def _normalize(string):
    """ Normalize whitepace in a string according to PDS4 Standards.

    Notes
    -----
    There are a number of ways to implement this method. The employed implementation is generally
    either the fastest or close to the fastest between the various platforms.

    Parameters
    ----------
    string : str or unicode
        String to normalize.

    Returns
    -------
    str or unicode
        Whitespace-collapsed string. Collapses contiguous spaces (including line feeds, carriage returns,
        tabs) into a single space character, and removes leading and trailing spaces entirely.
         white space collapsed
    """
    return ' '.join(string.split())
