org.apache.lucene.analysis.ngram

Class 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):

    Termababcbcbcdcdcdede
    Position increment1111111
    Position length1111111
    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).

    • Constructor Detail

      • NGramTokenizer

        public NGramTokenizer(int minGram,
                              int maxGram)
        Creates NGramTokenizer with given min and max n-grams.
        Parameters:
        minGram - the smallest n-gram to generate
        maxGram - the largest n-gram to generate
      • NGramTokenizer

        public NGramTokenizer(AttributeFactory factory,
                              int minGram,
                              int maxGram)
        Creates NGramTokenizer with given min and max n-grams.
        Parameters:
        factory - AttributeFactory to use
        minGram - the smallest n-gram to generate
        maxGram - the largest n-gram to generate
      • NGramTokenizer

        public NGramTokenizer()
        Creates NGramTokenizer with default min and max n-grams.
    • Method Detail

      • 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
      • isTokenChar

        protected boolean isTokenChar(int chr)
        Only collect characters which satisfy this condition.
      • 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