NiCkEl5/testmodulo5.sol

## testmodulo5.sol
pragma solidity ^0.4.24;

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {
/**
  * @dev Multiplies two numbers, throws on overflow.
  */
  function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
    // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
    if (a == 0) {
      return 0;
    }
    c = a * b;
    assert(c / a == b);
    return c;
  }
/**
  * @dev Integer division of two numbers, truncating the quotient.
  */
  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    // uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return a / b;
  }
/**
  * @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    assert(b <= a);
    return a - b;
  }
/**
  * @dev Adds two numbers, throws on overflow.
  */
  function add(uint256 a, uint256 b) internal pure returns (uint256 c) {
    c = a + b;
    assert(c >= a);
    return c;
  }
}

/**
  * @title ERC165
  * @dev https://github.com/ethereum/EIPs/blob/master/EIPS/eip-165.md
  */
interface ERC165 {
  /**
    * @notice Query if a contract implements an interface
	* @param _interfaceId The interface identifier, as specified in ERC-165
	* @dev Interface identification is specified in ERC-165. This function
	* uses less than 30,000 gas.
	*/
	function supportsInterface(bytes4 _interfaceId) external view returns (bool);
}

/**
  * @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,address,uint256,bytes)"))`,
  *  which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
  */
  bytes4 internal constant ERC721_RECEIVED = 0x150b7a02;
  /**
    * @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. Return of other than the magic value MUST result in the
	* transaction being reverted.
	* Note: the contract address is always the message sender.
	* @param _operator The address which called `safeTransferFrom` function
	* @param _from The address which previously owned the token
	* @param _tokenId The NFT identifier which is being transfered
	* @param _data Additional data with no specified format
	* @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
	*/
  function onERC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) public returns(bytes4);
}


/**
 * Utility library of inline functions on addresses
 */
library AddressUtils {
/**
   * Returns whether the target address is a contract
   * @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 to check
   * @return whether the target address is a contract
   */
    function isContract(address addr) internal view returns (bool) {
        uint256 size;
        // XXX Currently there is no better way to check if there is a
        // contract in an address
        // than to check the size of the code at that address.
        // See https://ethereum.stackexchange.com/a/14016/36603
        // for more details about how this works.
        // TODO Check this again before the Serenity release, because
        // all addresses will be
        // contracts then.
        // solium-disable-next-line security/no-inline-assembly

        assembly { size := extcodesize(addr) }
        return size > 0;
      }
}

  contract Ownable {
    address public owner;
    event OwnershipRenounced(address indexed _previousOwner);
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
    /**
     * @dev The Ownable constructor sets the original `owner` of the contract to the sender
     * account.
     */
    constructor() public {
      owner = msg.sender;
    }
  /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
      require(msg.sender == owner);
      _;
    }
  /**
     * @dev Allows the current owner to relinquish control of the contract.
     * @notice Renouncing to ownership will leave the contract without an owner.
     * It will not be possible to call the functions with the `onlyOwner`
     * modifier anymore.
     */

  function renounceOwnership() public onlyOwner {
      emit OwnershipRenounced(owner);
      owner = address(0);
    }
  /**
     * @dev Allows the current owner to transfer control of the contract to a newOwner.
     * @param _newOwner The address to transfer ownership to.
     */
    function transferOwnership(address _newOwner) public onlyOwner {
      _transferOwnership(_newOwner);
    }
  /**
     * @dev Transfers control of the contract to a newOwner.
     * @param _newOwner The address to transfer ownership to.
     */

  function _transferOwnership(address _newOwner) internal {
      require(_newOwner != address(0));
      emit OwnershipTransferred(owner, _newOwner);
      owner = _newOwner;
    }
  }


