Theta Smart Contracts | ThetaScan.io

Node:

Token Contract Details

Contract:

0xb34c2a3d3eb5b869a3263f23e5da14d671b70b0c

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Published Contract:

pragma solidity 0.5.16; // compiled with: v0.5.16+commit.9c3226ce
pragma experimental ABIEncoderV2;

// https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/utils/Address.sol
/**
 * @dev Collection of functions related to the address type,
 */
library Address {
 /**
 * @dev Returns true if `account` is a contract.
 *
 * This test is non-exhaustive, and there may be false-negatives: during the
 * execution of a contract9s constructor, its address will be reported as
 * not containing a contract.
 *
 * > It is unsafe to assume that an address for which this function returns
 * false is an externally-owned account (EOA) and not a contract.
 */
 function isContract(address account) internal view returns (bool) {
 // This method relies in extcodesize, which returns 0 for contracts in
 // construction, since the code is only stored at the end of the
 // constructor execution.

 uint256 size;
 // solhint-disable-next-line no-inline-assembly
 assembly { size := extcodesize(account) }
 return size > 0;
 }
}

// https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/math/SafeMath.sol
/**
 * @dev Wrappers over Solidity9s 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 it9s recommended to use it always.
 */
library SafeMath {
 /**
 * @dev Returns the addition of two unsigned integers, reverting on
 * overflow.
 *
 * Counterpart to Solidity9s `+` operator.
 *
 * Requirements:
 * - Addition cannot overflow.
 */
 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 Solidity9s `-` operator.
 *
 * Requirements:
 * - Subtraction cannot overflow.
 */
 function sub(uint256 a, uint256 b) internal pure returns (uint256) {
 require(b <= a, "SafeMath: subtraction overflow");
 uint256 c = a - b;

 return c;
 }

 /**
 * @dev Returns the multiplication of two unsigned integers, reverting on
 * overflow.
 *
 * Counterpart to Solidity9s `*` operator.
 *
 * Requirements:
 * - Multiplication cannot overflow.
 */
 function mul(uint256 a, uint256 b) internal pure returns (uint256) {
 // Gas optimization: this is cheaper than requiring 9a9 not being zero, but the
 // benefit is lost if 9b9 is also tested.
 // See: https://github.com/OpenZeppelin/openzeppelin-solidity/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 Solidity9s `/` 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) {
 // Solidity only automatically asserts when dividing by 0
 require(b > 0, "SafeMath: division by zero");
 uint256 c = a / b;
 // assert(a == b * c + a % b); // There is no case in which this doesn9t hold

 return c;
 }

 /**
 * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
 * Reverts when dividing by zero.
 *
 * Counterpart to Solidity9s `%` 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) {
 require(b != 0, "SafeMath: modulo by zero");
 return a % b;
 }
}

// https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/drafts/Counters.sol
/**
 * @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 ERC721 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 library9s 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 {
 counter._value += 1;
 }

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

/**
 * @dev Interface of the ERC165 standard, as defined in the
 * [EIP](https://eips.ethereum.org/EIPS/eip-165).
 *
 * Implementers can declare support of contract interfaces, which can then be
 * queried by others (`ERC165Checker`).
 *
 * For an implementation, see `ERC165`.
 */
interface IERC165 {
 /**
 * @dev Returns true if this contract implements the interface defined by
 * `interfaceId`. See the corresponding
 * [EIP section](https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified)
 * to learn more about how these ids are created.
 *
 * This function call must use less than 30 000 gas.
 */
 function supportsInterface(bytes4 interfaceId) external view returns (bool);
}

/**
 * @dev Implementation of the `IERC165` interface.
 *
 * Contracts may inherit from this and call `_registerInterface` to declare
 * their support of an interface.
 */
contract ERC165 is IERC165 {
 /*
 * bytes4(keccak256(9supportsInterface(bytes4)9)) == 0x01ffc9a7
 */
 bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;

