Atomic Order Matching - Smart Contracts.

13 files changed, 957 insertions, 114 deletions

diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/Exchange.sol b/packages/contracts/src/contracts/current/protocol/Exchange/Exchange.sol
index 85e2bc47e..438d50ecf 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/Exchange.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/Exchange.sol
@@ -25,24 +25,28 @@ import "./MixinSettlement.sol";
 import "./MixinWrapperFunctions.sol";
 import "./MixinAssetProxyDispatcher.sol";
 import "./MixinTransactions.sol";
+import "./MixinMatchOrders.sol";
 
 contract Exchange is
-    MixinWrapperFunctions,
     MixinExchangeCore,
+    MixinMatchOrders,
     MixinSettlement,
     MixinSignatureValidator,
     MixinTransactions,
-    MixinAssetProxyDispatcher
+    MixinAssetProxyDispatcher,
+    MixinWrapperFunctions
 {
+
     string constant public VERSION = "2.0.1-alpha";
 
     constructor (bytes memory _zrxProxyData)
         public
         MixinExchangeCore()
-        MixinSignatureValidator()
+        MixinMatchOrders()
         MixinSettlement(_zrxProxyData)
-        MixinWrapperFunctions()
-        MixinAssetProxyDispatcher()
+        MixinSignatureValidator()
         MixinTransactions()
+        MixinAssetProxyDispatcher()
+        MixinWrapperFunctions()
     {}
 }
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinExchangeCore.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinExchangeCore.sol
index 82c7a2ddc..58b5fa6b1 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/MixinExchangeCore.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinExchangeCore.sol
@@ -22,6 +22,7 @@ pragma experimental ABIEncoderV2;
 import "./libs/LibFillResults.sol";
 import "./libs/LibOrder.sol";
 import "./libs/LibMath.sol";
+import "./libs/LibStatus.sol";
 import "./libs/LibExchangeErrors.sol";
 import "./mixins/MExchangeCore.sol";
 import "./mixins/MSettlement.sol";
@@ -29,9 +30,11 @@ import "./mixins/MSignatureValidator.sol";
 import "./mixins/MTransactions.sol";
 
 contract MixinExchangeCore is
+    SafeMath,
+    LibMath,
+    LibStatus,
     LibOrder,
     LibFillResults,
-    LibMath,
     LibExchangeErrors,
     MExchangeCore,
     MSettlement,
@@ -48,64 +51,108 @@ contract MixinExchangeCore is
     // Orders with a salt less than their maker's epoch are considered cancelled
     mapping (address => uint256) public makerEpoch;
 
-    /// @dev Cancels all orders reated by sender with a salt less than or equal to the specified salt value.
-    /// @param salt Orders created with a salt less or equal to this value will be cancelled.
-    function cancelOrdersUpTo(uint256 salt)
-        external
-    {
-        uint256 newMakerEpoch = salt + 1;  // makerEpoch is initialized to 0, so to cancelUpTo we need salt + 1
-        require(
-            newMakerEpoch > makerEpoch[msg.sender],  // epoch must be monotonically increasing
-            INVALID_NEW_MAKER_EPOCH
-        );
-        makerEpoch[msg.sender] = newMakerEpoch;
-        emit CancelUpTo(msg.sender, newMakerEpoch);
-    }
+    ////// Core exchange functions //////
 
-    /// @dev Fills the input order.
-    /// @param order Order struct containing order specifications.
-    /// @param takerAssetFillAmount Desired amount of takerAsset to sell.
-    /// @param signature Proof that order has been created by maker.
-    /// @return Amounts filled and fees paid by maker and taker.
-    function fillOrder(
-        Order memory order,
-        uint256 takerAssetFillAmount,
-        bytes memory signature)
+    /// @dev Gets information about an order.
+    /// @param order Order to gather information on.
+    /// @return status Status of order. Statuses are defined in the LibStatus.Status struct.
+    /// @return orderHash Keccak-256 EIP712 hash of the order.
+    /// @return takerAssetFilledAmount Amount of order that has been filled.
+    function getOrderInfo(Order memory order)
         public
-        returns (FillResults memory fillResults)
+        view
+        returns (
+            uint8 orderStatus,
+            bytes32 orderHash,
+            uint256 takerAssetFilledAmount)
     {
-        // Compute the order hash
-        bytes32 orderHash = getOrderHash(order);
+        // Compute the order hash and fetch filled amount
+        orderHash = getOrderHash(order);
+        takerAssetFilledAmount = filled[orderHash];
 
-        // Check if order has been cancelled by salt value
-        if (order.salt < makerEpoch[order.makerAddress]) {
-            emit ExchangeError(uint8(Errors.ORDER_CANCELLED), orderHash);
-            return fillResults;
+        // If order.takerAssetAmount is zero, then the order will always
+        // be considered filled because 0 == takerAssetAmount == takerAssetFilledAmount
+        // Instead of distinguishing between unfilled and filled zero taker
+        // amount orders, we choose not to support them.
+        if (order.takerAssetAmount == 0) {
+            orderStatus = uint8(Status.ORDER_INVALID_TAKER_ASSET_AMOUNT);
+            return (orderStatus, orderHash, takerAssetFilledAmount);
+        }
+
+        // If order.makerAssetAmount is zero, we also reject the order.
+        // While the Exchange contract handles them correctly, they create
+        // edge cases in the supporting infrastructure because they have
+        // an 'infinite' price when computed by a simple division.
+        if (order.makerAssetAmount == 0) {
+            orderStatus = uint8(Status.ORDER_INVALID_MAKER_ASSET_AMOUNT);
+            return (orderStatus, orderHash, takerAssetFilledAmount);
+        }
+
+        // Validate order expiration
+        if (block.timestamp >= order.expirationTimeSeconds) {
+            orderStatus = uint8(Status.ORDER_EXPIRED);
+            return (orderStatus, orderHash, takerAssetFilledAmount);
         }
 
-        // Check if order has been cancelled by orderHash
+        // Validate order availability
+        if (takerAssetFilledAmount >= order.takerAssetAmount) {
+            orderStatus = uint8(Status.ORDER_FULLY_FILLED);
+            return (orderStatus, orderHash, takerAssetFilledAmount);
+        }
+
+        // Check if order has been cancelled
         if (cancelled[orderHash]) {
-            emit ExchangeError(uint8(Errors.ORDER_CANCELLED), orderHash);
-            return fillResults;
+            orderStatus = uint8(Status.ORDER_CANCELLED);
+            return (orderStatus, orderHash, takerAssetFilledAmount);
         }
 
