pragma solidity ^0.4.25;

/* 
	********************
	- HashKings Planet -
	********************
	v1.0
	
	Daniel Pittman - Qwoyn.io
	
	*Note:
	*
	*Holds all the plots from HashKings
	***********************************
	
*/

/**
 * @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 Helps contracts guard against reentrancy attacks.
* @author Remco Bloemen <remco@2π.com>, Eenae <alexey@mixbytes.io>
* @dev If you mark a function `nonReentrant`, you should also
* mark it `external`.
*/
contract ReentrancyGuard {

/// @dev counter to allow mutex lock with only one SSTORE operation
uint256 private guardCounter = 1;

/**
* @dev Prevents a contract from calling itself, directly or indirectly.
* If you mark a function `nonReentrant`, you should also
* mark it `external`. Calling one `nonReentrant` function from
* another is not supported. Instead, you can implement a
* `private` function doing the actual work, and an `external`
* wrapper marked as `nonReentrant`.
*/
	modifier nonReentrant() {
		guardCounter += 1;
		uint256 localCounter = guardCounter;
		_;
		require(localCounter == guardCounter);
	}
}

/**
 * @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;
  }

}

/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions". 
 */
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 onlyOwner {
	require(_newOwner != address(0));
	emit OwnershipTransferred(owner, _newOwner);
	owner = _newOwner;
  }
}

contract Fallback is Ownable {

  function withdraw() public onlyOwner {
	owner.transfer(address(this).balance);
  }
}

/**
 * @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 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 HashKingsPlanet is SupportsInterfaceWithLookup, ERC721BasicToken, ERC721, Ownable, Fallback {

  /*** EVENTS ***/
  /// The event emitted (useable by web3) when a token is purchased
  event BoughtToken(address indexed buyer, uint256 tokenId);

  /*** CONSTANTS ***/
  string public constant company = "Qwoyn, LLC ";
  string public constant contact = "https://qwoyn.io";
  string public constant author  = "Daniel Pittman";

  
  uint8 constant TITLE_MAX_LENGTH = 64;
  uint256 constant DESCRIPTION_MAX_LENGTH = 100000;

  /*** 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 = 0;
  
  mapping(uint256 => string)  tokenTitles;	  
  mapping(uint256 => string)  tokenDescriptions;
  

  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);
  }

  /// 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, dappCap and slammers noted here.
  /// @param _title The short title of the token
  /// @param _description Description of the token
  function buyToken (
	string  _title,
	string  _description
  ) public onlyOwner {
	bytes memory _titleBytes = bytes(_title);
	require(_titleBytes.length <= TITLE_MAX_LENGTH, "Desription is too long");
	
	bytes memory _descriptionBytes = bytes(_description);
	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);

	tokenTitles[index]       = _title;
	tokenDescriptions[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 viewTokenMeta(uint256 _tokenId)
	external
	view
	returns (
	  string  tokenTitle_,
	  string  tokenDescription_
  ) {
	  tokenTitle_       = tokenTitles[_tokenId];
	  tokenDescription_ = tokenDescriptions[_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;
  }
  
  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 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) public onlyOwner {
	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);
  }
}