pragma solidity ^0.4.23;

import 'openzeppelin-solidity/contracts/token/ERC20/ERC20.sol';
import "openzeppelin-solidity/contracts/math/SafeMath.sol";

contract StandardERC20Base is ERC20 {
    using SafeMath for uint256;
    
    uint256                                            _supply;
    mapping (address => uint256)                       _balances;
    mapping (address => mapping (address => uint256))  _approvals;

    function totalSupply() public view returns (uint) {
        return _supply;
    }
    function balanceOf(address src) public view returns (uint) {
        return _balances[src];
    }
    function allowance(address src, address guy) public view returns (uint) {
        return _approvals[src][guy];
    }

    function transfer(address dst, uint wad) public returns (bool) {
        return transferFrom(msg.sender, dst, wad);
    }

    function transferFrom(address src, address dst, uint wad)
        public
        returns (bool)
    {
        if (src != msg.sender) {
            _approvals[src][msg.sender] = _approvals[src][msg.sender].sub(wad);
        }

        _balances[src] = _balances[src].sub(wad);
        _balances[dst] = _balances[dst].add(wad);

        emit Transfer(src, dst, wad);

        return true;
    }

    function approve(address guy, uint wad) public returns (bool) {
        _approvals[msg.sender][guy] = wad;

        emit Approval(msg.sender, guy, wad);

        return true;
    }
}