Package org.apache.lucene.util.packed

Class PackedInts

java.lang.Object

org.apache.lucene.util.packed.PackedInts

public class PackedInts
extends Object

Simplistic compression for array of unsigned long values. Each value is >= 0 and <= a specified maximum value. The values are stored as packed ints, with each value consuming a fixed number of bits.

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static interface	PackedInts.Decoder	A decoder for packed integers.
static interface	PackedInts.Encoder	An encoder for packed integers.
static class	PackedInts.Format	A format to write packed ints.
static class	PackedInts.FormatAndBits	Simple class that holds a format and a number of bits per value.
static class	PackedInts.Mutable	A packed integer array that can be modified.
static class	PackedInts.NullReader	A PackedInts.Reader which has all its values equal to 0 (bitsPerValue = 0).
static class	PackedInts.Reader	A read-only random access array of positive integers.
static interface	PackedInts.ReaderIterator	Run-once iterator interface, to decode previously saved PackedInts.
static class	PackedInts.Writer	A write-once Writer.

Field Summary

Fields

Modifier and Type	Field	Description
static String	CODEC_NAME
static float	COMPACT	No memory overhead at all, but the returned implementation may be slow.
static float	DEFAULT	At most 25% memory overhead.
static int	DEFAULT_BUFFER_SIZE	Default amount of memory to use for bulk operations.
static float	FAST	At most 50% memory overhead, always select a reasonably fast implementation.
static float	FASTEST	At most 700% memory overhead, always select a direct implementation.
static int	VERSION_CURRENT
static int	VERSION_MONOTONIC_WITHOUT_ZIGZAG
static int	VERSION_START

Constructor Summary

Constructors

Constructor	Description
PackedInts()

Method Summary

Modifier and Type	Method	Description
static int	bitsRequired(long maxValue)	Returns how many bits are required to hold values up to and including maxValue NOTE: This method returns at least 1.
static void	checkVersion(int version)	Check the validity of a version number.
static void	copy(PackedInts.Reader src, int srcPos, PackedInts.Mutable dest, int destPos, int len, int mem)	Copy src[srcPos:srcPos+len] into dest[destPos:destPos+len] using at most mem bytes.
static PackedInts.FormatAndBits	fastestFormatAndBits(int valueCount, int bitsPerValue, float acceptableOverheadRatio)	Try to find the PackedInts.Format and number of bits per value that would restore from disk the fastest reader whose overhead is less than acceptableOverheadRatio.
static PackedInts.Decoder	getDecoder(PackedInts.Format format, int version, int bitsPerValue)	Get a PackedInts.Decoder.
static PackedInts.Encoder	getEncoder(PackedInts.Format format, int version, int bitsPerValue)	Get an PackedInts.Encoder.
static PackedInts.Mutable	getMutable(int valueCount, int bitsPerValue, float acceptableOverheadRatio)	Create a packed integer array with the given amount of values initialized to 0.
static PackedInts.Mutable	getMutable(int valueCount, int bitsPerValue, PackedInts.Format format)	Same as getMutable(int, int, float) with a pre-computed number of bits per value and format.
static PackedInts.ReaderIterator	getReaderIteratorNoHeader(DataInput in, PackedInts.Format format, int version, int valueCount, int bitsPerValue, int mem)	Expert: Restore a PackedInts.ReaderIterator from a stream without reading metadata at the beginning of the stream.
static PackedInts.Writer	getWriterNoHeader(DataOutput out, PackedInts.Format format, int valueCount, int bitsPerValue, int mem)	Expert: Create a packed integer array writer for the given output, format, value count, and number of bits per value.
static long	maxValue(int bitsPerValue)	Calculates the maximum unsigned long that can be expressed with the given number of bits.
static int	unsignedBitsRequired(long bits)	Returns how many bits are required to store bits, interpreted as an unsigned value.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

FASTEST

public static final float FASTEST

At most 700% memory overhead, always select a direct implementation.

See Also:

Constant Field Values

FAST

public static final float FAST

At most 50% memory overhead, always select a reasonably fast implementation.

See Also:

Constant Field Values

DEFAULT

public static final float DEFAULT

At most 25% memory overhead.

See Also:

Constant Field Values

COMPACT

public static final float COMPACT

No memory overhead at all, but the returned implementation may be slow.

See Also:

Constant Field Values

DEFAULT_BUFFER_SIZE

public static final int DEFAULT_BUFFER_SIZE

Default amount of memory to use for bulk operations.

See Also:

Constant Field Values

CODEC_NAME

public static final String CODEC_NAME

See Also:

Constant Field Values

VERSION_MONOTONIC_WITHOUT_ZIGZAG

public static final int VERSION_MONOTONIC_WITHOUT_ZIGZAG

See Also:

Constant Field Values

VERSION_START

public static final int VERSION_START

See Also:

Constant Field Values

VERSION_CURRENT

public static final int VERSION_CURRENT

See Also:

Constant Field Values

Constructor Detail

PackedInts

public PackedInts()

Method Detail

checkVersion

public static void checkVersion(int version)

Check the validity of a version number.

fastestFormatAndBits

public static PackedInts.FormatAndBits fastestFormatAndBits(int valueCount,
                                                            int bitsPerValue,
                                                            float acceptableOverheadRatio)

Try to find the PackedInts.Format and number of bits per value that would restore from disk the fastest reader whose overhead is less than acceptableOverheadRatio.

