content: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.beam.sdk.util.construction;
  
  import static org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Preconditions.checkArgument;
  
  import java.util.Map;
  import org.apache.beam.sdk.coders.VoidCoder;
  import org.apache.beam.sdk.runners.AppliedPTransform;
  import org.apache.beam.sdk.runners.PTransformOverrideFactory;
  import org.apache.beam.sdk.transforms.Create;
  import org.apache.beam.sdk.transforms.Flatten;
  import org.apache.beam.sdk.transforms.Flatten.PCollections;
  import org.apache.beam.sdk.transforms.PTransform;
  import org.apache.beam.sdk.values.PCollection;
  import org.apache.beam.sdk.values.PCollectionList;
  import org.apache.beam.sdk.values.TupleTag;
  
  /**
   * A {@link PTransformOverrideFactory} that provides an empty {@link Create} to replace a {@link
   * Flatten.PCollections} that takes no input {@link PCollection PCollections}.
   */
  public class EmptyFlattenAsCreateFactory<T>
      implements PTransformOverrideFactory<
          PCollectionList<T>, PCollection<T>, Flatten.PCollections<T>> {
    private static final EmptyFlattenAsCreateFactory<Object> INSTANCE =
        new EmptyFlattenAsCreateFactory<>();
  
    public static <T> EmptyFlattenAsCreateFactory<T> instance() {
      return (EmptyFlattenAsCreateFactory<T>) INSTANCE;
    }
  
    private EmptyFlattenAsCreateFactory() {}
  
    @Override
    public PTransformReplacement<PCollectionList<T>, PCollection<T>> getReplacementTransform(
        AppliedPTransform<PCollectionList<T>, PCollection<T>, PCollections<T>> transform) {
      checkArgument(
          transform.getInputs().isEmpty(),
          "Unexpected nonempty input %s for %s",
          transform.getInputs(),
          getClass().getSimpleName());
      return PTransformReplacement.of(
          PCollectionList.empty(transform.getPipeline()), new CreateEmptyFromList<T>());
    }
  
    @Override
    public Map<PCollection<?>, ReplacementOutput> mapOutputs(
        Map<TupleTag<?>, PCollection<?>> outputs, PCollection<T> newOutput) {
      return ReplacementOutputs.singleton(outputs, newOutput);
    }
  
    private static class CreateEmptyFromList<T>
        extends PTransform<PCollectionList<T>, PCollection<T>> {
      @Override
      public PCollection<T> expand(PCollectionList<T> input) {
        return (PCollection) input.getPipeline().apply(Create.empty(VoidCoder.of()));
      }
    }
  }
  