# Contract Example

``````pragma solidity ^0.5.0;

/**
* @dev Wrappers over Solidity's arithmetic operations with added overflow
* checks.
*
* Arithmetic operations in Solidity wrap on overflow. This can easily result
* in bugs, because programmers usually assume that an overflow raises an
* error, which is the standard behavior in high level programming languages.
* `SafeMath` restores this intuition by reverting the transaction when an
* operation overflows.
*
* Using this library instead of the unchecked operations eliminates an entire
* class of bugs, so it's recommended to use it always.
*/
library SafeMath {
/**
* @dev Returns the addition of two unsigned integers, reverting on
* overflow.
*
* Counterpart to Solidity's `+` operator.
*
* Requirements:
*/
function add(uint256 a, uint256 b) internal pure returns (uint256) {
uint256 c = a + b;
require(c >= a, "SafeMath: addition overflow");

return c;
}

/**
* @dev Returns the subtraction of two unsigned integers, reverting on
* overflow (when the result is negative).
*
* Counterpart to Solidity's `-` operator.
*
* Requirements:
* - Subtraction cannot overflow.
*/
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
return sub(a, b, "SafeMath: subtraction overflow");
}

/**
* @dev Returns the subtraction of two unsigned integers, reverting with custom message on
* overflow (when the result is negative).
*
* Counterpart to Solidity's `-` operator.
*
* Requirements:
* - Subtraction cannot overflow.
*
* _Available since v2.4.0._
*/
function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
require(b <= a, errorMessage);
uint256 c = a - b;

return c;
}

/**
* @dev Returns the multiplication of two unsigned integers, reverting on
* overflow.
*
* Counterpart to Solidity's `*` operator.
*
* Requirements:
* - Multiplication cannot overflow.
*/
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
// Gas optimization: this is cheaper than requiring 'a' not being zero, but the
// benefit is lost if 'b' is also tested.
// See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
if (a == 0) {
return 0;
}

uint256 c = a * b;
require(c / a == b, "SafeMath: multiplication overflow");

return c;
}

/**
* @dev Returns the integer division of two unsigned integers. Reverts on
* division by zero. The result is rounded towards zero.
*
* Counterpart to Solidity's `/` operator. Note: this function uses a
* `revert` opcode (which leaves remaining gas untouched) while Solidity
* uses an invalid opcode to revert (consuming all remaining gas).
*
* Requirements:
* - The divisor cannot be zero.
*/
function div(uint256 a, uint256 b) internal pure returns (uint256) {
return div(a, b, "SafeMath: division by zero");
}

/**
* @dev Returns the integer division of two unsigned integers. Reverts with custom message on
* division by zero. The result is rounded towards zero.
*
* Counterpart to Solidity's `/` operator. Note: this function uses a
* `revert` opcode (which leaves remaining gas untouched) while Solidity
* uses an invalid opcode to revert (consuming all remaining gas).
*
* Requirements:
* - The divisor cannot be zero.
*
* _Available since v2.4.0._
*/
function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
// Solidity only automatically asserts when dividing by 0
require(b > 0, errorMessage);
uint256 c = a / b;
// assert(a == b * c + a % b); // There is no case in which this doesn't hold

return c;
}

/**
* @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
* Reverts when dividing by zero.
*
* Counterpart to Solidity's `%` operator. This function uses a `revert`
* opcode (which leaves remaining gas untouched) while Solidity uses an
* invalid opcode to revert (consuming all remaining gas).
*
* Requirements:
* - The divisor cannot be zero.
*/
function mod(uint256 a, uint256 b) internal pure returns (uint256) {
return mod(a, b, "SafeMath: modulo by zero");
}

/**
* @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
* Reverts with custom message when dividing by zero.
*
* Counterpart to Solidity's `%` operator. This function uses a `revert`
* opcode (which leaves remaining gas untouched) while Solidity uses an
* invalid opcode to revert (consuming all remaining gas).
*
* Requirements:
* - The divisor cannot be zero.
