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A module that is everything required to understands Druid Segments
There is a newer version: 0.12.3
/*
 * Licensed to Metamarkets Group Inc. (Metamarkets) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. Metamarkets 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 io.druid.query;

import com.google.common.base.Function;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.Lists;
import com.metamx.common.ISE;
import com.metamx.common.guava.Sequence;
import com.metamx.common.guava.Sequences;
import io.druid.query.aggregation.MetricManipulationFn;
import io.druid.query.aggregation.MetricManipulatorFns;

import java.util.Map;

/**
 */
public class FinalizeResultsQueryRunner implements QueryRunner
{
  private final QueryRunner baseRunner;
  private final QueryToolChest> toolChest;

  public FinalizeResultsQueryRunner(
      QueryRunner baseRunner,
      QueryToolChest> toolChest
  )
  {
    this.baseRunner = baseRunner;
    this.toolChest = toolChest;
  }

  @Override
  public Sequence run(final Query query, Map responseContext)
  {
    final boolean isBySegment = BaseQuery.getContextBySegment(query, false);
    final boolean shouldFinalize = BaseQuery.getContextFinalize(query, true);

    final Query queryToRun;
    final Function finalizerFn;
    final MetricManipulationFn metricManipulationFn;

    if (shouldFinalize) {
      queryToRun = query.withOverriddenContext(ImmutableMap.of("finalize", false));
      metricManipulationFn = MetricManipulatorFns.finalizing();

    } else {
      queryToRun = query;
      metricManipulationFn = MetricManipulatorFns.identity();
    }
    if (isBySegment) {
      finalizerFn = new Function()
      {
        final Function baseFinalizer = toolChest.makePostComputeManipulatorFn(
            query,
            metricManipulationFn
        );

        @Override
        @SuppressWarnings("unchecked")
        public T apply(T input)
        {
          Result> result = (Result>) input;

          if (input == null) {
            throw new ISE("Cannot have a null result!");
          }

          BySegmentResultValue resultsClass = result.getValue();

          return (T) new Result(
              result.getTimestamp(),
              new BySegmentResultValueClass(
                  Lists.transform(resultsClass.getResults(), baseFinalizer),
                  resultsClass.getSegmentId(),
                  resultsClass.getInterval()
              )
          );
        }
      };
    } else {
      finalizerFn = toolChest.makePostComputeManipulatorFn(query, metricManipulationFn);
    }


    return Sequences.map(
        baseRunner.run(queryToRun, responseContext),
        finalizerFn
    );

  }
}