-        // Validate order and maker only if first time seen
-        // TODO: Read filled and cancelled only once
-        if (filled[orderHash] == 0) {
-            require(
-                order.makerAssetAmount > 0,
-                GT_ZERO_AMOUNT_REQUIRED
-            );
-            require(
-                order.takerAssetAmount > 0,
-                GT_ZERO_AMOUNT_REQUIRED
-            );
+        // Validate order is not cancelled
+        if (makerEpoch[order.makerAddress] > order.salt) {
+            orderStatus = uint8(Status.ORDER_CANCELLED);
+            return (orderStatus, orderHash, takerAssetFilledAmount);
+        }
+
+        // Order is Fillable
+        orderStatus = uint8(Status.ORDER_FILLABLE);
+        return (orderStatus, orderHash, takerAssetFilledAmount);
+    }
+
+    /// @dev Validates context for fillOrder. Succeeds or throws.
+    /// @param order to be filled.
+    /// @param orderStatus Status of order to be filled.
+    /// @param orderHash Hash of order to be filled.
+    /// @param takerAssetFilledAmount Amount of order already filled.
+    /// @param signature Proof that the orders was created by its maker.
+    /// @param takerAddress Address of order taker.
+    /// @param takerAssetFillAmount Desired amount of order to fill by taker.
+    function validateFillOrderContextOrRevert(
+        Order memory order,
+        uint8 orderStatus,
+        bytes32 orderHash,
+        uint256 takerAssetFilledAmount,
+        bytes memory signature,
+        address takerAddress,
+        uint256 takerAssetFillAmount)
+        internal
+    {
+        // Ensure order is valid
+        require(
+            orderStatus != uint8(Status.ORDER_INVALID_MAKER_ASSET_AMOUNT),
+            INVALID_ORDER_MAKER_ASSET_AMOUNT
+        );
+        require(
+            orderStatus != uint8(Status.ORDER_INVALID_TAKER_ASSET_AMOUNT),
+            INVALID_ORDER_TAKER_ASSET_AMOUNT
+        );
+
+        // Validate Maker signature (check only if first time seen)
+        if (takerAssetFilledAmount == 0) {
             require(
                 isValidSignature(orderHash, order.makerAddress, signature),
                 SIGNATURE_VALIDATION_FAILED
             );
         }
-        
+
         // Validate sender is allowed to fill this order
         if (order.senderAddress != address(0)) {
             require(
@@ -115,7 +162,6 @@ contract MixinExchangeCore is
         }
 
         // Validate taker is allowed to fill this order
-        address takerAddress = getCurrentContextAddress();
         if (order.takerAddress != address(0)) {
             require(
                 order.takerAddress == takerAddress,
@@ -126,88 +172,194 @@ contract MixinExchangeCore is
             takerAssetFillAmount > 0,
             GT_ZERO_AMOUNT_REQUIRED
         );
+    }
 
-        // Validate order expiration
-        if (block.timestamp >= order.expirationTimeSeconds) {
-            emit ExchangeError(uint8(Errors.ORDER_EXPIRED), orderHash);
-            return fillResults;
+    /// @dev Calculates amounts filled and fees paid by maker and taker.
+    /// @param order to be filled.
+    /// @param orderStatus Status of order to be filled.
+    /// @param takerAssetFilledAmount Amount of order already filled.
+    /// @param takerAssetFillAmount Desired amount of order to fill by taker.
+    /// @return status Return status of calculating fill amounts. Returns Status.SUCCESS on success.
+    /// @return fillResults Amounts filled and fees paid by maker and taker.
+    function calculateFillResults(
+        Order memory order,
+        uint8 orderStatus,
+        uint256 takerAssetFilledAmount,
+        uint256 takerAssetFillAmount)
+        public
+        pure
+        returns (
+            uint8 status,
+            FillResults memory fillResults)
+    {
+        // Fill Amount must be greater than 0
+        if (takerAssetFillAmount <= 0) {
+            status = uint8(Status.TAKER_ASSET_FILL_AMOUNT_TOO_LOW);
+            return;
         }
 
-        // Validate order availability
-        uint256 remainingTakerAssetFillAmount = safeSub(order.takerAssetAmount, filled[orderHash]);
-        if (remainingTakerAssetFillAmount == 0) {
-            emit ExchangeError(uint8(Errors.ORDER_FULLY_FILLED), orderHash);
-            return fillResults;
+        // Ensure the order is fillable
+        if (orderStatus != uint8(Status.ORDER_FILLABLE)) {
+            status = uint8(orderStatus);
+            return;
         }
 
+        // Compute takerAssetFilledAmount
+        uint256 remainingtakerAssetAmount = safeSub(order.takerAssetAmount, takerAssetFilledAmount);
+        fillResults.takerAssetFilledAmount = min256(takerAssetFillAmount, remainingtakerAssetAmount);
+
         // Validate fill order rounding
-        fillResults.takerAssetFilledAmount = min256(takerAssetFillAmount, remainingTakerAssetFillAmount);
-        if (isRoundingError(fillResults.takerAssetFilledAmount, order.takerAssetAmount, order.makerAssetAmount)) {
-            emit ExchangeError(uint8(Errors.ROUNDING_ERROR_TOO_LARGE), orderHash);
+        if (isRoundingError(
+            fillResults.takerAssetFilledAmount,
+            order.takerAssetAmount,
+            order.makerAssetAmount))
+        {
+            status = uint8(Status.ROUNDING_ERROR_TOO_LARGE);
             fillResults.takerAssetFilledAmount = 0;
-            return fillResults;
+            return;
         }
 
+        // Compute proportional transfer amounts
+        // TODO: All three are multiplied by the same fraction. This can
+        // potentially be optimized.
+        fillResults.makerAssetFilledAmount = getPartialAmount(
+            fillResults.takerAssetFilledAmount,
+            order.takerAssetAmount,
+            order.makerAssetAmount);
+        fillResults.makerFeePaid = getPartialAmount(
+            fillResults.takerAssetFilledAmount,
+            order.takerAssetAmount,
+            order.makerFee);
+        fillResults.takerFeePaid = getPartialAmount(
+            fillResults.takerAssetFilledAmount,
+            order.takerAssetAmount,
+            order.takerFee);
+
+        status = uint8(Status.SUCCESS);
+        return;
+    }
+
+    /// @dev Updates state with results of a fill order.
+    /// @param order that was filled.
+    /// @param takerAddress Address of taker who filled the order.
+    /// @return fillResults Amounts filled and fees paid by maker and taker.
+    function updateFilledState(
+        Order memory order,
+        address takerAddress,
+        bytes32 orderHash,
+        FillResults memory fillResults)
+        internal
+    {
         // Update state
         filled[orderHash] = safeAdd(filled[orderHash], fillResults.takerAssetFilledAmount);
 
