src/java/org/apache/cassandra/io/sstable/IndexSummaryBuilder.java - cassandra - Git at Google

 /*
  * 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.io.sstable;

 import java.io.IOException;
 import java.nio.ByteOrder;
 import java.util.Map;
 import java.util.TreeMap;

 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;

 import org.apache.cassandra.db.DecoratedKey;
 import org.apache.cassandra.dht.IPartitioner;
 import org.apache.cassandra.io.util.Memory;
 import org.apache.cassandra.io.util.SafeMemoryWriter;

 import static org.apache.cassandra.io.sstable.Downsampling.BASE_SAMPLING_LEVEL;

 public class IndexSummaryBuilder implements AutoCloseable
 {
     private static final Logger logger = LoggerFactory.getLogger(IndexSummaryBuilder.class);

     // the offset in the keys memory region to look for a given summary boundary
     private final SafeMemoryWriter offsets;
     private final SafeMemoryWriter entries;

     private final int minIndexInterval;
     private final int samplingLevel;
     private final int[] startPoints;
     private long keysWritten = 0;
     private long indexIntervalMatches = 0;
     private long nextSamplePosition;

     // for each ReadableBoundary, we map its dataLength property to itself, permitting us to lookup the
     // last readable boundary from the perspective of the data file
     // [data file position limit] => [ReadableBoundary]
     private TreeMap<Long, ReadableBoundary> lastReadableByData = new TreeMap<>();
     // for each ReadableBoundary, we map its indexLength property to itself, permitting us to lookup the
     // last readable boundary from the perspective of the index file
     // [index file position limit] => [ReadableBoundary]
     private TreeMap<Long, ReadableBoundary> lastReadableByIndex = new TreeMap<>();
     // the last synced data file position
     private long dataSyncPosition;
     // the last synced index file position
     private long indexSyncPosition;

     // the last summary interval boundary that is fully readable in both data and index files
     private ReadableBoundary lastReadableBoundary;

     /**
      * Represents a boundary that is guaranteed fully readable in the summary, index file and data file.
      * The key contained is the last key readable if the index and data files have been flushed to the
      * stored lengths.
      */
     public static class ReadableBoundary
     {
         public final DecoratedKey lastKey;
         public final long indexLength;
         public final long dataLength;
         public final int summaryCount;
         public final long entriesLength;
         public ReadableBoundary(DecoratedKey lastKey, long indexLength, long dataLength, int summaryCount, long entriesLength)
         {
             this.lastKey = lastKey;
             this.indexLength = indexLength;
             this.dataLength = dataLength;
             this.summaryCount = summaryCount;
             this.entriesLength = entriesLength;
         }
     }

     @SuppressWarnings("resource")
     public IndexSummaryBuilder(long expectedKeys, int minIndexInterval, int samplingLevel)
     {
         this.samplingLevel = samplingLevel;
         this.startPoints = Downsampling.getStartPoints(BASE_SAMPLING_LEVEL, samplingLevel);

         long maxExpectedEntries = expectedKeys / minIndexInterval;
         if (maxExpectedEntries > Integer.MAX_VALUE)
         {
             // that's a _lot_ of keys, and a very low min index interval
             int effectiveMinInterval = (int) Math.ceil((double) Integer.MAX_VALUE / expectedKeys);
             maxExpectedEntries = expectedKeys / effectiveMinInterval;
             assert maxExpectedEntries <= Integer.MAX_VALUE : maxExpectedEntries;
             logger.warn("min_index_interval of {} is too low for {} expected keys; using interval of {} instead",
                         minIndexInterval, expectedKeys, effectiveMinInterval);
             this.minIndexInterval = effectiveMinInterval;
         }
         else
         {
             this.minIndexInterval = minIndexInterval;
         }

         // for initializing data structures, adjust our estimates based on the sampling level
         maxExpectedEntries = Math.max(1, (maxExpectedEntries * samplingLevel) / BASE_SAMPLING_LEVEL);
         offsets = new SafeMemoryWriter(4 * maxExpectedEntries).order(ByteOrder.nativeOrder());
         entries = new SafeMemoryWriter(40 * maxExpectedEntries).order(ByteOrder.nativeOrder());

         // the summary will always contain the first index entry (downsampling will never remove it)
         nextSamplePosition = 0;
         indexIntervalMatches++;
     }

     // the index file has been flushed to the provided position; stash it and use that to recalculate our max readable boundary
     public void markIndexSynced(long upToPosition)
     {
         indexSyncPosition = upToPosition;
         refreshReadableBoundary();
     }

