public class NGramTokenizer extends Tokenizer
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:
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
.
AttributeSource.AttributeFactory, AttributeSource.State
Modifier and Type | Field and Description |
---|---|
static int |
DEFAULT_MAX_NGRAM_SIZE |
static int |
DEFAULT_MIN_NGRAM_SIZE |
Constructor and Description |
---|
NGramTokenizer(Version version,
AttributeSource.AttributeFactory factory,
Reader input,
int minGram,
int maxGram)
Creates NGramTokenizer with given min and max n-grams.
|
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.
|
Modifier and Type | Method and Description |
---|---|
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). |
boolean |
incrementToken()
Consumers (i.e.,
IndexWriter ) use this method to advance the stream to
the next token. |
void |
reset()
This method is called by a consumer before it begins consumption using
TokenStream.incrementToken() . |
addAttribute, addAttributeImpl, captureState, clearAttributes, cloneAttributes, copyTo, equals, getAttribute, getAttributeClassesIterator, getAttributeFactory, getAttributeImplsIterator, hasAttribute, hasAttributes, hashCode, reflectAsString, reflectWith, restoreState, toString
public static final int DEFAULT_MIN_NGRAM_SIZE
public static final int DEFAULT_MAX_NGRAM_SIZE
public NGramTokenizer(Version version, Reader input, int minGram, int maxGram)
public NGramTokenizer(Version version, AttributeSource.AttributeFactory factory, Reader input, int minGram, int maxGram)
version
- the lucene compatibility versionfactory
- AttributeSource.AttributeFactory
to useinput
- Reader
holding the input to be tokenizedminGram
- the smallest n-gram to generatemaxGram
- the largest n-gram to generatepublic final boolean incrementToken() throws IOException
TokenStream
IndexWriter
) use this method to advance the stream to
the next token. Implementing classes must implement this method and update
the appropriate AttributeImpl
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)
and AttributeSource.getAttribute(Class)
,
references to all AttributeImpl
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()
.
incrementToken
in class TokenStream
IOException
public final void end() throws IOException
TokenStream
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()
.
end
in class TokenStream
IOException
- If an I/O error occurspublic final void reset() throws IOException
TokenStream
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).
reset
in class Tokenizer
IOException
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