MatchingFacetSetsCounts decodes the facet-set binary doc value from position 0 of BytesRef.bytes instead of BytesRef.offset, so a value that a reader returns as a slice of a larger array is counted from the wrong bytes (silently wrong facet-set counts, or NegativeArraySizeException / ArrayIndexOutOfBoundsException)

Finding LFV-405Area: facet / facet sets (MatchingFacetSetsCounts.count, a BinaryDocValues consumer; reached through MemoryIndex, custom readers and the backward-codecs Lucene80 BEST_COMPRESSION doc-values producer)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-405

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

What it shows

You count facet sets with MatchingFacetSetsCounts over a reader that hands back a document's facet-set binary doc value as a BytesRef slice of a larger array (offset != 0), for example a MemoryIndex holding a value that was packed into a shared buffer, and expect the same counts as for a compact copy of the same bytes. Instead count() reads the dimension count from bytes[0] and the dimension values from bytes[4..length) of the backing array, ignoring the offset, so the document is counted under whatever bytes start that array: a document whose set is (3,4), stored right after an encoding of (1,2), is counted as (1,2)=1 and (3,4)=0, with no error. When the leading bytes do not decode to a sane dimension count, the MatchingFacetSetsCounts constructor throws NegativeArraySizeException or ArrayIndexOutOfBoundsException instead, while the same value indexed through IndexWriter with the default codec is counted correctly.

The test shows that MatchingFacetSetsCounts ignores BytesRef.offset of a facet-set binary doc value: a MemoryIndex document whose value is the (3,4) encoding as a slice after an encoding of (1,2) is counted as (1,2).

Expected vs actual

Expected counts (3,4)=1, (1,2)=0; actual (3,4)=0, (1,2)=1 (expected:<[(3,4) (1), (1,2) (0)]> but was:<[(3,4) (0), (1,2) (1)]>), with -ea and -da.

Files

Run it

# 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/facet/build/libs/lucene-facet-$V.jar
CP=$CP:$LUCENE/lucene/memory/build/libs/lucene-memory-$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.facet.facetset.TestMatchingFacetSetsCountsSlicedValue

The module(s) facet also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.

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.