pragma solidity ^0.4.18;

import { Token } from "../Token/Token.sol";

contract ERC20Token is Token {

    function transfer(address _to, uint _value)
        public
        returns (bool)
    {
        require(balances[msg.sender] >= _value && balances[_to] + _value >= balances[_to]);
        balances[msg.sender] -= _value;
        balances[_to] += _value;
        Transfer(msg.sender, _to, _value);
        return true;
    }

    function transferFrom(address _from, address _to, uint _value)
        public
        returns (bool)
    {
        require(balances[_from] >= _value && allowed[_from][msg.sender] >= _value && balances[_to] + _value >= balances[_to]);
        balances[_to] += _value;
        balances[_from] -= _value;
        allowed[_from][msg.sender] -= _value;
        Transfer(_from, _to, _value);
        return true;
    }

    function approve(address _spender, uint _value)
        public
        returns (bool)
    {
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        return true;
    }

    function balanceOf(address _owner)
        public
        view
        returns (uint)
    {
        return balances[_owner];
    }

    function allowance(address _owner, address _spender)
        public
        view
        returns (uint)
    {
        return allowed[_owner][_spender];
    }

    mapping (address => uint) balances;
    mapping (address => mapping (address => uint)) allowed;
    uint public totalSupply;
}