Chainlink VRF HousePicker

A Foundry project for HousePicker.sol, a Chainlink VRF v2.5 consumer that rolls a 4-sided dice and maps the result to a Hogwarts house.

Based on the Cyfrin Updraft Chainlink Fundamentals lesson "VRF in a smart contract", with one bug fixed (see Fix: house id collision).

• src/: Solidity sources
• lib/chainlink-brownie-contracts: Chainlink contracts, pinned at tag 1.3.0
• lib/forge-std: Foundry standard library

The Remix-style import prefix @chainlink/contracts@1.3.0/ is remapped to lib/chainlink-brownie-contracts/contracts/ in foundry.toml, so the imports work unchanged outside Remix.

forge build
forge test
forge fmt

No setup is needed for the test suite. It runs fully offline against a mocked coordinator. For anything that touches Sepolia, foundry.toml defines an rpc alias pointing at a keyless public endpoint, so --rpc-url sepolia works out of the box:

cast chain-id --rpc-url sepolia   # 11155111

Copy .env.example to .env (gitignored) to override it with your own provider, and to set PRIVATE_KEY for deployments. Never put a provider URL containing an API key in .env.example.

test/HousePicker.t.sol deploys Chainlink's VRFCoordinatorV2_5Mock and repoints the consumer at it via setCoordinator, so the tests run the full request and fulfill round trip. fulfillRandomWordsWithOverride supplies exact random words, which makes every house outcome deterministic.

Coverage includes the house mapping for all four outcomes, the "not rolled" and "roll in progress" guards, coordinator-only access control on the callback, event payloads, and a fuzz test asserting that house() resolves for any random word.

Four of the tests are regression tests for the bug below and fail against the original contract. The fuzz test finds the bad case within a handful of runs.

The constructor takes a funded VRF v2.5 subscription ID; the coordinator address and key hash are hardcoded for Sepolia. script/DeployHousePicker.s.sol defaults to this project's subscription and can be overridden with VRF_SUBSCRIPTION_ID.

cp .env.example .env   # then fill in SEPOLIA_RPC_URL and PRIVATE_KEY
forge script script/DeployHousePicker.s.sol --rpc-url "$SEPOLIA_RPC_URL" \
  --private-key "$PRIVATE_KEY" --broadcast

Dropping --broadcast simulates without sending a transaction. After deploying, add the contract as a consumer on the subscription at https://vrf.chain.link/sepolia, or rollDice() will revert.

Three rolls against a deployed instance on Sepolia. rollDice() is one-shot per address, so each roll came from a fresh wallet.

The first roll landed on Gryffindor, id 1, which is the outcome the original contract could not report. Before the fix it would have stored 0 and house() would have reverted with "Dice not rolled" until that wallet rolled again.

The second and third rolls both drew Hufflepuff from different addresses, a 1-in-4 repeat. This confirms that identical results stored against separate keys in s_results read back independently, with no interference between players.

Registering the contract as a consumer on the subscription is required before rollDice() will work.

A subscription funded with 10 LINK can't fulfill a request, even though a fulfillment costs about 0.04 LINK. Chainlink reserves funds at the gas lane's maximum gas price, and KEY_HASH here selects the 500 gwei lane:

(115,000 verification + 40,000 callback + 38,900 gasAfterPayment)
  x 500 gwei x 1.20 link premium / 5.347e15 wei-per-link  =~ 22 LINK

Below that, the request sits in Pending on the subscription page and is dropped after 24 hours. If you top up the balance within that window, the request clears on its own with no need to roll again. Paying in native ETH (nativePayment: true) avoids the LINK reserve entirely but needs a contract change.

The lesson's contract stores house ids as randomWords[0] % 4, giving the range 0–3, while s_results uses 0 as its "never rolled" sentinel, the default value of an unwritten mapping entry. Gryffindor is therefore indistinguishable from not having rolled at all, which breaks two guards:

• house() reverts with "Dice not rolled" for a player who rolled Gryffindor. The result is stored but can never be read, and the revert lasts until the player rolls again.
• rollDice() gates on s_results[msg.sender] == 0, so that same player passes the "Already rolled" check and can roll again, overwriting their result.

The % 4 also dropped the + 1 that the comment on that line still refers to, which is what collapsed the ids onto the sentinel in the first place.

The fix shifts house ids to 1-based so 0 is only ever the sentinel:

The two requires in house() and the == 0 guard in rollDice() are unchanged. They work as written once 0 is an unambiguous sentinel.

This is example, unaudited code. Do not use in production.