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from __future__ import absolute_import

from apache_beam import ExternalTransform
from apache_beam import pvalue
from apache_beam.coders import VarIntCoder
from apache_beam.portability.api.external_transforms_pb2 import ConfigValue
from apache_beam.portability.api.external_transforms_pb2 import ExternalConfigurationPayload
from apache_beam.transforms import ptransform


class GenerateSequence(ptransform.PTransform):
  """
    An external PTransform which provides a bounded or unbounded stream of
    integers.

    Note: To use this transform, you need to start the Java expansion service.
    Please refer to the portability documentation on how to do that. The
    expansion service address has to be provided when instantiating this
    transform. During pipeline translation this transform will be replaced by
    the Java SDK's GenerateSequence.

    If you start Flink's job server, the expansion service will be started on
    port 8097. This is also the configured default for this transform. For a
    different address, please set the expansion_service parameter.

    For more information see:
    - https://beam.apache.org/documentation/runners/flink/
    - https://beam.apache.org/roadmap/portability/

    Note: Runners need to support translating Read operations in order to use
    this source. At the moment only the Flink Runner supports this.
  """

  def __init__(self, start, stop=None,
               elements_per_period=None, max_read_time=None,
               expansion_service='localhost:8097'):
    super(GenerateSequence, self).__init__()
    self._urn = 'beam:external:java:generate_sequence:v1'
    self.start = start
    self.stop = stop
    self.elements_per_period = elements_per_period
    self.max_read_time = max_read_time
    self.expansion_service = expansion_service

  def expand(self, pbegin):
    if not isinstance(pbegin, pvalue.PBegin):
      raise Exception("GenerateSequence must be a root transform")

    coder = VarIntCoder()
    coder_urn = ['beam:coder:varint:v1']
    args = {
        'start':
        ConfigValue(
            coder_urn=coder_urn,
            payload=coder.encode(self.start))
    }
    if self.stop:
      args['stop'] = ConfigValue(
          coder_urn=coder_urn,
          payload=coder.encode(self.stop))
    if self.elements_per_period:
      args['elements_per_period'] = ConfigValue(
          coder_urn=coder_urn,
          payload=coder.encode(self.elements_per_period))
    if self.max_read_time:
      args['max_read_time'] = ConfigValue(
          coder_urn=coder_urn,
          payload=coder.encode(self.max_read_time))

    payload = ExternalConfigurationPayload(configuration=args)
    return pbegin.apply(
        ExternalTransform(
            self._urn,
            payload.SerializeToString(),
            self.expansion_service))
