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A module that is everything required to understands Druid Segments
There is a newer version: 31.0.0
/*
 * 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.segment.data;

import org.apache.druid.java.util.common.IAE;

import java.nio.ByteBuffer;

public class TableLongEncodingReader implements CompressionFactory.LongEncodingReader
{
  private final long[] table;
  private final int bitsPerValue;
  private final ByteBuffer buffer;
  private VSizeLongSerde.LongDeserializer deserializer;

  public TableLongEncodingReader(ByteBuffer fromBuffer)
  {
    this.buffer = fromBuffer.asReadOnlyBuffer();
    byte version = buffer.get();
    if (version == CompressionFactory.TABLE_ENCODING_VERSION) {
      int tableSize = buffer.getInt();
      if (tableSize < 0 || tableSize > CompressionFactory.MAX_TABLE_SIZE) {
        throw new IAE("Invalid table size[%s]", tableSize);
      }
      bitsPerValue = VSizeLongSerde.getBitsForMax(tableSize);
      table = new long[tableSize];
      for (int i = 0; i < tableSize; i++) {
        table[i] = buffer.getLong();
      }
      fromBuffer.position(buffer.position());
      deserializer = VSizeLongSerde.getDeserializer(bitsPerValue, buffer, buffer.position());
    } else {
      throw new IAE("Unknown version[%s]", version);
    }
  }

  private TableLongEncodingReader(ByteBuffer buffer, long[] table, int bitsPerValue)
  {
    this.buffer = buffer;
    this.table = table;
    this.bitsPerValue = bitsPerValue;
    deserializer = VSizeLongSerde.getDeserializer(bitsPerValue, buffer, buffer.position());
  }

  @Override
  public void setBuffer(ByteBuffer buffer)
  {
    deserializer = VSizeLongSerde.getDeserializer(bitsPerValue, buffer, buffer.position());
  }

  @Override
  public long read(int index)
  {
    return table[(int) deserializer.get(index)];
  }

  @Override
  public void read(long[] out, int outPosition, int startIndex, int length)
  {
    deserializer.getTable(out, outPosition, startIndex, length, table);
  }

  @Override
  public int read(long[] out, int outPosition, int[] indexes, int length, int indexOffset, int limit)
  {
    return deserializer.getTable(out, outPosition, indexes, length, indexOffset, limit, table);
  }

  @Override
  public CompressionFactory.LongEncodingReader duplicate()
  {
    return new TableLongEncodingReader(buffer.duplicate(), table, bitsPerValue);
  }
}