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.dataflow.worker.windmill.state;
  
  import java.util.ArrayList;
  import java.util.Iterator;
  import java.util.List;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Iterables;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Iterators;
  
  class ConcatIterables<T> implements Iterable<T> {
    // List of component iterables.  Should only be appended to in order to support snapshot().
    private final List<Iterable<T>> iterables;
  
    public ConcatIterables() {
      this.iterables = new ArrayList<>();
    }
  
    public void extendWith(Iterable<T> iterable) {
      iterables.add(iterable);
    }
  
    @Override
    public Iterator<T> iterator() {
      return Iterators.concat(Iterables.transform(iterables, Iterable::iterator).iterator());
    }
  
    /**
     * Returns a view of the current state of this iterable. Remembers the current length of iterables
     * so that the returned value Will not change due to future extendWith() calls.
     */
    public Iterable<T> snapshot() {
      final int limit = iterables.size();
      final List<Iterable<T>> iterablesList = iterables;
      return () ->
          Iterators.concat(
              Iterators.transform(
                  Iterators.limit(iterablesList.iterator(), limit), Iterable::iterator));
    }
  }
  