interface ISimpleFunctionRegistry {

    /// @dev A function implementation was updated via `extend()` or `rollback()`.
    /// @param selector The function selector.
    /// @param oldImpl The implementation contract address being replaced.
    /// @param newImpl The replacement implementation contract address.
    event ProxyFunctionUpdated(bytes4 indexed selector, address oldImpl, address newImpl);

    /// @dev Roll back to a prior implementation of a function.
    /// @param selector The function selector.
    /// @param targetImpl The address of an older implementation of the function.
    function rollback(bytes4 selector, address targetImpl) external;

    /// @dev Register or replace a function.
    /// @param selector The function selector.
    /// @param impl The implementation contract for the function.
    function extend(bytes4 selector, address impl) external;

    /// @dev Retrieve the length of the rollback history for a function.
    /// @param selector The function selector.
    /// @return rollbackLength The number of items in the rollback history for
    ///         the function.
    function getRollbackLength(bytes4 selector)
        external
        view
        returns (uint256 rollbackLength);

    /// @dev Retrieve an entry in the rollback history for a function.
    /// @param selector The function selector.
    /// @param idx The index in the rollback history.
    /// @return impl An implementation address for the function at
    ///         index `idx`.
    function getRollbackEntryAtIndex(bytes4 selector, uint256 idx)
        external
        view
        returns (address impl);
}
/// @dev Basic registry management features.
function extend(bytes4 selector, address impl) external;

/// @dev Register or replace a function.
/// @param selector The function selector.
/// @param impl The implementation contract for the function.