LexiSharp

A composable information retrieval toolkit for .NET — build, measure and inspect search pipelines, from lexical BM25 to hybrid and reranked retrieval.

Index, retrieve, rank and judge a search pipeline: an in-memory inverted index, four ranking strategies and their BM25 variants, rank fusion, reranking, optional PostgreSQL backends, and model-agnostic seams for dense, learned-sparse and neural scoring — the models stay in your application. The core package references no NuGet package at all.

📖 Full documentation → — the guide, the reference, and every measurement with the command that reproduces it. Published from docs/; this README is the short version and the details are delegated to those pages.

Install

dotnet add package LexiSharp   # the core: index, scorers, engines, decorators — no dependencies

net10.0, MIT. Optional: LexiSharp.MessagePack (binary index persistence), LexiSharp.AspNetCore (a GET /search minimal-API endpoint), LexiSharp.Postgres (lexical, vector, sparse, fuzzy and true BM25 backends) — packages.

Use it

using LexiSharp.Core;
using LexiSharp.Indexing;
using LexiSharp.Ranking;

// Three pieces, one contract: the index owns the corpus statistics, the scorer is a pure
// ranking strategy reading from it, the engine orchestrates. Each of the three is an
// interface, so you replace one without touching the others.
ITextSearchEngine engine = new RankedTextSearchEngine(
    new InMemoryTextIndex(),
    new Bm25Scorer());

engine.Index(new[]
{
    new SearchDocument("1", "The search engine uses BM25 to rank the results"),
    new SearchDocument("2", "TF-IDF is a classic method of textual search"),
    new SearchDocument("3", "Italian cuisine is renowned in Rome"),
});

foreach (var result in engine.Search("textual search"))
    Console.WriteLine($"{result.DocumentId} - {result.Score:0.###}: {result.Document.Text}");

That is the smallest thing the library does. The rest is composition: every engine above implements ITextSearchEngine, and a pipeline is stages wrapping each other. Given two engines over one index, HashingEmbeddingProvider standing in for your IEmbeddingProvider (no model, no service):

var hybrid = new HybridTextSearchEngine(
    new[] { lexical, dense },
    new ReciprocalRankFusionMerger());   // a BM25 score and a cosine, fused by rank, uncalibrated

ITextSearchEngine pipeline = new RerankedTextSearchEngine(
    hybrid, new ProximityReranker(index));   // ...or MMR, a cascade, MaxSim, a cross-encoder

Replacing a piece is the whole extension model — a PostgreSQL, vector, sparse or fuzzy backend takes the same slot, and IEmbeddingProvider, ISparseEmbeddingProvider and ICrossEncoderScorer are yours to implement: pipelines, and backends.

LexiSharpIndex<T> is the typed facade over the same engine if you would rather hand it your own objects — Getting started.

See it running

dotnet run --project samples/LexiSharp.Demo    # → http://localhost:5000

Five retrieval strategies over one corpus, compared live — BM25, corpus-derived semantic expansion, dense hashing embeddings, RRF fusion and a term-overlap rerank — with per-lane latency, highlighting and a click-through "why did this rank here?" panel. No model, no external service.

What it does not do

Stated plainly, so nothing is implied. The full list, with the measurement behind each claim, is Scope and limits.

• It matches the published BM25 baseline on all three corpora it can be compared on. On NFCorpus and SciFact, with the analysis, the BM25 parameters and the metric convention aligned to those the reference figures were produced with, the plain BM25 scorer reaches nDCG@10 0.3215 and 0.6788 against 0.3218 and 0.6789 — equal to the fourth decimal. On ArguAna, at the reference's own k1=0.9/b=0.4, it reaches 0.3970 against the 0.3970 that implementation publishes, and recall@100 0.9324 against 0.9324. That last one is not measured only by this harness: trec_eval — the standard evaluator, not this repository's code — reads both figures off a run this harness writes, and the 15,466 returned scores for the 1,406 queries match the reference's own searcher on the raw bits, so the ranking is that ranking rather than a lookalike. The same scores read under the library defaults are 0.308 / 0.662 / 0.320; that difference is the analyzer, the parameters and one task convention, not the ranking. Corpora are md5-verified on download, and the numbers are pinned and re-checked by the Pinned reference workflow, which replays every pinned configuration and exits non-zero on drift. It runs on a dispatch, on a push that touches the library or the harness, and weekly — see evaluation.
• An earlier figure published for ArguAna was withdrawn; it is not reproducible by any code path in this repository. See the changelog.
• No scorer here has a measured win over a tuned BM25. BM25+ and BM25L, tuned on their own δ, tie a tuned BM25 on the reference corpus and NFCorpus and edge it by 0.002–0.004 on SciFact — an in-sample margin, so an upper bound rather than a result. On ArguAna, untuned, they lose, and no δ-tuned ArguAna row exists, so whether tuning closes that gap is unmeasured (ranking).
• The SQL backends' retrieval quality is unmeasured. The BEIR numbers come from the in-memory engines; the live integration tests cover schema, query paths and cosine behaviour, not relevance (backends).
• Not every combination is tested. Engines, scorers, rerankers and mergers are tested individually and in the combinations described, but not every pairing — treat an unusual one as supported but unproven until you test it on your data.
• Version 0.6.0, one maintainer. The public API may still change between minor versions — pin a version and read the release notes.

Development

dotnet build LexiSharp.slnx
dotnet test  tests/LexiSharp.Tests   # xUnit suite; the Postgres suites need POSTGRES_TEST_CONNECTION

The retrieval quality gate replays every pinned configuration on the three BEIR corpora and exits non-zero on any drift. It downloads the corpora on first run, so it is not part of the xUnit suite:

dotnet run --project bench/LexiSharp.Eval -c Release -- --verify-reference

Benchmarks, the evaluation harness and the behavioural gate: Reference and Benchmarks.

License

MIT — see LICENSE. The ParadeDB pg_search extension used by the BM25 backend is licensed separately, under AGPL-3.