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;
  
  import org.apache.beam.sdk.coders.KvCoder;
  import org.apache.beam.sdk.coders.NullableCoder;
  import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
  import org.apache.beam.sdk.transforms.windowing.DefaultTrigger;
  import org.apache.beam.sdk.transforms.windowing.PaneInfo;
  import org.apache.beam.sdk.values.KV;
  import org.apache.beam.sdk.values.PCollection;
  import org.apache.beam.sdk.values.TypeDescriptor;
  import org.apache.beam.sdk.values.WindowingStrategy;
  import org.checkerframework.checker.nullness.qual.Nullable;
  import org.joda.time.Duration;
  
  /**
   * {@code Distinct<T>} takes a {@code PCollection<T>} and returns a {@code PCollection<T>} that has
   * all distinct elements of the input. Thus, each element is unique within each window.
   *
   * <p>Two values of type {@code T} are compared for equality <b>not</b> by regular Java {@link
   * Object#equals}, but instead by first encoding each of the elements using the {@code
   * PCollection}'s {@code Coder}, and then comparing the encoded bytes. This admits efficient
   * parallel evaluation.
   *
   * <p>Optionally, a function may be provided that maps each element to a representative value. In
   * this case, two elements will be considered duplicates if they have equal representative values,
   * with equality being determined as above.
   *
   * <p>By default, the {@code Coder} of the output {@code PCollection} is the same as the {@code
   * Coder} of the input {@code PCollection}.
   *
   * <p>Each output element is in the same window as its corresponding input element, and has the
   * timestamp of the end of that window. The output {@code PCollection} has the same {@link
   * org.apache.beam.sdk.transforms.windowing.WindowFn} as the input.
   *
   * <p>Does not preserve any order the input PCollection might have had.
   *
   * <p>Example of use:
   *
   * <pre>{@code
   * PCollection<String> words = ...;
   * PCollection<String> uniqueWords =
   *     words.apply(Distinct.<String>create());
   * }</pre>
   *
   * @param <T> the type of the elements of the input and output {@code PCollection}s
   */
  @SuppressWarnings({
    "nullness" // TODO(https://github.com/apache/beam/issues/20497)
  })
  public class Distinct<T> extends PTransform<PCollection<T>, PCollection<T>> {
  
    /**
     * Returns a {@code Distinct<T>} {@code PTransform}.
     *
     * @param <T> the type of the elements of the input and output {@code PCollection}s
     */
    public static <T> Distinct<T> create() {
      return new Distinct<>();
    }
  
    /**
     * Returns a {@code Distinct<T, IdT>} {@code PTransform}.
     *
     * @param <T> the type of the elements of the input and output {@code PCollection}s
     * @param <IdT> the type of the representative value used to dedup
     */
    public static <T, IdT> WithRepresentativeValues<T, IdT> withRepresentativeValueFn(
        SerializableFunction<T, IdT> fn) {
      return new WithRepresentativeValues<>(fn, null);
    }
  
    private static <T, W extends BoundedWindow> void validateWindowStrategy(
        WindowingStrategy<T, W> strategy) {
      if (strategy.needsMerge()
          && (!strategy.getTrigger().getClass().equals(DefaultTrigger.class)
              || strategy.getAllowedLateness().isLongerThan(Duration.ZERO))) {
        throw new UnsupportedOperationException(
            String.format(
                "%s does not support merging windowing strategies, except when using the default "
                    + "trigger and zero allowed lateness.",
                Distinct.class.getSimpleName()));
      }
    }
  
