// SPDX-License-Identifier: MIT

pragma solidity >=0.7.0 <0.9.0;
pragma experimental ABIEncoderV2;

import "../patterns/Upgradeable.sol";

/// @title WitnetProxy: upgradable delegate-proxy contract. 
/// @author Guillermo Díaz <guillermo@witnet.io>
contract WitnetProxy {

    /// Event emitted every time the implementation gets updated.
    event Upgraded(address indexed implementation);  

    /// Constructor with no params as to ease eventual support of Singleton pattern (i.e. ERC-2470).
    constructor () {}

    receive() virtual external payable {}

    /// Payable fallback accepts delegating calls to payable functions.  
    fallback() external payable { /* solhint-disable no-complex-fallback */
        address _implementation = implementation();
        assembly { /* solhint-disable avoid-low-level-calls */
            // Gas optimized delegate call to 'implementation' contract.
            // Note: `msg.data`, `msg.sender` and `msg.value` will be passed over 
            //       to actual implementation of `msg.sig` within `implementation` contract.
            let ptr := mload(0x40)
            calldatacopy(ptr, 0, calldatasize())
            let result := delegatecall(gas(), _implementation, ptr, calldatasize(), 0, 0)
            let size := returndatasize()
            returndatacopy(ptr, 0, size)
            switch result
                case 0  { 
                    // pass back revert message:
                    revert(ptr, size) 
                }
                default {
                  // pass back same data as returned by 'implementation' contract:
                  return(ptr, size) 
                }
        }
    }

    /// Returns proxy's current implementation address.
    function implementation() public view returns (address) {
        return __proxySlot().implementation;
    }

    /// Upgrades the `implementation` address.
    /// @param _newImplementation New implementation address.
    /// @param _initData Raw data with which new implementation will be initialized.
    /// @return Returns whether new implementation would be further upgradable, or not.
    function upgradeTo(address _newImplementation, bytes memory _initData)
        public returns (bool)
    {
        // New implementation cannot be null:
        require(_newImplementation != address(0), "WitnetProxy: null implementation");

        address _oldImplementation = implementation();
        if (_oldImplementation != address(0)) {
            // New implementation address must differ from current one:
            require(_newImplementation != _oldImplementation, "WitnetProxy: nothing to upgrade");

            // Assert whether current implementation is intrinsically upgradable:
            try Upgradeable(_oldImplementation).isUpgradable() returns (bool _isUpgradable) {
                require(_isUpgradable, "WitnetProxy: not upgradable");
            } catch {
                revert("WitnetProxy: unable to check upgradability");
            }

            // Assert whether current implementation allows `msg.sender` to upgrade the proxy:
            (bool _wasCalled, bytes memory _result) = _oldImplementation.delegatecall(
                abi.encodeWithSignature(
                    "isUpgradableFrom(address)",
                    msg.sender
                )
            );
            require(_wasCalled, "WitnetProxy: uncompliant implementation");
            require(abi.decode(_result, (bool)), "WitnetProxy: not authorized");
            require(
                Upgradeable(_oldImplementation).proxiableUUID() == Upgradeable(_newImplementation).proxiableUUID(),
                "WitnetProxy: proxiableUUIDs mismatch"
            );
        }

        // Initialize new implementation within proxy-context storage:
        (bool _wasInitialized, bytes memory _returnData) = _newImplementation.delegatecall(
            abi.encodeWithSignature(
                "initialize(bytes)",
                _initData
            )
        );
        if (!_wasInitialized) {
            if (_returnData.length < 68) {
                revert("WitnetProxy: initialization failed");
            } else {
                assembly {
                    _returnData := add(_returnData, 0x04)
                }
                revert(abi.decode(_returnData, (string)));
            }
        }

        // If all checks and initialization pass, update implementation address:
        __proxySlot().implementation = _newImplementation;
    
        emit Upgraded(_newImplementation);

        // Asserts new implementation complies w/ minimal implementation of Upgradeable interface:
        try Upgradeable(_newImplementation).isUpgradable() returns (bool _isUpgradable) {
            return _isUpgradable;
        }
        catch {
            revert ("WitnetProxy: uncompliant implementation");
        }
    }

    /// @dev Complying with EIP-1967, retrieves storage struct containing proxy's current implementation address.
    function __proxySlot() private pure returns (Proxiable.ProxiableSlot storage _slot) {
        assembly {
            // bytes32(uint256(keccak256('eip1967.proxy.implementation')) - 1)
            _slot.slot := 0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc
        }
    }

}