class TransformAddDummyStratigraphy(base.BaseTransform):
    """
    Adds and extra horizon above the first horizon
    from the ground level. MD of the top of the new
    horizon begins a the point where a record of all
    logs appears.

    Parameters
    ----------
    logs: list of str
        list of logs that need to be present at the top
        of the new horizon
    gap_threshold: int
        gap .......
    horizon_name: str
        name of new horizon

    """

    def __init__(self, logs, gap_threshold, horizon_name):
        """

        """
        self._logs = logs
        self._horizon_name = horizon_name
        self._gap_threshold = gap_threshold

    def __call__(self, data):
        log_top_md = data.dropna(subset=self._logs)['MD'].min()
        horizon_top_md = data[data['Layer'] != 'nan']['MD'].min()
        gap = horizon_top_md - log_top_md
        if gap > self._gap_threshold:
            new_horizon_top_md = horizon_top_md - self._gap_threshold
        elif gap <= self._gap_threshold and gap > 0:
            new_horizon_top_md = log_top_md
        else:
            return data

        where_new_layer = ((data['MD'] >= new_horizon_top_md)
                           & (data['MD'] <= horizon_top_md))
        data.loc[where_new_layer, 'Layer'] = self._horizon_name
        return data