text: # -*- coding: utf-8 -*-
  #
  # Copyright 2018 - Swiss Data Science Center (SDSC)
  # A partnership between École Polytechnique Fédérale de Lausanne (EPFL) and
  # Eidgenössische Technische Hochschule Zürich (ETHZ).
  #
  # 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.
  """Process Git repository."""
  
  from collections import deque
  
  GRAY, BLACK = 0, 1
  
  
  def topological(nodes):
      """Return nodes in a topological order."""
      order, enter, state = deque(), set(nodes), {}
  
      def dfs(node):
          """Visit nodes in depth-first order."""
          state[node] = GRAY
          for parent in nodes.get(node, ()):
              color = state.get(parent, None)
              if color == GRAY:
                  raise ValueError('cycle')
              if color == BLACK:
                  continue
              enter.discard(parent)
              dfs(parent)
          order.appendleft(node)
          state[node] = BLACK
  
      while enter:
          dfs(enter.pop())
  
      return order
  