# Bitcoin Coin-Selection
This library provides efficient algorithms to compose a set of unspent transaction outputs (UTXOs). When a Bitcoin wallet creates a transaction, there is a diverse set of trade-offs to decide which UTXOs to choose. The trade-offs for deciding which set of UTXOs to use are described in depth here: [An Evaluation of Coin Selection Stratagies](https://murch.one/wp-content/uploads/2016/11/erhardt2016coinselection.pdf) as well as here: [What is the Waste Metric?](https://murch.one/posts/waste-metric/).
## Usage
The current interface is provided via `select_coins()` function. The required parameters are:
`target` - *The desired transaction amount.*
`cost_of_change` - *How expensive it is to create a new output (UTXO).*
`max_weight` - *The maximum allowed UTXO selection weight.*
`weighted_utxos` - *The set of possible weighted UTXOs to choose from.*
As discussed in the literature above, we want to find a "changeless" solution. A changeless solution is one that exceeds the `target` however is less than `target` + `cost_of_change`. If no changeless solution can be found, then creating a change output by splitting a UTXO is the next best outcome. To that end, `select_coins()` initially attempts a Branch and Bound selection algorithm to find a changeless solution. If no changeless solution is found, then `select_coins()` falls back to a Single Random Draw selection strategy.
## Fuzz tests
To run fuzz tests, install [cargo fuzz](https://crates.io/crates/cargo-fuzz).
The following fuzz tests can then be run:
```
> cargo fuzz run single_random_draw
> cargo fuzz run branch_and_bound
> cargo fuzz run select_coins
```
## Property tests
This project has a number of property tests created using [arbtest](https://github.com/matklad/arbtest). The property tests build a random pool of UTXOs and random selection parameters and then assert the results are correct. To continuously run only the property tests, a simple shell script runs them in a loop.
To continuously run the property tests:
```
> run_proptests.sh
```
## Benchmarks
Benchmarks for the `Branch and Bound Selection` algorithm are written using [Criterion]( https://github.com/bheisler/criterion.rs).
To run the benchmarks use:
```
> cargo bench
```
Note: criterion requires rustc version 1.65 to run the benchmarks.
### performance comparison
A basic performance comparison between implementations using commodity hardware (My rather old laptop).
|implementation|pool size|ns/iter|
|-------------:|---------|-------|
| Rust SRD| 1,000| 22,446|
| Rust BnB| 1,000|393.410|
| C++ Core BnB| 1,000|816,374|
Note: The measurements where recorded using rustc 1.90 stable. Expect worse performance with MSRV.
## Minimum Supported Rust Version (MSRV)
This library should always compile with any combination of features on **Rust 1.63.0**.
## Release Notes
- [CHANGELOG](CHANGELOG.md)