 /**
 * @dev Mapping of interface ids to whether or not it9s supported.
 */
 mapping(bytes4 => bool) private _supportedInterfaces;

 constructor () internal {
 // Derived contracts need only register support for their own interfaces,
 // we register support for ERC165 itself here
 _registerInterface(_INTERFACE_ID_ERC165);
 }

 /**
 * @dev See `IERC165.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 ERC165 interface is automatic and
 * registering its interface id is not required.
 *
 * See `IERC165.supportsInterface`.
 *
 * Requirements:
 *
 * - `interfaceId` cannot be the ERC165 invalid interface (`0xffffffff`).
 */
 function _registerInterface(bytes4 interfaceId) internal {
 require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
 _supportedInterfaces[interfaceId] = true;
 }
}

/**
 * @dev Required interface of an ERC721 compliant contract.
 */
contract IERC721 is IERC165 {
 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);

 /**
 * @dev Returns the number of NFTs in `owner`9s 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 `setApproveForAll`.
 */
 function safeTransferFrom(address from, address to, uint256 tokenId) public;
 /**
 * @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 `setApproveForAll`.
 */
 function transferFrom(address from, address to, uint256 tokenId) public;
 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 safeTransferFrom(address from, address to, uint256 tokenId, bytes memory data) public;
}

// https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/token/ERC721/IERC721Receiver.sol
/**
 * @title ERC721 token receiver interface
 * @dev Interface for any contract that wants to support safeTransfers
 * from ERC721 asset contracts.
 */
contract IERC721Receiver {
 /**
 * @notice Handle the receipt of an NFT
 * @dev The ERC721 smart contract calls this function on the recipient
 * after a `safeTransfer`. This function MUST return the function selector,
 * otherwise the caller will revert the transaction. The selector to be
 * returned can be obtained as `this.onERC721Received.selector`. This
 * function MAY throw to revert and reject the transfer.
 * Note: the ERC721 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
 * @return bytes4 `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
 */
 function onERC721Received(address operator, address from, uint256 tokenId, bytes memory data)
 public returns (bytes4);
}

// https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/token/ERC721/ERC721.sol
contract ERC721 is ERC165, IERC721 {
 using SafeMath for uint256;
 using Address for address;
 using Counters for Counters.Counter;

 // Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
 // which can be also obtained as `IERC721Receiver(0).onERC721Received.selector`
 bytes4 private constant _ERC721_RECEIVED = 0x150b7a02;

 // 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
 mapping (address => mapping (address => bool)) private _operatorApprovals;

 /*
 * bytes4(keccak256(9balanceOf(address)9)) == 0x70a08231
 * bytes4(keccak256(9ownerOf(uint256)9)) == 0x6352211e
 * bytes4(keccak256(9approve(address,uint256)9)) == 0x095ea7b3
 * bytes4(keccak256(9getApproved(uint256)9)) == 0x081812fc
 * bytes4(keccak256(9setApprovalForAll(address,bool)9)) == 0xa22cb465
 * bytes4(keccak256(9isApprovedForAll(address,address)9)) == 0xe985e9c
 * bytes4(keccak256(9transferFrom(address,address,uint256)9)) == 0x23b872dd
 * bytes4(keccak256(9safeTransferFrom(address,address,uint256)9)) == 0x42842e0e
 * bytes4(keccak256(9safeTransferFrom(address,address,uint256,bytes)9)) == 0xb88d4fde
 *
 * => 0x70a08231 ^ 0x6352211e ^ 0x095ea7b3 ^ 0x081812fc ^
 * 0xa22cb465 ^ 0xe985e9c ^ 0x23b872dd ^ 0x42842e0e ^ 0xb88d4fde == 0x80ac58cd
 */
 bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;

 constructor () public {
 // register the supported interfaces to conform to ERC721 via ERC165
 _registerInterface(_INTERFACE_ID_ERC721);
 }

