Perform the division after multiplication to reduce compounding the rounding errors

2 files changed, 11 insertions, 10 deletions

diff --git a/src/utils/order_state_utils.ts b/src/utils/order_state_utils.ts
index 4f57de9fe..7c10a5480 100644
--- a/src/utils/order_state_utils.ts
+++ b/src/utils/order_state_utils.ts
@@ -66,13 +66,13 @@ export class OrderStateUtils {
         const totalTakerTokenAmount = signedOrder.takerTokenAmount;
         const remainingTakerTokenAmount = totalTakerTokenAmount.minus(unavailableTakerTokenAmount);
         // 200 in order, 100 unavailable  = 100 remaning, 0.5 remaining in taker proportion
-        const remainingTakerProportion = remainingTakerTokenAmount.dividedBy(totalTakerTokenAmount);
-        const remainingMakerTokenAmount = remainingTakerProportion.times(totalMakerTokenAmount);
+        const remainingMakerTokenAmount = remainingTakerTokenAmount.times(totalMakerTokenAmount)
+                                                                   .dividedToIntegerBy(totalTakerTokenAmount);
         // min allowance, balance in account of maker
         const fillableMakerTokenAmount = BigNumber.min([makerProxyAllowance, makerBalance]);
         // min ^, remaining order maker token amount
         const remainingFillableMakerTokenAmount = BigNumber.min(fillableMakerTokenAmount, remainingMakerTokenAmount);
-        // edge case when maker token is ZRX
+        // TODO: Handle edge case where maker token is ZRX with fee
         const orderRelevantState = {
             makerBalance,
             makerProxyAllowance,
diff --git a/test/order_state_watcher_test.ts b/test/order_state_watcher_test.ts
index 07d18ae61..924e54772 100644
--- a/test/order_state_watcher_test.ts
+++ b/test/order_state_watcher_test.ts
@@ -190,7 +190,8 @@ describe('OrderStateWatcher', () => {
                     const orderRelevantState = validOrderState.orderRelevantState;
                     const remainingMakerBalance = makerBalance.sub(fillAmountInBaseUnits);
                     const remainingFillable = fillableAmount.minus(fillAmountInBaseUnits);
-                    expect(remainingFillable).to.be.bignumber.equal(fillableAmount.minus(fillAmountInBaseUnits));
+                    expect(orderRelevantState.remainingFillableMakerTokenAmount).to.be.bignumber.equal(
+                        remainingFillable);
                     expect(orderRelevantState.makerBalance).to.be.bignumber.equal(remainingMakerBalance);
                     if (eventCount === 2) {
                         done();
@@ -204,7 +205,7 @@ describe('OrderStateWatcher', () => {
             })().catch(done);
         });
         describe('remainingFillableMakerTokenAmount', () => {
-          it('should calculate correct reamining fillable', (done: DoneCallback) => {
+          it('should calculate correct remaining fillable', (done: DoneCallback) => {
               (async () => {
                   const takerFillableAmount = new BigNumber(10);
                   const makerFillableAmount = new BigNumber(20);
@@ -265,20 +266,20 @@ describe('OrderStateWatcher', () => {
 
                   const makerBalance = await zeroEx.token.getBalanceAsync(makerToken.address, maker);
 
-                  const transferAmount = new BigNumber(1);
+                  const remainingAmount = new BigNumber(1);
+                  const transferAmount = makerBalance.sub(remainingAmount);
                   zeroEx.orderStateWatcher.addOrder(signedOrder);
 
                   const callback = reportCallbackErrors(done)((orderState: OrderState) => {
                       const validOrderState = orderState as OrderStateValid;
                       const orderRelevantState = validOrderState.orderRelevantState;
-
                       expect(orderRelevantState.remainingFillableMakerTokenAmount).to.be.bignumber.equal(
-                          transferAmount);
+                          remainingAmount);
                       done();
                   });
                   zeroEx.orderStateWatcher.subscribe(callback);
-                  await zeroEx.token.transferAsync(makerToken.address, maker, ZeroEx.NULL_ADDRESS,
-                                                   makerBalance.minus(transferAmount));
+                  await zeroEx.token.transferAsync(
+                      makerToken.address, maker, ZeroEx.NULL_ADDRESS, transferAmount);
               })().catch(done);
           });
         });