Today's quest objective is to master Non Fungible Tokens, NFTs.
To represent a simple token, we used a smart contract with with each blockchain address amount of tokens. We talk about fungible token because each token has the same value. A Non-Fungible Token is a token with a unique identifier, usually part of a collection. NFTs are used to represent playing cards, works of art, financial securities and even physical objects.
Internally fungible token that we have seen in the prior quest are represented with a mapping from addresses to an amount:
Address ---> Amount
0x2FE34 20
Non-Fungible Tokens are represented with a mapping of unique identifiers to an owner
Identifier ---> Address
123455 0x2FE34
In addition, each token is linked to an Uniform Resource Identifier (URI) where additional information about the NFT can be found, such as metadata, an image...
In this exercise we will create a basic NFT smart contract, and use it to instantiate an NFT collection with some custom functions
### Instructions
#### Basic NFT
- Create a smart contract `basicNFT`
- Create a `constructor` function that takes a string that will be used as a name for the NFT.
- Create a public function `ownerOf()` that associate a token to the address of its owner
- Create a public function `TokenURI()` that associate a token to a resource
- Create a function `transfer(uint, address)` that takes as parameters an identifier (a positive integer) and an address to allow the owner of a NFT to transfer it.
_Optionally you can implement or reuse the full ERC721 standard_
#### Napping cats
- Create a smart contract `Napping Cats` that inherits from basicNFT
- Its constructor function takes a new parameter, initial price, that fixes the initial sale price of the NFT collection
- Instantiate the first three cats inside the smart contract with for each a json file (0.json, 1.json, 2.json) that contain a field name and a field image that points to a local or online image. NFTs initially don't have an owner
#### Trading functions
- Add to `Napping Cats` a function `listToken(uint256 id, uint256 price)` that lists the token for sale at the proposed price
- Add to `Napping Cats` a function `buyToken(uint256 id)`, payable, that allows an user to buy a token
- For the initial sale price defined by the constructor for new tokens
- For the listed price by the function listToken
_Optionally, the price for new tokens is an auction. It starts at 100x of the initial sale price or the last paid price for a new token and decreases over time to reach 0 after 24 hours._