-        // Settle order
-        (fillResults.makerAssetFilledAmount, fillResults.makerFeePaid, fillResults.takerFeePaid) =
-            settleOrder(order, takerAddress, fillResults.takerAssetFilledAmount);
-
         // Log order
         emitFillEvent(order, takerAddress, orderHash, fillResults);
-        return fillResults;
     }
 
-    /// @dev After calling, the order can not be filled anymore.
+    /// @dev Fills the input order.
     /// @param order Order struct containing order specifications.
-    /// @return True if the order state changed to cancelled.
-    ///         False if the transaction was already cancelled or expired.
-    function cancelOrder(Order memory order)
+    /// @param takerAssetFillAmount Desired amount of takerToken to sell.
+    /// @param signature Proof that order has been created by maker.
+    /// @return Amounts filled and fees paid by maker and taker.
+    function fillOrder(
+        Order memory order,
+        uint256 takerAssetFillAmount,
+        bytes memory signature)
         public
-        returns (bool)
+        returns (FillResults memory fillResults)
     {
-        // Compute the order hash
-        bytes32 orderHash = getOrderHash(order);
+        // Fetch current order status
+        bytes32 orderHash;
+        uint8 orderStatus;
+        uint256 takerAssetFilledAmount;
+        (orderStatus, orderHash, takerAssetFilledAmount) = getOrderInfo(order);
 
-        // Validate the order
+        // Fetch taker address
+        address takerAddress = getCurrentContextAddress();
+
+        // Either our context is valid or we revert
+        validateFillOrderContextOrRevert(order, orderStatus, orderHash, takerAssetFilledAmount, signature, takerAddress, takerAssetFillAmount);
+
+        // Compute proportional fill amounts
+        uint8 status;
+        (status, fillResults) = calculateFillResults(order, orderStatus, takerAssetFilledAmount, takerAssetFillAmount);
+        if (status != uint8(Status.SUCCESS)) {
+            emit ExchangeStatus(uint8(status), orderHash);
+            return fillResults;
+        }
+
+        // Settle order
+        (fillResults.makerAssetFilledAmount, fillResults.makerFeePaid, fillResults.takerFeePaid) =
+            settleOrder(order, takerAddress, fillResults.takerAssetFilledAmount);
+
+        // Update exchange internal state
+        updateFilledState(order, takerAddress, orderHash, fillResults);
+        return fillResults;
+    }
+
+    /// @dev Validates context for cancelOrder. Succeeds or throws.
+    /// @param order that was cancelled.
+    /// @param orderStatus Status of order that was cancelled.
+    /// @param orderHash Hash of order that was cancelled.
+    function validateCancelOrderContextOrRevert(
+        Order memory order,
+        uint8 orderStatus,
+        bytes32 orderHash)
+        internal
+    {
+        // Ensure order is valid
         require(
-            order.makerAssetAmount > 0,
-            GT_ZERO_AMOUNT_REQUIRED
+            orderStatus != uint8(Status.ORDER_INVALID_MAKER_ASSET_AMOUNT),
+            INVALID_ORDER_MAKER_ASSET_AMOUNT
         );
         require(
-            order.takerAssetAmount > 0,
-            GT_ZERO_AMOUNT_REQUIRED
+            orderStatus != uint8(Status.ORDER_INVALID_TAKER_ASSET_AMOUNT),
+            INVALID_ORDER_TAKER_ASSET_AMOUNT
         );
 
-        // Validate sender is allowed to cancel this order
-        if (order.senderAddress != address(0)) {
-            require(
-                order.senderAddress == msg.sender,
-                INVALID_SENDER
-            );
-        }
-        
         // Validate transaction signed by maker
         address makerAddress = getCurrentContextAddress();
         require(
             order.makerAddress == makerAddress,
             INVALID_CONTEXT
         );
-        
-        if (block.timestamp >= order.expirationTimeSeconds) {
-            emit ExchangeError(uint8(Errors.ORDER_EXPIRED), orderHash);
-            return false;
+
+        // Validate sender is allowed to cancel this order
+        if (order.senderAddress != address(0)) {
+            require(
+                order.senderAddress == msg.sender,
+                INVALID_SENDER
+            );
         }
+    }
 
-        if (cancelled[orderHash]) {
-            emit ExchangeError(uint8(Errors.ORDER_CANCELLED), orderHash);
-            return false;
+    /// @dev Updates state with results of cancelling an order.
+    ///      State is only updated if the order is currently fillable.
+    ///      Otherwise, updating state would have no effect.
+    /// @param order that was cancelled.
+    /// @param orderStatus Status of order that was cancelled.
+    /// @param orderHash Hash of order that was cancelled.
+    /// @return stateUpdated Returns true only if state was updated.
+    function updateCancelledState(
+        Order memory order,
+        uint8 orderStatus,
+        bytes32 orderHash)
+        internal
+        returns (bool stateUpdated)
+    {
+        // Ensure order is fillable (otherwise cancelling does nothing)
+        if (orderStatus != uint8(Status.ORDER_FILLABLE)) {
+            emit ExchangeStatus(uint8(orderStatus), orderHash);
+            stateUpdated = false;
+            return stateUpdated;
         }
 
+        // Perform cancel
         cancelled[orderHash] = true;
+        stateUpdated = true;
 
+        // Log cancel
         emit Cancel(
             order.makerAddress,
             order.feeRecipientAddress,
@@ -215,7 +367,43 @@ contract MixinExchangeCore is
             order.makerAssetData,
             order.takerAssetData
         );
-        return true;
+
+        return stateUpdated;
+    }
+
+    /// @dev After calling, the order can not be filled anymore.
+    /// @param order Order struct containing order specifications.
+    /// @return True if the order state changed to cancelled.
+    ///         False if the transaction was already cancelled or expired.
+    function cancelOrder(Order memory order)
+        public
+        returns (bool)
+    {
+        // Fetch current order status
+        bytes32 orderHash;
+        uint8 orderStatus;
+        uint256 takerAssetFilledAmount;
+        (orderStatus, orderHash, takerAssetFilledAmount) = getOrderInfo(order);
+
+        // Validate context
+        validateCancelOrderContextOrRevert(order, orderStatus, orderHash);
+
+        // Perform cancel
+        return updateCancelledState(order, orderStatus, orderHash);
+    }
+
+    /// @dev Cancels all orders reated by sender with a salt less than or equal to the specified salt value.
+    /// @param salt Orders created with a salt less or equal to this value will be cancelled.
+    function cancelOrdersUpTo(uint256 salt)
+        external
+    {
+        uint256 newMakerEpoch = salt + 1;  // makerEpoch is initialized to 0, so to cancelUpTo we need salt + 1
+        require(
+            newMakerEpoch > makerEpoch[msg.sender],  // epoch must be monotonically increasing
+            INVALID_NEW_MAKER_EPOCH
+        );
+        makerEpoch[msg.sender] = newMakerEpoch;
+        emit CancelUpTo(msg.sender, newMakerEpoch);
     }
 
