Optimized ERC721 transfers. (#1539)

* Added _transferToken.

* _transferFrom is now usable by derived contracts, abstracted away enumerable behavior.

* Removed unnecesary check from _clearApprovals

contracts/token/ERC721/ERC721.sol

@@ -130,13 +130,8 @@ contract ERC721 is ERC165, IERC721 {
     */
     function transferFrom(address from, address to, uint256 tokenId) public {
         require(_isApprovedOrOwner(msg.sender, tokenId));
-        require(to != address(0));
-
-        _clearApproval(from, tokenId);
-        _removeTokenFrom(from, tokenId);
-        _addTokenTo(to, tokenId);
 
-        emit Transfer(from, to, tokenId);
+        _transferFrom(from, to, tokenId);
     }
 
     /**
@@ -217,7 +212,7 @@ contract ERC721 is ERC165, IERC721 {
      * @param tokenId uint256 ID of the token being burned by the msg.sender
      */
     function _burn(address owner, uint256 tokenId) internal {
-        _clearApproval(owner, tokenId);
+        _clearApproval(tokenId);
         _removeTokenFrom(owner, tokenId);
         emit Transfer(owner, address(0), tokenId);
     }
@@ -249,6 +244,27 @@ contract ERC721 is ERC165, IERC721 {
         _tokenOwner[tokenId] = address(0);
     }
 
+    /**
+     * @dev Internal function to transfer ownership of a given token ID to another address.
+     * As opposed to transferFrom, this imposes no restrictions on msg.sender.
+     * @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) internal {
+        require(ownerOf(tokenId) == from);
+        require(to != address(0));
+
+        _clearApproval(tokenId);
+
+        _ownedTokensCount[from] = _ownedTokensCount[from].sub(1);
+        _ownedTokensCount[to] = _ownedTokensCount[to].add(1);
+
+        _tokenOwner[tokenId] = to;
+
+        emit Transfer(from, to, tokenId);
+    }
+
     /**
      * @dev Internal function to invoke `onERC721Received` on a target address
      * The call is not executed if the target address is not a contract
@@ -269,12 +285,9 @@ contract ERC721 is ERC165, IERC721 {
 
     /**
      * @dev Private function to clear current approval of a given token ID
-     * 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) private {
-        require(ownerOf(tokenId) == owner);
+    function _clearApproval(uint256 tokenId) private {
         if (_tokenApprovals[tokenId] != address(0)) {
             _tokenApprovals[tokenId] = address(0);
         }

contracts/token/ERC721/ERC721Enumerable.sol

@@ -76,9 +76,8 @@ contract ERC721Enumerable is ERC165, ERC721, IERC721Enumerable {
      */
     function _addTokenTo(address to, uint256 tokenId) internal {
         super._addTokenTo(to, tokenId);
-        uint256 length = _ownedTokens[to].length;
-        _ownedTokens[to].push(tokenId);
-        _ownedTokensIndex[tokenId] = length;
+
+        _addTokenToOwnerEnumeration(to, tokenId);
     }
 
     /**
@@ -92,22 +91,26 @@ contract ERC721Enumerable is ERC165, ERC721, IERC721Enumerable {
     function _removeTokenFrom(address from, uint256 tokenId) internal {
         super._removeTokenFrom(from, tokenId);
 
-        // To prevent a gap in the array, we store the last token in the index of the token to delete, and
-        // then delete the last slot.
-        uint256 tokenIndex = _ownedTokensIndex[tokenId];
-        uint256 lastTokenIndex = _ownedTokens[from].length.sub(1);
-        uint256 lastToken = _ownedTokens[from][lastTokenIndex];
-
-        _ownedTokens[from][tokenIndex] = lastToken;
-        // This also deletes the contents at the last position of the array
-        _ownedTokens[from].length--;
-
-        // Note that this will handle single-element arrays. In that case, both tokenIndex and lastTokenIndex are going to
-        // be zero. Then we can make sure that we will remove tokenId from the ownedTokens list since we are first swapping
-        // the lastToken to the first position, and then dropping the element placed in the last position of the list
+        _removeTokenFromOwnerEnumeration(from, tokenId);
 
+        // Since the token is being destroyed, we also clear its index
+        // TODO(nventuro): 0 is still a valid index, so arguably this isnt really helpful, remove?
         _ownedTokensIndex[tokenId] = 0;
-        _ownedTokensIndex[lastToken] = tokenIndex;
+    }
+
+    /**
+     * @dev Internal function to transfer ownership of a given token ID to another address.
+     * As opposed to transferFrom, this imposes no restrictions on msg.sender.
+     * @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) internal {
+        super._transferFrom(from, to, tokenId);
+
+        _removeTokenFromOwnerEnumeration(from, tokenId);
+
+        _addTokenToOwnerEnumeration(to, tokenId);
     }
 
     /**
@@ -144,7 +147,7 @@ contract ERC721Enumerable is ERC165, ERC721, IERC721Enumerable {
         _allTokensIndex[tokenId] = 0;
         _allTokensIndex[lastToken] = tokenIndex;
     }
-
+    
     /**
      * @dev Gets the list of token IDs of the requested owner
      * @param owner address owning the tokens 
@@ -153,4 +156,46 @@ contract ERC721Enumerable is ERC165, ERC721, IERC721Enumerable {
     function _tokensOfOwner(address owner) internal view returns (uint256[] storage) {
         return _ownedTokens[owner];
     }
+
+    /**
+     * @dev Private function to add a token to this extension's ownership-tracking data structures.
+     * @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 _addTokenToOwnerEnumeration(address to, uint256 tokenId) private {
+        uint256 newOwnedTokensLength = _ownedTokens[to].push(tokenId);
+        // No need to use SafeMath since the length after a push cannot be zero
+        _ownedTokensIndex[tokenId] = newOwnedTokensLength - 1;
+    }
+
+    /**
+     * @dev Private function to remove a token from this extension's ownership-tracking data structures. Note that
+     * while the token is not assigned a new owner, the _ownedTokensIndex mapping is _not_ updated: this allows for
+     * gas optimizations e.g. when performing a transfer operation (avoiding double writes).
+     * This has O(1) time complexity, but alters the order of the _ownedTokens array.
+     * @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 _removeTokenFromOwnerEnumeration(address from, uint256 tokenId) private {
+        // To prevent a gap in from's tokens array, we store the last token in the index of the token to delete, and
+        // then delete the last slot (swap and pop).
+
+        uint256 lastTokenIndex = _ownedTokens[from].length.sub(1);
+        uint256 lastTokenId = _ownedTokens[from][lastTokenIndex];
+
+        uint256 tokenIndex = _ownedTokensIndex[tokenId];
+
+        _ownedTokens[from][tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
+        _ownedTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index
+
+        // Note that this will handle single-element arrays. In that case, both tokenIndex and lastTokenIndex are going
+        // to be zero. The swap operation will therefore have no effect, but the token _will_ be deleted during the
+        // 'pop' operation.
+
+        // This also deletes the contents at the last position of the array
+        _ownedTokens[from].length--;
+
+        // Note that _ownedTokensIndex[tokenId] hasn't been cleared: it still points to the old slot (now occcupied by
+        // lasTokenId).
+    }
 }