contract StandardToken is ERC20Token {

    using SafeMath for uint256;

    mapping (address => uint256) balances;

    mapping (address => mapping (address => uint256)) allowed;

    uint256 public totalSupply;

   /// @dev transfer token for a specified address
   /// @param _to The address to transfer to.
   /// @param _value The amount to be transferred.
   function transfer(address _to, uint256 _value) public returns (bool) {
        balances[msg.sender] = balances[msg.sender].safeSub(_value);
        balances[_to] = balances[_to].safeAdd(_value);

        emit Transfer(msg.sender, _to, _value);            

        return true;
    }

    /// @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) {
        uint256 _allowance = allowed[_from][msg.sender];

        // Check is not needed because sub(_allowance, _value) will already throw if this condition is not met
        // require (_value <= _allowance);        if (balances[_from] >= _value && allowed[_from][msg.sender] >= _value && balances[_to] + _value > balances[_to]) {

        balances[_to] = balances[_to].safeAdd(_value);
        balances[_from] = balances[_from].safeSub(_value);
        allowed[_from][msg.sender] = _allowance.safeSub(_value);

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

    /// @dev Gets the balance of the specified address.
    /// @param _owner The address to query the the balance of. 
    /// @return An uint256 representing the amount owned by the passed address.
    function balanceOf(address _owner) view public returns (uint256) {
        return balances[_owner];
    }

   /// @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
   /// @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) {
        // To change the approve amount you first have to reduce the addresses`
        //  allowance to zero by calling `approve(_spender, 0)` if it is not
        //  already 0 to mitigate the race condition described here:
        //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
        require((_value == 0) || (allowed[msg.sender][_spender] == 0));
        
        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) view public returns (uint256) {
        return allowed[_owner][_spender];
    }

    /// @notice approve should be called when allowed[_spender] == 0. To increment
    /// allowed value it is better to use this function to avoid 2 calls (and wait until 
    /// the first transaction is mined)
    function increaseApproval (address _spender, uint256 _addedValue) public returns (bool) {
        allowed[msg.sender][_spender] = allowed[msg.sender][_spender].safeAdd(_addedValue);

        emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);

        return true;
    }

    /// @notice approve should be called when allowed[_spender] == 0. To decrement
    /// allowed value it is better to use this function to avoid 2 calls (and wait until 
    /// the first transaction is mined)
    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 - _subtractedValue;
        }

        emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);

        return true;
    }

}
/// @title Standard ERC20 compliant token
/// @author Hoard Team
/// @notice Original taken from https://github.com/ethereum/EIPs/issues/20
/// @notice SafeMath used as specified by OpenZeppelin
/// @notice Comments and additional approval code from https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/token
function allowance(address _owner, address _spender) view public 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.