Calculate brevity penalty.

    Parameters
    ----------
    ref_length: int
        Sum of all closest references'lengths for every translations in a corpus
    trans_length: int
        Sum of all translations's lengths in a corpus.

    Returns
    -------
    bleu's brevity penalty: float

def _brevity_penalty(ref_length, trans_length):
    """Calculate brevity penalty.

    Parameters
    ----------
    ref_length: int
        Sum of all closest references'lengths for every translations in a corpus
    trans_length: int
        Sum of all translations's lengths in a corpus.

    Returns
    -------
    bleu's brevity penalty: float
    """
    if trans_length > ref_length:
        return 1
    # If translation is empty, brevity penalty = 0 should result in BLEU = 0.0
    elif trans_length == 0:
        return 0
    else:
        return math.exp(1 - float(ref_length) / trans_length)