     // the data file has been flushed to the provided position; stash it and use that to recalculate our max readable boundary
     public void markDataSynced(long upToPosition)
     {
         dataSyncPosition = upToPosition;
         refreshReadableBoundary();
     }

     private void refreshReadableBoundary()
     {
         // grab the readable boundary prior to the given position in either the data or index file
         Map.Entry<?, ReadableBoundary> byData = lastReadableByData.floorEntry(dataSyncPosition);
         Map.Entry<?, ReadableBoundary> byIndex = lastReadableByIndex.floorEntry(indexSyncPosition);
         if (byData == null || byIndex == null)
             return;

         // take the lowest of the two, and stash it
         lastReadableBoundary = byIndex.getValue().indexLength < byData.getValue().indexLength
                                ? byIndex.getValue() : byData.getValue();

         // clear our data prior to this, since we no longer need it
         lastReadableByData.headMap(lastReadableBoundary.dataLength, false).clear();
         lastReadableByIndex.headMap(lastReadableBoundary.indexLength, false).clear();
     }

     public ReadableBoundary getLastReadableBoundary()
     {
         return lastReadableBoundary;
     }

     public IndexSummaryBuilder maybeAddEntry(DecoratedKey decoratedKey, long indexStart) throws IOException
     {
         return maybeAddEntry(decoratedKey, indexStart, 0, 0);
     }

     /**
      *
      * @param decoratedKey the key for this record
      * @param indexStart the position in the index file this record begins
      * @param indexEnd the position in the index file we need to be able to read to (exclusive) to read this record
      * @param dataEnd the position in the data file we need to be able to read to (exclusive) to read this record
      *                a value of 0 indicates we are not tracking readable boundaries
      */
     public IndexSummaryBuilder maybeAddEntry(DecoratedKey decoratedKey, long indexStart, long indexEnd, long dataEnd) throws IOException
     {
         if (keysWritten == nextSamplePosition)
         {
             assert entries.length() <= Integer.MAX_VALUE;
             offsets.writeInt((int) entries.length());
             entries.write(decoratedKey.getKey());
             entries.writeLong(indexStart);
             setNextSamplePosition(keysWritten);
         }
         else if (dataEnd != 0 && keysWritten + 1 == nextSamplePosition)
         {
             // this is the last key in this summary interval, so stash it
             ReadableBoundary boundary = new ReadableBoundary(decoratedKey, indexEnd, dataEnd, (int)(offsets.length() / 4), entries.length());
             lastReadableByData.put(dataEnd, boundary);
             lastReadableByIndex.put(indexEnd, boundary);
         }
         keysWritten++;

         return this;
     }

     // calculate the next key we will store to our summary
     private void setNextSamplePosition(long position)
     {
         tryAgain: while (true)
         {
             position += minIndexInterval;
             long test = indexIntervalMatches++;
             for (int start : startPoints)
                 if ((test - start) % BASE_SAMPLING_LEVEL == 0)
                     continue tryAgain;

             nextSamplePosition = position;
             return;
         }
     }

     public void prepareToCommit()
     {
         // this method should only be called when we've finished appending records, so we truncate the
         // memory we're using to the exact amount required to represent it before building our summary
         entries.setCapacity(entries.length());
         offsets.setCapacity(offsets.length());
     }

     public IndexSummary build(IPartitioner partitioner)
     {
         return build(partitioner, null);
     }

     // build the summary up to the provided boundary; this is backed by shared memory between
     // multiple invocations of this build method
     public IndexSummary build(IPartitioner partitioner, ReadableBoundary boundary)
     {
         assert entries.length() > 0;

         int count = (int) (offsets.length() / 4);
         long entriesLength = entries.length();
         if (boundary != null)
         {
             count = boundary.summaryCount;
             entriesLength = boundary.entriesLength;
         }

         int sizeAtFullSampling = (int) Math.ceil(keysWritten / (double) minIndexInterval);
         assert count > 0;
         return new IndexSummary(partitioner, offsets.currentBuffer().sharedCopy(),
                                 count, entries.currentBuffer().sharedCopy(), entriesLength,
                                 sizeAtFullSampling, minIndexInterval, samplingLevel);
     }

