     private static final BoundBlockState AIR = BoundBlockState.of("AIR");
+    private static final VarHandle COLUMN_TOPS = MethodHandles.arrayElementVarHandle(int[].class);
 
     private final ChunkData chunk;
+    private final NativeBlockState[] data;
+    private final int[] columnTops;
+    private final int zStride;
 
     public ChunkDataHunkHolder(ChunkData chunk) {
         super(16, chunk.getMaxHeight() - chunk.getMinHeight(), 16);
         this.chunk = chunk;
+        this.zStride = 16 * getHeight();
+        this.data = new NativeBlockState[zStride * 16];
+        this.columnTops = new int[256];
+        Arrays.fill(columnTops, -1);
     }
 
     @Override
@@ -56,7 +72,16 @@ public class ChunkDataHunkHolder extends AtomicHunk<NativeBlockState> implements
         if (t == null) {
             return;
         }
-        super.setRaw(x, y, z, t);
+        data[index(x, y, z)] = t;
+        int column = (z << 4) | x;
+        int top = (int) COLUMN_TOPS.getOpaque(columnTops, column);
+        while (y > top) {
+            int witness = (int) COLUMN_TOPS.compareAndExchange(columnTops, column, top, y);
+            if (witness == top) {
+                break;
+            }
+            top = witness;
+        }
     }
 
     @Override
@@ -65,13 +90,23 @@ public class ChunkDataHunkHolder extends AtomicHunk<NativeBlockState> implements
             return AIR.get();
         }
 
-        NativeBlockState b = super.getRaw(x, y, z);
+        NativeBlockState b = data[index(x, y, z)];
 
         return b != null ? b : AIR.get();
     }
 
+    @Override
     public NativeBlockState getStoredRaw(int x, int y, int z) {
-        return super.getRaw(x, y, z);
+        return data[index(x, y, z)];
+    }
+
+    @Override
+    public int highestStoredY(int x, int z) {
+        return (int) COLUMN_TOPS.getOpaque(columnTops, (z << 4) | x);
+    }
+
+    private int index(int x, int y, int z) {
+        return (z * zStride) + (y << 4) + x;
     }
 
     public void apply() {
@@ -90,7 +125,7 @@ public class ChunkDataHunkHolder extends AtomicHunk<NativeBlockState> implements
                 int runStart = -1;
 
                 for (int y = 0; y < height; y++) {
-                    NativeBlockState state = super.getRaw(x, y, z);
+                    NativeBlockState state = getStoredRaw(x, y, z);
                     BlockData block = state == null ? null : (BlockData) state.nativeHandle();
                     // Custom wrappers are not real Bukkit data; write the vanilla base like the
                     // NMS fast path (NMSBinding.applyChunkDataBlocks) does.

core/src/main/java/art/arcane/iris/generation/chunk/ColumnExtent.java

@@ -0,0 +1,33 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.generation.chunk;
+
+import art.arcane.volmlib.util.hunk.Hunk;
+
+/**
+ * A block volume that knows, per column, the highest y it holds a value for. Every cell above that y reads as
+ * unwritten, so column scans can stop there without changing what they observe.
+ */
+public interface ColumnExtent {
+    int highestStoredY(int x, int z);
+
+    static int highestStoredY(Hunk<?> hunk, int x, int z) {
+        return hunk instanceof ColumnExtent extent ? extent.highestStoredY(x, z) : hunk.getHeight() - 1;
+    }
+}

core/src/main/java/art/arcane/iris/generation/chunk/ColumnExtentListeningHunk.java

@@ -0,0 +1,37 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.generation.chunk;
+
+import art.arcane.volmlib.util.function.Consumer4;
+import art.arcane.volmlib.util.hunk.Hunk;
+import art.arcane.volmlib.util.hunk.view.ListeningHunk;
+
+public final class ColumnExtentListeningHunk<T> extends ListeningHunk<T> implements ColumnExtent {
+    private final Hunk<T> source;
+
+    public ColumnExtentListeningHunk(Hunk<T> source, Consumer4<Integer, Integer, Integer, T> listener) {
+        super(source, listener);
+        this.source = source;
+    }
+
+    @Override
+    public int highestStoredY(int x, int z) {
+        return ColumnExtent.highestStoredY(source, x, z);
+    }
+}

