Shared bounds biomes and paged erosion raster

- Bounds starcasts share whole-column base biome samples across threads through a heap-sized lock-free memo
- Erosion raster geometry tracks station owners in paged local arrays and builds its index map in first-visit order

core/src/main/java/art/arcane/iris/generation/hydrology/surface/ErosionFieldCompiler.java

@@ -18,6 +18,7 @@ import it.unimi.dsi.fastutil.longs.LongIterator;
 import it.unimi.dsi.fastutil.longs.LongArrayFIFOQueue;
 import it.unimi.dsi.fastutil.longs.LongOpenHashSet;
 
+import java.util.Arrays;
 import java.util.Objects;
 
 public final class ErosionFieldCompiler {
@@ -159,44 +160,7 @@ public final class ErosionFieldCompiler {
         IntArrayList stations = geometry.stations;
         DoubleArrayList distance = geometry.distance;
         if (!geometry.prepared) {
-            for (int station = 0; station < count; station++) {
+            rasterize(centerline, prepared.radii(), count, samplingBounds, indices, stations, distance);
-                int radius = prepared.radii()[station];
-                int stationX = centerline.x()[station];
-                int stationZ = centerline.z()[station];
-                if (samplingBounds != null && !samplingBounds.intersects(stationX, stationZ, radius)) {
-                    continue;
-                }
-                int minimumZ = samplingBounds == null ? -radius : Math.max(-radius, samplingBounds.minimumZ() - stationZ);
-                int maximumZ = samplingBounds == null ? radius : Math.min(radius, samplingBounds.maximumZ() - stationZ);
-                int minimumX = samplingBounds == null ? -radius : Math.max(-radius, samplingBounds.minimumX() - stationX);
-                int maximumX = samplingBounds == null ? radius : Math.min(radius, samplingBounds.maximumX() - stationX);
-                for (int deltaZ = minimumZ; deltaZ <= maximumZ; deltaZ++) {
-                    for (int deltaX = minimumX; deltaX <= maximumX; deltaX++) {
-                        int cellX = stationX + deltaX;
-                        int cellZ = stationZ + deltaZ;
-                        double cellDistance = centerline.distanceToSegment(station, cellX, cellZ, SurfaceDistanceTable.SHARED);
-                        if (cellDistance > radius) {
-                            continue;
-                        }
-                        long key = RiverFootprint.pack(cellX, cellZ);
-                        int index = indices.putIfAbsent(key, stations.size());
-                        if (index < 0) {
-                            stations.add(station);
-                            distance.add(cellDistance);
-                            continue;
-                        }
-                        int existing = stations.getInt(index);
-                        double existingDistance = distance.getDouble(index);
-                        // A cell on a station's cross-section row ties between the two segments meeting
-                        // there; the later station owns it so the row uses the head its own ring bounded.
-                        if (cellDistance < existingDistance - EPSILON
-                                || Math.abs(cellDistance - existingDistance) <= EPSILON && station > existing) {
-                            stations.set(index, station);
-                            distance.set(index, cellDistance);
-                        }
-                    }
-                }
-            }
             geometry.prepared = true;
         }
         int oceanStart = oceanStart(centerline, valley, terminal);
@@ -912,12 +876,90 @@ public final class ErosionFieldCompiler {
         return 1D - t * t * (3D - 2D * t);
     }
 
