A0-1140: Leave dust in the dex

contracts/simple_dex/Cargo.toml

@@ -21,6 +21,9 @@ openbrush = { git = "https://github.com/Supercolony-net/openbrush-contracts.git"
 access_control = { path = "../access_control", default-features = false, features = ["ink-as-dependency"] }
 game_token = { path = "../game_token", default-features = false, features = ["ink-as-dependency"] }
 
+[dev-dependencies]
+proptest = "1.0"
+
 [lib]
 name = "simple_dex"
 path = "lib.rs"
@@ -47,4 +50,4 @@ std = [
 ink-as-dependency = []
 
 [profile.dev]
-codegen-units = 16
+codegen-units = 16

contracts/simple_dex/lib.rs

@@ -375,8 +375,22 @@ mod simple_dex {
             let balance_token_in = self.balance_of(token_in, this)?;
             let balance_token_out = self.balance_of(token_out, this)?;
 
+            Self::_out_given_in(
+                amount_token_in,
+                balance_token_in,
+                balance_token_out,
+                self.swap_fee_percentage,
+            )
+        }
+
+        fn _out_given_in(
+            amount_token_in: Balance,
+            balance_token_in: Balance,
+            balance_token_out: Balance,
+            swap_fee_percentage: Balance,
+        ) -> Result<Balance, DexError> {
             let op0 = amount_token_in
-                .checked_mul(self.swap_fee_percentage)
+                .checked_mul(swap_fee_percentage)
                 .ok_or(DexError::Arithmethic)?;
 
             let op1 = balance_token_in
@@ -395,6 +409,8 @@ mod simple_dex {
 
             balance_token_out
                 .checked_sub(op4)
+                // If the division is not even, leave the 1 unit of dust in the exchange instead of paying it out.
+                .and_then(|result| result.checked_sub(if op3 % op2 > 0 { 1 } else { 0 }))
                 .ok_or(DexError::Arithmethic)
         }
 
@@ -509,4 +525,30 @@ mod simple_dex {
             SimpleDex::new()
         }
     }
+
+    #[cfg(test)]
+    mod test {
+        use proptest::prelude::*;
+
+        use super::*;
+
+        proptest! {
+            #[test]
+            fn rounding_benefits_dex(
+                balance_token_a in 1..1000u128,
+                balance_token_b in 1..1000u128,
+                pay_token_a in 1..1000u128,
+                fee_percentage in 0..10u128
+            ) {
+                let get_token_b =
+                    SimpleDex::_out_given_in(pay_token_a, balance_token_a, balance_token_b, fee_percentage).unwrap();
+                let balance_token_a = balance_token_a + pay_token_a;
+                let balance_token_b = balance_token_b - get_token_b;
+                let get_token_a =
+                    SimpleDex::_out_given_in(get_token_b, balance_token_b, balance_token_a, fee_percentage).unwrap();
+
+                assert!(get_token_a <= pay_token_a);
+            }
+        }
+    }
 }