src/main/java/org/apache/datasketches/hive/tuple/ArrayOfDoublesSketchToVariancesUDF.java

/*
 * 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.datasketches.hive.tuple;

import java.util.ArrayList;
import java.util.List;

import org.apache.commons.math3.stat.descriptive.SummaryStatistics;
import org.apache.datasketches.memory.Memory;
import org.apache.datasketches.tuple.ArrayOfDoublesSketch;
import org.apache.datasketches.tuple.ArrayOfDoublesSketches;
import org.apache.hadoop.hive.ql.exec.Description;
import org.apache.hadoop.hive.ql.exec.UDF;
import org.apache.hadoop.io.BytesWritable;

@Description(
    name = "ArrayOfDoublesSketchToVariances",
    value = "_FUNC_(sketch)",
    extended = "Returns a list of variance values from a given ArrayOfDoublesSketch."
    + " The result will be N double values, where N is the number of double values kept in the"
    + " sketch per key.")
@SuppressWarnings("javadoc")
public class ArrayOfDoublesSketchToVariancesUDF extends UDF {

  /**
   * Get variances from a given ArrayOfDoublesSketch
   * @param serializedSketch ArrayOfDoublesSketch in as serialized binary
   * @return list of variances
   */
  public List<Double> evaluate(final BytesWritable serializedSketch) {
    if (serializedSketch == null) { return null; }
    final ArrayOfDoublesSketch sketch = ArrayOfDoublesSketches.wrapSketch(
        Memory.wrap(serializedSketch.getBytes()));

    if (sketch.getRetainedEntries() < 1) {
      return null;
    }

    final SummaryStatistics[] summaries = ArrayOfDoublesSketchStats.sketchToSummaryStatistics(sketch);
    final List<Double> variances = new ArrayList<>(sketch.getNumValues());
    for (int i = 0; i < sketch.getNumValues(); i++) {
      variances.add(summaries[i].getVariance());
    }
    return variances;
  }

}