// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

// Educational example: deploy only in Remix VM for this course.
contract Attendance {
    address public immutable owner;
    mapping(bytes32 => bool) public events;
    mapping(bytes32 => bool) public certificates;
    event Issued(bytes32 indexed eventId, bytes32 indexed guestId, bytes32 certificateId);

    constructor() { owner = msg.sender; }
    modifier onlyOwner() {
        require(msg.sender == owner, "Not authorized");
        _;
    }
    function createEvent(string calldata id) external onlyOwner {
        require(bytes(id).length > 0, "Empty event");
        bytes32 key = keccak256(bytes(id));
        require(!events[key], "Duplicate event");
        events[key] = true;
    }
    function issue(string calldata eventId, string calldata guestId) external onlyOwner returns (bytes32) {
        require(bytes(guestId).length > 0, "Empty guest");
        bytes32 eventKey = keccak256(bytes(eventId));
        require(events[eventKey], "Unknown event");
        bytes32 key = keccak256(abi.encode(eventId, guestId));
        require(!certificates[key], "Duplicate certificate");
        certificates[key] = true;
        emit Issued(eventKey, keccak256(bytes(guestId)), key);
        return key;
    }
    function verify(string calldata eventId, string calldata guestId) external view returns (bool) {
        return certificates[keccak256(abi.encode(eventId, guestId))];
    }
}