FeeRate calculation fixes

Weight units should always be whole units. However, virtual bytes will
need decimal places and the conversion from weight unit to vbytes should
not be rounded.

This commit has the following changes:
* vbytes should always be represented in `f32`
* conversion between vbytes and weight units should never be rounded
* simple calculations (such as those for feerates) should be explicitly
  defined (instead of using multiple small functions)

Author: evanlinjin

src/types.rs

@@ -98,13 +98,15 @@ impl FeeRate {
 
     /// Calculate fee rate from `fee` and weight units (`wu`).
     pub fn from_wu(fee: u64, wu: usize) -> FeeRate {
-        Self::from_vb(fee, wu.vbytes())
+        let vbytes = wu as f32 / 4.0;
+        let rate = fee as f32 / vbytes;
+        Self::new_checked(rate)
     }
 
     /// Calculate fee rate from `fee` and `vbytes`.
-    pub fn from_vb(fee: u64, vbytes: usize) -> FeeRate {
-        let rate = fee as f32 / vbytes as f32;
-        Self::from_sat_per_vb(rate)
+    pub fn from_vb(fee: u64, vbytes: f32) -> FeeRate {
+        let rate = fee as f32 / vbytes;
+        Self::new_checked(rate)
     }
 
     /// Return the value as satoshi/vbyte
@@ -114,12 +116,13 @@ impl FeeRate {
 
     /// Calculate absolute fee in Satoshis using size in weight units.
     pub fn fee_wu(&self, wu: usize) -> u64 {
-        self.fee_vb(wu.vbytes())
+        let vbytes = wu as f32 / 4.0;
+        (self.0 * vbytes).ceil() as u64
     }
 
     /// Calculate absolute fee in Satoshis using size in virtual bytes.
-    pub fn fee_vb(&self, vbytes: usize) -> u64 {
-        (self.as_sat_per_vb() * vbytes as f32).ceil() as u64
+    pub fn fee_vb(&self, vbytes: f32) -> u64 {
+        (self.0 * vbytes).ceil() as u64
     }
 }
 
@@ -140,13 +143,13 @@ impl Sub for FeeRate {
 /// Trait implemented by types that can be used to measure weight units.
 pub trait Vbytes {
     /// Convert weight units to virtual bytes.
-    fn vbytes(self) -> usize;
+    fn vbytes(self) -> f32;
 }
 
 impl Vbytes for usize {
-    fn vbytes(self) -> usize {
+    fn vbytes(self) -> f32 {
         // ref: https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#transaction-size-calculations
-        (self as f32 / 4.0).ceil() as usize
+        self as f32 / 4.0
     }
 }
 

src/wallet/coin_selection.rs

@@ -304,9 +304,8 @@ impl<D: Database> CoinSelectionAlgorithm<D> for OldestFirstCoinSelection {
 /// - `fee_rate`: required fee rate for the current selection
 /// - `drain_script`: script to consider change creation
 pub fn decide_change(remaining_amount: u64, fee_rate: FeeRate, drain_script: &Script) -> Excess {
-    // drain_output_len = size(len(script_pubkey)) + len(script_pubkey) + size(output_value)
-    let drain_output_len = serialize(drain_script).len() + 8usize;
-    let change_fee = fee_rate.fee_vb(drain_output_len);
+    let drain_output_len = serialize(drain_script).len() + 8_usize;
+    let change_fee = fee_rate.fee_vb(drain_output_len as f32);
     let drain_val = remaining_amount.saturating_sub(change_fee);
 
     if drain_val.is_dust(drain_script) {

src/wallet/mod.rs

@@ -2649,7 +2649,7 @@ pub(crate) mod test {
         builder
             .drain_to(addr.script_pubkey())
             .drain_wallet()
-            .fee_rate(FeeRate::from_sat_per_vb(453.0));
+            .fee_rate(FeeRate::from_sat_per_vb(455.0));
         builder.finish().unwrap();
     }