Apply single biome Age distribution

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 = 7;
+    public static final int CURRENT_GENERATION_VERSION = 8;
 
     public AgeDefinition {
         Objects.requireNonNull(id, "id");

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

@@ -5,6 +5,7 @@ import dev.galacticraft.dynamicdimensions.api.event.DynamicDimensionLoadCallback
 import java.util.Optional;
 import java.util.OptionalLong;
 import java.util.List;
+import java.util.Comparator;
 import net.dnorberg.rivencraft.Rivencraft;
 import net.dnorberg.rivencraft.compat.ImmersivePortalsChunkLoaders;
 import net.minecraft.core.BlockPos;
@@ -20,6 +21,9 @@ import net.minecraft.util.valueproviders.UniformInt;
 import net.minecraft.world.level.Level;
 import net.minecraft.world.level.block.Blocks;
 import net.minecraft.world.level.biome.Biomes;
+import net.minecraft.world.level.biome.Biome;
+import net.minecraft.world.level.biome.BiomeSource;
+import net.minecraft.world.level.biome.FixedBiomeSource;
 import net.minecraft.world.level.biome.MultiNoiseBiomeSource;
 import net.minecraft.world.level.biome.MultiNoiseBiomeSourceParameterLists;
 import net.minecraft.world.level.chunk.ChunkGenerator;
@@ -263,12 +267,31 @@ public final class AgeDimensionService {
         // island. Both use Overworld biomes and dimension environment.
         // random_vanilla is retained as a migration alias for existing Ages.
         return new NoiseBasedChunkGenerator(
-                MultiNoiseBiomeSource.createFromPreset(
-                        biomeParameters.getOrThrow(MultiNoiseBiomeSourceParameterLists.OVERWORLD)),
+                createBiomeSource(server, age, biomeParameters),
                 noiseSettings.getOrThrow(settingsKey)
         );
     }
 
+    private static BiomeSource createBiomeSource(
+            MinecraftServer server,
+            AgeDefinition age,
+            net.minecraft.core.HolderGetter<net.minecraft.world.level.biome.MultiNoiseBiomeSourceParameterList> biomeParameters
+    ) {
+        if (age.traits().biomeDistribution() == AgeTraits.BiomeDistribution.SINGLE) {
+            List<net.minecraft.core.Holder.Reference<Biome>> biomes = server.registryAccess()
+                    .lookupOrThrow(Registries.BIOME)
+                    .listElements()
+                    .sorted(Comparator.comparing(holder -> holder.key().location().toString()))
+                    .toList();
+            if (!biomes.isEmpty()) {
+                int index = Math.floorMod(Long.hashCode(age.seed() ^ 0x53494E474C454249L), biomes.size());
+                return new FixedBiomeSource(biomes.get(index));
+            }
+        }
+        return MultiNoiseBiomeSource.createFromPreset(
+                biomeParameters.getOrThrow(MultiNoiseBiomeSourceParameterLists.OVERWORLD));
+    }
+
     private static DimensionType createDimensionType(AgeDefinition age) {
         AgeTraits.Lighting lighting = age.traits().lighting();
         boolean hasSkyLight = lighting != AgeTraits.Lighting.DARK;

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

@@ -166,7 +166,16 @@ public final class LinkingGameTests {
                             Rivencraft.MOD_ID, "gametest/" + profile.getPath() + "_" + id
                     ),
                     null,
-                    AgeTraits.select(id.getMostSignificantBits())
+                    profile.equals(AgeGenerationProfiles.STANDARD)
+                            ? new AgeTraits(
+                                    AgeTraits.BiomeDistribution.SINGLE,
+                                    AgeTraits.Lighting.NORMAL,
+                                    AgeTraits.Weather.NORMAL,
+                                    1.0D,
+                                    0.0F,
+                                    0.0F
+                            )
+                            : AgeTraits.select(id.getMostSignificantBits())
             );
             var level = AgeDimensionService.ensureLoaded(helper.getLevel().getServer(), definition);
             helper.assertTrue(level != null, "The " + profile + " profile must load a runtime dimension");
@@ -176,6 +185,13 @@ public final class LinkingGameTests {
 
             helper.assertTrue(level.dimension().location().equals(definition.dimensionId()),
                     "The " + profile + " profile must retain its assigned dimension id");
+            if (profile.equals(AgeGenerationProfiles.STANDARD)) {
+                helper.assertValueEqual(
+                        level.getChunkSource().getGenerator().getBiomeSource().possibleBiomes().size(),
+                        1,
+                        "Single Biome Distribution must expose exactly one biome"
+                );
+            }
             if (definition.traits().lighting() == AgeTraits.Lighting.DARK) {
                 helper.assertFalse(level.dimensionType().hasSkyLight(),
                         "Dark Lighting Ages must suppress dimension skylight");