Implement tiny biome distribution

src/main/java/net/dnorberg/rivencraft/Rivencraft.java

@@ -7,6 +7,7 @@ import net.dnorberg.rivencraft.registry.ModItems;
 import net.dnorberg.rivencraft.registry.ModSounds;
 import net.dnorberg.rivencraft.registry.ModBlocks;
 import net.dnorberg.rivencraft.registry.ModBlockEntities;
+import net.dnorberg.rivencraft.registry.ModBiomeSources;
 import net.dnorberg.rivencraft.event.BookStandConversionHandler;
 import net.dnorberg.rivencraft.network.ModNetworking;
 import net.dnorberg.rivencraft.config.RivencraftConfig;
@@ -31,6 +32,7 @@ public final class Rivencraft {
         modContainer.registerConfig(ModConfig.Type.CLIENT, RivencraftClientConfig.SPEC);
         modEventBus.addListener(ModNetworking::register);
         ModDataComponents.register(modEventBus);
+        ModBiomeSources.register(modEventBus);
         ModBlocks.register(modEventBus);
         ModBlockEntities.register(modEventBus);
         ModItems.register(modEventBus);

src/main/java/net/dnorberg/rivencraft/book/AgeDefinition.java

@@ -21,7 +21,7 @@ public record AgeDefinition(
     public static final ResourceLocation RANDOM_VANILLA_PROFILE = ResourceLocation.fromNamespaceAndPath(
             Rivencraft.MOD_ID, "random_vanilla"
     );
-    public static final int CURRENT_GENERATION_VERSION = 11;
+    public static final int CURRENT_GENERATION_VERSION = 12;
 
     public AgeDefinition {
         Objects.requireNonNull(id, "id");

src/main/java/net/dnorberg/rivencraft/book/AgeDimensionService.java

@@ -29,6 +29,7 @@ import net.minecraft.world.level.biome.CheckerboardColumnBiomeSource;
 import net.minecraft.world.level.biome.FixedBiomeSource;
 import net.minecraft.world.level.biome.MultiNoiseBiomeSource;
 import net.minecraft.world.level.biome.MultiNoiseBiomeSourceParameterLists;
+import net.dnorberg.rivencraft.world.ScaledBiomeSource;
 import net.minecraft.world.level.chunk.ChunkGenerator;
 import net.minecraft.world.level.dimension.BuiltinDimensionTypes;
 import net.minecraft.world.level.dimension.DimensionType;
@@ -307,6 +308,15 @@ public final class AgeDimensionService {
                             : TILED_BIOME_SCALE
             );
         }
+        if (distribution == AgeTraits.BiomeDistribution.TINY && biomes.size() >= 3) {
+            List<net.minecraft.core.Holder.Reference<Biome>> selected = new java.util.ArrayList<>(biomes);
+            Collections.shuffle(selected, new java.util.Random(age.seed() ^ 0x54494E5942494F4DL));
+            return new ScaledBiomeSource(
+                    HolderSet.direct(List.of(selected.get(0), selected.get(1), selected.get(2))),
+                    age.seed(),
+                    0
+            );
+        }
         return MultiNoiseBiomeSource.createFromPreset(
                 biomeParameters.getOrThrow(MultiNoiseBiomeSourceParameterLists.OVERWORLD));
     }

src/main/java/net/dnorberg/rivencraft/gametest/LinkingGameTests.java

@@ -183,6 +183,15 @@ public final class LinkingGameTests {
                                             selectedTraits.celestialAngle(),
                                             selectedTraits.celestialPhase()
                                     )
+                            : profile.equals(AgeGenerationProfiles.CAVE)
+                                    ? new AgeTraits(
+                                            AgeTraits.BiomeDistribution.TINY,
+                                            selectedTraits.lighting(),
+                                            selectedTraits.weather(),
+                                            selectedTraits.celestialPeriod(),
+                                            selectedTraits.celestialAngle(),
+                                            selectedTraits.celestialPhase()
+                                    )
                             : selectedTraits;
             AgeDefinition definition = new AgeDefinition(
                     id,
@@ -245,6 +254,30 @@ public final class LinkingGameTests {
                 helper.assertTrue(origin.equals(diagonalChunk),
                         "Diagonal Grid chunks must repeat the biome");
             }
+            if (profile.equals(AgeGenerationProfiles.CAVE)) {
+                var biomeSource = level.getChunkSource().getGenerator().getBiomeSource();
+                helper.assertTrue(biomeSource instanceof net.dnorberg.rivencraft.world.ScaledBiomeSource,
+                        "Tiny Biome Distribution must use the serializable scaled biome source");
+                helper.assertValueEqual(
+                        biomeSource.possibleBiomes().size(),
+                        3,
+                        "Tiny Biome Distribution must expose exactly three seeded biomes"
+                );
+                java.util.Set<net.minecraft.core.Holder<net.minecraft.world.level.biome.Biome>> sampled =
+                        new java.util.HashSet<>();
+                for (int x = -32; x <= 32; x += 4) {
+                    for (int z = -32; z <= 32; z += 4) {
+                        sampled.add(biomeSource.getNoiseBiome(x, 0, z, null));
+                    }
+                }
+                helper.assertTrue(sampled.size() >= 2,
+                        "Tiny Biome Distribution must form multiple seeded regions near spawn");
+                helper.assertValueEqual(
+                        biomeSource.getNoiseBiome(7, 0, -5, null),
+                        biomeSource.getNoiseBiome(7, 0, -5, null),
+                        "Tiny biome sampling must be deterministic"
+                );
+            }
             if (definition.traits().lighting() == AgeTraits.Lighting.DARK) {
                 helper.assertFalse(level.dimensionType().hasSkyLight(),
                         "Dark Lighting Ages must suppress dimension skylight");

