bm25

A Rust crate that computes BM25 embeddings for information retrieval. You can use these embeddings in vector databases that support the storage of sparse vectors, e.g. Qdrant, Pinecone, Milvus, etc.

This crate also contains a light-weight, in-memory full-text search engine built on top of the embedder.

Features

• Fast
• Multilingual
• Language detection
• Stop word removal
• Parallelism for fast batch-embedding
• Customisable embedding space
• Full access to BM25 parameters

The BM25 algorithm

BM25 is an algorithm for scoring the relevance of a query to documents in a corpus. You can make this scoring more efficient by pre-computing a 'sparse embedding' of each document. You can use these sparse embeddings directly, or upload them to a vector database and query them from there.

BM25 assumes that you know the average (meaningful) word count of your documents ahead of time. This crate provides utilities to compute this. If this assumption doesn't hold for your use-case, you have two options: (1) make a sensible guess (e.g. based on a sample); or (2) configure the algorithm to disregard document length. The former is recommended if most of your documents are around the same size.

BM25 has three parameters: b, k1 and avgdl. These terms match the formula given on Wikipedia. avgdl ('average document length') is the aforementioned average meaningful word count; you should always provide a value for this and the crate can fit this for you. b controls document length normalization; 0 means no normalisation (length will not affect score) while 1 means full normalisation. If you know avgdl, 0.75 is typically a good choice for b. If you're guessing avgdl, you can use a slightly lower b to reduce the effect of document length on score. If you have no idea what avgdl is, set b to 0. k1 controls how much weight is given to recurring tokens. For almost all use-cases, a value of 1.2 is suitable.

Getting started

Add bm25 to your project with

cargo add bm25

Embed

The best way to embed some text is to fit an embedder to your corpus.

use bm25::{Embedder, EmbedderBuilder, Embedding, Language};

let corpus = [
    "The sky blushed pink as the sun dipped below the horizon.",
    "Apples, oranges, papayas, and more papayas.",
    "She found a forgotten letter tucked inside an old book.",
    "A single drop of rain fell, followed by a thousand more.",
];

let embedder: Embedder = EmbedderBuilder::with_fit_to_corpus(Language::English, &corpus)
    .build();

assert_eq!(embedder.avgdl(), 5.75);

let embedding = embedder.embed(corpus[1]);

assert_eq!(
    embedding,
    Embedding {
        indices: vec![1777144781, 3887370161, 2177600299, 2177600299],
        values:  vec![1.1422123 , 1.1422123 , 1.5037148 , 1.5037148 ],
    }
)

BM25 parameters

For cases where you don't have the full corpus ahead of time, but have an approximate idea of the average meaningful word count you expect, you can construct an embedder with your avgdl guess.

use bm25::{Embedder, EmbedderBuilder};

let embedder: Embedder = EmbedderBuilder::with_avgdl(7.0)
    .build();

If you want to disregard document length altogether, set b to 0.

use bm25::{Embedder, EmbedderBuilder};

let embedder: Embedder = EmbedderBuilder::with_avgdl(1.0)
    .b(0.0) // if b = 0, avgdl has no effect
    .build();

Language

By default, this crate tokenizes text in English. If you are working with a different language, you can set the embedder language mode.

use bm25::{Embedder, EmbedderBuilder, Language};

let embedder: Embedder = EmbedderBuilder::with_avgdl(256.0)
    .language_mode(Language::German)
    .build();

If your corpus is multilingual, or you don't know the language ahead of time, you can enable the language_detection feature.

cargo add bm25 --features language_detection

This unlocks the LanguageMode::Detect enum value. In this mode, the embedder will try to detect the language of each piece of input text before tokenizing. Note that there is a small performance overhead when embedding in this mode.

use bm25::{Embedder, EmbedderBuilder, LanguageMode};

let embedder: Embedder = EmbedderBuilder::with_avgdl(64.0)
    .language_mode(LanguageMode::Detect)
    .build();

Stop words

By default, this crate removes stop words according to the NLTK stop words list for the given/detected language before embedding. This removes noise from insignificant words. If you do not want to remove stop words, disable default features.

cargo add bm25 --no-default-features

If you would rather use the Stopwords ISO stop words collection:

cargo add bm25 --no-default-features --features iso_stopwords

Stop word lists are provided by the stop-words crate. For more information, see the documentation therein.

