org.apache.lucene.analysis.commongrams

## Class CommonGramsFilter

- java.lang.Object
- - org.apache.lucene.util.AttributeSource
  - - org.apache.lucene.analysis.TokenStream
    - - org.apache.lucene.analysis.TokenFilter
      - - org.apache.lucene.analysis.commongrams.CommonGramsFilter

- All Implemented Interfaces:
    Closeable, AutoCloseable

      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"

- - ### Nested Class Summary

    - ### Nested classes/interfaces inherited from class org.apache.lucene.util.AttributeSource

      `AttributeSource.AttributeFactory, AttributeSource.State`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static String` | `GRAM_TYPE`

    - ### Fields inherited from class org.apache.lucene.analysis.TokenFilter

      `input`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `CommonGramsFilter(Version matchVersion, TokenStream input, CharArraySet commonWords)` Construct a token stream filtering the given input using a Set of common words to create bigrams.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `incrementToken()` Inserts bigrams for common words into a token stream.
    `void` | `reset()`

    - ### Methods inherited from class org.apache.lucene.analysis.TokenFilter

      `close, end`

    - ### Methods inherited from class org.apache.lucene.util.AttributeSource

      `addAttribute, addAttributeImpl, captureState, clearAttributes, cloneAttributes, copyTo, equals, getAttribute, getAttributeClassesIterator, getAttributeFactory, getAttributeImplsIterator, hasAttribute, hasAttributes, hashCode, reflectAsString, reflectWith, restoreState`

    - ### Methods inherited from class java.lang.Object

      `clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait`

- - ### Field Detail

    - #### GRAM_TYPE

          public static final String GRAM_TYPE

      See Also:
        Constant Field Values

  - ### Constructor Detail

    - #### CommonGramsFilter

          public CommonGramsFilter(Version matchVersion,
                           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 .
      Parameters:
        `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`
      Throws:
        `IOException`

    - #### reset

          public void reset()
                     throws IOException

      Overrides:
        `reset` in class `TokenFilter`
      Throws:
        `IOException`

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