LibMem + TestLibMem + LibAssetProxyDecoder + DummyERC721Receiver

7 files changed, 579 insertions, 27 deletions

diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC20Proxy.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC20Proxy.sol
index 2c321e134..017f94b1a 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC20Proxy.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC20Proxy.sol
@@ -20,12 +20,14 @@ pragma solidity ^0.4.24;
 pragma experimental ABIEncoderV2;
 
 import "../../utils/LibBytes/LibBytes.sol";
+import "../../utils/LibAssetProxyDecoder/LibAssetProxyDecoder.sol";
 import "./MixinAssetProxy.sol";
 import "./MixinAuthorizable.sol";
 import "../../tokens/ERC20Token/IERC20Token.sol";
 
 contract ERC20Proxy is
     LibBytes,
+    LibAssetProxyDecoder,
     MixinAssetProxy,
     MixinAuthorizable
 {
@@ -34,34 +36,32 @@ contract ERC20Proxy is
     uint8 constant PROXY_ID = 1;
 
     /// @dev Internal version of `transferFrom`.
-    /// @param assetMetadata Encoded byte array.
+    /// @param proxyData Encoded byte array.
     /// @param from Address to transfer asset from.
     /// @param to Address to transfer asset to.
     /// @param amount Amount of asset to transfer.
     function transferFromInternal(
-        bytes memory assetMetadata,
+        bytes memory proxyData,
         address from,
         address to,
         uint256 amount
     )
         internal
     {
+        // Decode proxy data.
+        (
+            uint8 proxyId,
+            address token
+        ) = decodeERC20Data(proxyData);
+
         // Data must be intended for this proxy.
         uint256 length = assetMetadata.length;
 
         require(
-            length == 21,
-            LENGTH_21_REQUIRED
-        );
-        // TODO: Is this too inflexible in the future?
-        require(
-            uint8(assetMetadata[length - 1]) == PROXY_ID,
-            ASSET_PROXY_ID_MISMATCH
+            proxyId == PROXY_ID,
+            PROXY_ID_MISMATCH
         );
 
-        // Decode metadata.
-        address token = readAddress(assetMetadata, 0);
-
         // Transfer tokens.
         bool success = IERC20Token(token).transferFrom(from, to, amount);
         require(
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC721Proxy.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC721Proxy.sol
index 07e01c774..f35e48eee 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC721Proxy.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC721Proxy.sol
@@ -20,12 +20,14 @@ pragma solidity ^0.4.24;
 pragma experimental ABIEncoderV2;
 
 import "../../utils/LibBytes/LibBytes.sol";
+import "../../utils/LibAssetProxyDecoder/LibAssetProxyDecoder.sol";
 import "./MixinAssetProxy.sol";
 import "./MixinAuthorizable.sol";
 import "../../tokens/ERC721Token/ERC721Token.sol";
 
 contract ERC721Proxy is
     LibBytes,
+    LibAssetProxyDecoder,
     MixinAssetProxy,
     MixinAuthorizable
 {
@@ -33,19 +35,29 @@ contract ERC721Proxy is
     // Id of this proxy.
     uint8 constant PROXY_ID = 2;
 
+    string constant PROXY_ID_MISMATCH = "Proxy id in metadata does not match this proxy id.";
+
     /// @dev Internal version of `transferFrom`.
-    /// @param assetMetadata Encoded byte array.
+    /// @param proxyData Encoded byte array.
     /// @param from Address to transfer asset from.
     /// @param to Address to transfer asset to.
     /// @param amount Amount of asset to transfer.
     function transferFromInternal(
-        bytes memory assetMetadata,
+        bytes memory proxyData,
         address from,
         address to,
         uint256 amount
     )
         internal
     {
+        // Decode proxy data.
+        (
+            uint8 proxyId,
+            address token,
+            uint256 tokenId,
+            bytes memory data
+        ) = decodeERC721Data(proxyData);
+
         // Data must be intended for this proxy.
         uint256 length = assetMetadata.length;
 
@@ -56,8 +68,8 @@ contract ERC721Proxy is
 
         // TODO: Is this too inflexible in the future?
         require(
-            uint8(assetMetadata[length - 1]) == PROXY_ID,
-            ASSET_PROXY_ID_MISMATCH
+            proxyId == PROXY_ID,
+            PROXY_ID_MISMATCH
         );
 
         // There exists only 1 of each token.
@@ -66,15 +78,9 @@ contract ERC721Proxy is
             INVALID_AMOUNT
         );
 