+    static void rasterize(SurfaceCenterline centerline, int[] radii, int count, SurfaceBounds samplingBounds,
+                          Long2IntOpenHashMap indices, IntArrayList stations, DoubleArrayList distance) {
+        CellPages pages = new CellPages();
+        LongArrayList order = new LongArrayList();
+        for (int station = 0; station < count; station++) {
+            int radius = radii[station];
+            int stationX = centerline.x()[station];
+            int stationZ = centerline.z()[station];
+            if (samplingBounds != null && !samplingBounds.intersects(stationX, stationZ, radius)) {
+                continue;
+            }
+            int minimumZ = samplingBounds == null ? -radius : Math.max(-radius, samplingBounds.minimumZ() - stationZ);
+            int maximumZ = samplingBounds == null ? radius : Math.min(radius, samplingBounds.maximumZ() - stationZ);
+            int minimumX = samplingBounds == null ? -radius : Math.max(-radius, samplingBounds.minimumX() - stationX);
+            int maximumX = samplingBounds == null ? radius : Math.min(radius, samplingBounds.maximumX() - stationX);
+            for (int deltaZ = minimumZ; deltaZ <= maximumZ; deltaZ++) {
+                for (int deltaX = minimumX; deltaX <= maximumX; deltaX++) {
+                    int cellX = stationX + deltaX;
+                    int cellZ = stationZ + deltaZ;
+                    double cellDistance = centerline.distanceToSegment(station, cellX, cellZ, SurfaceDistanceTable.SHARED);
+                    if (cellDistance > radius) {
+                        continue;
+                    }
+                    int[] page = pages.page(cellX, cellZ);
+                    int offset = CellPages.offset(cellX, cellZ);
+                    int index = page[offset];
+                    if (index < 0) {
+                        page[offset] = stations.size();
+                        stations.add(station);
+                        distance.add(cellDistance);
+                        order.add(RiverFootprint.pack(cellX, cellZ));
+                        continue;
+                    }
+                    int existing = stations.getInt(index);
+                    double existingDistance = distance.getDouble(index);
+                    // A cell on a station's cross-section row ties between the two segments meeting
+                    // there; the later station owns it so the row uses the head its own ring bounded.
+                    if (cellDistance < existingDistance - EPSILON
+                            || Math.abs(cellDistance - existingDistance) <= EPSILON && station > existing) {
+                        stations.set(index, station);
+                        distance.set(index, cellDistance);
+                    }
+                }
+            }
+        }
+        for (int index = 0; index < order.size(); index++) {
+            indices.put(order.getLong(index), index);
+        }
+    }
+
     private static double side(SurfaceCenterline centerline, int station, int cellX, int cellZ) {
         double deltaX = cellX - centerline.x()[station];
         double deltaZ = cellZ - centerline.z()[station];
         return deltaX * centerline.normalX(station) + deltaZ * centerline.normalZ(station);
     }
 
+    private static final class CellPages {
+        private static final int SHIFT = 6;
+        private static final int MASK = (1 << SHIFT) - 1;
+        private final Long2ObjectOpenHashMap<int[]> pages = new Long2ObjectOpenHashMap<>();
+        private long currentKey;
+        private int[] current;
+
+        private int[] page(int x, int z) {
+            long key = RiverFootprint.pack(x >> SHIFT, z >> SHIFT);
+            if (current != null && key == currentKey) {
+                return current;
+            }
+            int[] page = pages.get(key);
+            if (page == null) {
+                page = new int[1 << (SHIFT * 2)];
+                Arrays.fill(page, -1);
+                pages.put(key, page);
+            }
+            current = page;
+            currentKey = key;
+            return page;
+        }
+
+        private static int offset(int x, int z) {
+            return ((z & MASK) << SHIFT) | (x & MASK);
+        }
+    }
+
     private static final class RasterGeometry {
         private final Long2IntOpenHashMap indices = new Long2IntOpenHashMap();
         private final IntArrayList stations = new IntArrayList();

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

@@ -128,8 +128,8 @@ import java.nio.file.Path;
 import java.util.function.BiFunction;
 