*
* _Available since v2.4.0._
*/
function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
require(b != 0, errorMessage);
return a % b;
}
}

/*
* @dev Provides information about the current execution context, including the
* sender of the transaction and its data. While these are generally available
* via msg.sender and msg.data, they should not be accessed in such a direct
* manner, since when dealing with GSN meta-transactions the account sending and
* paying for execution may not be the actual sender (as far as an application
* is concerned).
*
* This contract is only required for intermediate, library-like contracts.
*/
contract Context {
// Empty internal constructor, to prevent people from mistakenly deploying
// an instance of this contract, which should be used via inheritance.
constructor () internal { }
// solhint-disable-previous-line no-empty-blocks

function _msgSender() internal view returns (address payable) {
return msg.sender;
}

function _msgData() internal view returns (bytes memory) {
this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
return msg.data;
}
}

/**
* @title Roles
* @dev Library for managing addresses assigned to a Role.
*/
library Roles {
struct Role {
}

/**
*/
require(!has(role, account), "Roles: account already has role");
role.bearer[account] = true;
}

/**
*/
function remove(Role storage role, address account) internal {
require(has(role, account), "Roles: account does not have role");
role.bearer[account] = false;
}

/**
* @dev Check if an account has this role.
* @return bool
*/
function has(Role storage role, address account) internal view returns (bool) {
return role.bearer[account];
}
}
pragma solidity ^0.5.5;

/**
* @dev Collection of functions related to the address type
*/
/**
* @dev Converts an `address` into `address payable`. Note that this is
* simply a type cast: the actual underlying value is not changed.
*
* _Available since v2.4.0._
*/
}

/**
* @dev Replacement for Solidity's `transfer`: sends `amount` wei to
* `recipient`, forwarding all available gas and reverting on errors.
*
*
* IMPORTANT: because control is transferred to `recipient`, care must be
* taken to not create reentrancy vulnerabilities. Consider using
* {ReentrancyGuard} or the
*
* _Available since v2.4.0._
*/
function sendValue(address payable recipient, uint256 amount) internal {

// solhint-disable-next-line avoid-call-value
(bool success, ) = recipient.call.value(amount)("");
require(success, "Address: unable to send value, recipient may have reverted");
}
}

/**
* @title Counters
* @author Matt Condon (@shrugs)
* @dev Provides counters that can only be incremented or decremented by one. This can be used e.g. to track the number
* of elements in a mapping, issuing TRC721 ids, or counting request ids.
*
* Include with `using Counters for Counters.Counter;`
* Since it is not possible to overflow a 256 bit integer with increments of one, `increment` can skip the {SafeMath}
* overflow check, thereby saving gas. This does assume however correct usage, in that the underlying `_value` is never
* directly accessed.
*/
library Counters {
using SafeMath for uint256;

struct Counter {
// This variable should never be directly accessed by users of the library: interactions must be restricted to
// the library's function. As of Solidity v0.5.2, this cannot be enforced, though there is a proposal to add
// this feature: see https://github.com/ethereum/solidity/issues/4637
uint256 _value; // default: 0
}

function current(Counter storage counter) internal view returns (uint256) {
return counter._value;
}

function increment(Counter storage counter) internal {
// The {SafeMath} overflow check can be skipped here, see the comment at the top
counter._value += 1;
}

function decrement(Counter storage counter) internal {
counter._value = counter._value.sub(1);
}
}

contract MinterRole is Context {
using Roles for Roles.Role;

Roles.Role private _minters;

constructor () internal {
}

modifier onlyMinter() {
require(isMinter(_msgSender()), "MinterRole: caller does not have the Minter role");
_;
}

function isMinter(address account) public view returns (bool) {
return _minters.has(account);
}

}

function renounceMinter() public {
_removeMinter(_msgSender());
}

}

_minters.remove(account);
emit MinterRemoved(account);
}
}

/**
* @dev Interface of the TRC165 standard.
*
* Implementers can declare support of contract interfaces, which can then be
* queried by others ({TRC165Checker}).
*
* For an implementation, see {TRC165}.
