Module bdk::wallet::coin_selection

Expand description

Coin selection

This module provides the trait CoinSelectionAlgorithm that can be implemented to define custom coin selection algorithms.

You can specify a custom coin selection algorithm through the coin_selection method on TxBuilder. DefaultCoinSelectionAlgorithm aliases the coin selection algorithm that will be used if it is not explicitly set.

Example

#[derive(Debug)]
struct AlwaysSpendEverything;

impl CoinSelectionAlgorithm for AlwaysSpendEverything {
    fn coin_select(
        &self,
        required_utxos: Vec<WeightedUtxo>,
        optional_utxos: Vec<WeightedUtxo>,
        fee_rate: FeeRate,
        target_amount: u64,
        drain_script: &Script,
    ) -> Result<CoinSelectionResult, coin_selection::Error> {
        let mut selected_amount = 0;
        let mut additional_weight = Weight::ZERO;
        let all_utxos_selected = required_utxos
            .into_iter()
            .chain(optional_utxos)
            .scan(
                (&mut selected_amount, &mut additional_weight),
                |(selected_amount, additional_weight), weighted_utxo| {
                    **selected_amount += weighted_utxo.utxo.txout().value.to_sat();
                    **additional_weight += Weight::from_wu(
                        (TxIn::default().segwit_weight().to_wu()
                            + weighted_utxo.satisfaction_weight as u64)
                            as u64,
                    );
                    Some(weighted_utxo.utxo)
                },
            )
            .collect::<Vec<_>>();
        let additional_fees = (fee_rate * additional_weight).to_sat();
        let amount_needed_with_fees = additional_fees + target_amount;
        if selected_amount < amount_needed_with_fees {
            return Err(coin_selection::Error::InsufficientFunds {
                needed: amount_needed_with_fees,
                available: selected_amount,
            });
        }

        let remaining_amount = selected_amount - amount_needed_with_fees;

        let excess = decide_change(remaining_amount, fee_rate, drain_script);

        Ok(CoinSelectionResult {
            selected: all_utxos_selected,
            fee_amount: additional_fees,
            excess,
        })
    }
}

// create wallet, sync, ...

let to_address = Address::from_str("2N4eQYCbKUHCCTUjBJeHcJp9ok6J2GZsTDt")
    .unwrap()
    .require_network(Network::Testnet)
    .unwrap();
let psbt = {
    let mut builder = wallet.build_tx().coin_selection(AlwaysSpendEverything);
    builder.add_recipient(to_address.script_pubkey(), 50_000);
    builder.finish()?
};

// inspect, sign, broadcast, ...