/**
 *Submitted for verification at Etherscan.io on 2022-07-01
*/

/**
 *Submitted for verification at polygonscan.com on 2022-06-23
*/

pragma experimental ABIEncoderV2;
// File: @openzeppelin/contracts/utils/Strings.sol
pragma solidity >=0.6.0 <0.8.0;

/**

 * @dev String operations.
   */
   library Strings {
   /**

    * @dev Converts a `uint256` to its ASCII `string` representation.
      */
      function toString(uint256 value) internal pure returns (string memory) {
      // Inspired by OraclizeAPI's implementation - MIT licence
      // https://github.com/oraclize/ethereum-api/blob/b42146b063c7d6ee1358846c198246239e9360e8/oraclizeAPI_0.4.25.sol

      if (value == 0) {
          return "0";
      }
      uint256 temp = value;
      uint256 digits;
      while (temp != 0) {
          digits++;
          temp /= 10;
      }
      bytes memory buffer = new bytes(digits);
      uint256 index = digits - 1;
      temp = value;
      while (temp != 0) {
          buffer[index--] = bytes1(uint8(48 + temp % 10));
          temp /= 10;
      }
      return string(buffer);
      }
      }

// File: @openzeppelin/contracts/utils/EnumerableMap.sol



pragma solidity >=0.6.0 <0.8.0;

/**

 * @dev Library for managing an enumerable variant of Solidity's

 * https://solidity.readthedocs.io/en/latest/types.html#mapping-types[`mapping`]

 * type.
   *

 * Maps have the following properties:
   *

 * - Entries are added, removed, and checked for existence in constant time

 * (O(1)).

 * - Entries are enumerated in O(n). No guarantees are made on the ordering.
     *

 * ```
   
   ```

 * contract Example {

 * // Add the library methods

 * using EnumerableMap for EnumerableMap.UintToAddressMap;
    *

 * // Declare a set state variable

 * EnumerableMap.UintToAddressMap private myMap;

 * }

 * ```
   *
   ```

 * As of v3.0.0, only maps of type `uint256 -> address` (`UintToAddressMap`) are

 * supported.
   */
   library EnumerableMap {
   // To implement this library for multiple types with as little code
   // repetition as possible, we write it in terms of a generic Map type with
   // bytes32 keys and values.
   // The Map implementation uses private functions, and user-facing
   // implementations (such as Uint256ToAddressMap) are just wrappers around
   // the underlying Map.
   // This means that we can only create new EnumerableMaps for types that fit
   // in bytes32.

   struct MapEntry {
       bytes32 _key;
       bytes32 _value;
   }

   struct Map {
       // Storage of map keys and values
       MapEntry[] _entries;

       // Position of the entry defined by a key in the `entries` array, plus 1
       // because index 0 means a key is not in the map.
       mapping(bytes32 => uint256) _indexes;

   }

   /**

    * @dev Adds a key-value pair to a map, or updates the value for an existing

    * key. O(1).
      *

    * Returns true if the key was added to the map, that is if it was not

    * already present.
      */
      function _set(Map storage map, bytes32 key, bytes32 value) private returns (bool) {
      // We read and store the key's index to prevent multiple reads from the same storage slot
      uint256 keyIndex = map._indexes[key];

      if (keyIndex == 0) {// Equivalent to !contains(map, key)
          map._entries.push(MapEntry({_key : key, _value : value}));
          // The entry is stored at length-1, but we add 1 to all indexes
          // and use 0 as a sentinel value
          map._indexes[key] = map._entries.length;
          return true;
      } else {
          map._entries[keyIndex - 1]._value = value;
          return false;
      }
      }

   /**

    * @dev Removes a key-value pair from a map. O(1).
      *

    * Returns true if the key was removed from the map, that is if it was present.
      */
      function _remove(Map storage map, bytes32 key) private returns (bool) {
      // We read and store the key's index to prevent multiple reads from the same storage slot
      uint256 keyIndex = map._indexes[key];

      if (keyIndex != 0) {// Equivalent to contains(map, key)
          // To delete a key-value pair from the _entries array in O(1), we swap the entry to delete with the last one
          // in the array, and then remove the last entry (sometimes called as 'swap and pop').
          // This modifies the order of the array, as noted in {at}.

          uint256 toDeleteIndex = keyIndex - 1;
          uint256 lastIndex = map._entries.length - 1;
          
          // When the entry to delete is the last one, the swap operation is unnecessary. However, since this occurs
          // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.
          
          MapEntry storage lastEntry = map._entries[lastIndex];
          
          // Move the last entry to the index where the entry to delete is
          map._entries[toDeleteIndex] = lastEntry;
          // Update the index for the moved entry
          map._indexes[lastEntry._key] = toDeleteIndex + 1;
          // All indexes are 1-based
          
          // Delete the slot where the moved entry was stored
          map._entries.pop();
          
          // Delete the index for the deleted slot
          delete map._indexes[key];
          
          return true;

      } else {
          return false;
      }
      }

   /**

    * @dev Returns true if the key is in the map. O(1).
      */
      function _contains(Map storage map, bytes32 key) private view returns (bool) {
      return map._indexes[key] != 0;
      }

   /**

    * @dev Returns the number of key-value pairs in the map. O(1).
      */
      function _length(Map storage map) private view returns (uint256) {
      return map._entries.length;
      }

   /**

    * @dev Returns the key-value pair stored at position `index` in the map. O(1).
      *

   * Note that there are no guarantees on the ordering of entries inside the

   * array, and it may change when more entries are added or removed.
     *

   * Requirements:
     *

   * - `index` must be strictly less than {length}.
       */
       function _at(Map storage map, uint256 index) private view returns (bytes32, bytes32) {
       require(map._entries.length > index, "EnumerableMap: index out of bounds");

       MapEntry storage entry = map._entries[index];
       return (entry._key, entry._value);
       }

   /**

    * @dev Tries to returns the value associated with `key`.  O(1).
    * Does not revert if `key` is not in the map.
      */
      function _tryGet(Map storage map, bytes32 key) private view returns (bool, bytes32) {
      uint256 keyIndex = map._indexes[key];
      if (keyIndex == 0) return (false, 0);
      // Equivalent to contains(map, key)
      return (true, map._entries[keyIndex - 1]._value);
      // All indexes are 1-based
      }

   /**

    * @dev Returns the value associated with `key`.  O(1).
      *
    * Requirements:
      *
    * - `key` must be in the map.
        */
        function _get(Map storage map, bytes32 key) private view returns (bytes32) {
        uint256 keyIndex = map._indexes[key];
        require(keyIndex != 0, "EnumerableMap: nonexistent key");
        // Equivalent to contains(map, key)
        return map._entries[keyIndex - 1]._value;
        // All indexes are 1-based
        }

