# coding=utf-8
# Copyright 2019 The Google Research Authors.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# Lint as: python2, python3
"""A library for tokenizing text."""

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

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
