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.util.common.worker;
  
  /**
   * Implements a group of linked {@link ProgressTracker ProgressTrackers} that collectively track how
   * far a processing loop has gotten through the elements it's processing. Individual {@code
   * ProgressTracker} instances may be copied, capturing an independent view of the progress of the
   * system; this turns out to be useful for some non-trivial processing loops. The furthest point
   * reached by any {@code ProgressTracker} is the one reported.
   *
   * <p>This class is abstract. Its single extension point is {@link #report}, which should be
   * overridden to provide a function that handles the reporting of the supplied element, as
   * appropriate.
   *
   * @param <T> the type of elements being tracked
   */
  public abstract class ProgressTrackerGroup<T> {
    // TODO: Instead of an abstract class, strongly consider adding an
    // interface like Receiver to the SDK, so that this class can be final and all
    // that good stuff.
    private long nextIndexToReport = 0;
  
    public ProgressTrackerGroup() {}
  
    public final ProgressTracker<T> start() {
      return new Tracker(0);
    }
  
    /** Reports the indicated element. */
    protected abstract void report(T element);
  
    private final class Tracker implements ProgressTracker<T> {
      private long nextElementIndex;
  
      private Tracker(long nextElementIndex) {
        this.nextElementIndex = nextElementIndex;
      }
  
      @Override
      public ProgressTracker<T> copy() {
        return new Tracker(nextElementIndex);
      }
  
      @Override
      public void saw(T element) {
        long thisElementIndex = nextElementIndex;
        nextElementIndex++;
        if (thisElementIndex == nextIndexToReport) {
          nextIndexToReport = nextElementIndex;
          report(element);
        }
      }
    }
  }
  