 @Data
-@EqualsAndHashCode(exclude = {"data", "gridBoundsCache", "sharedCornerBounds", "terrainProgram", "bakeCache", "regionSource", "baseBiomeSource", "caveSource", "rockSource", "fluidSource", "childStyleSeed", "inferredByType", "naturalHeightSource", "naturalTrueBiomeSource", "trueBiomeSource", "frozenInterpolators", "frozenGenerators", "inferredBiomeStreams", "hydrologyRuntime", "imageMapRuntime", "terrainEngine", "terrain3D", "hydrologyBanks3D"})
+@EqualsAndHashCode(exclude = {"data", "gridBoundsCache", "sharedCornerBounds", "sharedBoundsBiomes", "terrainProgram", "bakeCache", "regionSource", "baseBiomeSource", "caveSource", "rockSource", "fluidSource", "childStyleSeed", "inferredByType", "naturalHeightSource", "naturalTrueBiomeSource", "trueBiomeSource", "frozenInterpolators", "frozenGenerators", "inferredBiomeStreams", "hydrologyRuntime", "imageMapRuntime", "terrainEngine", "terrain3D", "hydrologyBanks3D"})
-@ToString(exclude = {"data", "gridBoundsCache", "sharedCornerBounds", "terrainProgram", "bakeCache", "regionSource", "baseBiomeSource", "caveSource", "rockSource", "fluidSource", "childStyleSeed", "inferredByType", "naturalHeightSource", "naturalTrueBiomeSource", "trueBiomeSource", "frozenInterpolators", "frozenGenerators", "inferredBiomeStreams", "hydrologyRuntime", "imageMapRuntime", "terrainEngine", "terrain3D", "hydrologyBanks3D"})
+@ToString(exclude = {"data", "gridBoundsCache", "sharedCornerBounds", "sharedBoundsBiomes", "terrainProgram", "bakeCache", "regionSource", "baseBiomeSource", "caveSource", "rockSource", "fluidSource", "childStyleSeed", "inferredByType", "naturalHeightSource", "naturalTrueBiomeSource", "trueBiomeSource", "frozenInterpolators", "frozenGenerators", "inferredBiomeStreams", "hydrologyRuntime", "imageMapRuntime", "terrainEngine", "terrain3D", "hydrologyBanks3D"})
 public class IrisComplex implements DataProvider {
     private static final NoiseBounds ZERO_NOISE_BOUNDS = new NoiseBounds(0D, 0D);
     private static final AtomicLong lastBoundsFailureLog = new AtomicLong(0L);
@@ -163,6 +163,7 @@ public class IrisComplex implements DataProvider {
     @Getter(AccessLevel.NONE)
     private final int biomeBoundsSamplingStep;
     private transient volatile SharedCornerBounds sharedCornerBounds;
+    private transient volatile SharedBoundsBiomes sharedBoundsBiomes;
     @Getter(AccessLevel.NONE)
     @Setter(AccessLevel.NONE)
     private transient volatile TerrainProgram terrainProgram;
@@ -2054,6 +2055,21 @@ public class IrisComplex implements DataProvider {
         return shared;
     }
 
+    private SharedBoundsBiomes sharedBoundsBiomes(ProceduralStream<IrisBiome> source) {
+        SharedBoundsBiomes shared = sharedBoundsBiomes;
+        if (shared != null && shared.source() == source) {
+            return shared;
+        }
+        synchronized (this) {
+            shared = sharedBoundsBiomes;
+            if (shared == null || shared.source() != source) {
+                shared = new SharedBoundsBiomes(source, SharedBoundsBiomes.capacityFor(Runtime.getRuntime().maxMemory()));
+                sharedBoundsBiomes = shared;
+            }
+            return shared;
+        }
+    }
+
     private long computePackedBounds(GridBoundsCache cache, Engine engine, IrisInterpolator interpolator, IrisGenerator[] generators, int gx, int gz) {
         NoiseBounds bounds = sampleBoundsRaw(cache, engine, interpolator, generators, gx, gz);
         return (((long) Float.floatToRawIntBits((float) bounds.min())) << 32) | (Float.floatToRawIntBits((float) bounds.max()) & 0xFFFFFFFFL);
@@ -2688,6 +2704,7 @@ public class IrisComplex implements DataProvider {
         private Engine engine;
         private IrisGenerator[] generators;
         private IdentityHashMap<IrisBiome, GeneratorBounds> cachedBounds;
+        private SharedBoundsBiomes sharedBiomes;
         private boolean integerCoordinates;
         private boolean inUse;
 
@@ -2702,6 +2719,7 @@ public class IrisComplex implements DataProvider {
             this.cachedBounds = cachedBounds;
             this.integerCoordinates = complex.baseBiomeStream instanceof BakedStream<?>
                     || complex.baseBiomeStream instanceof CachedStream2D<?>;
+            this.sharedBiomes = integerCoordinates ? complex.sharedBoundsBiomes(complex.baseBiomeStream) : null;
             this.inUse = true;
             if (sampleSource.get() != complex.baseBiomeStream) {
                 sampleCache.clear();
@@ -2718,6 +2736,7 @@ public class IrisComplex implements DataProvider {
             engine = null;
             generators = null;
             cachedBounds = null;
+            sharedBiomes = null;
             inUse = false;
         }
 
@@ -2728,11 +2747,17 @@ public class IrisComplex implements DataProvider {
                 double sampleZ = integerCoordinates ? (int) zz : zz;
                 IrisBiome bx = sampleCache.get(sampleX, sampleZ);
                 if (bx == null) {
-                    ChunkBakeCache.enterSparse();
+                    bx = sharedBiomes == null ? null : sharedBiomes.get((int) sampleX, (int) sampleZ);
-                    try {
+                    if (bx == null) {
-                        bx = complex.baseBiomeStream.get(xx, zz);
+                        ChunkBakeCache.enterSparse();
-                    } finally {
+                        try {
-                        ChunkBakeCache.exitSparse();
+                            bx = complex.baseBiomeStream.get(xx, zz);
+                        } finally {
+                            ChunkBakeCache.exitSparse();
+                        }
+                        if (sharedBiomes != null) {
+                            sharedBiomes.put((int) sampleX, (int) sampleZ, bx);
+                        }
                     }
                     sampleCache.put(sampleX, sampleZ, bx);
                 }

