text: import ray
  from ray.util.annotations import PublicAPI
  
  
  @PublicAPI(stability="beta")
  class ActorPool:
      """Utility class to operate on a fixed pool of actors.
  
      Arguments:
          actors (list): List of Ray actor handles to use in this pool.
  
      Examples:
          >>> import ray
          >>> from ray.util.actor_pool import ActorPool
          >>> @ray.remote # doctest: +SKIP
          >>> class Actor: # doctest: +SKIP
          ...     ... # doctest: +SKIP
          >>> a1, a2 = Actor.remote(), Actor.remote() # doctest: +SKIP
          >>> pool = ActorPool([a1, a2]) # doctest: +SKIP
          >>> print(list(pool.map(lambda a, v: a.double.remote(v), # doctest: +SKIP
          ...                     [1, 2, 3, 4]))) # doctest: +SKIP
          [2, 4, 6, 8]
      """
  
      def __init__(self, actors):
          # actors to be used
          self._idle_actors = list(actors)
  
          # get actor from future
          self._future_to_actor = {}
  
          # get future from index
          self._index_to_future = {}
  
          # next task to do
          self._next_task_index = 0
  
          # next task to return
          self._next_return_index = 0
  
          # next work depending when actors free
          self._pending_submits = []
  
      def map(self, fn, values):
          """Apply the given function in parallel over the actors and values.
  
          This returns an ordered iterator that will return results of the map
          as they finish. Note that you must iterate over the iterator to force
          the computation to finish.
  
          Arguments:
              fn (func): Function that takes (actor, value) as argument and
                  returns an ObjectRef computing the result over the value. The
                  actor will be considered busy until the ObjectRef completes.
              values (list): List of values that fn(actor, value) should be
                  applied to.
  
          Returns:
              Iterator over results from applying fn to the actors and values.
  
          Examples:
              >>> from ray.util.actor_pool import ActorPool
              >>> pool = ActorPool(...) # doctest: +SKIP
              >>> print(list(pool.map(lambda a, v: a.double.remote(v),
              ...                     [1, 2, 3, 4]))) # doctest: +SKIP
              [2, 4, 6, 8]
          """
          # Ignore/Cancel all the previous submissions
          # by calling `has_next` and `gen_next` repeteadly.
          while self.has_next():
              try:
                  self.get_next(timeout=0)
              except TimeoutError:
                  pass
  
          for v in values:
              self.submit(fn, v)
          while self.has_next():
              yield self.get_next()
  
      def map_unordered(self, fn, values):
          """Similar to map(), but returning an unordered iterator.
  
          This returns an unordered iterator that will return results of the map
          as they finish. This can be more efficient that map() if some results
          take longer to compute than others.
  
          Arguments:
              fn (func): Function that takes (actor, value) as argument and
                  returns an ObjectRef computing the result over the value. The
                  actor will be considered busy until the ObjectRef completes.
              values (list): List of values that fn(actor, value) should be
                  applied to.
  
          Returns:
              Iterator over results from applying fn to the actors and values.
  
          Examples:
              >>> from ray.util.actor_pool import ActorPool
              >>> pool = ActorPool(...) # doctest: +SKIP
              >>> print(list(pool.map_unordered(lambda a, v: a.double.remote(v),
              ...                               [1, 2, 3, 4]))) # doctest: +SKIP
              [6, 2, 4, 8]
          """
          # Ignore/Cancel all the previous submissions
          # by calling `has_next` and `gen_next_unordered` repeteadly.
          while self.has_next():
              try:
                  self.get_next_unordered(timeout=0)
              except TimeoutError:
                  pass
  
          for v in values:
              self.submit(fn, v)
          while self.has_next():
              yield self.get_next_unordered()
  
      def submit(self, fn, value):
          """Schedule a single task to run in the pool.
  
          This has the same argument semantics as map(), but takes on a single
          value instead of a list of values. The result can be retrieved using
          get_next() / get_next_unordered().
  
          Arguments:
              fn (func): Function that takes (actor, value) as argument and
                  returns an ObjectRef computing the result over the value. The
                  actor will be considered busy until the ObjectRef completes.
              value (object): Value to compute a result for.
  
          Examples:
              >>> from ray.util.actor_pool import ActorPool
              >>> pool = ActorPool(...) # doctest: +SKIP
              >>> pool.submit(lambda a, v: a.double.remote(v), 1) # doctest: +SKIP
              >>> pool.submit(lambda a, v: a.double.remote(v), 2) # doctest: +SKIP
              >>> print(pool.get_next(), pool.get_next()) # doctest: +SKIP
              2, 4
          """
          if self._idle_actors:
              actor = self._idle_actors.pop()
              future = fn(actor, value)
              future_key = tuple(future) if isinstance(future, list) else future
              self._future_to_actor[future_key] = (self._next_task_index, actor)
              self._index_to_future[self._next_task_index] = future
              self._next_task_index += 1
          else:
              self._pending_submits.append((fn, value))
  
      def has_next(self):
          """Returns whether there are any pending results to return.
  
          Returns:
              True if there are any pending results not yet returned.
  
          Examples:
              >>> from ray.util.actor_pool import ActorPool
              >>> pool = ActorPool(...) # doctest: +SKIP
              >>> pool.submit(lambda a, v: a.double.remote(v), 1) # doctest: +SKIP
              >>> print(pool.has_next()) # doctest: +SKIP
              True
              >>> print(pool.get_next()) # doctest: +SKIP
              2
              >>> print(pool.has_next()) # doctest: +SKIP
              False
          """
          return bool(self._future_to_actor)
  
      def get_next(self, timeout=None):
          """Returns the next pending result in order.
  