core/src/main/java/art/arcane/iris/generation/runtime/IrisEngine.java

@@ -48,6 +48,7 @@ import art.arcane.volmlib.nativelib.terrain.NativeBlockState;
 import art.arcane.volmlib.util.atomics.AtomicRollingSequence;
 import art.arcane.iris.generation.context.ChunkContext;
 import art.arcane.iris.generation.context.IrisContext;
+import art.arcane.iris.generation.chunk.ColumnExtentListeningHunk;
 import art.arcane.volmlib.util.documentation.BlockCoordinates;
 import art.arcane.volmlib.util.documentation.ChunkCoordinates;
 import art.arcane.volmlib.util.hunk.Hunk;
@@ -1465,7 +1466,7 @@ public class IrisEngine implements Engine {
              IrisContext.Scope generationScope = IrisContext.open(this, lease.sessionId(), null)) {
             getEngineData().getStatistics().generatedChunk();
             PrecisionStopwatch p = PrecisionStopwatch.start();
-            Hunk<NativeBlockState> blocks = vblocks.listen((xx, y, zz, t) -> catchBlockUpdates(x + xx, y, z + zz, t));
+            Hunk<NativeBlockState> blocks = new ColumnExtentListeningHunk<>(vblocks, (xx, y, zz, t) -> catchBlockUpdates(x + xx, y, z + zz, t));
 
             if (getDimension().isDebugChunkCrossSections() && ((x >> 4) % getDimension().getDebugCrossSectionsMod() == 0 || (z >> 4) % getDimension().getDebugCrossSectionsMod() == 0)) {
                 NativeBlockState crossSection = B.getState("CRYING_OBSIDIAN");

core/src/main/java/art/arcane/iris/generation/stage/IrisCarveModifier.java

@@ -40,6 +40,7 @@ import art.arcane.iris.generation.decoration.IrisProceduralBlocks;
 import art.arcane.iris.generation.hydrology.IrisRiverMaterialConfig;
 import art.arcane.iris.generation.hydrology.cave.HydrologyCaveAction;
 import art.arcane.iris.generation.hydrology.cave.HydrologyCaveCell;
+import art.arcane.iris.generation.chunk.ColumnExtent;
 import art.arcane.iris.generation.context.ChunkContext;
 import art.arcane.iris.generation.context.IrisContext;
 import art.arcane.iris.generation.block.B;
@@ -896,12 +897,16 @@ public class IrisCarveModifier extends EngineAssignedModifier<NativeBlockState>
             int width = output.getWidth();
             int depth = output.getDepth();
             int height = output.getHeight();
+            ColumnExtent extent = output instanceof ColumnExtent columnExtent ? columnExtent : null;
             for (int localX = 0; localX < width; localX++) {
                 for (int localZ = 0; localZ < depth; localZ++) {
                     int worldX = blockX + localX;
                     int worldZ = blockZ + localZ;
                     int floor = -1;
                     for (int y = 1; y < height; y++) {
+                        if (extent != null && y > extent.highestStoredY(localX, localZ)) {
+                            break;
+                        }
                         NativeBlockState state = output.getRaw(localX, y, localZ);
                         if (B.isSolid(state)) {
                             if (floor >= 0) {

core/src/main/java/art/arcane/iris/generation/stage/IrisPerfectionModifier.java

@@ -18,6 +18,7 @@
 
 package art.arcane.iris.generation.stage;
 
+import art.arcane.iris.generation.chunk.ColumnExtent;
 import art.arcane.iris.generation.decoration.IrisSpeleothems;
 import art.arcane.iris.generation.runtime.Engine;
 import art.arcane.iris.generation.runtime.EngineAssignedModifier;
@@ -203,7 +204,7 @@ public class IrisPerfectionModifier extends EngineAssignedModifier<NativeBlockSt
     }
 
     private int getHeight(Hunk<NativeBlockState> output, int x, int z) {
-        for (int i = output.getHeight() - 1; i >= 0; i--) {
+        for (int i = Math.min(output.getHeight() - 1, ColumnExtent.highestStoredY(output, x, z)); i >= 0; i--) {
             NativeBlockState b = output.get(x, i, z);
 
             if (b != null) {