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 java.util.Collection;
  import org.apache.beam.sdk.coders.Coder;
  import org.apache.beam.sdk.transforms.display.DisplayData;
  import org.apache.beam.sdk.transforms.display.HasDisplayData;
  import org.apache.beam.sdk.values.TypeDescriptor;
  import org.joda.time.Instant;
  
  /**
   * The argument to the {@link Window} transform used to assign elements into windows and to
   * determine how windows are merged. See {@link Window} for more information on how {@code
   * WindowFn}s are used and for a library of predefined {@link WindowFn WindowFns}.
   *
   * <p>Users will generally want to use the predefined {@link WindowFn WindowFns}, but it is also
   * possible to create new subclasses.
   *
   * <p>To create a custom {@link WindowFn}, inherit from this class and override all required
   * methods. If no merging is required, inherit from {@link NonMergingWindowFn} instead. If no
   * merging is required and each element is assigned to a single window, inherit from {@link
   * PartitioningWindowFn}. Inheriting from the most specific subclass will enable more optimizations
   * in the runner.
   *
   * @param <T> type of elements being windowed
   * @param <W> {@link BoundedWindow} subclass used to represent the windows used by this {@link
   *     WindowFn}
   */
  public abstract class WindowFn<T, W extends BoundedWindow> implements Serializable, HasDisplayData {
    /** Information available when running {@link #assignWindows}. */
    public abstract class AssignContext {
      /** Returns the current element. */
      public abstract T element();
  
      /** Returns the timestamp of the current element. */
      public abstract Instant timestamp();
  
      /** Returns the window of the current element prior to this {@code WindowFn} being called. */
      public abstract BoundedWindow window();
    }
  
    /** Given a timestamp and element, returns the set of windows into which it should be placed. */
    public abstract Collection<W> assignWindows(AssignContext c) throws Exception;
  
    /** Information available when running {@link #mergeWindows}. */
    public abstract class MergeContext {
      /** Returns the current set of windows. */
      public abstract Collection<W> windows();
  
      /**
       * Signals to the framework that the windows in {@code toBeMerged} should be merged together to
       * form {@code mergeResult}.
       *
       * <p>{@code toBeMerged} should be a subset of {@link #windows} and disjoint from the {@code
       * toBeMerged} set of previous calls to {@code merge}.
       *
       * <p>{@code mergeResult} must either not be in {@link #windows} or be in {@code toBeMerged}.
       *
       * @throws IllegalArgumentException if any elements of toBeMerged are not in windows(), or have
       *     already been merged
       */
      public abstract void merge(Collection<W> toBeMerged, W mergeResult) throws Exception;
    }
  
    /**
     * Does whatever merging of windows is necessary.
     *
     * <p>See {@link MergeOverlappingIntervalWindows#mergeWindows} for an example of how to override
     * this method.
     */
    public abstract void mergeWindows(MergeContext c) throws Exception;
  
    /**
     * Returns whether this performs the same merging as the given {@code WindowFn}.
     *
     * @deprecated please override verifyCompatibility to throw a useful error message; we will remove
     *     isCompatible at version 3.0.0
     */
    @Deprecated
    public abstract boolean isCompatible(WindowFn<?, ?> other);
  
    /**
     * Throw {@link IncompatibleWindowException} if this WindowFn does not perform the same merging as
     * the given ${@code WindowFn}.
     *
     * @throws IncompatibleWindowException if compared WindowFns are not compatible.
     */
    public void verifyCompatibility(WindowFn<?, ?> other) throws IncompatibleWindowException {
      if (!this.isCompatible(other)) {
        throw new IncompatibleWindowException(
            other,
            String.format(
                "%s is not compatible with %s",
                this.getClass().getSimpleName(), other.getClass().getSimpleName()));
      }
    }
  
    /** Returns the {@link Coder} used for serializing the windows used by this windowFn. */
    public abstract Coder<W> windowCoder();
  
    /**
     * Returns the default {@link WindowMappingFn} to use to map main input windows to side input
     * windows. This should accept arbitrary main input windows, and produce a {@link BoundedWindow}
     * that can be produced by this {@link WindowFn}.
     */
    public abstract WindowMappingFn<W> getDefaultWindowMappingFn();
  
    /** Returns true if this {@code WindowFn} never needs to merge any windows. */
    public boolean isNonMerging() {
      return false;
    }
  
    /**
     * Returns true if this {@link WindowFn} always assigns an element to exactly one window.
     *
     * <p>If this varies per-element, or cannot be determined, conservatively return false.
     *
     * <p>By default, returns false.
     */
    public boolean assignsToOneWindow() {
      return false;
    }
  
    /**
     * Returns a {@link TypeDescriptor} capturing what is known statically about the window type of
     * this {@link WindowFn} instance's most-derived class.
     *
     * <p>In the normal case of a concrete {@link WindowFn} subclass with no generic type parameters
     * of its own (including anonymous inner classes), this will be a complete non-generic type.
     */
    public TypeDescriptor<W> getWindowTypeDescriptor() {
      return new TypeDescriptor<W>(this) {};
    }
  
    /**
     * {@inheritDoc}
     *
     * <p>By default, does not register any display data. Implementors may override this method to
     * provide their own display data.
     */
    @Override
    public void populateDisplayData(DisplayData.Builder builder) {}
  }
  