Session 6.1 - Ethereum Tokens

pragma solidity ^0.8.0;

interface IERC20 {
    function totalSupply() external view returns (uint256);
    function balanceOf(address account) external view returns (uint256);
    function transfer(address to, uint256 amount) external returns (bool);
    function allowance(address owner, address spender) external view returns (uint256);
    function approve(address spender, uint256 amount) external returns (bool);
    function transferFrom(address from, address to, uint256 amount) external returns (bool);
    
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

contract MyToken is IERC20 {
    mapping(address => uint256) private _balances;
    mapping(address => mapping(address => uint256)) private _allowances;
    
    uint256 private _totalSupply;
    string public name;
    string public symbol;
    uint8 public decimals;
    
    constructor(string memory _name, string memory _symbol, uint256 _totalSupply) {
        name = _name;
        symbol = _symbol;
        decimals = 18;
        _totalSupply = _totalSupply * 10**decimals;
        _balances[msg.sender] = _totalSupply;
        emit Transfer(address(0), msg.sender, _totalSupply);
    }
    
    function totalSupply() public view override returns (uint256) {
        return _totalSupply;
    }
    
    function balanceOf(address account) public view override returns (uint256) {
        return _balances[account];
    }
    
    function transfer(address to, uint256 amount) public override returns (bool) {
        require(to != address(0), "Transfer to zero address");
        require(_balances[msg.sender] >= amount, "Insufficient balance");
        
        _balances[msg.sender] -= amount;
        _balances[to] += amount;
        emit Transfer(msg.sender, to, amount);
        return true;
    }
    
    function approve(address spender, uint256 amount) public override returns (bool) {
        _allowances[msg.sender][spender] = amount;
        emit Approval(msg.sender, spender, amount);
        return true;
    }
    
    function allowance(address owner, address spender) public view override returns (uint256) {
        return _allowances[owner][spender];
    }
    
    function transferFrom(address from, address to, uint256 amount) public override returns (bool) {
        require(to != address(0), "Transfer to zero address");
        require(_balances[from] >= amount, "Insufficient balance");
        require(_allowances[from][msg.sender] >= amount, "Insufficient allowance");
        
        _balances[from] -= amount;
        _balances[to] += amount;
        _allowances[from][msg.sender] -= amount;
        
        emit Transfer(from, to, amount);
        return true;
    }
}

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract MyNFT is ERC721, Ownable {
    uint256 private _tokenIdCounter;
    mapping(uint256 => string) private _tokenURIs;
    
    constructor() ERC721("MyNFT", "MNFT") {}
    
    function mint(address to, string memory tokenURI) public onlyOwner {
        uint256 tokenId = _tokenIdCounter;
        _tokenIdCounter++;
        
        _mint(to, tokenId);
        _setTokenURI(tokenId, tokenURI);
    }
    
    function _setTokenURI(uint256 tokenId, string memory tokenURI) internal {
        require(_exists(tokenId), "Token does not exist");
        _tokenURIs[tokenId] = tokenURI;
    }
    
    function tokenURI(uint256 tokenId) public view override returns (string memory) {
        require(_exists(tokenId), "Token does not exist");
        return _tokenURIs[tokenId];
    }
    
    function burn(uint256 tokenId) public {
        require(_isApprovedOrOwner(msg.sender, tokenId), "Not approved or owner");
        _burn(tokenId);
        delete _tokenURIs[tokenId];
    }
}