// SPDX-License-Identifier: MIT

/* solhint-disable var-name-mixedcase */

pragma solidity >=0.6.0 <0.9.0;

import "@openzeppelin/contracts/utils/introspection/ERC165.sol";

import "./Initializable.sol";
import "./Proxiable.sol";

abstract contract Upgradeable is Initializable, Proxiable {

    address internal immutable _BASE;
    bytes32 internal immutable _CODEHASH;
    bool internal immutable _UPGRADABLE;

    modifier onlyDelegateCalls virtual {
        require(
            address(this) != _BASE,
            "Upgradeable: not a delegate call"
        );
        _;
    }

    /// Emitted every time the contract gets upgraded.
    /// @param from The address who ordered the upgrading. Namely, the WRB operator in "trustable" implementations.
    /// @param baseAddr The address of the new implementation contract.
    /// @param baseCodehash The EVM-codehash of the new implementation contract.
    /// @param versionTag Ascii-encoded version literal with which the implementation deployer decided to tag it.
    event Upgraded(
        address indexed from,
        address indexed baseAddr,
        bytes32 indexed baseCodehash,
        string  versionTag
    );

    constructor (bool _isUpgradable) {
        address _base = address(this);
        _BASE = _base;
        _UPGRADABLE = _isUpgradable;
    }

    /// @dev Retrieves base contract. Differs from address(this) when called via delegate-proxy pattern.
    function base() public view returns (address) {
        return _BASE;
    }

    /// @dev Retrieves the immutable codehash of this contract, even if invoked as delegatecall.
    function codehash() public view returns (bytes32 _codehash) {
        address _base = _BASE;
        assembly {
            _codehash := extcodehash(_base)
        }
    }

    /// @dev Determines whether the logic of this contract is potentially upgradable.
    function isUpgradable() public view returns (bool) {
        return _UPGRADABLE;
    }

    /// @dev Tells whether provided address could eventually upgrade the contract.
    function isUpgradableFrom(address from) virtual external view returns (bool);

    /// @notice Re-initialize contract's storage context upon a new upgrade from a proxy.    
    /// @dev Must fail when trying to upgrade to same logic contract more than once.
    function initialize(bytes memory) virtual external;

    /// @dev Retrieves human-redable named version of current implementation.
    function version() virtual public view returns (string memory); 
}