   /**

    * @dev Same as {_get}, with a custom error message when `key` is not in the map.
      *
    * CAUTION: This function is deprecated because it requires allocating memory for the error
    * message unnecessarily. For custom revert reasons use {_tryGet}.
      */
      function _get(Map storage map, bytes32 key, string memory errorMessage) private view returns (bytes32) {
      uint256 keyIndex = map._indexes[key];
      require(keyIndex != 0, errorMessage);
      // Equivalent to contains(map, key)
      return map._entries[keyIndex - 1]._value;
      // All indexes are 1-based
      }

   // UintToAddressMap

   struct UintToAddressMap {
       Map _inner;
   }

   /**

    * @dev Adds a key-value pair to a map, or updates the value for an existing
    * key. O(1).
      *
    * Returns true if the key was added to the map, that is if it was not
    * already present.
      */
      function set(UintToAddressMap storage map, uint256 key, address value) internal returns (bool) {
      return _set(map._inner, bytes32(key), bytes32(uint256(uint160(value))));
      }

   /**

    * @dev Removes a value from a set. O(1).
      *
    * Returns true if the key was removed from the map, that is if it was present.
      */
      function remove(UintToAddressMap storage map, uint256 key) internal returns (bool) {
      return _remove(map._inner, bytes32(key));
      }

   /**

    * @dev Returns true if the key is in the map. O(1).
      */
      function contains(UintToAddressMap storage map, uint256 key) internal view returns (bool) {
      return _contains(map._inner, bytes32(key));
      }

   /**

    * @dev Returns the number of elements in the map. O(1).
      */
      function length(UintToAddressMap storage map) internal view returns (uint256) {
      return _length(map._inner);
      }

   /**

    * @dev Returns the element stored at position `index` in the set. O(1).
   * Note that there are no guarantees on the ordering of values inside the
   * array, and it may change when more values are added or removed.
     *
   * Requirements:
     *
   * - `index` must be strictly less than {length}.
       */
       function at(UintToAddressMap storage map, uint256 index) internal view returns (uint256, address) {
       (bytes32 key, bytes32 value) = _at(map._inner, index);
       return (uint256(key), address(uint160(uint256(value))));
       }

   /**

    * @dev Tries to returns the value associated with `key`.  O(1).
    * Does not revert if `key` is not in the map.
      *
    * _Available since v3.4._
      */
      function tryGet(UintToAddressMap storage map, uint256 key) internal view returns (bool, address) {
      (bool success, bytes32 value) = _tryGet(map._inner, bytes32(key));
      return (success, address(uint160(uint256(value))));
      }

   /**

    * @dev Returns the value associated with `key`.  O(1).
      *
    * Requirements:
      *
    * - `key` must be in the map.
        */
        function get(UintToAddressMap storage map, uint256 key) internal view returns (address) {
        return address(uint160(uint256(_get(map._inner, bytes32(key)))));
        }

   /**

    * @dev Same as {get}, with a custom error message when `key` is not in the map.
      *
    * CAUTION: This function is deprecated because it requires allocating memory for the error
    * message unnecessarily. For custom revert reasons use {tryGet}.
      */
      function get(UintToAddressMap storage map, uint256 key, string memory errorMessage) internal view returns (address) {
      return address(uint160(uint256(_get(map._inner, bytes32(key), errorMessage))));
      }
      }

// File: @openzeppelin/contracts/utils/EnumerableSet.sol



pragma solidity >=0.6.0 <0.8.0;

/**

 * @dev Library for managing

 * https://en.wikipedia.org/wiki/Set_(abstract_data_type)[sets] of primitive

 * types.
   *

 * Sets have the following properties:
   *

 * - Elements are added, removed, and checked for existence in constant time

 * (O(1)).

 * - Elements are enumerated in O(n). No guarantees are made on the ordering.
     *

 * ```
   
   ```

 * contract Example {

 * // Add the library methods

 * using EnumerableSet for EnumerableSet.AddressSet;
    *

 * // Declare a set state variable

 * EnumerableSet.AddressSet private mySet;

 * }

 * ```
   *
   ```

 * As of v3.3.0, sets of type `bytes32` (`Bytes32Set`), `address` (`AddressSet`)

 * and `uint256` (`UintSet`) are supported.
   */
   library EnumerableSet {
   // To implement this library for multiple types with as little code
   // repetition as possible, we write it in terms of a generic Set type with
   // bytes32 values.
   // The Set implementation uses private functions, and user-facing
   // implementations (such as AddressSet) are just wrappers around the
   // underlying Set.
   // This means that we can only create new EnumerableSets for types that fit
   // in bytes32.

   struct Set {
       // Storage of set values
       bytes32[] _values;

       // Position of the value in the `values` array, plus 1 because index 0
       // means a value is not in the set.
       mapping(bytes32 => uint256) _indexes;

   }

   /**

    * @dev Add a value to a set. O(1).
      *
    * Returns true if the value was added to the set, that is if it was not
    * already present.
      */
      function _add(Set storage set, bytes32 value) private returns (bool) {
      if (!_contains(set, value)) {
          set._values.push(value);
          // The value is stored at length-1, but we add 1 to all indexes
          // and use 0 as a sentinel value
          set._indexes[value] = set._values.length;
          return true;
      } else {
          return false;
      }
      }

   /**

    * @dev Removes a value from a set. O(1).
      *

    * Returns true if the value was removed from the set, that is if it was

    * present.
      */
      function _remove(Set storage set, bytes32 value) private returns (bool) {
      // We read and store the value's index to prevent multiple reads from the same storage slot
      uint256 valueIndex = set._indexes[value];

      if (valueIndex != 0) {// Equivalent to contains(set, value)
          // To delete an element from the _values array in O(1), we swap the element to delete with the last one in
          // the array, and then remove the last element (sometimes called as 'swap and pop').
          // This modifies the order of the array, as noted in {at}.

          uint256 toDeleteIndex = valueIndex - 1;
          uint256 lastIndex = set._values.length - 1;
          
          // When the value to delete is the last one, the swap operation is unnecessary. However, since this occurs
          // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.
          
          bytes32 lastvalue = set._values[lastIndex];
          
          // Move the last value to the index where the value to delete is
          set._values[toDeleteIndex] = lastvalue;
          // Update the index for the moved value
          set._indexes[lastvalue] = toDeleteIndex + 1;
          // All indexes are 1-based
          
          // Delete the slot where the moved value was stored
          set._values.pop();
          
          // Delete the index for the deleted slot
          delete set._indexes[value];
          
          return true;

      } else {
          return false;
      }
      }

   /**

    * @dev Returns true if the value is in the set. O(1).
      */
      function _contains(Set storage set, bytes32 value) private view returns (bool) {
      return set._indexes[value] != 0;
      }

   /**

    * @dev Returns the number of values on the set. O(1).
      */
      function _length(Set storage set) private view returns (uint256) {
      return set._values.length;
      }

