registry.sol

pragma solidity^0.5.0;

contract Registry {
    
    address owner;
    
    constructor() public {
        owner = msg.sender;
    }
    
    mapping(address => bool) hasUpdated;

    address[] implementations;
    
    function update() public {
        hasUpdated[msg.sender] = true;
    }
    
    function addImplementation(address _impl) public {
        require(msg.sender == owner);
        implementations.push(_impl);
    }
    
    function getImplementation(address _user) public view returns (address) {
        if (hasUpdated[_user]) {
            return implementations[1];
        }
        else {
            return implementations[0];
        }
    }
    
}