text: # Copyright 2019 Google LLC All Rights Reserved.
  #
  # 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.
  
  """ This function handles messages posted to a pubsub topic by translating
      the data in the message as requested. The message must be a JSON encoded
      dictionary with fields:
  
      Original - the string to translate
      Language - the language to translate the string to
  
      The dictionary may have other fields, which will be ignored.
  """
  
  # [START getting_started_background_translate_setup]
  import base64
  import hashlib
  import json
  
  from google.cloud import firestore
  from google.cloud import translate
  # [END getting_started_background_translate_setup]
  
  # [START getting_started_background_translate_init]
  # Get client objects once to reuse over multiple invocations.
  xlate = translate.Client()
  db = firestore.Client()
  # [END getting_started_background_translate_init]
  
  
  # [START getting_started_background_translate_string]
  def translate_string(from_string, to_language):
      """ Translates a string to a specified language.
  
      from_string - the original string before translation
  
      to_language - the language to translate to, as a two-letter code (e.g.,
          'en' for english, 'de' for german)
  
      Returns the translated string and the code for original language
      """
      result = xlate.translate(from_string, target_language=to_language)
      return result['translatedText'], result['detectedSourceLanguage']
  # [END getting_started_background_translate_string]
  
  
  # [START getting_started_background_translate]
  def document_name(message):
      """ Messages are saved in a Firestore database with document IDs generated
          from the original string and destination language. If the exact same
          translation is requested a second time, the result will overwrite the
          prior result.
  
          message - a dictionary with fields named Language and Original, and
              optionally other fields with any names
  
          Returns a unique name that is an allowed Firestore document ID
      """
      key = '{}/{}'.format(message['Language'], message['Original'])
      hashed = hashlib.sha512(key.encode()).digest()
  
      # Note that document IDs should not contain the '/' character
      name = base64.b64encode(hashed, altchars=b'+-').decode('utf-8')
      return name
  
  
  @firestore.transactional
  def update_database(transaction, message):
      name = document_name(message)
      doc_ref = db.collection('translations').document(document_id=name)
  
      try:
          doc_ref.get(transaction=transaction)
      except firestore.NotFound:
          return  # Don't replace an existing translation
  
      transaction.set(doc_ref, message)
  
  
  def translate_message(event, context):
      """ Process a pubsub message requesting a translation
      """
      message_data = base64.b64decode(event['data']).decode('utf-8')
      message = json.loads(message_data)
  
      from_string = message['Original']
      to_language = message['Language']
  
      to_string, from_language = translate_string(from_string, to_language)
  
      message['Translated'] = to_string
      message['OriginalLanguage'] = from_language
  
      transaction = db.transaction()
      update_database(transaction, message)
  # [END getting_started_background_translate]
  