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.extensions.sql.impl.transform.agg;
  
  import com.google.auto.value.AutoValue;
  import java.io.Serializable;
  import java.math.BigDecimal;
  
  /** Accumulates current variance of a sample, its sum, and number of elements. */
  @AutoValue
  abstract class VarianceAccumulator implements Serializable {
    static final VarianceAccumulator EMPTY =
        newVarianceAccumulator(BigDecimal.ZERO, BigDecimal.ZERO, BigDecimal.ZERO);
  
    abstract BigDecimal variance();
  
    abstract BigDecimal count();
  
    abstract BigDecimal sum();
  
    static VarianceAccumulator newVarianceAccumulator(
        BigDecimal variance, BigDecimal count, BigDecimal sum) {
  
      return new AutoValue_VarianceAccumulator(variance, count, sum);
    }
  
    static VarianceAccumulator ofZeroElements() {
      return EMPTY;
    }
  
    static VarianceAccumulator ofSingleElement(BigDecimal inputElement) {
      return newVarianceAccumulator(BigDecimal.ZERO, BigDecimal.ONE, inputElement);
    }
  
    /** See {@link VarianceFn} doc above for explanation. */
    VarianceAccumulator combineWith(VarianceAccumulator otherVariance) {
      if (EMPTY.equals(this)) {
        return otherVariance;
      }
  
      if (EMPTY.equals(otherVariance)) {
        return this;
      }
  
      BigDecimal increment = calculateIncrement(this, otherVariance);
      BigDecimal combinedVariance = this.variance().add(otherVariance.variance()).add(increment);
  
      return newVarianceAccumulator(
          combinedVariance,
          this.count().add(otherVariance.count()),
          this.sum().add(otherVariance.sum()));
    }
  
    /** Implements this part: {@code increment = m/(n(m+n)) * (sum(x) * n/m - sum(y))^2 }. */
    private BigDecimal calculateIncrement(
        VarianceAccumulator varianceX, VarianceAccumulator varianceY) {
  
      BigDecimal m = varianceX.count();
      BigDecimal n = varianceY.count();
      BigDecimal sumX = varianceX.sum();
      BigDecimal sumY = varianceY.sum();
  
      // m/(n(m+n))
      BigDecimal multiplier = m.divide(n.multiply(m.add(n)), VarianceFn.MATH_CTX);
  
      // (n/m * sum(x) - sum(y))^2
      BigDecimal square = sumX.multiply(n).divide(m, VarianceFn.MATH_CTX).subtract(sumY).pow(2);
  
      return multiplier.multiply(square);
    }
  }
  