ERC721 full implementation (#803)

* Rename current ERC721 implementation to BaseERC721

* Implement ERC721 optional & approveAll functionality

* Support for new ERC721 interface

- Tests for new features are pending
- ERC721 is abstract, since it requires metadata implementation
- Move some methods into DeprecatedERC721 contract
- Reorganise base vs full implementation
- Pending tokenByIndex

* Add more tests for ERC721

* Implement suggestions by @dekz

* Update comments in ERC721 contracts

* Implement tokensByIndex extension

- Remove restrictions from mock mint and burn calls

* Add default implementation for metadata URI

This allows token implementation to be non-abstract

* Allow operators to call approve on a token

* Remove gas stipend restriction in call to 721 receiver

* Remove deprecated implementation

We only want to keep the interface, for interacting with already deployed contracts

* Add notice to isContract helper on constract constructors

* Change natspec delimiters for consistency

* Minor linting fixes

* Add constant modifier to ERC721_RECEIVED magic value

* Use 4-params safeTransferFrom for implementing the 3-params overload

* Minor text changes in natspec comments

* Use address(0) instead of 0 or 0x0

* Use if-statements instead of boolean one-liners for clarity

:-(

* Keep ownedTokensCount state var in sync in full ERC721 implementation

* Fix incorrect comparison when burning ERC721 tokens with metadata

* Use address(0) instead of 0 in one more place in ERC721

* Throw when querying balance for the zero address

Required by the spec

* Update links to approved version of EIP721

* Use explicit size for uint

* Remove unneeded internal function in ERC721

Also rename addToken and removeToken for added clarity

* Use underscore instead of 'do' prefix for internal methods in ERC721

* Fix failing test due to events reordering in ERC721 safe transfer

* Fix bug introduced in 74db03ba06

* Remove do prefix for internal setTokenUri method

* Allow transfers to self in ERC721

contracts/AddressUtils.sol

+pragma solidity ^0.4.18;
+
+/**
+ * Utility library of inline functions on addresses
+ */
+library AddressUtils {
+
+  /**
+   * Returns whether there is code in the target address
+   * @dev This function will return false if invoked during the constructor of a contract,
+   *  as the code is not actually created until after the constructor finishes.
+   * @param addr address address to check
+   * @return whether there is code in the target address
+   */
+  function isContract(address addr) internal view returns (bool) {
+    uint256 size;
+    assembly { size := extcodesize(addr) }
+    return size > 0;
+  }
+
+}

contracts/mocks/ERC721BasicTokenMock.sol

+pragma solidity ^0.4.18;
+
+import "../token/ERC721/ERC721BasicToken.sol";
+
+/**
+ * @title ERC721BasicTokenMock
+ * This mock just provides a public mint and burn functions for testing purposes
+ */
+contract ERC721BasicTokenMock is ERC721BasicToken {
+  function mint(address _to, uint256 _tokenId) public {
+    super._mint(_to, _tokenId);
+  }
+
+  function burn(uint256 _tokenId) public {
+    super._burn(ownerOf(_tokenId), _tokenId);
+  }
+}

contracts/mocks/ERC721ReceiverMock.sol

+pragma solidity ^0.4.18;
+
+import "../token/ERC721/ERC721Receiver.sol";
+
+contract ERC721ReceiverMock is ERC721Receiver {
+  bytes4 retval;
+  bool reverts;
+  
+  event Received(address _address, uint256 _tokenId, bytes _data, uint256 _gas);
+
+  function ERC721ReceiverMock(bytes4 _retval, bool _reverts) public {
+    retval = _retval;
+    reverts = _reverts;
+  }
+
+  function onERC721Received(address _address, uint256 _tokenId, bytes _data) public returns(bytes4) {
+    require(!reverts);
+    Received(_address, _tokenId, _data, msg.gas);
+    return retval;
+  }
+}

contracts/mocks/ERC721TokenMock.sol

@@ -4,16 +4,23 @@ import "../token/ERC721/ERC721Token.sol";
 
 /**
  * @title ERC721TokenMock
- * This mock just provides a public mint and burn functions for testing purposes.
+ * This mock just provides a public mint and burn functions for testing purposes,
+ * and a public setter for metadata URI
  */
 contract ERC721TokenMock is ERC721Token {
-  function ERC721TokenMock() ERC721Token() public { }
+  function ERC721TokenMock(string name, string symbol) public
+    ERC721Token(name, symbol)
+  { }
 
   function mint(address _to, uint256 _tokenId) public {
     super._mint(_to, _tokenId);
   }
 
   function burn(uint256 _tokenId) public {
-    super._burn(_tokenId);
+    super._burn(ownerOf(_tokenId), _tokenId);
   }
-}
+
+  function setTokenURI(uint256 _tokenId, string _uri) public {
+    super._setTokenURI(_tokenId, _uri);
+  }
+}
\ No newline at end of file

contracts/token/ERC721/DeprecatedERC721.sol

+pragma solidity ^0.4.18;
+
+import "./ERC721.sol";
+
+/**
+ * @title ERC-721 methods shipped in OpenZeppelin v1.7.0, removed in the latest version of the standard
+ * @dev Only use this interface for compatibility with previously deployed contracts
+ * @dev Use ERC721 for interacting with new contracts which are standard-compliant
+ */
+contract DeprecatedERC721 is ERC721 {
+  function takeOwnership(uint256 _tokenId) public;
+  function transfer(address _to, uint256 _tokenId) public;
+  function tokensOf(address _owner) public view returns (uint256[]);
+}
+

contracts/token/ERC721/ERC721.sol

 pragma solidity ^0.4.18;
 
+import "./ERC721Basic.sol";
+
+/**
+ * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
+ * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
+ */
+contract ERC721Enumerable is ERC721Basic {
+  function totalSupply() public view returns (uint256);
+  function tokenOfOwnerByIndex(address _owner, uint256 _index) public view returns (uint256 _tokenId);
+  function tokenByIndex(uint256 _index) public view returns (uint256);
+}
+
 /**
- * @title ERC721 interface
- * @dev see https://github.com/ethereum/eips/issues/721
+ * @title ERC-721 Non-Fungible Token Standard, optional metadata extension
+ * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
-contract ERC721 {
-  event Transfer(address indexed _from, address indexed _to, uint256 _tokenId);
-  event Approval(address indexed _owner, address indexed _approved, uint256 _tokenId);
+contract ERC721Metadata is ERC721Basic {
+  function name() public view returns (string _name);
+  function symbol() public view returns (string _symbol);
+  function tokenURI(uint256 _tokenId) public view returns (string);
+}
 
-  function balanceOf(address _owner) public view returns (uint256 _balance);
-  function ownerOf(uint256 _tokenId) public view returns (address _owner);
-  function transfer(address _to, uint256 _tokenId) public;
-  function approve(address _to, uint256 _tokenId) public;
-  function takeOwnership(uint256 _tokenId) public;
+/**
+ * @title ERC-721 Non-Fungible Token Standard, full implementation interface
+ * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
+ */
+contract ERC721 is ERC721Basic, ERC721Enumerable, ERC721Metadata {
 }

contracts/token/ERC721/ERC721Basic.sol

+pragma solidity ^0.4.18;
+
+/**
+ * @title ERC721 Non-Fungible Token Standard basic interface
+ * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
+ */
+contract ERC721Basic {
+  event Transfer(address indexed _from, address indexed _to, uint256 _tokenId);
+  event Approval(address indexed _owner, address indexed _approved, uint256 _tokenId);
+  event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved);  
+
+  function balanceOf(address _owner) public view returns (uint256 _balance);
+  function ownerOf(uint256 _tokenId) public view returns (address _owner);
+  function exists(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;
+  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;
+}

contracts/token/ERC721/ERC721BasicToken.sol

+pragma solidity ^0.4.18;
+
+import "./ERC721Basic.sol";
+import "./ERC721Receiver.sol";
+import "../../math/SafeMath.sol";
+import "../../AddressUtils.sol";
+
+/**
+ * @title ERC721 Non-Fungible Token Standard basic implementation
+ * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
+ */
+contract ERC721BasicToken is ERC721Basic {
+  using SafeMath for uint256;
+  using AddressUtils for address;
+  
+  // Equals to `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`
+  // which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
+  bytes4 constant ERC721_RECEIVED = 0xf0b9e5ba; 
+
+  // Mapping from token ID to owner
+  mapping (uint256 => address) internal tokenOwner;
+
+  // Mapping from token ID to approved address
+  mapping (uint256 => address) internal tokenApprovals;
+
+  // Mapping from owner to number of owned token
+  mapping (address => uint256) internal ownedTokensCount;
+
+  // Mapping from owner to operator approvals
+  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));
+    _;
+  }
+
+  /**
+  * @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
+  */
+  function balanceOf(address _owner) public view returns (uint256) {
+    require(_owner != address(0));
+    return ownedTokensCount[_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) public view returns (bool) {
+    address owner = tokenOwner[_tokenId];
+    return owner != address(0);
+  }
+
+  /**
+  * @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;
+      Approval(owner, _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) public view returns (address) {
+    return tokenApprovals[_tokenId];
+  }
+
+
+  /**
+  * @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) public {
+    require(_to != msg.sender);
+    operatorApprovals[msg.sender][_to] = _approved;
+    ApprovalForAll(msg.sender, _to, _approved);
+  }
+
+  /**
+   * @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
+   */
+  function isApprovedForAll(address _owner, address _operator) public view returns (bool) {
+    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);
+    
+    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) {
+    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);
+    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);
+    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);
+    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);
+      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] = ownedTokensCount[_to].add(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] = ownedTokensCount[_from].sub(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 (!_to.isContract()) {
+      return true;
+    }
+    bytes4 retval = ERC721Receiver(_to).onERC721Received(_from, _tokenId, _data);
+    return (retval == ERC721_RECEIVED);
+  }
+}