-        // Decode metadata
-        address token = readAddress(assetMetadata, 0);
-        uint256 tokenId = readUint256(assetMetadata, 20);
-
         // Transfer token.
         // Either succeeds or throws.
-        // @TODO: Call safeTransferFrom if there is additional
-        //        data stored in `assetMetadata`.
-        ERC721Token(token).transferFrom(from, to, tokenId);
+        ERC721Token(token).safeTransferFrom(from, to, tokenId, data);
     }
 
     /// @dev Gets the proxy id associated with the proxy address.
diff --git a/packages/contracts/src/contracts/current/test/DummyERC721Receiver/DummyERC721Receiver.sol b/packages/contracts/src/contracts/current/test/DummyERC721Receiver/DummyERC721Receiver.sol
new file mode 100644
index 000000000..1596f3357
--- /dev/null
+++ b/packages/contracts/src/contracts/current/test/DummyERC721Receiver/DummyERC721Receiver.sol
@@ -0,0 +1,62 @@
+/*
+The MIT License (MIT)
+
+Copyright (c) 2016 Smart Contract Solutions, Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be included
+in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+*/
+
+pragma solidity ^0.4.24;
+
+import "../../tokens/ERC721Token/IERC721Receiver.sol";
+
+contract DummyERC721Receiver is
+    IERC721Receiver
+{
+
+    event TokenReceived(
+        address from,
+        uint256 tokenId,
+        bytes data
+    );
+
+    /**
+    * @notice Handle the receipt of an NFT
+    * @dev The ERC721 smart contract calls this function on the recipient
+    *  after a `safetransfer`. This function MAY throw to revert and reject the
+    *  transfer. This function MUST use 50,000 gas or less. Return of other
+    *  than the magic value MUST result in the transaction being reverted.
+    *  Note: the contract address is always the message sender.
+    * @param _from The sending address
+    * @param _tokenId The NFT identifier which is being transfered
+    * @param _data Additional data with no specified format
+    * @return `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`
+    */
+    function onERC721Received(
+        address _from,
+        uint256 _tokenId,
+        bytes _data)
+        public
+        returns (bytes4)
+    {
+        emit TokenReceived(_from, _tokenId, _data);
+        return ERC721_RECEIVED;
+    }
+}
diff --git a/packages/contracts/src/contracts/current/test/TestLibMem/TestLibMem.sol b/packages/contracts/src/contracts/current/test/TestLibMem/TestLibMem.sol
new file mode 100644
index 000000000..4cf62bf3a
--- /dev/null
+++ b/packages/contracts/src/contracts/current/test/TestLibMem/TestLibMem.sol
@@ -0,0 +1,238 @@
+/*
+
+  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 ^0.4.24;
+
+import "../../utils/LibMem/LibMem.sol";
+import "../../utils/LibBytes/LibBytes.sol";
+
+contract TestLibMem is
+    LibMem,
+    LibBytes
+{
+
+    function test1()
+        public
+        pure
+    {
+        // Length of array & length to copy
+        uint256 length = 0;
+
+        // Create source array
+        bytes memory sourceArray = new bytes(length);
+
+        // Create dest array with same contents as source array
+        bytes memory destArray = new bytes(length);
+        memcpy(
+            getMemAddress(destArray) + 32,   // skip copying array length
+            getMemAddress(sourceArray) + 32, // skip copying array length
+            length
+        );
+
+        // Verify contents of source & dest arrays match
+        require(
+            areBytesEqual(sourceArray, destArray),
+            "Test #1 failed. Array contents are not the same."
+        );
+    }
+
+    function test2()
+        public
+        pure
+    {
+        // Length of array & length to copy
+        uint256 length = 1;
+
+        // Create source array
+        bytes memory sourceArray = new bytes(length);
+        sourceArray[0] = byte(1);
+
+        // Create dest array with same contents as source array
+        bytes memory destArray = new bytes(length);
+        memcpy(
+            getMemAddress(destArray) + 32,   // skip copying array length
+            getMemAddress(sourceArray) + 32, // skip copying array length
+            length
+        );
+
+        // Verify contents of source & dest arrays match
+        require(
+            areBytesEqual(sourceArray, destArray),
+            "Test #2 failed. Array contents are not the same."
+        );
+    }
+
+    function test3()
+        public
+        pure
+    {
+        // Length of array & length to copy
+        uint256 length = 11;
+
+        // Create source array
+        bytes memory sourceArray = new bytes(length);
+        for(uint256 i = 0; i < length; ++i) {
+            sourceArray[i] = byte((i % 0xF) + 1); // [1..f]
+        }
+
+        // Create dest array with same contents as source array
+        bytes memory destArray = new bytes(length);
+        memcpy(
+            getMemAddress(destArray) + 32,   // skip copying array length
+            getMemAddress(sourceArray) + 32, // skip copying array length
+            length
+        );
+
+        // Verify contents of source & dest arrays match
+        require(
+            areBytesEqual(sourceArray, destArray),
+            "Test #3 failed. Array contents are not the same."
+        );
+    }
+
+    function test4()
+        public
+        pure
+    {
+        // Length of array & length to copy
+        uint256 length = 32;
+
+        // Create source array
+        bytes memory sourceArray = new bytes(length);
+        for(uint256 i = 0; i < length; ++i) {
+            sourceArray[i] = byte((i % 0xF) + 1); // [1..f]
+        }
+
+        // Create dest array with same contents as source array
+        bytes memory destArray = new bytes(length);
+        memcpy(
+            getMemAddress(destArray) + 32,   // skip copying array length
+            getMemAddress(sourceArray) + 32, // skip copying array length
+            length
+        );
+
