Map Star Fissures to modern world minimum Y (#58)

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

@@ -103,7 +103,8 @@ public final class AgeDimensionService {
                     .anyMatch(choice -> choice.id().equals(AgeFeatureSelection.STAR_FISSURES))
                     || definition.randomFeatures().stream()
                     .anyMatch(choice -> choice.id().equals(AgeFeatureSelection.STAR_FISSURES))) {
-                StarFissureFeature.generate(level, definition.seed(), spawn);
+                StarFissureFeature.generate(level, definition.seed(), spawn,
+                        AgeBiomeSelection.isCave(runtimeProfile(definition)));
             }
             definition = AgeRegistrySavedData.get(level).setSpawn(definition.id(), spawn);
         }

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

@@ -155,6 +155,8 @@ public final class AgeFlatChunkGenerator extends ChunkGenerator {
     @Override public void applyCarvers(WorldGenRegion level, long seed, RandomState random, BiomeManager biomes,
                                        StructureManager structures, ChunkAccess chunk, GenerationStep.Carving step) { }
     @Override public void spawnOriginalMobs(WorldGenRegion level) { }
+    // Keep vanilla FlatLevelSource's generator API values. Actual layer
+    // placement uses chunk.getMinBuildHeight() above, not this metadata value.
     @Override public int getMinY() { return 0; }
     @Override public int getGenDepth() { return 384; }
     @Override public int getSeaLevel() { return -63; }

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

@@ -49,29 +49,38 @@ public final class StarFissureFeature {
 
     private static void addRow(List<BlockPos> columns, int originX, int originZ, int row, int left, int right) {
         for (int x = left; x <= right; x++) {
+            // Footprint is a 2D column map; this placeholder Y is never used
+            // for placement. generate() resolves the real floor from the level.
             columns.add(new BlockPos(originX + x, 0, originZ + row * 2));
             columns.add(new BlockPos(originX + x, 0, originZ + row * 2 + 1));
         }
     }
 
-    public static void generate(ServerLevel level, long ageSeed, BlockPos spawn) {
+    public static void generate(ServerLevel level, long ageSeed, BlockPos spawn, boolean caveTerrain) {
         List<BlockPos> columns = footprint(ageSeed, spawn);
-        BlockPos first = columns.getFirst();
-        // 1.12 worlds began at y=0. Modern superflat can sit below it, so use the
-        // modern world floor there. Empty Void columns keep Legacy's y=0 portal.
-        int surface = level.getHeight(Heightmap.Types.WORLD_SURFACE, first.getX(), first.getZ());
-        int minY = level.getMinBuildHeight();
-        int floorY = surface > minY && surface <= 0 ? minY + 1 : Math.max(0, minY + 1);
+        // Legacy's y=0 was its minimum build height. The equivalent in 1.21.1
+        // is the dimension's minimum Y, including the bedrock layer itself.
+        int floorY = level.getMinBuildHeight();
         BlockPos.MutableBlockPos mutable = new BlockPos.MutableBlockPos();
         for (BlockPos column : columns) {
             int top = level.getHeight(Heightmap.Types.WORLD_SURFACE, column.getX(), column.getZ());
-            level.setBlock(mutable.set(column.getX(), floorY, column.getZ()),
-                    ModBlocks.STAR_FISSURE.get().defaultBlockState(), 2);
-            // Legacy clears all blocks above each portal tile to the sky. Heightmap
-            // bounds the same work without iterating already-empty world height.
+            if (caveTerrain) {
+                // Cave terrain has a bedrock roof. Open the fissure into the
+                // cavern, but never punch through its upper boundary.
+                for (int y = Math.max(spawn.getY(), floorY + 1); y < top; y++) {
+                    if (level.getBlockState(mutable.set(column.getX(), y, column.getZ())).is(Blocks.BEDROCK)) {
+                        top = y;
+                        break;
+                    }
+                }
+            }
+            // Clear the full column first, including water. Place the portal
+            // last so an existing fluid cannot occupy its block during carving.
             for (int y = floorY + 1; y < top; y++) {
                 level.setBlock(mutable.set(column.getX(), y, column.getZ()), Blocks.AIR.defaultBlockState(), 2);
             }
+            level.setBlock(mutable.set(column.getX(), floorY, column.getZ()),
+                    ModBlocks.STAR_FISSURE.get().defaultBlockState(), 2);
         }
     }
 }