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 edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.OutputStream;
  import java.util.Objects;
  import org.apache.beam.sdk.coders.AtomicCoder;
  import org.apache.beam.sdk.coders.BigEndianLongCoder;
  import org.apache.beam.sdk.coders.Coder;
  import org.apache.beam.sdk.coders.CoderException;
  import org.apache.beam.sdk.coders.CoderRegistry;
  import org.apache.beam.sdk.coders.DoubleCoder;
  import org.apache.beam.sdk.transforms.Combine.AccumulatingCombineFn.Accumulator;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.MoreObjects;
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * {@code PTransform}s for computing the arithmetic mean (a.k.a. average) of the elements in a
   * {@code PCollection}, or the mean of the values associated with each key in a {@code PCollection}
   * of {@code KV}s.
   *
   * <p>Example 1: get the mean of a {@code PCollection} of {@code Long}s.
   *
   * <pre>{@code
   * PCollection<Long> input = ...;
   * PCollection<Double> mean = input.apply(Mean.<Long>globally());
   * }</pre>
   *
   * <p>Example 2: calculate the mean of the {@code Integer}s associated with each unique key (which
   * is of type {@code String}).
   *
   * <pre>{@code
   * PCollection<KV<String, Integer>> input = ...;
   * PCollection<KV<String, Double>> meanPerKey =
   *     input.apply(Mean.<String, Integer>perKey());
   * }</pre>
   */
  public class Mean {
  
    private Mean() {} // Namespace only
  
    /**
     * Returns a {@code PTransform} that takes an input {@code PCollection<NumT>} and returns a {@code
     * PCollection<Double>} whose contents is the mean of the input {@code PCollection}'s elements, or
     * {@code 0} if there are no elements.
     *
     * @param <NumT> the type of the {@code Number}s being combined
     */
    public static <NumT extends Number> Combine.Globally<NumT, Double> globally() {
      return Combine.globally(Mean.of());
    }
  
    /**
     * Returns a {@code PTransform} that takes an input {@code PCollection<KV<K, N>>} and returns a
     * {@code PCollection<KV<K, Double>>} that contains an output element mapping each distinct key in
     * the input {@code PCollection} to the mean of the values associated with that key in the input
     * {@code PCollection}.
     *
     * <p>See {@link Combine.PerKey} for how this affects timestamps and bucketing.
     *
     * @param <K> the type of the keys
     * @param <NumT> the type of the {@code Number}s being combined
     */
    public static <K, NumT extends Number> Combine.PerKey<K, NumT, Double> perKey() {
      return Combine.perKey(Mean.of());
    }
  
    /**
     * A {@code Combine.CombineFn} that computes the arithmetic mean (a.k.a. average) of an {@code
     * Iterable} of numbers of type {@code N}, useful as an argument to {@link Combine#globally} or
     * {@link Combine#perKey}.
     *
     * <p>Returns {@code Double.NaN} if combining zero elements.
     *
     * @param <NumT> the type of the {@code Number}s being combined
     */
    public static <NumT extends Number>
        Combine.AccumulatingCombineFn<NumT, CountSum<NumT>, Double> of() {
      return new MeanFn<>();
    }
  
    /////////////////////////////////////////////////////////////////////////////
  
    private static class MeanFn<NumT extends Number>
        extends Combine.AccumulatingCombineFn<NumT, CountSum<NumT>, Double> {
      /**
       * Constructs a combining function that computes the mean over a collection of values of type
       * {@code N}.
       */
      @Override
      public CountSum<NumT> createAccumulator() {
        return new CountSum<>();
      }
  
      @Override
      public Coder<CountSum<NumT>> getAccumulatorCoder(
          CoderRegistry registry, Coder<NumT> inputCoder) {
        return new CountSumCoder<>();
      }
    }
  
    /** Accumulator class for {@link MeanFn}. */
    static class CountSum<NumT extends Number> implements Accumulator<NumT, CountSum<NumT>, Double> {
  
      long count = 0;
      double sum = 0.0;
  
      public CountSum() {
        this(0, 0);
      }
  
      public CountSum(long count, double sum) {
        this.count = count;
        this.sum = sum;
      }
  
      @Override
      public void addInput(NumT element) {
        count++;
        sum += element.doubleValue();
      }
  
      @Override
      public void mergeAccumulator(CountSum<NumT> accumulator) {
        count += accumulator.count;
        sum += accumulator.sum;
      }
  
      @Override
      public Double extractOutput() {
        return count == 0 ? Double.NaN : sum / count;
      }
  
      @Override
      // Comparing doubles directly since class is package private and equals method is only used in
      // coder test.
      @SuppressFBWarnings("FE_FLOATING_POINT_EQUALITY")
      public boolean equals(@Nullable Object other) {
        if (!(other instanceof CountSum)) {
          return false;
        }
        @SuppressWarnings("unchecked")
        CountSum<?> otherCountSum = (CountSum<?>) other;
        return (count == otherCountSum.count) && (sum == otherCountSum.sum);
      }
  
      @Override
      public int hashCode() {
        return Objects.hash(count, sum);
      }
  
      @Override
      public String toString() {
        return MoreObjects.toStringHelper(this).add("count", count).add("sum", sum).toString();
      }
    }
  
    static class CountSumCoder<NumT extends Number> extends AtomicCoder<CountSum<NumT>> {
      private static final Coder<Long> LONG_CODER = BigEndianLongCoder.of();
      private static final Coder<Double> DOUBLE_CODER = DoubleCoder.of();
  
      @Override
      public void encode(CountSum<NumT> value, OutputStream outStream)
          throws CoderException, IOException {
        LONG_CODER.encode(value.count, outStream);
        DOUBLE_CODER.encode(value.sum, outStream);
      }
  
      @Override
      public CountSum<NumT> decode(InputStream inStream) throws CoderException, IOException {
        return new CountSum<>(LONG_CODER.decode(inStream), DOUBLE_CODER.decode(inStream));
      }
  
      @Override
      public void verifyDeterministic() throws NonDeterministicException {
        LONG_CODER.verifyDeterministic();
        DOUBLE_CODER.verifyDeterministic();
      }
    }
  }
  