class RuboCop::Formatter::FormatterSet

def builtin_formatter_class(specified_key)
  matching_keys = BUILTIN_FORMATTERS_FOR_KEYS.keys.select do |key|
    /^\[#{specified_key}\]/.match?(key) || specified_key == key.delete('[]')
  end
  raise %(No formatter for "#{specified_key}") if matching_keys.empty?
  raise %(Cannot determine formatter for "#{specified_key}") if matching_keys.size > 1
  formatter_name = BUILTIN_FORMATTERS_FOR_KEYS[matching_keys.first]
  RuboCop::Formatter.const_get(formatter_name)
end