Compacted biome selection

- Biome and carving children share compact rarity selection
- Weighted choices preserve alternating insertion order
- Rarity totals use long counts without expanded lists
- Natural child selection preserves its existing noise calls
- Differential tests cover normal and extreme rarity values

core/src/main/java/art/arcane/iris/engine/IrisComplex.java

@@ -37,6 +37,7 @@ import art.arcane.iris.engine.image.IrisImageMapRuntime;
 import art.arcane.iris.engine.mantle.components.MantleHydrologyCaveVoxelView;
 import art.arcane.iris.engine.object.InferredType;
 import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.object.IrisRaritySelection;
 import art.arcane.iris.engine.object.IrisDecorationPart;
 import art.arcane.iris.engine.object.IrisDecorator;
 import art.arcane.iris.engine.object.IrisGenerator;
@@ -1708,74 +1709,25 @@ public class IrisComplex implements DataProvider {
     }
 
     static final class ChildSelectionPlan {
-        private final IrisBiome[] mappedBiomes;
-        private final int maxIndex;
+        private final IrisRaritySelection<IrisBiome> selection;
 
-        private ChildSelectionPlan(IrisBiome[] mappedBiomes) {
-            this.mappedBiomes = mappedBiomes;
-            this.maxIndex = mappedBiomes.length - 1;
+        private ChildSelectionPlan(IrisRaritySelection<IrisBiome> selection) {
+            this.selection = selection;
         }
 
         static ChildSelectionPlan create(KList<IrisBiome> options) {
-            if (options.isEmpty()) {
-                return new ChildSelectionPlan(new IrisBiome[0]);
-            }
-
-            int maxRarity = 1;
-            for (IrisBiome biome : options) {
-                if (biome != null && biome.getRarity() > maxRarity) {
-                    maxRarity = biome.getRarity();
-                }
-            }
-
-            int rarityMax = maxRarity + 1;
-            boolean flip = false;
-            KList<IrisBiome> mapped = new KList<>();
-            for (IrisBiome biome : options) {
-                if (biome == null) {
-                    continue;
-                }
-
-                int rarity = Math.max(1, biome.getRarity());
-                int count = rarityMax - rarity;
-                for (int index = 0; index < count; index++) {
-                    flip = !flip;
-                    if (flip) {
-                        mapped.add(biome);
-                    } else {
-                        mapped.add(0, biome);
-                    }
-                }
-            }
-
-            if (mapped.isEmpty()) {
-                IrisBiome[] fallback = new IrisBiome[]{options.get(0)};
-                return new ChildSelectionPlan(fallback);
-            }
-
-            IrisBiome[] mappedBiomes = mapped.toArray(new IrisBiome[0]);
-            return new ChildSelectionPlan(mappedBiomes);
+            return new ChildSelectionPlan(IrisRaritySelection.create(options));
         }
 
         IrisBiome select(CNG childCell, double x, double z) {
-            if (mappedBiomes.length == 0) {
-                return null;
+            long maximumIndex = selection.size() - 1L;
+            if (maximumIndex <= 0L) {
+                return selection.get(0L);
             }
-
-            if (mappedBiomes.length == 1) {
-                return mappedBiomes[0];
+            if (maximumIndex <= Integer.MAX_VALUE) {
+                return selection.get(childCell.fit2D(0, (int) maximumIndex, x, z));
             }
-
-            int selectedIndex = childCell.fit2D(0, maxIndex, x, z);
-            if (selectedIndex < 0) {
-                return mappedBiomes[0];
-            }
-
-            if (selectedIndex > maxIndex) {
-                return mappedBiomes[maxIndex];
-            }
-
-            return mappedBiomes[selectedIndex];
+            return selection.select(childCell.noiseFast2D(x, z));
         }
     }
 

core/src/main/java/art/arcane/iris/engine/object/IrisDimensionCarvingResolver.java

@@ -2,7 +2,6 @@ package art.arcane.iris.engine.object;
 
 import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.framework.Engine;
-import art.arcane.iris.util.project.interpolation.IrisInterpolation;
 import art.arcane.volmlib.util.collection.KList;
 import art.arcane.iris.util.project.noise.CNG;
 
@@ -149,21 +148,20 @@ public final class IrisDimensionCarvingResolver {
             return parent;
         }
 
