Uniq Random Numbers - Without Chainlink

Question

I am trying to assign unique tickets for users. The user can choose the number of tickets and get an allotment. The allocated tickets will be mapped to users.

pragma solidity ^0.8.11;
contract UniqTicket
{
    // Initializing the state variable
    uint public randNonce = 0;

    mapping(address => uint256[]) public participants;
    uint256[] public allTickets;

 
    function randMod(uint _modulus) public returns(uint){
        randNonce++;
        return uint(keccak256(abi.encodePacked(block.timestamp, msg.sender, randNonce, block.difficulty))) % _modulus;
    }


    function allotTickets(uint256 size) public{
        for(uint i=0; i<size; i++) {
            uint256 ticket = randMod(1000);
            participants[msg.sender].push(ticket);
            allTickets.push(ticket);
        }
    }

    function getTickets(address _user) public view returns(uint256[] memory) {
      return participants[_user];
    }

    function fetchAllTickets() public view returns(uint256[] memory) {
        return allTickets;
    }

}

When users choose allotTickets(200). The user is getting duplicate numbers. Though randNonce is a unique counter.

Looking out for a solution, how to get unique numbers.

Sample Output:

[427,257,673,731,304,357,2,959,236,931,766,322,929,42,69,897,902,116,972,408,27,397,13,141,337,238,520,680,847,657,44,630,649,722,715,81,126,569,952,874,925,354,907,101,649,548,259,8,678,10,297,256,371,369,338,355,446,248,36,639,169,742,802,391,886,117,230,781,230,279,693,9,532,456,276,148,105,863,134,161,19,954,231,949,849,477,211,305,570,281,575,548,340,348,758,15,794,311,634,639,753,50,33,856,59,142,505,201,331,513,913,801,660,739,830,641,888,975,481,236,473,624,243,86,976,871,606,736,726,780,571,53,98,470,702,437,33,688,332,639,392,720,751,107,377,544,235,773,625,544,451,315,68,559,217,491,401,75,549,781,267,217,308,430,776,163,300,438,631,125,689,787,503,805,277,828,430,377,133,122,343,913,637,712,862,247,348,964,824,189,711,666,730,43,704,896,829,771,696,983]

Thank you

Answer 1

To ensure unique ticket numbers, you can use a mapping to keep track of the allocated tickets. Please use this code below:

pragma solidity ^0.8.11;

contract UniqTicket {
    // Initializing the state variable
    uint public randNonce = 0;

    mapping(address => uint256[]) public participants;
    mapping(uint256 => bool) public allocatedTickets;
    uint256[] public allTickets;

    function randMod(uint _modulus) public returns(uint){
        randNonce++;
        return uint(keccak256(abi.encodePacked(block.timestamp, msg.sender, randNonce, block.difficulty))) % _modulus;
    }

    function allotTickets(uint256 size) public {
        for(uint i = 0; i < size; i++) {
            uint256 ticket = randMod(1000);
            while (allocatedTickets[ticket]) {
                ticket = randMod(1000);
            }
            allocatedTickets[ticket] = true;
            participants[msg.sender].push(ticket);
            allTickets.push(ticket);
        }
    }

    function getTickets(address _user) public view returns(uint256[] memory) {
        return participants[_user];
    }

    function fetchAllTickets() public view returns(uint256[] memory) {
        return allTickets;
    }
}

I added a new mapping called allocated tickets to keep track of the allocated ticket numbers. In the allotTickets function, added a while loop to generate a new random ticket number if the current ticket number is already allocated. This ensures that each ticket number is unique. Please note that this method may not be efficient for a large number of tickets, as the while loop may run for a long time if there are only a few unallocated tickets left. In such cases, you may want to consider alternative methods for generating unique ticket numbers.