Remove UNIT (#2499)

# Description
Addresses
#2448 (comment)

Apply function needs to return the same denomination AFAIU.

crates/driver/src/domain/competition/auction.rs

@@ -404,6 +404,9 @@ pub struct Token {
 pub struct Price(eth::Ether);
 
 impl Price {
+    /// The base Ether amount for pricing.
+    const BASE: u128 = 10_u128.pow(18);
+
     pub fn new(value: eth::Ether) -> Result<Self, InvalidPrice> {
         if value.0.is_zero() {
             Err(InvalidPrice)
@@ -413,8 +416,22 @@ impl Price {
     }
 
     /// Apply this price to some token amount, converting that token into ETH.
-    pub fn apply(self, amount: eth::TokenAmount) -> eth::Ether {
-        (amount.0 * self.0 .0).into()
+    ///
+    /// # Examples
+    ///
+    /// Converting 1 ETH expressed in `eth::TokenAmount` into `eth::Ether`
+    ///
+    /// ```
+    /// use driver::domain::{competition::auction::Price, eth};
+    ///
+    /// let amount = eth::TokenAmount::from(eth::U256::exp10(18));
+    /// let price = Price::new(eth::Ether::from(eth::U256::exp10(18))).unwrap();
+    ///
+    /// let eth = price.in_eth(amount);
+    /// assert_eq!(eth, eth::Ether::from(eth::U256::exp10(18)));
+    /// ```
+    pub fn in_eth(self, amount: eth::TokenAmount) -> eth::Ether {
+        (amount.0 * self.0 .0 / Self::BASE).into()
     }
 }
 

crates/driver/src/domain/competition/order/mod.rs

@@ -208,8 +208,8 @@ impl Order {
             tokens.get(self.sell.token).price,
         ) {
             (Some(buy_price), Some(sell_price)) => {
-                let buy = buy_price.apply(self.buy.amount);
-                let sell = sell_price.apply(self.sell.amount);
+                let buy = buy_price.in_eth(self.buy.amount);
+                let sell = sell_price.in_eth(self.sell.amount);
                 sell.0
                     .to_big_rational()
                     .checked_div(&buy.0.to_big_rational())

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

@@ -129,13 +129,12 @@ impl Trade {
     ///
     /// Denominated in NATIVE token
     fn native_surplus(&self, prices: &auction::Prices) -> Result<eth::Ether, Error> {
-        let surplus = self.surplus_token_price(prices)?.apply(
+        let surplus = self.surplus_token_price(prices)?.in_eth(
             self.surplus()
                 .ok_or(Error::Surplus(self.sell, self.buy))?
                 .amount,
         );
-        // normalize
-        Ok((surplus.0 / *UNIT).into())
+        Ok(surplus)
     }
 
     /// Protocol fee is defined by fee policies attached to the order.
@@ -215,9 +214,8 @@ impl Trade {
     fn native_protocol_fee(&self, prices: &auction::Prices) -> Result<eth::Ether, Error> {
         let protocol_fee = self
             .surplus_token_price(prices)?
-            .apply(self.protocol_fee()?.amount);
-        // normalize
-        Ok((protocol_fee.0 / *UNIT).into())
+            .in_eth(self.protocol_fee()?.amount);
+        Ok(protocol_fee)
     }
 
     fn surplus_token(&self) -> eth::TokenAddress {
@@ -263,7 +261,3 @@ pub enum Error {
     #[error(transparent)]
     Math(#[from] Math),
 }
-
-lazy_static::lazy_static! {
-    static ref UNIT: eth::U256 = eth::U256::from(1_000_000_000_000_000_000_u128);
-}