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

@@ -0,0 +1,86 @@
+package art.arcane.iris.generation.runtime;
+
+import art.arcane.iris.generation.biome.IrisBiome;
+
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.VarHandle;
+import java.util.Objects;
+
+final class SharedBoundsBiomes {
+    static final int MINIMUM_CAPACITY = 1 << 16;
+    static final int MAXIMUM_CAPACITY = 1 << 21;
+    static final long BYTES_PER_ENTRY = 16L;
+    static final long HEAP_SHARE = 256L;
+    private static final VarHandle INTS = MethodHandles.arrayElementVarHandle(int[].class);
+    private static final VarHandle LONGS = MethodHandles.arrayElementVarHandle(long[].class);
+    private static final VarHandle OBJECTS = MethodHandles.arrayElementVarHandle(Object[].class);
+
+    private final Object source;
+    private final long[] keys;
+    private final Object[] values;
+    private final int[] stamps;
+    private final int mask;
+
+    SharedBoundsBiomes(Object source, int capacity) {
+        if (capacity < 2 || Integer.bitCount(capacity) != 1) {
+            throw new IllegalArgumentException("capacity must be a power of two of at least 2");
+        }
+        this.source = Objects.requireNonNull(source, "source");
+        this.keys = new long[capacity];
+        this.values = new Object[capacity];
+        this.stamps = new int[capacity];
+        this.mask = capacity - 1;
+    }
+
+    static int capacityFor(long maxMemory) {
+        long entries = Math.max(0L, maxMemory) / HEAP_SHARE / BYTES_PER_ENTRY;
+        long bounded = Math.max(MINIMUM_CAPACITY, Math.min(MAXIMUM_CAPACITY, entries));
+        return Integer.highestOneBit((int) bounded);
+    }
+
+    Object source() {
+        return source;
+    }
+
+    int slot(int x, int z) {
+        long mixed = key(x, z);
+        mixed = (mixed ^ (mixed >>> 30)) * 0xBF58476D1CE4E5B9L;
+        mixed = (mixed ^ (mixed >>> 27)) * 0x94D049BB133111EBL;
+        return (int) ((mixed ^ (mixed >>> 31)) & mask);
+    }
+
+    IrisBiome get(int x, int z) {
+        long key = key(x, z);
+        int slot = slot(x, z);
+        int before = (int) INTS.getAcquire(stamps, slot);
+        if (before == 0 || (before & 1) != 0) {
+            return null;
+        }
+        long storedKey = (long) LONGS.getAcquire(keys, slot);
+        Object value = OBJECTS.getAcquire(values, slot);
+        VarHandle.loadLoadFence();
+        int after = (int) INTS.getAcquire(stamps, slot);
+        if (before != after || storedKey != key) {
+            return null;
+        }
+        return (IrisBiome) value;
+    }
+
+    void put(int x, int z, IrisBiome biome) {
+        if (biome == null) {
+            return;
+        }
+        int slot = slot(x, z);
+        int stamp = (int) INTS.getAcquire(stamps, slot);
+        if ((stamp & 1) != 0 || !INTS.compareAndSet(stamps, slot, stamp, stamp + 1)) {
+            return;
+        }
+        LONGS.setRelease(keys, slot, key(x, z));
+        OBJECTS.setRelease(values, slot, biome);
+        INTS.setRelease(stamps, slot, stamp + 2);
+    }
+
+    private static long key(int x, int z) {
+        return ((long) x << 32) | (z & 0xFFFFFFFFL);
+    }
+}

core/src/test/java/art/arcane/iris/generation/hydrology/surface/ErosionRasterGeometryTest.java

