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.runners.samza.util;
  
  import java.util.Collection;
  import java.util.List;
  import java.util.concurrent.CompletableFuture;
  import java.util.concurrent.CompletionStage;
  import java.util.stream.Collectors;
  import java.util.stream.Stream;
  
  /** A util class to handle java 8 {@link CompletableFuture} and {@link CompletionStage}. */
  @SuppressWarnings({"rawtypes"})
  public final class FutureUtils {
    /**
     * Flattens the input future collection and returns a single future comprising the results of all
     * the futures.
     *
     * @param inputFutures input future collection
     * @param <T> result type of the input future
     * @return a single {@link CompletionStage} that contains the results of all the input futures.
     */
    public static <T> CompletionStage<Collection<T>> flattenFutures(
        Collection<CompletionStage<T>> inputFutures) {
      CompletableFuture<T>[] futures = inputFutures.toArray(new CompletableFuture[0]);
  
      return CompletableFuture.allOf(futures)
          .thenApply(
              ignored -> {
                final List<T> result =
                    Stream.of(futures).map(CompletableFuture::join).collect(Collectors.toList());
                return result;
              });
    }
  
    public static <T> CompletionStage<Collection<T>> combineFutures(
        CompletionStage<Collection<T>> future1, CompletionStage<Collection<T>> future2) {
  
      if (future1 == null) {
        return future2;
      } else if (future2 == null) {
        return future1;
      } else {
        return future1.thenCombine(
            future2,
            (c1, c2) -> {
              c1.addAll(c2);
              return c1;
            });
      }
    }
  }
  