Optimize ERC721Token

5 files changed, 462 insertions, 510 deletions

diff --git a/packages/contracts/src/2.0.0/test/DummyERC721Token/DummyERC721Token.sol b/packages/contracts/src/2.0.0/test/DummyERC721Token/DummyERC721Token.sol
index 627746a52..7c64005bd 100644
--- a/packages/contracts/src/2.0.0/test/DummyERC721Token/DummyERC721Token.sol
+++ b/packages/contracts/src/2.0.0/test/DummyERC721Token/DummyERC721Token.sol
@@ -18,59 +18,35 @@
 
 pragma solidity 0.4.24;
 
-import "../../tokens/ERC721Token/ERC721Token.sol";
+import "../../tokens/ERC721Token/MintableERC721Token.sol";
 import "../../utils/Ownable/Ownable.sol";
 
 
 // solhint-disable no-empty-blocks
 contract DummyERC721Token is
     Ownable,
-    ERC721Token
+    MintableERC721Token
 {
 
-    /**
-    * @dev Constructor passes its arguments to the base ERC721Token constructor
-    * @param name of token
-    * @param symbol of token
-    */
-    constructor (
-        string name,
-        string symbol
-    )
-        public
-        ERC721Token(name, symbol)
-    {}
-
-    /**
-    * @dev Function to mint a new token
-    * @dev Reverts if the given token ID already exists
-    * @param to address the beneficiary that will own the minted token
-    * @param tokenId uint256 ID of the token to be minted by the msg.sender
-    */
-    function mint(address to, uint256 tokenId)
-        public
+    /// @dev Function to mint a new token
+    ///      Reverts if the given token ID already exists
+    /// @param _to Address of the beneficiary that will own the minted token
+    /// @param _tokenId ID of the token to be minted by the msg.sender    
+    function mint(address _to, uint256 _tokenId)
+        external
         onlyOwner
     {
-        require(
-            !exists(tokenId),
-            "Token with tokenId already exists."
-        );
-        _mint(to, tokenId);
+        _mint(_to, _tokenId);
     }
 
-    /**
-    * @dev Function to burn a token
-    * @dev Reverts if the given token ID doesn't exist
-    * @param tokenId uint256 ID of the token to be minted by the msg.sender
-    */
-    function burn(address owner, uint256 tokenId)
-        public
+    /// @dev Function to burn a token
+    ///      Reverts if the given token ID doesn't exist
+    /// @param _owner Owner of token with given token ID
+    /// @param _tokenId ID of the token to be burned by the msg.sender
+    function burn(address _owner, uint256 _tokenId)
+        external
         onlyOwner
     {
-        require(
-            exists(tokenId),
-            "Token with tokenId does not exist."
-        );
-        _burn(owner, tokenId);
+        _burn(_owner, _tokenId);
     }
 }
diff --git a/packages/contracts/src/2.0.0/tokens/ERC721Token/ERC721Token.sol b/packages/contracts/src/2.0.0/tokens/ERC721Token/ERC721Token.sol
index 60603aa19..94e084731 100644
--- a/packages/contracts/src/2.0.0/tokens/ERC721Token/ERC721Token.sol
+++ b/packages/contracts/src/2.0.0/tokens/ERC721Token/ERC721Token.sol
@@ -1,26 +1,19 @@
 /*
-The MIT License (MIT)
 
-Copyright (c) 2016 Smart Contract Solutions, Inc.
+  Copyright 2018 ZeroEx Intl.
 
-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:
+  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
 
-The above copyright notice and this permission notice shall be included
-in all copies or substantial portions of the Software.
+    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.
 
-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;
@@ -30,179 +23,250 @@ import "./IERC721Receiver.sol";
 import "../../utils/SafeMath/SafeMath.sol";
 
 
-/**
- * @title ERC721 Non-Fungible Token Standard basic implementation
- * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
- * Modified from https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/token/ERC721/ERC721BasicToken.sol
- */
 contract ERC721Token is
     IERC721Token,
     SafeMath
 {
-    // Equals to `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`
-    // which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
-    bytes4 constant internal ERC721_RECEIVED = 0xf0b9e5ba;
+    // Function selector for ERC721Receiver.onERC721Received
+    // 0x150b7a02
+    bytes4 constant internal ERC721_RECEIVED = bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"));
 