contracts/token/ERC721/ERC721Holder.sol

+pragma solidity ^0.4.18;
+
+import "./ERC721Receiver.sol";
+
+contract ERC721Holder is ERC721Receiver {
+  function onERC721Received(address, uint256, bytes) public returns(bytes4) {
+    return ERC721_RECEIVED;
+  }
+}

contracts/token/ERC721/ERC721Receiver.sol

+pragma solidity ^0.4.18;
+
+/**
+ * @title ERC721 token receiver interface
+ * @dev Interface for any contract that wants to support safeTransfers
+ *  from ERC721 asset contracts.
+ */
+contract ERC721Receiver {
+  /**
+   * @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 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)"))`
+   */
+  function onERC721Received(address _from, uint256 _tokenId, bytes _data) public returns(bytes4);
+}

contracts/token/ERC721/ERC721Token.sol

 pragma solidity ^0.4.18;
 
 import "./ERC721.sol";
-import "../../math/SafeMath.sol";
+import "./DeprecatedERC721.sol";
+import "./ERC721BasicToken.sol";
+
 
 /**
- * @title ERC721Token
- * Generic implementation for the required functionality of the ERC721 standard
+ * @title Full ERC721 Token
+ * This implementation includes all the required and some optional functionality of the ERC721 standard
+ * Moreover, it includes approve all functionality using operator terminology
+ * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
-contract ERC721Token is ERC721 {
-  using SafeMath for uint256;
-
-  // Total amount of tokens
-  uint256 private totalTokens;
+contract ERC721Token is ERC721, ERC721BasicToken {
+  // Token name
+  string internal name_;
 
-  // Mapping from token ID to owner
-  mapping (uint256 => address) private tokenOwner;
-
-  // Mapping from token ID to approved address
-  mapping (uint256 => address) private tokenApprovals;
+  // Token symbol
+  string internal symbol_;
 
   // Mapping from owner to list of owned token IDs
-  mapping (address => uint256[]) private ownedTokens;
+  mapping (address => uint256[]) internal ownedTokens;
 
   // Mapping from token ID to index of the owner tokens list
-  mapping(uint256 => uint256) private ownedTokensIndex;
+  mapping(uint256 => uint256) internal ownedTokensIndex;
 
-  /**
-  * @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);
-    _;
-  }
+  // Array with all token ids, used for enumeration
+  uint256[] internal allTokens;
 
-  /**
-  * @dev Gets the total amount of tokens stored by the contract
-  * @return uint256 representing the total amount of tokens
-  */
-  function totalSupply() public view returns (uint256) {
-    return totalTokens;
-  }
+  // Mapping from token id to position in the allTokens array
+  mapping(uint256 => uint256) internal allTokensIndex;
 
-  /**
-  * @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
-  */
-  function balanceOf(address _owner) public view returns (uint256) {
-    return ownedTokens[_owner].length;
-  }
-
-  /**
-  * @dev Gets the list of tokens owned by a given address
-  * @param _owner address to query the tokens of
-  * @return uint256[] representing the list of tokens owned by the passed address
-  */
-  function tokensOf(address _owner) public view returns (uint256[]) {
-    return ownedTokens[_owner];
-  }
+  // Optional mapping for token URIs 
+  mapping(uint256 => string) internal tokenURIs;
 
   /**
-  * @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
+  * @dev Constructor function
   */
-  function ownerOf(uint256 _tokenId) public view returns (address) {
-    address owner = tokenOwner[_tokenId];
-    require(owner != address(0));
-    return owner;
+  function ERC721Token(string _name, string _symbol) public {
+    name_ = _name;
+    symbol_ = _symbol;
   }
 
   /**
-   * @dev Gets the approved address to take ownership of a given token ID
-   * @param _tokenId uint256 ID of the token to query the approval of
-   * @return address currently approved to take ownership of the given token ID
-   */
-  function approvedFor(uint256 _tokenId) public view returns (address) {
-    return tokenApprovals[_tokenId];
-  }
-
-  /**
-  * @dev Transfers the ownership of a given token ID to another address
-  * @param _to address to receive the ownership of the given token ID
-  * @param _tokenId uint256 ID of the token to be transferred
+  * @dev Gets the token name
+  * @return string representing the token name
   */
-  function transfer(address _to, uint256 _tokenId) public onlyOwnerOf(_tokenId) {
-    clearApprovalAndTransfer(msg.sender, _to, _tokenId);
+  function name() public view returns (string) {
+    return name_;
   }
 
   /**
-  * @dev Approves another address to claim for the ownership of the given token ID
-  * @param _to address to be approved for the given token ID
-  * @param _tokenId uint256 ID of the token to be approved
+  * @dev Gets the token symbol
+  * @return string representing the token symbol
   */
-  function approve(address _to, uint256 _tokenId) public onlyOwnerOf(_tokenId) {
-    address owner = ownerOf(_tokenId);
-    require(_to != owner);
-    if (approvedFor(_tokenId) != 0 || _to != 0) {
-      tokenApprovals[_tokenId] = _to;
-      Approval(owner, _to, _tokenId);
-    }
+  function symbol() public view returns (string) {
+    return symbol_;
   }
 
   /**
-  * @dev Claims the ownership of a given token ID
-  * @param _tokenId uint256 ID of the token being claimed by the msg.sender
+  * @dev Returns an URI for a given token ID
+  * @dev Throws if the token ID does not exist. May return an empty string.
+  * @param _tokenId uint256 ID of the token to query
   */
-  function takeOwnership(uint256 _tokenId) public {
-    require(isApprovedFor(msg.sender, _tokenId));
-    clearApprovalAndTransfer(ownerOf(_tokenId), msg.sender, _tokenId);
+  function tokenURI(uint256 _tokenId) public view returns (string) {
+    require(exists(_tokenId));
+    return tokenURIs[_tokenId];
   }
 
   /**
-  * @dev Mint token function
-  * @param _to The address that will own the minted token
-  * @param _tokenId uint256 ID of the token to be minted by the msg.sender
+  * @dev Internal function to set the token URI for a given token
+  * @dev Reverts if the token ID does not exist
+  * @param _tokenId uint256 ID of the token to set its URI
+  * @param _uri string URI to assign
   */
-  function _mint(address _to, uint256 _tokenId) internal {
-    require(_to != address(0));
-    addToken(_to, _tokenId);
-    Transfer(0x0, _to, _tokenId);
+  function _setTokenURI(uint256 _tokenId, string _uri) internal {
+    require(exists(_tokenId));
+    tokenURIs[_tokenId] = _uri;
   }
 
   /**
-  * @dev Burns a specific token
-  * @param _tokenId uint256 ID of the token being burned by the msg.sender
+  * @dev Gets the token ID at a given index of the tokens list of the requested owner
+  * @param _owner address owning the tokens list to be accessed
+  * @param _index uint256 representing the index to be accessed of the requested tokens list
+  * @return uint256 token ID at the given index of the tokens list owned by the requested address
   */
-  function _burn(uint256 _tokenId) onlyOwnerOf(_tokenId) internal {
-    if (approvedFor(_tokenId) != 0) {
-      clearApproval(msg.sender, _tokenId);
-    }
-    removeToken(msg.sender, _tokenId);
-    Transfer(msg.sender, 0x0, _tokenId);
+  function tokenOfOwnerByIndex(address _owner, uint256 _index) public view returns (uint256) {
+    require(_index < balanceOf(_owner));
+    return ownedTokens[_owner][_index];
   }
 
