contract StandardToken is Ownable, ERC20, BasicToken {
    mapping(address => mapping(address => uint256)) internal allowed;

    /**
     * @dev Transfer tokens from one address to another
     * @param _from address The address which you want to send tokens from
     * @param _to address The address which you want to transfer to
     * @param _value uint256 the amount of tokens to be transferred
     **/
    function transferFrom(
        address _from,
        address _to,
        uint256 _value
    ) public returns (bool) {
        require(
            _to != address(0),
            "transfer to zero address is not  allowed"
        );
        require(
            _value <= balances[_from],
            "from address does not have enough balance"
        );
        require(
            _value <= allowed[_from][msg.sender],
            "sender does not have enough allowance"
        );

        balances[_from] = balances[_from]
        .sub(_value);

        balances[_to] = balances[_to]
        .add(_value);

        allowed[_from][msg.sender] = allowed[
        _from
        ]
        [
        msg.sender
        ].sub(_value);

        emit Transfer(
            _from,
            _to,
            _value
        );
        return true;
    }

    function transfer(
        address to,
        uint256 value
    ) public returns(bool) {
        return _transfer(to, value);
    }

    /**
     * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
     *
     * Beware that changing an allowance with this method brings the risk that someone may use both the old
     * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
     * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     * @param _spender The address which will spend the funds.
     * @param _value The amount of tokens to be spent.
     **/
    function approve(address _spender, uint256 _value) public returns (bool) {
        allowed[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }

    /**
     * @dev Function to check the amount of tokens that an owner allowed to a spender.
     * @param _owner address The address which owns the funds.
     * @param _spender address The address which will spend the funds.
     * @return A uint256 specifying the amount of tokens still available for the spender.
     **/
    function allowance(address _owner, address _spender)
    public
    view
    returns (uint256)
    {
        return allowed[
        _owner
        ]
        [
        _spender
        ];
    }

    /**
     * @dev Increase the amount of tokens that an owner allowed to a spender.
     *
     * approve should be called when allowed[_spender] == 0. To increment
     * allowed value is better to use this function to avoid 2 calls (and wait until
     * the first transaction is mined)
     * @param _spender The address which will spend the funds.
     * @param _addedValue The amount of tokens to increase the allowance by.
     **/
    function increaseApproval(address _spender, uint256 _addedValue)
    public
    returns (bool)
    {
        allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(
            _addedValue
        );
        emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
        return true;
    }

    /**
     * @dev Decrease the amount of tokens that an owner allowed to a spender.
     *
     * approve should be called when allowed[_spender] == 0. To decrement
     * allowed value is better to use this function to avoid 2 calls (and wait until
     * the first transaction is mined)
     * @param _spender The address which will spend the funds.
     * @param _subtractedValue The amount of tokens to decrease the allowance by.
     **/
    function decreaseApproval(address _spender, uint256 _subtractedValue)
    public
    returns (bool)
    {
        uint256 oldValue = allowed[msg.sender][_spender];
        if (_subtractedValue > oldValue) {
            allowed[msg.sender][_spender] = 0;
        } else {
            allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
        }
        emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
        return true;
    }
}
function allowance(address _owner, address _spender)
public
view
returns (uint256)
{
    return allowed[
    _owner
    ]
    [
    _spender
    ];
}

/**
 * @dev Function to check the amount of tokens that an owner allowed to a spender.
 * @param _owner address The address which owns the funds.
 * @param _spender address The address which will spend the funds.
 * @return A uint256 specifying the amount of tokens still available for the spender.
 **/