*/
interface ITRC165 {
/**
* @dev Returns true if this contract implements the interface defined by
* `interfaceId`.
*
* This function call must use less than 30 000 gas.
*/
function supportsInterface(bytes4 interfaceId) external view returns (bool);
}

/**
* @dev Required interface of an TRC721 compliant contract.
*/
contract ITRC721 is ITRC165 {

/**
* @dev Returns the number of NFTs in `owner`'s account.
*/
function balanceOf(address owner) public view returns (uint256 balance);

/**
* @dev Returns the owner of the NFT specified by `tokenId`.
*/
function ownerOf(uint256 tokenId) public view returns (address owner);

/**
* @dev Transfers a specific NFT (`tokenId`) from one account (`from`) to
* another (`to`).
*
*
*
* Requirements:
* - `from`, `to` cannot be zero.
* - `tokenId` must be owned by `from`.
* - If the caller is not `from`, it must be have been allowed to move this
* NFT by either {approve} or {setApprovalForAll}.
*/
/**
* @dev Transfers a specific NFT (`tokenId`) from one account (`from`) to
* another (`to`).
*
* Requirements:
* - If the caller is not `from`, it must be approved to move this NFT by
* either {approve} or {setApprovalForAll}.
*/
function approve(address to, uint256 tokenId) public;
function getApproved(uint256 tokenId) public view returns (address operator);

function setApprovalForAll(address operator, bool _approved) public;

}

/**
* @title TRC-721 Non-Fungible Token Standard, optional metadata extension
*/
function name() external view returns (string memory);
function symbol() external view returns (string memory);
function tokenURI(uint256 tokenId) external view returns (string memory);
}

/**
* @title TRC721 token receiver interface
* @dev Interface for any contract that wants to support safeTransfers
* from TRC721 asset contracts.
*/
/**
* @notice Handle the receipt of an NFT
* @dev The TRC721 smart contract calls this function on the recipient
* after a {ITRC721-safeTransferFrom}. This function MUST return the function selector,
* otherwise the caller will revert the transaction. The selector to be
* returned can be obtained as `this.onTRC721Received.selector`. This
* function MAY throw to revert and reject the transfer.
* Note: the TRC721 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 transferred
* @param data Additional data with no specified format
*/
public returns (bytes4);
}

/**
* @dev Implementation of the {ITRC165} interface.
*
* Contracts may inherit from this and call {_registerInterface} to declare
* their support of an interface.
*/
contract TRC165 is ITRC165 {
/*
* bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
*/
bytes4 private constant _INTERFACE_ID_TRC165 = 0x01ffc9a7;

/**
* @dev Mapping of interface ids to whether or not it's supported.
*/
mapping(bytes4 => bool) private _supportedInterfaces;

constructor () internal {
// Derived contracts need only register support for their own interfaces,
// we register support for TRC165 itself here
_registerInterface(_INTERFACE_ID_TRC165);
}

/**
* @dev See {ITRC165-supportsInterface}.
*
* Time complexity O(1), guaranteed to always use less than 30 000 gas.
*/
function supportsInterface(bytes4 interfaceId) external view returns (bool) {
return _supportedInterfaces[interfaceId];
}

/**
* @dev Registers the contract as an implementer of the interface defined by
* `interfaceId`. Support of the actual TRC165 interface is automatic and
* registering its interface id is not required.
*
* See {ITRC165-supportsInterface}.
*
* Requirements:
*
* - `interfaceId` cannot be the TRC165 invalid interface (`0xffffffff`).
