def retention_schedule(name, retain, strptime_format=None, timezone=None):
    '''
    Apply retention scheduling to backup storage directory.

    .. versionadded:: 2016.11.0

    :param name:
        The filesystem path to the directory containing backups to be managed.

    :param retain:
        Delete the backups, except for the ones we want to keep.
        The N below should be an integer but may also be the special value of ``all``,
        which keeps all files matching the criteria.
        All of the retain options default to None,
        which means to not keep files based on this criteria.

        :most_recent N:
            Keep the most recent N files.

        :first_of_hour N:
            For the last N hours from now, keep the first file after the hour.

        :first_of_day N:
            For the last N days from now, keep the first file after midnight.
            See also ``timezone``.

        :first_of_week N:
            For the last N weeks from now, keep the first file after Sunday midnight.

        :first_of_month N:
            For the last N months from now, keep the first file after the start of the month.

        :first_of_year N:
            For the last N years from now, keep the first file after the start of the year.

    :param strptime_format:
        A python strptime format string used to first match the filenames of backups
        and then parse the filename to determine the datetime of the file.
        https://docs.python.org/2/library/datetime.html#datetime.datetime.strptime
        Defaults to None, which considers all files in the directory to be backups eligible for deletion
        and uses ``os.path.getmtime()`` to determine the datetime.

    :param timezone:
        The timezone to use when determining midnight.
        This is only used when datetime is pulled from ``os.path.getmtime()``.
        Defaults to ``None`` which uses the timezone from the locale.

    Usage example:

    .. code-block:: yaml

        /var/backups/example_directory:
          file.retention_schedule:
            - retain:
                most_recent: 5
                first_of_hour: 4
                first_of_day: 7
                first_of_week: 6    # NotImplemented yet.
                first_of_month: 6
                first_of_year: all
            - strptime_format: example_name_%Y%m%dT%H%M%S.tar.bz2
            - timezone: None

    '''
    name = os.path.expanduser(name)
    ret = {'name': name,
           'changes': {'retained': [], 'deleted': [], 'ignored': []},
           'result': True,
           'comment': ''}
    if not name:
        return _error(ret, 'Must provide name to file.retention_schedule')
    if not os.path.isdir(name):
        return _error(ret, 'Name provided to file.retention must be a directory')

    # get list of files in directory
    all_files = __salt__['file.readdir'](name)

    # if strptime_format is set, filter through the list to find names which parse and get their datetimes.
    beginning_of_unix_time = datetime(1970, 1, 1)

    def get_file_time_from_strptime(f):
        try:
            ts = datetime.strptime(f, strptime_format)
            ts_epoch = salt.utils.dateutils.total_seconds(ts - beginning_of_unix_time)
            return (ts, ts_epoch)
        except ValueError:
            # Files which don't match the pattern are not relevant files.
            return (None, None)

    def get_file_time_from_mtime(f):
        if f == '.' or f == '..':
            return (None, None)
        lstat = __salt__['file.lstat'](os.path.join(name, f))
        if lstat:
            mtime = lstat['st_mtime']
            return (datetime.fromtimestamp(mtime, timezone), mtime)
        else:   # maybe it was deleted since we did the readdir?
            return (None, None)

    get_file_time = get_file_time_from_strptime if strptime_format else get_file_time_from_mtime

    # data structures are nested dicts:
    # files_by_ymd = year.month.day.hour.unixtime: filename
    # files_by_y_week_dow = year.week_of_year.day_of_week.unixtime: filename
    # http://the.randomengineer.com/2015/04/28/python-recursive-defaultdict/
    # TODO: move to an ordered dict model and reduce the number of sorts in the rest of the code?
    def dict_maker():
        return defaultdict(dict_maker)
    files_by_ymd = dict_maker()
    files_by_y_week_dow = dict_maker()
    relevant_files = set()
    ignored_files = set()
    for f in all_files:
        ts, ts_epoch = get_file_time(f)
        if ts:
            files_by_ymd[ts.year][ts.month][ts.day][ts.hour][ts_epoch] = f
            week_of_year = ts.isocalendar()[1]
            files_by_y_week_dow[ts.year][week_of_year][ts.weekday()][ts_epoch] = f
            relevant_files.add(f)
        else:
            ignored_files.add(f)

