runners/core-java/src/main/java/org/apache/beam/runners/core/PeekingReiterator.java

/*
 * 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.v26_0_jre.com.google.common.base.Preconditions.checkNotNull;
import static org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions.checkState;

import java.util.NoSuchElementException;
import javax.annotation.Nullable;
import org.apache.beam.sdk.util.common.Reiterator;

/**
 * A {@link Reiterator} that supports one-element lookahead during iteration.
 *
 * @param <T> the type of elements returned by this iterator
 */
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;
  }
}