module RuboCop::YAMLDuplicationChecker

def self.check(yaml_string, filename, &on_duplicated)
  # Specify filename to display helpful message when it raises an error.
  tree = YAML.parse(yaml_string, filename: filename)
  return unless tree
  traverse(tree, &on_duplicated)
end

def self.traverse(tree, &on_duplicated)
  case tree
  when Psych::Nodes::Mapping
    tree.children.each_slice(2).with_object([]) do |(key, value), keys|
      exist = keys.find { |key2| key2.value == key.value }
      on_duplicated.call(exist, key) if exist
      keys << key
      traverse(value, &on_duplicated)
    end
  else
    children = tree.children
    return unless children
    children.each { |c| traverse(c, &on_duplicated) }
  end
end