     // close the builder and release any associated memory
     public void close()
     {
         entries.close();
         offsets.close();
     }

     public Throwable close(Throwable accumulate)
     {
         accumulate = entries.close(accumulate);
         accumulate = offsets.close(accumulate);
         return accumulate;
     }

     public static int entriesAtSamplingLevel(int samplingLevel, int maxSummarySize)
     {
         return (int) Math.ceil((samplingLevel * maxSummarySize) / (double) BASE_SAMPLING_LEVEL);
     }

     public static int calculateSamplingLevel(int currentSamplingLevel, int currentNumEntries, long targetNumEntries, int minIndexInterval, int maxIndexInterval)
     {
         // effective index interval == (BASE_SAMPLING_LEVEL / samplingLevel) * minIndexInterval
         // so we can just solve for minSamplingLevel here:
         // maxIndexInterval == (BASE_SAMPLING_LEVEL / minSamplingLevel) * minIndexInterval
         int effectiveMinSamplingLevel = Math.max(1, (int) Math.ceil((BASE_SAMPLING_LEVEL * minIndexInterval) / (double) maxIndexInterval));

         // Algebraic explanation for calculating the new sampling level (solve for newSamplingLevel):
         // originalNumEntries = (baseSamplingLevel / currentSamplingLevel) * currentNumEntries
         // newSpaceUsed = (newSamplingLevel / baseSamplingLevel) * originalNumEntries
         // newSpaceUsed = (newSamplingLevel / baseSamplingLevel) * (baseSamplingLevel / currentSamplingLevel) * currentNumEntries
         // newSpaceUsed = (newSamplingLevel / currentSamplingLevel) * currentNumEntries
         // (newSpaceUsed * currentSamplingLevel) / currentNumEntries = newSamplingLevel
         int newSamplingLevel = (int) (targetNumEntries * currentSamplingLevel) / currentNumEntries;
         return Math.min(BASE_SAMPLING_LEVEL, Math.max(effectiveMinSamplingLevel, newSamplingLevel));
     }

     /**
      * Downsamples an existing index summary to a new sampling level.
      * @param existing an existing IndexSummary
      * @param newSamplingLevel the target level for the new IndexSummary.  This must be less than the current sampling
      *                         level for `existing`.
      * @param partitioner the partitioner used for the index summary
      * @return a new IndexSummary
      */
     public static IndexSummary downsample(IndexSummary existing, int newSamplingLevel, int minIndexInterval, IPartitioner partitioner)
     {
         // To downsample the old index summary, we'll go through (potentially) several rounds of downsampling.
         // Conceptually, each round starts at position X and then removes every Nth item.  The value of X follows
         // a particular pattern to evenly space out the items that we remove.  The value of N decreases by one each
         // round.

         int currentSamplingLevel = existing.getSamplingLevel();
         assert currentSamplingLevel > newSamplingLevel;
         assert minIndexInterval == existing.getMinIndexInterval();

         // calculate starting indexes for downsampling rounds
         int[] startPoints = Downsampling.getStartPoints(currentSamplingLevel, newSamplingLevel);

         // calculate new off-heap size
         int newKeyCount = existing.size();
         long newEntriesLength = existing.getEntriesLength();
         for (int start : startPoints)
         {
             for (int j = start; j < existing.size(); j += currentSamplingLevel)
             {
                 newKeyCount--;
                 long length = existing.getEndInSummary(j) - existing.getPositionInSummary(j);
                 newEntriesLength -= length;
             }
         }

         Memory oldEntries = existing.getEntries();
         Memory newOffsets = Memory.allocate(newKeyCount * 4);
         Memory newEntries = Memory.allocate(newEntriesLength);

         // Copy old entries to our new Memory.
         int i = 0;
         int newEntriesOffset = 0;
         outer:
         for (int oldSummaryIndex = 0; oldSummaryIndex < existing.size(); oldSummaryIndex++)
         {
             // to determine if we can skip this entry, go through the starting points for our downsampling rounds
             // and see if the entry's index is covered by that round
             for (int start : startPoints)
             {
                 if ((oldSummaryIndex - start) % currentSamplingLevel == 0)
                     continue outer;
             }

