Class NGramTokenizer
- All Implemented Interfaces:
Closeable
,AutoCloseable
- Direct Known Subclasses:
EdgeNGramTokenizer
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).
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Nested Class Summary
Nested classes/interfaces inherited from class org.apache.lucene.util.AttributeSource
AttributeSource.State
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Field Summary
FieldsModifier and TypeFieldDescriptionprivate int[]
private int
private int
private CharacterUtils.CharacterBuffer
static final int
static final int
private boolean
private boolean
private int
private int
private int
private int
private int
private int
private final OffsetAttribute
private final PositionIncrementAttribute
private final PositionLengthAttribute
private final CharTermAttribute
Fields inherited from class org.apache.lucene.analysis.TokenStream
DEFAULT_TOKEN_ATTRIBUTE_FACTORY
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Constructor Summary
ConstructorsConstructorDescriptionCreates NGramTokenizer with default min and max n-grams.NGramTokenizer
(int minGram, int maxGram) Creates NGramTokenizer with given min and max n-grams.NGramTokenizer
(int minGram, int maxGram, boolean edgesOnly) NGramTokenizer
(AttributeFactory factory, int minGram, int maxGram) Creates NGramTokenizer with given min and max n-grams.NGramTokenizer
(AttributeFactory factory, int minGram, int maxGram, boolean edgesOnly) -
Method Summary
Modifier and TypeMethodDescriptionprivate void
consume()
Consume one code point.final void
end()
This method is called by the consumer after the last token has been consumed, afterTokenStream.incrementToken()
returnedfalse
(using the newTokenStream
API).final boolean
Consumers (i.e.,IndexWriter
) use this method to advance the stream to the next token.private void
init
(int minGram, int maxGram, boolean edgesOnly) protected boolean
isTokenChar
(int chr) Only collect characters which satisfy this condition.final void
reset()
This method is called by a consumer before it begins consumption usingTokenStream.incrementToken()
.private void
Methods inherited from class org.apache.lucene.analysis.Tokenizer
close, correctOffset, setReader, setReaderTestPoint
Methods inherited from class org.apache.lucene.util.AttributeSource
addAttribute, addAttributeImpl, captureState, clearAttributes, cloneAttributes, copyTo, endAttributes, equals, getAttribute, getAttributeClassesIterator, getAttributeFactory, getAttributeImplsIterator, hasAttribute, hasAttributes, hashCode, reflectAsString, reflectWith, removeAllAttributes, restoreState, toString
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Field Details
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DEFAULT_MIN_NGRAM_SIZE
public static final int DEFAULT_MIN_NGRAM_SIZE- See Also:
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DEFAULT_MAX_NGRAM_SIZE
public static final int DEFAULT_MAX_NGRAM_SIZE- See Also:
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charBuffer
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buffer
private int[] buffer -
bufferStart
private int bufferStart -
bufferEnd
private int bufferEnd -
offset
private int offset -
gramSize
private int gramSize -
minGram
private int minGram -
maxGram
private int maxGram -
exhausted
private boolean exhausted -
lastCheckedChar
private int lastCheckedChar -
lastNonTokenChar
private int lastNonTokenChar -
edgesOnly
private boolean edgesOnly -
termAtt
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posIncAtt
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posLenAtt
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offsetAtt
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Constructor Details
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NGramTokenizer
NGramTokenizer(int minGram, int maxGram, boolean edgesOnly) -
NGramTokenizer
public NGramTokenizer(int minGram, int maxGram) Creates NGramTokenizer with given min and max n-grams.- Parameters:
minGram
- the smallest n-gram to generatemaxGram
- the largest n-gram to generate
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NGramTokenizer
NGramTokenizer(AttributeFactory factory, int minGram, int maxGram, boolean edgesOnly) -
NGramTokenizer
Creates NGramTokenizer with given min and max n-grams.- Parameters:
factory
-AttributeFactory
to useminGram
- the smallest n-gram to generatemaxGram
- the largest n-gram to generate
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NGramTokenizer
public NGramTokenizer()Creates NGramTokenizer with default min and max n-grams.
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Method Details
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init
private void init(int minGram, int maxGram, boolean edgesOnly) -
incrementToken
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 appropriateAttributeImpl
s 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)
andAttributeSource.getAttribute(Class)
, references to allAttributeImpl
s 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 classTokenStream
- Returns:
- false for end of stream; true otherwise
- Throws:
IOException
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updateLastNonTokenChar
private void updateLastNonTokenChar() -
consume
private void consume()Consume one code point. -
isTokenChar
protected boolean isTokenChar(int chr) Only collect characters which satisfy this condition. -
end
Description copied from class:TokenStream
This method is called by the consumer after the last token has been consumed, afterTokenStream.incrementToken()
returnedfalse
(using the newTokenStream
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 classTokenStream
- Throws:
IOException
- If an I/O error occurs
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reset
Description copied from class:TokenStream
This method is called by a consumer before it begins consumption usingTokenStream.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 throwIllegalStateException
on further usage).- Overrides:
reset
in classTokenizer
- Throws:
IOException
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