*/
function _registerInterface(bytes4 interfaceId) internal {
require(interfaceId != 0xffffffff, "TRC165: invalid interface id");
_supportedInterfaces[interfaceId] = true;
}
}

/**
* @title TRC721 Non-Fungible Token Standard basic implementation
*/
contract TRC721 is Context, TRC165, ITRC721 {
using SafeMath for uint256;
using Counters for Counters.Counter;

//
// NOTE: TRC721 uses 0x150b7a02, TRC721 uses 0x5175f878.
bytes4 private constant _TRC721_RECEIVED = 0x5175f878;

// 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 number of owned token
mapping (address => Counters.Counter) private _ownedTokensCount;

// Mapping from owner to operator approvals

/*
*     bytes4(keccak256('ownerOf(uint256)')) == 0x6352211e
*     bytes4(keccak256('getApproved(uint256)')) == 0x081812fc
*
*     => 0x70a08231 ^ 0x6352211e ^ 0x095ea7b3 ^ 0x081812fc ^
*        0xa22cb465 ^ 0xe985e9c ^ 0x23b872dd ^ 0x42842e0e ^ 0xb88d4fde == 0x80ac58cd
*/
bytes4 private constant _INTERFACE_ID_TRC721 = 0x80ac58cd;

constructor () public {
// register the supported interfaces to conform to TRC721 via TRC165
_registerInterface(_INTERFACE_ID_TRC721);
}

/**
* @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 _ownedTokensCount[owner].current();
}

/**
* @dev Gets the owner of the specified token ID.
* @param tokenId uint256 ID of the token to query the owner of
* @return address currently marked as the owner of the given token ID
*/
function ownerOf(uint256 tokenId) public view returns (address) {
require(owner != address(0), "TRC721: owner query for nonexistent token");

return owner;
}

/**
* @dev Approves another address to transfer the given token ID
* 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 {
require(to != owner, "TRC721: approval to current owner");

require(_msgSender() == owner || isApprovedForAll(owner, _msgSender()),
"TRC721: approve caller is not owner nor approved for all"
);

_tokenApprovals[tokenId] = to;
emit Approval(owner, to, tokenId);
}

/**
* @dev Gets the approved address for a token ID, or zero if no address set
* Reverts if the token ID does not exist.
* @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) {
require(_exists(tokenId), "TRC721: approved query for nonexistent token");

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 != _msgSender(), "TRC721: approve to caller");

_operatorApprovals[_msgSender()][to] = approved;
emit ApprovalForAll(_msgSender(), 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
*/
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
*/
//solhint-disable-next-line max-line-length
require(_isApprovedOrOwner(_msgSender(), tokenId), "TRC721: transfer caller is not owner nor approved");

_transferFrom(from, to, tokenId);
}

/**
* @dev Safely transfers the ownership of a given token ID to another address
* which is called upon a safe transfer, and return the magic value
* 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
*/
safeTransferFrom(from, to, tokenId, "");
}

/**
* @dev Safely transfers the ownership of a given token ID to another address
* which is called upon a safe transfer, and return the magic value
* the transfer is reverted.
* Requires the _msgSender() 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 memory _data) public {
require(_isApprovedOrOwner(_msgSender(), tokenId), "TRC721: transfer caller is not owner nor approved");
_safeTransferFrom(from, to, tokenId, _data);
}

/**
* @dev Safely transfers the ownership of a given token ID to another address
* If the target address is a contract, it must implement `onTRC721Received`,
* which is called upon a safe transfer, and return the magic value
* 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 memory _data) internal {
_transferFrom(from, to, tokenId);
}

/**
* @dev Returns whether the specified token exists.
* @param tokenId uint256 ID of the token to query the existence of
* @return bool whether the token exists
*/
function _exists(uint256 tokenId) internal view returns (bool) {
}

/**
* @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) {
require(_exists(tokenId), "TRC721: operator query for nonexistent token");
return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
}

/**
* @dev Internal function to safely mint a new token.
* Reverts if the given token ID already exists.
* If the target address is a contract, it must implement `onTRC721Received`,
* which is called upon a safe transfer, and return the magic value
* the transfer is reverted.
* @param to The address that will own the minted token
* @param tokenId uint256 ID of the token to be minted
*/
function _safeMint(address to, uint256 tokenId) internal {
_safeMint(to, tokenId, "");
}

/**
* @dev Internal function to safely mint a new token.
* Reverts if the given token ID already exists.
* If the target address is a contract, it must implement `onTRC721Received`,
* which is called upon a safe transfer, and return the magic value
* the transfer is reverted.
