01b_erc20better.bbo

contract PreToken
(uint256 totalSupply
,address => uint256 balances
,address => address => uint256 allowances
)
{
    default
    {
        balances[this] = totalSupply;
        return then become Token(totalSupply, balances, allowances);
    }
}

event Transfer(address indexed _from, address indexed _to, uint256 _amount);
event Buy(address indexed _buyer, uint256 _amount, uint256 _value);
event Sell(address indexed _buyer, uint256 _amount, uint256 _value);
event Approval(address indexed _owner, address indexed _spender, uint256 _value);

contract Token
(uint256 totalSupply
,address => uint256 balances
,address => address => uint256 allowances
)
{
    case(uint256 totalSupply())
    {
        return totalSupply then become Token(totalSupply, balances, allowances);
    }
    case(uint256 balanceOf(address a))
    {
        return balances[a] then become Token(totalSupply, balances, allowances);
    }

    case(bool transfer(address _to, uint256 _amount))
    {
        if (balances[sender(msg)] < _amount) abort;
        if (sender(msg) == _to)
        {
            log Transfer(sender(msg), sender(msg), _amount);
            return true then become Token(totalSupply, balances, allowances);
        }
        balances[sender(msg)] = balances[sender(msg)] - _amount;
        if (balances[_to] + _amount < balances[_to]) abort;
        balances[_to] = balances[_to] + _amount;
        log Transfer(sender(msg), _to, _amount);
        return true then become Token(totalSupply, balances, allowances);
    }

    case(bool approve(address _spender, uint256 _amount))
    {
        if (balances[sender(msg)] < _amount) abort;
        if (sender(msg) == _spender) abort;
        allowances[sender(msg)][_spender] = _amount;
        log Approval(sender(msg), _spender, _amount);
        return true then become Token(totalSupply, balances, allowances);
    }
    case(uint256 allowance(address _owner, address _spender))
    {
        return allowances[_owner][_spender] then become Token(totalSupply, balances, allowances);
    }
    case(bool transferFrom(address _from, address _to, uint256 _amount))
    {
        if (balances[_from] < _amount) abort;
        if (allowances[_from][sender(msg)] < _amount) abort;
        if (_from == _to)
        {
            log Transfer(_from, _to, _amount);
            return true then become Token(totalSupply, balances, allowances);
        }

        balances[_from] = balances[_from] - _amount;
        allowances[_from][sender(msg)] = allowances[_from][sender(msg)] - _amount;
        balances[_to] = balances[_to] + _amount;
        log Transfer(_from, _to, _amount);
        return true then become Token(totalSupply, balances, allowances);
    }

    case(bool buy(uint256 _amount))
    {
        if (balances[this] < _amount) abort;
        if (balances[sender(msg)] + _amount < balances[sender(msg)]) abort;

        uint256 old_eth_balance = balance(this) - value(msg);
        if (balance(this) * _amount > (balances[this] - _amount) * value(msg)) abort;

        balances[this] = balances[this] - _amount;
        balances[sender(msg)] = balances[sender(msg)] + _amount;

        log Buy(sender(msg), _amount, value(msg));
        return true then become Token(totalSupply, balances, allowances);
    }
    case (bool sell(uint256 _amount, uint256 _value))
    {
        if (balance(this) < _value) abort;
        if (balances[sender(msg)] < _amount) abort;
        if (balances[this] + _amount < balances[this]) abort;
        if (not (iszero(value(msg)))) abort;

        uint256 old_eth_balance = balance(this);
        uint256 new_eth_balance = balance(this) - _value;
        uint256 new_amount = balances[this] + _amount;
        if (new_eth_balance * _amount < new_amount * _value) abort;

        balances[this] = new_amount;
        balances[sender(msg)] = balances[sender(msg)] - _amount;
        log Sell(sender(msg), _amount, _value);
        void = sender(msg).default() with _value reentrance { abort; };
        return true then become Token(totalSupply, balances, allowances);
    }
}