Class CommonGramsFilter

  • All Implemented Interfaces:
    Closeable, AutoCloseable, Unwrappable<TokenStream>

    public final class CommonGramsFilter
    extends TokenFilter
    Construct bigrams for frequently occurring terms while indexing. Single terms are still indexed too, with bigrams overlaid. This is achieved through the use of PositionIncrementAttribute.setPositionIncrement(int). Bigrams have a type of GRAM_TYPE Example:
    • input:"the quick brown fox"
    • output:|"the","the-quick"|"brown"|"fox"|
    • "the-quick" has a position increment of 0 so it is in the same position as "the" "the-quick" has a term.type() of "gram"
    • Constructor Detail

      • CommonGramsFilter

        public CommonGramsFilter​(TokenStream input,
                                 CharArraySet commonWords)
        Construct a token stream filtering the given input using a Set of common words to create bigrams. Outputs both unigrams with position increment and bigrams with position increment 0 type=gram where one or both of the words in a potential bigram are in the set of common words .
        input - TokenStream input in filter chain
        commonWords - The set of common words.
    • Method Detail

      • incrementToken

        public boolean incrementToken()
                               throws IOException
        Inserts bigrams for common words into a token stream. For each input token, output the token. If the token and/or the following token are in the list of common words also output a bigram with position increment 0 and type="gram"

        TODO:Consider adding an option to not emit unigram stopwords as in CDL XTF BigramStopFilter, CommonGramsQueryFilter would need to be changed to work with this.

        TODO: Consider optimizing for the case of three commongrams i.e "man of the year" normally produces 3 bigrams: "man-of", "of-the", "the-year" but with proper management of positions we could eliminate the middle bigram "of-the"and save a disk seek and a whole set of position lookups.

        Specified by:
        incrementToken in class TokenStream