/*

  Copyright 2018 ZeroEx Intl.

  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.

*/

pragma solidity 0.4.24;

import "./IERC20Token.sol";


contract ERC20Token is IERC20Token {

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

    uint256 public totalSupply;

    function transfer(address _to, uint256 _value)
        public
        returns (bool)
    {
        require(
            balances[msg.sender] >= _value,
            "ERC20_INSUFFICIENT_BALANCE"
        );
        require(
            balances[_to] + _value >= balances[_to],
            "OVERFLOW"
        );
        balances[msg.sender] -= _value;
        balances[_to] += _value;
        emit Transfer(msg.sender, _to, _value);
        return true;
    }

    function transferFrom(address _from, address _to, uint256 _value)
        public
        returns (bool)
    {
        require(
            balances[_from] >= _value,
            "ERC20_INSUFFICIENT_BALANCE"
        );
        require(
            allowed[_from][msg.sender] >= _value,
            "ERC20_INSUFFICIENT_ALLOWANCE"
        );
        require(
            balances[_to] + _value >= balances[_to],
            "OVERFLOW"
        );
        balances[_to] += _value;
        balances[_from] -= _value;
        allowed[_from][msg.sender] -= _value;
        emit Transfer(_from, _to, _value);
        return true;
    }

    function approve(address _spender, uint256 _value)
        public
        returns (bool)
    {
        allowed[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }

    function balanceOf(address _owner)
        public
        view
        returns (uint256)
    {
        return balances[_owner];
    }

    function allowance(address _owner, address _spender)
        public
        view
        returns (uint256)
    {
        return allowed[_owner][_spender];
    }
}