// SPDX-License-Identifier: agpl-3.0
pragma solidity ^0.6.8;

import "./UpgradeabilityProxy.sol";

/**
 * @title BaseAdminUpgradeabilityProxy
 * @dev This contract combines an upgradeability proxy with an authorization
 * mechanism for administrative tasks.
 * All external functions in this contract must be guarded by the
 * `ifAdmin` modifier. See ethereum/solidity#3864 for a Solidity
 * feature proposal that would enable this to be done automatically.
 */
contract BaseAdminUpgradeabilityProxy is BaseUpgradeabilityProxy {
    /**
   * @dev Emitted when the administration has been transferred.
   * @param previousAdmin Address of the previous admin.
   * @param newAdmin Address of the new admin.
   */
    event AdminChanged(address previousAdmin, address newAdmin);

    /**
   * @dev Storage slot with the admin of the contract.
   * This is the keccak-256 hash of "eip1967.proxy.admin" subtracted by 1, and is
   * validated in the constructor.
   */
    bytes32 internal constant ADMIN_SLOT = 0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6103;

    /**
   * @dev Modifier to check whether the `msg.sender` is the admin.
   * If it is, it will run the function. Otherwise, it will delegate the call
   * to the implementation.
   */
    modifier ifAdmin() {
        if (msg.sender == _admin()) {
            _;
        } else {
            _fallback();
        }
    }

    /**
   * @return The address of the proxy admin.
   */
    function admin() external ifAdmin returns (address) {
        return _admin();
    }

    /**
   * @return The address of the implementation.
   */
    function implementation() external ifAdmin returns (address) {
        return _implementation();
    }

    /**
   * @dev Changes the admin of the proxy.
   * Only the current admin can call this function.
   * @param newAdmin Address to transfer proxy administration to.
   */
    function changeAdmin(address newAdmin) external ifAdmin {
        require(newAdmin != address(0), "Cannot change the admin of a proxy to the zero address");
        emit AdminChanged(_admin(), newAdmin);
        _setAdmin(newAdmin);
    }

    /**
   * @dev Upgrade the backing implementation of the proxy.
   * Only the admin can call this function.
   * @param newImplementation Address of the new implementation.
   */
    function upgradeTo(address newImplementation) external ifAdmin {
        _upgradeTo(newImplementation);
    }

    /**
   * @dev Upgrade the backing implementation of the proxy and call a function
   * on the new implementation.
   * This is useful to initialize the proxied contract.
   * @param newImplementation Address of the new implementation.
   * @param data Data to send as msg.data in the low level call.
   * It should include the signature and the parameters of the function to be called, as described in
   * https://solidity.readthedocs.io/en/v0.4.24/abi-spec.html#function-selector-and-argument-encoding.
   */
    function upgradeToAndCall(address newImplementation, bytes calldata data) external payable ifAdmin {
        _upgradeTo(newImplementation);
        (bool success, ) = newImplementation.delegatecall(data);
        require(success);
    }

    /**
   * @return adm The admin slot.
   */
    function _admin() internal view returns (address adm) {
        bytes32 slot = ADMIN_SLOT;
        //solium-disable-next-line
        assembly {
            adm := sload(slot)
        }
    }

    /**
   * @dev Sets the address of the proxy admin.
   * @param newAdmin Address of the new proxy admin.
   */
    function _setAdmin(address newAdmin) internal {
        bytes32 slot = ADMIN_SLOT;
        //solium-disable-next-line
        assembly {
            sstore(slot, newAdmin)
        }
    }

    /**
   * @dev Only fall back when the sender is not the admin.
   */
    function _willFallback() internal override virtual {
        require(msg.sender != _admin(), "Cannot call fallback function from the proxy admin");
        super._willFallback();
    }
}