Implement grid biome 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 = 10;
+    public static final int CURRENT_GENERATION_VERSION = 11;
 
     public AgeDefinition {
         Objects.requireNonNull(id, "id");

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

@@ -45,6 +45,7 @@ public final class AgeDimensionService {
     private static final int SURFACE_SEARCH_RADIUS = 16;
     private static final int FALLBACK_PLATFORM_Y = 63;
     private static final int CAVE_SPAWN_SEARCH_Y = 64;
+    private static final int GRID_BIOME_SCALE = 0;
     private static final int TILED_BIOME_SCALE = 4;
 
     private AgeDimensionService() {
@@ -291,12 +292,19 @@ public final class AgeDimensionService {
             int index = Math.floorMod(Long.hashCode(age.seed() ^ 0x53494E474C454249L), biomes.size());
             return new FixedBiomeSource(biomes.get(index));
         }
-        if (age.traits().biomeDistribution() == AgeTraits.BiomeDistribution.TILED && biomes.size() >= 2) {
+        AgeTraits.BiomeDistribution distribution = age.traits().biomeDistribution();
+        if ((distribution == AgeTraits.BiomeDistribution.TILED
+                || distribution == AgeTraits.BiomeDistribution.GRID) && biomes.size() >= 2) {
             List<net.minecraft.core.Holder.Reference<Biome>> selected = new java.util.ArrayList<>(biomes);
-            Collections.shuffle(selected, new java.util.Random(age.seed() ^ 0x54494C454442494FL));
+            long salt = distribution == AgeTraits.BiomeDistribution.GRID
+                    ? 0x4752494442494F4DL
+                    : 0x54494C454442494FL;
+            Collections.shuffle(selected, new java.util.Random(age.seed() ^ salt));
             return new CheckerboardColumnBiomeSource(
                     HolderSet.direct(List.of(selected.get(0), selected.get(1))),
-                    TILED_BIOME_SCALE
+                    distribution == AgeTraits.BiomeDistribution.GRID
+                            ? GRID_BIOME_SCALE
+                            : TILED_BIOME_SCALE
             );
         }
         return MultiNoiseBiomeSource.createFromPreset(

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

@@ -174,6 +174,15 @@ public final class LinkingGameTests {
                                     selectedTraits.celestialAngle(),
                                     selectedTraits.celestialPhase()
                             )
+                            : profile.equals(AgeGenerationProfiles.ISLAND)
+                                    ? new AgeTraits(
+                                            AgeTraits.BiomeDistribution.GRID,
+                                            selectedTraits.lighting(),
+                                            selectedTraits.weather(),
+                                            selectedTraits.celestialPeriod(),
+                                            selectedTraits.celestialAngle(),
+                                            selectedTraits.celestialPhase()
+                                    )
                             : selectedTraits;
             AgeDefinition definition = new AgeDefinition(
                     id,
@@ -218,6 +227,24 @@ public final class LinkingGameTests {
                 helper.assertTrue(origin.equals(diagonalTile),
                         "Diagonal 256-block Tiled squares must repeat the biome");
             }
+            if (profile.equals(AgeGenerationProfiles.ISLAND)) {
+                var biomeSource = level.getChunkSource().getGenerator().getBiomeSource();
+                helper.assertValueEqual(
+                        biomeSource.possibleBiomes().size(),
+                        2,
+                        "Grid Biome Distribution must expose exactly two seeded biomes"
+                );
+                var origin = biomeSource.getNoiseBiome(0, 0, 0, null);
+                var insideOriginChunk = biomeSource.getNoiseBiome(3, 0, 3, null);
+                var eastChunk = biomeSource.getNoiseBiome(4, 0, 0, null);
+                var diagonalChunk = biomeSource.getNoiseBiome(4, 0, 4, null);
+                helper.assertTrue(origin.equals(insideOriginChunk),
+                        "Grid biomes must remain constant inside each 16-block chunk");
+                helper.assertFalse(origin.equals(eastChunk),
+                        "Adjacent Grid chunks must alternate biomes");
+                helper.assertTrue(origin.equals(diagonalChunk),
+                        "Diagonal Grid chunks must repeat the biome");
+            }
             if (definition.traits().lighting() == AgeTraits.Lighting.DARK) {
                 helper.assertFalse(level.dimensionType().hasSkyLight(),
                         "Dark Lighting Ages must suppress dimension skylight");