   /**

    * @dev Returns the value stored at position `index` in the set. O(1).
      *
   * Note that there are no guarantees on the ordering of values inside the
   * array, and it may change when more values are added or removed.
     *
   * Requirements:
     *
   * - `index` must be strictly less than {length}.
       */
       function _at(Set storage set, uint256 index) private view returns (bytes32) {
       require(set._values.length > index, "EnumerableSet: index out of bounds");
       return set._values[index];
       }

   // Bytes32Set

   struct Bytes32Set {
       Set _inner;
   }

   /**

    * @dev Add a value to a set. O(1).
      *
    * Returns true if the value was added to the set, that is if it was not
    * already present.
      */
      function add(Bytes32Set storage set, bytes32 value) internal returns (bool) {
      return _add(set._inner, value);
      }

   /**

    * @dev Removes a value from a set. O(1).
      *
    * Returns true if the value was removed from the set, that is if it was
    * present.
      */
      function remove(Bytes32Set storage set, bytes32 value) internal returns (bool) {
      return _remove(set._inner, value);
      }

   /**

    * @dev Returns true if the value is in the set. O(1).
      */
      function contains(Bytes32Set storage set, bytes32 value) internal view returns (bool) {
      return _contains(set._inner, value);
      }

   /**

    * @dev Returns the number of values in the set. O(1).
      */
      function length(Bytes32Set storage set) internal view returns (uint256) {
      return _length(set._inner);
      }

   /**

    * @dev Returns the value stored at position `index` in the set. O(1).
      *
   * Note that there are no guarantees on the ordering of values inside the
   * array, and it may change when more values are added or removed.
     *
   * Requirements:
     *
   * - `index` must be strictly less than {length}.
       */
       function at(Bytes32Set storage set, uint256 index) internal view returns (bytes32) {
       return _at(set._inner, index);
       }

   // AddressSet

   struct AddressSet {
       Set _inner;
   }

   /**

    * @dev Add a value to a set. O(1).
      *
    * Returns true if the value was added to the set, that is if it was not
    * already present.
      */
      function add(AddressSet storage set, address value) internal returns (bool) {
      return _add(set._inner, bytes32(uint256(uint160(value))));
      }

   /**

    * @dev Removes a value from a set. O(1).
      *
    * Returns true if the value was removed from the set, that is if it was
    * present.
      */
      function remove(AddressSet storage set, address value) internal returns (bool) {
      return _remove(set._inner, bytes32(uint256(uint160(value))));
      }

   /**

    * @dev Returns true if the value is in the set. O(1).
      */
      function contains(AddressSet storage set, address value) internal view returns (bool) {
      return _contains(set._inner, bytes32(uint256(uint160(value))));
      }

   /**

    * @dev Returns the number of values in the set. O(1).
      */
      function length(AddressSet storage set) internal view returns (uint256) {
      return _length(set._inner);
      }

   /**

    * @dev Returns the value stored at position `index` in the set. O(1).
      *
   * Note that there are no guarantees on the ordering of values inside the
   * array, and it may change when more values are added or removed.
     *
   * Requirements:
     *
   * - `index` must be strictly less than {length}.
       */
       function at(AddressSet storage set, uint256 index) internal view returns (address) {
       return address(uint160(uint256(_at(set._inner, index))));
       }


    // UintSet
    
    struct UintSet {
        Set _inner;
    }
    
    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function add(UintSet storage set, uint256 value) internal returns (bool) {
        return _add(set._inner, bytes32(value));
    }
    
    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function remove(UintSet storage set, uint256 value) internal returns (bool) {
        return _remove(set._inner, bytes32(value));
    }
    
    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function contains(UintSet storage set, uint256 value) internal view returns (bool) {
        return _contains(set._inner, bytes32(value));
    }
    
    /**
     * @dev Returns the number of values on the set. O(1).
     */
    function length(UintSet storage set) internal view returns (uint256) {
        return _length(set._inner);
    }
    
    /**
     * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function at(UintSet storage set, uint256 index) internal view returns (uint256) {
        return uint256(_at(set._inner, index));
    }

}

// File: @openzeppelin/contracts/utils/Address.sol



pragma solidity >=0.6.2 <0.8.0;

/**

 * @dev Collection of functions related to the address type
   */
   library Address {
   /**

    * @dev Returns true if `account` is a contract.
      *

    * [IMPORTANT]

    * ====

    * It is unsafe to assume that an address for which this function returns

    * false is an externally-owned account (EOA) and not a contract.
      *

    * Among others, `isContract` will return false for the following

    * types of addresses:
      *

    * - an externally-owned account

    * - a contract in construction

    * - an address where a contract will be created

    * - an address where a contract lived, but was destroyed

    * ====
      */
      function isContract(address account) internal view returns (bool) {
      // This method relies on 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;
      }

   /**

    * @dev Replacement for Solidity's `transfer`: sends `amount` wei to

    * `recipient`, forwarding all available gas and reverting on errors.
      *

    * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost

    * of certain opcodes, possibly making contracts go over the 2300 gas limit

    * imposed by `transfer`, making them unable to receive funds via

    * `transfer`. {sendValue} removes this limitation.
      *

    * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
      *

    * IMPORTANT: because control is transferred to `recipient`, care must be

    * taken to not create reentrancy vulnerabilities. Consider using

    * {ReentrancyGuard} or the

    * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
      */
      function sendValue(address payable recipient, uint256 amount) internal {
      require(address(this).balance >= amount, "Address: insufficient balance");

      // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
      (bool success,) = recipient.call{value : amount}("");
      require(success, "Address: unable to send value, recipient may have reverted");
      }

   /**

    * @dev Performs a Solidity function call using a low level `call`. A
    * plain`call` is an unsafe replacement for a function call: use this
    * function instead.
      *
    * If `target` reverts with a revert reason, it is bubbled up by this
    * function (like regular Solidity function calls).
      *
    * Returns the raw returned data. To convert to the expected return value,
    * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
      *
    * Requirements:
      *
    * - `target` must be a contract.
    * - calling `target` with `data` must not revert.
        *
    * _Available since v3.1._
      */
      function functionCall(address target, bytes memory data) internal returns (bytes memory) {
      return functionCall(target, data, "Address: low-level call failed");
      }

   /**

    * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
    * `errorMessage` as a fallback revert reason when `target` reverts.
      *
    * _Available since v3.1._
      */
      function functionCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
      return functionCallWithValue(target, data, 0, errorMessage);
      }

   /**

    * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
    * but also transferring `value` wei to `target`.
      *
    * Requirements:
      *
    * - the calling contract must have an ETH balance of at least `value`.
    * - the called Solidity function must be `payable`.
        *
    * _Available since v3.1._
      */
      function functionCallWithValue(address target, bytes memory data, uint256 value) internal returns (bytes memory) {
      return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
      }