+        // Verify contents of source & dest arrays match
+        require(
+            areBytesEqual(sourceArray, destArray),
+            "Test #4 failed. Array contents are not the same."
+        );
+    }
+
+    function test5()
+        public
+        pure
+    {
+        // Length of array & length to copy
+        uint256 length = 72;
+
+        // Create source array
+        bytes memory sourceArray = new bytes(length);
+        for(uint256 i = 0; i < length; ++i) {
+            sourceArray[i] = byte((i % 0xF) + 1); // [1..f]
+        }
+
+        // Create dest array with same contents as source array
+        bytes memory destArray = new bytes(length);
+        memcpy(
+            getMemAddress(destArray) + 32,   // skip copying array length
+            getMemAddress(sourceArray) + 32, // skip copying array length
+            length
+        );
+
+        // Verify contents of source & dest arrays match
+        require(
+            areBytesEqual(sourceArray, destArray),
+            "Test #5 failed. Array contents are not the same."
+        );
+    }
+
+
+    function test6()
+        public
+        pure
+    {
+        // Length of arrays
+        uint256 length1 = 72;
+        uint256 length2 = 100;
+
+        // The full source array is used for comparisons at the end
+        bytes memory fullSourceArray = new bytes(length1 + length2);
+
+        // First source array
+        bytes memory sourceArray1 = new bytes(length1);
+        for(uint256 i = 0; i < length1; ++i) {
+            sourceArray1[i] = byte((i % 0xF) + 1); // [1..f]
+            fullSourceArray[i] = byte((i % 0xF) + 1); // [1..f]
+        }
+
+        // Second source array
+        bytes memory sourceArray2 = new bytes(length2);
+        for(uint256 j = 0; i < length2; ++i) {
+            sourceArray2[j] = byte((j % 0xF) + 1); // [1..f]
+            fullSourceArray[length1+j] = byte((j % 0xF) + 1); // [1..f]
+        }
+
+        // Create dest array with same contents as source arrays
+        bytes memory destArray = new bytes(length1 + length2);
+        memcpy(
+            getMemAddress(destArray) + 32,    // skip copying array length
+            getMemAddress(sourceArray1) + 32, // skip copying array length
+            length1
+        );
+        memcpy(
+            getMemAddress(destArray) + 32 + length1,   // skip copying array length + sourceArray1 bytes
+            getMemAddress(sourceArray2) + 32,          // skip copying array length
+            length2
+        );
+
+        // Verify contents of source & dest arrays match
+        require(
+            areBytesEqual(fullSourceArray, destArray),
+            "Test #6 failed. Array contents are not the same."
+        );
+    }
+
+    function test7()
+        public
+        pure
+    {
+        // Length of array & length to copy
+        uint256 length = 72;
+
+        // Create source array
+        bytes memory sourceArray = new bytes(length);
+        for(uint256 i = 0; i < length; ++i) {
+            sourceArray[i] = byte((i % 0xF) + 1); // [1..f]
+        }
+
+        // Create dest array with same contents as source array
+        bytes memory destArray = new bytes(length);
+        memcpy(
+            getMemAddress(destArray) + 32,   // skip copying array length
+            getMemAddress(sourceArray) + 32, // skip copying array length
+            length - 8                       // Copy all but last byte.
+        );
+
+        // Verify contents of source & dest arrays match
+        // We expect this to fail
+        require(
+            areBytesEqual(sourceArray, destArray),
+            "Test #7 failed. Array contents are not the same."
+        );
+    }
+}
diff --git a/packages/contracts/src/contracts/current/utils/LibAssetProxyDecoder/LibAssetProxyDecoder.sol b/packages/contracts/src/contracts/current/utils/LibAssetProxyDecoder/LibAssetProxyDecoder.sol
new file mode 100644
index 000000000..ba53f2769
--- /dev/null
+++ b/packages/contracts/src/contracts/current/utils/LibAssetProxyDecoder/LibAssetProxyDecoder.sol
@@ -0,0 +1,74 @@
+/*
+
+  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 ^0.4.24;
+pragma experimental ABIEncoderV2;
+
+import "../LibBytes/LibBytes.sol";
+
+contract LibAssetProxyDecoder is
+    LibBytes
+{
+
+    string constant INVALID_ERC20_METADATA_LENGTH = "Metadata must have a length of 21.";
+    string constant INVALID_ERC721_METADATA_LENGTH = "Metadata must have a length of at least 53.";
+
+    /// @dev Decodes ERC721 Asset Proxy data
+    function decodeERC20Data(bytes memory proxyData)
+        internal
+        pure
+        returns (
+            uint8 proxyId,
+            address token
+        )
+    {
+        require(
+            proxyData.length == 21,
+            INVALID_ERC20_METADATA_LENGTH
+        );
+        proxyId = uint8(proxyData[0]);
+        token = readAddress(proxyData, 1);
+
+        return (proxyId, token);
+    }
+
+    /// @dev Decodes ERC721 Asset Proxy data
+    function decodeERC721Data(bytes memory proxyData)
+        internal
+        pure
+        returns (
+            uint8 proxyId,
+            address token,
+            uint256 tokenId,
+            bytes memory data
+        )
+    {
+        require(
+            proxyData.length >= 53,
+            INVALID_ERC721_METADATA_LENGTH
+        );
+        proxyId = uint8(proxyData[0]);
+        token = readAddress(proxyData, 1);
+        tokenId = readUint256(proxyData, 21);
+        if (proxyData.length > 53) {
+            data = readBytes(proxyData, 53);
+        }
+
+        return (proxyId, token, tokenId, data);
+    }
+}
diff --git a/packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol b/packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol
index df2221c93..fb8359462 100644
--- a/packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol
+++ b/packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol
@@ -18,7 +18,11 @@
 