 /**
  * @title SupportsInterfaceWithLookup
  * @author Matt Condon (@shrugs)
  * @dev Implements ERC165 using a lookup table.
  */
 contract SupportsInterfaceWithLookup is ERC165 {
   bytes4 public constant InterfaceId_ERC165 = 0x01ffc9a7;
   /**
    * 0x01ffc9a7 ===
    *   bytes4(keccak256('supportsInterface(bytes4)'))
    */
    /**
    * @dev a mapping of interface id to whether or not it's supported
    */
   mapping(bytes4 => bool) internal supportedInterfaces;
 /**
    * @dev A contract implementing SupportsInterfaceWithLookup
    * implement ERC165 itself
    */
   constructor() public
   {
     _registerInterface(InterfaceId_ERC165);
   }
   /**
    * @dev implement supportsInterface(bytes4) using a lookup table
    */
   function supportsInterface(bytes4 _interfaceId) external view returns (bool) {
     return supportedInterfaces[_interfaceId];
   }
   /**
    * @dev private method for registering an interface
    */
   function _registerInterface(bytes4 _interfaceId) internal {
     require(_interfaceId != 0xffffffff);
     supportedInterfaces[_interfaceId] = true;
   }
 }

 /**
  * @title ERC721 Non-Fungible Token Standard basic interface
  * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
 contract ERC721Basic is ERC165 {
    event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);
    event Approval(address indexed _owner, address indexed _approved, uint256 indexed _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;
 }
 /**
  * @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 ERC-721 Non-Fungible Token Standard, optional metadata extension
  * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
 contract ERC721Metadata is ERC721Basic {
    function name() external view returns (string _name);
    function symbol() external view returns (string _symbol);
    function tokenURI(uint256 _tokenId) public view returns (string);
 }
 /**
  * @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 {
 }
 /**
  * @title ERC721 Non-Fungible Token Standard basic implementation
  * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
 contract ERC721BasicToken is SupportsInterfaceWithLookup, ERC721Basic {
    bytes4 private constant InterfaceId_ERC721 = 0x80ac58cd;
   /*
    * 0x80ac58cd ===
    *   bytes4(keccak256('balanceOf(address)')) ^
    *   bytes4(keccak256('ownerOf(uint256)')) ^
    *   bytes4(keccak256('approve(address,uint256)')) ^
    *   bytes4(keccak256('getApproved(uint256)')) ^
    *   bytes4(keccak256('setApprovalForAll(address,bool)')) ^
    *   bytes4(keccak256('isApprovedForAll(address,address)')) ^
    *   bytes4(keccak256('transferFrom(address,address,uint256)')) ^
    *   bytes4(keccak256('safeTransferFrom(address,address,uint256)')) ^
    *   bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)'))
    */
    bytes4 private constant InterfaceId_ERC721Exists = 0x4f558e79;
   /*
    * 0x4f558e79 ===
    *   bytes4(keccak256('exists(uint256)'))
    */
    using SafeMath for uint256;
    using AddressUtils for address;
 // Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
   // which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
    bytes4 private constant ERC721_RECEIVED = 0x150b7a02;
 // 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));
     _;
   }

