Flatten transferFrom to minimize state reads

contracts/token/ERC721/ERC721BasicToken.sol

@@ -161,13 +161,25 @@ contract ERC721BasicToken is SupportsInterfaceWithLookup, ERC721Basic {
   )
     public
   {
-    require(isApprovedOrOwner(msg.sender, _tokenId));
-    require(_from != address(0));
+    address owner = ownerOf(_tokenId);
+    require(_from == owner);
     require(_to != address(0));
 
-    clearApproval(_from, _tokenId);
-    removeTokenFrom(_from, _tokenId);
-    addTokenTo(_to, _tokenId);
+    address spender = msg.sender;
+    require(
+      spender == owner ||
+      isApprovedForAll(owner, spender) ||
+      getApproved(_tokenId) == spender
+    );
+
+    if (tokenApprovals[_tokenId] != address(0)) {
+      tokenApprovals[_tokenId] = address(0);
+      emit Approval(owner, address(0), _tokenId);
+    }
+
+    tokenOwner[_tokenId] = _to;
+    ownedTokensCount[_from] = ownedTokensCount[_from].sub(1);
+    ownedTokensCount[_to] = ownedTokensCount[_to].add(1);
 
     emit Transfer(_from, _to, _tokenId);
   }

contracts/token/ERC721/ERC721Token.sol

@@ -106,6 +106,41 @@ contract ERC721Token is SupportsInterfaceWithLookup, ERC721BasicToken, ERC721 {
     return ownedTokens[_owner][_index];
   }
 
+  /**
+   * @dev Transfers the ownership of a given token ID to another address
+   * Usage of this method is discouraged, use `safeTransferFrom` whenever possible
+   * 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
+  {
+    super.transferFrom(_from, _to, _tokenId);
+
+    uint256 tokenIndex = ownedTokensIndex[_tokenId];
+    uint256 lastTokenIndex = ownedTokens[_from].length.sub(1);
+    uint256 lastToken = ownedTokens[_from][lastTokenIndex];
+
+    ownedTokens[_from][tokenIndex] = lastToken;
+    ownedTokens[_from][lastTokenIndex] = 0;
+    // 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
+
+    ownedTokens[_from].length--;
+    ownedTokensIndex[_tokenId] = 0;
+    ownedTokensIndex[lastToken] = tokenIndex;
+
+    uint256 length = ownedTokens[_to].length;
+    ownedTokens[_to].push(_tokenId);
+    ownedTokensIndex[_tokenId] = length;
+  }
   /**
    * @dev Gets the total amount of tokens stored by the contract
    * @return uint256 representing the total amount of tokens