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Bm25Index

BM25 (Okapi) scored over a document-term matrix of raw counts.

public sealed class Bm25Index

Example — three documents, and the rare term that separates them.

using Lodestar.Abstractions;
using Lodestar.Text.Search;
using Lodestar.Text.Vectorization;

var vectorizer = new CountVectorizer();
CsrMatrix counts = vectorizer.FitTransform(
    ["the cat sat", "the dog sat sat", "a bird flew far away today"]);
int cat = vectorizer.GetFeatureNames().ToList().IndexOf("cat");

var index = new Bm25Index(counts);
double[] scores = index.Score([cat]);

double first = scores[0];  // => 0.5755…
double second = scores[1];  // => 0
int documents = index.DocumentCount;  // => 3

Remarks — the matrix is the one CountVectorizer produced, so everything that shaped it — the analyzer, the n-gram range, the stop words, MinDf and MaxDf — has already happened and is not re-decided here. Bm25Index adds the weighting and nothing else.

Document length comes free. BM25's |D| is the L1 norm of a raw-count row, which CsrMatrix.RowL1Norm already exposes; document frequency is one pass over the column indices. There is no new numerical machinery in this type.

Building transposes, so a query does not scan. CSR is row-major, and reaching the documents that hold one term through it means reading every row. The constructor sorts the non-zeros by column once — one extra copy of them — after which a query term reads its own postings and a rare term costs what it matches rather than what the corpus holds.

Queries are column indices, not strings. One entry per query occurrence, so a term given twice counts twice — which is how the reference counts it, and a set-based query would halve the contribution. An index outside the vocabulary is ignored, which is how a term the corpus never saw behaves. GetFeatureNames() is the map from term to column.

Immutable once built, and safe to score from any number of threads at once.

Reference behaviour is rank_bm25 0.2.2's BM25Okapi, matched over 8 corpora.

Applies to — net10.0, netstandard2.0.

See also — the search index, Bm25Options, RankFusion.

Members

Member	What it does
Bm25Index.Score	Scores every document against a query.
Bm25Index.Top	The best documents for a query, best first.