-    // Mapping from token ID to owner
-    mapping (uint256 => address) internal tokenOwner;
+    // Mapping of tokenId => owner
+    mapping (uint256 => address) internal owners;
 
-    // Mapping from token ID to approved address
-    mapping (uint256 => address) internal tokenApprovals;
+    // Mapping of tokenId => approved address
+    mapping (uint256 => address) internal approvals;
 
-    // Mapping from owner to number of owned token
-    mapping (address => uint256) internal ownedTokensCount;
+    // Mapping of owner => number of tokens owned
+    mapping (address => uint256) internal balances;
 
-    // Mapping from owner to operator approvals
+    // Mapping of owner => operator => approved
     mapping (address => mapping (address => bool)) internal operatorApprovals;
 
-    /**
-    * @dev Guarantees msg.sender is owner of the given token
-    * @param _tokenId uint256 ID of the token to validate its ownership belongs to msg.sender
-    */
-    modifier onlyOwnerOf(uint256 _tokenId) {
-        require(ownerOf(_tokenId) == msg.sender);
-        _;
-    }
-
-    /**
-    * @dev Checks msg.sender can transfer a token, by being owner, approved, or operator
-    * @param _tokenId uint256 ID of the token to validate
-    */
-    modifier canTransfer(uint256 _tokenId) {
-        require(isApprovedOrOwner(msg.sender, _tokenId));
-        _;
+    /// @notice Transfers the ownership of an NFT from one address to another address
+    /// @dev Throws unless `msg.sender` is the current owner, an authorized
+    ///      perator, or the approved address for this NFT. Throws if `_from` is
+    ///      not the current owner. Throws if `_to` is the zero address. Throws if
+    ///      `_tokenId` is not a valid NFT. When transfer is complete, this function
+    ///      checks if `_to` is a smart contract (code size > 0). If so, it calls
+    ///      `onERC721Received` on `_to` and throws if the return value is not
+    ///      `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`.
+    /// @param _from The current owner of the NFT
+    /// @param _to The new owner
+    /// @param _tokenId The NFT to transfer
+    /// @param _data Additional data with no specified format, sent in call to `_to`
+    function safeTransferFrom(
+        address _from,
+        address _to,
+        uint256 _tokenId,
+        bytes _data
+    )
+        external
+    {
+        transferFrom(
+            _from,
+            _to,
+            _tokenId
+        );
+
+        uint256 receiverCodeSize;
+        assembly {
+            receiverCodeSize := extcodesize(_to)
+        }
+        if (receiverCodeSize > 0) {
+            bytes4 selector = IERC721Receiver(_to).onERC721Received(
+                msg.sender,
+                _from,
+                _tokenId,
+                _data
+            );
+            require(
+                selector == ERC721_RECEIVED,
+                "ERC721_INVALID_SELECTOR"
+            );
+        }
     }
 
-    constructor (
-        string _name,
-        string _symbol)
-        public
+    /// @notice Transfers the ownership of an NFT from one address to another address
+    /// @dev This works identically to the other function with an extra data parameter,
+    ///      except this function just sets data to "".
+    /// @param _from The current owner of the NFT
+    /// @param _to The new owner
+    /// @param _tokenId The NFT to transfer
+    function safeTransferFrom(
+        address _from,
+        address _to,
+        uint256 _tokenId
+    )
+        external
     {
-        name_ = _name;
-        symbol_ = _symbol;
+        transferFrom(
+            _from,
+            _to,
+            _tokenId
+        );
+
+        uint256 receiverCodeSize;
+        assembly {
+            receiverCodeSize := extcodesize(_to)
+        }
+        if (receiverCodeSize > 0) {
+            bytes4 selector = IERC721Receiver(_to).onERC721Received(
+                msg.sender,
+                _from,
+                _tokenId,
+                ""
+            );
+            require(
+                selector == ERC721_RECEIVED,
+                "ERC721_INVALID_SELECTOR"
+            );
+        }
     }
 
