FSTCompletionLookup sorts weights as unsigned bytes, so negative weights get the highest buckets and are suggested before positive ones

Finding LFV-479Area: suggest / FST completion suggester (FSTCompletionLookup.build weight bucketing)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-479

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

What it shows

You build an FSTCompletionLookup from two suggestions, apple with weight 10 and apricot with weight -10, and ask lookup("ap", true, 2) for the best completions first. You expect apple first, because the class javadoc says buckets come from 'sorting the input by weight', InputIterator says 'higher numbers mean better suggestions', and encodeWeight accepts every int. Instead you get [apricot/5, apple/0], with no error: build() sorts the weights by their unsigned big-endian bytes, where every negative int (first byte 0x80 or above) comes after every non-negative int, so all negative weights land in the highest buckets and are suggested first.

The test shows that FSTCompletionLookup sorts weights as unsigned bytes, so a negative weight lands in the highest bucket and is suggested before a positive one.

Expected vs actual

Expected apple (weight 10) before apricot (weight -10) for lookup("ap", true, 2); actual [apricot/5, apple/0], with -ea and -da.

Files

• src/org/apache/lucene/search/suggest/fst/TestFSTCompletionLookupNegativeWeights.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=$CP:$LUCENE/lucene/analysis/common/build/libs/lucene-analysis-common-$V.jar
CP=$CP:$LUCENE/lucene/suggest/build/libs/lucene-suggest-$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.suggest.fst.TestFSTCompletionLookupNegativeWeights

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