   /**
-   * @dev Tells whether the msg.sender is approved for the given token ID or not
-   * This function is not private so it can be extended in further implementations like the operatable ERC721
-   * @param _owner address of the owner to query the approval of
-   * @param _tokenId uint256 ID of the token to query the approval of
-   * @return bool whether the msg.sender is approved for the given token ID or not
-   */
-  function isApprovedFor(address _owner, uint256 _tokenId) internal view returns (bool) {
-    return approvedFor(_tokenId) == _owner;
-  }
-
-  /**
-  * @dev Internal function to clear current approval and transfer the ownership of a given token ID
-  * @param _from address which you want to send tokens from
-  * @param _to address which you want to transfer the token to
-  * @param _tokenId uint256 ID of the token to be transferred
+  * @dev Gets the total amount of tokens stored by the contract
+  * @return uint256 representing the total amount of tokens
   */
-  function clearApprovalAndTransfer(address _from, address _to, uint256 _tokenId) internal {
-    require(_to != address(0));
-    require(_to != ownerOf(_tokenId));
-    require(ownerOf(_tokenId) == _from);
-
-    clearApproval(_from, _tokenId);
-    removeToken(_from, _tokenId);
-    addToken(_to, _tokenId);
-    Transfer(_from, _to, _tokenId);
+  function totalSupply() public view returns (uint256) {
+    return allTokens.length;
   }
 
   /**
-  * @dev Internal function to clear current approval of a given token ID
-  * @param _tokenId uint256 ID of the token to be transferred
+  * @dev Gets the token ID at a given index of all the tokens in this contract
+  * @dev Reverts if the index is greater or equal to the total number of tokens
+  * @param _index uint256 representing the index to be accessed of the tokens list
+  * @return uint256 token ID at the given index of the tokens list
   */
-  function clearApproval(address _owner, uint256 _tokenId) private {
-    require(ownerOf(_tokenId) == _owner);
-    tokenApprovals[_tokenId] = 0;
-    Approval(_owner, 0, _tokenId);
+  function tokenByIndex(uint256 _index) public view returns (uint256) {
+    require(_index < totalSupply());
+    return allTokens[_index];
   }
 
   /**
@@ -178,13 +113,11 @@ contract ERC721Token is ERC721 {
   * @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 addToken(address _to, uint256 _tokenId) private {
-    require(tokenOwner[_tokenId] == address(0));
-    tokenOwner[_tokenId] = _to;
-    uint256 length = balanceOf(_to);
+  function addTokenTo(address _to, uint256 _tokenId) internal {
+    super.addTokenTo(_to, _tokenId);
+    uint256 length = ownedTokens[_to].length;
     ownedTokens[_to].push(_tokenId);
     ownedTokensIndex[_tokenId] = length;
-    totalTokens = totalTokens.add(1);
   }
 
   /**
@@ -192,14 +125,13 @@ contract ERC721Token is ERC721 {
   * @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 removeToken(address _from, uint256 _tokenId) private {
-    require(ownerOf(_tokenId) == _from);
+  function removeTokenFrom(address _from, uint256 _tokenId) internal {
+    super.removeTokenFrom(_from, _tokenId);
 
     uint256 tokenIndex = ownedTokensIndex[_tokenId];
-    uint256 lastTokenIndex = balanceOf(_from).sub(1);
+    uint256 lastTokenIndex = ownedTokens[_from].length.sub(1);
     uint256 lastToken = ownedTokens[_from][lastTokenIndex];
 
-    tokenOwner[_tokenId] = 0;
     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
@@ -209,6 +141,46 @@ contract ERC721Token is ERC721 {
     ownedTokens[_from].length--;
     ownedTokensIndex[_tokenId] = 0;
     ownedTokensIndex[lastToken] = tokenIndex;
-    totalTokens = totalTokens.sub(1);
   }
+
+  /**
+  * @dev Internal 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) internal {
+    super._mint(_to, _tokenId);
+    
+    allTokensIndex[_tokenId] = allTokens.length;
+    allTokens.push(_tokenId);
+  }
+
+  /**
+  * @dev Internal function to burn a specific token
+  * @dev Reverts if the token does not exist
+  * @param _owner owner of the token to burn
+  * @param _tokenId uint256 ID of the token being burned by the msg.sender
+  */
+  function _burn(address _owner, uint256 _tokenId) internal {
+    super._burn(_owner, _tokenId);
+
+    // Clear metadata (if any)
+    if (bytes(tokenURIs[_tokenId]).length != 0) {
+      delete tokenURIs[_tokenId];
+    }
+
+    // Reorg all tokens array
+    uint256 tokenIndex = allTokensIndex[_tokenId];
+    uint256 lastTokenIndex = allTokens.length.sub(1);
+    uint256 lastToken = allTokens[lastTokenIndex];
+
+    allTokens[tokenIndex] = lastToken;
+    allTokens[lastTokenIndex] = 0;
+
+    allTokens.length--;
+    allTokensIndex[_tokenId] = 0;
+    allTokensIndex[lastToken] = tokenIndex;
+  }
+
 }

test/helpers/sendTransaction.js

+const _ = require('lodash');
+const ethjsABI = require('ethjs-abi');
+
+export function findMethod (abi, name, args) {
+  for (var i = 0; i < abi.length; i++) {
+    const methodArgs = _.map(abi[i].inputs, 'type').join(',');
+    if ((abi[i].name === name) && (methodArgs === args)) {
+      return abi[i];
+    }
+  }
+}
+
+export default function sendTransaction (target, name, argsTypes, argsValues, opts) {
+  const abiMethod = findMethod(target.abi, name, argsTypes);
+  const encodedData = ethjsABI.encodeMethod(abiMethod, argsValues);
+  return target.sendTransaction(Object.assign({ data: encodedData }, opts));
+}

