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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def calculate_keys_by_mapreduce_state(cls, mapreduce_state):
 """Calculate all shard states keys for given mapreduce. Args: mapreduce_state: MapreduceState instance Returns: A list of keys for shard states, sorted by shard id. The corresponding shard states may not exist. """

    if mapreduce_state is None:
      return []

    keys = []
    for i in range(mapreduce_state.mapreduce_spec.mapper.shard_count):
      shard_id = cls.shard_id_from_number(mapreduce_state.key().name(), i)
      keys.append(cls.get_key_by_shard_id(shard_id))
    return keys