* @param to The address that will own the minted token
* @param tokenId uint256 ID of the token to be minted
* @param _data bytes data to send along with a safe transfer check
*/
function _safeMint(address to, uint256 tokenId, bytes memory _data) internal {
_mint(to, tokenId);
}

/**
* @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
*/
function _mint(address to, uint256 tokenId) internal {

_tokenOwner[tokenId] = to;
_ownedTokensCount[to].increment();

}

/**
* @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
*/
function _burn(address owner, uint256 tokenId) internal {
require(ownerOf(tokenId) == owner, "TRC721: burn of token that is not own");

_clearApproval(tokenId);

_ownedTokensCount[owner].decrement();

}

/**
* @dev Internal function to burn a specific token.
* Reverts if the token does not exist.
* @param tokenId uint256 ID of the token being burned
*/
function _burn(uint256 tokenId) internal {
_burn(ownerOf(tokenId), tokenId);
}

/**
* @dev Internal function to transfer ownership of a given token ID to another address.
* As opposed to {transferFrom}, this imposes no restrictions on msg.sender.
* @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
*/
require(ownerOf(tokenId) == from, "TRC721: transfer of token that is not own");

_clearApproval(tokenId);

_ownedTokensCount[from].decrement();
_ownedTokensCount[to].increment();

_tokenOwner[tokenId] = to;

emit Transfer(from, to, tokenId);
}

/**
* The call is not executed if the target address is not a contract.
*
* This is an internal detail of the `TRC721` contract and its use is deprecated.
* @param from address representing the previous owner of the given token ID
* @param tokenId uint256 ID of the token to be transferred
* @param _data bytes optional data to send along with the call
* @return bool whether the call correctly returned the expected magic value
*/
internal returns (bool)
{
if (!to.isContract) {
return true;
}
// solhint-disable-next-line avoid-low-level-calls
(bool success, bytes memory returndata) = to.call(abi.encodeWithSelector(
_msgSender(),
from,
tokenId,
_data
));
if (!success) {
if (returndata.length > 0) {
// solhint-disable-next-line no-inline-assembly
assembly {
}
} else {
revert("TRC721: transfer to non TRC721Receiver implementer");
}
} else {
bytes4 retval = abi.decode(returndata, (bytes4));
}
}

/**
* @dev Private function to clear current approval of a given token ID.
* @param tokenId uint256 ID of the token to be transferred
*/
function _clearApproval(uint256 tokenId) private {
}
}
}

// Token name
string private _name;

// Token symbol
string private _symbol;

// Base URI
string private _baseURI;

// Optional mapping for token URIs
mapping(uint256 => string) private _tokenURIs;

/*
*     bytes4(keccak256('name()')) == 0x06fdde03
*     bytes4(keccak256('symbol()')) == 0x95d89b41
*     bytes4(keccak256('tokenURI(uint256)')) == 0xc87b56dd
*
*     => 0x06fdde03 ^ 0x95d89b41 ^ 0xc87b56dd == 0x5b5e139f
*/
bytes4 private constant _INTERFACE_ID_TRC721_METADATA = 0x5b5e139f;

/**
* @dev Constructor function
*/
constructor (string memory name, string memory symbol) public {
_name = name;
_symbol = symbol;

// register the supported interfaces to conform to TRC721 via TRC165
}

/**
* @dev Gets the token name.
* @return string representing the token name
*/
function name() external view returns (string memory) {
return _name;
}

/**
* @dev Gets the token symbol.
* @return string representing the token symbol
*/
function symbol() external view returns (string memory) {
return _symbol;
}

/**
* @dev Returns the URI for a given token ID. May return an empty string.
*
* If the token's URI is non-empty and a base URI was set (via
* {_setBaseURI}), it will be added to the token ID's URI as a prefix.
*
* Reverts if the token ID does not exist.
