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):

    ngram tokens example
    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.