Compute BLEU for two files (reference and hypothesis translation).

def bleu_wrapper(ref_filename, hyp_filename, case_sensitive=False):
  """Compute BLEU for two files (reference and hypothesis translation)."""
  ref_lines = tf.gfile.Open(ref_filename).read().strip().splitlines()
  hyp_lines = tf.gfile.Open(hyp_filename).read().strip().splitlines()

  if len(ref_lines) != len(hyp_lines):
    raise ValueError("Reference and translation files have different number of "
                     "lines.")
  if not case_sensitive:
    ref_lines = [x.lower() for x in ref_lines]
    hyp_lines = [x.lower() for x in hyp_lines]
  ref_tokens = [bleu_tokenize(x) for x in ref_lines]
  hyp_tokens = [bleu_tokenize(x) for x in hyp_lines]
  return metrics.compute_bleu(ref_tokens, hyp_tokens) * 100