     /// @dev Logs a Fill event with the given arguments.
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinMatchOrders.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinMatchOrders.sol
new file mode 100644
index 000000000..863128a06
--- /dev/null
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinMatchOrders.sol
@@ -0,0 +1,285 @@
+/*
+  Copyright 2018 ZeroEx Intl.
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+    http://www.apache.org/licenses/LICENSE-2.0
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+*/
+
+pragma solidity ^0.4.21;
+pragma experimental ABIEncoderV2;
+
+import "./mixins/MExchangeCore.sol";
+import "./mixins/MMatchOrders.sol";
+import "./mixins/MSettlement.sol";
+import "./mixins/MTransactions.sol";
+import "../../utils/SafeMath/SafeMath.sol";
+import "./libs/LibMath.sol";
+import "./libs/LibOrder.sol";
+import "./libs/LibStatus.sol";
+import "../../utils/LibBytes/LibBytes.sol";
+
+contract MixinMatchOrders is
+    SafeMath,
+    LibBytes,
+    LibMath,
+    LibStatus,
+    LibOrder,
+    MExchangeCore,
+    MMatchOrders,
+    MSettlement,
+    MTransactions
+    {
+
+    /// @dev Validates context for matchOrders. Succeeds or throws.
+    /// @param leftOrder First order to match.
+    /// @param rightOrder Second order to match.
+    function validateMatchOrdersContextOrRevert(
+        Order memory leftOrder,
+        Order memory rightOrder)
+        internal
+    {
+        // The leftOrder maker asset must be the same as the rightOrder taker asset.
+        require(areBytesEqual(leftOrder.makerAssetData, rightOrder.takerAssetData));
+
+        // The leftOrder taker asset must be the same as the rightOrder maker asset.
+        require(areBytesEqual(leftOrder.takerAssetData, rightOrder.makerAssetData));
+
+        // Make sure there is a positive spread.
+        // There is a positive spread iff the cost per unit bought (OrderA.MakerAmount/OrderA.TakerAmount) for each order is greater
+        // than the profit per unit sold of the matched order (OrderB.TakerAmount/OrderB.MakerAmount).
+        // This is satisfied by the equations below:
+        // <leftOrder.makerAssetAmount> / <leftOrder.takerAssetAmount> >= <rightOrder.takerAssetAmount> / <rightOrder.makerAssetAmount>
+        // AND
+        // <rightOrder.makerAssetAmount> / <rightOrder.takerAssetAmount> >= <leftOrder.takerAssetAmount> / <leftOrder.makerAssetAmount>
+        // These equations can be combined to get the following:
+        require(
+            safeMul(leftOrder.makerAssetAmount, rightOrder.makerAssetAmount) >=
+            safeMul(leftOrder.takerAssetAmount, rightOrder.takerAssetAmount)
+        );
+    }
+
+    /// @dev Validates matched fill results. Succeeds or throws.
+    /// @param matchedFillResults Amounts to fill and fees to pay by maker and taker of matched orders.
+    function validateMatchedOrderFillResultsOrThrow(MatchedFillResults memory matchedFillResults)
+        internal
+    {
+        // The right order must spend at least as much as we're transferring to the left order's maker.
+        // If the amount transferred from the right order is greater than what is transferred, it is a rounding error amount.
+        // Ensure this difference is negligible by dividing the values with each other. The result should equal to ~1.
+        require(matchedFillResults.right.makerAssetFilledAmount >= matchedFillResults.left.takerAssetFilledAmount);
+        require(!isRoundingError(matchedFillResults.right.makerAssetFilledAmount, matchedFillResults.left.takerAssetFilledAmount, 1));
+    }
+
+    /// @dev Calculates partial value given a numerator and denominator.
+    ///      Throws if there is a rounding error.
+    /// @param numerator Numerator.
+    /// @param denominator Denominator.
+    /// @param target Value to calculate partial of.
+    /// @return Partial value of target.
+    function safeGetPartialAmount(
+        uint256 numerator,
+        uint256 denominator,
+        uint256 target)
+        internal pure
+        returns (uint256 partialAmount)
+    {
+        require(!isRoundingError(numerator, denominator, target));
+        return getPartialAmount(numerator, denominator, target);
+    }
+
+    /// @dev Calculates fill amounts for the matched orders.
+    ///      Each order is filled at their respective price point. However, the calculations are
+    ///      carried out as though the orders are both being filled at the right order's price point.
+    ///      The profit made by the leftOrder order goes to the taker (who matched the two orders).
+    /// @param leftOrder First order to match.
+    /// @param rightOrder Second order to match.
+    /// @param leftOrderStatus Order status of left order.
+    /// @param rightOrderStatus Order status of right order.
+    /// @param leftOrderFilledAmount Amount of left order already filled.
+    /// @param rightOrderFilledAmount Amount of right order already filled.
+    /// @return status Return status of calculating fill amounts. Returns Status.SUCCESS on success.
+    /// @param matchedFillResults Amounts to fill and fees to pay by maker and taker of matched orders.
+    function calculateMatchedFillResults(
+        Order memory leftOrder,
+        Order memory rightOrder,
+        uint8 leftOrderStatus,
+        uint8 rightOrderStatus,
+        uint256 leftOrderFilledAmount,
+        uint256 rightOrderFilledAmount)
+        internal
+        returns (
+            uint8 status,
+            MatchedFillResults memory matchedFillResults)
+    {
+        // We settle orders at the price point defined by the right order (profit goes to the order taker)
+        // The constraint can be either on the left or on the right.
+        // The constraint is on the left iff the amount required to fill the left order
+        // is less than or equal to the amount we can spend from the right order:
+        //    <leftTakerAssetAmountRemaining> <= <rightTakerAssetAmountRemaining> * <rightMakerToTakerRatio>
+        //    <leftTakerAssetAmountRemaining> <= <rightTakerAssetAmountRemaining> * <rightOrder.makerAssetAmount> / <rightOrder.takerAssetAmount>
+        //    <leftTakerAssetAmountRemaining> * <rightOrder.takerAssetAmount> <= <rightTakerAssetAmountRemaining> * <rightOrder.makerAssetAmount>
+        uint256 rightTakerAssetAmountRemaining = safeSub(rightOrder.takerAssetAmount, rightOrderFilledAmount);
+        uint256 leftTakerAssetAmountRemaining = safeSub(leftOrder.takerAssetAmount, leftOrderFilledAmount);
+        uint256 leftOrderAmountToFill = 0;
+        uint256 rightOrderAmountToFill = 0;
+        if (
+            safeMul(leftTakerAssetAmountRemaining, rightOrder.takerAssetAmount) <=
+            safeMul(rightTakerAssetAmountRemaining, rightOrder.makerAssetAmount)
+        ) {
+            // Left order is the constraint: maximally fill left
+            leftOrderAmountToFill = leftTakerAssetAmountRemaining;
+
+            // The right order receives an amount proportional to how much was spent.
+            // TODO: Ensure rounding error is in the correct direction.
+            rightOrderAmountToFill = safeGetPartialAmount(
+                rightOrder.takerAssetAmount,
+                rightOrder.makerAssetAmount,
+                leftOrderAmountToFill);
+        } else {
+            // Right order is the constraint: maximally fill right
+            rightOrderAmountToFill = rightTakerAssetAmountRemaining;
+
+            // The left order receives an amount proportional to how much was spent.
+            // TODO: Ensure rounding error is in the correct direction.
+            leftOrderAmountToFill = safeGetPartialAmount(
+                rightOrder.makerAssetAmount,
+                rightOrder.takerAssetAmount,
+                rightOrderAmountToFill);
+        }
+
+        // Calculate fill results for left order
+        (   status,
+            matchedFillResults.left
+        ) = calculateFillResults(
+            leftOrder,
+            leftOrderStatus,
+            leftOrderFilledAmount,
+            leftOrderAmountToFill);
+        if (status != uint8(Status.SUCCESS)) {
+            return (status, matchedFillResults);
+        }
+
+        // Calculate fill results for right order
+        (   status,
+            matchedFillResults.right
+        ) = calculateFillResults(
+            rightOrder,
+            rightOrderStatus,
+            rightOrderFilledAmount,
+            rightOrderAmountToFill);
+        if (status != uint8(Status.SUCCESS)) {
+            return (status, matchedFillResults);
+        }
+
+        // Validate the fill results
+        validateMatchedOrderFillResultsOrThrow(matchedFillResults);
+
+        // Return status & fill results
+        return (status, matchedFillResults);
+    }
+
+    /// @dev Match two complementary orders that have a positive spread.
+    ///      Each order is filled at their respective price point. However, the calculations are
+    ///      carried out as though the orders are both being filled at the right order's price point.
+    ///      The profit made by the left order goes to the taker (who matched the two orders).
+    /// @param leftOrder First order to match.
+    /// @param rightOrder Second order to match.
+    /// @param leftSignature Proof that order was created by the left maker.
+    /// @param rightSignature Proof that order was created by the right maker.
+    /// @return matchedFillResults Amounts filled and fees paid by maker and taker of matched orders.
+    function matchOrders(
+        Order memory leftOrder,
+        Order memory rightOrder,
+        bytes leftSignature,
+        bytes rightSignature)
+        public
+        returns (MatchedFillResults memory matchedFillResults)
+    {
+        // Get left status
+        OrderInfo memory leftOrderInfo;
+        (   leftOrderInfo.orderStatus,
+            leftOrderInfo.orderHash,
+            leftOrderInfo.orderFilledAmount
+        ) = getOrderInfo(leftOrder);
+        if (leftOrderInfo.orderStatus != uint8(Status.ORDER_FILLABLE)) {
+            emit ExchangeStatus(uint8(leftOrderInfo.orderStatus), leftOrderInfo.orderHash);
+            return matchedFillResults;
+        }
+
+        // Get right status
+        OrderInfo memory rightOrderInfo;
+        (   rightOrderInfo.orderStatus,
+            rightOrderInfo.orderHash,
+            rightOrderInfo.orderFilledAmount
+        ) = getOrderInfo(rightOrder);
+        if (rightOrderInfo.orderStatus != uint8(Status.ORDER_FILLABLE)) {
+            emit ExchangeStatus(uint8(rightOrderInfo.orderStatus), rightOrderInfo.orderHash);
+            return matchedFillResults;
+        }
+
+        // Fetch taker address
+        address takerAddress = getCurrentContextAddress();
+
+        // Either our context is valid or we revert
+        validateMatchOrdersContextOrRevert(leftOrder, rightOrder);
+
+        // Compute proportional fill amounts
+        uint8 matchedFillAmountsStatus;
+        (   matchedFillAmountsStatus,
+            matchedFillResults
+        ) = calculateMatchedFillResults(
+            leftOrder,
+            rightOrder,
+            leftOrderInfo.orderStatus,
+            rightOrderInfo.orderStatus,
+            leftOrderInfo.orderFilledAmount,
+            rightOrderInfo.orderFilledAmount);
+        if (matchedFillAmountsStatus != uint8(Status.SUCCESS)) {
+            return matchedFillResults;
+        }
+
+        // Validate fill contexts
+        validateFillOrderContextOrRevert(
+            leftOrder,
+            leftOrderInfo.orderStatus,
+            leftOrderInfo.orderHash,
+            leftOrderInfo.orderFilledAmount,
+            leftSignature,
+            rightOrder.makerAddress,
+            matchedFillResults.left.takerAssetFilledAmount);
+        validateFillOrderContextOrRevert(
+            rightOrder,
+            rightOrderInfo.orderStatus,
+            rightOrderInfo.orderHash,
+            rightOrderInfo.orderFilledAmount,
+            rightSignature,
+            leftOrder.makerAddress,
+            matchedFillResults.right.takerAssetFilledAmount);
+
+        // Settle matched orders. Succeeds or throws.
+        settleMatchedOrders(leftOrder, rightOrder, matchedFillResults, takerAddress);
+
+        // Update exchange state
+        updateFilledState(
+            leftOrder,
+            rightOrder.makerAddress,
+            leftOrderInfo.orderHash,
+            matchedFillResults.left
+        );
+        updateFilledState(
+            rightOrder,
+            leftOrder.makerAddress,
+            rightOrderInfo.orderHash,
+            matchedFillResults.right
+        );
+
+        // Return results
+        return matchedFillResults;
+    }
+}
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinSettlement.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinSettlement.sol
index 30f1bb49b..3b67051d2 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/MixinSettlement.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinSettlement.sol
@@ -22,9 +22,11 @@ import "./mixins/MSettlement.sol";
 import "./mixins/MAssetProxyDispatcher.sol";
 import "./libs/LibOrder.sol";
 import "./libs/LibMath.sol";
+import "./mixins/MMatchOrders.sol";
 
