UnifiedHighlighter with postings or term-vector offsets highlights the wrong characters, or drops the match, in the second and later values of a multi-valued field when the analyzer's offset gap is not 1

Finding LFV-783Area: highlighter / UnifiedHighlighter (multi-valued content: postings and term-vector offset sources vs Analyzer#getOffsetGap; LimitedStoredFieldVisitor value join)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 index a field with two values, 'alpha one' and 'beta two', with an analyzer whose offset gap is 3 (for example CustomAnalyzer.builder().withOffsetGap(3)) and offsets in the postings or term vectors, then highlight the hits of body:beta and body:two with the UnifiedHighlighter; you expect 'beta two' and 'beta two', as with the default gap of 1. Instead you get 'beta two' and the unhighlighted 'alpha one', because indexing put the second value 3 characters after the first while the highlighter rebuilds the text with a single separator character between values, so every offset in the second value points 2 characters too far. The re-analysis offset source rejects the same analyzer with an IllegalArgumentException; the postings and term-vector sources accept it and return wrong snippets silently.

Expected vs actual

Expected: You expect 'beta two' and 'beta two', as with the default gap of 1.
Actual: You get 'beta two' and the unhighlighted 'alpha one', because indexing put the second value 3 characters after the first while the highlighter rebuilds the text with a single separator character between values, so every offset in the second value points 2 characters too far.

Files

src/org/apache/lucene/search/uhighlight/TestUnifiedHighlighterOffsetGap.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/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.uhighlight.TestUnifiedHighlighterOffsetGap

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.