-        ParentSelectionPlan selectionPlan = state.selectionPlans.get(parent);
+        IrisRaritySelection<CarvingChoice> selectionPlan = state.selectionPlans.get(parent);
         if (selectionPlan == null) {
             selectionPlan = buildSelectionPlan(engine, parent, parentBiome, entryIndex, state);
             state.selectionPlans.put(parent, selectionPlan);
         }
 
-        if (selectionPlan.parentOnly) {
+        if (selectionPlan.size() <= 1L) {
             return parent;
         }
 
         long seed = resolveChildSeed(engine, state);
         CNG childGenerator = parent.getChildrenGenerator(seed, engine.getData());
         double sample = childGenerator.noiseFast2D(worldX, worldZ);
-        int selectedIndex = (int) Math.round(IrisInterpolation.lerp(0, selectionPlan.maxIndex, sample));
-        CarvingChoice selected = selectionPlan.get(selectedIndex);
+        CarvingChoice selected = selectionPlan.select(sample);
         if (selected == null || selected.entry == null) {
             return parent;
         }
@@ -171,7 +169,7 @@ public final class IrisDimensionCarvingResolver {
         return selected.entry;
     }
 
-    private static ParentSelectionPlan buildSelectionPlan(
+    private static IrisRaritySelection<CarvingChoice> buildSelectionPlan(
             Engine engine,
             IrisDimensionCarvingEntry parent,
             IrisBiome parentBiome,
@@ -201,42 +199,7 @@ public final class IrisDimensionCarvingResolver {
         }
 
         options.add(new CarvingChoice(parent, rarity(parentBiome)));
-        if (options.size() <= 1) {
-            return ParentSelectionPlan.parentOnly();
-        }
-
-        CarvingChoice[] mappedChoices = buildRarityMappedChoices(options);
-        if (mappedChoices.length == 0) {
-            return ParentSelectionPlan.parentOnly();
-        }
-
-        return new ParentSelectionPlan(mappedChoices);
-    }
-
-    private static CarvingChoice[] buildRarityMappedChoices(List<CarvingChoice> choices) {
-        int max = 1;
-        for (CarvingChoice choice : choices) {
-            if (choice.rarity > max) {
-                max = choice.rarity;
-            }
-        }
-
-        max++;
-        List<CarvingChoice> mapped = new ArrayList<>();
-        boolean flip = false;
-        for (CarvingChoice choice : choices) {
-            int count = max - choice.rarity;
-            for (int index = 0; index < count; index++) {
-                flip = !flip;
-                if (flip) {
-                    mapped.add(choice);
-                } else {
-                    mapped.add(0, choice);
-                }
-            }
-        }
-
-        return mapped.toArray(new CarvingChoice[0]);
+        return IrisRaritySelection.create(options);
     }
 
     private static int rarity(IrisBiome biome) {
@@ -285,7 +248,7 @@ public final class IrisDimensionCarvingResolver {
 
     public static final class State {
         private final Map<Integer, IrisDimensionCarvingEntry> rootEntriesByWorldY = new HashMap<>();
-        private final Map<IrisDimensionCarvingEntry, ParentSelectionPlan> selectionPlans = new IdentityHashMap<>();
+        private final Map<IrisDimensionCarvingEntry, IrisRaritySelection<CarvingChoice>> selectionPlans = new IdentityHashMap<>();
         private final Map<IrisDimensionCarvingEntry, IrisBiome> biomeCache = new IdentityHashMap<>();
         private WeakReference<Engine> engineIdentity;
         private WeakReference<IrisDimension> dimensionIdentity;
@@ -316,44 +279,6 @@ public final class IrisDimensionCarvingResolver {
         }
     }
 
-    private static final class ParentSelectionPlan {
-        private final CarvingChoice[] mappedChoices;
-        private final int maxIndex;
-        private final boolean parentOnly;
-
-        private ParentSelectionPlan(CarvingChoice[] mappedChoices) {
-            this.mappedChoices = mappedChoices;
-            this.maxIndex = mappedChoices.length - 1;
-            this.parentOnly = false;
-        }
-
-        private ParentSelectionPlan() {
-            this.mappedChoices = null;
-            this.maxIndex = -1;
-            this.parentOnly = true;
-        }
-
-        private static ParentSelectionPlan parentOnly() {
-            return new ParentSelectionPlan();
-        }
-
-        private CarvingChoice get(int index) {
-            if (mappedChoices == null || mappedChoices.length == 0) {
-                return null;
-            }
-
-            if (index < 0) {
-                return mappedChoices[0];
-            }
-
-            if (index >= mappedChoices.length) {
-                return mappedChoices[mappedChoices.length - 1];
-            }
-
-            return mappedChoices[index];
-        }
-    }
-
     private static final class CarvingChoice implements IRare {
         private final IrisDimensionCarvingEntry entry;
         private final int rarity;

