class Sass::Value::String

def sass_index_to_string_index(sass_index, name = nil)
  index = sass_index.assert_number(name).assert_integer(name)
  raise error('String index may not be 0', name) if index.zero?
  if index.abs > text.length
    raise error("Invalid index #{sass_index} for a string with #{text.length} characters", name)
  end
  index.negative? ? text.length + index : index - 1
end