test/token/ERC721/ERC721BasicToken.behaviour.js

+import assertRevert from '../../helpers/assertRevert';
+import decodeLogs from '../../helpers/decodeLogs';
+import sendTransaction from '../../helpers/sendTransaction';
+import _ from 'lodash';
+
+const ERC721Receiver = artifacts.require('ERC721ReceiverMock.sol');
+const BigNumber = web3.BigNumber;
+
+require('chai')
+  .use(require('chai-as-promised'))
+  .use(require('chai-bignumber')(BigNumber))
+  .should();
+
+export default function shouldBehaveLikeERC721BasicToken (accounts) {
+  const firstTokenId = 1;
+  const secondTokenId = 2;
+  const unknownTokenId = 3;
+  const creator = accounts[0];
+  const ZERO_ADDRESS = '0x0000000000000000000000000000000000000000';
+  const RECEIVER_MAGIC_VALUE = '0xf0b9e5ba';
+
+  describe('like a ERC721BasicToken', function () {
+    beforeEach(async function () {
+      await this.token.mint(creator, firstTokenId, { from: creator });
+      await this.token.mint(creator, secondTokenId, { from: creator });
+    });
+
+    describe('balanceOf', function () {
+      describe('when the given address owns some tokens', function () {
+        it('returns the amount of tokens owned by the given address', async function () {
+          const balance = await this.token.balanceOf(creator);
+          balance.should.be.bignumber.equal(2);
+        });
+      });
+
+      describe('when the given address does not own any tokens', function () {
+        it('returns 0', async function () {
+          const balance = await this.token.balanceOf(accounts[1]);
+          balance.should.be.bignumber.equal(0);
+        });
+      });
+
+      describe('when querying the zero address', function () {
+        it('throws', async function () {
+          await assertRevert(this.token.balanceOf(0));
+        });
+      });
+    });
+
+    describe('exists', function () {
+      describe('when the token exists', function () {
+        const tokenId = firstTokenId;
+
+        it('should return true', async function () {
+          const result = await this.token.exists(tokenId);
+          result.should.be.true;
+        });
+      });
+
+      describe('when the token does not exist', function () {
+        const tokenId = unknownTokenId;
+
+        it('should return false', async function () {
+          const result = await this.token.exists(tokenId);
+          result.should.be.false;
+        });
+      });
+    });
+
+    describe('ownerOf', function () {
+      describe('when the given token ID was tracked by this token', function () {
+        const tokenId = firstTokenId;
+
+        it('returns the owner of the given token ID', async function () {
+          const owner = await this.token.ownerOf(tokenId);
+          owner.should.be.equal(creator);
+        });
+      });
+
+      describe('when the given token ID was not tracked by this token', function () {
+        const tokenId = unknownTokenId;
+
+        it('reverts', async function () {
+          await assertRevert(this.token.ownerOf(tokenId));
+        });
+      });
+    });
+
+    describe('transfers', function () {
+      const owner = accounts[0];
+      const approved = accounts[2];
+      const operator = accounts[3];
+      const unauthorized = accounts[4];
+      const tokenId = firstTokenId;
+      const data = '0x42';
+      
+      let logs = null;
+
+      beforeEach(async function () {
+        this.to = accounts[1];
+        await this.token.approve(approved, tokenId, { from: owner });
+        await this.token.setApprovalForAll(operator, true, { from: owner });
+      });
+
+      const transferWasSuccessful = function ({ owner, tokenId, approved }) {
+        it('transfers the ownership of the given token ID to the given address', async function () {
+          const newOwner = await this.token.ownerOf(tokenId);
+          newOwner.should.be.equal(this.to);
+        });
+
+        it('clears the approval for the token ID', async function () {
+          const approvedAccount = await this.token.getApproved(tokenId);
+          approvedAccount.should.be.equal(ZERO_ADDRESS);
+        });
+
+        if (approved) {
+          it('emits an approval and transfer events', async function () {
+            logs.length.should.be.equal(2);
+            logs[0].event.should.be.eq('Approval');
+            logs[0].args._owner.should.be.equal(owner);
+            logs[0].args._approved.should.be.equal(ZERO_ADDRESS);
+            logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
+  
+            logs[1].event.should.be.eq('Transfer');
+            logs[1].args._from.should.be.equal(owner);
+            logs[1].args._to.should.be.equal(this.to);
+            logs[1].args._tokenId.should.be.bignumber.equal(tokenId);
+          });
+        } else {
+          it('emits only a transfer event', async function () {
+            logs.length.should.be.equal(1);
+            logs[0].event.should.be.eq('Transfer');
+            logs[0].args._from.should.be.equal(owner);
+            logs[0].args._to.should.be.equal(this.to);
+            logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
+          });
+        }
+
+        it('adjusts owners balances', async function () {
+          const newOwnerBalance = await this.token.balanceOf(this.to);
+          newOwnerBalance.should.be.bignumber.equal(1);
+
+          const previousOwnerBalance = await this.token.balanceOf(owner);
+          previousOwnerBalance.should.be.bignumber.equal(1);
+        });
+
+        it('adjusts owners tokens by index', async function () {
+          if (!this.token.tokenOfOwnerByIndex) return;
+
+          const newOwnerToken = await this.token.tokenOfOwnerByIndex(this.to, 0);
+          newOwnerToken.toNumber().should.be.equal(tokenId);
+          
+          const previousOwnerToken = await this.token.tokenOfOwnerByIndex(owner, 0);
+          previousOwnerToken.toNumber().should.not.be.equal(tokenId);
+        });
+      };
+
+      const shouldTransferTokensByUsers = function (transferFunction) {
+        describe('when called by the owner', function () {
+          beforeEach(async function () {
+            ({ logs } = await transferFunction.call(this, owner, this.to, tokenId, { from: owner }));
+          });
+          transferWasSuccessful({ owner, tokenId, approved });
+        });
+
+        describe('when called by the approved individual', function () {
+          beforeEach(async function () {
+            ({ logs } = await transferFunction.call(this, owner, this.to, tokenId, { from: approved }));
+          });
+          transferWasSuccessful({ owner, tokenId, approved });
+        });
+
+        describe('when called by the operator', function () {
+          beforeEach(async function () {
+            ({ logs } = await transferFunction.call(this, owner, this.to, tokenId, { from: operator }));
+          });
+          transferWasSuccessful({ owner, tokenId, approved });
+        });
+
+        describe('when called by the owner without an approved user', function () {
+          beforeEach(async function () {
+            await this.token.approve(ZERO_ADDRESS, tokenId, { from: owner });
+            ({ logs } = await transferFunction.call(this, owner, this.to, tokenId, { from: operator }));
+          });
+          transferWasSuccessful({ owner, tokenId, approved: null });
+        });
+
+        describe('when sent to the owner', function () {
+          beforeEach(async function () {
+            ({ logs } = await transferFunction.call(this, owner, owner, tokenId, { from: owner }));
+          });
+
+          it('keeps ownership of the token', async function () {
+            const newOwner = await this.token.ownerOf(tokenId);
+            newOwner.should.be.equal(owner);
+          });
+  
+          it('clears the approval for the token ID', async function () {
+            const approvedAccount = await this.token.getApproved(tokenId);
+            approvedAccount.should.be.equal(ZERO_ADDRESS);
+          });
+  
+          it('emits an approval and transfer events', async function () {
+            logs.length.should.be.equal(2);
+            logs[0].event.should.be.eq('Approval');
+            logs[0].args._owner.should.be.equal(owner);
+            logs[0].args._approved.should.be.equal(ZERO_ADDRESS);
+            logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
+  
+            logs[1].event.should.be.eq('Transfer');
+            logs[1].args._from.should.be.equal(owner);
+            logs[1].args._to.should.be.equal(owner);
+            logs[1].args._tokenId.should.be.bignumber.equal(tokenId);
+          });
+  
+          it('keeps the owner balance', async function () {
+            const ownerBalance = await this.token.balanceOf(owner);
+            ownerBalance.should.be.bignumber.equal(2);
+          });
+  
+          it('keeps same tokens by index', async function () {
+            if (!this.token.tokenOfOwnerByIndex) return;
+            const tokensListed = await Promise.all(_.range(2).map(i => this.token.tokenOfOwnerByIndex(owner, i)));
+            tokensListed.map(t => t.toNumber()).should.have.members([firstTokenId, secondTokenId]);
+          });
+        });
+        
+        describe('when the address of the previous owner is incorrect', function () {
+          it('reverts', async function () {
+            await assertRevert(transferFunction.call(this, unauthorized, this.to, tokenId, { from: owner }));
+          });
+        });
+
+        describe('when the sender is not authorized for the token id', function () {
+          it('reverts', async function () {
+            await assertRevert(transferFunction.call(this, owner, this.to, tokenId, { from: unauthorized }));
+          });
+        });
+
+        describe('when the given token ID does not exist', function () {
+          it('reverts', async function () {
+            await assertRevert(transferFunction.call(this, owner, this.to, unknownTokenId, { from: owner }));
+          });
+        });
+
+        describe('when the address to transfer the token to is the zero address', function () {
+          it('reverts', async function () {
+            await assertRevert(transferFunction.call(this, owner, ZERO_ADDRESS, tokenId, { from: owner }));
+          });
+        });
+      };
+
+      describe('via transferFrom', function () {
+        shouldTransferTokensByUsers(function (from, to, tokenId, opts) {
+          return this.token.transferFrom(from, to, tokenId, opts);
+        });
+      });
+
+      describe('via safeTransferFrom', function () {
+        const safeTransferFromWithData = function (from, to, tokenId, opts) {
+          return sendTransaction(
+            this.token,
+            'safeTransferFrom',
+            'address,address,uint256,bytes',
+            [from, to, tokenId, data],
+            opts