The acceptableOverheadRatio parameter makes sense for random-access PackedInts.Readers. In case you only plan to perform sequential access on this stream later on, you should probably use COMPACT.

If you don't know how many values you are going to write, use valueCount = -1.

getDecoder

public static PackedInts.Decoder getDecoder(PackedInts.Format format,
                                            int version,
                                            int bitsPerValue)

Get a PackedInts.Decoder.

Parameters:

format - the format used to store packed ints

version - the compatibility version

bitsPerValue - the number of bits per value

Returns:

a decoder

getEncoder

public static PackedInts.Encoder getEncoder(PackedInts.Format format,
                                            int version,
                                            int bitsPerValue)

Get an PackedInts.Encoder.

Parameters:

format - the format used to store packed ints

version - the compatibility version

bitsPerValue - the number of bits per value

Returns:

an encoder

getReaderIteratorNoHeader

public static PackedInts.ReaderIterator getReaderIteratorNoHeader(DataInput in,
                                                                  PackedInts.Format format,
                                                                  int version,
                                                                  int valueCount,
                                                                  int bitsPerValue,
                                                                  int mem)

Expert: Restore a PackedInts.ReaderIterator from a stream without reading metadata at the beginning of the stream. This method is useful to restore data from streams which have been created using getWriterNoHeader(DataOutput, Format, int, int, int).

Parameters:

in - the stream to read data from, positioned at the beginning of the packed values

format - the format used to serialize

version - the version used to serialize the data

valueCount - how many values the stream holds

bitsPerValue - the number of bits per value

mem - how much memory the iterator is allowed to use to read-ahead (likely to speed up iteration)

Returns:

a ReaderIterator

See Also:

getWriterNoHeader(DataOutput, Format, int, int, int)

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

getMutable

public static PackedInts.Mutable getMutable(int valueCount,
                                            int bitsPerValue,
                                            float acceptableOverheadRatio)

Create a packed integer array with the given amount of values initialized to 0. the valueCount and the bitsPerValue cannot be changed after creation. All Mutables known by this factory are kept fully in RAM.

Positive values of acceptableOverheadRatio will trade space for speed by selecting a faster but potentially less memory-efficient implementation. An acceptableOverheadRatio of COMPACT will make sure that the most memory-efficient implementation is selected whereas FASTEST will make sure that the fastest implementation is selected.

Parameters:

valueCount - the number of elements

bitsPerValue - the number of bits available for any given value

acceptableOverheadRatio - an acceptable overhead ratio per value

Returns:

a mutable packed integer array

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

getMutable

public static PackedInts.Mutable getMutable(int valueCount,
                                            int bitsPerValue,
                                            PackedInts.Format format)

Same as getMutable(int, int, float) with a pre-computed number of bits per value and format.

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

getWriterNoHeader

public static PackedInts.Writer getWriterNoHeader(DataOutput out,
                                                  PackedInts.Format format,
                                                  int valueCount,
                                                  int bitsPerValue,
                                                  int mem)

Expert: Create a packed integer array writer for the given output, format, value count, and number of bits per value.

The resulting stream will be long-aligned. This means that depending on the format which is used, up to 63 bits will be wasted. An easy way to make sure that no space is lost is to always use a valueCount that is a multiple of 64.

This method does not write any metadata to the stream, meaning that it is your responsibility to store it somewhere else in order to be able to recover data from the stream later on:

• format (using PackedInts.Format.getId()),
• valueCount,
• bitsPerValue,
• VERSION_CURRENT.

It is possible to start writing values without knowing how many of them you are actually going to write. To do this, just pass -1 as valueCount. On the other hand, for any positive value of valueCount, the returned writer will make sure that you don't write more values than expected and pad the end of stream with zeros in case you have written less than valueCount when calling PackedInts.Writer.finish().

The mem parameter lets you control how much memory can be used to buffer changes in memory before flushing to disk. High values of mem are likely to improve throughput. On the other hand, if speed is not that important to you, a value of 0 will use as little memory as possible and should already offer reasonable throughput.

Parameters:

out - the data output

format - the format to use to serialize the values

valueCount - the number of values

bitsPerValue - the number of bits per value

mem - how much memory (in bytes) can be used to speed up serialization

Returns:

a Writer

See Also:

getReaderIteratorNoHeader(DataInput, Format, int, int, int, int)

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

bitsRequired

public static int bitsRequired(long maxValue)

Returns how many bits are required to hold values up to and including maxValue NOTE: This method returns at least 1.

Parameters:

maxValue - the maximum value that should be representable.

Returns:

the amount of bits needed to represent values from 0 to maxValue.

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

unsignedBitsRequired

public static int unsignedBitsRequired(long bits)

Returns how many bits are required to store bits, interpreted as an unsigned value. NOTE: This method returns at least 1.

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

maxValue

public static long maxValue(int bitsPerValue)

Calculates the maximum unsigned long that can be expressed with the given number of bits.

Parameters:

bitsPerValue - the number of bits available for any given value.

Returns:

the maximum value for the given bits.

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

copy

public static void copy(PackedInts.Reader src,
                        int srcPos,
                        PackedInts.Mutable dest,
                        int destPos,
                        int len,
                        int mem)

Copy src[srcPos:srcPos+len] into dest[destPos:destPos+len] using at most mem bytes.