module Sass::Value

def sass_index_to_array_index(sass_index, name = nil)
  index = sass_index.assert_number(name).assert_integer(name)
  raise Sass::ScriptError.new('List index may not be 0', name) if index.zero?
  if index.abs > to_a_length
    raise Sass::ScriptError.new("Invalid index #{sass_index} for a list with #{to_a_length} elements", name)
  end
  index.negative? ? to_a_length + index : index - 1
end