Remove base_weight, put weight in Target

- CoinSelector no longer tracks anything but input weight
- Previously the value of the target outputs was in `Target` but the
  weights were accounted for in CoinSelector. Now they're in all in
  target.
- This allows us to actually figure out how many outputs there are and
  therefore the actual weight of the transaction accounting for varints.

Author: LLFourn

CHANGELOG.md

@@ -4,4 +4,7 @@
 - Remove `min_fee` in favour of `replace` which allows you to replace a transaction
 - Remove `Drain` argument from `CoinSelector::select_until_target_met` because adding a drain won't
   change when the target is met.
+- No more `base_weight` in `CoinSelector`. Weight of the outputs is tracked in `target`.
+- You now account for the number of outputs in both drain and target and their weight.
+- Removed waste metric because it was pretty broken and took a lot to maintain
 

README.md

@@ -1,36 +1,40 @@
 # BDK Coin Selection
 
-`bdk_coin_select` is a tool to help you select inputs for making Bitcoin (ticker: BTC) transactions.
-It's got zero dependencies so you can paste it into your project without concern.
+`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.
 
-## Constructing the `CoinSelector`
-
-The main structure is [`CoinSelector`](crate::CoinSelector). To construct it, we specify a list of
-candidate UTXOs and a transaction `base_weight`. The `base_weight` includes the recipient outputs
-and mandatory inputs (if any).
+## Synopis
 
 ```rust
 use std::str::FromStr;
-use bdk_coin_select::{ CoinSelector, Candidate, TR_KEYSPEND_TXIN_WEIGHT};
+use bdk_coin_select::{ CoinSelector, Candidate, TR_KEYSPEND_TXIN_WEIGHT, Drain, FeeRate, Target, ChangePolicy, TargetOutputs, TargetFee, DrainWeights};
 use bitcoin::{ Address, Network, Transaction, TxIn, TxOut };
 
-// The address where we want to send our coins.
 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).
+        // 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, 
+        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
@@ -45,91 +49,54 @@ let candidates = vec![
     }
 ];
 
-let base_tx = Transaction {
-    input: vec![],
-    // include your recipient outputs here
-    output: vec![TxOut {
-        value: 900_000,
-        script_pubkey: recipient_addr.payload.script_pubkey(),
-    }],
-    lock_time: bitcoin::absolute::LockTime::from_height(0).unwrap(),
-    version: 0x02,
-};
-let base_weight = base_tx.weight().to_wu() as u32;
-println!("base weight: {}", base_weight);
-
 // You can now select coins!
-let mut coin_selector = CoinSelector::new(&candidates, base_weight);
+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");
+}
 ```
 
-## Change Policy
+## Automatic selection with Branch and Bound
 
-A change policy determines whether the drain output(s) should be in the final solution. The
-determination is simple: if the excess value is above a threshold then the drain should be added. To
-construct a change policy you always provide `DrainWeights` which tell the coin selector the weight
-cost of adding the drain. `DrainWeights` includes two weights. One is the weight of the drain
-output(s). The other is the weight of spending the drain output later on (the input weight).
+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`](metrics::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.
 
 ```rust
 use std::str::FromStr;
-use bdk_coin_select::{CoinSelector, Candidate, DrainWeights, TXIN_BASE_WEIGHT, ChangePolicy, TR_KEYSPEND_TXIN_WEIGHT};
-use bitcoin::{Address, Network, Transaction, TxIn, TxOut};
-let base_tx = Transaction {
-    input: vec![],
-    output: vec![/* include your recipient outputs here */],
-    lock_time: bitcoin::absolute::LockTime::from_height(0).unwrap(),
-    version: 0x02,
-};
-let base_weight = base_tx.weight().to_wu() as u32;
-
-// The change output that may or may not be included in the final transaction.
-let drain_addr =
-    Address::from_str("tb1pvjf9t34fznr53u5tqhejz4nr69luzkhlvsdsdfq9pglutrpve2xq7hps46")
-    .expect("address must be valid")
-    .require_network(Network::Testnet)
-    .expect("network must match");
-
-// The drain output(s) may or may not be included in the final tx. We calculate
-// the drain weight to include the output length varint weight changes from
-// including the drain output(s).
-let drain_output_weight = {
-    let mut tx_with_drain = base_tx.clone();
-    tx_with_drain.output.push(TxOut {
-        script_pubkey: drain_addr.script_pubkey(),
-        ..Default::default()
-    });
-    tx_with_drain.weight().to_wu() as u32 - base_weight
-};
-println!("drain output weight: {}", drain_output_weight);
-
-let drain_weights = DrainWeights {
-    output_weight: drain_output_weight,
-    spend_weight: TR_KEYSPEND_TXIN_WEIGHT,
-};
-
-// This constructs a change policy that creates change when the change value is
-// greater than or equal to the dust limit.
-let change_policy = ChangePolicy::min_value(
-    drain_weights,
-    drain_addr.script_pubkey().dust_value().to_sat(),
-);
-```
-
-## Branch and Bound
+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 };
 
-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. However, if you 
-wish to automatically select coins to optimize for a given metric, [`CoinSelector::run_bnb`] can be
-used.
+let recipient_addr =
+    Address::from_str("tb1pvjf9t34fznr53u5tqhejz4nr69luzkhlvsdsdfq9pglutrpve2xq7hps46").unwrap();
 
-Built-in metrics are provided in the [`metrics`] submodule. Currently, only the 
-[`LowestFee`](metrics::LowestFee) metric is considered stable.
+let outputs = vec![TxOut {
+    value: 210_000,
+    script_pubkey: recipient_addr.payload.script_pubkey(),
+}];
 
-```rust
-use bdk_coin_select::{ Candidate, CoinSelector, FeeRate, Target, TargetFee, ChangePolicy, TR_KEYSPEND_TXIN_WEIGHT };
-use bdk_coin_select::metrics::LowestFee;
 let candidates = [
     Candidate {
         input_count: 1,
@@ -150,34 +117,35 @@ let candidates = [
         is_segwit: true
     }
 ];
-let base_weight = 0;
 let drain_weights = bdk_coin_select::DrainWeights::default();
-let dust_limit = 0;
+// 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, base_weight);
+let mut coin_selector = CoinSelector::new(&candidates);
 
 let target = Target {
     fee: TargetFee::from_feerate(FeeRate::from_sat_per_vb(15.0)),
-    value: 210_000,
+    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" and that the change output's value is at least that of the 
-// `dust_limit`.
+// 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,
 );
 
-// This metric minimizes transaction fees paid over time. The 
-// `long_term_feerate` is used to calculate the additional fee from spending 
-// the change output in the future.
+// The LowestFee metric tries make selections that minimize your total fees paid over time.
 let metric = LowestFee {
     target,
-    long_term_feerate,
+    long_term_feerate, // used to calculate the cost of spending th change output if the future
     change_policy
 };
 
@@ -203,79 +171,6 @@ match coin_selector.run_bnb(metric, 100_000) {
 
