content: /*
   * 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.cassandra.index.sai.disk.v1.bitpack;
  
  import java.io.IOException;
  
  import org.apache.lucene.store.IndexOutput;
  import org.apache.lucene.util.packed.DirectWriter;
  
  /**
   * A writer for large monotonically increasing sequences of positive longs.
   *
   * The writer is optimised for monotonic sequences and stores values as a series of deltas
   * from an expected value. The expected value is calculated from the minimum value in the block and the average
   * delta for the block. This means that stored values are generally smaller and can be packed
   * into a smaller number of bits, allowing for larger block sizes.
   *
   * Modified copy of {@link org.apache.lucene.util.packed.MonotonicBlockPackedWriter} to use {@link DirectWriter} for
   * optimised reads that doesn't require seeking through the whole file to open a thread-exclusive reader.
   */
  public class MonotonicBlockPackedWriter extends AbstractBlockPackedWriter
  {
      public MonotonicBlockPackedWriter(IndexOutput out, int blockSize)
      {
          super(out, blockSize);
      }
  
      @Override
      public void add(long l) throws IOException
      {
          assert l >= 0;
          super.add(l);
      }
  
      @Override
      protected void flushBlock() throws IOException
      {
          final float averageDelta = blockIndex == 1 ? 0f : (float) (blockValues[blockIndex - 1] - blockValues[0]) / (blockIndex - 1);
          long minimumValue = blockValues[0];
          // adjust minimumValue so that all deltas will be positive
          for (int index = 1; index < blockIndex; ++index)
          {
              long actual = blockValues[index];
              long expected = MonotonicBlockPackedReader.expected(minimumValue, averageDelta, index);
              if (expected > actual)
              {
                  minimumValue -= (expected - actual);
              }
          }
  
          long maxDelta = 0;
          for (int i = 0; i < blockIndex; ++i)
          {
              blockValues[i] = blockValues[i] - MonotonicBlockPackedReader.expected(minimumValue, averageDelta, i);
              maxDelta = Math.max(maxDelta, blockValues[i]);
          }
  
          blockMetaWriter.writeZLong(minimumValue);
          blockMetaWriter.writeInt(Float.floatToIntBits(averageDelta));
          if (maxDelta == 0)
          {
              blockMetaWriter.writeVInt(0);
          }
          else
          {
              final int bitsRequired = DirectWriter.bitsRequired(maxDelta);
              blockMetaWriter.writeVInt(bitsRequired);
              blockMetaWriter.writeVLong(indexOutput.getFilePointer());
              writeValues(blockIndex, bitsRequired);
          }
      }
  }
  