 contract MixinSettlement is
     LibMath,
+    MMatchOrders,
     MSettlement,
     MAssetProxyDispatcher
 {
@@ -96,4 +98,87 @@ contract MixinSettlement is
         );
         return (makerAssetFilledAmount, makerFeePaid, takerFeePaid);
     }
+
+    /// @dev Settles matched order by transferring appropriate funds between order makers, taker, and fee recipient.
+    /// @param leftOrder First matched order.
+    /// @param rightOrder Second matched order.
+    /// @param matchedFillResults Struct holding amounts to transfer between makers, taker, and fee recipients.
+    /// @param takerAddress Address that matched the orders. The taker receives the spread between orders as profit.
+    function settleMatchedOrders(
+        LibOrder.Order memory leftOrder,
+        LibOrder.Order memory rightOrder,
+        MatchedFillResults memory matchedFillResults,
+        address takerAddress)
+        internal
+    {
+        // Optimized for:
+        // * leftOrder.feeRecipient =?= rightOrder.feeRecipient
+
+        // Not optimized for:
+        // * {left, right}.{MakerAsset, TakerAsset} == ZRX
+        // * {left, right}.maker, takerAddress == {left, right}.feeRecipient
+
+        // leftOrder.MakerAsset == rightOrder.TakerAsset
+        // Taker should be left with a positive balance (the spread)
+        dispatchTransferFrom(
+            leftOrder.makerAssetData,
+            leftOrder.makerAddress,
+            takerAddress,
+            matchedFillResults.left.makerAssetFilledAmount);
+        dispatchTransferFrom(
+            leftOrder.makerAssetData,
+            takerAddress,
+            rightOrder.makerAddress,
+            matchedFillResults.right.takerAssetFilledAmount);
+
+        // rightOrder.MakerAsset == leftOrder.TakerAsset
+        // leftOrder.takerAssetFilledAmount ~ rightOrder.makerAssetFilledAmount
+        // The change goes to right, not to taker.
+        assert(matchedFillResults.right.makerAssetFilledAmount >= matchedFillResults.left.takerAssetFilledAmount);
+        dispatchTransferFrom(
+            rightOrder.makerAssetData,
+            rightOrder.makerAddress,
+            leftOrder.makerAddress,
+            matchedFillResults.right.makerAssetFilledAmount);
+
+        // Maker fees
+        dispatchTransferFrom(
+            ZRX_PROXY_DATA,
+            leftOrder.makerAddress,
+            leftOrder.feeRecipientAddress,
+            matchedFillResults.left.makerFeePaid);
+        dispatchTransferFrom(
+            ZRX_PROXY_DATA,
+            rightOrder.makerAddress,
+            rightOrder.feeRecipientAddress,
+            matchedFillResults.right.makerFeePaid);
+
+        // Taker fees
+        // If we assume distinct(left, right, takerAddress) and
+        // distinct(MakerAsset, TakerAsset, zrx) then the only remaining
+        // opportunity for optimization is when both feeRecipientAddress' are
+        // the same.
+        if (leftOrder.feeRecipientAddress == rightOrder.feeRecipientAddress) {
+            dispatchTransferFrom(
+                ZRX_PROXY_DATA,
+                takerAddress,
+                leftOrder.feeRecipientAddress,
+                safeAdd(
+                    matchedFillResults.left.takerFeePaid,
+                    matchedFillResults.right.takerFeePaid
+                )
+            );
+        } else {
+            dispatchTransferFrom(
+                ZRX_PROXY_DATA,
+                takerAddress,
+                leftOrder.feeRecipientAddress,
+                matchedFillResults.left.takerFeePaid);
+            dispatchTransferFrom(
+                ZRX_PROXY_DATA,
+                takerAddress,
+                rightOrder.feeRecipientAddress,
+                matchedFillResults.right.takerFeePaid);
+        }
+    }
 }
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinWrapperFunctions.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinWrapperFunctions.sol
index 42517221e..04018e09d 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/MixinWrapperFunctions.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinWrapperFunctions.sol
@@ -27,10 +27,11 @@ import "./libs/LibFillResults.sol";
 import "./libs/LibExchangeErrors.sol";
 
