Merge pull request #11 from euler-xyz/fix/_computeQuote

Fix _computeQuote()

Author: hoytech

TODO

@@ -3,8 +3,6 @@
 ! Better revert messages when a swap cannot be satisifed due to debt-limit/utilisation/etc
   * currently it's an arithmetic underflow
 ! Don't make quotes that would cause a swap to fail due to supply or borrow caps
-! In _computeQuote() use vault.cash() method instead of token.balanceOf() otherwise donations could cause quoted swaps to fail
-! In _computeQuote(), exactOut swaps should verify the *amount* (not quote) is withdrawable
 * ConstantSum: incorporate price multipliers in quote methods
 * natspec
 * permit2 instead of regular approval: measure gas savings

src/MaglevBase.sol

@@ -219,13 +219,17 @@ abstract contract MaglevBase is IMaglevBase, EVCUtil {
 
         uint256 quote = computeQuote(amount, exactIn, asset0IsInput);
 
-        require(quote <= (asset0IsInput ? reserve1 : reserve0), InsufficientReserves());
-        require(
-            quote <= IERC20(asset0IsInput ? asset1 : asset0).balanceOf(asset0IsInput ? vault1 : vault0),
-            InsufficientCash()
-        );
+        if (exactIn) {
+            // if `exactIn`, `quote` is the amount of assets to buy from the AMM
+            require(quote <= (asset0IsInput ? reserve1 : reserve0), InsufficientReserves());
+            require(quote <= IEVault(asset0IsInput ? vault1 : vault0).cash(), InsufficientCash());
+        } else {
+            // if `!exactIn`, `amount` is the amount of assets to buy from the AMM
+            require(amount <= (asset0IsInput ? reserve1 : reserve0), InsufficientReserves());
+            require(amount <= IEVault(asset0IsInput ? vault1 : vault0).cash(), InsufficientCash());
+        }
 
-        // exactOut: increase required amountIn, rounding up
+        // exactOut: increase required quote(amountIn), rounding up
         if (!exactIn) quote = (quote * 1e18 + (feeMultiplier - 1)) / feeMultiplier;
 
         return quote;