audit(12): Long-term orders may be filled with an incorrect amount

src/lib/NonlinearDutchDecayLib.sol

@@ -5,6 +5,7 @@ import {OutputToken, InputToken} from "../base/ReactorStructs.sol";
 import {V3DutchOutput, V3DutchInput, NonlinearDutchDecay} from "../lib/V3DutchOrderLib.sol";
 import {FixedPointMathLib} from "solmate/src/utils/FixedPointMathLib.sol";
 import {MathExt} from "./MathExt.sol";
+import {Math} from "openzeppelin-contracts/utils/math/Math.sol";
 import {Uint16ArrayLibrary, Uint16Array, fromUnderlying} from "../types/Uint16Array.sol";
 import {DutchDecayLib} from "./DutchDecayLib.sol";
 
@@ -39,7 +40,7 @@ library NonlinearDutchDecayLib {
             return startAmount.bound(minAmount, maxAmount);
         }
 
-        uint16 blockDelta = uint16(block.number - decayStartBlock);
+        uint16 blockDelta = uint16(Math.min(block.number - decayStartBlock, type(uint16).max));
         (uint16 startPoint, uint16 endPoint, int256 relStartAmount, int256 relEndAmount) =
             locateCurvePosition(curve, blockDelta);
         // get decay of only the relative amounts

test/lib/NonLinearDutchDecayLib.t.sol

@@ -474,4 +474,36 @@ contract NonlinearDutchDecayLibTest is Test, GasSnapshot {
         vm.expectRevert(NonlinearDutchDecayLib.InvalidDecayCurve.selector);
         mockNonlinearDutchDecayLibContract.decay(curve, startAmount, decayStartBlock, 1 ether, 1 ether);
     }
+
+    function testFuzzDutchDecayBeyondUint16Max(
+        uint256 startAmount,
+        uint256 blockDelta,
+        uint16 lastValidBlock,
+        uint256 decayAmountFuzz
+    ) public {
+        uint256 decayStartBlock = 100;
+        vm.assume(lastValidBlock > 0);
+        vm.assume(startAmount > 0);
+
+        // Testing that we get a fully decayed curve instead of overflowed mistake
+        vm.assume(blockDelta > uint256(type(uint16).max));
+        vm.assume(blockDelta < type(uint256).max - decayStartBlock);
+
+        // Bound decayAmountFuzz between 0 and startAmount
+        decayAmountFuzz = bound(decayAmountFuzz, 0, startAmount);
+
+        uint16[] memory blocks = new uint16[](1);
+        blocks[0] = lastValidBlock;
+
+        int256[] memory decayAmounts = new int256[](1);
+        decayAmounts[0] = int256(decayAmountFuzz);
+
+        NonlinearDutchDecay memory curve = CurveBuilder.multiPointCurve(blocks, decayAmounts);
+
+        uint256 currentBlock = decayStartBlock + blockDelta;
+        vm.roll(currentBlock);
+        uint256 decayed = NonlinearDutchDecayLib.decay(curve, startAmount, decayStartBlock, 0, startAmount);
+
+        assertEq(decayed, startAmount - decayAmountFuzz, "Should be fully decayed for block delta beyond uint16.max");
+    }
 }