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"""Create transform for streaming."""

from apache_beam import DoFn
from apache_beam import ParDo
from apache_beam import PTransform
from apache_beam import Windowing
from apache_beam import pvalue
from apache_beam.transforms.window import GlobalWindows


class StreamingCreate(PTransform):
  """A specialized implementation for ``Create`` transform in streaming mode.

  Note: There is no unbounded source API in python to wrap the Create source,
  so we map this to composite of Impulse primitive and an SDF.
  """

  def __init__(self, values, coder):
    self.coder = coder
    self.encoded_values = map(coder.encode, values)

  class DecodeAndEmitDoFn(DoFn):
    """A DoFn which stores encoded versions of elements.

    It also stores a Coder to decode and emit those elements.
    TODO: BEAM-2422 - Make this a SplittableDoFn.
    """

    def __init__(self, encoded_values, coder):
      self.encoded_values = encoded_values
      self.coder = coder

    def process(self, unused_element):
      for encoded_value in self.encoded_values:
        yield self.coder.decode(encoded_value)

  class Impulse(PTransform):
    """The Dataflow specific override for the impulse primitive."""

    def expand(self, pbegin):
      assert isinstance(pbegin, pvalue.PBegin), (
          'Input to Impulse transform must be a PBegin but found %s' % pbegin)
      return pvalue.PCollection(pbegin.pipeline)

    def get_windowing(self, inputs):
      return Windowing(GlobalWindows())

    def infer_output_type(self, unused_input_type):
      return bytes

  def expand(self, pbegin):
    return (pbegin
            | 'Impulse' >> self.Impulse()
            | 'Decode Values' >> ParDo(
                self.DecodeAndEmitDoFn(self.encoded_values, self.coder)))