 /**
 * @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), "ERC721: balance query for the zero address");

 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) {
 address owner = _tokenOwner[tokenId];
 require(owner != address(0), "ERC721: owner query for nonexistent token");

 return owner;
 }

 /**
 * @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, "ERC721: approval to current owner");

 require(msg.sender == owner || isApprovedForAll(owner, msg.sender),
 "ERC721: 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), "ERC721: 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 != msg.sender, "ERC721: approve to caller");

 _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 {
 //solhint-disable-next-line max-line-length
 require(_isApprovedOrOwner(msg.sender, tokenId), "ERC721: transfer caller is not owner nor approved");

 _transferFrom(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 {
 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 memory _data) public {
 transferFrom(from, to, tokenId);
 require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
 }

 /**
 * @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) {
 address owner = _tokenOwner[tokenId];
 return owner != address(0);
 }

 /**
 * @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), "ERC721: operator query for nonexistent token");
 address owner = ownerOf(tokenId);
 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
 */
 function _mint(address to, uint256 tokenId) internal {
 require(to != address(0), "ERC721: mint to the zero address");
 require(!_exists(tokenId), "ERC721: token already minted");

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

 emit Transfer(address(0), to, tokenId);
 }

 /**
 * @dev Internal function to burn a specific token.
 * Reverts if the token does not exist.
 * Deprecated, use _burn(uint256) instead.
 * @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, "ERC721: burn of token that is not own");

 _clearApproval(tokenId);

 _ownedTokensCount[owner].decrement();
 _tokenOwner[tokenId] = address(0);

 emit Transfer(owner, address(0), 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
 */
 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
 */
 function _transferFrom(address from, address to, uint256 tokenId) internal {
 require(ownerOf(tokenId) == from, "ERC721: transfer of token that is not own");
 require(to != address(0), "ERC721: transfer to the zero address");

 _clearApproval(tokenId);

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

 _tokenOwner[tokenId] = to;

 emit Transfer(from, to, tokenId);
 }

 /**
 * @dev Internal function to invoke `onERC721Received` on a target address.
 * The call is not executed if the target address is not a contract.
 *
 * This function is deprecated.
 * @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 bool whether the call correctly returned the expected magic value
 */
 function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data)
 internal returns (bool)
 {
 if (!to.isContract()) {
 return true;
 }

 bytes4 retval = IERC721Receiver(to).onERC721Received(msg.sender, from, tokenId, _data);
 return (retval == _ERC721_RECEIVED);
 }

 /**
 * @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 {
 if (_tokenApprovals[tokenId] != address(0)) {
 _tokenApprovals[tokenId] = address(0);
 }
 }
}

/**
 * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
contract IERC721Enumerable is IERC721 {
 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 with optional enumeration extension logic
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
contract ERC721Enumerable is ERC165, ERC721, IERC721Enumerable {
 // Mapping from owner to list of owned token IDs
 mapping(address => uint256[]) private _ownedTokens;

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

 // 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(9totalSupply()9)) == 0x18160ddd
 * bytes4(keccak256(9tokenOfOwnerByIndex(address,uint256)9)) == 0x2f745c59
 * bytes4(keccak256(9tokenByIndex(uint256)9)) == 0x4f6ccce7
 *
 * => 0x18160ddd ^ 0x2f745c59 ^ 0x4f6ccce7 == 0x780e9d63
 */
 bytes4 private constant _INTERFACE_ID_ERC721_ENUMERABLE = 0x780e9d63;

 /**
 * @dev Constructor function.
 */
 constructor () public {
 // register the supported interface to conform to ERC721Enumerable via ERC165
 _registerInterface(_INTERFACE_ID_ERC721_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), "ERC721Enumerable: 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(), "ERC721Enumerable: 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
 */
 function _transferFrom(address from, address to, uint256 tokenId) internal {
 super._transferFrom(from, to, tokenId);