+          );
+        };
+
+        const safeTransferFromWithoutData = function (from, to, tokenId, opts) {
+          return this.token.safeTransferFrom(from, to, tokenId, opts);
+        };
+
+        const shouldTransferSafely = function (transferFun, data) {
+          describe('to a user account', function () {
+            shouldTransferTokensByUsers(transferFun);
+          });
+  
+          describe('to a valid receiver contract', function () {
+            beforeEach(async function () {
+              this.receiver = await ERC721Receiver.new(RECEIVER_MAGIC_VALUE, false);
+              this.to = this.receiver.address;
+            });
+  
+            shouldTransferTokensByUsers(transferFun);
+  
+            it('should call onERC721Received', async function () {
+              const result = await transferFun.call(this, owner, this.to, tokenId, { from: owner });
+              result.receipt.logs.length.should.be.equal(3);
+              const [log] = decodeLogs([result.receipt.logs[2]], ERC721Receiver, this.receiver.address);
+              log.event.should.be.eq('Received');
+              log.args._address.should.be.equal(owner);
+              log.args._tokenId.toNumber().should.be.equal(tokenId);
+              log.args._data.should.be.equal(data);
+            });
+          });
+        };
+
+        describe('with data', function () {
+          shouldTransferSafely(safeTransferFromWithData, data);
+        });
+
+        describe('without data', function () {
+          shouldTransferSafely(safeTransferFromWithoutData, '0x');
+        });
+
+        describe('to a receiver contract returning unexpected value', function () {
+          it('reverts', async function () {
+            const invalidReceiver = await ERC721Receiver.new('0x42', false);
+            await assertRevert(this.token.safeTransferFrom(owner, invalidReceiver.address, tokenId, { from: owner }));
+          });
+        });
+
+        describe('to a receiver contract that throws', function () {
+          it('reverts', async function () {
+            const invalidReceiver = await ERC721Receiver.new(RECEIVER_MAGIC_VALUE, true);
+            await assertRevert(this.token.safeTransferFrom(owner, invalidReceiver.address, tokenId, { from: owner }));
+          });
+        });
+
+        describe('to a contract that does not implement the required function', function () {
+          it('reverts', async function () {
+            const invalidReceiver = this.token;
+            await assertRevert(this.token.safeTransferFrom(owner, invalidReceiver.address, tokenId, { from: owner }));
+          });
+        });
+      });
+    });
+
+    describe('approve', function () {
+      const tokenId = firstTokenId;
+      const sender = creator;
+      const to = accounts[1];
+
+      let logs = null;
+
+      const itClearsApproval = function () {
+        it('clears approval for the token', async function () {
+          const approvedAccount = await this.token.getApproved(tokenId);
+          approvedAccount.should.be.equal(ZERO_ADDRESS);
+        });
+      };
+
+      const itApproves = function (address) {
+        it('sets the approval for the target address', async function () {
+          const approvedAccount = await this.token.getApproved(tokenId);
+          approvedAccount.should.be.equal(address);
+        });
+      };
+
+      const itEmitsApprovalEvent = function (address) {
+        it('emits an approval event', async function () {
+          logs.length.should.be.equal(1);
+          logs[0].event.should.be.eq('Approval');
+          logs[0].args._owner.should.be.equal(sender);
+          logs[0].args._approved.should.be.equal(address);
+          logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
+        });
+      };
+      
+      describe('when clearing approval', function () {
+        describe('when there was no prior approval', function () {
+          beforeEach(async function () {
+            ({ logs } = await this.token.approve(ZERO_ADDRESS, tokenId, { from: sender }));
+          });
+
+          itClearsApproval();
+
+          it('does not emit an approval event', async function () {
+            logs.length.should.be.equal(0);
+          });
+        });
+    
+        describe('when there was a prior approval', function () {
+          beforeEach(async function () {
+            await this.token.approve(to, tokenId, { from: sender });
+            ({ logs } = await this.token.approve(ZERO_ADDRESS, tokenId, { from: sender }));
+          });
+
+          itClearsApproval();
+          itEmitsApprovalEvent(ZERO_ADDRESS);
+        });
+      });
+
+      describe('when approving a non-zero address', function () {
+        describe('when there was no prior approval', function () {
+          beforeEach(async function () {
+            ({ logs } = await this.token.approve(to, tokenId, { from: sender }));
+          });
+
+          itApproves(to);
+          itEmitsApprovalEvent(to);
+        });
+
+        describe('when there was a prior approval to the same address', function () {
+          beforeEach(async function () {
+            await this.token.approve(to, tokenId, { from: sender });
+            ({ logs } = await this.token.approve(to, tokenId, { from: sender }));
+          });
+
+          itApproves(to);
+          itEmitsApprovalEvent(to);
+        });
+
+        describe('when there was a prior approval to a different address', function () {
+          beforeEach(async function () {
+            await this.token.approve(accounts[2], tokenId, { from: sender });
+            ({ logs } = await this.token.approve(to, tokenId, { from: sender }));
+          });
+
+          itApproves(to);
+          itEmitsApprovalEvent(to);
+        });
+      });
+
+      describe('when the address that receives the approval is the owner', function () {
+        it('reverts', async function () {
+          await assertRevert(this.token.approve(sender, tokenId, { from: sender }));
+        });
+      });
+      
+      describe('when the sender does not own the given token ID', function () {
+        it('reverts', async function () {
+          await assertRevert(this.token.approve(to, tokenId, { from: accounts[2] }));
+        });
+      });
+
+      describe('when the sender is approved for the given token ID', function () {
+        it('reverts', async function () {
+          await this.token.approve(accounts[2], tokenId, { from: sender });
+          await assertRevert(this.token.approve(to, tokenId, { from: accounts[2] }));
+        });
+      });
+
+      describe('when the sender is an operator', function () {
+        const operator = accounts[2];
+        beforeEach(async function () {
+          await this.token.setApprovalForAll(operator, true, { from: sender });
+          ({ logs } = await this.token.approve(to, tokenId, { from: operator }));
+        });
+
+        itApproves(to);
+        itEmitsApprovalEvent(to);
+      });
+
+      describe('when the given token ID does not exist', function () {
+        it('reverts', async function () {
+          await assertRevert(this.token.approve(to, unknownTokenId, { from: sender }));
+        });
+      });
+    });
+
+    describe('setApprovalForAll', function () {
+      const sender = creator;
+
+      describe('when the operator willing to approve is not the owner', function () {
+        const operator = accounts[1];
+
+        describe('when there is no operator approval set by the sender', function () {
+          it('approves the operator', async function () {
+            await this.token.setApprovalForAll(operator, true, { from: sender });
+
+            const isApproved = await this.token.isApprovedForAll(sender, operator);
+            isApproved.should.be.true;
+          });
+
+          it('emits an approval event', async function () {
+            const { logs } = await this.token.setApprovalForAll(operator, true, { from: sender });
+
+            logs.length.should.be.equal(1);
+            logs[0].event.should.be.eq('ApprovalForAll');
+            logs[0].args._owner.should.be.equal(sender);
+            logs[0].args._operator.should.be.equal(operator);
+            logs[0].args._approved.should.be.true;
+          });
+        });
+
+        describe('when the operator was set as not approved', function () {
+          beforeEach(async function () {
+            await this.token.setApprovalForAll(operator, false, { from: sender });
+          });
+
+          it('approves the operator', async function () {
+            await this.token.setApprovalForAll(operator, true, { from: sender });
+
+            const isApproved = await this.token.isApprovedForAll(sender, operator);
+            isApproved.should.be.true;
+          });
+
+          it('emits an approval event', async function () {
+            const { logs } = await this.token.setApprovalForAll(operator, true, { from: sender });
+
+            logs.length.should.be.equal(1);
+            logs[0].event.should.be.eq('ApprovalForAll');
+            logs[0].args._owner.should.be.equal(sender);
+            logs[0].args._operator.should.be.equal(operator);
+            logs[0].args._approved.should.be.true;
+          });
+
+          it('can unset the operator approval', async function () {
+            await this.token.setApprovalForAll(operator, false, { from: sender });
+
+            const isApproved = await this.token.isApprovedForAll(sender, operator);
+            isApproved.should.be.false;
+          });
+        });
+
+        describe('when the operator was already approved', function () {
+          beforeEach(async function () {
+            await this.token.setApprovalForAll(operator, true, { from: sender });
+          });
+
+          it('keeps the approval to the given address', async function () {
+            await this.token.setApprovalForAll(operator, true, { from: sender });
+
+            const isApproved = await this.token.isApprovedForAll(sender, operator);
+            isApproved.should.be.true;
+          });
+
+          it('emits an approval event', async function () {
+            const { logs } = await this.token.setApprovalForAll(operator, true, { from: sender });
+
+            logs.length.should.be.equal(1);
+            logs[0].event.should.be.eq('ApprovalForAll');
+            logs[0].args._owner.should.be.equal(sender);
+            logs[0].args._operator.should.be.equal(operator);
+            logs[0].args._approved.should.be.true;
+          });
+        });
+      });
+
+      describe('when the operator is the owner', function () {
+        const operator = creator;
+
+        it('reverts', async function () {
+          await assertRevert(this.token.setApprovalForAll(operator, true, { from: sender }));
+        });
+      });
+    });
+  });
+};

