Add requested changes

Author: csralvall

src/wallet/coin_selection.rs

@@ -167,7 +167,7 @@ impl CoinSelectionResult {
 /// that output in the future.
 /// > change_cost = current_fee_rate * change_output_size + long_term_feerate * change_spend_size
 ///
-/// Excess happens when there is not change, and the surplus of coins is spend as part of the fees
+/// Excess happens when there is no change, and the surplus of coins is spend as part of the fees
 /// to the miner:
 /// > excess = tx_total_value - tx_fees - target
 ///
@@ -176,12 +176,13 @@ impl CoinSelectionResult {
 /// Creation cost can be zero if there is a perfect match as result of the coin selection
 /// algorithm.
 ///
-/// So, waste can be zero or negative if the creation cost is zero and the timing
-/// cost is less than or equal to zero.
+/// So, waste can be zero if creation and timing cost are zero. Or can be negative, if timing cost
+/// is negative and the creation cost is lower enough (less than the absolute value of timing
+/// cost).
 #[derive(Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub struct Waste(pub i64);
 
-const LONG_TERM_FEE_RATE: FeeRate = FeeRate::from_sat_per_vb(10.0);
+const LONG_TERM_FEE_RATE: FeeRate = FeeRate::from_sat_per_vb(5.0);
 
 impl Waste {
     /// Calculate the amount of waste for the given coin selection
@@ -1507,16 +1508,19 @@ mod test {
             .map(|u| OutputGroup::new(u, fee_rate))
             .collect();
 
-        let size_of_change = 31;
-        let cost_of_change: u64 = (size_of_change as f32 * fee_rate.as_sat_vb()) as u64;
+        let change_out_size = 31_f32;
+        let change_in_size = 68_f32;
+        let cost_of_change: u64 = (change_out_size * fee_rate.as_sat_vb()
+            - change_in_size * LONG_TERM_FEE_RATE.as_sat_vb())
+            as u64;
 
         let utxo_fee_diff: i64 = utxos.iter().fold(0, |acc, utxo| {
-            let fee_rate: i64 = utxo.fee as i64;
+            let fee: i64 = utxo.fee as i64;
             let long_term_fee: i64 = LONG_TERM_FEE_RATE
                 .fee_wu(TXIN_BASE_WEIGHT + utxo.weighted_utxo.satisfaction_weight)
                 as i64;
 
-            acc + fee_rate - long_term_fee
+            acc + fee - long_term_fee
         });
 
         // Waste with change is the change cost and difference between fee and long term fee
@@ -1541,12 +1545,12 @@ mod test {
         let utxos_fee: u64 = utxos.clone().iter().fold(0, |acc, utxo| acc + utxo.fee);
 
         let utxo_fee_diff: i64 = utxos.iter().fold(0, |acc, utxo| {
-            let fee_rate: i64 = utxo.fee as i64;
+            let fee: i64 = utxo.fee as i64;
             let long_term_fee: i64 = LONG_TERM_FEE_RATE
                 .fee_wu(TXIN_BASE_WEIGHT + utxo.weighted_utxo.satisfaction_weight)
                 as i64;
 
-            acc + fee_rate - long_term_fee
+            acc + fee - long_term_fee
         });
 
         let excess = available_value - utxos_fee - amount_needed;
@@ -1585,12 +1589,12 @@ mod test {
         let utxos_fee: u64 = utxos.clone().iter().fold(0, |acc, utxo| acc + utxo.fee);
 
         let utxo_fee_diff: i64 = utxos.iter().fold(0, |acc, utxo| {
-            let fee_rate: i64 = utxo.fee as i64;
+            let fee: i64 = utxo.fee as i64;
             let long_term_fee: i64 = LONG_TERM_FEE_RATE
                 .fee_wu(TXIN_BASE_WEIGHT + utxo.weighted_utxo.satisfaction_weight)
                 as i64;
 
-            acc + fee_rate - long_term_fee
+            acc + fee - long_term_fee
         });
 
         let excess = available_value - utxos_fee - amount_needed;
@@ -1603,7 +1607,7 @@ mod test {
 
     #[test]
     fn test_calculate_waste_with_no_timing_cost_and_no_creation_cost() {
-        let fee_rate = FeeRate::from_sat_per_vb(10.0);
+        let fee_rate = LONG_TERM_FEE_RATE;
         let utxo_values = vec![200_000_000];
         let selected = generate_utxos_of_values(utxo_values.clone());