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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 _wait_time(self, shard_state, secs, now=datetime.datetime.now):
 """Time to wait until slice_start_time is secs ago from now. Args: shard_state: shard state. secs: duration in seconds. now: a func that gets now. Returns: 0 if no wait. A positive int in seconds otherwise. Always around up. """

    assert shard_state.slice_start_time is not None
    delta = now() - shard_state.slice_start_time
    duration = datetime.timedelta(seconds=secs)
    if delta < duration:
      return util.total_seconds(duration - delta)
    else:
      return 0