computing the default_start_level just from the number of leaves; tes…

…ting it always produces the same output as when using the BVH; cleaned codebase

src/traverse/traverse_single.jl

@@ -1,5 +1,6 @@
 """
     default_start_level(bvh::BVH)::Int
+    default_start_level(num_leaves::Integer)::Int
 
 Compute the default start level when traversing a single BVH tree.
 """
@@ -8,6 +9,17 @@ function default_start_level(bvh::BVH)
 end
 
 
+function default_start_level(num_leaves::Integer)
+    # Compute the default start level from the number of leaves (geometries) only
+    @boundscheck if num_leaves < 1
+        throw(DomainError(num_leaves, "must have at least one geometry!"))
+    end
+
+    levels = @inbounds ilog2(num_leaves, RoundUp) + 1   # number of binary tree levels
+    maximum2(levels ÷ 2, 1)
+end
+
+
 """
     traverse(
         bvh::BVH,