Change logic how network fee is calculated (#2904)

# Description
Fix rounding issues for volume based fee policies (a follow up for
#2857)

# Changes
<!-- List of detailed changes (how the change is accomplished) -->

- [ ] Reuse `sell_amount` and `buy_amount` to calculate traded amounts
for volume based policy (there is one ceil_div hidden here that wasn't
used until now).
- [ ] `saturating_sub` instead of `checked_sub` to avoid rounding issues
when total fee is smaller than protocol fee by 1 wei (this happened on
xdai for a solution where network fee is 0 because total fee is
calculated as a difference of surpluses over uniform and clearing prices
and then returned back to sell token). But, since total fee is
99999999999999 which is equal [how the old code calculated
it](https://aws-es.cow.fi/_dashboards/app/discover#/doc/5ed837d0-4a67-11ef-8b11-a98c7c46873d/cowlogs-staging-2024.08.16?id=t06PWpEBxgnhShsyGxFw)
I still think this PR is an improvement and tackles differences between
old and new code for calculating fees.

crates/autopilot/src/domain/eth/mod.rs

@@ -103,6 +103,16 @@ impl num::CheckedSub for SellTokenAmount {
     }
 }
 
+impl num::Saturating for SellTokenAmount {
+    fn saturating_add(self, v: Self) -> Self {
+        self.0.saturating_add(v.0).into()
+    }
+
+    fn saturating_sub(self, v: Self) -> Self {
+        self.0.saturating_sub(v.0).into()
+    }
+}
+
 /// Gas amount in gas units.
 ///
 /// The amount of Ether that is paid in transaction fees is proportional to this

crates/autopilot/src/domain/settlement/solution/mod.rs

@@ -15,7 +15,7 @@ mod trade;
 pub use error::Error;
 use {
     crate::{domain, domain::fee},
-    num::CheckedSub,
+    num::Saturating,
     std::collections::HashMap,
 };
 
@@ -92,8 +92,8 @@ impl Solution {
                     match (total, protocol) {
                         (Ok(total), Ok(protocol)) => {
                             let network =
-                                total.checked_sub(&protocol.iter().map(|(fee, _)| *fee).sum());
-                            network.map(|network| ExecutedFee { protocol, network })
+                                total.saturating_sub(protocol.iter().map(|(fee, _)| *fee).sum());
+                            Some(ExecutedFee { protocol, network })
                         }
                         _ => None,
                     }

crates/autopilot/src/domain/settlement/solution/trade.rs

@@ -436,23 +436,10 @@ impl Trade {
         // Finally:
         // case Sell: fee = traded_buy_amount' * factor / (1 - factor)
         // case Buy: fee = traded_sell_amount' * factor / (1 + factor)
-        let executed_in_surplus_token: eth::TokenAmount = match self.side {
-            order::Side::Sell => self
-                .executed
-                .0
-                .checked_mul(self.prices.custom.sell)
-                .ok_or(error::Math::Overflow)?
-                .checked_div(self.prices.custom.buy)
-                .ok_or(error::Math::DivisionByZero)?,
-            order::Side::Buy => self
-                .executed
-                .0
-                .checked_mul(self.prices.custom.buy)
-                .ok_or(error::Math::Overflow)?
-                .checked_div(self.prices.custom.sell)
-                .ok_or(error::Math::DivisionByZero)?,
-        }
-        .into();
+        let executed_in_surplus_token = match self.side {
+            order::Side::Buy => self.sell_amount()?,
+            order::Side::Sell => self.buy_amount()?,
+        };
         let factor = match self.side {
             order::Side::Sell => factor / (1.0 - factor),
             order::Side::Buy => factor / (1.0 + factor),

crates/driver/src/domain/competition/solution/scoring.rs

@@ -382,23 +382,10 @@ impl Trade {
         // Finally:
         // case Sell: fee = traded_buy_amount' * factor / (1 - factor)
         // case Buy: fee = traded_sell_amount' * factor / (1 + factor)
-        let executed_in_surplus_token: eth::TokenAmount = match self.side {
-            Side::Sell => self
-                .executed
-                .0
-                .checked_mul(self.custom_price.sell)
-                .ok_or(Math::Overflow)?
-                .checked_div(self.custom_price.buy)
-                .ok_or(Math::DivisionByZero)?,
-            Side::Buy => self
-                .executed
-                .0
-                .checked_mul(self.custom_price.buy)
-                .ok_or(Math::Overflow)?
-                .checked_div(self.custom_price.sell)
-                .ok_or(Math::DivisionByZero)?,
-        }
-        .into();
+        let executed_in_surplus_token = match self.side {
+            order::Side::Buy => self.sell_amount()?,
+            order::Side::Sell => self.buy_amount()?,
+        };
         let factor = match self.side {
             Side::Sell => factor / (1.0 - factor),
             Side::Buy => factor / (1.0 + factor),