-    /**
-    * @dev Gets the token name
-    * @return string representing the token name
-    */
-    function name()
-        public
-        view
-        returns (string)
+    /// @notice Change or reaffirm the approved address for an NFT
+    /// @dev The zero address indicates there is no approved address.
+    ///      Throws unless `msg.sender` is the current NFT owner, or an authorized
+    ///      operator of the current owner.
+    /// @param _approved The new approved NFT controller
+    /// @param _tokenId The NFT to approve
+    function approve(address _approved, uint256 _tokenId)
+        external
     {
-        return name_;
+        address owner = ownerOf(_tokenId);
+        require(
+            msg.sender == owner || isApprovedForAll(owner, msg.sender),
+            "ERC721_INVALID_SENDER"
+        );
+
+        approvals[_tokenId] = _approved;
+        emit Approval(
+            owner,
+            _approved,
+            _tokenId
+        );
     }
 
-    /**
-    * @dev Gets the token symbol
-    * @return string representing the token symbol
-    */
-    function symbol()
-        public
-        view
-        returns (string)
+    /// @notice Enable or disable approval for a third party ("operator") to manage
+    ///         all of `msg.sender`'s assets
+    /// @dev Emits the ApprovalForAll event. The contract MUST allow
+    ///      multiple operators per owner.
+    /// @param _operator Address to add to the set of authorized operators
+    /// @param _approved True if the operator is approved, false to revoke approval
+    function setApprovalForAll(address _operator, bool _approved)
+        external
     {
-        return symbol_;
+        operatorApprovals[msg.sender][_operator] = _approved;
+        emit ApprovalForAll(
+            msg.sender,
+            _operator,
+            _approved
+        );
     }
-
-    /**
-    * @dev Gets the balance of the specified address
-    * @param _owner address to query the balance of
-    * @return uint256 representing the amount owned by the passed address
-    */
+    
+    /// @notice Count all NFTs assigned to an owner
+    /// @dev NFTs assigned to the zero address are considered invalid, and this
+    ///      function throws for queries about the zero address.
+    /// @param _owner An address for whom to query the balance
+    /// @return The number of NFTs owned by `_owner`, possibly zero
     function balanceOf(address _owner)
-        public
+        external
         view
         returns (uint256)
     {
-        require(_owner != address(0));
-        return ownedTokensCount[_owner];
+        require(
+            _owner != address(0),
+            "ERC721_ZERO_OWNER"
+        );
+        return balances[_owner];
     }
 
-    /**
-    * @dev Gets the owner of the specified token ID
-    * @param _tokenId uint256 ID of the token to query the owner of
-    * @return owner address currently marked as the owner of the given token ID
-    */
-    function ownerOf(uint256 _tokenId)
-        public
-        view
-        returns (address)
-    {
-        address owner = tokenOwner[_tokenId];
-        require(owner != address(0));
-        return owner;
-    }
-
-    /**
-    * @dev Returns whether the specified token exists
-    * @param _tokenId uint256 ID of the token to query the existance of
-    * @return whether the token exists
-    */
-    function exists(uint256 _tokenId)
+    /// @notice Transfer ownership of an NFT -- THE CALLER IS RESPONSIBLE
+    ///         TO CONFIRM THAT `_to` IS CAPABLE OF RECEIVING NFTS OR ELSE
+    ///         THEY MAY BE PERMANENTLY LOST
+    /// @dev Throws unless `msg.sender` is the current owner, an authorized
+    ///      operator, or the approved address for this NFT. Throws if `_from` is
+    ///      not the current owner. Throws if `_to` is the zero address. Throws if
+    ///      `_tokenId` is not a valid NFT.
+    /// @param _from The current owner of the NFT
+    /// @param _to The new owner
+    /// @param _tokenId The NFT to transfer
+    function transferFrom(
+        address _from,
+        address _to,
+        uint256 _tokenId
+    )
         public
-        view
-        returns (bool)
     {
-        address owner = tokenOwner[_tokenId];
-        return owner != address(0);
-    }
+        require(
+            _to != address(0),
+            "ERC721_ZERO_TO_ADDRESS"
+        );
 
