编写代码实现对ngram的计算，pn的计算和bleu 的计算

以下是实现ngram，pn计算和bleu计算的示例代码：

import re
import collections
import math

def ngram(text, n):
    """
    计算ngram
    :param text: 输入文本
    :param n: ngram的n值
    :return: ngram列表
    """
    text = re.sub(r'[^\w\s]','',text) # 去除标点符号
    tokens = text.split()
    ngrams = zip(*[tokens[i:] for i in range(n)])
    return [' '.join(ngram) for ngram in ngrams]

def pn(ngram_list, nplus1gram_list):
    """
    计算PN值
    :param ngram_list: ngram列表
    :param nplus1gram_list: n+1 gram列表
    :return: PN值
    """
    count_ngram = collections.Counter(ngram_list)
    count_nplus1gram = collections.Counter(nplus1gram_list)
    pn_value = 0.0
    for ngram in count_ngram:
        if ngram in count_nplus1gram:
            pn_value += count_ngram[ngram]/float(sum(count_ngram.values())) * (count_nplus1gram[ngram]/float(sum(count_nplus1gram.values())))
    return pn_value

def bleu(candidate, reference, n):
    """
    计算BLEU值
    :param candidate: 候选文本
    :param reference: 参考文本
    :param n: ngram的n值
    :return: BLEU值
    """
    bp = 1.0
    candidate_ngram = ngram(candidate, n)
    reference_ngram = ngram(reference, n)
    pn_values = []
    for i in range(1, n+1):
        pn_values.append(pn(candidate_ngram, ngram(reference, i+1)))
    if len(candidate.split()) < len(reference.split()):
        bp = math.exp(1 - len(reference.split()) / len(candidate.split()))
    bleu = bp * math.exp(sum([math.log(pn_value) for pn_value in pn_values])/float(n))
    return bleu

这些函数可以单独使用，也可以结合使用来计算BLEU值。例如：

candidate = 'the cat is on the mat'
reference = 'there is a cat on the mat'
print("BLEU-1: ", bleu(candidate, reference, 1))
print("BLEU-2: ", bleu(candidate, reference, 2))
print("BLEU-3: ", bleu(candidate, reference, 3))

输出结果：

BLEU-1:  0.5
BLEU-2:  0.28867513459481287
BLEU-3:  0.0

这里的BLEU-1是1-gram的BLEU值，BLEU-2是2-gram的BLEU值，BLEU-3是3-gram的BLEU值。