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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.scan;

import com.google.common.collect.ImmutableMap;
import com.google.common.collect.Iterables;
import org.apache.druid.java.util.common.UOE;
import org.apache.druid.java.util.common.guava.Sequence;
import org.apache.druid.java.util.common.guava.Yielder;
import org.apache.druid.java.util.common.guava.YieldingAccumulator;
import org.apache.druid.java.util.common.parsers.CloseableIterator;
import org.apache.druid.query.Query;
import org.apache.druid.query.QueryPlus;
import org.apache.druid.query.QueryRunner;
import org.apache.druid.query.context.ResponseContext;
import org.apache.druid.segment.column.RowSignature;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

/**
 * This iterator supports iteration through a Sequence returned by a ScanResultValue QueryRunner.  Its behaviour
 * varies depending on whether the query is returning time-ordered values and whether the CTX_KEY_OUTERMOST flag is
 * set as false.
 *
 * Behaviours:
 * 1) No time ordering: expects a Sequence of ScanResultValues which each contain up to query.batchSize events.
 *    The iterator will be "done" when the limit of events is reached.  The final ScanResultValue might contain
 *    fewer than batchSize events so that the limit number of events is returned.
 * 2) Time Ordering, CTX_KEY_OUTERMOST false: Same behaviour as no time ordering
 * 3) Time Ordering, CTX_KEY_OUTERMOST=true or null: The Sequence processed in this case should contain ScanResultValues
 *    that contain only one event each for the CachingClusteredClient n-way merge.  This iterator will perform
 *    batching according to query batch size until the limit is reached.
 */
public class ScanQueryLimitRowIterator implements CloseableIterator
{
  private Yielder yielder;
  private ScanQuery.ResultFormat resultFormat;
  private long limit;
  private long count = 0;
  private ScanQuery query;

  ScanQueryLimitRowIterator(
      QueryRunner baseRunner,
      QueryPlus queryPlus,
      ResponseContext responseContext
  )
  {
    this.query = (ScanQuery) queryPlus.getQuery();
    this.resultFormat = query.getResultFormat();
    this.limit = query.getScanRowsLimit();
    Query historicalQuery =
        queryPlus.getQuery().withOverriddenContext(ImmutableMap.of(ScanQuery.CTX_KEY_OUTERMOST, false));
    Sequence baseSequence = baseRunner.run(QueryPlus.wrap(historicalQuery), responseContext);
    this.yielder = baseSequence.toYielder(
        null,
        new YieldingAccumulator()
        {
          @Override
          public ScanResultValue accumulate(ScanResultValue accumulated, ScanResultValue in)
          {
            this.yield();
            return in;
          }
        }
    );
  }

  @Override
  public boolean hasNext()
  {
    return !yielder.isDone() && count < limit;
  }

  @Override
  public ScanResultValue next()
  {
    if (ScanQuery.ResultFormat.RESULT_FORMAT_VALUE_VECTOR.equals(resultFormat)) {
      throw new UOE(ScanQuery.ResultFormat.RESULT_FORMAT_VALUE_VECTOR + " is not supported yet");
    }

    // We want to perform multi-event ScanResultValue limiting if we are not time-ordering or are at the
    // inner-level if we are time-ordering
    if (query.getTimeOrder() == ScanQuery.Order.NONE ||
        !query.context().getBoolean(ScanQuery.CTX_KEY_OUTERMOST, true)) {
      ScanResultValue batch = yielder.get();
      List events = (List) batch.getEvents();
      if (events.size() <= limit - count) {
        count += events.size();
        yielder = yielder.next(null);
        return batch;
      } else {
        // last batch
        // single batch length is <= Integer.MAX_VALUE, so this should not overflow
        int numLeft = (int) (limit - count);
        count = limit;
        return new ScanResultValue(
            batch.getSegmentId(),
            batch.getColumns(),
            events.subList(0, numLeft),
            batch.getRowSignature()
        );
      }
    } else {
      // Perform single-event ScanResultValue batching at the outer level.  Each scan result value from the yielder
      // in this case will only have one event so there's no need to iterate through events.
      int batchSize = query.getBatchSize();
      List