-    /**
-    * @dev Approves another address to transfer the given token ID
-    * @dev The zero address indicates there is no approved address.
-    * @dev There can only be one approved address per token at a given time.
-    * @dev Can only be called by the token owner or an approved operator.
-    * @param _to address to be approved for the given token ID
-    * @param _tokenId uint256 ID of the token to be approved
-    */
-    function approve(address _to, uint256 _tokenId)
-        public
-    {
         address owner = ownerOf(_tokenId);
-        require(_to != owner);
-        require(msg.sender == owner || isApprovedForAll(owner, msg.sender));
-
-        if (getApproved(_tokenId) != address(0) || _to != address(0)) {
-            tokenApprovals[_tokenId] = _to;
-            emit Approval(owner, _to, _tokenId);
+        require(
+            _from == owner,
+            "ERC721_OWNER_MISMATCH"
+        );
+
+        address spender = msg.sender;
+        address approvedAddress = getApproved(_tokenId);
+        require(
+            spender == owner ||
+            isApprovedForAll(owner, spender) ||
+            approvedAddress == spender,
+            "ERC721_INVALID_SPENDER"
+        );
+
+        if (approvedAddress != address(0)) {
+            approvals[_tokenId] = address(0);
         }
+
+        owners[_tokenId] = _to;
+        balances[_from] = safeSub(balances[_from], 1);
+        balances[_to] = safeAdd(balances[_to], 1);
+    
+        emit Transfer(
+            _from,
+            _to,
+            _tokenId
+        );
     }
 
-    /**
-    * @dev Gets the approved address for a token ID, or zero if no address set
-    * @param _tokenId uint256 ID of the token to query the approval of
-    * @return address currently approved for a the given token ID
-    */
-    function getApproved(uint256 _tokenId)
+    /// @notice Find the owner of an NFT
+    /// @dev NFTs assigned to zero address are considered invalid, and queries
+    ///      about them do throw.
+    /// @param _tokenId The identifier for an NFT
+    /// @return The address of the owner of the NFT
+    function ownerOf(uint256 _tokenId)
         public
         view
         returns (address)
     {
-        return tokenApprovals[_tokenId];
+        address owner = owners[_tokenId];
+        require(
+            owner != address(0),
+            "ERC721_ZERO_OWNER"
+        );
+        return owner;
     }
 
-    /**
-    * @dev Sets or unsets the approval of a given operator
-    * @dev An operator is allowed to transfer all tokens of the sender on their behalf
-    * @param _to operator address to set the approval
-    * @param _approved representing the status of the approval to be set
-    */
-    function setApprovalForAll(address _to, bool _approved)
+    /// @notice Get the approved address for a single NFT
+    /// @dev Throws if `_tokenId` is not a valid NFT.
+    /// @param _tokenId The NFT to find the approved address for
+    /// @return The approved address for this NFT, or the zero address if there is none
+    function getApproved(uint256 _tokenId)
         public
+        view
+        returns (address)
     {
-        require(_to != msg.sender);
-        operatorApprovals[msg.sender][_to] = _approved;
-        emit ApprovalForAll(msg.sender, _to, _approved);
+        return approvals[_tokenId];
     }
 