    constructor() public {
     // register the supported interfaces to conform to ERC721 via ERC165
     _registerInterface(InterfaceId_ERC721);
     _registerInterface(InterfaceId_ERC721Exists);
    }
   /**
    * @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 existence 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
    * The zero address indicates there is no approved address.
    * There can only be one approved address per token at a given time.
    * 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));
        tokenApprovals[_tokenId] = _to;
        emit 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 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
    * 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;
     emit 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
    * Usage of this method is discouraged, use `safeTransferFrom` whenever possible
    * Requires the msg sender to be the owner, approved, or operator
    * @param _from current owner of the token
    * @param _to address to receive the ownership of the given token ID
    * @param _tokenId uint256 ID of the token to be transferred
   */
   function transferFrom(address _from, address _to, uint256 _tokenId) public canTransfer(_tokenId) {
    require(_from != address(0));
    require(_to != address(0));
    clearApproval(_from, _tokenId);
    removeTokenFrom(_from, _tokenId);
    addTokenTo(_to, _tokenId);
    emit Transfer(_from, _to, _tokenId);
   }
 /**
    * @dev Safely transfers the ownership of a given token ID to another address
    * 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,address,uint256,bytes)"))`; otherwise,
    * the transfer is reverted.
    *
    * 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) {
     // solium-disable-next-line arg-overflow
     safeTransferFrom(_from, _to, _tokenId, "");
   }
 /**
    * @dev Safely transfers the ownership of a given token ID to another address
    * 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,address,uint256,bytes)"))`; otherwise,
    * the transfer is reverted.
    * 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);
     // solium-disable-next-line arg-overflow
     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);
     // Disable solium check because of
     // https://github.com/duaraghav8/Solium/issues/175
     // solium-disable-next-line operator-whitespace
     return (_spender == owner || getApproved(_tokenId) == _spender || isApprovedForAll(owner, _spender));
   }
 /**
    * @dev Internal function to mint a new token
    * 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);
     emit Transfer(address(0), _to, _tokenId);
   }
 /**
    * @dev Internal function to burn a specific token
    * 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);
     emit Transfer(_owner, address(0), _tokenId);
   }
 /**
    * @dev Internal function to clear current approval of a given token ID
    * Reverts if the given address is not indeed the owner of the token
    * @param _owner owner of the token
    * @param _tokenId uint256 ID of the token to be transferred
    */
   function clearApproval(address _owner, uint256 _tokenId) internal {
     require(ownerOf(_tokenId) == _owner);
     if (tokenApprovals[_tokenId] != address(0)) {
       tokenApprovals[_tokenId] = address(0);
     }
   }
 /**
    * @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
    * 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(
     msg.sender, _from, _tokenId, _data);
     return (retval == ERC721_RECEIVED);
   }
 }
 /**
  * @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 SupportsInterfaceWithLookup, ERC721BasicToken, ERC721 {
    bytes4 private constant InterfaceId_ERC721Enumerable = 0x780e9d63;
   /**
    * 0x780e9d63 ===
    *   bytes4(keccak256('totalSupply()')) ^
    *   bytes4(keccak256('tokenOfOwnerByIndex(address,uint256)')) ^
    *   bytes4(keccak256('tokenByIndex(uint256)'))
    */
    bytes4 private constant InterfaceId_ERC721Metadata = 0x5b5e139f;
   /**
    * 0x5b5e139f ===
    *   bytes4(keccak256('name()')) ^
    *   bytes4(keccak256('symbol()')) ^
    *   bytes4(keccak256('tokenURI(uint256)'))
    */
 // 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;
 // Array with all token ids, used for enumeration
    uint256[] internal allTokens;
 // Mapping from token id to position in the allTokens array
    mapping(uint256 => uint256) internal allTokensIndex;
 // Optional mapping for token URIs
    mapping(uint256 => string) internal tokenURIs;
 /**
    * @dev Constructor function
    */
   constructor(string _name, string _symbol) public {
     name_ = _name;
     symbol_ = _symbol;
 // register the supported interfaces to conform to ERC721 via ERC165
     _registerInterface(InterfaceId_ERC721Enumerable);
     _registerInterface(InterfaceId_ERC721Metadata);
   }
 /**
    * @dev Gets the token name
    * @return string representing the token name
    */
   function name() external view returns (string) {
     return name_;
   }
 /**
    * @dev Gets the token symbol
    * @return string representing the token symbol
    */
   function symbol() external view returns (string) {
     return symbol_;
   }
 /**
    * @dev Returns an URI for a given token ID
    * Throws if the token ID does not exist. May return an empty string.
    * @param _tokenId uint256 ID of the token to query
    */
   function tokenURI(uint256 _tokenId) public view returns (string) {
     require(exists(_tokenId));
     return tokenURIs[_tokenId];
   }
 /**
    * @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 allTokens.length;
   }
 /**
    * @dev Gets the token ID at a given index of all the tokens in this contract
    * 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 tokenByIndex(uint256 _index) public view returns (uint256) {
     require(_index < totalSupply());
     return allTokens[_index];
   }
 /**
    * @dev Internal function to set the token URI for a given token
    * 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 _setTokenURI(uint256 _tokenId, string _uri) internal {
     require(exists(_tokenId));
     tokenURIs[_tokenId] = _uri;
   }
 /**
    * @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 {
     super.addTokenTo(_to, _tokenId);
     uint256 length = ownedTokens[_to].length;
     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 removeTokenFrom(address _from, uint256 _tokenId) internal {
        super.removeTokenFrom(_from, _tokenId);
        uint256 tokenIndex = ownedTokensIndex[_tokenId];
        uint256 lastTokenIndex = ownedTokens[_from].length.sub(1);
        uint256 lastToken = ownedTokens[_from][lastTokenIndex];
        ownedTokens[_from][tokenIndex] = lastToken;
        ownedTokens[_from][lastTokenIndex] = 0;
        // Note that this will handle single-element arrays. In that case, both tokenIndex and lastTokenIndex are going to
        // be zero. Then we can make sure that we will remove _tokenId from the ownedTokens list since we are first swapping
        // the lastToken to the first position, and then dropping the element placed in the last position of the list
        ownedTokens[_from].length--;
        ownedTokensIndex[_tokenId] = 0;
        ownedTokensIndex[lastToken] = tokenIndex;
   }
 /**
    * @dev Internal function to mint a new token
    * 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
    * 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;
   }
 }

