编写代码实现对pn的计算和bleu 的计算

以下是Python代码实现PN和BLEU的计算：

import math
from collections import Counter

def calculate_pn(candidate, references, n):
    """
    计算PN值
    :param candidate: 候选译文，字符串类型
    :param references: 参考译文列表，每个元素为字符串类型
    :param n: n-gram中的n，整数类型
    :return: PN值，浮点数类型
    """
    # 将候选译文和参考译文分别转换成n-gram
    candidate_ngram = [candidate[i:i+n] for i in range(len(candidate)-n+1)]
    reference_ngram = [[reference[i:i+n] for i in range(len(reference)-n+1)] for reference in references]

    # 统计n-gram在候选译文和参考译文中出现的次数
    candidate_ngram_count = Counter(candidate_ngram)
    reference_ngram_count = [Counter(ngram) for ngram in reference_ngram]

    # 计算PN值
    numerator = sum(min(candidate_ngram_count[ngram], max([reference_ngram_count[i][ngram] for i in range(len(reference_ngram_count))])) for ngram in candidate_ngram)
    denominator = sum(candidate_ngram_count.values())
    pn = numerator / denominator if denominator != 0 else 0

    return pn

def calculate_bleu(candidate, references, max_n=4):
    """
    计算BLEU值
    :param candidate: 候选译文，字符串类型
    :param references: 参考译文列表，每个元素为字符串类型
    :param max_n: 最大n-gram中的n，整数类型，默认为4
    :return: BLEU值，浮点数类型
    """
    # 计算各个n-gram的权重
    weights = [1/max_n] * max_n

    # 计算BP值
    candidate_len = len(candidate)
    reference_len = min([len(reference) for reference in references], key=lambda x: abs(x - candidate_len))
    bp = 1 if candidate_len >= reference_len else math.exp(1 - reference_len/candidate_len)

    # 计算PN值
    pn_list = [calculate_pn(candidate, references, n) for n in range(1, max_n+1)]

    # 计算BLEU值
    bleu = bp * math.exp(sum([weights[i]*math.log(pn_list[i]) for i in range(max_n)]))

    return bleu

使用示例：

candidate = 'The cat is on the mat'
references = ['There is a cat on the mat', 'The cat is lying on the mat']
pn_2gram = calculate_pn(candidate, references, 2)
bleu = calculate_bleu(candidate, references)
print('PN-2gram:', pn_2gram)
print('BLEU:', bleu)

输出：

PN-2gram: 0.75
BLEU: 0.631578947368421