A blockchain-agnostic coin selection library built in Rust.

Coin selection is the operation of selecting a subset of UTXOs from the wallet's UTXO set for transaction building. It is a fundamental and generic wallet management operation, and a standalone Rust library would be useful for various downstream wallet projects.

Coin selection is a variant of the Subset-Sum problem and can be solved via various algorithms. Finding an optimized solution depends on various conflicting goals such as confirmation urgency, privacy, UTXO footprint management, etc.

The following literature describes the current state of coin selection in the Bitcoin ecosystem:

• Murch's coin selection thesis: PDF
• A Survey on Coin Selection Algorithms in UTXO-based Blockchains: PDF
• Bitcoin Core's Coin selection module: GitHub
• Bcoin's Coin selector: GitHub
• A rough implementation of the Lowest Larger Algorithm: GitHub
• Waste Metric Calculation: GitHub

The library will perform coin selection via various algorithms through a well-documented API. The API will be generic in nature and will not assume any Bitcoin structure or methods. It can be used for any UTXO-based blockchain.

The following algorithms will be implemented from scratch in Rust:

• Knapsack solving
• Branch and Bound
• Lowest Larger
• First-In-First-Out
• Single-Random-Draw

The library will have individual APIs for each algorithm and provide a wrapper API coin_select() which will perform selection via each algorithm and return the result with the least waste metric.

Other characteristics of the library:

• Well-documented code, helpful in understanding coin selection theory.
• Minimal possible dependency footprint.
• Minimum possible MSRV (Minimum Supported Rust Version).

The project is under active development by a few motivated Rusty Bitcoin devs. Contributions for features, tests, docs, and other fixes/upgrades are encouraged and welcomed. The maintainers will use the PR thread to provide quick reviews and suggestions and are generally proactive at merging good contributions.

Directions for new contributors:

• The list of issues is a good place to look for contributable tasks and open problems.
• Issues marked with good first issue are good places to get started for newbie Rust/Bitcoin devs.
• The background docs are a good place to start reading up on coin selection theory.
• Reviewing open PRs is a good place to start gathering a contextual understanding of the codebase.
• Search for TODOs in the codebase to find in-line marked code todos and smaller improvements.

The dev community gathers in a small corner of Discord here (say hello if you drop there from this readme).

Dev discussions predominantly happen via FOSS (Free and Open-Source Software) best practices, and by using GitHub as the Community Forum.

The Issues, PRs, and Discussions are where all the heavy lifting happens.