core/src/main/java/art/arcane/iris/engine/object/IrisRaritySelection.java

@@ -0,0 +1,60 @@
+package art.arcane.iris.engine.object;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public final class IrisRaritySelection<T extends IRare> {
+    private final List<T> choices;
+    private final long[] cumulativeCounts;
+
+    private IrisRaritySelection(List<T> choices, long[] cumulativeCounts) {
+        this.choices = choices;
+        this.cumulativeCounts = cumulativeCounts;
+    }
+
+    public static <T extends IRare> IrisRaritySelection<T> create(List<T> options) {
+        ArrayList<T> choices = new ArrayList<>(options.size());
+        long maximumRarity = 1L;
+        for (T option : options) {
+            if (option != null) {
+                choices.add(option);
+                maximumRarity = Math.max(maximumRarity, option.getRarity());
+            }
+        }
+        long[] cumulativeCounts = new long[choices.size()];
+        long total = 0L;
+        for (int index = 0; index < choices.size(); index++) {
+            total += maximumRarity + 1L - Math.max(1, choices.get(index).getRarity());
+            cumulativeCounts[index] = total;
+        }
+        return new IrisRaritySelection<>(List.copyOf(choices), cumulativeCounts);
+    }
+
+    public long size() {
+        return cumulativeCounts.length == 0 ? 0L : cumulativeCounts[cumulativeCounts.length - 1];
+    }
+
+    public T select(double noiseValue) {
+        long maximumIndex = size() - 1L;
+        long selectedIndex = Math.round(noiseValue * maximumIndex);
+        return get(maximumIndex <= Integer.MAX_VALUE ? (int) selectedIndex : selectedIndex);
+    }
+
+    public T get(long index) {
+        long size = size();
+        if (size == 0L) {
+            return null;
+        }
+        if (choices.size() == 1) {
+            return choices.getFirst();
+        }
+        long selectedIndex = Math.max(0L, Math.min(size - 1L, index));
+        long frontSize = size / 2L;
+        long insertionIndex = selectedIndex < frontSize
+                ? 2L * (frontSize - selectedIndex) - 1L
+                : 2L * (selectedIndex - frontSize);
+        int choiceIndex = Arrays.binarySearch(cumulativeCounts, insertionIndex + 1L);
+        return choices.get(choiceIndex < 0 ? -choiceIndex - 1 : choiceIndex);
+    }
+}

core/src/test/java/art/arcane/iris/engine/IrisComplexImplodeParityTest.java

@@ -13,8 +13,12 @@ import java.util.List;
 
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertSame;
+import static org.mockito.ArgumentMatchers.anyDouble;
+import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
 
 public class IrisComplexImplodeParityTest {
     private static Method childSelectionCreateMethod;
@@ -48,6 +52,27 @@ public class IrisComplexImplodeParityTest {
         }
     }
 
+    @Test
+    public void extremeCountsKeepTheRareBiomeReachableAcrossTheIntIndexBoundary() {
+        IrisBiome rare = createBiome(Integer.MAX_VALUE);
+        IrisBiome common = createBiome(1);
+        CNG generator = mock(CNG.class);
+        doReturn(1_073_741_824).when(generator).fit2D(0, Integer.MAX_VALUE, 12D, -32D);
+        IrisComplex.ChildSelectionPlan pair = IrisComplex.ChildSelectionPlan.create(new KList<>(rare, common));
+
+        assertSame(rare, pair.select(generator, 12D, -32D));
+        verify(generator).fit2D(0, Integer.MAX_VALUE, 12D, -32D);
+        verify(generator, never()).noiseFast2D(anyDouble(), anyDouble());
+
+        CNG largeGenerator = mock(CNG.class);
+        doReturn(0.5D).when(largeGenerator).noiseFast2D(12D, -32D);
+        IrisComplex.ChildSelectionPlan large = IrisComplex.ChildSelectionPlan.create(
+                new KList<>(rare, common, createBiome(2)));
+        assertSame(rare, large.select(largeGenerator, 12D, -32D));
+        verify(largeGenerator).noiseFast2D(12D, -32D);
+        verify(largeGenerator, never()).fit2D(anyInt(), anyInt(), anyDouble(), anyDouble());
+    }
+
     @Test
     public void emptySelectionPlanMatchesLegacyEmptyBehavior() throws Exception {
         KList<IrisBiome> options = new KList<>();

core/src/test/java/art/arcane/iris/engine/object/IrisDimensionCarvingResolverParityTest.java

