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.sdk.transforms.windowing;
  
  import java.io.Serializable;
  import org.apache.beam.sdk.values.PCollectionView;
  import org.joda.time.Duration;
  
  /**
   * A function that takes the windows of elements in a main input and maps them to the appropriate
   * window in a {@link PCollectionView} consumed as a {@link
   * org.apache.beam.sdk.transforms.ParDo.MultiOutput#withSideInputs(PCollectionView[]) side input}.
   */
  public abstract class WindowMappingFn<TargetWindowT extends BoundedWindow> implements Serializable {
    private final Duration maximumLookback;
  
    /** Create a new {@link WindowMappingFn} with {@link Duration#ZERO zero} maximum lookback. */
    protected WindowMappingFn() {
      this(Duration.ZERO);
    }
  
    /** Create a new {@link WindowMappingFn} with the specified maximum lookback. */
    protected WindowMappingFn(Duration maximumLookback) {
      this.maximumLookback = maximumLookback;
    }
  
    /** Returns the window of the side input corresponding to the given window of the main input. */
    public abstract TargetWindowT getSideInputWindow(BoundedWindow mainWindow);
  
    /**
     * The maximum distance between the end of any main input window {@code mainWindow} and the end of
     * the side input window returned by {@link #getSideInputWindow(BoundedWindow)}
     *
     * <p>A side input window {@code w} becomes unreachable when the input watermarks for all
     * consumers surpasses the timestamp:
     *
     * <p>(end of side input window) + (maximum lookback) + (main input allowed lateness).
     *
     * <p>At this point, every main input window that could map to {@code w} is expired.
     */
    public final Duration maximumLookback() {
      return maximumLookback;
    }
  }
  