-    /**
-    * @dev Tells whether an operator is approved by a given owner
-    * @param _owner owner address which you want to query the approval of
-    * @param _operator operator address which you want to query the approval of
-    * @return bool whether the given operator is approved by the given owner
-    */
+    /// @notice Query if an address is an authorized operator for another address
+    /// @param _owner The address that owns the NFTs
+    /// @param _operator The address that acts on behalf of the owner
+    /// @return True if `_operator` is an approved operator for `_owner`, false otherwise
     function isApprovedForAll(address _owner, address _operator)
         public
         view
@@ -210,198 +274,4 @@ contract ERC721Token is
     {
         return operatorApprovals[_owner][_operator];
     }
-
-    /**
-    * @dev Transfers the ownership of a given token ID to another address
-    * @dev Usage of this method is discouraged, use `safeTransferFrom` whenever possible
-    * @dev Requires the msg sender to be the owner, approved, or operator
-    * @param _from current owner of the token
-    * @param _to address to receive the ownership of the given token ID
-    * @param _tokenId uint256 ID of the token to be transferred
-    */
-    function transferFrom(address _from, address _to, uint256 _tokenId)
-        public
-        canTransfer(_tokenId)
-    {
-        require(_from != address(0));
-        require(_to != address(0));
-
-        clearApproval(_from, _tokenId);
-        removeTokenFrom(_from, _tokenId);
-        addTokenTo(_to, _tokenId);
-
-        emit Transfer(_from, _to, _tokenId);
-    }
-
-    /**
-    * @dev Safely transfers the ownership of a given token ID to another address
-    * @dev If the target address is a contract, it must implement `onERC721Received`,
-    *  which is called upon a safe transfer, and return the magic value
-    *  `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`; otherwise,
-    *  the transfer is reverted.
-    * @dev Requires the msg sender to be the owner, approved, or operator
-    * @param _from current owner of the token
-    * @param _to address to receive the ownership of the given token ID
-    * @param _tokenId uint256 ID of the token to be transferred
-    */
-    function safeTransferFrom(
-        address _from,
-        address _to,
-        uint256 _tokenId)
-        public
-        canTransfer(_tokenId)
-    {
-        // solium-disable-next-line arg-overflow
-        safeTransferFrom(_from, _to, _tokenId, "");
-    }
-
-    /**
-    * @dev Safely transfers the ownership of a given token ID to another address
-    * @dev If the target address is a contract, it must implement `onERC721Received`,
-    *  which is called upon a safe transfer, and return the magic value
-    *  `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`; otherwise,
-    *  the transfer is reverted.
-    * @dev Requires the msg sender to be the owner, approved, or operator
-    * @param _from current owner of the token
-    * @param _to address to receive the ownership of the given token ID
-    * @param _tokenId uint256 ID of the token to be transferred
-    * @param _data bytes data to send along with a safe transfer check
-    */
-    function safeTransferFrom(
-        address _from,
-        address _to,
-        uint256 _tokenId,
-        bytes _data)
-        public
-        canTransfer(_tokenId)
-    {
-        transferFrom(_from, _to, _tokenId);
-        // solium-disable-next-line arg-overflow
-        require(checkAndCallSafeTransfer(_from, _to, _tokenId, _data));
-    }
-
-    /**
-    * @dev Returns whether the given spender can transfer a given token ID
-    * @param _spender address of the spender to query
-    * @param _tokenId uint256 ID of the token to be transferred
-    * @return bool whether the msg.sender is approved for the given token ID,
-    *  is an operator of the owner, or is the owner of the token
-    */
-    function isApprovedOrOwner(address _spender, uint256 _tokenId)
-        internal
-        view
-        returns (bool)
-    {
-        address owner = ownerOf(_tokenId);
-        return _spender == owner || getApproved(_tokenId) == _spender || isApprovedForAll(owner, _spender);
-    }
-
-    /**
-    * @dev Internal function to mint a new token
-    * @dev Reverts if the given token ID already exists
-    * @param _to The address that will own the minted token
-    * @param _tokenId uint256 ID of the token to be minted by the msg.sender
-    */
-    function _mint(address _to, uint256 _tokenId)
-        internal
-    {
-        require(_to != address(0));
-        addTokenTo(_to, _tokenId);
-        emit Transfer(address(0), _to, _tokenId);
-    }
-
-    /**
-    * @dev Internal function to burn a specific token
-    * @dev Reverts if the token does not exist
-    * @param _tokenId uint256 ID of the token being burned by the msg.sender
-    */
-    function _burn(address _owner, uint256 _tokenId)
-        internal
-    {
-        clearApproval(_owner, _tokenId);
-        removeTokenFrom(_owner, _tokenId);
-        emit Transfer(_owner, address(0), _tokenId);
-    }
-
-    /**
-    * @dev Internal function to clear current approval of a given token ID
-    * @dev Reverts if the given address is not indeed the owner of the token
-    * @param _owner owner of the token
-    * @param _tokenId uint256 ID of the token to be transferred
-    */
-    function clearApproval(address _owner, uint256 _tokenId)
-        internal
-    {
-        require(ownerOf(_tokenId) == _owner);
-        if (tokenApprovals[_tokenId] != address(0)) {
-            tokenApprovals[_tokenId] = address(0);
-            emit Approval(_owner, address(0), _tokenId);
-        }
-    }
-
-    /**
-    * @dev Internal function to add a token ID to the list of a given address
-    * @param _to address representing the new owner of the given token ID
-    * @param _tokenId uint256 ID of the token to be added to the tokens list of the given address
-    */
-    function addTokenTo(address _to, uint256 _tokenId)
-        internal
-    {
-        require(tokenOwner[_tokenId] == address(0));
-        tokenOwner[_tokenId] = _to;
-        ownedTokensCount[_to] = safeAdd(ownedTokensCount[_to], 1);
-    }
-
-    /**
-    * @dev Internal function to remove a token ID from the list of a given address
-    * @param _from address representing the previous owner of the given token ID
-    * @param _tokenId uint256 ID of the token to be removed from the tokens list of the given address
-    */
-    function removeTokenFrom(address _from, uint256 _tokenId)
-        internal
-    {
-        require(ownerOf(_tokenId) == _from);
-        ownedTokensCount[_from] = safeSub(ownedTokensCount[_from], 1);
-        tokenOwner[_tokenId] = address(0);
-    }
-
-    /**
-    * @dev Internal function to invoke `onERC721Received` on a target address
-    * @dev The call is not executed if the target address is not a contract
-    * @param _from address representing the previous owner of the given token ID
-    * @param _to target address that will receive the tokens
-    * @param _tokenId uint256 ID of the token to be transferred
-    * @param _data bytes optional data to send along with the call
-    * @return whether the call correctly returned the expected magic value
-    */
-    function checkAndCallSafeTransfer(
-        address _from,
-        address _to,
-        uint256 _tokenId,
-        bytes _data)
-        internal
-        returns (bool)
-    {
-        if (!isContract(_to)) {
-            return true;
-        }
-        bytes4 retval = IERC721Receiver(_to).onERC721Received(_from, _tokenId, _data);
-        return (retval == ERC721_RECEIVED);
-    }
-
-    function isContract(address addr)
-        internal
-        view
-        returns (bool)
-    {
-        uint256 size;
-        // XXX Currently there is no better way to check if there is a contract in an address
-        // than to check the size of the code at that address.
-        // See https://ethereum.stackexchange.com/a/14016/36603
-        // for more details about how this works.
-        // TODO Check this again before the Serenity release, because all addresses will be
-        // contracts then.
-        assembly { size := extcodesize(addr) }  // solium-disable-line security/no-inline-assembly
-        return size > 0;
-    }
 }
diff --git a/packages/contracts/src/2.0.0/tokens/ERC721Token/IERC721Receiver.sol b/packages/contracts/src/2.0.0/tokens/ERC721Token/IERC721Receiver.sol
index f2e8f3c88..8e0e32ab2 100644
--- a/packages/contracts/src/2.0.0/tokens/ERC721Token/IERC721Receiver.sol
+++ b/packages/contracts/src/2.0.0/tokens/ERC721Token/IERC721Receiver.sol
@@ -1,61 +1,44 @@
 /*
-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.
+
+  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;
 
