BackwardsCompatibleApprove.sol

pragma solidity ^0.4.11;


contract BackwardsCompatibleApprove {

    struct Allowance {
        uint amount;
        bool used;
    }
    
    mapping(address => mapping(address => Allowance)) allowed;
    mapping(address => uint256) balances;
    
    function approve(address _spender, uint _value) public returns (bool success) {
        if(allowed[msg.sender][_spender].used){
            allowed[msg.sender][_spender].amount = 0;
            allowed[msg.sender][_spender].used=false;
            Approval(msg.sender,_spender,0);
            return false;
        } else {
            allowed[msg.sender][_spender].amount=_value;
            Approval(msg.sender,_spender,_value);
            return true;
        }
    }
    
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
        require(_to != address(0));
        require(_value <= balances[_from]);
        require(_value <= allowed[_from][msg.sender].amount);
        
        balances[_from] -= _value;
        balances[_to] += _value;
        allowed[_from][msg.sender].amount -= _value;
        
        if(_value > 0){
            allowed[_from][msg.sender].used = true;
        }
        
        Transfer(_from, _to, _value);
        return true;
    }

    event Transfer(address indexed _from, address indexed _to, uint _value);
    event Approval(address indexed _owner, address indexed _spender, uint _value);
 
}