Class NGramTokenizer

Tokenizes the input into n-grams of the given size(s).

On the contrary to NGramTokenFilter, this class sets offsets so that characters between startOffset and endOffset in the original stream are the same as the term chars.

For example, "abcde" would be tokenized as (minGram=2, maxGram=3):

Term	Position increment	Position length	Offsets
ab	1	1	[0,2[
abc	1	1	[0,3[
bc	1	1	[1,3[
bcd	1	1	[1,4[
cd	1	1	[2,4[
cde	1	1	[2,5[
de	1	1	[3,5[

This tokenizer changed a lot in Lucene 4.4 in order to:

tokenize in a streaming fashion to support streams which are larger than 1024 chars (limit of the previous version),
count grams based on unicode code points instead of java chars (and never split in the middle of surrogate pairs),
give the ability to pre-tokenize the stream (IsTokenChar(int)) before computing n-grams.

Additionally, this class doesn't trim trailing whitespaces and emits tokens in a different order, tokens are now emitted by increasing start offsets while they used to be emitted by increasing lengths (which prevented from supporting large input streams).

Although highly discouraged, it is still possible to use the old behavior through Lucene43NGramTokenizer.

Inheritance

object

AttributeSource

TokenStream

Tokenizer

NGramTokenizer

EdgeNGramTokenizer

Implements

IDisposable

Inherited Members

Tokenizer.m_input

Tokenizer.Dispose(bool)

Tokenizer.CorrectOffset(int)

Tokenizer.SetReader(TextReader)

TokenStream.Dispose()

AttributeSource.GetAttributeFactory()

AttributeSource.GetAttributeClassesEnumerator()

AttributeSource.GetAttributeImplsEnumerator()

AttributeSource.AddAttributeImpl(Attribute)

AttributeSource.AddAttribute<T>()

AttributeSource.HasAttributes

AttributeSource.HasAttribute<T>()

AttributeSource.GetAttribute<T>()

AttributeSource.ClearAttributes()

AttributeSource.CaptureState()

AttributeSource.RestoreState(AttributeSource.State)

AttributeSource.GetHashCode()

AttributeSource.Equals(object)

AttributeSource.ReflectAsString(bool)

AttributeSource.ReflectWith(IAttributeReflector)

AttributeSource.CloneAttributes()

AttributeSource.CopyTo(AttributeSource)

AttributeSource.ToString()

object.Equals(object, object)

object.GetType()

object.MemberwiseClone()

object.ReferenceEquals(object, object)

Namespace: Lucene.Net.Analysis.NGram

Assembly: Lucene.Net.Analysis.Common.dll

Syntax

public class NGramTokenizer : Tokenizer, IDisposable

Constructors

NGramTokenizer(LuceneVersion, AttributeFactory, TextReader, int, int)

Creates NGramTokenizer with given min and max n-grams.

Declaration

public NGramTokenizer(LuceneVersion version, AttributeSource.AttributeFactory factory, TextReader input, int minGram, int maxGram)

Parameters

Type	Name	Description
LuceneVersion	version	the lucene compatibility version
AttributeSource.AttributeFactory	factory	Lucene.Net.Util.AttributeSource.AttributeFactory to use
TextReader	input	TextReader holding the input to be tokenized
int	minGram	the smallest n-gram to generate
int	maxGram	the largest n-gram to generate

NGramTokenizer(LuceneVersion, TextReader)

Creates NGramTokenizer with default min and max n-grams.

Declaration

public NGramTokenizer(LuceneVersion version, TextReader input)

Parameters

Type	Name	Description
LuceneVersion	version	the lucene compatibility version
TextReader	input	TextReader holding the input to be tokenized

NGramTokenizer(LuceneVersion, TextReader, int, int)

Creates NGramTokenizer with given min and max n-grams.

Declaration

public NGramTokenizer(LuceneVersion version, TextReader input, int minGram, int maxGram)

Parameters

Type	Name	Description
LuceneVersion	version	the lucene compatibility version
TextReader	input	TextReader holding the input to be tokenized
int	minGram	the smallest n-gram to generate
int	maxGram	the largest n-gram to generate

Fields

DEFAULT_MAX_NGRAM_SIZE

Tokenizes the input into n-grams of the given size(s).

On the contrary to NGramTokenFilter, this class sets offsets so that characters between startOffset and endOffset in the original stream are the same as the term chars.

