Table of Contents

Boosting and scoring

LeanCorpus defaults to BM25 (Bm25Similarity).

Available similarities

Similarity Model Notes
Bm25Similarity BM25 (k1=1.2, b=0.75) Default
Bm25PlusSimilarity BM25+ with lower-bound delta Avoids over-penalising long docs
Bm25LSimilarity BM25L with tf/(1+tf) modulated delta More nuanced lower-bound than BM25+
TfIdfSimilarity Classic TF-IDF sqrt(tf) * idf / sqrt(dl)
TfIdfAugmentedSimilarity Augmented TF-IDF 0.5 + 0.5 * tf/max_tf
TfIdfDoubleNormSimilarity Double-normalised TF-IDF Two-stage normalisation
TfIdfPivotedSimilarity Pivoted TF-IDF Pivoted document length normalisation
DirichletSimilarity LM with Dirichlet smoothing (μ=2000) Bayesian smoothing towards collection
LMAbsoluteDiscountingSimilarity LM with absolute discounting Subtracts constant δ from counts
LMJelinekMercerSimilarity LM with Jelinek-Mercer (λ=0.7) Linear interpolation with collection

All implement ISimilarity. Set on both writer (for norms) and searcher (for scoring):

var config = new IndexWriterConfig { Similarity = new Bm25PlusSimilarity() };
var searcherConfig = new IndexSearcherConfig { Similarity = new Bm25PlusSimilarity() };

Per-query boost

Every Query has a Boost (default 1.0). Multiplies that query's contribution within a BooleanQuery:

var q = new BooleanQuery.Builder()
    .Add(new TermQuery("title", "fox") { Boost = 3.0f }, Occur.Should)
    .Add(new TermQuery("body",  "fox") { Boost = 1.0f }, Occur.Should)
    .Build();

Constant scores

ConstantScoreQuery assigns a fixed score; skips BM25:

var filter = new ConstantScoreQuery(new TermQuery("status", "published"), score: 1.0f);

Function scores

FunctionScoreQuery blends BM25 with a numeric field:

ScoreMode Effect
Multiply (default) score * fieldValue
Replace fieldValue
Sum score + fieldValue
Max max(score, fieldValue) 
var boosted = new FunctionScoreQuery(
    new TermQuery("body", "phone"), "popularity", ScoreMode.Multiply);

Index-time field boosting

Set per-field boost factors at write time. They persist in index norms and apply to every query:

var config = new IndexWriterConfig
{
    FieldBoosts = new Dictionary<string, float>
    {
        ["title"] = 3.0f,
        ["body"]  = 1.0f,
    },
};

A field boost of 2.0 makes every hit in that field count twice as much. Set to 0.0 to disable a field for ranking.

See also

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