   /**

    * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but

    * with `errorMessage` as a fallback revert reason when `target` reverts.
      *

    * _Available since v3.1._
      */
      function functionCallWithValue(address target, bytes memory data, uint256 value, string memory errorMessage) internal returns (bytes memory) {
      require(address(this).balance >= value, "Address: insufficient balance for call");
      require(isContract(target), "Address: call to non-contract");

      // solhint-disable-next-line avoid-low-level-calls
      (bool success, bytes memory returndata) = target.call{value : value}(data);
      return _verifyCallResult(success, returndata, errorMessage);
      }

   /**

    * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
    * but performing a static call.
      *
    * _Available since v3.3._
      */
      function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
      return functionStaticCall(target, data, "Address: low-level static call failed");
      }

   /**

    * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],

    * but performing a static call.
      *

    * _Available since v3.3._
      */
      function functionStaticCall(address target, bytes memory data, string memory errorMessage) internal view returns (bytes memory) {
      require(isContract(target), "Address: static call to non-contract");

      // solhint-disable-next-line avoid-low-level-calls
      (bool success, bytes memory returndata) = target.staticcall(data);
      return _verifyCallResult(success, returndata, errorMessage);
      }

   /**

    * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
    * but performing a delegate call.
      *
    * _Available since v3.4._
      */
      function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
      return functionDelegateCall(target, data, "Address: low-level delegate call failed");
      }

   /**

    * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],

    * but performing a delegate call.
      *

    * _Available since v3.4._
      */
      function functionDelegateCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
      require(isContract(target), "Address: delegate call to non-contract");

      // solhint-disable-next-line avoid-low-level-calls
      (bool success, bytes memory returndata) = target.delegatecall(data);
      return _verifyCallResult(success, returndata, errorMessage);
      }

   function _verifyCallResult(bool success, bytes memory returndata, string memory errorMessage) private pure returns (bytes memory) {
       if (success) {
           return returndata;
       } else {
           // Look for revert reason and bubble it up if present
           if (returndata.length > 0) {
               // The easiest way to bubble the revert reason is using memory via assembly

               // solhint-disable-next-line no-inline-assembly
               assembly {
                   let returndata_size := mload(returndata)
                   revert(add(32, returndata), returndata_size)
               }
           } else {
               revert(errorMessage);
           }
       }

   }
   }

// File: @openzeppelin/contracts/math/SafeMath.sol



pragma solidity >=0.6.0 <0.8.0;

/**

 * @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, with an overflow flag.
      *
    * _Available since v3.4._
      */
      function tryAdd(uint256 a, uint256 b) internal pure returns (bool, uint256) {
      uint256 c = a + b;
      if (c < a) return (false, 0);
      return (true, c);
      }

   /**

    * @dev Returns the substraction of two unsigned integers, with an overflow flag.
      *
    * _Available since v3.4._
      */
      function trySub(uint256 a, uint256 b) internal pure returns (bool, uint256) {
      if (b > a) return (false, 0);
      return (true, a - b);
      }

   /**

    * @dev Returns the multiplication of two unsigned integers, with an overflow flag.
      *
    * _Available since v3.4._
      */
      function tryMul(uint256 a, uint256 b) internal pure returns (bool, 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 (true, 0);
      uint256 c = a * b;
      if (c / a != b) return (false, 0);
      return (true, c);
      }

   /**

    * @dev Returns the division of two unsigned integers, with a division by zero flag.
      *
    * _Available since v3.4._
      */
      function tryDiv(uint256 a, uint256 b) internal pure returns (bool, uint256) {
      if (b == 0) return (false, 0);
      return (true, a / b);
      }

   /**

    * @dev Returns the remainder of dividing two unsigned integers, with a division by zero flag.
      *
    * _Available since v3.4._
      */
      function tryMod(uint256 a, uint256 b) internal pure returns (bool, uint256) {
      if (b == 0) return (false, 0);
      return (true, a % b);
      }

   /**

    * @dev Returns the addition of two unsigned integers, reverting on
    * overflow.
      *
    * Counterpart to Solidity's `+` 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 Solidity's `-` operator.
      *
    * Requirements:
      *
    * - Subtraction cannot overflow.
        */
        function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "SafeMath: subtraction overflow");
        return a - b;
        }

   /**

    * @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) {
        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, reverting 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) {
        require(b > 0, "SafeMath: division by zero");
        return a / b;
        }

   /**

    * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
    * reverting 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) {
        require(b > 0, "SafeMath: modulo by zero");
        return a % b;
        }

   /**

    * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
    * overflow (when the result is negative).
      *
    * CAUTION: This function is deprecated because it requires allocating memory for the error
    * message unnecessarily. For custom revert reasons use {trySub}.
      *
    * Counterpart to Solidity's `-` operator.
      *
    * Requirements:
      *
    * - Subtraction cannot overflow.
        */
        function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        return a - b;
        }

   /**

    * @dev Returns the integer division of two unsigned integers, reverting with custom message on
    * division by zero. The result is rounded towards zero.
      *
    * CAUTION: This function is deprecated because it requires allocating memory for the error
    * message unnecessarily. For custom revert reasons use {tryDiv}.
      *
    * 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, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        return a / b;
        }

   /**

    * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
    * reverting with custom message when dividing by zero.
      *
    * CAUTION: This function is deprecated because it requires allocating memory for the error
    * message unnecessarily. For custom revert reasons use {tryMod}.
      *
    * 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, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        return a % b;
        }
        }

// File: @openzeppelin/contracts/token/ERC721/IERC721Receiver.sol



pragma solidity >=0.6.0 <0.8.0;

/**

 * @title ERC721 token receiver interface
 * @dev Interface for any contract that wants to support safeTransfers
 * from ERC721 asset contracts.
   */
   interface IERC721Receiver {
   /**
    * @dev Whenever an {IERC721} `tokenId` token is transferred to this contract via {IERC721-safeTransferFrom}
    * by `operator` from `from`, this function is called.
      *
    * It must return its Solidity selector to confirm the token transfer.
    * If any other value is returned or the interface is not implemented by the recipient, the transfer will be reverted.
      *
    * The selector can be obtained in Solidity with `IERC721.onERC721Received.selector`.
      */
      function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data) external returns (bytes4);
      }

// File: @openzeppelin/contracts/introspection/IERC165.sol



pragma solidity >=0.6.0 <0.8.0;

/**

 * @dev Interface of the ERC165 standard, as defined in the
 * https://eips.ethereum.org/EIPS/eip-165[EIP].
   *
 * 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
    * https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified[EIP section]
    * 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);
      }

// File: @openzeppelin/contracts/introspection/ERC165.sol



pragma solidity >=0.6.0 <0.8.0;