 contract MixinWrapperFunctions is
+    SafeMath,
+    LibBytes,
+    LibMath,
     LibOrder,
     LibFillResults,
-    LibMath,
-    LibBytes,
     LibExchangeErrors,
     MExchangeCore
 {
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/interfaces/IWrapperFunctions.sol b/packages/contracts/src/contracts/current/protocol/Exchange/interfaces/IWrapperFunctions.sol
index d23170b1c..3aaa9de9e 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/interfaces/IWrapperFunctions.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/interfaces/IWrapperFunctions.sol
@@ -23,10 +23,10 @@ import "./libs/LibOrder.sol";
 import "./libs/LibFillResults.sol";
 
 contract IWrapperFunctions is
+    LibBytes,
+    LibMath,
     LibOrder,
     LibFillResults,
-    LibMath,
-    LibBytes,
     LibExchangeErrors,
     MExchangeCore
 {
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibExchangeErrors.sol b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibExchangeErrors.sol
index 6ffbdfdca..bf048e815 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibExchangeErrors.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibExchangeErrors.sol
@@ -20,17 +20,6 @@ pragma solidity ^0.4.23;
 
 contract LibExchangeErrors {
 
-    // Error Codes
-    enum Errors {
-        ORDER_EXPIRED,                     // Order has already expired
-        ORDER_FULLY_FILLED,                // Order has already been fully filled
-        ORDER_CANCELLED,                   // Order has already been cancelled
-        ROUNDING_ERROR_TOO_LARGE,          // Rounding error too large
-        INSUFFICIENT_BALANCE_OR_ALLOWANCE  // Insufficient balance or allowance for token transfer
-    }
-
-    event ExchangeError(uint8 indexed errorId, bytes32 indexed orderHash);
-
     // Core revert reasons
     string constant GT_ZERO_AMOUNT_REQUIRED = "Amount must be greater than 0.";
     string constant SIGNATURE_VALIDATION_FAILED = "Signature validation failed.";
@@ -38,6 +27,10 @@ contract LibExchangeErrors {
     string constant INVALID_CONTEXT = "Function called in an invalid context.";
     string constant INVALID_NEW_MAKER_EPOCH = "Specified salt must be greater than or equal to existing makerEpoch.";
 
