Refactor terrain LOD mesh generation to use chunk-based data structure

src/geometry/terrain-mesh.ts

@@ -596,6 +596,53 @@ export function buildTerrainLodMeshData(
       startSampleX < maxCellX;
       startSampleX += chunkSizeCells, chunkX += 1
     ) {
+      const chunk = buildTerrainLodChunkMeshData(
+        terrain,
+        startSampleX,
+        startSampleZ,
+        chunkSizeCells
+      );
+
+      if (chunk === null) {
+        continue;
+      }
+
+      includePointInBounds(localBounds, chunk.localBounds.min);
+      includePointInBounds(localBounds, chunk.localBounds.max);
+
+      chunks.push({
+        ...chunk,
+        chunkX,
+        chunkZ
+      });
+    }
+  }
+
+  return {
+    chunkSizeCells,
+    chunks,
+    localBounds: cloneBounds(localBounds)
+  };
+}
+
+export function buildTerrainLodChunkMeshData(
+  terrain: Terrain,
+  startSampleX: number,
+  startSampleZ: number,
+  chunkSizeCells = TERRAIN_LOD_CHUNK_SIZE_CELLS
+): TerrainLodChunkMeshData | null {
+  const maxCellX = terrain.sampleCountX - 1;
+  const maxCellZ = terrain.sampleCountZ - 1;
+
+  if (
+    startSampleX < 0 ||
+    startSampleZ < 0 ||
+    startSampleX >= maxCellX ||
+    startSampleZ >= maxCellZ
+  ) {
+    return null;
+  }
+
   const endSampleX = Math.min(startSampleX + chunkSizeCells, maxCellX);
   const endSampleZ = Math.min(startSampleZ + chunkSizeCells, maxCellZ);
   const cellCountX = endSampleX - startSampleX;
@@ -630,12 +677,9 @@ export function buildTerrainLodMeshData(
     chunkBounds.max.z - chunkBounds.min.z
   );
 
-      includePointInBounds(localBounds, chunkBounds.min);
-      includePointInBounds(localBounds, chunkBounds.max);
-
-      chunks.push({
-        chunkX,
-        chunkZ,
+  return {
+    chunkX: Math.floor(startSampleX / chunkSizeCells),
+    chunkZ: Math.floor(startSampleZ / chunkSizeCells),
     startSampleX,
     startSampleZ,
     endSampleX,
@@ -646,14 +690,6 @@ export function buildTerrainLodMeshData(
     localBounds: cloneBounds(chunkBounds),
     localCenter,
     diagonal
-      });
-    }
-  }
-
-  return {
-    chunkSizeCells,
-    chunks,
-    localBounds: cloneBounds(localBounds)
   };
 }