HarbergerNFT

This is an experiment to create an NFT with the Harberger Tax and a cheap mass minting feature.

How to use

The HarbergerNFT contract is in the src folder. And all tests are in the test folder.

To run tests use forge test -vv (you need to install Foundry first). To turn off some of the test functions change vm.skip(false) to vm.skip(true) and vice versa.

How it works

• In constructor(uint256 _maxAmount, uint256 _defaultPrice, uint256 _taxRate) we save the deployer address, the default price and the tax rate. We also reserve _maxAmount of bits in storage starting from LIST_SLOT, which creates an "allowlist", where every bit corresponds to the position in the list.

• In mint(bytes calldata signature, uint256 userId, uint128 newPrice) users provide their signature with the corresponding userId, which is signed by the deployer off-chain. userId corresponds to the position in the allowlist. Then the contract checks if signature is valid and if the bit in the userId position equals 1. If it is, it mints a token and updates the allowlist to avoid the signature replay attack. And finally, the contract updates the token info: the amount of ETH deposited to pay for tax, the new price, the time when the price was changed, and the owner address.

• Using the buy(uint256 tokenId, uint128 newPrice) function any user can buy any minted token. The price depends on the deposit left of the current owner. If the deposit left is less than the tax owned, then the price is DEFAULT_PRICE. Otherwise, the price is the one set by the owner. Then the contract updates the token info and sends ETH to the old owner.

• To change the price of a token owners can use the setPrice(uint256 tokenId, uint128 newPrice) function. The contract will recalculate the deposit and update the token info.

• Any user can deposit ETH to a minted token using the deposit(uint256 tokenId) function. And only owners can withdraw ETH from their deposit using withdraw(uint256 tokenId, uint256 amount).

• To get the token info users can use the getTokenInfo(uint256 tokenId) function, which will return the address of the owner, the price of the token, the deposit left (minus tax) and the time left (in seconds) until the price is set to the default one.

Additionally, the contract uses transient storage for the reentrancy guard.

TODO

Make it a proper ERC721 token, maybe add frontend.