text: import numpy as np
  from collections import Counter
  
  
  def BLEU(references, generated, max_grams=4, weights=None):
      ref_list = [ref.lower().split(" ") for ref in references]
      gen = generated.lower().split(" ")
      cpn = np.empty((max_grams,), dtype=np.float32)
      for n in range(1, max_grams+1):
          gen_gram = [" ".join(gen[i:i+n]) for i in range(0, len(gen)-n+1)]
          refs_gram = [[" ".join(ref[i:i+n]) for i in range(0, len(ref)-n+1)] for ref in ref_list]
          g_counter = Counter(gen_gram)
          r_counters = [Counter(ref_gram) for ref_gram in refs_gram]
          count_clip = 0
          for k, v in g_counter.items():
              count_clip += min(v, max([r.get(k, 0) for r in r_counters]))
          cpn[n-1] = count_clip/sum(g_counter.values())
  
      ls = len(gen)
      lc = max([len(ref) for ref in ref_list])
      brevity_penalty = 1 if lc > ls else np.exp(1-ls/lc)
  
      if weights is None:
          weights = np.ones_like(cpn)
      bleu = brevity_penalty * np.exp(np.mean(weights * np.log(cpn)))
      return bleu
  
  
  bleu = BLEU(["The cat is on the mat", "There is a cat on the mat"], "The cat is on the mat", 3)
  print(bleu)