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A module that is everything required to understands Druid Segments
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/*
 * 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.druid.query;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.google.common.base.Function;
import org.apache.druid.data.input.MapBasedRow;
import org.apache.druid.java.util.common.DateTimes;
import org.apache.druid.java.util.common.guava.Sequence;
import org.apache.druid.java.util.common.guava.Sequences;
import org.apache.druid.query.context.ResponseContext;
import org.apache.druid.query.spec.MultipleIntervalSegmentSpec;
import org.apache.druid.query.timeboundary.TimeBoundaryQuery;
import org.apache.druid.query.timeboundary.TimeBoundaryResultValue;
import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
import org.joda.time.Interval;
import org.joda.time.Period;

import java.util.Collections;

/**
 * TimewarpOperator is an example post-processing operator that maps current time
 * to the latest period ending withing the specified data interval and truncates
 * the query interval to discard data that would be mapped to the future.
 */
public class TimewarpOperator implements PostProcessingOperator
{
  private final Interval dataInterval;
  private final long periodMillis;
  private final long originMillis;

  /**
   * @param dataInterval interval containing the actual data
   * @param period       time will be offset by a multiple of the given period
   *                     until there is at least a full period ending within the data interval
   * @param origin       origin to be used to align time periods
   *                     (e.g. to determine on what day of the week a weekly period starts)
   */
  @JsonCreator
  public TimewarpOperator(
      @JsonProperty("dataInterval") Interval dataInterval,
      @JsonProperty("period") Period period,
      @JsonProperty("origin") DateTime origin
  )
  {
    this.originMillis = origin.getMillis();
    this.dataInterval = dataInterval;
    // this will fail for periods that do not map to millis (e.g. P1M)
    this.periodMillis = period.toStandardDuration().getMillis();
  }

  @Override
  public QueryRunner postProcess(QueryRunner baseQueryRunner)
  {
    return postProcess(baseQueryRunner, DateTimes.nowUtc().getMillis());
  }

  public QueryRunner postProcess(final QueryRunner baseRunner, final long now)
  {
    return new QueryRunner()
    {
      @Override
      public Sequence run(final QueryPlus queryPlus, final ResponseContext responseContext)
      {
        final DateTimeZone tz = queryPlus.getQuery().getTimezone();
        final long offset = computeOffset(now, tz);

        final Interval interval = queryPlus.getQuery().getIntervals().get(0);
        final Interval modifiedInterval = new Interval(
            Math.min(interval.getStartMillis() + offset, now + offset),
            Math.min(interval.getEndMillis() + offset, now + offset),
            interval.getChronology()
        );
        return Sequences.map(
            baseRunner.run(
                queryPlus.withQuery(
                    queryPlus.getQuery().withQuerySegmentSpec(
                        new MultipleIntervalSegmentSpec(Collections.singletonList(modifiedInterval))
                    )
                ),
                responseContext
            ),
            new Function()
            {
              @Override
              public T apply(T input)
              {
                if (input instanceof Result) {
                  Result res = (Result) input;
                  Object value = res.getValue();
                  if (value instanceof TimeBoundaryResultValue) {
                    TimeBoundaryResultValue boundary = (TimeBoundaryResultValue) value;

                    DateTime minTime;
                    try {
                      minTime = boundary.getMinTime();
                    }
                    catch (IllegalArgumentException e) {
                      minTime = null;
                    }

                    final DateTime maxTime = boundary.getMaxTime();

                    return (T) ((TimeBoundaryQuery) queryPlus.getQuery()).buildResult(
                        DateTimes.utc(Math.min(res.getTimestamp().getMillis() - offset, now)),
                        minTime != null ? minTime.minus(offset) : null,
                        maxTime != null ? DateTimes.utc(Math.min(maxTime.getMillis() - offset, now)) : null
                    ).iterator().next();
                  }
                  return (T) new Result(res.getTimestamp().minus(offset), value);
                } else if (input instanceof MapBasedRow) {
                  MapBasedRow row = (MapBasedRow) input;
                  return (T) new MapBasedRow(row.getTimestamp().minus(offset), row.getEvent());
                }

                // default to noop for unknown result types
                return input;
              }
            }
        );
      }
    };
  }

  /**
   * Map time t into the last `period` ending within `dataInterval`
   *
   * @param t the current time to be mapped into `dataInterval`
   *
   * @return the offset between the mapped time and time t
   */
  protected long computeOffset(final long t, final DateTimeZone tz)
  {
    // start is the beginning of the last period ending within dataInterval
    long start = dataInterval.getEndMillis() - periodMillis;
    long startOffset = start % periodMillis - originMillis % periodMillis;
    if (startOffset < 0) {
      startOffset += periodMillis;
    }

    start -= startOffset;

    // tOffset is the offset time t within the last period
    long tOffset = t % periodMillis - originMillis % periodMillis;
    if (tOffset < 0) {
      tOffset += periodMillis;
    }
    tOffset += start;
    return tOffset - t - (tz.getOffset(tOffset) - tz.getOffset(t));
  }
}