from Michael: helper function to compute the post-swap derivative (marginal price)

Author: hoytech

src/libraries/CurveLib.sol

@@ -140,4 +140,15 @@ library CurveLib {
         }
         return xMin;
     }
+
+    /// @dev EulerSwap derivative helper function to find the price after a swap
+    /// Pre-conditions: 0 < x <= x0, 1 <= {px,py} <= 1e36, {x0,y0} <= type(uint112).max, c <= 1e18
+    function df_dx(uint256 x, uint256 px, uint256 py, uint256 x0, uint256 y0, uint256 c)
+        internal
+        pure
+        returns (int256)
+    {
+        uint256 r = Math.mulDiv(x0 * x0 / x, 1e18, x, Math.Rounding.Ceil);
+        return -int256(px * (c + (1e18 - c) * r / 1e18) / py);
+    }
 }