@@ -35,11 +35,16 @@ import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.Answers.CALLS_REAL_METHODS;
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyDouble;
 import static org.mockito.ArgumentMatchers.anyInt;
+import static org.mockito.ArgumentMatchers.anyLong;
 import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.verify;
 
 public class IrisDimensionCarvingResolverParityTest {
     private static final int MAX_CHILD_DEPTH = 32;
@@ -71,6 +76,31 @@ public class IrisDimensionCarvingResolverParityTest {
         return data;
     }
 
+    @Test
+    public void extremeChildRarityRemainsReachableWithOneNoiseSample() {
+        Fixture fixture = createFixture();
+        Engine engine = fixture.engine;
+        IrisData data = engine.getData();
+        IrisBiome rare = data.getBiomeLoader().load("child-a");
+        IrisBiome common = data.getBiomeLoader().load("child-b");
+        IrisBiome parentBiome = data.getBiomeLoader().load("root-low");
+        doReturn(Integer.MAX_VALUE).when(rare).getRarity();
+        doReturn(1).when(common).getRarity();
+        doReturn(2).when(parentBiome).getRarity();
+        IrisDimensionCarvingEntry parent = spy(engine.getDimension().getCarvingEntryIndex().get("root-low"));
+        parent.setChildRecursionDepth(1);
+        CNG generator = mock(CNG.class);
+        doReturn(0.5D).when(generator).noiseFast2D(-17D, 23D);
+        doReturn(generator).when(parent).getChildrenGenerator(anyLong(), any(IrisData.class));
+
+        IrisDimensionCarvingEntry selected = IrisDimensionCarvingResolver.resolveFromRoot(
+                engine, parent, -17, 23, new IrisDimensionCarvingResolver.State());
+
+        assertSame(engine.getDimension().getCarvingEntryIndex().get("child-a"), selected);
+        verify(generator).noiseFast2D(-17D, 23D);
+        verify(generator, never()).fit2D(anyInt(), anyInt(), anyDouble(), anyDouble());
+    }
+
     @Test
     public void resolverStatefulOverloadsMatchLegacyResolverAcrossSampleGrid() {
         Fixture fixture = createFixture();