@@ -0,0 +1,128 @@
+package art.arcane.iris.generation.hydrology.surface;
+
+import art.arcane.iris.generation.hydrology.HydrologyPoint;
+import art.arcane.iris.generation.hydrology.RiverFootprint;
+import it.unimi.dsi.fastutil.doubles.DoubleArrayList;
+import it.unimi.dsi.fastutil.ints.IntArrayList;
+import it.unimi.dsi.fastutil.longs.Long2IntOpenHashMap;
+import it.unimi.dsi.fastutil.longs.LongIterator;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.SplittableRandom;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class ErosionRasterGeometryTest {
+    private static final double EPSILON = 1.0E-9D;
+
+    @Test
+    public void rasterMatchesTheReferenceOwnersDistancesAndIterationOrder() {
+        SplittableRandom random = new SplittableRandom(0x5EED_E05EL);
+        int compared = 0;
+        for (int trial = 0; trial < 160; trial++) {
+            SurfaceCenterline centerline = randomCenterline(random);
+            int count = Math.max(1, centerline.size() - random.nextInt(Math.min(4, centerline.size())));
+            int[] radii = new int[count];
+            for (int station = 0; station < count; station++) {
+                radii[station] = 1 + random.nextInt(trial % 3 == 0 ? 48 : 12);
+            }
+            SurfaceBounds bounds = trial % 4 == 0 ? null : randomBounds(random, centerline);
+            Long2IntOpenHashMap expectedIndices = new Long2IntOpenHashMap();
+            expectedIndices.defaultReturnValue(-1);
+            IntArrayList expectedStations = new IntArrayList();
+            DoubleArrayList expectedDistances = new DoubleArrayList();
+            reference(centerline, radii, count, bounds, expectedIndices, expectedStations, expectedDistances);
+            Long2IntOpenHashMap actualIndices = new Long2IntOpenHashMap();
+            actualIndices.defaultReturnValue(-1);
+            IntArrayList actualStations = new IntArrayList();
+            DoubleArrayList actualDistances = new DoubleArrayList();
+            ErosionFieldCompiler.rasterize(centerline, radii, count, bounds, actualIndices, actualStations, actualDistances);
+
+            assertEquals(expectedIndices.size(), actualIndices.size());
+            LongIterator expectedKeys = expectedIndices.keySet().iterator();
+            LongIterator actualKeys = actualIndices.keySet().iterator();
+            while (expectedKeys.hasNext()) {
+                long expectedKey = expectedKeys.nextLong();
+                long actualKey = actualKeys.nextLong();
+                assertEquals(expectedKey, actualKey);
+                int expectedIndex = expectedIndices.get(expectedKey);
+                int actualIndex = actualIndices.get(actualKey);
+                assertEquals(expectedIndex, actualIndex);
+                assertEquals(expectedStations.getInt(expectedIndex), actualStations.getInt(actualIndex));
+                assertEquals(Double.doubleToLongBits(expectedDistances.getDouble(expectedIndex)),
+                        Double.doubleToLongBits(actualDistances.getDouble(actualIndex)));
+                compared++;
+            }
+            assertEquals(expectedStations.size(), actualStations.size());
+        }
+        assertTrue(compared > 100_000);
+    }
+
+    private static SurfaceCenterline randomCenterline(SplittableRandom random) {
+        int originX = random.nextInt(-40_000, 40_000);
+        int originZ = random.nextInt(-40_000, 40_000);
+        int points = 2 + random.nextInt(6);
+        List<HydrologyPoint> path = new ArrayList<>(points);
+        int x = originX;
+        int z = originZ;
+        for (int point = 0; point < points; point++) {
+            path.add(new HydrologyPoint(x, 64, z));
+            x += random.nextInt(-90, 91);
+            z += random.nextInt(-90, 91);
+        }
+        return SurfaceCenterline.densify(path);
+    }
+
+    private static SurfaceBounds randomBounds(SplittableRandom random, SurfaceCenterline centerline) {
+        int station = random.nextInt(centerline.size());
+        int centerX = centerline.x()[station];
+        int centerZ = centerline.z()[station];
+        int halfWidth = 4 + random.nextInt(60);
+        int halfDepth = 4 + random.nextInt(60);
+        return new SurfaceBounds(centerX - halfWidth, centerZ - halfDepth, centerX + halfWidth, centerZ + halfDepth)
+                .expand(2);
+    }
+
+    private static void reference(SurfaceCenterline centerline, int[] radii, int count, SurfaceBounds samplingBounds,
+                                  Long2IntOpenHashMap indices, IntArrayList stations, DoubleArrayList distance) {
+        for (int station = 0; station < count; station++) {
+            int radius = radii[station];
+            int stationX = centerline.x()[station];
+            int stationZ = centerline.z()[station];
+            if (samplingBounds != null && !samplingBounds.intersects(stationX, stationZ, radius)) {
+                continue;
+            }
+            int minimumZ = samplingBounds == null ? -radius : Math.max(-radius, samplingBounds.minimumZ() - stationZ);
+            int maximumZ = samplingBounds == null ? radius : Math.min(radius, samplingBounds.maximumZ() - stationZ);
+            int minimumX = samplingBounds == null ? -radius : Math.max(-radius, samplingBounds.minimumX() - stationX);
+            int maximumX = samplingBounds == null ? radius : Math.min(radius, samplingBounds.maximumX() - stationX);
+            for (int deltaZ = minimumZ; deltaZ <= maximumZ; deltaZ++) {
+                for (int deltaX = minimumX; deltaX <= maximumX; deltaX++) {
+                    int cellX = stationX + deltaX;
+                    int cellZ = stationZ + deltaZ;
+                    double cellDistance = centerline.distanceToSegment(station, cellX, cellZ, SurfaceDistanceTable.SHARED);
+                    if (cellDistance > radius) {
+                        continue;
+                    }
+                    long key = RiverFootprint.pack(cellX, cellZ);
+                    int index = indices.putIfAbsent(key, stations.size());
+                    if (index < 0) {
+                        stations.add(station);
+                        distance.add(cellDistance);
+                        continue;
+                    }
+                    int existing = stations.getInt(index);
+                    double existingDistance = distance.getDouble(index);
+                    if (cellDistance < existingDistance - EPSILON
+                            || Math.abs(cellDistance - existingDistance) <= EPSILON && station > existing) {
+                        stations.set(index, station);
+                        distance.set(index, cellDistance);
+                    }
+                }
+            }
+        }
+    }
+}

