How to keep track of already minted NFTs while minting ERC721 NFTs Randomly?

Question

I am working on random minting of ERC721 Tokens... I am using Chainlink VRF For randomisation.

The confusion I have is that how can I keep track of NFTs that have been already minted... Because VRF can generate a number(NFT ID) that has already been minted... One option is to use while loop or use an array which will be way too expensive

Any other optimised solution please???

Answer 1

something like this is a bit expensive to set up, but minting is cheap, so I think it's along the lines of what you want to do.

Rather than keeping track of what's been minted already, it keeps a set of IDs that haven't been minted and selects from that. The set gets smaller as NFTs are minted.

    pragma solidity ^0.8.7

    import "hardhat/console.sol";

    contract RandomMints {

    uint[] notMinted;

    function createSet() external {
        // this is a bit expensive but a one off
        // txn gas 2282618 in remix
        for(uint i; i < 100; ++i){
            notMinted.push(i);
        }
    }

    function pickId() external returns (uint) {
        // txn gas 37430 in remix
        uint numLeft = notMinted.length;
        console.log("num of NFTs not minted:", numLeft);

        // pick a number between 0 - num of unminted NFTs -1
        uint rand = block.timestamp % (numLeft - 1);  //not really random
        uint idSelected = notMinted[rand];

        // now reduce the set. it doesn't matter if we mix up the order as it adds more entropy
        // get rid of the picked id by overwriting it with the last id/element
        notMinted[rand] = notMinted[numLeft -1];
        // pop the last (now duplicated) id/element off the array
        notMinted.pop();

        console.log("id selected:", idSelected);
        return idSelected;
    }
}

Answer 2

What about to have a mapping(uint => bool) to know which ID number was already minted?

For example

mapping(uint => bool) minted;

function mint(uint id) {
  require(!minted[id],"already minted")
  //mint the NFT
}