NGramFilter -- construct n-grams from a TokenStream

• Key: LUCENE-400
• Type: Improvement
• Status: Closed
• Resolution: Fixed
• Priority: Minor
• Assignee: Grant Ingersoll
• Reporter: Sebastian Kirsch
• Component/s: Analysis
• Affects Version/s: unspecified
• Fix Version/s: 2.4

Environment

Operating System: All
Platform: All

Description

This filter constructs n-grams (token combinations up to a fixed size, sometimes
called "shingles") from a token stream.

The filter sets start offsets, end offsets and position increments, so
highlighting and phrase queries should work.

Position increments > 1 in the input stream are replaced by filler tokens
(tokens with termText "_" and endOffset - startOffset = 0) in the output
n-grams. (Position increments > 1 in the input stream are usually caused by
removing some tokens, eg. stopwords, from a stream.)

The filter uses CircularFifoBuffer and UnboundedFifoBuffer from Apache
Commons-Collections.

Filter, test case and an analyzer are attached.