Add further optmization to coreIssuanceOrder, remove slice

contracts/core/exchange-wrappers/lib/ZeroExOrderDataHandler.sol

@@ -44,8 +44,8 @@ import { LibOrder } from "../../../external/0x/Exchange/libs/LibOrder.sol";
  * |         | order                 | 160+signatureLength | orderLength     | ZeroEx Order                  |
  */
 library ZeroExOrderDataHandler {
-    using SafeMath for uint256;
     using LibBytes for bytes;
+    using SafeMath for uint256;
 
     // ============ Constants ============
 

contracts/core/extensions/CoreIssuanceOrder.sol

@@ -48,10 +48,6 @@ contract CoreIssuanceOrder is
     using SafeMath for uint256;
     using Math for uint256;
 
-    /* ============ Constants ============ */
-
-    uint256 constant EXCHANGE_HEADER_LENGTH = 160;
-
     /* ============ Events ============ */
 
     event LogFill(
@@ -256,14 +252,11 @@ contract CoreIssuanceOrder is
         uint256 scannedBytes;
         uint256 makerTokenUsed;
         while (scannedBytes < _orderData.length) {
-            // Read and parse the next exchange order header
-            bytes memory headerData = LibBytes.slice(
-                _orderData,
-                scannedBytes,
-                scannedBytes.add(EXCHANGE_HEADER_LENGTH)
-            );
+
+            // Parse next exchange header based on scannedBytes
             ExchangeHandler.ExchangeHeader memory header = ExchangeHandler.parseExchangeHeader(
-                headerData
+                _orderData,
+                scannedBytes
             );
 
             // Get exchange address from state mapping based on header exchange info
@@ -273,10 +266,10 @@ contract CoreIssuanceOrder is
             require(exchange != address(0));
 
             // Read the order body based on header order length info
-            uint256 exchangeDataLength = header.totalOrdersLength.add(EXCHANGE_HEADER_LENGTH);
+            uint256 exchangeDataLength = header.totalOrdersLength.add(160);
             bytes memory bodyData = LibBytes.slice(
                 _orderData,
-                scannedBytes.add(EXCHANGE_HEADER_LENGTH),
+                scannedBytes.add(160),
                 scannedBytes.add(exchangeDataLength)
             );
 
@@ -307,7 +300,7 @@ contract CoreIssuanceOrder is
 
             // Update scanned bytes with header and body lengths
             scannedBytes = scannedBytes.add(exchangeDataLength);
-            makerTokenUsed += header.makerTokenAmount;
+            makerTokenUsed = makerTokenUsed.add(header.makerTokenAmount);
         }
 
         return makerTokenUsed;

contracts/core/lib/ExchangeHandler.sol

@@ -17,6 +17,9 @@
 pragma solidity 0.4.24;
 pragma experimental "ABIEncoderV2";
 
+import { LibBytes } from "../../external/0x/LibBytes.sol";
+import { SafeMath } from "zeppelin-solidity/contracts/math/SafeMath.sol";
+
 
 /**
  * @title ExchangeHandler
@@ -25,6 +28,8 @@ pragma experimental "ABIEncoderV2";
  * This library contains functions and structs to assist with parsing exchange orders data
  */
 library ExchangeHandler {
+    using LibBytes for bytes;
+    using SafeMath for uint256;
 
     // ============ Structs ============
 
@@ -45,21 +50,23 @@ library ExchangeHandler {
      * @return ExchangeHeader  Struct containing data for a batch of exchange orders
      */
     function parseExchangeHeader(
-        bytes _headerData
+        bytes _headerData,
+        uint256 _offset
     )
         internal
         pure
         returns (ExchangeHeader memory)
     {
         ExchangeHeader memory header;
 
-        // Create ExchangeHeader struct
+        uint256 headerDataStart = _headerData.contentAddress().add(_offset);
+
         assembly {
-            mstore(header,          mload(add(_headerData, 32)))   // exchange
-            mstore(add(header, 32), mload(add(_headerData, 64)))   // orderCount
-            mstore(add(header, 64), mload(add(_headerData, 96)))   // makerTokenAddress
-            mstore(add(header, 96), mload(add(_headerData, 128)))  // makerTokenAmount
-            mstore(add(header, 128), mload(add(_headerData, 160))) // totalOrdersLength
+            mstore(header,          mload(headerDataStart))            // exchange
+            mstore(add(header, 32), mload(add(headerDataStart, 32)))   // orderCount
+            mstore(add(header, 64), mload(add(headerDataStart, 64)))   // makerTokenAddress
+            mstore(add(header, 96), mload(add(headerDataStart, 96)))   // makerTokenAmount
+            mstore(add(header, 128), mload(add(headerDataStart, 128))) // totalOrdersLength
         }
 
         return header;