 _removeTokenFromOwnerEnumeration(from, tokenId);

 _addTokenToOwnerEnumeration(to, 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);

 _addTokenToOwnerEnumeration(to, tokenId);

 _addTokenToAllTokensEnumeration(tokenId);
 }

 /**
 * @dev Internal function to burn a specific token.
 * Reverts if the token does not exist.
 * Deprecated, use _burn(uint256) instead.
 * @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 extension9s 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
 */
 function _addTokenToOwnerEnumeration(address to, uint256 tokenId) private {
 _ownedTokensIndex[tokenId] = _ownedTokens[to].length;
 _ownedTokens[to].push(tokenId);
 }

 /**
 * @dev Private function to add a token to this extension9s token tracking data structures.
 * @param tokenId uint256 ID of the token to be added to the tokens list
 */
 function _addTokenToAllTokensEnumeration(uint256 tokenId) private {
 _allTokensIndex[tokenId] = _allTokens.length;
 _allTokens.push(tokenId);
 }

 /**
 * @dev Private function to remove a token from this extension9s 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 from9s 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 token9s index
 }

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

 // Note that _ownedTokensIndex[tokenId] hasn9t 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 extension9s 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 9if9 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 token9s index

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

contract TNT721 is ERC721Enumerable {

 struct Instance {
 // the owner of the contract at the time the token was minted
 address minter;
 // the original account the token was minted to
 address original_owner;
 // the id given to the token off chain
 string off_chain_id;
 }

 // the original creator of the contract
 address private _creator;
 // the current owner and minter of the contract
 address private _owner;
 // the uri of the off chain metadata of the contract
 string private _metadata_uri;
 // the base uri of the off chain metadata for each token
 string private _base_token_uri;

 Instance[] public instances;

 constructor (string memory metadataUri, string memory baseTokenUri) public {
 _creator = msg.sender;
 _owner = msg.sender;
 _metadata_uri = metadataUri;
 _base_token_uri = baseTokenUri;
 }

 function creator() public view returns (address) {
 return _creator;
 }

 function owner() public view returns (address) {
 return _owner;
 }

 function mint(address account, string memory instanceId) public {
 require(_owner == msg.sender);
 uint256 tokenId = instances.length;
 instances.push(Instance(_owner, account, instanceId));
 _mint(account, tokenId);
 }

 function batchMint(address[] memory accounts, string[] memory instanceIds) public {
 require(accounts.length == instanceIds.length, "unmatched recipient and instance ids");
 require(_owner == msg.sender);
 for (uint i = 0; i < instanceIds.length; i++) {
 address account = accounts[i];
 string memory instanceId = instanceIds[i];
 mint(account, instanceId);
 }
 }

 function mintSet(address account, string[] memory instanceIds) public {
 require(_owner == msg.sender);
 for (uint i = 0; i < instanceIds.length; i++) {
 string memory instanceId = instanceIds[i];
 mint(account, instanceId);
 }
 }

 function transferOwnership(address newOwner) public {
 require(_owner == msg.sender);
 _owner = newOwner;
 }

 function burn(uint256 instanceId) public {
 _burn(ownerOf(instanceId), instanceId);
 }

 function contractURI() public view returns (string memory) {
 return _metadata_uri;
 }

 function baseTokenURI() public view returns (string memory) {
 return _base_token_uri;
 }

 function tokenURI(uint256 _tokenId) public view returns (string memory) {
 bytes memory _ba = bytes(_base_token_uri);
 bytes memory _bb = bytes(instances[_tokenId].off_chain_id);
 string memory ab = new string(_ba.length + _bb.length);
 bytes memory bab = bytes(ab);
 uint k = 0;
 for (uint i = 0; i < _ba.length; i++) bab[k++] = _ba[i];
 for (uint i = 0; i < _bb.length; i++) bab[k++] = _bb[i];
 return string(bab);
 }
}

Contract ABI:

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