Classic Highlighter's SimpleSpanFragmenter splits a short phrase match across two fragments because its position counter lags the real token positions: the Highlighter never shows it the first token or tokens that overlap the previous one, so the phrase is recognised one or more words late and getBestFragment can return only half of it

Finding LFV-859Area: highlighter / classic Highlighter fragmenting (SimpleSpanFragmenter.isNewFragment position counter; Highlighter.getBestTextFragments)Priority: lowImpact: silent wrong resultsNovelty: known upstream

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You highlight 'Lucene is a search library written in Java' with the classic Highlighter, a QueryScorer for PhraseQuery 'search library', the default StandardAnalyzer and SimpleSpanFragmenter(scorer, 20), whose javadoc says it 'breaks text up into same-size fragments but does not split up Spans', and expect the 14-character phrase to stay in one fragment. Instead the fragments are 'Lucene is a <B>search</B>' | ' <B>library</B> written in Java' and getBestFragment returns 'Lucene is a <B>search</B>'; the same cut happens at the default fragment size 100 whenever the size threshold falls at the phrase's second word, and after a leading stop word or a word split by WordDelimiterGraphFilter the lag grows to two positions, so even three-word phrases are cut. The fragmenter counts positions only from the tokens the Highlighter shows it, and the Highlighter never shows it the first token or tokens that overlap the previous one, so it recognises the phrase one or more words late.

Expected vs actual

• Expected: The test passes: it asserts the documented behaviour.
• Actual: The fragments are 'Lucene is a <B>search</B>' | ' <B>library</B> written in Java' and getBestFragment returns 'Lucene is a <B>search</B>'; the same cut happens at the default fragment size 100 whenever the size threshold falls at the phrase's second word, and after a leading stop word or a word split by WordDelimiterGraphFilter the lag grows to two positions, so even three-word phrases are cut.

Files

• src/org/apache/lucene/search/highlight/TestSimpleSpanFragmenterPositionCounter.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main a9779ef3c193 (2026-10-26): reproduced.
• 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:$LUCENE/lucene/highlighter/build/libs/lucene-highlighter-$V.jar
CP=$CP:$LUCENE/lucene/memory/build/libs/lucene-memory-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$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.search.highlight.TestSimpleSpanFragmenterPositionCounter

Gradle 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.