Embedding space

You can customise the dimensionality of your sparse vector via the generic parameter. Supported values are usize, u32 and u64. You can also use your own type (and inject your own embedding function) by implementing the EmbeddingDimension trait.

use bm25::{EmbedderBuilder, EmbeddingDimension};

let text = "cup of tea";

// Embed into a u32-dimensional space
let embedder = EmbedderBuilder::<u32>::with_avgdl(2.0).build();
let embedding = embedder.embed(text);
assert_eq!(embedding.indices, [2070875659, 415655421]);

// Embed into a u64-dimensional space
let embedder = EmbedderBuilder::<u64>::with_avgdl(2.0).build();
let embedding = embedder.embed(text);
assert_eq!(embedding.indices, [3288102823240002853, 7123809554392261272]);

// Embed into a usize-dimensional space
let embedder = EmbedderBuilder::<usize>::with_avgdl(2.0).build();
let embedding = embedder.embed(text);
assert_eq!(embedding.indices, [3288102823240002853, 7123809554392261272]);

#[derive(Eq, PartialEq, Hash, Clone, Debug)]
struct MyType(u32);
impl EmbeddingDimension for MyType {
    fn embed(_token: &str) -> Self {
        MyType(42)
    }
}

// Embed into a MyType-dimensional space
let embedder = EmbedderBuilder::<MyType>::with_avgdl(2.0).build();
let embedding = embedder.embed(text);
assert_eq!(embedding.indices, [MyType(42), MyType(42)]);

Search

This crate includes a light-weight, in-memory full-text search engine built on top of the embedder.

use bm25::{Document, Language, SearchEngineBuilder, SearchResult};

let corpus = [
    "The rabbit munched the orange carrot.",
    "The snake hugged the green lizard.",
    "The hedgehog impaled the orange orange.",
    "The squirrel buried the brown nut.",
];

let search_engine = SearchEngineBuilder::<u32>::with_corpus(Language::English, corpus)
    .build();

let limit = 3;
let search_results = search_engine.search("orange", limit);

assert_eq!(
    search_results,
    vec![
        SearchResult {
            document: Document {
                id: 2,
                contents: String::from("The hedgehog impaled the orange orange."),
            },
            score: 0.4904281,
        },
        SearchResult {
            document: Document {
                id: 0,
                contents: String::from("The rabbit munched the orange carrot."),
            },
            score: 0.35667497,
        },
    ]
);

You can construct a search engine with documents (allowing you to customise the id type and value), or with an average document length.

use bm25::{Document, Language, SearchEngineBuilder};

// Build a search engine from documents
let search_engine = SearchEngineBuilder::<&str>::with_documents(
    Language::English,
    [
        Document {
            id: "Guacamole",
            contents: String::from("avocado, lime juice, salt, onion, tomatoes, coriander."),
        },
        Document {
            id: "Hummus",
            contents: String::from("chickpeas, tahini, olive oil, garlic, lemon juice, salt."),
        },
    ],
)
.build();

// Build a search engine from avgdl
let search_engine = SearchEngineBuilder::<u32>::with_avgdl(128.0)
    .build();

You can upsert or remove documents from the search engine. Note that mutating the search corpus by upserting or removing documents will change the true value of avgdl. The more avgdl drifts from its true value, the less accurate the BM25 scores will be.

use bm25::{Document, SearchEngineBuilder};

let mut search_engine = SearchEngineBuilder::<u32>::with_avgdl(10.0)
    .build();

let document_id = 42;
let document = Document {
    id: document_id,
    contents: String::from(
        "A breeze carried the scent of blooming jasmine through the open window.",
    ),
};

search_engine.upsert(document.clone());
assert_eq!(search_engine.get(&document_id).unwrap(), document);

search_engine.remove(&document_id);
assert_eq!(search_engine.get(&document_id), None);

Working with a large corpus

If your corpus is large, fitting an embedder and generating embeddings can be slow. Fortunately, these tasks can both be trivially parallelised via the parallelism feature, which implements data parallelism using Rayon.

cargo add bm25 --features parallelism

License

MIT License