 
-/**
- * @title ERC721 token receiver interface
- * @dev Interface for any contract that wants to support safeTransfers
- *      rom ERC721 asset contracts.
- * Modified from https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/token/ERC721/ERC721Receiver.sol
- */
 contract IERC721Receiver {
-    /**
-    * @dev Magic value to be returned upon successful reception of an NFT
-    *  Equals to `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`,
-    *  which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
-    */
-    bytes4 constant internal ERC721_RECEIVED = 0xf0b9e5ba;
-
-    /**
-    * @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)"))`
-    */
+
+    /// @notice Handle the receipt of an NFT
+    /// @dev The ERC721 smart contract calls this function on the recipient
+    ///  after a `transfer`. This function MAY throw to revert and reject the
+    ///  transfer. Return of other than the magic value MUST result in the
+    ///  transaction being reverted.
+    ///  Note: the contract address is always the message sender.
+    /// @param _operator The address which called `safeTransferFrom` function
+    /// @param _from The address which previously owned the token
+    /// @param _tokenId The NFT identifier which is being transferred
+    /// @param _data Additional data with no specified format
+    /// @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
+    ///  unless throwing
     function onERC721Received(
+        address _operator,
         address _from,
         uint256 _tokenId,
-        bytes _data)
-        public
+        bytes _data
+    )
+        external
         returns (bytes4);
 }
diff --git a/packages/contracts/src/2.0.0/tokens/ERC721Token/IERC721Token.sol b/packages/contracts/src/2.0.0/tokens/ERC721Token/IERC721Token.sol
index 4d57ece38..ac992c80d 100644
--- a/packages/contracts/src/2.0.0/tokens/ERC721Token/IERC721Token.sol
+++ b/packages/contracts/src/2.0.0/tokens/ERC721Token/IERC721Token.sol
@@ -1,118 +1,158 @@
 /*
-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.
+
+  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;
 
 
-/**
- * @title ERC721 Non-Fungible Token Standard basic interface
- * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
- * Modified from https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/token/ERC721/ERC721Basic.sol
- */
 contract IERC721Token {
-    string internal name_;
-    string internal symbol_;
 
