fix(trading-function): reverts if reserve bounds are reached but not …

…opposite

contracts/strategies/NormalStrategyLib.sol

@@ -244,24 +244,32 @@ function tradingFunction(NormalCurve memory self)
 
     // Overwrite the reserves to be as close to its bounds as possible, to avoid reverting.
     uint256 invariantTermXInput; // x - 1
-    if (self.reserveXPerWad >= upperBoundX) {
-        // As x -> 1E18, 1E18 - x -> 0, Φ⁻¹(0) = -∞, therefore bound invariantTermXInput to 1E18 - 1E18 + 1.
-        invariantTermXInput = 1;
-    } else if (self.reserveXPerWad <= lowerBoundX) {
-        // As x -> 0, 1E18 - 0 -> 1E18, Φ⁻¹(1E18) = +∞, therefore bound invariantTermXInput to 1E18 - 0 - 1.
-        invariantTermXInput = WAD - 1;
-    } else {
-        invariantTermXInput = WAD - self.reserveXPerWad;
-    }
-
     uint256 invariantTermYInput =
         self.reserveYPerWad.divWadDown(self.strikePriceWad); // y/K -> [0,1]
+
     if (invariantTermYInput >= WAD) {
+        // If the Y term is gte the upper bound, the X term must be lte the lower bound.
+        require(
+            self.reserveXPerWad <= lowerBoundX,
+            "NormalStrategyLib: X reserves must be at lower bound to fill Y reserves."
+        );
         // As y -> K, y/K -> 1E18, Φ⁻¹(1E18) = +∞, therefore bound invariantTermYInput to 1E18 - 1.
         invariantTermYInput = WAD - 1;
+        // As x -> 0, 1E18 - 0 -> 1E18, Φ⁻¹(1E18) = +∞, therefore bound invariantTermXInput to 1E18 - 0 - 1.
+        invariantTermXInput = WAD - 1;
     } else if (invariantTermYInput <= 0) {
+        // Else if Y term is lte the lower bound, the X term must be gte the upper bound.
+        require(
+            self.reserveXPerWad >= upperBoundX,
+            "NormalStrategyLib: X reserves must be at upper bound to empty Y reserves."
+        );
         // As y -> 0, y/K -> 0, Φ⁻¹(0) = -∞, therefore bound invariantTermYInput to 0 + 1.
         invariantTermYInput = 1;
+        // As x -> 1E18, 1E18 - x -> 0, Φ⁻¹(0) = -∞, therefore bound invariantTermXInput to 1E18 - 1E18 + 1.
+        invariantTermXInput = 1;
+    } else {
+        // If neither bound is reached, set the invariant terms to their actual values.
+        invariantTermXInput = WAD - self.reserveXPerWad;
     }
 
     // Φ⁻¹(1-x)