 pragma solidity ^0.4.24;
 
-contract LibBytes {
+import "../LibMem/LibMem.sol";
+
+contract LibBytes is
+    LibMem
+{
 
     // Revert reasons
     string constant GT_ZERO_LENGTH_REQUIRED = "Length must be greater than 0.";
@@ -42,7 +46,7 @@ contract LibBytes {
 
         // Store last byte.
         result = b[b.length - 1];
-        
+
         assembly {
             // Decrement length of byte array.
             let newLen := sub(mload(b), 1)
@@ -125,7 +129,7 @@ contract LibBytes {
         require(
             b.length >= index + 20,  // 20 is length of address
             GTE_20_LENGTH_REQUIRED
-        ); 
+        );
 
         // Add offset to index:
         // 1. Arrays are prefixed by 32-byte length parameter (add 32 to index)
@@ -157,7 +161,7 @@ contract LibBytes {
         require(
             b.length >= index + 20,  // 20 is length of address
             GTE_20_LENGTH_REQUIRED
-        ); 
+        );
 
         // Add offset to index:
         // 1. Arrays are prefixed by 32-byte length parameter (add 32 to index)
@@ -264,6 +268,7 @@ contract LibBytes {
         writeBytes32(b, index, bytes32(input));
     }
 
+=======
     /// @dev Reads the first 4 bytes from a byte array of arbitrary length.
     /// @param b Byte array to read first 4 bytes from.
     /// @return First 4 bytes of data.
@@ -281,4 +286,67 @@ contract LibBytes {
         }
         return result;
     }
+
+    /// @dev Reads a uint256 value from a position in a byte array.
+    /// @param b Byte array containing a uint256 value.
+    /// @param index Index in byte array of uint256 value.
+    /// @return uint256 value from byte array.
+    function readBytes(
+        bytes memory b,
+        uint256 index
+    )
+        internal
+        pure
+        returns (bytes memory result)
+    {
+        // Read length of nested bytes
+        require(
+            b.length >= index + 32,
+            GTE_32_LENGTH_REQUIRED
+        );
+        uint256 nestedBytesLength = readUint256(b, index);
+
+        // Assert length of <b> is valid, given
+        // length of nested bytes
+        require(
+            b.length >= index + 32 + nestedBytesLength,
+            GTE_32_LENGTH_REQUIRED
+        );
+
+        // Allocate memory and copy value to result
+        result = new bytes(nestedBytesLength);
+        memcpy(
+            getMemAddress(result) + 32,    // +32 skips array length
+            getMemAddress(b) + index + 32, // +32 skips array length
+            nestedBytesLength
+        );
+
+        return result;
+    }
+
+    /// @dev Writes a uint256 into a specific position in a byte array.
+    /// @param b Byte array to insert <input> into.
+    /// @param index Index in byte array of <input>.
+    /// @param input uint256 to put into byte array.
+    function writeBytes(
+        bytes memory b,
+        uint256 index,
+        bytes memory input
+    )
+        internal
+        pure
+    {
+        // Read length of nested bytes
+        require(
+            b.length >= index + 32 /* 32 bytes to store length */ + input.length,
+            GTE_32_LENGTH_REQUIRED
+        );
+
+        // Copy <input> into <b>
+        memcpy(
+            getMemAddress(b) + index,
+            getMemAddress(input),
+            input.length + 32 /* 32 bytes to store length */
+        );
+    }
 }
diff --git a/packages/contracts/src/contracts/current/utils/LibMem/LibMem.sol b/packages/contracts/src/contracts/current/utils/LibMem/LibMem.sol
new file mode 100644
index 000000000..b07a5da54
--- /dev/null
+++ b/packages/contracts/src/contracts/current/utils/LibMem/LibMem.sol
@@ -0,0 +1,104 @@
+/*
+