test/token/ERC721/ERC721BasicToken.test.js

+import shouldBehaveLikeERC721BasicToken from './ERC721BasicToken.behaviour';
+import shouldMintAndBurnERC721Token from './ERC721MintBurn.behaviour';
+
+const BigNumber = web3.BigNumber;
+const ERC721BasicToken = artifacts.require('ERC721BasicTokenMock.sol');
+
+require('chai')
+  .use(require('chai-as-promised'))
+  .use(require('chai-bignumber')(BigNumber))
+  .should();
+
+contract('ERC721BasicToken', function (accounts) {
+  beforeEach(async function () {
+    this.token = await ERC721BasicToken.new({ from: accounts[0] });
+  });
+
+  shouldBehaveLikeERC721BasicToken(accounts);
+  shouldMintAndBurnERC721Token(accounts);
+});

test/token/ERC721/ERC721MintBurn.behaviour.js

+import assertRevert from '../../helpers/assertRevert';
+const BigNumber = web3.BigNumber;
+
+require('chai')
+  .use(require('chai-as-promised'))
+  .use(require('chai-bignumber')(BigNumber))
+  .should();
+
+export default function shouldMintAndBurnERC721Token (accounts) {
+  const firstTokenId = 1;
+  const secondTokenId = 2;
+  const unknownTokenId = 3;
+  const creator = accounts[0];
+  const ZERO_ADDRESS = '0x0000000000000000000000000000000000000000';
+
+  describe('like a mintable and burnable ERC721Token', function () {
+    beforeEach(async function () {
+      await this.token.mint(creator, firstTokenId, { from: creator });
+      await this.token.mint(creator, secondTokenId, { from: creator });
+    });
+
+    describe('mint', function () {
+      const to = accounts[1];
+      const tokenId = unknownTokenId;
+      let logs = null;
+      
+      describe('when successful', function () {
+        beforeEach(async function () {
+          const result = await this.token.mint(to, tokenId);
+          logs = result.logs;
+        });
+
+        it('assigns the token to the new owner', async function () {
+          const owner = await this.token.ownerOf(tokenId);
+          owner.should.be.equal(to);
+        });
+
+        it('increases the balance of its owner', async function () {
+          const balance = await this.token.balanceOf(to);
+          balance.should.be.bignumber.equal(1);
+        });
+
+        it('emits a transfer event', async function () {
+          logs.length.should.be.equal(1);
+          logs[0].event.should.be.eq('Transfer');
+          logs[0].args._from.should.be.equal(ZERO_ADDRESS);
+          logs[0].args._to.should.be.equal(to);
+          logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
+        });
+      });
+
+      describe('when the given owner address is the zero address', function () {
+        it('reverts', async function () {
+          await assertRevert(this.token.mint(ZERO_ADDRESS, tokenId));
+        });
+      });
+
+      describe('when the given token ID was already tracked by this contract', function () {
+        it('reverts', async function () {
+          await assertRevert(this.token.mint(accounts[1], firstTokenId));
+        });
+      });
+    });
+
+    describe('burn', function () {
+      const tokenId = firstTokenId;
+      const sender = creator;
+      let logs = null;
+
+      describe('when successful', function () {
+        beforeEach(async function () {
+          const result = await this.token.burn(tokenId, { from: sender });
+          logs = result.logs;
+        });
+
+        it('burns the given token ID and adjusts the balance of the owner', async function () {
+          await assertRevert(this.token.ownerOf(tokenId));
+          const balance = await this.token.balanceOf(sender);
+          balance.should.be.bignumber.equal(1);
+        });
+
+        it('emits a burn event', async function () {
+          logs.length.should.be.equal(1);
+          logs[0].event.should.be.eq('Transfer');
+          logs[0].args._from.should.be.equal(sender);
+          logs[0].args._to.should.be.equal(ZERO_ADDRESS);
+          logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
+        });
+      });
+
+      describe('when there is a previous approval', function () {
+        beforeEach(async function () {
+          await this.token.approve(accounts[1], tokenId, { from: sender });
+          const result = await this.token.burn(tokenId, { from: sender });
+          logs = result.logs;
+        });
+
+        it('clears the approval', async function () {
+          const approvedAccount = await this.token.getApproved(tokenId);
+          approvedAccount.should.be.equal(ZERO_ADDRESS);
+        });
+
+        it('emits an approval event', async function () {
+          logs.length.should.be.equal(2);
+
+          logs[0].event.should.be.eq('Approval');
+          logs[0].args._owner.should.be.equal(sender);
+          logs[0].args._approved.should.be.equal(ZERO_ADDRESS);
+          logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
+
+          logs[1].event.should.be.eq('Transfer');
+        });
+      });
+
+      describe('when the given token ID was not tracked by this contract', function () {
+        it('reverts', async function () {
+          await assertRevert(this.token.burn(unknownTokenId, { from: creator }));
+        });
+      });
+    });
+  });
+};

test/token/ERC721/ERC721Token.test.js

 import assertRevert from '../../helpers/assertRevert';
+import shouldBehaveLikeERC721BasicToken from './ERC721BasicToken.behaviour';
+import shouldMintAndBurnERC721Token from './ERC721MintBurn.behaviour';
+import _ from 'lodash';
+
 const BigNumber = web3.BigNumber;
 const ERC721Token = artifacts.require('ERC721TokenMock.sol');
 
@@ -7,527 +11,188 @@ require('chai')
   .use(require('chai-bignumber')(BigNumber))
   .should();
 
-contract('ERC721Token', accounts => {
-  let token = null;
-  const _firstTokenId = 1;
-  const _secondTokenId = 2;
-  const _unknownTokenId = 3;
-  const _creator = accounts[0];
-  const ZERO_ADDRESS = '0x0000000000000000000000000000000000000000';
+contract('ERC721Token', function (accounts) {
+  const name = 'Non Fungible Token';
+  const symbol = 'NFT';
+  const firstTokenId = 100;
+  const secondTokenId = 200;
+  const creator = accounts[0];
 
   beforeEach(async function () {
-    token = await ERC721Token.new({ from: _creator });
-    await token.mint(_creator, _firstTokenId, { from: _creator });
-    await token.mint(_creator, _secondTokenId, { from: _creator });
+    this.token = await ERC721Token.new(name, symbol, { from: creator });
   });
 
-  describe('totalSupply', function () {
-    it('has a total supply equivalent to the inital supply', async function () {
-      const totalSupply = await token.totalSupply();
-      totalSupply.should.be.bignumber.equal(2);
-    });
-  });
-
-  describe('balanceOf', function () {
-    describe('when the given address owns some tokens', function () {
-      it('returns the amount of tokens owned by the given address', async function () {
-        const balance = await token.balanceOf(_creator);
-        balance.should.be.bignumber.equal(2);
-      });
-    });
+  shouldBehaveLikeERC721BasicToken(accounts);
+  shouldMintAndBurnERC721Token(accounts);
 
-    describe('when the given address does not own any tokens', function () {
-      it('returns 0', async function () {
-        const balance = await token.balanceOf(accounts[1]);
-        balance.should.be.bignumber.equal(0);
-      });
+  describe('like a full ERC721', function () {
+    beforeEach(async function () {
+      await this.token.mint(creator, firstTokenId, { from: creator });
+      await this.token.mint(creator, secondTokenId, { from: creator });
     });
-  });
 
-  describe('ownerOf', function () {
-    describe('when the given token ID was tracked by this token', function () {
-      const tokenId = _firstTokenId;
+    describe('mint', function () {
+      const to = accounts[1];
+      const tokenId = 3;
 
-      it('returns the owner of the given token ID', async function () {
-        const owner = await token.ownerOf(tokenId);
-        owner.should.be.equal(_creator);
+      beforeEach(async function () {
+        await this.token.mint(to, tokenId);
       });
-    });
 
-    describe('when the given token ID was not tracked by this token', function () {
-      const tokenId = _unknownTokenId;
+      it('adjusts owner tokens by index', async function () {
+        const token = await this.token.tokenOfOwnerByIndex(to, 0);
+        token.toNumber().should.be.equal(tokenId);
+      });
 
-      it('reverts', async function () {
-        await assertRevert(token.ownerOf(tokenId));
+      it('adjusts all tokens list', async function () {
+        const newToken = await this.token.tokenByIndex(2);
+        newToken.toNumber().should.be.equal(tokenId);
       });
     });
-  });
-
-  describe('mint', function () {
-    describe('when the given token ID was not tracked by this contract', function () {
-      const tokenId = _unknownTokenId;
-
-      describe('when the given address is not the zero address', function () {
-        const to = accounts[1];
-
-        it('mints the given token ID to the given address', async function () {
-          const previousBalance = await token.balanceOf(to);
-
-          await token.mint(to, tokenId);
-
-          const owner = await token.ownerOf(tokenId);
-          owner.should.be.equal(to);
-
-          const balance = await token.balanceOf(to);
-          balance.should.be.bignumber.equal(previousBalance + 1);
-        });
 
-        it('adds that token to the token list of the owner', async function () {
-          await token.mint(to, tokenId);
+    describe('burn', function () {
+      const tokenId = firstTokenId;
+      const sender = creator;
 
-          const tokens = await token.tokensOf(to);
-          tokens.length.should.be.equal(1);
-          tokens[0].should.be.bignumber.equal(tokenId);
-        });
-
-        it('emits a transfer event', async function () {
-          const { logs } = await token.mint(to, tokenId);
-
-          logs.length.should.be.equal(1);
-          logs[0].event.should.be.eq('Transfer');
-          logs[0].args._from.should.be.equal(ZERO_ADDRESS);
-          logs[0].args._to.should.be.equal(to);
-          logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
-        });
+      beforeEach(async function () {
+        await this.token.burn(tokenId, { from: sender });
       });
 
-      describe('when the given address is the zero address', function () {
-        const to = ZERO_ADDRESS;
-
-        it('reverts', async function () {
-          await assertRevert(token.mint(to, tokenId));
-        });
+      it('removes that token from the token list of the owner', async function () {
+        const token = await this.token.tokenOfOwnerByIndex(sender, 0);
+        token.toNumber().should.be.equal(secondTokenId);
       });
-    });
 
-    describe('when the given token ID was already tracked by this contract', function () {
-      const tokenId = _firstTokenId;
+      it('adjusts all tokens list', async function () {
+        const token = await this.token.tokenByIndex(0);
+        token.toNumber().should.be.equal(secondTokenId);
+      });
 
