exercise: Simple contract to store software licenses in solidity

simpleLicense.sol

pragma solidity ^0.5.11;

contract SoftwareLicense {
        
    struct License {
        uint serial;
        address user;
        uint datetime;
    }
    
    mapping (address => License) license;
    mapping (uint => address) serialOwner;
    
    function registerLicense(uint _serial) public {
        license[msg.sender].user = msg.sender;
        license[msg.sender].serial = _serial;
        license[msg.sender].datetime = now;
    }
    
    function getLicenseFromUser(address user) public view returns (uint, address, uint) {
        return (license[user].serial, license[user].user, license[user].datetime);
    }
    
    function getLicenseFromSerial(uint serial) public view returns (uint, address, uint) {
        return getLicenseFromUser(serialOwner[serial]);
    }
}