For example, "abcde" would be tokenized as (minGram=2, maxGram=3):

Term	Position increment	Position length	Offsets
ab	1	1	[0,2[
abc	1	1	[0,3[
bc	1	1	[1,3[
bcd	1	1	[1,4[
cd	1	1	[2,4[
cde	1	1	[2,5[
de	1	1	[3,5[

This tokenizer changed a lot in Lucene 4.4 in order to:

tokenize in a streaming fashion to support streams which are larger than 1024 chars (limit of the previous version),
count grams based on unicode code points instead of java chars (and never split in the middle of surrogate pairs),
give the ability to pre-tokenize the stream (IsTokenChar(int)) before computing n-grams.

Although highly discouraged, it is still possible to use the old behavior through Lucene43NGramTokenizer.

Declaration

public const int DEFAULT_MAX_NGRAM_SIZE = 2

Field Value

Type	Description
int

DEFAULT_MIN_NGRAM_SIZE

Tokenizes the input into n-grams of the given size(s).

On the contrary to NGramTokenFilter, this class sets offsets so that characters between startOffset and endOffset in the original stream are the same as the term chars.

For example, "abcde" would be tokenized as (minGram=2, maxGram=3):

Term	Position increment	Position length	Offsets
ab	1	1	[0,2[
abc	1	1	[0,3[
bc	1	1	[1,3[
bcd	1	1	[1,4[
cd	1	1	[2,4[
cde	1	1	[2,5[
de	1	1	[3,5[

This tokenizer changed a lot in Lucene 4.4 in order to:

tokenize in a streaming fashion to support streams which are larger than 1024 chars (limit of the previous version),
count grams based on unicode code points instead of java chars (and never split in the middle of surrogate pairs),
give the ability to pre-tokenize the stream (IsTokenChar(int)) before computing n-grams.

Although highly discouraged, it is still possible to use the old behavior through Lucene43NGramTokenizer.

Declaration

public const int DEFAULT_MIN_NGRAM_SIZE = 1

Field Value

Type	Description
int

Methods

End()

This method is called by the consumer after the last token has been consumed, after Lucene.Net.Analysis.TokenStream.IncrementToken() returned false (using the new Lucene.Net.Analysis.TokenStream API). Streams implementing the old API should upgrade to use this feature.

This method can be used to perform any end-of-stream operations, such as setting the final offset of a stream. The final offset of a stream might differ from the offset of the last token eg in case one or more whitespaces followed after the last token, but a WhitespaceTokenizer was used.

Additionally any skipped positions (such as those removed by a stopfilter) can be applied to the position increment, or any adjustment of other attributes where the end-of-stream value may be important.

If you override this method, always call base.End();.

Declaration

public override sealed void End()

Overrides

Lucene.Net.Analysis.TokenStream.End()

Exceptions

Type	Condition
IOException	If an I/O error occurs

IncrementToken()

Consumers (i.e., Lucene.Net.Index.IndexWriter) use this method to advance the stream to the next token. Implementing classes must implement this method and update the appropriate Lucene.Net.Util.IAttributes with the attributes of the next token.

The producer must make no assumptions about the attributes after the method has been returned: the caller may arbitrarily change it. If the producer needs to preserve the state for subsequent calls, it can use Lucene.Net.Util.AttributeSource.CaptureState() to create a copy of the current attribute state.

this method is called for every token of a document, so an efficient implementation is crucial for good performance. To avoid calls to Lucene.Net.Util.AttributeSource.AddAttribute<T>() and Lucene.Net.Util.AttributeSource.GetAttribute<T>(), references to all Lucene.Net.Util.IAttributes that this stream uses should be retrieved during instantiation.

To ensure that filters and consumers know which attributes are available, the attributes must be added during instantiation. Filters and consumers are not required to check for availability of attributes in Lucene.Net.Analysis.TokenStream.IncrementToken().

Declaration

public override sealed bool IncrementToken()

Returns

Type	Description
bool	false for end of stream; true otherwise

Overrides

Lucene.Net.Analysis.TokenStream.IncrementToken()

IsTokenChar(int)

Only collect characters which satisfy this condition.

Declaration

protected virtual bool IsTokenChar(int chr)

Parameters

Type	Name	Description
int	chr

Returns

Type	Description
bool

Reset()

This method is called by a consumer before it begins consumption using Lucene.Net.Analysis.TokenStream.IncrementToken().

Resets this stream to a clean state. Stateful implementations must implement this method so that they can be reused, just as if they had been created fresh.

If you override this method, always call base.Reset(), otherwise some internal state will not be correctly reset (e.g., Lucene.Net.Analysis.Tokenizer will throw InvalidOperationException on further usage).

Declaration

public override sealed void Reset()

Overrides

Lucene.Net.Analysis.Tokenizer.Reset()

Implements

IDisposable