Add waste metric for coin selection

CHANGELOG.md

@@ -6,7 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- added `OldestFirstCoinSelection` impl to `CoinSelectionAlgorithm`
+- Added `OldestFirstCoinSelection` impl to `CoinSelectionAlgorithm`
 - New MSRV set to `1.56`
 - Unpinned tokio to `1`
 - Add traits to reuse `Blockchain`s across multiple wallets (`BlockchainFactory` and `StatelessBlockchain`).
@@ -19,6 +19,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - Signing Taproot PSBTs (key spend and script spend)
   - Support for `tr()` descriptors in the `descriptor!()` macro
 - Add support for Bitcoin Core 23.0 when using the `rpc` blockchain
+- Added `Waste` struct to `coinselection` module, with impl of
+  `Waste::calculate` to compute waste metric for coin selection algorithms.
+- Added new type `WeightedTxOut`. It joins a TxOut with the satisfaction weight
+  needed to spend it in the future.
+- Added new field `drain_output` to struct `CoinSelectionResult`. It'll hold
+  the created change output if needed.
+- Added `weighted_drain_output` parameter for
+  `CoinSelectionAlgorithm::coin_select` to pass the TxOut to drain the change
+  joined with the satisfaction weight to spend it in the future.
+- Changed `OutputGroup` owned `weighted_utxo` value to borrowed one.
 
 ## [v0.18.0] - [v0.17.0]
 

src/types.rs

@@ -147,6 +147,19 @@ pub struct WeightedUtxo {
     pub utxo: Utxo,
 }
 
+/// A [`TxOut`] with its `satisfaction_weight`, if it were spend by the wallet owning the produced
+/// UTXO from this TxOut.
+#[derive(Debug, Clone, PartialEq)]
+pub struct WeightedTxOut {
+    /// The weight of the witness data and `scriptSig` expressed in [weight units]. This is used to
+    /// properly compute the cost of change for waste metric.
+    ///
+    /// [weight units]: https://en.bitcoin.it/wiki/Weight_units
+    pub satisfaction_weight: usize,
+    /// The TxOut
+    pub txout: TxOut,
+}
+
 #[derive(Debug, Clone, PartialEq)]
 /// An unspent transaction output (UTXO).
 pub enum Utxo {