+    // Order revert reasons
+    string constant INVALID_ORDER_TAKER_ASSET_AMOUNT = "Invalid order taker asset amount: expected a non-zero value.";
+    string constant INVALID_ORDER_MAKER_ASSET_AMOUNT = "Invalid order maker asset amount: expected a non-zero value.";
+
     // Transaction revert reasons
     string constant DUPLICATE_TRANSACTION_HASH = "Transaction has already been executed.";
     string constant TRANSACTION_EXECUTION_FAILED = "Transaction execution failed.";
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibStatus.sol b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibStatus.sol
new file mode 100644
index 000000000..42b9cb65d
--- /dev/null
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibStatus.sol
@@ -0,0 +1,49 @@
+/*
+
+  Copyright 2018 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity ^0.4.21;
+pragma experimental ABIEncoderV2;
+
+contract LibStatus {
+
+    // Exchange Status Codes
+    enum Status {
+        /// Default Status ///
+        INVALID,                                // General invalid status
+
+        /// General Exchange Statuses ///
+        SUCCESS,                                // Indicates a successful operation
+        ROUNDING_ERROR_TOO_LARGE,               // Rounding error too large
+        INSUFFICIENT_BALANCE_OR_ALLOWANCE,      // Insufficient balance or allowance for token transfer
+        TAKER_ASSET_FILL_AMOUNT_TOO_LOW,        // takerAssetFillAmount is <= 0
+        INVALID_SIGNATURE,                      // Invalid signature
+        INVALID_SENDER,                         // Invalid sender
+        INVALID_TAKER,                          // Invalid taker
+        INVALID_MAKER,                          // Invalid maker
+
+        /// Order State Statuses ///
+        ORDER_INVALID_MAKER_ASSET_AMOUNT,       // Order does not have a valid maker asset amount
+        ORDER_INVALID_TAKER_ASSET_AMOUNT,       // Order does not have a valid taker asset amount
+        ORDER_FILLABLE,                         // Order is fillable
+        ORDER_EXPIRED,                          // Order has already expired
+        ORDER_FULLY_FILLED,                     // Order is fully filled
+        ORDER_CANCELLED                         // Order has been cancelled
+    }
+
+    event ExchangeStatus(uint8 indexed statusId, bytes32 indexed orderHash);
+}
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MAssetProxyDispatcher.sol b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MAssetProxyDispatcher.sol
index 77ee77797..67752d3c8 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MAssetProxyDispatcher.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MAssetProxyDispatcher.sol
@@ -17,6 +17,7 @@
 */
 
 pragma solidity ^0.4.23;
+pragma experimental ABIEncoderV2;
 
 import "../interfaces/IAssetProxyDispatcher.sol";
 
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MExchangeCore.sol b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MExchangeCore.sol
index 5b78e70da..eb97be8b0 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MExchangeCore.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MExchangeCore.sol
@@ -17,6 +17,7 @@
 */
 
 pragma solidity ^0.4.23;
+pragma experimental ABIEncoderV2;
 
