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.core;
  
  import static org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Preconditions.checkNotNull;
  import static org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Preconditions.checkState;
  
  import java.util.NoSuchElementException;
  import org.apache.beam.sdk.util.common.Reiterator;
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * A {@link Reiterator} that supports one-element lookahead during iteration.
   *
   * @param <T> the type of elements returned by this iterator
   */
  @SuppressWarnings({
    "nullness" // TODO(https://github.com/apache/beam/issues/20497)
  })
  public final class PeekingReiterator<T> implements Reiterator<T> {
    private @Nullable T nextElement;
    private boolean nextElementComputed;
    private final Reiterator<T> iterator;
  
    public PeekingReiterator(Reiterator<T> iterator) {
      this.iterator = checkNotNull(iterator);
    }
  
    PeekingReiterator(PeekingReiterator<T> it) {
      this.iterator = checkNotNull(checkNotNull(it).iterator.copy());
      this.nextElement = it.nextElement;
      this.nextElementComputed = it.nextElementComputed;
    }
  
    @Override
    public boolean hasNext() {
      computeNext();
      return nextElementComputed;
    }
  
    @Override
    public T next() {
      T result = peek();
      nextElementComputed = false;
      return result;
    }
  
    /**
     * {@inheritDoc}
     *
     * <p>If {@link #peek} is called, {@code remove} is disallowed until {@link #next} has been
     * subsequently called.
     */
    @Override
    public void remove() {
      checkState(!nextElementComputed, "After peek(), remove() is disallowed until next() is called");
      iterator.remove();
    }
  
    @Override
    public PeekingReiterator<T> copy() {
      return new PeekingReiterator<>(this);
    }
  
    /**
     * Returns the element that would be returned by {@link #next}, without actually consuming the
     * element.
     *
     * @throws NoSuchElementException if there is no next element
     */
    public T peek() {
      computeNext();
      if (!nextElementComputed) {
        throw new NoSuchElementException();
      }
      return nextElement;
    }
  
    private void computeNext() {
      if (nextElementComputed) {
        return;
      }
      if (!iterator.hasNext()) {
        return;
      }
      nextElement = iterator.next();
      nextElementComputed = true;
    }
  }
  