SpellChecker.suggestSimilar with SuggestWordFrequencyComparator drops more frequent suggestions: once the queue is full it raises the accuracy cutoff to the score of the least frequent queued word, which is a valid bound only for a score-first order (since 3.1, LUCENE-2479)
Finding LFV-566Area: suggest / spell: SpellChecker.suggestSimilar (index-based n-gram spell checker; comparator vs. full-queue accuracy cutoff)Priority: lowImpact: silent wrong resultsNovelty: new
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 index-based SpellChecker with the shipped SuggestWordFrequencyComparator ('Frequency first, then score') and ask for suggestions restricted to a field, expecting the most frequent qualifying words among the retrieved candidates. Once the queue holds numSug words, suggestSimilar raises its accuracy cutoff to the score of the queue's head, which under this comparator is the least frequent queued word, so a later candidate that is more frequent but scores slightly lower is rejected before its frequency is even read. With 'receive' in 50 documents and one-edit neighbours such as 'relieve', 'reprieve' and 'retrieve' in one document each, suggestSimilar('recieve', 1, reader, field, SUGGEST_WHEN_NOT_IN_INDEX) returns [relieve] instead of receive (numSug 1-3 all leave it out; numSug 4 ranks it first).
The test shows that SpellChecker.suggestSimilar with SuggestWordFrequencyComparator ("frequency first, then score") drops a more frequent word once the queue is full, because it raises the accuracy cutoff to the score of the least frequent queued word (receive in 50 docs, relieve in 1, query recieve, numSug 1).
Expected vs actual
Expected [receive]; actual [relieve] (expected:<[re[cei]ve]> but was:<[re[lie]ve]>), with -ea and -da.
Files
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.spell.TestSpellCheckerFrequencyComparatorGradle 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.