@classmethod
def _get_rsi(cls, df, n_days):
    """ Calculate the RSI (Relative Strength Index) within N days

        calculated based on the formula at:
        https://en.wikipedia.org/wiki/Relative_strength_index

        :param df: data
        :param n_days: N days
        :return: None
        """
    n_days = int(n_days)
    d = df['close_-1_d']
    df['closepm'] = (d + d.abs()) / 2
    df['closenm'] = (-d + d.abs()) / 2
    closepm_smma_column = 'closepm_{}_smma'.format(n_days)
    closenm_smma_column = 'closenm_{}_smma'.format(n_days)
    p_ema = df[closepm_smma_column]
    n_ema = df[closenm_smma_column]
    rs_column_name = 'rs_{}'.format(n_days)
    rsi_column_name = 'rsi_{}'.format(n_days)
    df[rs_column_name] = rs = p_ema / n_ema
    df[rsi_column_name] = 100 - 100 / (1.0 + rs)
    columns_to_remove = ['closepm', 'closenm', closepm_smma_column,
        closenm_smma_column]
    cls._drop_columns(df, columns_to_remove)
