BDK Coin Selection

bdk_coin_select is a zero-dependency tool to help you select inputs for making Bitcoin (ticker: BTC) transactions.

⚠ This work is only ready to use by those who expect (potentially catastrophic) bugs and will have the time to investigate them and contribute back to this crate.

Synopis

use std::str::FromStr;
use bdk_coin_select::{ CoinSelector, Candidate, TR_KEYSPEND_TXIN_WEIGHT, Drain, FeeRate, Target, ChangePolicy, TargetOutputs, TargetFee, DrainWeights};
use bitcoin::{ Address, Network, Transaction, TxIn, TxOut };

let recipient_addr = 
    Address::from_str("tb1pvjf9t34fznr53u5tqhejz4nr69luzkhlvsdsdfq9pglutrpve2xq7hps46").unwrap();

let outputs = vec![TxOut {
    value: 3_500_000,
    script_pubkey: recipient_addr.payload.script_pubkey(),
}];

let target = Target {
    outputs: TargetOutputs::fund_outputs(outputs.iter().map(|output| (output.weight() as u32, output.value))),
    fee: TargetFee::from_feerate(FeeRate::from_sat_per_vb(42.0))
};

let candidates = vec![
    Candidate {
        // How many inputs does this candidate represents. Needed so we can 
        // figure out the weight of the varint that encodes the number of inputs
        input_count: 1,
        // the value of the input
        value: 1_000_000,
        // the total weight of the input(s) including their witness/scriptSig
        // you may need to use miniscript to figure out the correct value here.
        weight: TR_KEYSPEND_TXIN_WEIGHT,
        // wether it's a segwit input. Needed so we know whether to include the
        // segwit header in total weight calculations.
        is_segwit: true
    },
    Candidate {
        // A candidate can represent multiple inputs in the case where you 
        // always want some inputs to be spent together.
        input_count: 2,
        weight: 2*TR_KEYSPEND_TXIN_WEIGHT,
        value: 3_000_000,
        is_segwit: true
    }
];

// You can now select coins!
let mut coin_selector = CoinSelector::new(&candidates);
coin_selector.select(0);

assert!(!coin_selector.is_target_met(target), "we didn't select enough");
println!("we didn't select enough yet we're missing: {}", coin_selector.missing(target));
coin_selector.select(1);
assert!(coin_selector.is_target_met(target), "we should have enough now");

// Now we need to know if we need a change output to drain the excess if we overshot too much
//
// We don't need to know exactly which change output we're going to use yet but we assume it's a taproot output
// that we'll use a keyspend to spend from.
let drain_weights = DrainWeights::TR_KEYSPEND; 
// Our policy is to only add a change output if the value is over 1_000 sats
let change_policy = ChangePolicy::min_value(drain_weights, 1_000);
let change = coin_selector.drain(target, change_policy);
if change.is_some() {
    println!("We need to add our change output to the transaction with {} value", change.value);
} else {
    println!("Yay we don't need to add a change output");
}

Automatic selection with Branch and Bound

You can use methods such as CoinSelector::select to manually select coins, or methods such as CoinSelector::select_until_target_met for a rudimentary automatic selection. Probably you want to use CoinSelector::run_bnb to do this in a smart way.

Built-in metrics are provided in the metrics submodule. Currently, only the LowestFee metric is considered stable. Note you can try and write your own metric by implementing the BnbMetric yourself but we don't recommend this.

use std::str::FromStr;
use bdk_coin_select::{ Candidate, CoinSelector, FeeRate, Target, TargetFee, TargetOutputs, ChangePolicy, TR_KEYSPEND_TXIN_WEIGHT, TR_DUST_RELAY_MIN_VALUE};
use bdk_coin_select::metrics::LowestFee;
use bitcoin::{ Address, Network, Transaction, TxIn, TxOut };

let recipient_addr =
    Address::from_str("tb1pvjf9t34fznr53u5tqhejz4nr69luzkhlvsdsdfq9pglutrpve2xq7hps46").unwrap();

let outputs = vec![TxOut {
    value: 210_000,
    script_pubkey: recipient_addr.payload.script_pubkey(),
}];

let candidates = [
    Candidate {
        input_count: 1,
        value: 400_000,
        weight: TR_KEYSPEND_TXIN_WEIGHT,
        is_segwit: true
    },
    Candidate {
        input_count: 1,
        value: 200_000,
        weight: TR_KEYSPEND_TXIN_WEIGHT,
        is_segwit: true
    },
    Candidate {
        input_count: 1,
        value: 11_000,
        weight: TR_KEYSPEND_TXIN_WEIGHT,
        is_segwit: true
    }
];
let drain_weights = bdk_coin_select::DrainWeights::default();
// You could determine this by looking at the user's transaction history and taking an average of the feerate.
let long_term_feerate = FeeRate::from_sat_per_vb(10.0);

let mut coin_selector = CoinSelector::new(&candidates);

let target = Target {
    fee: TargetFee::from_feerate(FeeRate::from_sat_per_vb(15.0)),
    outputs: TargetOutputs::fund_outputs(outputs.iter().map(|output| (output.weight() as u32, output.value))),
};

// The change output must be at least this size to be relayed.
// To choose it you need to know the kind of script pubkey on your change txout.
// Here we assume it's a taproot output
let dust_limit = TR_DUST_RELAY_MIN_VALUE;

// We use a change policy that introduces a change output if doing so reduces
// the "waste" (i.e. adding change doesn't increase the fees we'd pay if we factor in the cost to spend the output later on).
let change_policy = ChangePolicy::min_value_and_waste(
    drain_weights,
    dust_limit,
    target.fee.rate,
    long_term_feerate,
);

// The LowestFee metric tries make selections that minimize your total fees paid over time.
let metric = LowestFee {
    target,
    long_term_feerate, // used to calculate the cost of spending th change output if the future
    change_policy
};

// We run the branch and bound algorithm with a max round limit of 100,000.
match coin_selector.run_bnb(metric, 100_000) {
    Err(err) => {
        println!("failed to find a solution: {}", err);
        // fall back to naive selection
        coin_selector.select_until_target_met(target).expect("a selection was impossible!");
    }
    Ok(score) => {
        println!("we found a solution with score {}", score);

        let selection = coin_selector
            .apply_selection(&candidates)
            .collect::<Vec<_>>();
        let change = coin_selector.drain(target, change_policy);

        println!("we selected {} inputs", selection.len());
        println!("We are including a change output of {} value (0 means not change)", change.value);
    }
};

Minimum Supported Rust Version (MSRV)

This library is compiles on rust v1.54 and above