Update UniswapV2IndexExchangeAdapter to support two-hop trades (#110)

* update adapter and tests

* add integration test

* improve comments

* review changes

Author: ncitron

contracts/protocol/integration/index-exchange/UniswapV2IndexExchangeAdapter.sol

@@ -21,12 +21,16 @@ pragma experimental "ABIEncoderV2";
 
 import { IIndexExchangeAdapter } from "../../../interfaces/IIndexExchangeAdapter.sol";
 
+
 /**
  * @title UniswapV2IndexExchangeAdapter
  * @author Set Protocol
  *
  * A Uniswap Router02 exchange adapter that returns calldata for trading with GeneralIndexModule, allows encoding a trade with a fixed input quantity or
  * a fixed output quantity.
+ *
+ * CHANGELOG: 7/8/2021
+ * - Update getTradeCalldata to allow for the intermediate token of the path to be passed in through the _data parameter
  */
 contract UniswapV2IndexExchangeAdapter is IIndexExchangeAdapter {
 
@@ -65,6 +69,8 @@ contract UniswapV2IndexExchangeAdapter is IIndexExchangeAdapter {
      * @param  _isSendTokenFixed         Boolean indicating if the send quantity is fixed, used to determine correct trade interface
      * @param  _sourceQuantity           Fixed/Max amount of source token to sell
      * @param  _destinationQuantity      Min/Fixed amount of destination token to buy
+     * @param  _data                     Encoded address intermediary token in the trade path. If empty, path is the input and output tokens. 
+                                         Note: only allows one intermediary asset
      *
      * @return address                   Target contract address
      * @return uint256                   Call value
@@ -77,16 +83,26 @@ contract UniswapV2IndexExchangeAdapter is IIndexExchangeAdapter {
         bool _isSendTokenFixed,
         uint256 _sourceQuantity,
         uint256 _destinationQuantity,
-        bytes memory /*_data*/
+        bytes memory _data
     )
         external
         view
         override
         returns (address, uint256, bytes memory)
     {
-        address[] memory path = new address[](2);
-        path[0] = _sourceToken;
-        path[1] = _destinationToken;
+        address[] memory path;
+
+        if (_data.length == 0) {
+            path = new address[](2);
+            path[0] = _sourceToken;
+            path[1] = _destinationToken;
+        } else {
+            address intermediateToken = abi.decode(_data, (address));
+            path = new address[](3);
+            path[0] = _sourceToken;
+            path[1] = intermediateToken;
+            path[2] = _destinationToken;
+        }
 
         bytes memory callData = abi.encodeWithSignature(
             _isSendTokenFixed ? SWAP_EXACT_TOKENS_FOR_TOKENS : SWAP_TOKENS_FOR_EXACT_TOKENS,