# L-SEQ - Fractional Indexing for List CRDTs

This repository contains implementations of the L-SEQ algorithm for fractional indexing, used to create naive list CRDTs (Conflict-free Replicated Data Types). L-SEQ provides a way to generate unique, sortable identifiers that can be inserted between any two existing identifiers, making it ideal for collaborative editing and distributed systems.

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  <img class="hero-readme-img" src="docs/lseq-explained.webp" alt="Visualization of LSEQ tree structure" style="margin: 0 auto; max-height: 400px;" />
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## Implementations

- **TypeScript**: `@peoplesgrocers/lseq` - Located in the `typescript/` directory
- **Rust**: `peoplesgrocers-lseq` - Located in the `rust/` directory

## What is L-SEQ?

L-SEQ is a sequence CRDT that uses fractional indexing to maintain a total order of elements in a distributed system. It allows multiple users to concurrently insert elements into a list without conflicts, automatically resolving the order based on the generated identifiers.

## Usage

Each implementation provides:
- Allocation of new identifiers between existing ones
- Comparison functions for sorting
- Serialization to/from strings

See the individual directories for language-specific documentation and examples.

## References

This library implements:

[Brice Nédelec, Pascal Molli, Achour Mostefaoui, Emmanuel Desmontils. LSEQ: an Adaptive Structure
for Sequences in Distributed Collaborative Editing. 13th ACM Symposium on Document Engineering
(DocEng), Sep 2013, Florence, Italy. pp.37–46, 10.1145/2494266.2494278. hal-00921633](https://hal.science/hal-00921633/document)

I recommend also reading:

[Marc Shapiro, Nuno Preguiça, Carlos Baquero, Marek Zawirski. A comprehensive study of Conver-
gent and Commutative Replicated Data Types. [Research Report] RR-7506, Inria – Centre Paris-
Rocquencourt; INRIA. 2011, pp.50. inria-00555588](https://inria.hal.science/inria-00555588/document)