ShingleFilter with minShingleSize >= 3 ends the token stream early when a hole (for example a removed stop word) is near the end, so the last word(s) are never output as unigrams and are missing from the index ('a cup of tea' loses 'tea'; 'the cat' yields no token at all)
Finding LFV-528Area: analysis-common / shingle (ShingleFilter, ShingleFilterFactory, ShingleAnalyzerWrapper)Priority: mediumImpact: silent wrong resultsNovelty: new
AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.
What it shows
You index 'a cup of tea' with WhitespaceTokenizer, StopFilter{a, of, the} and ShingleFilter(min 3, max 3) with unigrams on (the default), and expect every real word to be indexed as a unigram, because the javadoc says unigrams 'are always output, regardless of whether any shingles are available' and that the unigram option 'is independent of the min shingle size'. Instead, when the window starts with the filler left by the removed stop word, incrementToken() skips the all-filler unigram, finds fewer than minShingleSize positions left for a shingle and returns false, which ends the stream although 'tea' is still in its window: 'tea' is never indexed and TermQuery(tea) finds 0 documents. With min 4 up to two trailing words are lost, a value such as 'the cat' yields no token at all, and nothing throws, with assertions on or off.
The test shows that ShingleFilter(3, 3) with unigrams on ends the stream early after a removed stop word, so "the cat" yields no token at all.
Expected vs actual
Expected: [cat] (the javadoc says unigrams are always output, even when no shingle fits). Actual (-ea and -da): [].
Files
Run it
- Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
- Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
- Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
- How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
- Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.
# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:$LUCENE/lucene/analysis/common/build/libs/lucene-analysis-common-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar # JUnit Platform console launcher (Maven Central)
javac -d classes -cp "$CP" $(find src -name '*.java')
# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
-cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
execute --select-class org.apache.lucene.analysis.shingle.TestShingleFilterHoleBeforeLastWordGradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).
Feedback
Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.
Snapshot of 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.