Prevent adding ancestor as child

Author: evolve75

lib/tree.rb

@@ -394,6 +394,10 @@ def add(child, at_index = -1)
 
       raise ArgumentError, 'Attempting add root as a child' if child.equal?(root)
 
+      if (ancestors = parentage) && ancestors.include?(child)
+        raise ArgumentError, 'Attempting add ancestor as a child'
+      end
+
       # Lazy man's unique test, won't test if children of child are unique in
       # this tree too.
       raise "Child #{child.name} already added!"\

test/test_tree.rb

@@ -491,6 +491,10 @@ def test_add
 
       # Test the addition of a nil node.
       assert_raise(ArgumentError) { @root.add(nil) }
+
+      # Test adding an ancestor as a child (cycle prevention).
+      error = assert_raise(ArgumentError) { @child4.add(@child3) }
+      assert_match(/Attempting add ancestor as a child/, error.message)
     end
 
     # Test the addition of a duplicate node (duplicate being defined as a node with the same name).