AxiomaticF1EXP, F1LOG, F3EXP and F3LOG explain tf as 'computed as 1 + log(1 + log(freq))' of the raw freq, but the value they show is 1 + log(1 + log(freq + 1)); the score itself is correct

Finding LFV-920Area: core / search similarities (Axiomatic F1/F3 term-frequency explanation, tfExplain)Priority: lowImpact: minorNovelty: new

Finding page for LFV-920

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

What it shows

You search with AxiomaticF1EXP (or F1LOG, F3EXP, F3LOG) and call IndexSearcher.explain on a hit. The tf node says 'tf, term frequency computed as 1 + log(1 + log(freq)) from:' and its only child is 'freq, number of occurrences of term in the document', the raw count, so you expect its value to be that formula of that child. Instead tf() first adds 1 to freq (since LUCENE-8010, 2017), so for freq = 1 the node shows 1.5265890 where the stated formula gives 1.0, and for freq = 3 it shows 1.8697417 instead of 1.7412763; the score is right and only the formula text is stale.

The test shows that AxiomaticF1EXP's explanation describes the tf node as 1 + log(1 + log(freq)) while its value is tf(), which adds 1 to freq first (F1LOG, F3EXP and F3LOG share the same string); the score itself is right.

Expected vs actual

For one document with freq 1 the tf value is 1.526589 = 1 + log(1 + log(freq + 1)); expected the description to state that formula; actual: 1 + log(1 + log(freq)), which gives 1.0 (tf value 1.526589 expected:<...log(freq[ + 1])) from:> but was:<...log(freq[])) from:>), with -ea and -da.

Files

• src/org/apache/lucene/search/similarities/TestAxiomaticF1EXPTfExplain.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.search.similarities.TestAxiomaticF1EXPTfExplain

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: explanation-formula-strings-vs-their-own-operand-values

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.