          This returns the next result produced by submit(), blocking for up to
          the specified timeout until it is available.
  
          Returns:
              The next result.
  
          Raises:
              TimeoutError if the timeout is reached.
  
          Examples:
              >>> from ray.util.actor_pool import ActorPool
              >>> pool = ActorPool(...) # doctest: +SKIP
              >>> pool.submit(lambda a, v: a.double.remote(v), 1) # doctest: +SKIP
              >>> print(pool.get_next()) # doctest: +SKIP
              2
          """
          if not self.has_next():
              raise StopIteration("No more results to get")
          if self._next_return_index >= self._next_task_index:
              raise ValueError(
                  "It is not allowed to call get_next() after get_next_unordered()."
              )
          future = self._index_to_future[self._next_return_index]
          if timeout is not None:
              res, _ = ray.wait([future], timeout=timeout)
              if not res:
                  raise TimeoutError("Timed out waiting for result")
          del self._index_to_future[self._next_return_index]
          self._next_return_index += 1
  
          future_key = tuple(future) if isinstance(future, list) else future
          i, a = self._future_to_actor.pop(future_key)
  
          self._return_actor(a)
          return ray.get(future)
  
      def get_next_unordered(self, timeout=None):
          """Returns any of the next pending results.
  
          This returns some result produced by submit(), blocking for up to
          the specified timeout until it is available. Unlike get_next(), the
          results are not always returned in same order as submitted, which can
          improve performance.
  
          Returns:
              The next result.
  
          Raises:
              TimeoutError if the timeout is reached.
  
          Examples:
              >>> from ray.util.actor_pool import ActorPool
              >>> pool = ActorPool(...) # doctest: +SKIP
              >>> pool.submit(lambda a, v: a.double.remote(v), 1) # doctest: +SKIP
              >>> pool.submit(lambda a, v: a.double.remote(v), 2) # doctest: +SKIP
              >>> print(pool.get_next_unordered()) # doctest: +SKIP
              4
              >>> print(pool.get_next_unordered()) # doctest: +SKIP
              2
          """
          if not self.has_next():
              raise StopIteration("No more results to get")
          # TODO(ekl) bulk wait for performance
          res, _ = ray.wait(list(self._future_to_actor), num_returns=1, timeout=timeout)
          if res:
              [future] = res
          else:
              raise TimeoutError("Timed out waiting for result")
          i, a = self._future_to_actor.pop(future)
          self._return_actor(a)
          del self._index_to_future[i]
          self._next_return_index = max(self._next_return_index, i + 1)
          return ray.get(future)
  
      def _return_actor(self, actor):
          self._idle_actors.append(actor)
          if self._pending_submits:
              self.submit(*self._pending_submits.pop(0))
  
      def has_free(self):
          """Returns whether there are any idle actors available.
  
          Returns:
              True if there are any idle actors and no pending submits.
  
          Examples:
              >>> @ray.remote # doctest: +SKIP
              >>> class Actor: # doctest: +SKIP
              ...     ... # doctest: +SKIP
              >>> a1 = Actor.remote() # doctest: +SKIP
              >>> pool = ActorPool(a1) # doctest: +SKIP
              >>> pool.submit(lambda a, v: a.double.remote(v), 1) # doctest: +SKIP
              >>> print(pool.has_free()) # doctest: +SKIP
              False
              >>> print(pool.get_next()) # doctest: +SKIP
              2
              >>> print(pool.has_free()) # doctest: +SKIP
              True
          """
          return len(self._idle_actors) > 0 and len(self._pending_submits) == 0
  
      def pop_idle(self):
          """Removes an idle actor from the pool.
  
          Returns:
              An idle actor if one is available.
              None if no actor was free to be removed.
  
          Examples:
              >>> @ray.remote # doctest: +SKIP
              >>> class Actor: # doctest: +SKIP
              ...     ... # doctest: +SKIP
              >>> a1 = Actor.remote() # doctest: +SKIP
              >>> pool = ActorPool([a1]) # doctest: +SKIP
              >>> pool.submit(lambda a, v: a.double.remote(v), 1) # doctest: +SKIP
              >>> print(pool.pop_idle()) # doctest: +SKIP
              None
              >>> print(pool.get_next()) # doctest: +SKIP
              2
              >>> print(pool.pop_idle()) # doctest: +SKIP
              <ptr to a1>
          """
          if self.has_free():
              return self._idle_actors.pop()
          return None
  
      def push(self, actor):
          """Pushes a new actor into the current list of idle actors.
  
          Examples:
              >>> @ray.remote # doctest: +SKIP
              >>> class Actor: # doctest: +SKIP
              ...     ... # doctest: +SKIP
              >>> a1, b1 = Actor.remote(), Actor.remote() # doctest: +SKIP
              >>> pool = ActorPool([a1]) # doctest: +SKIP
              >>> pool.submit(lambda a, v: a.double.remote(v), 1) # doctest: +SKIP
              >>> print(pool.get_next()) # doctest: +SKIP
              2
              >>> pool2 = ActorPool([b1]) # doctest: +SKIP
              >>> pool2.push(pool.pop_idle()) # doctest: +SKIP
          """
          busy_actors = []
          if self._future_to_actor.values():
              _, busy_actors = zip(*self._future_to_actor.values())
          if actor in self._idle_actors or actor in busy_actors:
              raise ValueError("Actor already belongs to current ActorPool")
          else:
              self._idle_actors.append(actor)
  