*/
function tokenURI(uint256 tokenId) external view returns (string memory) {
require(_exists(tokenId), "TRC721Metadata: URI query for nonexistent token");

string memory _tokenURI = _tokenURIs[tokenId];

// Even if there is a base URI, it is only appended to non-empty token-specific URIs
if (bytes(_tokenURI).length == 0) {
return "";
} else {
// abi.encodePacked is being used to concatenate strings
return string(abi.encodePacked(_baseURI, _tokenURI));
}
}

/**
* @dev Internal function to set the token URI for a given token.
*
* Reverts if the token ID does not exist.
*
* TIP: if all token IDs share a prefix (e.g. if your URIs look like
* `http://api.myproject.com/token/<id>`), use {_setBaseURI} to store
* it and save gas.
*/
function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal {
require(_exists(tokenId), "TRC721Metadata: URI set of nonexistent token");
_tokenURIs[tokenId] = _tokenURI;
}

/**
* @dev Internal function to set the base URI for all token IDs. It is
* automatically added as a prefix to the value returned in {tokenURI}.
*
* _Available since v2.5.0._
*/
function _setBaseURI(string memory baseURI) internal {
_baseURI = baseURI;
}

/**
* @dev Returns the base URI set via {_setBaseURI}. This will be
* automatically added as a preffix in {tokenURI} to each token's URI, when
* they are non-empty.
*
* _Available since v2.5.0._
*/
function baseURI() external view returns (string memory) {
return _baseURI;
}

/**
* @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);

if (bytes(_tokenURIs[tokenId]).length != 0) {
delete _tokenURIs[tokenId];
}
}
}

/**
* @dev TRC721 minting logic with metadata.
*/
/**
* @dev Function to mint tokens.
* @param tokenId The token id to mint.
* @param tokenURI The token URI of the minted token.
* @return A boolean that indicates if the operation was successful.
*/
function mintWithTokenURI(address to, uint256 tokenId, string memory tokenURI) public onlyMinter returns (bool) {
_mint(to, tokenId);
_setTokenURI(tokenId, tokenURI);
return true;
}
}

/**
* @title TRC721Mintable
* @dev TRC721 minting logic.
*/
contract TRC721Mintable is TRC721, MinterRole {
/**
* @dev Function to mint tokens.
* @param tokenId The token id to mint.
* @return A boolean that indicates if the operation was successful.
*/
function mint(address to, uint256 tokenId) public onlyMinter returns (bool) {
_mint(to, tokenId);
return true;
}

/**
* @dev Function to safely mint tokens.
* @param tokenId The token id to mint.
* @return A boolean that indicates if the operation was successful.
*/
function safeMint(address to, uint256 tokenId) public onlyMinter returns (bool) {
_safeMint(to, tokenId);
return true;
}

/**
* @dev Function to safely mint tokens.
* @param tokenId The token id to mint.
* @param _data bytes data to send along with a safe transfer check.
* @return A boolean that indicates if the operation was successful.
*/
function safeMint(address to, uint256 tokenId, bytes memory _data) public onlyMinter returns (bool) {
_safeMint(to, tokenId, _data);
return true;
}
}

/**
* @title TRC-721 Non-Fungible Token Standard, optional enumeration extension
*/
contract ITRC721Enumerable is ITRC721 {
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 TRC-721 Non-Fungible Token with optional enumeration extension logic
*/
contract TRC721Enumerable is Context, TRC165, TRC721, ITRC721Enumerable {
// Mapping from owner to list of owned token IDs

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

// Array with all token ids, used for enumeration
uint256[] private _allTokens;

// Mapping from token id to position in the allTokens array
mapping(uint256 => uint256) private _allTokensIndex;

/*
*     bytes4(keccak256('totalSupply()')) == 0x18160ddd
*     bytes4(keccak256('tokenByIndex(uint256)')) == 0x4f6ccce7
*
*     => 0x18160ddd ^ 0x2f745c59 ^ 0x4f6ccce7 == 0x780e9d63
*/
bytes4 private constant _INTERFACE_ID_TRC721_ENUMERABLE = 0x780e9d63;

/**
* @dev Constructor function.
