ERC20Aula.sol

pragma solidity ^0.4.18;

contract ERC20Interface {
    function totalSupply() public constant returns (uint);
    function balanceOf(address tokenOwner) public constant returns (uint balance);
    function allowance(address tokenOwner, address spender) public constant returns (uint remaining);
    function transfer(address to, uint tokens) public returns (bool success);
    function approve(address spender, uint tokens) public returns (bool success);
    function transferFrom(address from, address to, uint tokens) public returns (bool success);

    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
}

contract GoBlockchainERC20 is ERC20Interface {
    address public dono;
    
    string public symbol;
    string public name;
    uint8 public decimals;
    uint256 public _totalSupply;
    
    //Henrique = 200 tokens;
    mapping(address => uint256) balances;
    
    //Henrique = (Joao = 100);
    mapping(address => mapping(address => uint256)) allowed;
    
    function GoBlockchainERC20() public {
        symbol = "$GB";
        name = "Go";
        dono = msg.sender;
        decimals = 0;
        _totalSupply = 0;
        balances[dono] = _totalSupply;
    }
    
    function () public payable {
        uint tokens;
        tokens = msg.value * 2;
        balances[msg.sender] += tokens;
        _totalSupply += tokens;
        dono.transfer(msg.value);
        emit Transfer(this, msg.sender, tokens);
    }
     
    function totalSupply() public view returns(uint256) {
        return _totalSupply;
    }
    
    function balanceOf(address tokensDono) public view returns(uint256) {
        return balances[tokensDono];
    } 
    
    // exercicio/
    // verificar se o msg.sender tem o saldo suficiente para liberar os tokens para o amigo
    function approve(address _amigo, uint _qtdTokens) public returns (bool) {
        // if (balanceOf[msg.sender] > _qtdTokens) 
        allowed[msg.sender][_amigo] = _qtdTokens;
        return true;
     }
     
     // aplicar regras necessárias
     function transferFrom(address _from, address _to, uint _qtdTokens) public returns (bool) {
        balances[_from] -= _qtdTokens;
        allowed[_from][msg.sender] -= _qtdTokens;
        balances[_to] += _qtdTokens;
        return true;
     }
     
     function transfer(address to, uint tokens) public returns (bool success) {
         balances[msg.sender] -= tokens;
         balances[to] += tokens;
         return true;
     }

     function allowance (address _dono, address _amigo) public view returns(uint256 saldo) {
         return allowed[_dono][_amigo];
     }
    
}