Proof of Existence.sol

// SPDX-License-Identifier: MIT

pragma solidity >=0.8.0 < 0.9.0;

/**
 * @title Certificate Proof of Existence Contract
 * @dev This PoE stores University Certificate document with time stamp as Hash values.
        The unique identifiers of the certificates are stored on the Ethereum Blockchain.
        It is publicly verifiable to prove their existence.
 *
 */

contract ProofOfExistence {

    address admin;
    constructor(address _admin) {
        admin = _admin;
    }

    modifier onlyAdmin {
        require(msg.sender == admin, "Insufficient access");
        _;
    }

    struct CertificateDetails {  
       uint certificateID;
       string candidateName;
       string courseName;
       string grade;
       uint date;
    }

    uint certificateID = 200;

    mapping(address => uint) getID;
    mapping(uint => CertificateDetails) storeID;

    CertificateDetails[] certificateRecord;       

    function proofCertificateExist (
      address _candidate,
      uint _certificateID,
      string memory _candidateName,
      string memory _courseName,
      string memory _grade,
      uint _date
        ) public onlyAdmin returns(bool, uint) {
            
            if(certificateIdTaken(_candidate)) {
                return (false, getID[_candidate]);
            }
            certificateID = certificateID + 1;

            CertificateDetails memory newCertificate = CertificateDetails(_certificateID, _candidateName, _courseName, _grade, _date);

            certificateRecord.push(newCertificate);

            getID[_candidate] = certificateID;
            storeID[certificateID] = newCertificate;
            return(true, certificateID);
    }

    function verifyId(uint _certificateID) external view returns(CertificateDetails memory) {
        return storeID[_certificateID];
    }

    function retrieveID() external view returns(bool, uint) {
        uint id = getID[msg.sender];
        if(id == 0) return (false, 0);
        return (true, id);
    }
    
    function certificateIdTaken(address _candidate) internal view returns(bool) {
        return !(getID[_candidate] == 0);
    }
}