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A module that is everything required to understands Druid Segments
There is a newer version: 30.0.1
/*
 * 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.frame.processor;

import it.unimi.dsi.fastutil.ints.IntAVLTreeSet;
import it.unimi.dsi.fastutil.ints.IntSet;
import org.apache.druid.frame.Frame;
import org.apache.druid.frame.channel.ReadableFrameChannel;
import org.apache.druid.frame.channel.WritableFrameChannel;

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

/**
 * Processor that merges frames from inputChannels into a single outputChannel. No sorting is done: input frames are
 * simply written to the output channel as they come in.
 *
 * For sorted output, use {@link FrameChannelMerger} instead.
 */
public class FrameChannelMixer implements FrameProcessor
{
  private final List inputChannels;
  private final WritableFrameChannel outputChannel;

  private final IntSet awaitSet;
  private long rowsRead = 0L;

  public FrameChannelMixer(
      final List inputChannels,
      final WritableFrameChannel outputChannel
  )
  {
    this.inputChannels = inputChannels;
    this.outputChannel = outputChannel;
    this.awaitSet = FrameProcessors.rangeSet(inputChannels.size());
  }

  @Override
  public List inputChannels()
  {
    return inputChannels;
  }

  @Override
  public List outputChannels()
  {
    return Collections.singletonList(outputChannel);
  }

  @Override
  public ReturnOrAwait runIncrementally(final IntSet readableInputs) throws IOException
  {
    final IntSet readySet = new IntAVLTreeSet(readableInputs);

    for (int channelNumber : readableInputs) {
      final ReadableFrameChannel channel = inputChannels.get(channelNumber);

      if (channel.isFinished()) {
        awaitSet.remove(channelNumber);
        readySet.remove(channelNumber);
      }
    }

    if (!readySet.isEmpty()) {
      // Read a random channel: avoid biasing towards lower-numbered channels.
      final int channelNumber = FrameProcessors.selectRandom(readySet);
      final ReadableFrameChannel channel = inputChannels.get(channelNumber);

      if (!channel.isFinished()) {
        final Frame frame = channel.read();
        outputChannel.write(frame);
        rowsRead += frame.numRows();
      }
    }

    if (awaitSet.isEmpty()) {
      return ReturnOrAwait.returnObject(rowsRead);
    } else {
      return ReturnOrAwait.awaitAny(awaitSet);
    }
  }

  @Override
  public void cleanup() throws IOException
  {
    FrameProcessors.closeAll(inputChannels(), outputChannels());
  }
}