Package org.apache.lucene.analysis.ngram

Class NGramTokenizer

java.lang.Object

org.apache.lucene.util.AttributeSource

org.apache.lucene.analysis.TokenStream

org.apache.lucene.analysis.Tokenizer

org.apache.lucene.analysis.ngram.NGramTokenizer

All Implemented Interfaces:

Closeable, AutoCloseable

Direct Known Subclasses:

EdgeNGramTokenizer

public class NGramTokenizer
extends Tokenizer

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

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.lucene.util.AttributeSource

AttributeSource.AttributeFactory, AttributeSource.State

Field Summary

Fields

Modifier and Type	Field	Description
static final int	DEFAULT_MAX_NGRAM_SIZE
static final int	DEFAULT_MIN_NGRAM_SIZE

Constructor Summary

Constructors

Constructor	Description
NGramTokenizer(Version version, Reader input)	Creates NGramTokenizer with default min and max n-grams.
NGramTokenizer(Version version, Reader input, int minGram, int maxGram)	Creates NGramTokenizer with given min and max n-grams.
NGramTokenizer(Version version, AttributeSource.AttributeFactory factory, Reader input, int minGram, int maxGram)	Creates NGramTokenizer with given min and max n-grams.

Method Summary

Modifier and Type	Method	Description
final void	end()	This method is called by the consumer after the last token has been consumed, after TokenStream.incrementToken() returned false (using the new TokenStream API).
final boolean	incrementToken()	Consumers (i.e., IndexWriter) use this method to advance the stream to the next token.
final void	reset()	This method is called by a consumer before it begins consumption using TokenStream.incrementToken().

Methods inherited from class org.apache.lucene.analysis.Tokenizer

close, setReader

Methods inherited from class org.apache.lucene.util.AttributeSource

addAttribute, addAttributeImpl, captureState, clearAttributes, cloneAttributes, copyTo, equals, getAttribute, getAttributeClassesIterator, getAttributeFactory, getAttributeImplsIterator, hasAttribute, hasAttributes, hashCode, reflectAsString, reflectWith, restoreState, toString

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Field Details

DEFAULT_MIN_NGRAM_SIZE

public static final int DEFAULT_MIN_NGRAM_SIZE

See Also:

• Constant Field Values

DEFAULT_MAX_NGRAM_SIZE

public static final int DEFAULT_MAX_NGRAM_SIZE

See Also:

• Constant Field Values

Constructor Details

NGramTokenizer

public NGramTokenizer(Version version,
 Reader input,
 int minGram,
 int maxGram)

Creates NGramTokenizer with given min and max n-grams.

Parameters:

version - the lucene compatibility version

input - Reader holding the input to be tokenized

minGram - the smallest n-gram to generate

maxGram - the largest n-gram to generate

NGramTokenizer

public NGramTokenizer(Version version,
 AttributeSource.AttributeFactory factory,
 Reader input,
 int minGram,
 int maxGram)

Creates NGramTokenizer with given min and max n-grams.

Parameters:

version - the lucene compatibility version

factory - AttributeSource.AttributeFactory to use

input - Reader holding the input to be tokenized

minGram - the smallest n-gram to generate

maxGram - the largest n-gram to generate

NGramTokenizer

public NGramTokenizer(Version version,
 Reader input)

Creates NGramTokenizer with default min and max n-grams.

Parameters:

version - the lucene compatibility version

input - Reader holding the input to be tokenized

Method Details

incrementToken

public final boolean incrementToken()
                             throws IOException

Description copied from class: TokenStream

Consumers (i.e., IndexWriter) use this method to advance the stream to the next token. Implementing classes must implement this method and update the appropriate AttributeImpls 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 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 AttributeSource.addAttribute(Class) and AttributeSource.getAttribute(Class), references to all AttributeImpls 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 TokenStream.incrementToken().

Specified by:

incrementToken in class TokenStream

Returns:

false for end of stream; true otherwise

Throws:

IOException

end

public final void end()
               throws IOException

Description copied from class: TokenStream

This method is called by the consumer after the last token has been consumed, after TokenStream.incrementToken() returned false (using the new 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 super.end().

Overrides:

end in class TokenStream

Throws:

IOException - If an I/O error occurs

reset

public final void reset()
                 throws IOException

Description copied from class: TokenStream

This method is called by a consumer before it begins consumption using 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 super.reset(), otherwise some internal state will not be correctly reset (e.g., Tokenizer will throw IllegalStateException on further usage).

Overrides:

reset in class Tokenizer

Throws:

IOException