Calculates n-grams.

  Args:
    n: which n-grams to calculate
    text: An array of tokens

  Returns:
    A set of n-grams

def _get_ngrams(n, text):
  """Calculates n-grams.

  Args:
    n: which n-grams to calculate
    text: An array of tokens

  Returns:
    A set of n-grams
  """
  ngram_set = set()
  text_length = len(text)
  max_index_ngram_start = text_length - n
  for i in range(max_index_ngram_start + 1):
    ngram_set.add(tuple(text[i:i + n]))
  return ngram_set