*/
constructor () public {
// register the supported interface to conform to TRC721Enumerable via TRC165
_registerInterface(_INTERFACE_ID_TRC721_ENUMERABLE);
}

/**
* @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), "TRC721Enumerable: owner index out of bounds");
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(), "TRC721Enumerable: global index out of bounds");
return _allTokens[index];
}

/**
* @dev Internal function to transfer ownership of a given token ID to another address.
* As opposed to transferFrom, this imposes no restrictions on msg.sender.
* @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
*/
super._transferFrom(from, to, tokenId);

_removeTokenFromOwnerEnumeration(from, tokenId);

}

/**
* @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
*/
function _mint(address to, uint256 tokenId) internal {
super._mint(to, 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
*/
function _burn(address owner, uint256 tokenId) internal {
super._burn(owner, tokenId);

_removeTokenFromOwnerEnumeration(owner, tokenId);
// Since tokenId will be deleted, we can clear its slot in _ownedTokensIndex to trigger a gas refund
_ownedTokensIndex[tokenId] = 0;

_removeTokenFromAllTokensEnumeration(tokenId);
}

/**
* @dev Gets the list of token IDs of the requested owner.
* @param owner address owning the tokens
* @return uint256[] List of token IDs owned by the requested address
*/
function _tokensOfOwner(address owner) internal view returns (uint256[] storage) {
return _ownedTokens[owner];
}

/**
* @dev Private function to add a token to this extension's ownership-tracking data structures.
* @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
*/
_ownedTokensIndex[tokenId] = _ownedTokens[to].length;
_ownedTokens[to].push(tokenId);
}

/**
* @dev Private function to add a token to this extension's token tracking data structures.
* @param tokenId uint256 ID of the token to be added to the tokens list
*/
_allTokensIndex[tokenId] = _allTokens.length;
_allTokens.push(tokenId);
}

/**
* @dev Private function to remove a token from this extension's ownership-tracking data structures. Note that
* while the token is not assigned a new owner, the `_ownedTokensIndex` mapping is _not_ updated: this allows for
* gas optimizations e.g. when performing a transfer operation (avoiding double writes).
* This has O(1) time complexity, but alters the order of the _ownedTokens array.
* @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 _removeTokenFromOwnerEnumeration(address from, uint256 tokenId) private {
// To prevent a gap in from's tokens array, we store the last token in the index of the token to delete, and
// then delete the last slot (swap and pop).

uint256 lastTokenIndex = _ownedTokens[from].length.sub(1);
uint256 tokenIndex = _ownedTokensIndex[tokenId];

// When the token to delete is the last token, the swap operation is unnecessary
if (tokenIndex != lastTokenIndex) {
uint256 lastTokenId = _ownedTokens[from][lastTokenIndex];

_ownedTokens[from][tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
_ownedTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index
}

// This also deletes the contents at the last position of the array
_ownedTokens[from].length--;

// Note that _ownedTokensIndex[tokenId] hasn't been cleared: it still points to the old slot (now occupied by
// lastTokenId, or just over the end of the array if the token was the last one).
}

/**
* @dev Private function to remove a token from this extension's token tracking data structures.
* This has O(1) time complexity, but alters the order of the _allTokens array.
* @param tokenId uint256 ID of the token to be removed from the tokens list
*/
function _removeTokenFromAllTokensEnumeration(uint256 tokenId) private {
// To prevent a gap in the tokens array, we store the last token in the index of the token to delete, and
// then delete the last slot (swap and pop).

uint256 lastTokenIndex = _allTokens.length.sub(1);
uint256 tokenIndex = _allTokensIndex[tokenId];

// When the token to delete is the last token, the swap operation is unnecessary. However, since this occurs so
// rarely (when the last minted token is burnt) that we still do the swap here to avoid the gas cost of adding
// an 'if' statement (like in _removeTokenFromOwnerEnumeration)
uint256 lastTokenId = _allTokens[lastTokenIndex];

_allTokens[tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
_allTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index

// This also deletes the contents at the last position of the array
_allTokens.length--;
_allTokensIndex[tokenId] = 0;
}
}

contract TRC721Token is TRC721, TRC721Enumerable, TRC721MetadataMintable {