+    /// @dev This emits when ownership of any NFT changes by any mechanism.
+    ///      This event emits when NFTs are created (`from` == 0) and destroyed
+    ///      (`to` == 0). Exception: during contract creation, any number of NFTs
+    ///      may be created and assigned without emitting Transfer. At the time of
+    ///      any transfer, the approved address for that NFT (if any) is reset to none.
     event Transfer(
         address indexed _from,
         address indexed _to,
-        uint256 _tokenId
+        uint256 indexed _tokenId
     );
 
+    /// @dev This emits when the approved address for an NFT is changed or
+    ///      reaffirmed. The zero address indicates there is no approved address.
+    ///      When a Transfer event emits, this also indicates that the approved
+    ///      address for that NFT (if any) is reset to none.
     event Approval(
         address indexed _owner,
         address indexed _approved,
-        uint256 _tokenId
+        uint256 indexed _tokenId
     );
 
+    /// @dev This emits when an operator is enabled or disabled for an owner.
+    ///      The operator can manage all NFTs of the owner.
     event ApprovalForAll(
         address indexed _owner,
         address indexed _operator,
         bool _approved
     );
 
-    function name()
-        public
-        view
-        returns (string);
-
-    function symbol()
-        public
-        view
-        returns (string);
+    /// @notice Transfers the ownership of an NFT from one address to another address
+    /// @dev Throws unless `msg.sender` is the current owner, an authorized
+    ///      perator, or the approved address for this NFT. Throws if `_from` is
+    ///      not the current owner. Throws if `_to` is the zero address. Throws if
+    ///      `_tokenId` is not a valid NFT. When transfer is complete, this function
+    ///      checks if `_to` is a smart contract (code size > 0). If so, it calls
+    ///      `onERC721Received` on `_to` and throws if the return value is not
+    ///      `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`.
+    /// @param _from The current owner of the NFT
+    /// @param _to The new owner
+    /// @param _tokenId The NFT to transfer
+    /// @param _data Additional data with no specified format, sent in call to `_to`
+    function safeTransferFrom(
+        address _from,
+        address _to,
+        uint256 _tokenId,
+        bytes _data
+    )
+        external;
+
+    /// @notice Transfers the ownership of an NFT from one address to another address
+    /// @dev This works identically to the other function with an extra data parameter,
+    ///      except this function just sets data to "".
+    /// @param _from The current owner of the NFT
+    /// @param _to The new owner
+    /// @param _tokenId The NFT to transfer
+    function safeTransferFrom(
+        address _from,
+        address _to,
+        uint256 _tokenId
+    )
+        external;
+
+    /// @notice Change or reaffirm the approved address for an NFT
+    /// @dev The zero address indicates there is no approved address.
+    ///      Throws unless `msg.sender` is the current NFT owner, or an authorized
+    ///      operator of the current owner.
+    /// @param _approved The new approved NFT controller
+    /// @param _tokenId The NFT to approve
+    function approve(address _approved, uint256 _tokenId)
+        external;
+
+    /// @notice Enable or disable approval for a third party ("operator") to manage
+    ///         all of `msg.sender`'s assets
+    /// @dev Emits the ApprovalForAll event. The contract MUST allow
+    ///      multiple operators per owner.
+    /// @param _operator Address to add to the set of authorized operators
+    /// @param _approved True if the operator is approved, false to revoke approval
+    function setApprovalForAll(address _operator, bool _approved)
+        external;
 
+    /// @notice Count all NFTs assigned to an owner
+    /// @dev NFTs assigned to the zero address are considered invalid, and this
+    ///      function throws for queries about the zero address.
+    /// @param _owner An address for whom to query the balance
+    /// @return The number of NFTs owned by `_owner`, possibly zero
     function balanceOf(address _owner)
-        public
+        external
         view
-        returns (uint256 _balance);
+        returns (uint256);
+
+    /// @notice Transfer ownership of an NFT -- THE CALLER IS RESPONSIBLE
+    ///         TO CONFIRM THAT `_to` IS CAPABLE OF RECEIVING NFTS OR ELSE
+    ///         THEY MAY BE PERMANENTLY LOST
+    /// @dev Throws unless `msg.sender` is the current owner, an authorized
+    ///      operator, or the approved address for this NFT. Throws if `_from` is
+    ///      not the current owner. Throws if `_to` is the zero address. Throws if
+    ///      `_tokenId` is not a valid NFT.
+    /// @param _from The current owner of the NFT
+    /// @param _to The new owner
+    /// @param _tokenId The NFT to transfer
+    function transferFrom(
+        address _from,
+        address _to,
+        uint256 _tokenId
+    )
+        public;
 