 import "../libs/LibOrder.sol";
 import "../libs/LibFillResults.sol";
@@ -63,4 +64,122 @@ contract MExchangeCore is
         bytes32 orderHash,
         LibFillResults.FillResults memory fillResults)
         internal;
+
+    /// @dev Gets information about an order.
+    /// @param order Order to gather information on.
+    /// @return status Status of order. Statuses are defined in the LibStatus.Status struct.
+    /// @return orderHash Keccak-256 EIP712 hash of the order.
+    /// @return takerAssetFilledAmount Amount of order that has been filled.
+    function getOrderInfo(LibOrder.Order memory order)
+        public
+        view
+        returns (
+            uint8 orderStatus,
+            bytes32 orderHash,
+            uint256 takerAssetFilledAmount);
+
+    /// @dev Validates context for fillOrder. Succeeds or throws.
+    /// @param order to be filled.
+    /// @param orderStatus Status of order to be filled.
+    /// @param orderHash Hash of order to be filled.
+    /// @param takerAssetFilledAmount Amount of order already filled.
+    /// @param signature Proof that the orders was created by its maker.
+    /// @param takerAddress Address of order taker.
+    /// @param takerAssetFillAmount Desired amount of order to fill by taker.
+    function validateFillOrderContextOrRevert(
+        LibOrder.Order memory order,
+        uint8 orderStatus,
+        bytes32 orderHash,
+        uint256 takerAssetFilledAmount,
+        bytes memory signature,
+        address takerAddress,
+        uint256 takerAssetFillAmount)
+        internal;
+
+    /// @dev Calculates amounts filled and fees paid by maker and taker.
+    /// @param order to be filled.
+    /// @param orderStatus Status of order to be filled.
+    /// @param takerAssetFilledAmount Amount of order already filled.
+    /// @param takerAssetFillAmount Desired amount of order to fill by taker.
+    /// @return status Return status of calculating fill amounts. Returns Status.SUCCESS on success.
+    /// @return fillResults Amounts filled and fees paid by maker and taker.
+    function calculateFillResults(
+        LibOrder.Order memory order,
+        uint8 orderStatus,
+        uint256 takerAssetFilledAmount,
+        uint256 takerAssetFillAmount)
+        public
+        pure
+        returns (
+            uint8 status,
+            LibFillResults.FillResults memory fillResults);
+
+    /// @dev Updates state with results of a fill order.
+    /// @param order that was filled.
+    /// @param takerAddress Address of taker who filled the order.
+    /// @return fillResults Amounts filled and fees paid by maker and taker.
+    function updateFilledState(
+        LibOrder.Order memory order,
+        address takerAddress,
+        bytes32 orderHash,
+        LibFillResults.FillResults memory fillResults)
+        internal;
+
+    /// @dev Fills the input order.
+    /// @param order Order struct containing order specifications.
+    /// @param takerAssetFillAmount Desired amount of takerToken to sell.
+    /// @param signature Proof that order has been created by maker.
+    /// @return Amounts filled and fees paid by maker and taker.
+    function fillOrder(
+        LibOrder.Order memory order,
+        uint256 takerAssetFillAmount,
+        bytes memory signature)
+        public
+        returns (LibFillResults.FillResults memory fillResults);
+
+    /// @dev Validates context for cancelOrder. Succeeds or throws.
+    /// @param order that was cancelled.
+    /// @param orderStatus Status of order that was cancelled.
+    /// @param orderHash Hash of order that was cancelled.
+    function validateCancelOrderContextOrRevert(
+        LibOrder.Order memory order,
+        uint8 orderStatus,
+        bytes32 orderHash)
+        internal;
+
+    /// @dev Updates state with results of cancelling an order.
+    ///      State is only updated if the order is currently fillable.
+    ///      Otherwise, updating state would have no effect.
+    /// @param order that was cancelled.
+    /// @param orderStatus Status of order that was cancelled.
+    /// @param orderHash Hash of order that was cancelled.
+    /// @return stateUpdated Returns true only if state was updated.
+    function updateCancelledState(
+        LibOrder.Order memory order,
+        uint8 orderStatus,
+        bytes32 orderHash)
+        internal
+        returns (bool stateUpdated);
+
+    /// @dev After calling, the order can not be filled anymore.
+    /// @param order Order struct containing order specifications.
+    /// @return True if the order state changed to cancelled.
+    ///         False if the transaction was already cancelled or expired.
+    function cancelOrder(LibOrder.Order memory order)
+        public
+        returns (bool);
+
+    /// @param salt Orders created with a salt less or equal to this value will be cancelled.
+    function cancelOrdersUpTo(uint256 salt)
+        external;
+
+/*
+    /// @dev Checks if rounding error > 0.1%.
+    /// @param numerator Numerator.
+    /// @param denominator Denominator.
+    /// @param target Value to multiply with numerator/denominator.
+    /// @return Rounding error is present.
+    function isRoundingError(uint256 numerator, uint256 denominator, uint256 target)
+        public pure
+        returns (bool isError); */
 }
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MMatchOrders.sol b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MMatchOrders.sol
new file mode 100644
index 000000000..abd15182e
--- /dev/null
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MMatchOrders.sol
@@ -0,0 +1,106 @@
+/*
+
+  Copyright 2018 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+pragma solidity ^0.4.21;
+pragma experimental ABIEncoderV2;
+
+import "../libs/LibOrder.sol";
+import "../libs/LibFillResults.sol";
+import "./MExchangeCore.sol";
+
+contract MMatchOrders {
+
+    struct MatchedFillResults {
+        LibFillResults.FillResults left;
+        LibFillResults.FillResults right;
+    }
+
+    /// This struct exists solely to avoid the stack limit constraint
+    /// in matchOrders
+    struct OrderInfo {
+        uint8 orderStatus;
+        bytes32 orderHash;
+        uint256 orderFilledAmount;
+    }
+
+    /// @dev Validates context for matchOrders. Succeeds or throws.
+    /// @param leftOrder First order to match.
+    /// @param rightOrder Second order to match.
+    function validateMatchOrdersContextOrRevert(
+        LibOrder.Order memory leftOrder,
+        LibOrder.Order memory rightOrder)
+        internal;
+
+    /// @dev Validates matched fill results. Succeeds or throws.
+    /// @param matchedFillResults Amounts to fill and fees to pay by maker and taker of matched orders.
+    function validateMatchedOrderFillResultsOrThrow(MatchedFillResults memory matchedFillResults)
+        internal;
+
+    /// @dev Calculates partial value given a numerator and denominator.
+    ///      Throws if there is a rounding error.
+    /// @param numerator Numerator.
+    /// @param denominator Denominator.
+    /// @param target Value to calculate partial of.
+    /// @return Partial value of target.
+    function safeGetPartialAmount(
+        uint256 numerator,
+        uint256 denominator,
+        uint256 target)
+        internal pure
+        returns (uint256 partialAmount);
+
+    /// @dev Calculates fill amounts for the matched orders.
+    ///      Each order is filled at their respective price point. However, the calculations are
+    ///      carried out as though the orders are both being filled at the right order's price point.
+    ///      The profit made by the leftOrder order goes to the taker (who matched the two orders).
+    /// @param leftOrder First order to match.
+    /// @param rightOrder Second order to match.
+    /// @param leftOrderStatus Order status of left order.
+    /// @param rightOrderStatus Order status of right order.
+    /// @param leftOrderFilledAmount Amount of left order already filled.
+    /// @param rightOrderFilledAmount Amount of right order already filled.
+    /// @return status Return status of calculating fill amounts. Returns Status.SUCCESS on success.
+    /// @param matchedFillResults Amounts to fill and fees to pay by maker and taker of matched orders.
+    function calculateMatchedFillResults(
+        LibOrder.Order memory leftOrder,
+        LibOrder.Order memory rightOrder,
+        uint8 leftOrderStatus,
+        uint8 rightOrderStatus,
+        uint256 leftOrderFilledAmount,
+        uint256 rightOrderFilledAmount)
+        internal
+        returns (
+            uint8 status,
+            MatchedFillResults memory matchedFillResults);
+
+    /// @dev Match two complementary orders that have a positive spread.
+    ///      Each order is filled at their respective price point. However, the calculations are
+    ///      carried out as though the orders are both being filled at the right order's price point.
+    ///      The profit made by the left order goes to the taker (who matched the two orders).
+    /// @param leftOrder First order to match.
+    /// @param rightOrder Second order to match.
+    /// @param leftSignature Proof that order was created by the left maker.
+    /// @param rightSignature Proof that order was created by the right maker.
+    /// @return matchedFillResults Amounts filled and fees paid by maker and taker of matched orders.
+    function matchOrders(
+        LibOrder.Order memory leftOrder,
+        LibOrder.Order memory rightOrder,
+        bytes leftSignature,
+        bytes rightSignature)
+        public
+        returns (MatchedFillResults memory matchedFillResults);
+}
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MSettlement.sol b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MSettlement.sol
index 5e2edbf99..ac38ede95 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MSettlement.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MSettlement.sol
@@ -19,6 +19,7 @@
 pragma solidity ^0.4.23;
 
 import "../libs/LibOrder.sol";
+import "./MMatchOrders.sol";
 
 contract MSettlement {
 
@@ -37,5 +38,16 @@ contract MSettlement {
             uint256 makerFeePaid,
             uint256 takerFeePaid
         );
-}
 
+    /// @dev Settles matched order by transferring appropriate funds between order makers, taker, and fee recipient.
+    /// @param leftOrder First matched order.
+    /// @param rightOrder Second matched order.
+    /// @param matchedFillResults Struct holding amounts to transfer between makers, taker, and fee recipients.
+    /// @param takerAddress Address that matched the orders. The taker receives the spread between orders as profit.
+    function settleMatchedOrders(
+        LibOrder.Order memory leftOrder,
+        LibOrder.Order memory rightOrder,
+        MMatchOrders.MatchedFillResults memory matchedFillResults,
+        address takerAddress)
+        internal;
+}
diff --git a/packages/contracts/src/contracts/current/test/TestSignatureValidator/TestSignatureValidator.sol b/packages/contracts/src/contracts/current/test/TestSignatureValidator/TestSignatureValidator.sol
index 20202cd7b..44565b361 100644
--- a/packages/contracts/src/contracts/current/test/TestSignatureValidator/TestSignatureValidator.sol
+++ b/packages/contracts/src/contracts/current/test/TestSignatureValidator/TestSignatureValidator.sol
@@ -38,4 +38,4 @@ contract TestSignatureValidator is MixinSignatureValidator {
         );
         return isValid;
     }
-}
\ No newline at end of file
+}