Class FuzzySet

A class used to represent a set of many, potentially large, values (e.g. many long strings such as URLs), using a significantly smaller amount of memory.

The set is "lossy" in that it cannot definitively state that is does contain a value but it can definitively say if a value is not in the set. It can therefore be used as a Bloom Filter.

Another application of the set is that it can be used to perform fuzzy counting because it can estimate reasonably accurately how many unique values are contained in the set.

This class is NOT threadsafe.

Internally a Bitset is used to record values and once a client has finished recording a stream of values the Downsize(Single) method can be used to create a suitably smaller set that is sized appropriately for the number of values recorded and desired saturation levels.

This is a Lucene.NET EXPERIMENTAL API, use at your own risk

Inheritance

System.Object

FuzzySet

Namespace: Lucene.Net.Codecs.Bloom

Assembly: Lucene.Net.Codecs.dll

Syntax

public class FuzzySet : object

Fields

VERSION_CURRENT

Declaration

public static readonly int VERSION_CURRENT

Field Value

Type	Description
System.Int32

VERSION_SPI

Declaration

public static readonly int VERSION_SPI

Field Value

Type	Description
System.Int32

VERSION_START

Declaration

public static readonly int VERSION_START

Field Value

Type	Description
System.Int32

Methods

AddValue(BytesRef)

Records a value in the set. The referenced bytes are hashed and then modulo n'd where n is the chosen size of the internal bitset.

Declaration

public virtual void AddValue(BytesRef value)

Parameters

Type	Name	Description
BytesRef	value	The Key value to be hashed.

Contains(BytesRef)

The main method required for a Bloom filter which, given a value determines set membership. Unlike a conventional set, the fuzzy set returns NO or MAYBE rather than true or false.

Declaration

public virtual FuzzySet.ContainsResult Contains(BytesRef value)

Parameters

Type	Name	Description
BytesRef	value

Returns

Type	Description
FuzzySet.ContainsResult	NO or MAYBE

CreateSetBasedOnMaxMemory(Int32)

Declaration

public static FuzzySet CreateSetBasedOnMaxMemory(int maxNumBytes)

Parameters

Type	Name	Description
System.Int32	maxNumBytes

Returns

Type	Description
FuzzySet

CreateSetBasedOnQuality(Int32, Single)

Declaration

public static FuzzySet CreateSetBasedOnQuality(int maxNumUniqueValues, float desiredMaxSaturation)

Parameters

Type	Name	Description
System.Int32	maxNumUniqueValues
System.Single	desiredMaxSaturation

Returns

Type	Description
FuzzySet

Deserialize(DataInput)

Declaration

public static FuzzySet Deserialize(DataInput input)

Parameters

Type	Name	Description
DataInput	input

Returns

Type	Description
FuzzySet

Downsize(Single)

Declaration

public virtual FuzzySet Downsize(float targetMaxSaturation)

Parameters

Type	Name	Description
System.Single	targetMaxSaturation	A number between 0 and 1 describing the % of bits that would ideally be set in the result. Lower values have better accuracy but require more space.

Returns

Type	Description
FuzzySet

GetEstimatedNumberUniqueValuesAllowingForCollisions(Int32, Int32)

Given a setSize and a the number of set bits, produces an estimate of the number of unique values recorded.

Declaration

public static int GetEstimatedNumberUniqueValuesAllowingForCollisions(int setSize, int numRecordedBits)

Parameters

Type	Name	Description
System.Int32	setSize
System.Int32	numRecordedBits

Returns

Type	Description
System.Int32

GetEstimatedUniqueValues()

Declaration

public virtual int GetEstimatedUniqueValues()

Returns

Type	Description
System.Int32

GetNearestSetSize(Int32)

Rounds down required maxNumberOfBits to the nearest number that is made up of all ones as a binary number.
Use this method where controlling memory use is paramount.

Declaration

public static int GetNearestSetSize(int maxNumberOfBits)

Parameters

Type	Name	Description
System.Int32	maxNumberOfBits

Returns

Type	Description
System.Int32

GetNearestSetSize(Int32, Single)

Use this method to choose a set size where accuracy (low content saturation) is more important than deciding how much memory to throw at the problem.

Declaration

public static int GetNearestSetSize(int maxNumberOfValuesExpected, float desiredSaturation)

Parameters

Type	Name	Description
System.Int32	maxNumberOfValuesExpected
System.Single	desiredSaturation	A number between 0 and 1 expressing the % of bits set once all values have been recorded.

Returns

Type	Description
System.Int32	The size of the set nearest to the required size.

GetSaturation()

Declaration

public virtual float GetSaturation()

Returns

Type	Description
System.Single

HashFunctionForVersion(Int32)

Declaration

public static HashFunction HashFunctionForVersion(int version)

Parameters

Type	Name	Description
System.Int32	version

Returns

Type	Description
HashFunction

RamBytesUsed()

Declaration

public virtual long RamBytesUsed()

Returns

Type	Description
System.Int64

Serialize(DataOutput)

Serializes the data set to file using the following format:

• FuzzySet -->FuzzySetVersion,HashFunctionName,BloomSize, NumBitSetWords,BitSetWord^{NumBitSetWords}
• HashFunctionName --> String () The name of a ServiceProvider registered HashFunction
• FuzzySetVersion --> Uint32 () The version number of the FuzzySet class
• BloomSize --> Uint32 () The modulo value used to project hashes into the field's Bitset
• NumBitSetWords --> Uint32 () The number of longs (as returned from GetBits())
• BitSetWord --> Long () A long from the array returned by GetBits()

Declaration

public virtual void Serialize(DataOutput output)

Parameters

Type	Name	Description
DataOutput	output	Data output stream.