 ```
 
-## Finalizing a Selection
-
-- [`is_target_met`] checks whether the current state of [`CoinSelector`] meets the [`Target`].
-- [`apply_selection`] applies the selection to the original list of candidate `TxOut`s.
-
-[`is_target_met`]: crate::CoinSelector::is_target_met
-[`apply_selection`]: crate::CoinSelector::apply_selection
-[`CoinSelector`]: crate::CoinSelector
-[`Target`]: crate::Target
-
-```rust
-use bdk_coin_select::{CoinSelector, Candidate, DrainWeights, Target, ChangePolicy, TR_KEYSPEND_TXIN_WEIGHT, Drain};
-use bitcoin::{Amount, TxOut, Address};
-let base_weight = 0_u32;
-let drain_weights = DrainWeights::TR_KEYSPEND;
-use core::str::FromStr;
-
-// A random target, as an example.
-let target = Target {
-    value: 21_000,
-    ..Default::default()
-};
-// Am arbitary drain policy, for the example.
-let change_policy = ChangePolicy::min_value(drain_weights, 1337);
-
-// This is a list of candidate txouts for coin selection. If a txout is picked,
-// our transaction's input will spend it.
-let candidate_txouts = vec![
-    TxOut {
-        value: 100_000,
-        script_pubkey: Address::from_str("bc1p5cyxnuxmeuwuvkwfem96lqzszd02n6xdcjrs20cac6yqjjwudpxqkedrcr").unwrap().payload.script_pubkey(),
-    },
-    TxOut {
-        value: 150_000,
-        script_pubkey: Address::from_str("bc1p4qhjn9zdvkux4e44uhx8tc55attvtyu358kutcqkudyccelu0was9fqzwh").unwrap().payload.script_pubkey(),
-    },
-    TxOut {
-        value: 200_000,
-        script_pubkey: Address::from_str("bc1p0d0rhyynq0awa9m8cqrcr8f5nxqx3aw29w4ru5u9my3h0sfygnzs9khxz8").unwrap().payload.script_pubkey()
-    }
-];
-// We transform the candidate txouts into something `CoinSelector` can
-// understand.
-let candidates = candidate_txouts
-    .iter()
-    .map(|txout| Candidate {
-        input_count: 1,
-        value: txout.value,
-        weight: TR_KEYSPEND_TXIN_WEIGHT, // you need to figure out the weight of the txin somehow
-        is_segwit: txout.script_pubkey.is_witness_program(),
-    })
-    .collect::<Vec<_>>();
-
-let mut selector = CoinSelector::new(&candidates, base_weight);
-selector
-    .select_until_target_met(target)
-    .expect("we've got enough coins");
-
-// Get a list of coins that are selected.
-let selected_coins = selector
-    .apply_selection(&candidate_txouts)
-    .collect::<Vec<_>>();
-assert_eq!(selected_coins.len(), 1);
-
-// Determine whether we should add a change output.
-let drain = selector.drain(target, change_policy);
-
-if drain.is_some() {
-    // add our change output to the transaction
-    let change_value = drain.value;
-}
-```
-
 # Minimum Supported Rust Version (MSRV)
 
 This library is compiles on rust v1.54 and above

src/change_policy.rs

@@ -39,7 +39,11 @@ impl ChangePolicy {
     ) -> Self {
         // The output waste of a changeless solution is the excess.
         let waste_with_change = drain_weights
-            .waste(target_feerate, long_term_feerate)
+            .waste(
+                target_feerate,
+                long_term_feerate,
+                0, /* ignore varint cost for now */
+            )
             .ceil() as u64;
 
         Self {