+  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 ^0.4.24;
+
+contract LibMem {
+
+    function getMemAddress(bytes memory input)
+        internal
+        pure
+        returns (uint256 address_)
+    {
+        assembly {
+            address_ := input
+        }
+        return address_;
+    }
+
+    /// @dev Writes a uint256 into a specific position in a byte array.
+    /// @param dest memory adress to copy bytes to
+    function memcpy(
+        uint256 dest,
+        uint256 source,
+        uint256 length
+    )
+        internal
+        pure
+    {
+        // Base cases
+        if(length == 0) return;
+        if(source == dest) return;
+
+        // Copy bytes from source to dest
+        assembly {
+            // Compute number of complete words to copy + remaining bytes
+            let lenFullWords := div(add(length, 0x1F), 0x20)
+            let remainder := mod(length, 0x20)
+            if gt(remainder, 0) {
+                lenFullWords := sub(lenFullWords, 1)
+            }
+
+            // Copy full words from source to dest
+            let offset := 0
+            let maxOffset := mul(0x20, lenFullWords)
+            for {offset := 0} lt(offset, maxOffset) {offset := add(offset, 0x20)} {
+                mstore(add(dest, offset),  mload(add(source, offset)))
+            }
+
+            // Copy remaining bytes
+            if gt(remainder, 0) {
+                // Read a full word from source, containing X bytes to copy to dest.
+                // We only want to keep the X bytes, zeroing out the remaining bytes.
+                // We accomplish this by a right shift followed by a left shift.
+                // Example:
+                //   Suppose a word of 8 bits has all 1's: [11111111]
+                //   Let X = 7 (we want to copy the first 7 bits)
+                //   Apply a right shift of 1: [01111111]
+                //   Apply a left shift of 1: [11111110]
+                let sourceShiftFactor := exp(2, mul(8, sub(0x20, remainder)))
+                let sourceWord := mload(add(source, offset))
+                let sourceBytes := mul(div(sourceWord, sourceShiftFactor), sourceShiftFactor)
+
+                // Read a full word from dest, containing (32-X) bytes to retain.
+                // We need to zero out the remaining bytes to be overwritten by source,
+                // while retaining the (32-X) bytes we don't want to overwrite.
+                // We accomplish this by a left shift followed by a right shift.
+                // Example:
+                //   Suppose a word of 8 bits has all 1's: [11111111]
+                //   Let X = 7 (we want to free the first 7 bits, and retain the last bit)
+                //   Apply a left shift of 1: [11111110]
+                //   Apply a right shift of 1: [01111111]
+                let destShiftFactor := exp(2, mul(8, remainder))
+                let destWord := mload(add(dest, offset))
+                let destBytes := div(mul(destWord, destShiftFactor), destShiftFactor)
+
+                // Combine the source and dest bytes. There should be no overlap:
+                // The source bytes run from [0..X-1] and the dest bytes from [X..31].
+                // Example:
+                //   Following the example from above, we have [11111110]
+                //   from the source word and [01111111] from the dest word.
+                //   Combine these words using <or> to get [11111111].
+                let combinedDestWord := or(sourceBytes, destBytes)
+
+                // Store the combined word into dest
+                mstore(add(dest, offset), combinedDestWord)
+            }
+        }
+    }
+}