core/src/test/java/art/arcane/iris/generation/runtime/IrisComplexBoundsBiomeCacheTest.java

@@ -71,6 +71,66 @@ public class IrisComplexBoundsBiomeCacheTest {
         }
     }
 
+    @Test
+    public void wholeColumnSamplesAreSharedAcrossThreadMemos() throws Exception {
+        IrisBiome first = new IrisBiome();
+        IrisBiome second = new IrisBiome();
+        ProceduralStream<IrisBiome> source = ProceduralStream.of(
+                (x, z) -> x.intValue() < 0 || z.intValue() < 0 ? first : second,
+                Interpolated.of(value -> 0D, value -> first));
+        try (MockedStatic<IrisServices> services = mockStatic(IrisServices.class)) {
+            services.when(() -> IrisServices.get(PreservationRegistry.class))
+                    .thenReturn(mock(PreservationRegistry.class));
+            CachedStream2D<IrisBiome> stream = spy(new CachedStream2D<>("bounds", mock(Engine.class), source, 16));
+            Fixture fixture = new Fixture(stream);
+            IrisInterpolator interpolator = new IrisInterpolator()
+                    .setFunction(InterpolationMethod.STARCAST_12).setHorizontalScale(1.25D);
+            fixture.setBounds(interpolator, first, -15.25D, 63.5D);
+            fixture.setBounds(interpolator, second, 7.75D, 127.25D);
+            Object otherThread = fixture.newGridCache();
+            for (int z = -2; z <= 2; z++) {
+                for (int x = -2; x <= 2; x++) {
+                    fixture.sample(interpolator, x, z);
+                }
+            }
+            long firstReads = streamReads(stream);
+            for (int z = -2; z <= 2; z++) {
+                for (int x = -2; x <= 2; x++) {
+                    NoiseBounds expected = interpolator.interpolateBounds(x, z, (xx, zz) ->
+                            (int) xx < 0 || (int) zz < 0
+                                    ? new NoiseBounds(-15.25D, 63.5D) : new NoiseBounds(7.75D, 127.25D));
+                    assertBits(expected, fixture.sample(otherThread, interpolator, x, z));
+                }
+            }
+            assertTrue(firstReads > 0L);
+            assertEquals(firstReads, streamReads(stream));
+        }
+    }
+
+    @Test
+    public void replacingAWholeColumnStreamDropsSharedSamples() throws Exception {
+        IrisBiome first = new IrisBiome();
+        IrisBiome second = new IrisBiome();
+        try (MockedStatic<IrisServices> services = mockStatic(IrisServices.class)) {
+            services.when(() -> IrisServices.get(PreservationRegistry.class))
+                    .thenReturn(mock(PreservationRegistry.class));
+            Fixture fixture = new Fixture(new CachedStream2D<>("bounds", mock(Engine.class), constant(first), 16));
+            IrisInterpolator interpolator = new IrisInterpolator();
+            fixture.setBounds(interpolator, first, -10D, 20D);
+            fixture.setBounds(interpolator, second, 50D, 80D);
+
+            assertBits(new NoiseBounds(-10D, 20D), fixture.sample(interpolator, 0, 0));
+            fixture.complex.setBaseBiomeStream(new CachedStream2D<>("bounds", mock(Engine.class), constant(second), 16));
+            assertBits(new NoiseBounds(50D, 80D), fixture.sample(fixture.newGridCache(), interpolator, 0, 0));
+        }
+    }
+
+    private static long streamReads(CachedStream2D<IrisBiome> stream) {
+        return mockingDetails(stream).getInvocations().stream()
+                .filter(invocation -> invocation.getMethod().getName().equals("get"))
+                .count();
+    }
+
     @Test
     public void uncachedBiomeStreamsPreserveFractionalSamplesWithinOneColumn() throws Exception {
         IrisBiome first = new IrisBiome();
@@ -248,7 +308,19 @@ public class IrisComplexBoundsBiomeCacheTest {
         }
 