/**

 * @dev Implementation of the {IERC165} interface.
   *

 * Contracts may inherit from this and call {_registerInterface} to declare

 * their support of an interface.
   */
   abstract contract ERC165 is IERC165 {
   /*

    * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
      */
      bytes4 private constant _INTERFACE_ID_ERC165 = 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 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) public view virtual override 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 virtual {
        require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
        _supportedInterfaces[interfaceId] = true;
        }
        }

// File: @openzeppelin/contracts/token/ERC721/IERC721.sol



pragma solidity >=0.6.2 <0.8.0;


/**

 * @dev Required interface of an ERC721 compliant contract.
   */
   interface IERC721 is IERC165 {
   /**

    * @dev Emitted when `tokenId` token is transferred from `from` to `to`.
      */
      event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);

   /**

    * @dev Emitted when `owner` enables `approved` to manage the `tokenId` token.
      */
      event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);

   /**

    * @dev Emitted when `owner` enables or disables (`approved`) `operator` to manage all of its assets.
      */
      event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

   /**

    * @dev Returns the number of tokens in ``owner``'s account.
      */
      function balanceOf(address owner) external view returns (uint256 balance);

   /**

    * @dev Returns the owner of the `tokenId` token.
      *
    * Requirements:
      *
    * - `tokenId` must exist.
        */
        function ownerOf(uint256 tokenId) external view returns (address owner);

   /**

    * @dev Safely transfers `tokenId` token from `from` to `to`, checking first that contract recipients
    * are aware of the ERC721 protocol to prevent tokens from being forever locked.
      *
    * Requirements:
      *
    * - `from` cannot be the zero address.
    * - `to` cannot be the zero address.
    * - `tokenId` token must exist and be owned by `from`.
    * - If the caller is not `from`, it must be have been allowed to move this token by either {approve} or {setApprovalForAll}.
    * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
        *
    * Emits a {Transfer} event.
      */
      function safeTransferFrom(address from, address to, uint256 tokenId) external;

   /**

    * @dev Transfers `tokenId` token from `from` to `to`.
      *
    * WARNING: Usage of this method is discouraged, use {safeTransferFrom} whenever possible.
      *
    * Requirements:
      *
    * - `from` cannot be the zero address.
    * - `to` cannot be the zero address.
    * - `tokenId` token must be owned by `from`.
    * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
        *
    * Emits a {Transfer} event.
      */
      function transferFrom(address from, address to, uint256 tokenId) external;

   /**

    * @dev Gives permission to `to` to transfer `tokenId` token to another account.
    * The approval is cleared when the token is transferred.
      *
    * Only a single account can be approved at a time, so approving the zero address clears previous approvals.
      *
    * Requirements:
      *
    * - The caller must own the token or be an approved operator.
    * - `tokenId` must exist.
        *
    * Emits an {Approval} event.
      */
      function approve(address to, uint256 tokenId) external;

   /**

    * @dev Returns the account approved for `tokenId` token.
      *
    * Requirements:
      *
    * - `tokenId` must exist.
        */
        function getApproved(uint256 tokenId) external view returns (address operator);

   /**

    * @dev Approve or remove `operator` as an operator for the caller.
    * Operators can call {transferFrom} or {safeTransferFrom} for any token owned by the caller.
      *
    * Requirements:
      *
    * - The `operator` cannot be the caller.
        *
    * Emits an {ApprovalForAll} event.
      */
      function setApprovalForAll(address operator, bool _approved) external;

   /**

    * @dev Returns if the `operator` is allowed to manage all of the assets of `owner`.
      *
    * See {setApprovalForAll}
      */
      function isApprovedForAll(address owner, address operator) external view returns (bool);

   /**

     * @dev Safely transfers `tokenId` token from `from` to `to`.
       *

     * Requirements:
       *

     * - `from` cannot be the zero address.

     * - `to` cannot be the zero address.

     * - `tokenId` token must exist and be owned by `from`.

     * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.

     * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
         *

       * Emits a {Transfer} event.
         */
         function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external;
         }

// File: @openzeppelin/contracts/token/ERC721/IERC721Enumerable.sol



pragma solidity >=0.6.2 <0.8.0;


/**

 * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension

 * @dev See https://eips.ethereum.org/EIPS/eip-721
   */
   interface IERC721Enumerable is IERC721 {

   /**

    * @dev Returns the total amount of tokens stored by the contract.
      */
      function totalSupply() external view returns (uint256);

   /**

    * @dev Returns a token ID owned by `owner` at a given `index` of its token list.
    * Use along with {balanceOf} to enumerate all of ``owner``'s tokens.
      */
      function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256 tokenId);

   /**

    * @dev Returns a token ID at a given `index` of all the tokens stored by the contract.
    * Use along with {totalSupply} to enumerate all tokens.
      */
      function tokenByIndex(uint256 index) external view returns (uint256);
      }

// File: @openzeppelin/contracts/token/ERC721/IERC721Metadata.sol



pragma solidity >=0.6.2 <0.8.0;


/**

 * @title ERC-721 Non-Fungible Token Standard, optional metadata extension

 * @dev See https://eips.ethereum.org/EIPS/eip-721
   */
   interface IERC721Metadata is IERC721 {

   /**

    * @dev Returns the token collection name.
      */
      function name() external view returns (string memory);

   /**

    * @dev Returns the token collection symbol.
      */
      function symbol() external view returns (string memory);

   /**

    * @dev Returns the Uniform Resource Identifier (URI) for `tokenId` token.
      */
      function tokenURI(uint256 tokenId) external view returns (string memory);
      }

// File: @openzeppelin/contracts/utils/Context.sol



pragma solidity >=0.6.0 <0.8.0;

/*

 * @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.
   */
   abstract contract Context {
   function _msgSender() internal view virtual returns (address payable) {
       return msg.sender;
   }

   function _msgData() internal view virtual returns (bytes memory) {
       this;
       // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
       return msg.data;
   }
   }

// File: @openzeppelin/contracts/token/ERC721/ERC721.sol

pragma solidity >=0.6.0 <0.8.0;

/**

 * @title ERC721 Non-Fungible Token Standard basic implementation

 * @dev see https://eips.ethereum.org/EIPS/eip-721
   */
   contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable {
   using SafeMath for uint256;
   using Address for address;
   using EnumerableSet for EnumerableSet.UintSet;
   using EnumerableMap for EnumerableMap.UintToAddressMap;
   using Strings for uint256;

   // 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 holder address to their (enumerable) set of owned tokens
   mapping(address => EnumerableSet.UintSet) private _holderTokens;

   // Enumerable mapping from token ids to their owners
   EnumerableMap.UintToAddressMap private _tokenOwners;

   // Mapping from token ID to approved address
   mapping(uint256 => address) private _tokenApprovals;

