fixes #3 (#30)

* use fixed point math lib * add test for precision loss
Uniswap · Jul 8, 2024 · bf1f79b · bf1f79b
1 parent 4ce5b58
commit bf1f79b
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Showing 2 changed files with 21 additions and 2 deletions.
diff --git a/src/FeeOnTransferDetector.sol b/src/FeeOnTransferDetector.sol
@@ -4,6 +4,7 @@ pragma abicoder v2;
 
 import "solmate/tokens/ERC20.sol";
 import "solmate/utils/SafeTransferLib.sol";
+import "solmate/utils/FixedPointMathLib.sol";
 import "v2-core/interfaces/IUniswapV2Pair.sol";
 import "./lib/UniswapV2Library.sol";
 
@@ -17,6 +18,7 @@ struct TokenFees {
 /// @notice Detects the buy and sell fee for a fee-on-transfer token
 contract FeeOnTransferDetector {
     using SafeTransferLib for ERC20;
+    using FixedPointMathLib for uint256;
 
     error SameToken();
     error PairLookupFailed();
@@ -129,7 +131,7 @@ contract FeeOnTransferDetector {
         pure
         returns (uint256 buyFeeBps)
     {
-        buyFeeBps = (amountRequestedToBorrow - amountBorrowed) * BPS / amountRequestedToBorrow;
+        buyFeeBps = (amountRequestedToBorrow - amountBorrowed).mulDivUp(BPS, amountRequestedToBorrow);
     }
 
     /// @notice helper function to calculate the sell fee in bps
@@ -142,7 +144,7 @@ contract FeeOnTransferDetector {
         try this.callTransfer(tokenBorrowed, address(pair), amountBorrowed) {
             uint256 amountSold = tokenBorrowed.balanceOf(address(pair)) - pairBalanceBeforeSell;
             uint256 sellFee = amountBorrowed - amountSold;
-            sellFeeBps = sellFee * BPS / amountBorrowed;
+            sellFeeBps = sellFee.mulDivUp(BPS, amountBorrowed);
         } catch (bytes memory) {
             sellFeeBps = buyFeeBps;
         }

diff --git a/test/FeeOnTransferDetector.t.sol b/test/FeeOnTransferDetector.t.sol
@@ -39,6 +39,23 @@ contract FeeOnTransferDetectorTest is Test {
         assertEq(fees.externalTransferFailed, false);
     }
 
+    function testBasicFotTokenNoPrecisionLoss() public {
+        MockFotToken fotToken = new MockFotToken(200, 500);
+        MockToken otherToken = new MockToken();
+        address pair = factory.deployPair(address(fotToken), address(otherToken));
+        fotToken.setPair(pair);
+        fotToken.mint(pair, 100 ether);
+        otherToken.mint(pair, 100 ether);
+        IUniswapV2Pair(pair).sync();
+
+        // previously used to fail due to precision loss from integer division
+        TokenFees memory fees = detector.validate(address(fotToken), address(otherToken), 9999);
+        assertEq(fees.buyFeeBps, 200);
+        assertEq(fees.sellFeeBps, 500);
+        assertEq(fees.feeTakenOnTransfer, false);
+        assertEq(fees.externalTransferFailed, false);
+    }
+
     function testBasicFotTokenWithExternalFees() public {
         MockFotTokenWithExternalFees fotToken = new MockFotTokenWithExternalFees(500);
         MockToken otherToken = new MockToken();