         private NoiseBounds sample(IrisInterpolator interpolator, double x, double z) throws ReflectiveOperationException {
-            return (NoiseBounds) sample.invoke(complex, cache, null, interpolator, new IrisGenerator[0], x, z);
+            return sample(cache, interpolator, x, z);
+        }
+
+        private NoiseBounds sample(Object gridCache, IrisInterpolator interpolator, double x, double z)
+                throws ReflectiveOperationException {
+            return (NoiseBounds) sample.invoke(complex, gridCache, null, interpolator, new IrisGenerator[0], x, z);
+        }
+
+        private Object newGridCache() throws ReflectiveOperationException {
+            Class<?> cacheClass = Class.forName("art.arcane.iris.generation.runtime.IrisComplex$GridBoundsCache");
+            Constructor<?> constructor = cacheClass.getDeclaredConstructor();
+            constructor.setAccessible(true);
+            return constructor.newInstance();
         }
     }
 }

core/src/test/java/art/arcane/iris/generation/runtime/SharedBoundsBiomesTest.java

@@ -0,0 +1,169 @@
+package art.arcane.iris.generation.runtime;
+
+import art.arcane.iris.generation.biome.IrisBiome;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+
+public class SharedBoundsBiomesTest {
+    private static final Object SOURCE = new Object();
+
+    @Test
+    public void storedColumnsComeBackAndUnknownColumnsMiss() {
+        SharedBoundsBiomes cache = new SharedBoundsBiomes(SOURCE, 1 << 10);
+        IrisBiome biome = new IrisBiome();
+
+        assertNull(cache.get(12, -40));
+        cache.put(12, -40, biome);
+
+        assertSame(biome, cache.get(12, -40));
+        assertNull(cache.get(-40, 12));
+        assertSame(SOURCE, cache.source());
+    }
+
+    @Test
+    public void originMissesUntilItIsStored() {
+        SharedBoundsBiomes cache = new SharedBoundsBiomes(SOURCE, 2);
+        IrisBiome biome = new IrisBiome();
+
+        assertNull(cache.get(0, 0));
+        cache.put(0, 0, biome);
+        assertSame(biome, cache.get(0, 0));
+    }
+
+    @Test
+    public void nullBiomesAreNeverStored() {
+        SharedBoundsBiomes cache = new SharedBoundsBiomes(SOURCE, 2);
+        IrisBiome biome = new IrisBiome();
+        cache.put(3, 4, biome);
+        cache.put(3, 4, null);
+
+        assertSame(biome, cache.get(3, 4));
+    }
+
+    @Test
+    public void fullCoordinateDomainKeepsDistinctColumns() {
+        int[] coordinates = {0, 1, -1, 1 << 21, -(1 << 21), 30_000_000, -30_000_000,
+                Integer.MIN_VALUE, Integer.MAX_VALUE};
+        IrisBiome biome = new IrisBiome();
+        for (int x : coordinates) {
+            for (int z : coordinates) {
+                SharedBoundsBiomes cache = new SharedBoundsBiomes(SOURCE, 2);
+                cache.put(x, z, biome);
+                for (int otherX : coordinates) {
+                    for (int otherZ : coordinates) {
+                        if (x == otherX && z == otherZ) {
+                            assertSame(biome, cache.get(otherX, otherZ));
+                        } else {
+                            assertNull(cache.get(otherX, otherZ));
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    public void aCollidingColumnReplacesTheOldOne() {
+        SharedBoundsBiomes cache = new SharedBoundsBiomes(SOURCE, 2);
+        int colliding = 0;
+        for (int x = 1; x < 100_000; x++) {
+            if (cache.slot(x, 0) == cache.slot(0, 0)) {
+                colliding = x;
+                break;
+            }
+        }
+        assertNotEquals(0, colliding);
+        IrisBiome first = new IrisBiome();