   // Mapping from owner to operator approvals
   mapping(address => mapping(address => bool)) private _operatorApprovals;

   // Token name
   string internal _name;

   // Token symbol
   string internal _symbol;

   // Optional mapping for token URIs
   mapping(uint256 => string) private _tokenURIs;

   // Base URI
   string private _baseURI;

   address public exPlatform;

   /*

    *     bytes4(keccak256('balanceOf(address)')) == 0x70a08231
    *     bytes4(keccak256('ownerOf(uint256)')) == 0x6352211e
    *     bytes4(keccak256('approve(address,uint256)')) == 0x095ea7b3
    *     bytes4(keccak256('getApproved(uint256)')) == 0x081812fc
    *     bytes4(keccak256('setApprovalForAll(address,bool)')) == 0xa22cb465
    *     bytes4(keccak256('isApprovedForAll(address,address)')) == 0xe985e9c5
    *     bytes4(keccak256('transferFrom(address,address,uint256)')) == 0x23b872dd
    *     bytes4(keccak256('safeTransferFrom(address,address,uint256)')) == 0x42842e0e
    *     bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)')) == 0xb88d4fde
           *
    *     => 0x70a08231 ^ 0x6352211e ^ 0x095ea7b3 ^ 0x081812fc ^
    *     0xa22cb465 ^ 0xe985e9c5 ^ 0x23b872dd ^ 0x42842e0e ^ 0xb88d4fde == 0x80ac58cd
           */
          bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;

   /*

    *     bytes4(keccak256('name()')) == 0x06fdde03
    *     bytes4(keccak256('symbol()')) == 0x95d89b41
    *     bytes4(keccak256('tokenURI(uint256)')) == 0xc87b56dd
           *
    *     => 0x06fdde03 ^ 0x95d89b41 ^ 0xc87b56dd == 0x5b5e139f
           */
          bytes4 private constant _INTERFACE_ID_ERC721_METADATA = 0x5b5e139f;

   /*

    *     bytes4(keccak256('totalSupply()')) == 0x18160ddd
    *     bytes4(keccak256('tokenOfOwnerByIndex(address,uint256)')) == 0x2f745c59
    *     bytes4(keccak256('tokenByIndex(uint256)')) == 0x4f6ccce7
           *
    *     => 0x18160ddd ^ 0x2f745c59 ^ 0x4f6ccce7 == 0x780e9d63
           */
          bytes4 private constant _INTERFACE_ID_ERC721_ENUMERABLE = 0x780e9d63;

   /**

    * @dev Initializes the contract by setting a `name` and a `symbol` to the token collection.
      */
      constructor () public {
      // register the supported interfaces to conform to ERC721 via ERC165
      _registerInterface(_INTERFACE_ID_ERC721);
      _registerInterface(_INTERFACE_ID_ERC721_METADATA);
      _registerInterface(_INTERFACE_ID_ERC721_ENUMERABLE);
      }

   /**

    * @dev See {IERC721-balanceOf}.
      */
      function balanceOf(address owner) public view virtual override returns (uint256) {
      require(owner != address(0), "ERC721: balance query for the zero address");
      return _holderTokens[owner].length();
      }

   /**

    * @dev See {IERC721-ownerOf}.
      */
      function ownerOf(uint256 tokenId) public view virtual override returns (address) {
      return _tokenOwners.get(tokenId, "ERC721: owner query for nonexistent token");
      }

   /**

    * @dev See {IERC721Metadata-name}.
      */
      function name() public view virtual override returns (string memory) {
      return _name;
      }

   /**

    * @dev See {IERC721Metadata-symbol}.
      */
      function symbol() public view virtual override returns (string memory) {
      return _symbol;
      }

   /**

    * @dev See {IERC721Metadata-tokenURI}.
      */
      function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
      require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");

      string memory _tokenURI = _tokenURIs[tokenId];
      string memory base = baseURI();

      // If there is no base URI, return the token URI.
      if (bytes(base).length == 0) {
          return _tokenURI;
      }
      // If both are set, concatenate the baseURI and tokenURI (via abi.encodePacked).
      if (bytes(_tokenURI).length > 0) {
          return string(abi.encodePacked(base, _tokenURI));
      }
      // If there is a baseURI but no tokenURI, concatenate the tokenID to the baseURI.
      return string(abi.encodePacked(base, tokenId.toString()));
      }

   /**

   * @dev Returns the base URI set via {_setBaseURI}. This will be
   * automatically added as a prefix in {tokenURI} to each token's URI, or
   * to the token ID if no specific URI is set for that token ID.
     */
     function baseURI() public view virtual returns (string memory) {
       return _baseURI;
     }

   /**

    * @dev See {IERC721Enumerable-tokenOfOwnerByIndex}.
      */
      function tokenOfOwnerByIndex(address owner, uint256 index) public view virtual override returns (uint256) {
      return _holderTokens[owner].at(index);
      }

   /**

    * @dev See {IERC721Enumerable-totalSupply}.
      */
      function totalSupply() public view virtual override returns (uint256) {
      // _tokenOwners are indexed by tokenIds, so .length() returns the number of tokenIds
      return _tokenOwners.length();
      }

   /**

    * @dev See {IERC721Enumerable-tokenByIndex}.
      */
      function tokenByIndex(uint256 index) public view virtual override returns (uint256) {
      (uint256 tokenId,) = _tokenOwners.at(index);
      return tokenId;
      }

   /**

    * @dev See {IERC721-approve}.
      */
      function approve(address to, uint256 tokenId) public virtual override {
      address owner = ownerOf(tokenId);
      require(to != owner, "ERC721: approval to current owner");

      require(_msgSender() == owner || isApprovedForAll(owner, _msgSender()),
          "ERC721: approve caller is not owner nor approved for all"
      );

      _approve(to, tokenId);
      }

   /**

    * @dev See {IERC721-getApproved}.
      */
      function getApproved(uint256 tokenId) public view virtual override returns (address) {
      require(_exists(tokenId), "ERC721: approved query for nonexistent token");

      return _tokenApprovals[tokenId];
      }

   /**

    * @dev See {IERC721-setApprovalForAll}.
      */
      function setApprovalForAll(address operator, bool approved) public virtual override {
      require(operator != _msgSender(), "ERC721: approve to caller");

      _operatorApprovals[_msgSender()][operator] = approved;
      emit ApprovalForAll(_msgSender(), operator, approved);
      }

   /**

    * @dev See {IERC721-isApprovedForAll}.
      */
      function isApprovedForAll(address owner, address operator) public view virtual override returns (bool) {
      return _operatorApprovals[owner][operator];
      }

   /**

    * @dev See {IERC721-transferFrom}.
      */
      function transferFrom(address from, address to, uint256 tokenId) public virtual override {
      //solhint-disable-next-line max-line-length
      require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");