             // write the position of the actual entry in the index summary (4 bytes)
             newOffsets.setInt(i * 4, newEntriesOffset);
             i++;
             long start = existing.getPositionInSummary(oldSummaryIndex);
             long length = existing.getEndInSummary(oldSummaryIndex) - start;
             newEntries.put(newEntriesOffset, oldEntries, start, length);
             newEntriesOffset += length;
         }
         assert newEntriesOffset == newEntriesLength;
         return new IndexSummary(partitioner, newOffsets, newKeyCount, newEntries, newEntriesLength,
                                 existing.getMaxNumberOfEntries(), minIndexInterval, newSamplingLevel);
     }
 }
	/*
	* 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.io.sstable;

	import java.io.IOException;
	import java.nio.ByteOrder;
	import java.util.Map;
	import java.util.TreeMap;

	import org.slf4j.Logger;
	import org.slf4j.LoggerFactory;

	import org.apache.cassandra.db.DecoratedKey;
	import org.apache.cassandra.dht.IPartitioner;
	import org.apache.cassandra.io.util.Memory;
	import org.apache.cassandra.io.util.SafeMemoryWriter;

	import static org.apache.cassandra.io.sstable.Downsampling.BASE_SAMPLING_LEVEL;

	public class IndexSummaryBuilder implements AutoCloseable
	{
	private static final Logger logger = LoggerFactory.getLogger(IndexSummaryBuilder.class);

	// the offset in the keys memory region to look for a given summary boundary
	private final SafeMemoryWriter offsets;
	private final SafeMemoryWriter entries;

	private final int minIndexInterval;
	private final int samplingLevel;
	private final int[] startPoints;
	private long keysWritten = 0;
	private long indexIntervalMatches = 0;
	private long nextSamplePosition;

	// for each ReadableBoundary, we map its dataLength property to itself, permitting us to lookup the
	// last readable boundary from the perspective of the data file
	// [data file position limit] => [ReadableBoundary]
	private TreeMap<Long, ReadableBoundary> lastReadableByData = new TreeMap<>();
	// for each ReadableBoundary, we map its indexLength property to itself, permitting us to lookup the
	// last readable boundary from the perspective of the index file
	// [index file position limit] => [ReadableBoundary]
	private TreeMap<Long, ReadableBoundary> lastReadableByIndex = new TreeMap<>();
	// the last synced data file position
	private long dataSyncPosition;
	// the last synced index file position
	private long indexSyncPosition;

	// the last summary interval boundary that is fully readable in both data and index files
	private ReadableBoundary lastReadableBoundary;

	/**
	* Represents a boundary that is guaranteed fully readable in the summary, index file and data file.
	* The key contained is the last key readable if the index and data files have been flushed to the
	* stored lengths.
	*/
	public static class ReadableBoundary
	{
	public final DecoratedKey lastKey;
	public final long indexLength;
	public final long dataLength;
	public final int summaryCount;
	public final long entriesLength;
	public ReadableBoundary(DecoratedKey lastKey, long indexLength, long dataLength, int summaryCount, long entriesLength)
	{
	this.lastKey = lastKey;
	this.indexLength = indexLength;
	this.dataLength = dataLength;
	this.summaryCount = summaryCount;
	this.entriesLength = entriesLength;
	}
	}

	@SuppressWarnings("resource")
	public IndexSummaryBuilder(long expectedKeys, int minIndexInterval, int samplingLevel)
	{
	this.samplingLevel = samplingLevel;
	this.startPoints = Downsampling.getStartPoints(BASE_SAMPLING_LEVEL, samplingLevel);

	long maxExpectedEntries = expectedKeys / minIndexInterval;
	if (maxExpectedEntries > Integer.MAX_VALUE)
	{
	// that's a _lot_ of keys, and a very low min index interval
	int effectiveMinInterval = (int) Math.ceil((double) Integer.MAX_VALUE / expectedKeys);
	maxExpectedEntries = expectedKeys / effectiveMinInterval;
	assert maxExpectedEntries <= Integer.MAX_VALUE : maxExpectedEntries;
	logger.warn("min_index_interval of {} is too low for {} expected keys; using interval of {} instead",
	minIndexInterval, expectedKeys, effectiveMinInterval);
	this.minIndexInterval = effectiveMinInterval;
	}
	else
	{
	this.minIndexInterval = minIndexInterval;
	}

	// for initializing data structures, adjust our estimates based on the sampling level
	maxExpectedEntries = Math.max(1, (maxExpectedEntries * samplingLevel) / BASE_SAMPLING_LEVEL);
	offsets = new SafeMemoryWriter(4 * maxExpectedEntries).order(ByteOrder.nativeOrder());
	entries = new SafeMemoryWriter(40 * maxExpectedEntries).order(ByteOrder.nativeOrder());

	// the summary will always contain the first index entry (downsampling will never remove it)
	nextSamplePosition = 0;
	indexIntervalMatches++;
	}

	// the index file has been flushed to the provided position; stash it and use that to recalculate our max readable boundary
	public void markIndexSynced(long upToPosition)
	{
	indexSyncPosition = upToPosition;
	refreshReadableBoundary();
	}

