Missing section misdirection

- Avoid adding all sections every frame
- Remove sections when they are no longer needed
- Rebuild the lut when sections are removed
- Properly detect missing sections by writing 1-based indices to the lut

common/src/backend/java/dev/engine_room/flywheel/backend/engine/EngineImpl.java

@@ -46,7 +46,8 @@ public class EngineImpl implements Engine {
 
 	@Override
 	public Plan<RenderContext> createFramePlan() {
-		return lightStorage.createFramePlan().then(SyncedPlan.of(this::flush));
+		return lightStorage.createFramePlan()
+				.then(SyncedPlan.of(this::flush));
 	}
 
 	@Override

common/src/backend/java/dev/engine_room/flywheel/backend/engine/embed/LightLut.java

@@ -80,7 +80,8 @@ public class LightLut {
 				zLookup.ensureCapacity(zIndex + 3);
 				zLookup.size(zIndex + 3);
 			}
-			zLookup.set(zIndex + 2, sectionIndicesMaps.get(position));
+			// Add 1 to the actual index so that 0 indicates a missing section.
+			zLookup.set(zIndex + 2, sectionIndicesMaps.get(position) + 1);
 		}
 		return indices;
 	}

common/src/backend/java/dev/engine_room/flywheel/backend/engine/embed/LightStorage.java

@@ -12,6 +12,7 @@ import dev.engine_room.flywheel.lib.task.SimplePlan;
 import it.unimi.dsi.fastutil.ints.IntArrayList;
 import it.unimi.dsi.fastutil.longs.Long2IntMap;
 import it.unimi.dsi.fastutil.longs.Long2IntOpenHashMap;
+import it.unimi.dsi.fastutil.longs.LongSet;
 import net.minecraft.core.BlockPos;
 import net.minecraft.core.SectionPos;
 import net.minecraft.world.level.LevelAccessor;
@@ -47,7 +48,7 @@ public class LightStorage {
 	}
 
 	private final BitSet changed = new BitSet();
-	private boolean newSections = false;
+	private boolean needsLutRebuild = false;
 
 	public LightStorage(LevelAccessor level, EnvironmentStorage environmentStorage) {
 		this.level = level;
@@ -58,11 +59,34 @@ public class LightStorage {
 
 	public Plan<RenderContext> createFramePlan() {
 		return SimplePlan.of(() -> {
-			var longs = environmentStorage.allLightSections();
+			var allLightSections = environmentStorage.allLightSections();
-			longs.forEach(this::addSection);
+
+			removeUnusedSections(allLightSections);
+
+			// Only add the new sections.
+			allLightSections.removeAll(section2ArenaIndex.keySet());
+
+			for (var section : allLightSections) {
+				addSection(section);
+			}
 		});
 	}
 
+	private void removeUnusedSections(LongSet allLightSections) {
+		var entries = section2ArenaIndex.long2IntEntrySet();
+		var it = entries.iterator();
+		while (it.hasNext()) {
+			var entry = it.next();
+			var section = entry.getLongKey();
+
+			if (!allLightSections.contains(section)) {
+				arena.free(entry.getIntValue());
+				needsLutRebuild = true;
+				it.remove();
+			}
+		}
+	}
+
 	public int capacity() {
 		return arena.capacity();
 	}
@@ -179,15 +203,11 @@ public class LightStorage {
 		if (out == INVALID_SECTION) {
 			out = arena.alloc();
 			section2ArenaIndex.put(section, out);
-			newSections = true;
+			needsLutRebuild = true;
 		}
 		return out;
 	}
 
-	public void removeSection(long section) {
-
-	}
-
 	public void delete() {
 		arena.delete();
 	}
@@ -196,8 +216,10 @@ public class LightStorage {
 		return !changed.isEmpty();
 	}
 
-	public boolean hasNewSections() {
+	public boolean checkNeedsLutRebuildAndClear() {
-		return newSections;
+		var out = needsLutRebuild;
+		needsLutRebuild = false;
+		return out;
 	}
 
 	public void uploadChangedSections(StagingBuffer staging, int dstVbo) {
@@ -208,6 +230,7 @@ public class LightStorage {
 	}
 
 	public IntArrayList createLut() {
+		// TODO: incremental lut updates
 		return LightLut.buildLut(section2ArenaIndex);
 	}
 }

common/src/backend/java/dev/engine_room/flywheel/backend/engine/indirect/LightBuffers.java

@@ -22,7 +22,7 @@ public class LightBuffers {
 		lightArena.ensureCapacity(capacity);
 		light.uploadChangedSections(staging, lightArena.handle());
 
-		if (light.hasNewSections()) {
+		if (light.checkNeedsLutRebuildAndClear()) {
 			var lut = light.createLut();
 
 			this.lut.ensureCapacity(lut.size());

common/src/backend/resources/assets/flywheel/flywheel/internal/indirect/main.frag

@@ -42,15 +42,24 @@ bool _flw_nextLut(uint base, int coord, out uint next) {
 
 bool _flw_chunkCoordToSectionIndex(ivec3 sectionPos, out uint index) {
     uint y;
-    if (_flw_nextLut(0, sectionPos.x, y)) {
+    if (_flw_nextLut(0, sectionPos.x, y) || y == 0) {
         return true;
     }
 
     uint z;
-    if (_flw_nextLut(y, sectionPos.y, z)) {
+    if (_flw_nextLut(y, sectionPos.y, z) || z == 0) {
         return true;
     }
-    return _flw_nextLut(z, sectionPos.z, index);
+
+    uint sectionIndex;
+    if (_flw_nextLut(z, sectionPos.z, index) || index == 0) {
+        return true;
+    }
+
+    // The index is written as 1-based so we can properly detect missing sections.
+    index = sectionIndex - 1;
+
+    return false;
 }
 
 vec2 _flw_lightAt(uint sectionOffset, uvec3 blockInSectionPos) {