      _transfer(from, to, tokenId);
      }

   /**

    * @dev See {IERC721-safeTransferFrom}.
      */
      function safeTransferFrom(address from, address to, uint256 tokenId) public virtual override {
      safeTransferFrom(from, to, tokenId, "");
      }

   /**

    * @dev See {IERC721-safeTransferFrom}.
      */
      function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public virtual override {
      require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");
      _safeTransfer(from, to, tokenId, _data);
      }

   /**

    * @dev Safely transfers `tokenId` token from `from` to `to`, checking first that contract recipients
    * are aware of the ERC721 protocol to prevent tokens from being forever locked.
      *
    * `_data` is additional data, it has no specified format and it is sent in call to `to`.
      *
    * This internal function is equivalent to {safeTransferFrom}, and can be used to e.g.
    * implement alternative mechanisms to perform token transfer, such as signature-based.
      *
    * Requirements:
      *
    * - `from` cannot be the zero address.
    * - `to` cannot be the zero address.
    * - `tokenId` token must exist and be owned by `from`.
    * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
        *
    * Emits a {Transfer} event.
      */
      function _safeTransfer(address from, address to, uint256 tokenId, bytes memory _data) internal virtual {
      _transfer(from, to, tokenId);
      require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
      }

   /**

    * @dev Returns whether `tokenId` exists.
      *
    * Tokens can be managed by their owner or approved accounts via {approve} or {setApprovalForAll}.
      *
    * Tokens start existing when they are minted (`_mint`),
    * and stop existing when they are burned (`_burn`).
      */
      function _exists(uint256 tokenId) internal view virtual returns (bool) {
      return _tokenOwners.contains(tokenId);
      }

   /**

    * @dev Returns whether `spender` is allowed to manage `tokenId`.
      *
    * Requirements:
      *
    * - `tokenId` must exist.
        */
        function _isApprovedOrOwner(address spender, uint256 tokenId) internal view virtual returns (bool) {
        require(_exists(tokenId), "ERC721: operator query for nonexistent token");
        address owner = ownerOf(tokenId);
        return (spender == owner || spender == exPlatform || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
        }

   /**

    * @dev Safely mints `tokenId` and transfers it to `to`.
      *
    * Requirements:
      d*
    * - `tokenId` must not exist.
    * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
        *
    * Emits a {Transfer} event.
      */
      function _safeMint(address to, uint256 tokenId) internal virtual {
      _safeMint(to, tokenId, "");
      }

   /**

    * @dev Same as {xref-ERC721-_safeMint-address-uint256-}[`_safeMint`], with an additional `data` parameter which is
    * forwarded in {IERC721Receiver-onERC721Received} to contract recipients.
      */
      function _safeMint(address to, uint256 tokenId, bytes memory _data) internal virtual {
      _mint(to, tokenId);
      require(_checkOnERC721Received(address(0), to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
      }

   /**

    * @dev Mints `tokenId` and transfers it to `to`.
      *

    * WARNING: Usage of this method is discouraged, use {_safeMint} whenever possible
      *

    * Requirements:
      *

    * - `tokenId` must not exist.

    * - `to` cannot be the zero address.
        *

    * Emits a {Transfer} event.
      */
      function _mint(address to, uint256 tokenId) internal virtual {
      require(to != address(0), "ERC721: mint to the zero address");
      require(!_exists(tokenId), "ERC721: token already minted");

      _beforeTokenTransfer(address(0), to, tokenId);

      _holderTokens[to].add(tokenId);

      _tokenOwners.set(tokenId, to);

      emit Transfer(address(0), to, tokenId);
      }

   /**

    * @dev Destroys `tokenId`.

    * The approval is cleared when the token is burned.
      *

    * Requirements:
      *

    * - `tokenId` must exist.
        *

    * Emits a {Transfer} event.
      */
      function _burn(uint256 tokenId) internal virtual {
      address owner = ownerOf(tokenId);
      // internal owner

      _beforeTokenTransfer(owner, address(0), tokenId);

      // Clear approvals
      _approve(address(0), tokenId);

      // Clear metadata (if any)
      if (bytes(_tokenURIs[tokenId]).length != 0) {
          delete _tokenURIs[tokenId];
      }

      _holderTokens[owner].remove(tokenId);

      _tokenOwners.remove(tokenId);

      emit Transfer(owner, address(0), tokenId);
      }

   /**

    * @dev Transfers `tokenId` from `from` to `to`.

    * As opposed to {transferFrom}, this imposes no restrictions on msg.sender.
      *

    * Requirements:
      *

    * - `to` cannot be the zero address.

    * - `tokenId` token must be owned by `from`.
        *

    * Emits a {Transfer} event.
      */
      function _transfer(address from, address to, uint256 tokenId) internal virtual {
      require(ownerOf(tokenId) == from, "ERC721: transfer of token that is not own");
      // internal owner
      require(to != address(0), "ERC721: transfer to the zero address");

      _beforeTokenTransfer(from, to, tokenId);

      // Clear approvals from the previous owner
      _approve(address(0), tokenId);

      _holderTokens[from].remove(tokenId);
      _holderTokens[to].add(tokenId);

      _tokenOwners.set(tokenId, to);

      emit Transfer(from, to, tokenId);
      }

   /**

    * @dev Sets `_tokenURI` as the tokenURI of `tokenId`.
      *
    * Requirements:
      *
    * - `tokenId` must exist.
        */
        function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal virtual {
        require(_exists(tokenId), "ERC721Metadata: 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},
    * or to the token ID if {tokenURI} is empty.
      */
      function _setBaseURI(string memory baseURI_) internal virtual {
      _baseURI = baseURI_;
      }

   /**

    * @dev Internal function to invoke {IERC721Receiver-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 bool whether the call correctly returned the expected magic value
      */
      function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data)
      private returns (bool)
      {
      if (!to.isContract()) {
          return true;
      }
      bytes memory returndata = to.functionCall(abi.encodeWithSelector(
              IERC721Receiver(to).onERC721Received.selector,
              _msgSender(),
              from,
              tokenId,
              _data
          ), "ERC721: transfer to non ERC721Receiver implementer");
      bytes4 retval = abi.decode(returndata, (bytes4));
      return (retval == _ERC721_RECEIVED);
      }

   function _approve(address to, uint256 tokenId) internal {
       _tokenApprovals[tokenId] = to;
       emit Approval(ownerOf(tokenId), to, tokenId);
       // internal owner
   }

   /**

    * @dev Hook that is called before any token transfer. This includes minting
    * and burning.
      *
    * Calling conditions:
      *
    * - When `from` and `to` are both non-zero, ``from``'s `tokenId` will be
    * transferred to `to`.
    * - When `from` is zero, `tokenId` will be minted for `to`.
    * - When `to` is zero, ``from``'s `tokenId` will be burned.
    * - `from` cannot be the zero address.
    * - `to` cannot be the zero address.
        *
    * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
      */
      function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal virtual {}
      }