 contract Document is ERC721Token, Ownable {
 /*** EVENTS ***/
   /// The event emitted (useable by web3) when a token is purchased
   event BoughtToken(address indexed buyer, uint256 tokenId);
 /*** CONSTANTS ***/
   uint8 constant TITLE_MIN_LENGTH = 1;
   uint8 constant TITLE_MAX_LENGTH = 64;
   uint256 constant DESCRIPTION_MIN_LENGTH = 1;
   uint256 constant DESCRIPTION_MAX_LENGTH = 10000;
 /*** DATA TYPES ***/
 /// Price set by contract owner for each token in Wei.
 /// @dev If you'd like a different price for each token type, you
 /// will need to use a mapping like: `mapping(uint256 => uint256)
 /// tokenTypePrices;`
   uint256 currentPrice = 3000000000000000;
 /// The token type (1 for idea, 2 for belonging, etc)
   mapping(uint256 => uint256) tokenTypes;
 /// The title of the token
   mapping(uint256 => string) tokenTitles;
 /// The description of the token
   mapping(uint256 => string) tokenDescription;

   constructor() ERC721Token("Document", "DQMT") public {
     // any init code when you deploy the contract would run here
   }
 /// Requires the amount of Ether be at least or more of the
 /// currentPrice
 /// @dev Creates an instance of an token and mints it to the
 /// purchaser
 /// @param _type The token type as an integer
 /// @param _title The short title of the token
 /// @param _description Description of the token
 function buyToken (uint256 _type, string _title, string _description) external payable {
    bytes memory _titleBytes = bytes(_title);
    require(_titleBytes.length >= TITLE_MIN_LENGTH, "Title is too short");
    require(_titleBytes.length <= TITLE_MAX_LENGTH, "Title is too long");

    bytes memory _descriptionBytes = bytes(_description);
    require(_descriptionBytes.length >= DESCRIPTION_MIN_LENGTH, "Description is too short");
    require(_descriptionBytes.length <= DESCRIPTION_MAX_LENGTH, "Description is too long");
    require(msg.value >= currentPrice, "Amount of Ether sent too small");
    uint256 index = allTokens.length + 1;
    _mint(msg.sender, index);
    tokenTypes[index] = _type;
    tokenTitles[index] = _title;
    tokenDescription[index] = _description;
    emit BoughtToken(msg.sender, index);

 }
   /**
    * @dev Returns all of the tokens that the user owns
    * @return An array of token indices
    */
    function myTokens() external view returns (uint256[]) {
     return ownedTokens[msg.sender];
    }
 /// @notice Returns all the relevant information about a specific /// token
 /// @param _tokenId The ID of the token of interest
   function viewToken(uint256 _tokenId) external view returns (uint256 tokenType_, string tokenTitle_, string tokenDescription_) {
    tokenType_ = tokenTypes[_tokenId];
    tokenTitle_ = tokenTitles[_tokenId];
    tokenDescription_ = tokenDescription[_tokenId];
   }
 /// @notice Allows the owner of this contract to set the
 /// currentPrice for each token
   function setCurrentPrice(uint256 newPrice) public onlyOwner {
       currentPrice = newPrice;
   }
 /// @notice Returns the currentPrice for each token
   function getCurrentPrice() external view returns (uint256 price) {
       price = currentPrice;
   }
 /// @notice allows the owner of this contract to destroy the ///contract
    function kill() public {
     if(msg.sender == owner) selfdestruct(owner);
    }
 }