+        IrisBiome second = new IrisBiome();
+
+        cache.put(0, 0, first);
+        cache.put(colliding, 0, second);
+
+        assertNull(cache.get(0, 0));
+        assertSame(second, cache.get(colliding, 0));
+    }
+
+    @Test
+    public void capacityFollowsTheHeapWithinItsBounds() {
+        long megabyte = 1024L * 1024L;
+        assertEquals(SharedBoundsBiomes.MINIMUM_CAPACITY, SharedBoundsBiomes.capacityFor(0L));
+        assertEquals(SharedBoundsBiomes.MINIMUM_CAPACITY, SharedBoundsBiomes.capacityFor(256L * megabyte));
+        assertEquals(1 << 20, SharedBoundsBiomes.capacityFor(6L * 1024L * megabyte));
+        assertEquals(SharedBoundsBiomes.MAXIMUM_CAPACITY, SharedBoundsBiomes.capacityFor(64L * 1024L * megabyte));
+        long previous = 0L;
+        for (long heap = 64L * megabyte; heap <= 128L * 1024L * megabyte; heap *= 2L) {
+            int capacity = SharedBoundsBiomes.capacityFor(heap);
+            assertEquals(1, Integer.bitCount(capacity));
+            assertTrue(capacity >= previous);
+            assertTrue((long) capacity * SharedBoundsBiomes.BYTES_PER_ENTRY <= Math.max(heap / SharedBoundsBiomes.HEAP_SHARE,
+                    (long) SharedBoundsBiomes.MINIMUM_CAPACITY * SharedBoundsBiomes.BYTES_PER_ENTRY));
+            previous = capacity;
+        }
+    }
+
+    @Test
+    public void concurrentReadersNeverSeeAnotherColumnsBiome() throws InterruptedException {
+        SharedBoundsBiomes cache = new SharedBoundsBiomes(SOURCE, 8);
+        int[] coordinates = {0, 1 << 21, -(1 << 21), 30_000_000, -30_000_000,
+                Integer.MIN_VALUE, Integer.MAX_VALUE};
+        IrisBiome[][] biomes = new IrisBiome[coordinates.length][coordinates.length];
+        for (int x = 0; x < coordinates.length; x++) {
+            for (int z = 0; z < coordinates.length; z++) {
+                biomes[x][z] = new IrisBiome();
+            }
+        }
+        AtomicBoolean torn = new AtomicBoolean();
+        AtomicInteger hits = new AtomicInteger();
+        CountDownLatch start = new CountDownLatch(1);
+        List<Thread> threads = new ArrayList<>();
+        for (int worker = 0; worker < 4; worker++) {
+            int seed = worker;
+            threads.add(new Thread(() -> {
+                try {
+                    start.await();
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                    torn.set(true);
+                    return;
+                }
+                long state = 0x9E3779B97F4A7C15L * (seed + 1);
+                for (int step = 0; step < 200_000; step++) {
+                    state ^= state << 13;
+                    state ^= state >>> 7;
+                    state ^= state << 17;
+                    int xi = (int) ((state & Long.MAX_VALUE) % coordinates.length);
+                    int zi = (int) ((state >>> 10) % coordinates.length);
+                    cache.put(coordinates[xi], coordinates[zi], biomes[xi][zi]);
+                    IrisBiome actual = cache.get(coordinates[xi], coordinates[zi]);
+                    if (actual != null) {
+                        hits.incrementAndGet();
+                        if (actual != biomes[xi][zi]) {
+                            torn.set(true);
+                        }
+                    }
+                }
+            }));
+        }
+        for (Thread thread : threads) {
+            thread.start();
+        }
+        start.countDown();
+        for (Thread thread : threads) {
+            thread.join();
+        }
+        assertTrue(hits.get() > 0);
+        assertFalse(torn.get());
+    }
+}