-      it('reverts', async function () {
-        await assertRevert(token.mint(accounts[1], tokenId));
+      it('burns all tokens', async function () {
+        await this.token.burn(secondTokenId, { from: sender });
+        const total = await this.token.totalSupply();
+        total.toNumber().should.be.equal(0);
+        await assertRevert(this.token.tokenByIndex(0));
       });
     });
-  });
-
-  describe('burn', function () {
-    describe('when the given token ID was tracked by this contract', function () {
-      const tokenId = _firstTokenId;
-
-      describe('when the msg.sender owns given token', function () {
-        const sender = _creator;
-
-        it('burns the given token ID and adjusts the balance of the owner', async function () {
-          const previousBalance = await token.balanceOf(sender);
-
-          await token.burn(tokenId, { from: sender });
+    
+    describe('metadata', function () {
+      const sampleUri = 'mock://mytoken';
 
-          await assertRevert(token.ownerOf(tokenId));
-          const balance = await token.balanceOf(sender);
-          balance.should.be.bignumber.equal(previousBalance - 1);
-        });
-
-        it('removes that token from the token list of the owner', async function () {
-          await token.burn(tokenId, { from: sender });
-
-          const tokens = await token.tokensOf(sender);
-          tokens.length.should.be.equal(1);
-          tokens[0].should.be.bignumber.equal(_secondTokenId);
-        });
-
-        it('emits a burn event', async function () {
-          const { logs } = await token.burn(tokenId, { from: sender });
-
-          logs.length.should.be.equal(1);
-          logs[0].event.should.be.eq('Transfer');
-          logs[0].args._from.should.be.equal(sender);
-          logs[0].args._to.should.be.equal(ZERO_ADDRESS);
-          logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
-        });
-
-        describe('when there is an approval for the given token ID', function () {
-          beforeEach(async function () {
-            await token.approve(accounts[1], tokenId, { from: sender });
-          });
-
-          it('clears the approval', async function () {
-            await token.burn(tokenId, { from: sender });
-            const approvedAccount = await token.approvedFor(tokenId);
-            approvedAccount.should.be.equal(ZERO_ADDRESS);
-          });
-
-          it('emits an approval event', async function () {
-            const { logs } = await token.burn(tokenId, { from: sender });
-
-            logs.length.should.be.equal(2);
-
-            logs[0].event.should.be.eq('Approval');
-            logs[0].args._owner.should.be.equal(sender);
-            logs[0].args._approved.should.be.equal(ZERO_ADDRESS);
-            logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
-          });
-        });
+      it('has a name', async function () {
+        const name = await this.token.name();
+        name.should.be.equal(name);
       });
 
-      describe('when the msg.sender does not own given token', function () {
-        const sender = accounts[1];
-
-        it('reverts', async function () {
-          await assertRevert(token.burn(tokenId, { from: sender }));
-        });
+      it('has a symbol', async function () {
+        const symbol = await this.token.symbol();
+        symbol.should.be.equal(symbol);
       });
-    });
-
-    describe('when the given token ID was not tracked by this contract', function () {
-      const tokenID = _unknownTokenId;
 
-      it('reverts', async function () {
-        await assertRevert(token.burn(tokenID, { from: _creator }));
+      it('sets and returns metadata for a token id', async function () {
+        await this.token.setTokenURI(firstTokenId, sampleUri);
+        const uri = await this.token.tokenURI(firstTokenId);
+        uri.should.be.equal(sampleUri);
       });
-    });
-  });
-
-  describe('transfer', function () {
-    describe('when the address to transfer the token to is not the zero address', function () {
-      const to = accounts[1];
-
-      describe('when the given token ID was tracked by this token', function () {
-        const tokenId = _firstTokenId;
-
-        describe('when the msg.sender is the owner of the given token ID', function () {
-          const sender = _creator;
-
-          it('transfers the ownership of the given token ID to the given address', async function () {
-            await token.transfer(to, tokenId, { from: sender });
-
-            const newOwner = await token.ownerOf(tokenId);
-            newOwner.should.be.equal(to);
-          });
-
-          it('clears the approval for the token ID', async function () {
-            await token.approve(accounts[2], tokenId, { from: sender });
-
-            await token.transfer(to, tokenId, { from: sender });
-
-            const approvedAccount = await token.approvedFor(tokenId);
-            approvedAccount.should.be.equal(ZERO_ADDRESS);
-          });
-
-          it('emits an approval and transfer events', async function () {
-            const { logs } = await token.transfer(to, tokenId, { from: sender });
-
-            logs.length.should.be.equal(2);
-
-            logs[0].event.should.be.eq('Approval');
-            logs[0].args._owner.should.be.equal(sender);
-            logs[0].args._approved.should.be.equal(ZERO_ADDRESS);
-            logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
-
-            logs[1].event.should.be.eq('Transfer');
-            logs[1].args._from.should.be.equal(sender);
-            logs[1].args._to.should.be.equal(to);
-            logs[1].args._tokenId.should.be.bignumber.equal(tokenId);
-          });
-
-          it('adjusts owners balances', async function () {
-            const previousBalance = await token.balanceOf(sender);
-            await token.transfer(to, tokenId, { from: sender });
-
-            const newOwnerBalance = await token.balanceOf(to);
-            newOwnerBalance.should.be.bignumber.equal(1);
-
-            const previousOwnerBalance = await token.balanceOf(_creator);
-            previousOwnerBalance.should.be.bignumber.equal(previousBalance - 1);
-          });
-
-          it('adds the token to the tokens list of the new owner', async function () {
-            await token.transfer(to, tokenId, { from: sender });
-
-            const tokenIDs = await token.tokensOf(to);
-            tokenIDs.length.should.be.equal(1);
-            tokenIDs[0].should.be.bignumber.equal(tokenId);
-          });
-        });
-
-        describe('when the msg.sender is not the owner of the given token ID', function () {
-          const sender = accounts[2];
 
-          it('reverts', async function () {
-            await assertRevert(token.transfer(to, tokenId, { from: sender }));
-          });
-        });
+      it('can burn token with metadata', async function () {
+        await this.token.setTokenURI(firstTokenId, sampleUri);
+        await this.token.burn(firstTokenId);
+        const exists = await this.token.exists(firstTokenId);
+        exists.should.be.false;
       });
 
-      describe('when the given token ID was not tracked by this token', function () {
-        let tokenId = _unknownTokenId;
+      it('returns empty metadata for token', async function () {
+        const uri = await this.token.tokenURI(firstTokenId);
+        uri.should.be.equal('');
+      });
 
-        it('reverts', async function () {
-          await assertRevert(token.transfer(to, tokenId, { from: _creator }));
-        });
+      it('reverts when querying metadata for non existant token id', async function () {
+        await assertRevert(this.token.tokenURI(500));
       });
     });
 
-    describe('when the address to transfer the token to is the zero address', function () {
-      const to = ZERO_ADDRESS;
-
-      it('reverts', async function () {
-        await assertRevert(token.transfer(to, 0, { from: _creator }));
+    describe('totalSupply', function () {
+      it('returns total token supply', async function () {
+        const totalSupply = await this.token.totalSupply();
+        totalSupply.should.be.bignumber.equal(2);
       });
     });
-  });
-
-  describe('approve', function () {
-    describe('when the given token ID was already tracked by this contract', function () {
-      const tokenId = _firstTokenId;
-
-      describe('when the sender owns the given token ID', function () {
-        const sender = _creator;
-
-        describe('when the address that receives the approval is the 0 address', function () {
-          const to = ZERO_ADDRESS;
-
-          describe('when there was no approval for the given token ID before', function () {
-            it('clears the approval for that token', async function () {
-              await token.approve(to, tokenId, { from: sender });
-
-              const approvedAccount = await token.approvedFor(tokenId);
-              approvedAccount.should.be.equal(to);
-            });
-
-            it('does not emit an approval event', async function () {
-              const { logs } = await token.approve(to, tokenId, { from: sender });
-
-              logs.length.should.be.equal(0);
-            });
-          });
-
-          describe('when the given token ID was approved for another account', function () {
-            beforeEach(async function () {
-              await token.approve(accounts[2], tokenId, { from: sender });
-            });
-
-            it('clears the approval for the token ID', async function () {
-              await token.approve(to, tokenId, { from: sender });
-
-              const approvedAccount = await token.approvedFor(tokenId);
-              approvedAccount.should.be.equal(to);
-            });
-
-            it('emits an approval event', async function () {
-              const { logs } = await token.approve(to, tokenId, { from: sender });
-
-              logs.length.should.be.equal(1);
-              logs[0].event.should.be.eq('Approval');
-              logs[0].args._owner.should.be.equal(sender);
-              logs[0].args._approved.should.be.equal(to);
-              logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
-            });
-          });
-        });
-
-        describe('when the address that receives the approval is not the 0 address', function () {
-          describe('when the address that receives the approval is different than the owner', function () {
-            const to = accounts[1];
-
-            describe('when there was no approval for the given token ID before', function () {
-              it('approves the token ID to the given address', async function () {
-                await token.approve(to, tokenId, { from: sender });
-
-                const approvedAccount = await token.approvedFor(tokenId);
-                approvedAccount.should.be.equal(to);
-              });
-
-              it('emits an approval event', async function () {
-                const { logs } = await token.approve(to, tokenId, { from: sender });
-
-                logs.length.should.be.equal(1);
-                logs[0].event.should.be.eq('Approval');
-                logs[0].args._owner.should.be.equal(sender);
-                logs[0].args._approved.should.be.equal(to);
-                logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
-              });
-            });
-
-            describe('when the given token ID was approved for the same account', function () {
-              beforeEach(async function () {
-                await token.approve(to, tokenId, { from: sender });
-              });
-
-              it('keeps the approval to the given address', async function () {
-                await token.approve(to, tokenId, { from: sender });
-
-                const approvedAccount = await token.approvedFor(tokenId);
-                approvedAccount.should.be.equal(to);
-              });
-
-              it('emits an approval event', async function () {
-                const { logs } = await token.approve(to, tokenId, { from: sender });
-
-                logs.length.should.be.equal(1);
-                logs[0].event.should.be.eq('Approval');
-                logs[0].args._owner.should.be.equal(sender);
-                logs[0].args._approved.should.be.equal(to);
-                logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
-              });
-            });
-
-            describe('when the given token ID was approved for another account', function () {
-              beforeEach(async function () {
-                await token.approve(accounts[2], tokenId, { from: sender });
-              });
-
-              it('changes the approval to the given address', async function () {
-                await token.approve(to, tokenId, { from: sender });
-
-                const approvedAccount = await token.approvedFor(tokenId);
-                approvedAccount.should.be.equal(to);
-              });
-
-              it('emits an approval event', async function () {
-                const { logs } = await token.approve(to, tokenId, { from: sender });
-
-                logs.length.should.be.equal(1);
-                logs[0].event.should.be.eq('Approval');
-                logs[0].args._owner.should.be.equal(sender);
-                logs[0].args._approved.should.be.equal(to);
-                logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
-              });
-            });
-          });
-
-          describe('when the address that receives the approval is the owner', function () {
-            const to = _creator;
-
-            describe('when there was no approval for the given token ID before', function () {
-              it('reverts', async function () {
-                await assertRevert(token.approve(to, tokenId, { from: sender }));
-              });
-            });
-
-            describe('when the given token ID was approved for another account', function () {
-              beforeEach(async function () {
-                await token.approve(accounts[2], tokenId, { from: sender });
-              });
 
