calculate brevity penalty

def BP(candidate, references):
    """
    calculate brevity penalty
    """
    c = len(candidate)
    ref_lens = (len(reference) for reference in references)
    r = min(ref_lens, key=lambda ref_len: (abs(ref_len - c), ref_len))

    if c > r:
        return 1
    else:
        return math.exp(1 - r / c)