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;
}
}
/**
 * @dev Implementation of the {IERC165} interface.
 *
 * Contracts may inherit from this and call {_registerInterface} to declare
 * their support of an interface.
 */
function _registerInterface(bytes4 interfaceId) internal virtual {
require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
_supportedInterfaces[interfaceId] = true;
}

/**
 * @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`).
 */