A staking rewards contract with fair accounting

#Brief

Staking and liquidity-mining contracts must share rewards fairly among people who join and leave at different times, without looping over every staker. The accumulator pattern solves this, and it is a favourite interview topic. Use a local Anvil chain and, when you deploy publicly, a testnet such as Sepolia with free test ether from a faucet. Never use real funds or a wallet that holds any, and never paste a private key into code or a website.

#What to build

1. Implement stake, withdraw and claim for an ERC-20 staking token and an ERC-20 reward token

2. Track reward per token stored and each user reward-per-token paid, updated on every action

3. Let the owner fund a reward period with a fixed duration and rate, and handle top-ups mid-period

4. Write a Python model that computes exact rewards for random sequences of actions, and differential fuzz tests against it

5. Write invariant tests: rewards paid never exceed rewards funded, and total staked equals the sum of balances

#Deliverables

• Repository with src/, test/, model/ and DESIGN.md

#How it is graded

Reviewers run long fuzz campaigns and check that no user can claim more than their share and that dust does not accumulate unfairly.

#Stretch

Add a boost for long lock-ups and show how it changes the accounting.