core/src/test/java/art/arcane/iris/engine/object/IrisRaritySelectionTest.java

@@ -0,0 +1,141 @@
+package art.arcane.iris.engine.object;
+
+import com.google.gson.Gson;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Random;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+
+public class IrisRaritySelectionTest {
+    @Test
+    public void everyIndexMatchesExpandedInsertionOrderForRandomizedChoices() {
+        Random random = new Random(718938L);
+        for (int scenario = 0; scenario < 500; scenario++) {
+            ArrayList<Choice> choices = new ArrayList<>();
+            int count = random.nextInt(12);
+            for (int index = 0; index < count; index++) {
+                choices.add(random.nextInt(7) == 0 ? null : new Choice(index, random.nextInt(35) - 3));
+            }
+            List<Choice> expanded = expand(choices);
+            IrisRaritySelection<Choice> selection = IrisRaritySelection.create(choices);
+            assertEquals(expanded.size(), selection.size());
+            if (expanded.isEmpty()) {
+                assertNull(selection.get(0));
+                continue;
+            }
+            for (int index = 0; index < expanded.size(); index++) {
+                assertSame(expanded.get(index), selection.get(index));
+            }
+            assertSame(expanded.getFirst(), selection.get(Long.MIN_VALUE));
+            assertSame(expanded.getLast(), selection.get(Long.MAX_VALUE));
+            for (double sample : new double[]{-3D, -0D, 0.125D, 0.5D, 0.875D, 1D, 3D,
+                    Double.NaN, Double.NEGATIVE_INFINITY, Double.POSITIVE_INFINITY}) {
+                int index = (int) Math.round(sample * (expanded.size() - 1));
+                assertSame(expanded.get(Math.max(0, Math.min(expanded.size() - 1, index))), selection.select(sample));
+            }
+        }
+    }
+
+    @Test(timeout = 1000)
+    public void maximumRarityCountsStayPositiveWithoutExpandingBillionsOfEntries() {
+        Choice rare = new Choice(0, Integer.MAX_VALUE);
+        Choice common = new Choice(1, 1);
+        Choice next = new Choice(2, 2);
+        IrisRaritySelection<Choice> selection = IrisRaritySelection.create(List.of(rare, common, next));
+
+        assertEquals(4_294_967_294L, selection.size());
+        assertSame(next, selection.get(0L));
+        assertSame(common, selection.get(2_147_483_646L));
+        assertSame(rare, selection.get(2_147_483_647L));
+        assertSame(common, selection.get(2_147_483_648L));
+        assertSame(next, selection.get(selection.size() - 1L));
+        assertSame(rare, selection.select(0.5D));
+        assertSame(next, selection.select(0D));
+        assertSame(next, selection.select(1D));
+    }
+
+    @Test
+    public void equalMaximumRaritiesRetainEveryChoice() {
+        Choice first = new Choice(0, Integer.MAX_VALUE);
+        Choice second = new Choice(1, Integer.MAX_VALUE);
+        Choice third = new Choice(2, Integer.MAX_VALUE);
+        IrisRaritySelection<Choice> selection = IrisRaritySelection.create(List.of(first, second, third));
+
+        assertEquals(3L, selection.size());
+        assertSame(second, selection.get(0L));
+        assertSame(first, selection.get(1L));
+        assertSame(third, selection.get(2L));
+    }
+
+    @Test
+    public void duplicateChoicesKeepTheirSeparateInsertionPositions() {
+        Choice repeated = new Choice(0, 1);
+        Choice other = new Choice(1, 3);
+        List<Choice> choices = List.of(repeated, other, repeated);
+        IrisRaritySelection<Choice> selection = IrisRaritySelection.create(choices);
+        List<Choice> expanded = expand(choices);
+        for (int index = 0; index < expanded.size(); index++) {
+            assertSame(expanded.get(index), selection.get(index));
+        }
+    }
+
+    @Test
+    public void deserializedRuntimeRarityValuesUseTheExistingMinimumClamp() {
+        Gson gson = new Gson();
+        IrisBiome zero = gson.fromJson("{\"rarity\":0}", IrisBiome.class);
+        IrisBiome negative = gson.fromJson("{\"rarity\":-2147483648}", IrisBiome.class);
+        IrisBiome maximum = gson.fromJson("{\"rarity\":2147483647}", IrisBiome.class);
+        IrisRaritySelection<IrisBiome> selection = IrisRaritySelection.create(List.of(zero, negative, maximum));
+
+        assertEquals(4_294_967_295L, selection.size());
+        assertSame(zero, selection.get(2_147_483_647L));
+        assertSame(maximum, selection.get(selection.size() - 1L));
+        assertEquals(0, zero.getRarity());
+        assertEquals(Integer.MIN_VALUE, negative.getRarity());
+    }
+
+    @Test
+    public void nullAndEmptyOptionsDoNotProduceCandidates() {
+        assertNull(IrisRaritySelection.<Choice>create(List.of()).select(0.5D));
+        assertNull(IrisRaritySelection.<Choice>create(Arrays.asList(null, null)).get(0L));
+    }
+
+    private static List<Choice> expand(List<Choice> choices) {
+        int maximum = 1;
+        for (Choice choice : choices) {
+            if (choice != null) {
+                maximum = Math.max(maximum, choice.rarity());
+            }
+        }
+        ArrayList<Choice> expanded = new ArrayList<>();
+        boolean append = false;
+        for (Choice choice : choices) {
+            if (choice == null) {
+                continue;
+            }
+            int count = maximum + 1 - Math.max(1, choice.rarity());
+            for (int index = 0; index < count; index++) {
+                append = !append;
+                if (append) {
+                    expanded.add(choice);
+                } else {
+                    expanded.add(0, choice);
+                }
+            }
+        }
+        return expanded;
+    }
+
+    private record Choice(int id, int rarity) implements IRare {
+        @Override
+        public int getRarity() {
+            return rarity;
+        }
+    }
+}