+    /// @notice Find the owner of an NFT
+    /// @dev NFTs assigned to zero address are considered invalid, and queries
+    ///      about them do throw.
+    /// @param _tokenId The identifier for an NFT
+    /// @return The address of the owner of the NFT
     function ownerOf(uint256 _tokenId)
         public
         view
-        returns (address _owner);
+        returns (address);
 
-    function exists(uint256 _tokenId)
+    /// @notice Get the approved address for a single NFT
+    /// @dev Throws if `_tokenId` is not a valid NFT.
+    /// @param _tokenId The NFT to find the approved address for
+    /// @return The approved address for this NFT, or the zero address if there is none
+    function getApproved(uint256 _tokenId) 
         public
         view
-        returns (bool _exists);
-
-    function approve(address _to, uint256 _tokenId)
-        public;
-
-    function getApproved(uint256 _tokenId)
-        public
-        view
-        returns (address _operator);
-
-    function setApprovalForAll(address _operator, bool _approved)
-        public;
-
+        returns (address);
+    
+    /// @notice Query if an address is an authorized operator for another address
+    /// @param _owner The address that owns the NFTs
+    /// @param _operator The address that acts on behalf of the owner
+    /// @return True if `_operator` is an approved operator for `_owner`, false otherwise
     function isApprovedForAll(address _owner, address _operator)
         public
         view
         returns (bool);
-
-    function transferFrom(
-        address _from,
-        address _to,
-        uint256 _tokenId
-    )
-        public;
-
-    function safeTransferFrom(
-        address _from,
-        address _to,
-        uint256 _tokenId
-    )
-        public;
-
-    function safeTransferFrom(
-        address _from,
-        address _to,
-        uint256 _tokenId,
-        bytes _data
-    )
-        public;
 }
diff --git a/packages/contracts/src/2.0.0/tokens/ERC721Token/MintableERC721Token.sol b/packages/contracts/src/2.0.0/tokens/ERC721Token/MintableERC721Token.sol
new file mode 100644
index 000000000..85d192779
--- /dev/null
+++ b/packages/contracts/src/2.0.0/tokens/ERC721Token/MintableERC721Token.sol
@@ -0,0 +1,83 @@
+/*
+
+  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 "./ERC721Token.sol";
+
+
+contract MintableERC721Token is
+    ERC721Token
+{
+
+    /// @dev Function to mint a new token
+    ///      Reverts if the given token ID already exists
+    /// @param _to Address of the beneficiary that will own the minted token
+    /// @param _tokenId ID of the token to be minted by the msg.sender    
+    function _mint(address _to, uint256 _tokenId)
+        internal
+    {
+        require(
+            _to != address(0),
+            "ERC721_ZERO_TO_ADDRESS"
+        );
+
+        address owner = owners[_tokenId];
+        require(
+            owner == address(0),
+            "ERC721_OWNER_ALREADY_EXISTS"
+        );
+
+        owners[_tokenId] = _to;
+        balances[_to] = safeAdd(balances[_to], 1);
+
+        emit Transfer(
+            address(0),
+            _to,
+            _tokenId
+        );
+    }
+
+    /// @dev Function to burn a token
+    ///      Reverts if the given token ID doesn't exist
+    /// @param _owner Owner of token with given token ID
+    /// @param _tokenId ID of the token to be burned by the msg.sender
+    function _burn(address _owner, uint256 _tokenId)
+        internal
+    {
+        require(
+            _owner != address(0),
+            "ERC721_ZERO_OWNER_ADDRESS"
+        );
+
+        address owner = owners[_tokenId];
+        require(
+            owner == _owner,
+            "ERC721_OWNER_MISMATCH"
+        );
+
+        owners[_tokenId] = address(0);
+        balances[_owner] = safeSub(balances[_owner], 1);
+
+        emit Transfer(
+            _owner,
+            address(0),
+            _tokenId
+        );
+    }
+}