All contracts and tests are in the src folder. There are multiple implementations and you can select between:

• nf-token.sol: This is the base ERC-721 token implementation (with support for ERC-165).
• nf-token-metadata.sol: This implements optional ERC-721 metadata features for the token contract. It implements a token name, a symbol and a distinct URI pointing to a publicly exposed ERC-721 JSON metadata file.
• nf-token-enumerable.sol: This implements optional ERC-721 support for enumeration. It is useful if you want to know the total supply of tokens, to query a token by index, etc.

Other files in the tokens and utils directories named erc*.sol are interfaces and define the respective standards.

buggy_nft is a buggy implementation of ERC721

1. burn 调用之后，totalSupply 不会减一
2. 没有 Approve 权限也可以通过 safeTransferFrom 转移别人的 NFT
3. 调用 transferFrom 和 safetransferFrom 之后，owner不会改变
4. mint 和 burn 调用之后，用户的 balance 不会改变