// File: @openzeppelin/contracts/access/Ownable.sol



pragma solidity >=0.6.0 <0.8.0;

/**

 * @dev Contract module which provides a basic access control mechanism, where

 * there is an account (an owner) that can be granted exclusive access to

 * specific functions.
   *

 * By default, the owner account will be the one that deploys the contract. This

 * can later be changed with {transferOwnership}.
   *

 * This module is used through inheritance. It will make available the modifier

 * `onlyOwner`, which can be applied to your functions to restrict their use to

 * the owner.
   */
   abstract contract Ownable is Context {
   address internal _owner;

   event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

   /**

    * @dev Initializes the contract setting the deployer as the initial owner.
      */
      constructor () internal {
      address msgSender = _msgSender();
      _owner = msgSender;
      emit OwnershipTransferred(address(0), msgSender);
      }

   /**

    * @dev Returns the address of the current owner.
      */
      function owner() public view virtual returns (address) {
      return _owner;
      }

   /**

    * @dev Throws if called by any account other than the owner.
      */
      modifier onlyOwner() {
      require(owner() == _msgSender(), "Ownable: caller is not the owner");
      _;
      }

   /**

    * @dev Leaves the contract without owner. It will not be possible to call
    * `onlyOwner` functions anymore. Can only be called by the current owner.
      *
    * NOTE: Renouncing ownership will leave the contract without an owner,
    * thereby removing any functionality that is only available to the owner.
      */
      function renounceOwnership() public virtual onlyOwner {
      emit OwnershipTransferred(_owner, address(0));
      _owner = address(0);
      }

   /**

    * @dev Transfers ownership of the contract to a new account (`newOwner`).
    * Can only be called by the current owner.
      */
      function transferOwnership(address newOwner) public virtual onlyOwner {
      require(newOwner != address(0), "Ownable: new owner is the zero address");
      emit OwnershipTransferred(_owner, newOwner);
      _owner = newOwner;
      }
      }

// File: localhost/contract/interface/INFT721.sol



pragma solidity 0.6.6;

interface INFT721 {
    struct CopyrightTax {
        address payable copyrightHolder;
        uint256 taxFeeRate;// ?/10000
    }

    function copyrightTax(uint256 tokenId) external view returns (CopyrightTax memory);
    
    function burn(uint256 tokenId) external;
    
    function safeMint(address to, uint256 tokenId, string calldata name, string calldata des, string calldata data) external;
    
    function safeMint(address to, uint256 tokenId, string calldata name, string calldata des, string calldata data, address payable copyrightHolder, uint256 taxFeeRate) external;

}
// File: localhost/contract/implement/NFT721.sol

pragma solidity 0.6.6;

contract NFT721 is INFT721, ERC721, Ownable {

    event Mint(address indexed from, address indexed to, uint256 indexed tokenId, string name, string des, string uri);
    event MintWithCopyright(address indexed from, address indexed to, uint256 indexed tokenId, string name, string des, string uri, address copyrightHolder, uint256 taxFeeRate);
    
    mapping(address => bool) public miners;
    mapping(uint256 => string) public names;
    mapping(uint256 => string) public descriptions;
    mapping(uint256 => CopyrightTax) public copyrightTaxes;
    
    bytes32 public DOMAIN_SEPARATOR;
    // keccak256("Permit(address spender, uint256 tokenId, uint256 nonce,uint256 deadline)");
    bytes32 public constant PERMIT_TYPEHASH = 0x5770bb3a28b9dfc7db536f474ee81d23db80ccbd9d0c80e812238c1f77823448;
    mapping(address => uint256) public nonces;
    
    modifier onlyMiner() {
        require(miners[msg.sender], "caller is not the miner");
        _;
    }
    
    constructor(address pl, string memory name, string memory symbol) public {
        uint chainId;
        assembly {
            chainId := chainId
        }
        DOMAIN_SEPARATOR = keccak256(
            abi.encode(
                keccak256('EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)'),
                keccak256(bytes(name)),
                keccak256(bytes('1')),
                chainId,
                address(this)
            )
        );
    
        exPlatform = pl;
        _name = name;
        _symbol = symbol;
    }
    
    receive() external payable {}
    
    function burn(uint256 tokenId) override external {
        _burn(tokenId);
        delete names[tokenId];
        delete descriptions[tokenId];
        delete copyrightTaxes[tokenId];
    }
    
    function copyrightTax(uint256 tokenId) override external view returns (CopyrightTax memory){
        return copyrightTaxes[tokenId];
    }
    
    function safeMint(address to, uint256 tokenId, string calldata name, string calldata des, string calldata data) override external onlyMiner {
        _commonSafeMint(to, tokenId, name, des, data);
        emit Mint(address(0), to, tokenId, name, des, data);
    }
    
    function safeMint(address to, uint256 tokenId, string calldata name, string calldata des, string calldata data, address payable copyrightHolder, uint256 taxFeeRate) override external onlyMiner {
        require(taxFeeRate < 10000, "must be less than 10000");
        _commonSafeMint(to, tokenId, name, des, data);
    
        CopyrightTax storage cpt = copyrightTaxes[tokenId];
        cpt.copyrightHolder = copyrightHolder;
        cpt.taxFeeRate = taxFeeRate;
    
        emit MintWithCopyright(address(0), to, tokenId, name, des, data, copyrightHolder, taxFeeRate);
    }
    
    function _commonSafeMint(address to, uint256 tokenId, string memory name, string memory des, string memory data) internal {
        _safeMint(to, tokenId);
        _setTokenURI(tokenId, data);
        names[tokenId] = name;
        descriptions[tokenId] = des;
    }
    
    function setMiner(address miner, bool state) external onlyOwner {
        miners[miner] = state;
    }
    
    function setBaseURI(string calldata _baseURI) external onlyOwner {
        _setBaseURI(_baseURI);
    }
    
    function permit(address spender, uint256 tokenId, uint256 deadline, uint8 v, bytes32 r, bytes32 s) external {
        require(deadline >= block.timestamp, 'expired');
        address owner = ownerOf(tokenId);
        bytes32 hash = keccak256(
            abi.encodePacked(
                '\x19\x01',
                DOMAIN_SEPARATOR,
                keccak256(abi.encode(PERMIT_TYPEHASH, owner, spender, tokenId, nonces[owner]++, deadline))
            )
        );
    
        address recoveredAddress = ecrecover(hash, v, r, s);
        require(recoveredAddress != address(0) && (recoveredAddress == owner || isApprovedForAll(owner, recoveredAddress)), 'invalid signature');
        require(spender != owner, "ERC721: approval to current owner");
    
        _approve(spender, tokenId);
    }

}