Per-chunk hydrology samples

- terrain painter reads the chunk's column array once instead of 256 cache lookups
- test pins chunk samples against per-column lookups on a real plan

core/src/main/java/art/arcane/iris/generation/hydrology/HydrologyTileCache.java

@@ -537,6 +537,14 @@ public final class HydrologyTileCache implements AutoCloseable {
         chunkColumns(blockX, blockZ);
     }
 
+    /**
+     * Every column of the block's chunk, indexed {@code localZ * 16 + localX}; a column without hydrology
+     * is null. The array is shared with the cache and must not be modified.
+     */
+    public HydrologyColumnSample[] chunkSamples(int blockX, int blockZ) {
+        return chunkColumns(blockX, blockZ).columns();
+    }
+
     /**
      * Whether the column's chunk can be answered without planning: every tile it composes from is
      * already planned. A caller that must not wait (the server thread answering a height or biome

core/src/main/java/art/arcane/iris/generation/hydrology/runtime/IrisHydrologyRuntime.java

@@ -195,6 +195,11 @@ public final class IrisHydrologyRuntime implements AutoCloseable {
         cache.prepareChunkColumns(blockX, blockZ);
     }
 
+    /** The block's chunk columns indexed {@code localZ * 16 + localX}; shared, never modified by callers. */
+    public HydrologyColumnSample[] chunkSamples(int blockX, int blockZ) {
+        return cache.chunkSamples(blockX, blockZ);
+    }
+
     /** Whether the chunk or one of its eight neighbours holds a planned footprint column. */
     public boolean hasColumnsAround(int chunkX, int chunkZ) {
         return cache.hasColumnsAround(chunkX, chunkZ);

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

@@ -1062,6 +1062,31 @@ public class IrisComplex implements DataProvider {
         return transitionHydrologySample(sample, hydrologyWeight, transitionDisplacement);
     }
 
+    /**
+     * The raw hydrology columns of the chunk holding the block, for a caller that samples every column of
+     * one chunk through {@link #sampleHydrologyColumn(HydrologyColumnSample[], int, int)}; null without
+     * hydrology.
+     */
+    public HydrologyColumnSample[] hydrologyChunkSamples(int x, int z) {
+        return hydrologyRuntime == null ? null : hydrologyRuntime.chunkSamples(x, z);
+    }
+
+    /** {@link #sampleHydrologyColumn(double, double)} for a column of the chunk {@code chunkSamples} came from. */
+    public HydrologyColumnSample sampleHydrologyColumn(HydrologyColumnSample[] chunkSamples, int x, int z) {
+        if (chunkSamples == null) {
+            return null;
+        }
+        HydrologyColumnSample sample = chunkSamples[(z & 15) << 4 | (x & 15)];
+        if (transitionGenerationPlan == null) {
+            return sample;
+        }
+        double hydrologyWeight = transitionHydrologyWeight(x, z);
+        if (sample == null) {
+            return sample;
+        }
+        return transitionHydrologySample(sample, hydrologyWeight, transitionDisplacement);
+    }
+
     private HydrologyColumnSnapshot sampleHydrologySnapshot(int x, int z) {
         if (hydrologyRuntime == null) {
             return HydrologyColumnSnapshot.ready(null);

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

@@ -154,6 +154,7 @@ public class IrisTerrainNormalActuator extends EngineAssignedActuator<NativeBloc
         private final IrisData data;
         private final IrisComplex complex;
         private final ProceduralStream<Double> riverWaterSurfaceStream;
+        private final HydrologyColumnSample[] hydrologySamples;
         private final boolean bedrockEnabled;
         private final boolean hideOres;
         private final boolean transitionDisplaced;
@@ -186,6 +187,7 @@ public class IrisTerrainNormalActuator extends EngineAssignedActuator<NativeBloc
             data = getData();
             complex = getComplex();
             riverWaterSurfaceStream = complex.getRiverWaterSurfaceStream();
+            hydrologySamples = complex.hydrologyChunkSamples(x, z);
             bedrockEnabled = dimension.isBedrock();
             hideOres = dimension.isHideOresForHiddenOre();
             transitionDisplaced = complex.getTransitionDisplacement() != null;
@@ -226,7 +228,7 @@ public class IrisTerrainNormalActuator extends EngineAssignedActuator<NativeBloc
             }
 
             int topY = Math.min(hf, chunkHeight - 1);
-            HydrologyColumnSample hydrology = complex.sampleHydrologyColumn(realX, realZ);
+            HydrologyColumnSample hydrology = complex.sampleHydrologyColumn(hydrologySamples, realX, realZ);
             HydrologyColumnLayer hydrologyFluid = hydrology == null
                     ? null
                     : hydrology.primarySurfaceFluidLayerOrNull();

core/src/test/java/art/arcane/iris/generation/hydrology/HydrologyTileCacheTest.java

@@ -1125,6 +1125,34 @@ public class HydrologyTileCacheTest {
         }
     }
 
+    @Test
+    public void chunkSamplesMatchPerColumnLookups() {
+        HydrologyTileCache perColumn = new HydrologyTileCache(new HydrologyPlanner(811L, featureSettings(), this::featureTerrain), 8);
+        HydrologyTileCache perChunk = new HydrologyTileCache(new HydrologyPlanner(811L, featureSettings(), this::featureTerrain), 8);
+        HydrologyColumnSample plannedColumn = perColumn.get(new HydrologyTileKey(0, 0)).footprint().columns().values().stream()
+                .filter(HydrologyColumnSample::present)
+                .findFirst()
+                .orElseThrow();
+        int centreChunkX = Math.floorDiv(plannedColumn.x(), 16);
+        int centreChunkZ = Math.floorDiv(plannedColumn.z(), 16);
+        int present = 0;
+        for (int chunkZ = centreChunkZ - 2; chunkZ <= centreChunkZ + 2; chunkZ++) {
+            for (int chunkX = centreChunkX - 2; chunkX <= centreChunkX + 2; chunkX++) {
+                HydrologyColumnSample[] samples = perChunk.chunkSamples(chunkX * 16 + 5, chunkZ * 16 + 11);
+                assertEquals(256, samples.length);
+                for (int localZ = 0; localZ < 16; localZ++) {
+                    for (int localX = 0; localX < 16; localX++) {
+                        HydrologyColumnSample expected = perColumn
+                                .columnAt(chunkX * 16 + localX, chunkZ * 16 + localZ).orElse(null);
+                        assertEquals(expected, samples[localZ * 16 + localX]);
+                        present += expected == null ? 0 : 1;
+                    }
+                }
+            }
+        }
+        assertTrue(present > 0);
+    }
+
     @Test
     public void singleColumnContributionReusesItsImmutableSample() {
         HydrologyColumnSample sample = compositionSample(90, List.of(compositionLayer(11L, 80)));