-              it('reverts', async function () {
-                await assertRevert(token.approve(to, tokenId, { from: sender }));
-              });
-            });
-          });
+    describe('tokenOfOwnerByIndex', function () {
+      const owner = creator;
+      const another = accounts[1];
+        
+      describe('when the given index is lower than the amount of tokens owned by the given address', function () {
+        it('returns the token ID placed at the given index', async function () {
+          const tokenId = await this.token.tokenOfOwnerByIndex(owner, 0);
+          tokenId.should.be.bignumber.equal(firstTokenId);
         });
       });
 
-      describe('when the sender does not own the given token ID', function () {
-        const sender = accounts[1];
-
+      describe('when the index is greater than or equal to the total tokens owned by the given address', function () {
         it('reverts', async function () {
-          await assertRevert(token.approve(accounts[2], tokenId, { from: sender }));
+          await assertRevert(this.token.tokenOfOwnerByIndex(owner, 2));
         });
       });
-    });
-
-    describe('when the given token ID was not tracked by the contract before', function () {
-      const tokenId = _unknownTokenId;
 
-      it('reverts', async function () {
-        await assertRevert(token.approve(accounts[1], tokenId, { from: _creator }));
+      describe('when the given address does not own any token', function () {
+        it('reverts', async function () {
+          await assertRevert(this.token.tokenOfOwnerByIndex(another, 0));
+        });
       });
-    });
-  });
-
-  describe('takeOwnership', function () {
-    describe('when the given token ID was already tracked by this contract', function () {
-      const tokenId = _firstTokenId;
-
-      describe('when the sender has the approval for the token ID', function () {
-        const sender = accounts[1];
 
+      describe('after transferring all tokens to another user', function () {
         beforeEach(async function () {
-          await token.approve(sender, tokenId, { from: _creator });
-        });
-
-        it('transfers the ownership of the given token ID to the given address', async function () {
-          await token.takeOwnership(tokenId, { from: sender });
-
-          const newOwner = await token.ownerOf(tokenId);
-          newOwner.should.be.equal(sender);
-        });
-
-        it('clears the approval for the token ID', async function () {
-          await token.takeOwnership(tokenId, { from: sender });
-
-          const approvedAccount = await token.approvedFor(tokenId);
-          approvedAccount.should.be.equal(ZERO_ADDRESS);
-        });
-
-        it('emits an approval and transfer events', async function () {
-          const { logs } = await token.takeOwnership(tokenId, { from: sender });
-
-          logs.length.should.be.equal(2);
-
-          logs[0].event.should.be.eq('Approval');
-          logs[0].args._owner.should.be.equal(_creator);
-          logs[0].args._approved.should.be.equal(ZERO_ADDRESS);
-          logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
-
-          logs[1].event.should.be.eq('Transfer');
-          logs[1].args._from.should.be.equal(_creator);
-          logs[1].args._to.should.be.equal(sender);
-          logs[1].args._tokenId.should.be.bignumber.equal(tokenId);
-        });
-
-        it('adjusts owners balances', async function () {
-          const previousBalance = await token.balanceOf(_creator);
-
-          await token.takeOwnership(tokenId, { from: sender });
-
-          const newOwnerBalance = await token.balanceOf(sender);
-          newOwnerBalance.should.be.bignumber.equal(1);
-
-          const previousOwnerBalance = await token.balanceOf(_creator);
-          previousOwnerBalance.should.be.bignumber.equal(previousBalance - 1);
-        });
-
-        it('adds the token to the tokens list of the new owner', async function () {
-          await token.takeOwnership(tokenId, { from: sender });
-
-          const tokenIDs = await token.tokensOf(sender);
-          tokenIDs.length.should.be.equal(1);
-          tokenIDs[0].should.be.bignumber.equal(tokenId);
+          await this.token.transferFrom(owner, another, firstTokenId, { from: owner });
+          await this.token.transferFrom(owner, another, secondTokenId, { from: owner });
         });
-      });
-
-      describe('when the sender does not have an approval for the token ID', function () {
-        const sender = accounts[1];
 
-        it('reverts', async function () {
-          await assertRevert(token.takeOwnership(tokenId, { from: sender }));
+        it('returns correct token IDs for target', async function () {
+          const count = await this.token.balanceOf(another);
+          count.toNumber().should.be.equal(2);
+          const tokensListed = await Promise.all(_.range(2).map(i => this.token.tokenOfOwnerByIndex(another, i)));
+          tokensListed.map(t => t.toNumber()).should.have.members([firstTokenId, secondTokenId]);
         });
-      });
 
-      describe('when the sender is already the owner of the token', function () {
-        const sender = _creator;
-
-        it('reverts', async function () {
-          await assertRevert(token.takeOwnership(tokenId, { from: sender }));
+        it('returns empty collection for original owner', async function () {
+          const count = await this.token.balanceOf(owner);
+          count.toNumber().should.be.equal(0);
+          await assertRevert(this.token.tokenOfOwnerByIndex(owner, 0));
         });
       });
     });
 
-    describe('when the given token ID was not tracked by the contract before', function () {
-      const tokenId = _unknownTokenId;
-
-      it('reverts', async function () {
-        await assertRevert(token.takeOwnership(tokenId, { from: _creator }));
+    describe('tokenByIndex', function () {
+      it('should return all tokens', async function () {
+        const tokensListed = await Promise.all(_.range(2).map(i => this.token.tokenByIndex(i)));
+        tokensListed.map(t => t.toNumber()).should.have.members([firstTokenId, secondTokenId]);
+      });
+
+      it('should revert if index is greater than supply', async function () {
+        await assertRevert(this.token.tokenByIndex(2));
+      });
+
+      [firstTokenId, secondTokenId].forEach(function (tokenId) {
+        it(`should return all tokens after burning token ${tokenId} and minting new tokens`, async function () {
+          const owner = accounts[0];
+          const newTokenId = 300;
+          const anotherNewTokenId = 400;
+          
+          await this.token.burn(tokenId, { from: owner });
+          await this.token.mint(owner, newTokenId, { from: owner });
+          await this.token.mint(owner, anotherNewTokenId, { from: owner });
+  
+          const count = await this.token.totalSupply();
+          count.toNumber().should.be.equal(3);
+          
+          const tokensListed = await Promise.all(_.range(3).map(i => this.token.tokenByIndex(i)));
+          const expectedTokens = _.filter(
+            [firstTokenId, secondTokenId, newTokenId, anotherNewTokenId],
+            x => (x !== tokenId)
+          );
+          tokensListed.map(t => t.toNumber()).should.have.members(expectedTokens);
+        });
       });
     });
   });