    # This is tightly coupled with the file_with_times data-structure above.
    RETAIN_TO_DEPTH = {
        'first_of_year': 1,
        'first_of_month': 2,
        'first_of_day': 3,
        'first_of_hour': 4,
        'most_recent': 5,
    }

    def get_first(fwt):
        if isinstance(fwt, dict):
            first_sub_key = sorted(fwt.keys())[0]
            return get_first(fwt[first_sub_key])
        else:
            return set([fwt, ])

    def get_first_n_at_depth(fwt, depth, n):
        if depth <= 0:
            return get_first(fwt)
        else:
            result_set = set()
            for k in sorted(fwt.keys(), reverse=True):
                needed = n - len(result_set)
                if needed < 1:
                    break
                result_set |= get_first_n_at_depth(fwt[k], depth - 1, needed)
            return result_set

    # for each retain criteria, add filenames which match the criteria to the retain set.
    retained_files = set()
    for retention_rule, keep_count in retain.items():
        # This is kind of a hack, since 'all' should really mean all,
        # but I think it's a large enough number that even modern filesystems would
        # choke if they had this many files in a single directory.
        keep_count = sys.maxsize if 'all' == keep_count else int(keep_count)
        if 'first_of_week' == retention_rule:
            first_of_week_depth = 2   # year + week_of_year = 2
            # I'm adding 1 to keep_count below because it fixed an off-by one
            # issue in the tests. I don't understand why, and that bothers me.
            retained_files |= get_first_n_at_depth(files_by_y_week_dow,
                                                   first_of_week_depth,
                                                   keep_count + 1)
        else:
            retained_files |= get_first_n_at_depth(files_by_ymd,
                                                   RETAIN_TO_DEPTH[retention_rule],
                                                   keep_count)

    deletable_files = list(relevant_files - retained_files)
    deletable_files.sort(reverse=True)
    changes = {
            'retained': sorted(list(retained_files), reverse=True),
            'deleted': deletable_files,
            'ignored': sorted(list(ignored_files), reverse=True),
        }
    ret['changes'] = changes

    # TODO: track and report how much space was / would be reclaimed
    if __opts__['test']:
        ret['comment'] = '{0} backups would have been removed from {1}.\n'.format(len(deletable_files), name)
        if deletable_files:
            ret['result'] = None
    else:
        for f in deletable_files:
            __salt__['file.remove'](os.path.join(name, f))
        ret['comment'] = '{0} backups were removed from {1}.\n'.format(len(deletable_files), name)
        ret['changes'] = changes

    return ret
Apply retention scheduling to backup storage directory.

    .. versionadded:: 2016.11.0

    :param name:
        The filesystem path to the directory containing backups to be managed.

    :param retain:
        Delete the backups, except for the ones we want to keep.
        The N below should be an integer but may also be the special value of ``all``,
        which keeps all files matching the criteria.
        All of the retain options default to None,
        which means to not keep files based on this criteria.

        :most_recent N:
            Keep the most recent N files.

        :first_of_hour N:
            For the last N hours from now, keep the first file after the hour.

        :first_of_day N:
            For the last N days from now, keep the first file after midnight.
            See also ``timezone``.

        :first_of_week N:
            For the last N weeks from now, keep the first file after Sunday midnight.

        :first_of_month N:
            For the last N months from now, keep the first file after the start of the month.

        :first_of_year N:
            For the last N years from now, keep the first file after the start of the year.

    :param strptime_format:
        A python strptime format string used to first match the filenames of backups
        and then parse the filename to determine the datetime of the file.
        https://docs.python.org/2/library/datetime.html#datetime.datetime.strptime
        Defaults to None, which considers all files in the directory to be backups eligible for deletion
        and uses ``os.path.getmtime()`` to determine the datetime.

    :param timezone:
        The timezone to use when determining midnight.
        This is only used when datetime is pulled from ``os.path.getmtime()``.
        Defaults to ``None`` which uses the timezone from the locale.

    Usage example:

    .. code-block:: yaml

        /var/backups/example_directory:
          file.retention_schedule:
            - retain:
                most_recent: 5
                first_of_hour: 4
                first_of_day: 7
                first_of_week: 6    # NotImplemented yet.
                first_of_month: 6
                first_of_year: all
            - strptime_format: example_name_%Y%m%dT%H%M%S.tar.bz2
            - timezone: None