src/main/java/net/dnorberg/rivencraft/registry/ModBiomeSources.java

@@ -0,0 +1,26 @@
+package net.dnorberg.rivencraft.registry;
+
+import com.mojang.serialization.MapCodec;
+import net.dnorberg.rivencraft.Rivencraft;
+import net.dnorberg.rivencraft.world.ScaledBiomeSource;
+import net.minecraft.core.registries.Registries;
+import net.minecraft.world.level.biome.BiomeSource;
+import net.neoforged.bus.api.IEventBus;
+import net.neoforged.neoforge.registries.DeferredRegister;
+
+/** Serializable biome-source types used by generated Ages. */
+public final class ModBiomeSources {
+    private static final DeferredRegister<MapCodec<? extends BiomeSource>> BIOME_SOURCES =
+            DeferredRegister.create(Registries.BIOME_SOURCE, Rivencraft.MOD_ID);
+
+    static {
+        BIOME_SOURCES.register("scaled", () -> ScaledBiomeSource.CODEC);
+    }
+
+    private ModBiomeSources() {
+    }
+
+    public static void register(IEventBus modEventBus) {
+        BIOME_SOURCES.register(modEventBus);
+    }
+}

src/main/java/net/dnorberg/rivencraft/world/ScaledBiomeSource.java

@@ -0,0 +1,89 @@
+package net.dnorberg.rivencraft.world;
+
+import com.mojang.serialization.Codec;
+import com.mojang.serialization.MapCodec;
+import com.mojang.serialization.codecs.RecordCodecBuilder;
+import java.util.stream.Stream;
+import net.minecraft.core.Holder;
+import net.minecraft.core.HolderSet;
+import net.minecraft.world.level.biome.Biome;
+import net.minecraft.world.level.biome.BiomeSource;
+import net.minecraft.world.level.biome.Climate;
+
+/** Seeded, softly bounded biome regions at the five Mystcraft distribution scales. */
+public final class ScaledBiomeSource extends BiomeSource {
+    public static final MapCodec<ScaledBiomeSource> CODEC = RecordCodecBuilder.mapCodec(instance -> instance.group(
+            Biome.LIST_CODEC.fieldOf("biomes").forGetter(source -> source.biomes),
+            Codec.LONG.fieldOf("seed").forGetter(source -> source.seed),
+            Codec.intRange(0, 4).fieldOf("scale").forGetter(source -> source.scale)
+    ).apply(instance, ScaledBiomeSource::new));
+
+    private final HolderSet<Biome> biomes;
+    private final long seed;
+    private final int scale;
+    private final int cellWidth;
+
+    public ScaledBiomeSource(HolderSet<Biome> biomes, long seed, int scale) {
+        if (biomes.size() < 3) {
+            throw new IllegalArgumentException("Scaled biome distributions require at least three biomes");
+        }
+        this.biomes = biomes;
+        this.seed = seed;
+        this.scale = scale;
+        // BiomeSource coordinates are quart positions. Legacy applies two zooms
+        // before the selected scale, yielding 16-block Tiny regions that double
+        // with every subsequent distribution size.
+        this.cellWidth = 4 << scale;
+    }
+
+    @Override
+    protected MapCodec<? extends BiomeSource> codec() {
+        return CODEC;
+    }
+
+    @Override
+    protected Stream<Holder<Biome>> collectPossibleBiomes() {
+        return biomes.stream();
+    }
+
+    @Override
+    public Holder<Biome> getNoiseBiome(int x, int y, int z, Climate.Sampler sampler) {
+        int baseX = Math.floorDiv(x, cellWidth);
+        int baseZ = Math.floorDiv(z, cellWidth);
+        int closestX = baseX;
+        int closestZ = baseZ;
+        double closestDistance = Double.POSITIVE_INFINITY;
+
+        for (int cellX = baseX - 1; cellX <= baseX + 1; cellX++) {
+            for (int cellZ = baseZ - 1; cellZ <= baseZ + 1; cellZ++) {
+                long cellSeed = mix(seed, cellX, cellZ);
+                double centerX = (cellX + 0.5D + jitter(cellSeed) * 0.8D) * cellWidth;
+                double centerZ = (cellZ + 0.5D + jitter(mix64(cellSeed)) * 0.8D) * cellWidth;
+                double deltaX = x - centerX;
+                double deltaZ = z - centerZ;
+                double distance = deltaX * deltaX + deltaZ * deltaZ;
+                if (distance < closestDistance) {
+                    closestDistance = distance;
+                    closestX = cellX;
+                    closestZ = cellZ;
+                }
+            }
+        }
+
+        return biomes.get(Math.floorMod((int)mix(seed ^ 0x42494F4D45534CL, closestX, closestZ), biomes.size()));
+    }
+
+    private static double jitter(long value) {
+        return ((value >>> 11) * 0x1.0p-53) - 0.5D;
+    }
+
+    private static long mix(long seed, int x, int z) {
+        return mix64(seed ^ (long)x * 0x9E3779B97F4A7C15L ^ (long)z * 0xC2B2AE3D27D4EB4FL);
+    }
+
+    private static long mix64(long value) {
+        value = (value ^ value >>> 30) * 0xBF58476D1CE4E5B9L;
+        value = (value ^ value >>> 27) * 0x94D049BB133111EBL;
+        return value ^ value >>> 31;
+    }
+}