    @Override
    public PCollection<T> expand(PCollection<T> in) {
      validateWindowStrategy(in.getWindowingStrategy());
      PCollection<KV<T, Void>> combined =
          in.apply(
                  "KeyByElement",
                  MapElements.via(
                      new SimpleFunction<T, KV<T, Void>>() {
                        @Override
                        public KV<T, Void> apply(T element) {
                          return KV.of(element, (Void) null);
                        }
                      }))
              .apply(
                  "DropValues",
                  Combine.perKey(
                      new SerializableFunction<Iterable<Void>, Void>() {
                        @Override
                        public @Nullable Void apply(Iterable<Void> iter) {
                          return null; // ignore input
                        }
                      }));
      return combined.apply(
          "ExtractFirstKey",
          ParDo.of(
              new DoFn<KV<T, Void>, T>() {
                @ProcessElement
                public void processElement(
                    @Element KV<T, Void> element, PaneInfo paneInfo, OutputReceiver<T> receiver) {
                  if (paneInfo.isFirst()) {
                    // Only output the key if it's the first time it's been seen.
                    receiver.output(element.getKey());
                  }
                }
              }));
    }
  
    /**
     * A {@link Distinct} {@link PTransform} that uses a {@link SerializableFunction} to obtain a
     * representative value for each input element.
     *
     * <p>Construct via {@link Distinct#withRepresentativeValueFn(SerializableFunction)}.
     *
     * @param <T> the type of input and output element
     * @param <IdT> the type of representative values used to dedup
     */
    public static class WithRepresentativeValues<T, IdT>
        extends PTransform<PCollection<T>, PCollection<T>> {
      private final SerializableFunction<T, IdT> fn;
      private final TypeDescriptor<IdT> representativeType;
  
      private WithRepresentativeValues(
          SerializableFunction<T, IdT> fn, TypeDescriptor<IdT> representativeType) {
        this.fn = fn;
        this.representativeType = representativeType;
      }
  
      @Override
      public PCollection<T> expand(PCollection<T> in) {
        validateWindowStrategy(in.getWindowingStrategy());
        WithKeys<IdT, T> withKeys = WithKeys.of(fn);
        if (representativeType != null) {
          withKeys = withKeys.withKeyType(representativeType);
        }
  
        PCollection<KV<IdT, T>> keyed = in.apply("KeyByRepresentativeValue", withKeys);
        KvCoder<IdT, T> keyedCoder = (KvCoder<IdT, T>) keyed.getCoder();
        PCollection<KV<IdT, T>> combined =
            keyed
                .apply(
                    "OneValuePerKey",
                    Combine.perKey(
                        new Combine.BinaryCombineFn<T>() {
                          @Override
                          public T apply(T left, T right) {
                            return left;
                          }
                        }))
                // When there is no input, the combine outputs null. This can occur when the input
                // is in discarding mode with speculative triggers consuming all input prior to
                // on-time or GC firing. There is no reason that the input coder would necessarily
                // support nulls.
                .setCoder(
                    KvCoder.of(
                        keyedCoder.getKeyCoder(), NullableCoder.of(keyedCoder.getValueCoder())));
  
        return combined.apply(
            "KeepFirstPane",
            ParDo.of(
                new DoFn<KV<IdT, T>, T>() {
                  @ProcessElement
                  public void processElement(
                      @Element KV<IdT, T> element, PaneInfo paneInfo, OutputReceiver<T> receiver) {
                    // Only output the value if it's the first time it's been seen.
                    if (paneInfo.isFirst()) {
                      receiver.output(element.getValue());
                    }
                  }
                }));
      }
  
      /**
       * Return a {@code WithRepresentativeValues} {@link PTransform} that is like this one, but with
       * the specified output type descriptor.
       *
       * <p>Required for use of {@link Distinct#withRepresentativeValueFn(SerializableFunction)} in
       * Java 8 with a lambda as the fn.
       *
       * @param type a {@link TypeDescriptor} describing the representative type of this {@code
       *     WithRepresentativeValues}
       * @return A {@code WithRepresentativeValues} {@link PTransform} that is like this one, but with
       *     the specified output type descriptor.
       */
      public WithRepresentativeValues<T, IdT> withRepresentativeType(TypeDescriptor<IdT> type) {
        return new WithRepresentativeValues<>(fn, type);
      }
    }
  }
  