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bitcoindevkit / coin-select

Tool to help you select inputs for making bitcoin transactions.
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ChangePolicy should be decided by metrics in branch and bound #20

Open LLFourn opened 7 months ago

LLFourn commented 7 months ago

Right now we pass in a change policy when creating the metric type:

#[derive(Clone, Copy)]
pub struct LowestFee {
    /// The target parameters for the resultant selection.
    pub target: Target,
    /// The estimated feerate needed to spend our change output later.
    pub long_term_feerate: FeeRate,
    /// Policy to determine the change output (if any) of a given selection.
    pub change_policy: ChangePolicy,
}

But what if we pass in a really wacky change_policy that doesn't try and lower your fees? This means a whole lot of complicated logic is needed within the bound function of the metric to try and still get the correct bound with a ChangePolicy that is working against the metric!

This would allow us to remove this entire if/else statement:

https://github.com/bitcoindevkit/coin-select/blob/0f7cc313b792c4b4a9b58ece8396f02f71191d77/src/metrics/lowest_fee.rs#L56-L108

Can anyone think of a reason you'd want a change policy that is not inline with the metric you are trying to optimize for?