	// the data file has been flushed to the provided position; stash it and use that to recalculate our max readable boundary
	public void markDataSynced(long upToPosition)
	{
	dataSyncPosition = upToPosition;
	refreshReadableBoundary();
	}

	private void refreshReadableBoundary()
	{
	// grab the readable boundary prior to the given position in either the data or index file
	Map.Entry<?, ReadableBoundary> byData = lastReadableByData.floorEntry(dataSyncPosition);
	Map.Entry<?, ReadableBoundary> byIndex = lastReadableByIndex.floorEntry(indexSyncPosition);
	if (byData == null \|\| byIndex == null)
	return;

	// take the lowest of the two, and stash it
	lastReadableBoundary = byIndex.getValue().indexLength < byData.getValue().indexLength
	? byIndex.getValue() : byData.getValue();

	// clear our data prior to this, since we no longer need it
	lastReadableByData.headMap(lastReadableBoundary.dataLength, false).clear();
	lastReadableByIndex.headMap(lastReadableBoundary.indexLength, false).clear();
	}

	public ReadableBoundary getLastReadableBoundary()
	{
	return lastReadableBoundary;
	}

	public IndexSummaryBuilder maybeAddEntry(DecoratedKey decoratedKey, long indexStart) throws IOException
	{
	return maybeAddEntry(decoratedKey, indexStart, 0, 0);
	}

	/**
	*
	* @param decoratedKey the key for this record
	* @param indexStart the position in the index file this record begins
	* @param indexEnd the position in the index file we need to be able to read to (exclusive) to read this record
	* @param dataEnd the position in the data file we need to be able to read to (exclusive) to read this record
	* a value of 0 indicates we are not tracking readable boundaries
	*/
	public IndexSummaryBuilder maybeAddEntry(DecoratedKey decoratedKey, long indexStart, long indexEnd, long dataEnd) throws IOException
	{
	if (keysWritten == nextSamplePosition)
	{
	assert entries.length() <= Integer.MAX_VALUE;
	offsets.writeInt((int) entries.length());
	entries.write(decoratedKey.getKey());
	entries.writeLong(indexStart);
	setNextSamplePosition(keysWritten);
	}
	else if (dataEnd != 0 && keysWritten + 1 == nextSamplePosition)
	{
	// this is the last key in this summary interval, so stash it
	ReadableBoundary boundary = new ReadableBoundary(decoratedKey, indexEnd, dataEnd, (int)(offsets.length() / 4), entries.length());
	lastReadableByData.put(dataEnd, boundary);
	lastReadableByIndex.put(indexEnd, boundary);
	}
	keysWritten++;

	return this;
	}

	// calculate the next key we will store to our summary
	private void setNextSamplePosition(long position)
	{
	tryAgain: while (true)
	{
	position += minIndexInterval;
	long test = indexIntervalMatches++;
	for (int start : startPoints)
	if ((test - start) % BASE_SAMPLING_LEVEL == 0)
	continue tryAgain;

	nextSamplePosition = position;
	return;
	}
	}

	public void prepareToCommit()
	{
	// this method should only be called when we've finished appending records, so we truncate the
	// memory we're using to the exact amount required to represent it before building our summary
	entries.setCapacity(entries.length());
	offsets.setCapacity(offsets.length());
	}

	public IndexSummary build(IPartitioner partitioner)
	{
	return build(partitioner, null);
	}

	// build the summary up to the provided boundary; this is backed by shared memory between
	// multiple invocations of this build method
	public IndexSummary build(IPartitioner partitioner, ReadableBoundary boundary)
	{
	assert entries.length() > 0;

	int count = (int) (offsets.length() / 4);
	long entriesLength = entries.length();
	if (boundary != null)
	{
	count = boundary.summaryCount;
	entriesLength = boundary.entriesLength;
	}

	int sizeAtFullSampling = (int) Math.ceil(keysWritten / (double) minIndexInterval);
	assert count > 0;
	return new IndexSummary(partitioner, offsets.currentBuffer().sharedCopy(),
	count, entries.currentBuffer().sharedCopy(), entriesLength,
	sizeAtFullSampling, minIndexInterval, samplingLevel);
	}

