(Latent) MonotonicBlockPackedWriter writes a block whose values end close to Long.MAX_VALUE with too few bits, so MonotonicBlockPackedReader returns wrong values: min + (long) (avg * i) overflows in flush()

Finding LFV-148Area: util / packed ints (MonotonicBlockPackedWriter, MonotonicBlockPackedReader)Priority: lowImpact: minorNovelty: new

Finding page for LFV-148

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

What it shows

You write a block of non-negative, increasing longs with MonotonicBlockPackedWriter, for example {1, Long.MAX_VALUE}, and read it back with MonotonicBlockPackedReader. You expect the same values back: the writer is documented as 'A writer for large monotonically increasing sequences of positive longs' with no upper bound, and flush() says 'adjust min so that all deltas will be positive'. Instead the predicted value min + (long) (avg * i) wraps past Long.MAX_VALUE, so the adjustment of min is skipped and the resulting negative delta is ignored when the bit width is chosen; get(1) returns Long.MIN_VALUE with no error (a larger block trips an AssertionError in BulkOperationPacked.encode under -ea and reads back wrong values under -da).

The test shows that MonotonicBlockPackedWriter mis-encodes the block {1, Long.MAX_VALUE} (block size 64): min + (long) (avg * i) wraps in flush(), so the delta gets too few bits.

Expected vs actual

Expected reader.get(1) == Long.MAX_VALUE; actual Long.MIN_VALUE (same with -ea and -da).

Files

• src/org/apache/lucene/util/packed/TestMonotonicBlockPackedNearMaxValue.java (JUnit test)

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=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$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.util.packed.TestMonotonicBlockPackedNearMaxValue

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).

Formal models and scripts this finding's part of the hunt used: packed-ints-monotonic

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 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.