Chainlink VRF – Random Number from Chainlink VRF at Hardhat Test Always 0: Troubleshooting Guide

chainlinkrandomnessvrf

I tried to test chainlink vrf at hardhat test with vrfcoordinatormock. test is work fine but random words and request id are always 0.
is It correct that returning 0 from MockVRF?


    const deployContract = async(vrfCoordinatorContract : "MockVRFCoordinator" | "MockVRFCoordinatorUnfulfillable" = "MockVRFCoordinator") => {
        const Consumer = await ethers.getContractFactory("VRFv2Consumer");
        vrfCoordFactory = await ethers.getContractFactory(vrfCoordinatorContract);
        vrfCoordinatorV2Mock = await vrfCoordFactory.deploy();
        return await Consumer.deploy(MOCK_SUBSCRIPTION_ID,keyHash,vrfCoordinatorV2Mock.address);

    }

    before(async () => {
        const [owner] = await ethers.getSigners();
        owner_wallet = owner.address;
        const LotterPoly = await ethers.getContractFactory("LotterPoly");
    })
    it("Should successfully request a random number", async() => {
        vrfv2Consumer = await deployContract();

        let tx = await vrfv2Consumer.requestRandomness();
        await tx.wait();
        await new Promise(resolve => setTimeout(resolve, 30000));
        const requestId = await vrfv2Consumer.s_requestId();
        console.log(requestId);
        const randomWord = await vrfv2Consumer.s_randomWords(0);
        console.log(randomWord);


       
    })

Best Answer

Random words and request id should not be 0. If you check VRF mock contract in VRFCoordinatorV2Mock.sol, you will find that the logic to generate random words is defined in the function fulfillRandomWordsWithOverride, and the code shown below:

if (_words.length == 0) {
  _words = new uint256[](req.numWords);
  for (uint256 i = 0; i < req.numWords; i++) {
    _words[i] = uint256(keccak256(abi.encode(_requestId, i)));
  }
} else if (_words.length != req.numWords) {
  revert InvalidRandomWords();
}

The snippet of codes shows that random words are generated by encoding the _requestId and i in the for loop, so the random number should not be 0.

In your test, actually, you do not verify any result at all. If you want to verify the random words is received and greater than 0, maybe you can get the random words first and use assertion to verify it.

assert(RandomNumber.gt(ethers.constants.Zero))

Please check the unit test in the repository Chainlink hardhat starter kit.

Related Solutions

[Ethereum] What are the costs of using Chainlink to generate a random number using VRF

I've had a similar question. As far as I can tell, at the time of writing, the cost of a request to the Chainlink VRF (Verifiable Random Function) v2 on the Ethereum Mainnet is 0.25 LINK, which is around $4.

Quite expensive to pay $4 for one random number. Thoughts anyone?

Edit:

It's true that one request csts 0.25 LINK, however, it seems up to 500 random values can be requested at once, see MAX_NUM_WORDS parameter in the contract.

References

Chainlink Docs: https://docs.chain.link/docs/vrf-contracts/#config

Etherscan, see getFeeConfig: https://etherscan.io/token/0x271682DEB8C4E0901D1a1550aD2e64D568E69909#readContract

solidity – How to Request Multiple Random Numbers at Once Using Chainlink VRF

If you want to get multiple random numbers from a single VRF response, you should create an array where the randomValue is your original returned VRF number and n is the desired number of random numbers.

function expand(uint256 randomValue, uint256 n) public pure returns (uint256[] memory expandedValues) {
    expandedValues = new uint256[](n);
    for (uint256 i = 0; i < n; i++) {
        expandedValues[i] = uint256(keccak256(abi.encode(randomValue, i)));
    }
    return expandedValues;
}

This code snippet takes the initial random value, puts it inside a for loop, combines it with an incrementing number by ABI encoding the random number and incremented number, hashes that encoding using Keccak256, and finally type casts the hash to a type uint to be used as that now expanded random number. The loop then continues for as many random numbers needed, which were set with the n parameter of the function.

Reference: VRF Best Practices

Best Answer

Related Solutions

[Ethereum] What are the costs of using Chainlink to generate a random number using VRF

solidity – How to Request Multiple Random Numbers at Once Using Chainlink VRF

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