openzeppelin-contracts/contracts/token/ERC721/ERC721Token.sol

pragma solidity ^0.4.18;

import "./ERC721.sol";
import "./DeprecatedERC721.sol";
import "./ERC721BasicToken.sol";


/**
 * @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 operatable terminology
 * @dev see https://github.com/ethereum/eips/issues/721
 */
contract ERC721Token is ERC721, ERC721BasicToken {
  // Token name
  string internal name_;

  // Token symbol
  string internal symbol_;

  // Mapping from owner to list of owned token IDs
  mapping (address => uint256[]) internal ownedTokens;

  // Mapping from token ID to index of the owner tokens list
  mapping(uint256 => uint256) internal ownedTokensIndex;

  /**
  * @dev Constructor function
  */
  function ERC721Token(string _name, string _symbol) public {
    name_ = _name;
    symbol_ = _symbol;
  }

  /**
  * @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 token name
  * @return string representing the token name
  */
  function name() public view returns (string) {
    return name_;
  }

  /**
  * @dev Gets the token symbol
  * @return string representing the token symbol
  */
  function symbol() public view returns (string) {
    return symbol_;
  }

  /**
  * @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 tokenOfOwnerByIndex(address _owner, uint256 _index) public view returns (uint256) {
    require(_index < balanceOf(_owner));
    return ownedTokens[_owner][_index];
  }

  /**
  * @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 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;
  }

  /**
  * @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(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;
    // Note that this will handle single-element arrays. In that case, both tokenIndex and lastTokenIndex are going to
    // be zero. Then we can make sure that we will remove _tokenId from the ownedTokens list since we are first swapping
    // the lastToken to the first position, and then dropping the element placed in the last position of the list

    ownedTokens[_from].length--;
    ownedTokensIndex[_tokenId] = 0;
    ownedTokensIndex[lastToken] = tokenIndex;
  }

}