Brian/full scenario analysis

contracts/core/extensions/CoreIssuance.sol

@@ -266,7 +266,7 @@ contract CoreIssuance is
                     state.vaultAddress
                 );
 
-                // Log transfer of component from issuer waller
+                // Log transfer of component from issuer wallet
                 emit IssuanceComponentDeposited(
                     _setAddress,
                     component,

contracts/core/extensions/CoreIssuanceOrder.sol

@@ -152,7 +152,7 @@ contract CoreIssuanceOrder is
         // Settle Order
         settleOrder(order, _fillQuantity, _orderData);
 
-        //Issue Set
+        // Issue Set
         issueInternal(
             order.makerAddress,
             order.setAddress,
@@ -364,7 +364,7 @@ contract CoreIssuanceOrder is
         );
 
         // Calculate fees required
-        uint requiredFees = _order.relayerTokenAmount.mul(_fillQuantity).div(_order.quantity);
+        uint requiredFees = OrderLibrary.getPartialAmount(_order.relayerTokenAmount, _fillQuantity, _order.quantity);
 
         //Send fees to relayer
         ITransferProxy(state.transferProxyAddress).transfer(
@@ -412,8 +412,7 @@ contract CoreIssuanceOrder is
         uint[] memory requiredBalances = new uint[](_order.requiredComponents.length);
 
         // Calculate amount of maker token required
-        // Look into rounding errors
-        uint requiredMakerTokenAmount = _order.makerTokenAmount.mul(_fillQuantity).div(_order.quantity);
+        uint requiredMakerTokenAmount = OrderLibrary.getPartialAmount(_order.makerTokenAmount, _fillQuantity, _order.quantity);
 
         // Calculate amount of component tokens required to issue
         for (uint16 i = 0; i < _order.requiredComponents.length; i++) {
@@ -424,7 +423,7 @@ contract CoreIssuanceOrder is
             );
 
             // Amount of component tokens to be added to Vault
-            uint requiredAddition = _order.requiredComponentAmounts[i].mul(_fillQuantity).div(_order.quantity);
+            uint requiredAddition = OrderLibrary.getPartialAmount(_order.requiredComponentAmounts[i], _fillQuantity, _order.quantity);
 
             // Required vault balances after exchange order executed
             requiredBalances[i] = tokenBalance.add(requiredAddition);

contracts/core/lib/OrderLibrary.sol

@@ -16,6 +16,8 @@
 
 pragma solidity 0.4.24;
 
+import { SafeMath } from "zeppelin-solidity/contracts/math/SafeMath.sol";
+
 
 /**
  * @title OrderLibrary
@@ -26,6 +28,10 @@ pragma solidity 0.4.24;
  */
 
 library OrderLibrary {
+    using SafeMath for uint256;
+
+    /* ============ Constants ============ */
+    string constant ROUNDING_ERROR_TOO_LARGE = "Rounding error too large.";
 
     /* ============ Structs ============ */
 
@@ -137,4 +143,23 @@ library OrderLibrary {
 
         return recAddress == _signerAddress;
     }
+
+    function getPartialAmount(
+        uint principal,
+        uint numerator,
+        uint denominator
+    )
+        internal
+        returns (uint256)
+    {
+        uint remainder = mulmod(principal, numerator, denominator);
+        if (remainder == 0) {
+            return principal.mul(numerator).div(denominator);
+        }
+
+        uint errPercentageTimes1000000 = remainder.mul(1000000).div(numerator.mul(principal));
+
+        require(errPercentageTimes1000000 < 1000, ROUNDING_ERROR_TOO_LARGE);
+        return principal.mul(numerator).div(denominator);
+    }
 }

test/contracts/core/extensions/coreIssuanceOrder.spec.ts

@@ -529,6 +529,26 @@ contract('CoreIssuanceOrder', accounts => {
         await expectRevertError(subject());
       });
     });
+
+    describe("when rounding error is too large", async () => {
+      before(async () => {
+        orderQuantity = ether(6);
+        makerTokenAmount = new BigNumber(10);
+      });
+
+      beforeEach(async () => {
+        subjectQuantityToIssue = ether(4);
+      });
+
+      after(async () => {
+        orderQuantity = undefined;
+        makerTokenAmount = undefined;
+      });
+
+      it("should revert", async () => {
+        await expectRevertError(subject());
+      });
+    });
   });
 
   describe('#cancelOrder', async () => {