Package org.apache.lucene.analysis.ngram

Class NGramTokenFilter

  • All Implemented Interfaces:
    java.io.Closeable, java.lang.AutoCloseable
    public final class NGramTokenFilter
extends TokenFilter
    Tokenizes the input into n-grams of the given size(s). As of Lucene 4.4, this token filter:
    • handles supplementary characters correctly,
    • emits all n-grams for the same token at the same position,
    • does not modify offsets,
    • sorts n-grams by their offset in the original token first, then increasing length (meaning that "abc" will give "a", "ab", "abc", "b", "bc", "c").

    If you were using this TokenFilter to perform partial highlighting, this won't work anymore since this filter doesn't update offsets. You should modify your analysis chain to use NGramTokenizer, and potentially override NGramTokenizer.isTokenChar(int) to perform pre-tokenization.

    • Field Detail

      • DEFAULT_PRESERVE_ORIGINAL

        public static final boolean DEFAULT_PRESERVE_ORIGINAL
        See Also:
        Constant Field Values

      • minGram

        private final int minGram

      • maxGram

        private final int maxGram

      • preserveOriginal

        private final boolean preserveOriginal

      • curTermBuffer

        private char[] curTermBuffer

      • curTermLength

        private int curTermLength

      • curTermCodePointCount

        private int curTermCodePointCount

      • curGramSize

        private int curGramSize

      • curPos

        private int curPos

      • curPosIncr

        private int curPosIncr

    • Constructor Detail

      • NGramTokenFilter

        public NGramTokenFilter​(TokenStream input,
                        int minGram,
                        int maxGram,
                        boolean preserveOriginal)
        Creates an NGramTokenFilter that, for a given input term, produces all contained n-grams with lengths >= minGram and <= maxGram. Will optionally preserve the original term when its length is outside of the defined range. Note: Care must be taken when choosing minGram and maxGram; depending on the input token size, this filter potentially produces a huge number of terms.
        Parameters:
        input - TokenStream holding the input to be tokenized
        minGram - the minimum length of the generated n-grams
        maxGram - the maximum length of the generated n-grams
        preserveOriginal - Whether or not to keep the original term when it is shorter than minGram or longer than maxGram

      • NGramTokenFilter

        public NGramTokenFilter​(TokenStream input,
                        int gramSize)
        Creates an NGramTokenFilter that produces n-grams of the indicated size.
        Parameters:
        input - TokenStream holding the input to be tokenized
        gramSize - the size of n-grams to generate.

    • Method Detail

      • incrementToken

        public final boolean incrementToken()
                             throws java.io.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:
        java.io.IOException

      • reset

        public void reset()
           throws java.io.IOException
        Description copied from class: TokenFilter
        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).

        NOTE: The default implementation chains the call to the input TokenStream, so be sure to call super.reset() when overriding this method.

        Overrides:
        reset in class TokenFilter
        Throws:
        java.io.IOException

      • end

        public void end()
         throws java.io.IOException
        Description copied from class: TokenFilter
        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().

        NOTE: The default implementation chains the call to the input TokenStream, so be sure to call super.end() first when overriding this method.

        Overrides:
        end in class TokenFilter
        Throws:
        java.io.IOException - If an I/O error occurs