BlendedInfixSuggester loses a distinct suggestion when a duplicate entry arrives while its result set is full: boundedTreeAdd evicts the lowest result before TreeSet.add rejects the duplicate, so lookup returns num-1 suggestions, or a lower-ranked fetched suggestion in place of a higher-ranked one

Finding LFV-700Area: suggest / analyzing: BlendedInfixSuggester result collectionPriority: mediumImpact: silent wrong resultsNovelty: new

Finding page for LFV-700

AI-assisted finding, confirmed by automated checks. Upstream status: Fix in progress, by sjs004 (PR #16993) on 2026-11-06. PR #16993 references this bug. Source. This site files nothing upstream.

What it shows

You build a BlendedInfixSuggester with its default settings (POSITION_LINEAR, numFactor 10) from {the lake:1000, lake:950, lake:950, my lake:920} and call lookup("lake", false, 2); blending gives lake 950, the lake 900 and my lake 828, num=3 returns [lake/950, the lake/900, my lake/828], so you expect [lake/950, the lake/900]. Instead you get [lake/950, my lake/828], and without 'my lake' only [lake/950]. When the bounded result set is full, boundedTreeAdd (BlendedInfixSuggester.java:294-306) removes the lowest result with pollFirst() before it calls results.add(result), and that add does nothing for the second 'lake' because LookUpComparator (:383-407) already holds an equal element, so 'the lake' is lost and a later, lower-ranked hit takes its slot.

The test shows that a default BlendedInfixSuggester drops a distinct suggestion when a duplicate arrives while its result set is full: boundedTreeAdd evicts the lowest result before TreeSet.add rejects the duplicate.

Expected vs actual

For {the lake:1000, lake:950, lake:950, my lake:920}, lookup("lake", false, 2) should be the top 2 of the num=3 result, [lake/950, the lake/900]; actual [lake/950, my lake/828], with -ea and -da.

Files

src/org/apache/lucene/search/suggest/analyzing/TestBlendedInfixDuplicateEviction.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.analyzing.TestBlendedInfixDuplicateEviction

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; triaged by a Lucene maintainer as noted above. Feedback: open an issue in lucene-ai-bug-hunt.