import re
import six


def tokenize(text, stemmer):

  """Tokenize input text into a list of tokens.
  This approach aims to replicate the approach taken by Chin-Yew Lin in
  the original ROUGE implementation.
  Args:
    text: A text blob to tokenize.
    stemmer: An optional stemmer.
  Returns:
    A list of string tokens extracted from input text.
  """

  # Convert everything to lowercase.
  text = text.lower()
  # Replace any non-alpha-numeric characters with spaces.
  text = re.sub(r"[^a-z0-9]+", " ", six.ensure_str(text))

  tokens = re.split(r"\s+", text)
  if stemmer:
    # Only stem words more than 3 characters long.
    tokens = [stemmer.stem(x) if len(x) > 3 else x for x in tokens]

  # One final check to drop any empty or invalid tokens.
  tokens = [x for x in tokens if re.match(r"^[a-z0-9]+$", six.ensure_str(x))]

  return tokens