Keep Cave Age spawns below the roof

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

@@ -37,6 +37,7 @@ public final class AgeDimensionService {
     private static final int PLATFORM_RADIUS = 2;
     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 AgeDimensionService() {
     }
@@ -64,7 +65,7 @@ public final class AgeDimensionService {
 
         BlockPos spawn = definition.spawn();
         if (spawn == null) {
-            spawn = createSafeSpawn(level);
+            spawn = createSafeSpawn(level, definition);
             definition = AgeRegistrySavedData.get(level).setSpawn(definition.id(), spawn);
         }
         return Optional.of(new LinkTarget(
@@ -105,10 +106,19 @@ public final class AgeDimensionService {
         return ResourceKey.create(net.minecraft.core.registries.Registries.DIMENSION, definition.dimensionId());
     }
 
-    private static BlockPos createSafeSpawn(ServerLevel level) {
-        BlockPos floorCenter = isNetherStyle(level)
-                ? netherSpawnFloor(level)
-                : highestFootedSurface(level).orElseGet(() -> fallbackSpawnFloor(level));
+    private static BlockPos createSafeSpawn(ServerLevel level, AgeDefinition age) {
+        BlockPos floorCenter;
+        if (age.generationProfile().equals(AgeGenerationProfiles.CAVE)) {
+            // Cave Ages are enclosed. Searching from the world ceiling would
+            // select or expose their roof, so begin with the floor beneath a
+            // player standing at Y=64 and search downward from there.
+            floorCenter = highestFootedSurface(level, CAVE_SPAWN_SEARCH_Y - 1)
+                    .orElseGet(() -> fallbackSpawnFloor(level));
+        } else if (isNetherStyle(level)) {
+            floorCenter = netherSpawnFloor(level);
+        } else {
+            floorCenter = highestFootedSurface(level).orElseGet(() -> fallbackSpawnFloor(level));
+        }
 
         for (int x = -PLATFORM_RADIUS; x <= PLATFORM_RADIUS; x++) {
             for (int z = -PLATFORM_RADIUS; z <= PLATFORM_RADIUS; z++) {
@@ -138,8 +148,13 @@ public final class AgeDimensionService {
 
     /** Selects the highest nearby terrain column that can naturally support a player. */
     private static Optional<BlockPos> highestFootedSurface(ServerLevel level) {
+        return highestFootedSurface(level, level.getMaxBuildHeight() - 5);
+    }
+
+    private static Optional<BlockPos> highestFootedSurface(ServerLevel level, int requestedMaximumFloorY) {
         BlockPos best = null;
-        int maximumFloorY = level.getMaxBuildHeight() - 5;
+        int maximumFloorY = Math.clamp(requestedMaximumFloorY,
+                level.getMinBuildHeight(), level.getMaxBuildHeight() - 5);
 
         for (int x = -SURFACE_SEARCH_RADIUS; x <= SURFACE_SEARCH_RADIUS; x++) {
             for (int z = -SURFACE_SEARCH_RADIUS; z <= SURFACE_SEARCH_RADIUS; z++) {