def METHOD_NAME(sandbox, output_filename, correct_output_filename):
    """Compare user output and correct output with a simple diff.
    It gives an outcome 1.0 if the output and the reference output are
    identical (or differ just by white spaces) and 0.0 if they don't (or if
    the output doesn't exist).
    sandbox (Sandbox): the sandbox we consider.
    output_filename (str): the filename of user's output in the sandbox.
    correct_output_filename (str): the same with reference output.
    return ((float, [str])): the outcome as above and a description text.
    """
    if sandbox.file_exists(output_filename):
        with sandbox.get_file(output_filename) as out_file, \
                sandbox.get_file(correct_output_filename) as res_file:
            return white_diff_fobj_step(out_file, res_file)
    else:
        return 0.0, [
            EVALUATION_MESSAGES.get("nooutput").message, output_filename]