private static <K, InputT, AccumT> Coder<AccumT> extractAccumulatorCoder(
GlobalCombineFn<InputT, AccumT, ?> combineFn,
AppliedPTransform<PCollection<KV<K, InputT>>, ?, Combine.PerKey<K, InputT, ?>> transform)
throws CannotProvideCoderException {
@SuppressWarnings("unchecked")
PCollection<KV<K, InputT>> mainInput =
(PCollection<KV<K, InputT>>)
Iterables.getOnlyElement(TransformInputs.nonAdditionalInputs(transform));
KvCoder<K, InputT> inputCoder = (KvCoder<K, InputT>) mainInput.getCoder();
return AppliedCombineFn.withInputCoder(
combineFn,
transform.getPipeline().getCoderRegistry(),
inputCoder,
transform.getTransform().getSideInputs(),
((PCollection<?>) Iterables.getOnlyElement(transform.getOutputs().values()))
.getWindowingStrategy())
.getAccumulatorCoder();
}