Implement ERC721 required functionality

contracts/token/ERC721Token.sol

+pragma solidity ^0.4.18;
+
+import './ERC721.sol';
+import '../math/SafeMath.sol';
+
+/**
+ * @title ERC721Token
+ * Generic implementation for the required functionality of the ERC721 standard
+ */
+contract ERC721Token is ERC721 {
+  using SafeMath for uint256;
+
+  // Total amount of tokens
+  uint256 private totalTokens;
+
+  // Mapping from token ID to owner
+  mapping (uint256 => address) private tokenOwner;
+
+  // Mapping from token ID to approved address
+  mapping (uint256 => address) private tokenApprovals;
+
+  // Mapping from owner to list of owned token IDs
+  mapping (address => uint256[]) private ownedTokens;
+
+  // Mapping from token ID to index of the owner tokens list
+  mapping(uint256 => uint256) private 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);
+    _;
+  }
+
+  /**
+  * @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;
+  }
+
+  /**
+  * @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];
+  }
+
+  /**
+  * @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 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
+  */
+  function transfer(address _to, uint256 _tokenId) public onlyOwnerOf(_tokenId) {
+    clearApprovalAndTransfer(msg.sender, _to, _tokenId);
+  }
+
+  /**
+  * @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
+  */
+  function approve(address _to, uint256 _tokenId) public onlyOwnerOf(_tokenId) {
+    address owner = ownerOf(_tokenId);
+    require(_to != owner);
+    if(approvedFor(_tokenId) == 0 && _to == 0) return;
+    tokenApprovals[_tokenId] = _to;
+    Approval(owner, _to, _tokenId);
+  }
+
+  /**
+  * @dev Claims the ownership of a given token ID
+  * @param _tokenId uint256 ID of the token being claimed by the msg.sender
+  */
+  function takeOwnership(uint256 _tokenId) public {
+    require(isApprovedFor(_tokenId));
+    clearApprovalAndTransfer(ownerOf(_tokenId), msg.sender, _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
+  */
+  function mint(address _to, uint256 _tokenId) internal {
+    require(_to != address(0));
+    addToken(_to, _tokenId);
+    Transfer(0x0, _to, _tokenId);
+  }
+
+  /**
+  * @dev Burns a specific token
+  * @param _tokenId uint256 ID of the token being burned by the msg.sender
+  */
+  function burn(uint256 _tokenId) onlyOwnerOf(_tokenId) internal {
+    if(approvedFor(_tokenId) != 0) {
+      clearApproval(msg.sender, _tokenId);
+    }
+    removeToken(msg.sender, _tokenId);
+    Transfer(msg.sender, 0x0, _tokenId);
+  }
+
+  /**
+  * @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
+  */
+  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);
+  }
+
+  /**
+  * @dev Internal function to clear current approval of a given token ID
+  * @param _tokenId uint256 ID of the token to be transferred
+  */
+  function clearApproval(address _owner, uint256 _tokenId) internal {
+    require(ownerOf(_tokenId) == _owner);
+    tokenApprovals[_tokenId] = 0;
+    Approval(_owner, 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 addToken(address _to, uint256 _tokenId) internal {
+    require(tokenOwner[_tokenId] == address(0));
+    tokenOwner[_tokenId] = _to;
+    uint256 length = balanceOf(_to);
+    ownedTokens[_to].push(_tokenId);
+    ownedTokensIndex[_tokenId] = length;
+    totalTokens = totalTokens.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 removeToken(address _from, uint256 _tokenId) internal {
+    require(balanceOf(_from) > 0);
+    require(ownerOf(_tokenId) == _from);
+
+    uint256 tokenIndex = ownedTokensIndex[_tokenId];
+    uint256 lastTokenIndex = balanceOf(_from).sub(1);
+    uint256 lastToken = ownedTokens[_from][lastTokenIndex];
+
+    tokenOwner[_tokenId] = 0;
+    ownedTokens[_from][tokenIndex] = lastToken;
+    ownedTokens[_from][lastTokenIndex] = 0;
+    ownedTokens[_from].length--;
+    ownedTokensIndex[_tokenId] = 0;
+    ownedTokensIndex[lastToken] = tokenIndex;
+    totalTokens = totalTokens.sub(1);
+  }
+
+  /**
+   * @dev Tells whether the msg.sender is approved for the given token ID or not
+   * @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(uint256 _tokenId) internal view returns (bool) {
+    return approvedFor(_tokenId) == msg.sender;
+  }
+}