	// close the builder and release any associated memory
	public void close()
	{
	entries.close();
	offsets.close();
	}

	public Throwable close(Throwable accumulate)
	{
	accumulate = entries.close(accumulate);
	accumulate = offsets.close(accumulate);
	return accumulate;
	}

	public static int entriesAtSamplingLevel(int samplingLevel, int maxSummarySize)
	{
	return (int) Math.ceil((samplingLevel * maxSummarySize) / (double) BASE_SAMPLING_LEVEL);
	}

	public static int calculateSamplingLevel(int currentSamplingLevel, int currentNumEntries, long targetNumEntries, int minIndexInterval, int maxIndexInterval)
	{
	// effective index interval == (BASE_SAMPLING_LEVEL / samplingLevel) * minIndexInterval
	// so we can just solve for minSamplingLevel here:
	// maxIndexInterval == (BASE_SAMPLING_LEVEL / minSamplingLevel) * minIndexInterval
	int effectiveMinSamplingLevel = Math.max(1, (int) Math.ceil((BASE_SAMPLING_LEVEL * minIndexInterval) / (double) maxIndexInterval));

	// Algebraic explanation for calculating the new sampling level (solve for newSamplingLevel):
	// originalNumEntries = (baseSamplingLevel / currentSamplingLevel) * currentNumEntries
	// newSpaceUsed = (newSamplingLevel / baseSamplingLevel) * originalNumEntries
	// newSpaceUsed = (newSamplingLevel / baseSamplingLevel) * (baseSamplingLevel / currentSamplingLevel) * currentNumEntries
	// newSpaceUsed = (newSamplingLevel / currentSamplingLevel) * currentNumEntries
	// (newSpaceUsed * currentSamplingLevel) / currentNumEntries = newSamplingLevel
	int newSamplingLevel = (int) (targetNumEntries * currentSamplingLevel) / currentNumEntries;
	return Math.min(BASE_SAMPLING_LEVEL, Math.max(effectiveMinSamplingLevel, newSamplingLevel));
	}

	/**
	* Downsamples an existing index summary to a new sampling level.
	* @param existing an existing IndexSummary
	* @param newSamplingLevel the target level for the new IndexSummary. This must be less than the current sampling
	* level for `existing`.
	* @param partitioner the partitioner used for the index summary
	* @return a new IndexSummary
	*/
	public static IndexSummary downsample(IndexSummary existing, int newSamplingLevel, int minIndexInterval, IPartitioner partitioner)
	{
	// To downsample the old index summary, we'll go through (potentially) several rounds of downsampling.
	// Conceptually, each round starts at position X and then removes every Nth item. The value of X follows
	// a particular pattern to evenly space out the items that we remove. The value of N decreases by one each
	// round.

	int currentSamplingLevel = existing.getSamplingLevel();
	assert currentSamplingLevel > newSamplingLevel;
	assert minIndexInterval == existing.getMinIndexInterval();

	// calculate starting indexes for downsampling rounds
	int[] startPoints = Downsampling.getStartPoints(currentSamplingLevel, newSamplingLevel);

	// calculate new off-heap size
	int newKeyCount = existing.size();
	long newEntriesLength = existing.getEntriesLength();
	for (int start : startPoints)
	{
	for (int j = start; j < existing.size(); j += currentSamplingLevel)
	{
	newKeyCount--;
	long length = existing.getEndInSummary(j) - existing.getPositionInSummary(j);
	newEntriesLength -= length;
	}
	}

	Memory oldEntries = existing.getEntries();
	Memory newOffsets = Memory.allocate(newKeyCount * 4);
	Memory newEntries = Memory.allocate(newEntriesLength);

	// Copy old entries to our new Memory.
	int i = 0;
	int newEntriesOffset = 0;
	outer:
	for (int oldSummaryIndex = 0; oldSummaryIndex < existing.size(); oldSummaryIndex++)
	{
	// to determine if we can skip this entry, go through the starting points for our downsampling rounds
	// and see if the entry's index is covered by that round
	for (int start : startPoints)
	{
	if ((oldSummaryIndex - start) % currentSamplingLevel == 0)
	continue outer;
	}

	// write the position of the actual entry in the index summary (4 bytes)
	newOffsets.setInt(i * 4, newEntriesOffset);
	i++;
	long start = existing.getPositionInSummary(oldSummaryIndex);
	long length = existing.getEndInSummary(oldSummaryIndex) - start;
	newEntries.put(newEntriesOffset, oldEntries, start, length);
	newEntriesOffset += length;
	}
	assert newEntriesOffset == newEntriesLength;
	return new IndexSummary(partitioner, newOffsets, newKeyCount, newEntries, newEntriesLength,
	existing.getMaxNumberOfEntries(), minIndexInterval, newSamplingLevel);
	}
	}