openzeppelin-contracts/contracts/Token.sol

// Source: https://github.com/nexusdev/erc20
// Flat file implementation of `dappsys/token/base.sol::DSTokenBase`

// Everything throws instead of returning false on failure.
import 'ERC20.sol';

contract Token is ERC20
{
    mapping( address => uint ) _balances;
    mapping( address => mapping( address => uint ) ) _approvals;
    uint _supply;

    function Token( uint initial_balance ) {
        _balances[msg.sender] = initial_balance;
        _supply = initial_balance;
    }
    function totalSupply() constant returns (uint supply) {
        return _supply;
    }
    function balanceOf( address who ) constant returns (uint value) {
        return _balances[who];
    }
    function transfer( address to, uint value) returns (bool ok) {
        if( _balances[msg.sender] < value ) {
            throw;
        }
        if( !safeToAdd(_balances[to], value) ) {
            throw;
        }
        _balances[msg.sender] -= value;
        _balances[to] += value;
        Transfer( msg.sender, to, value );
        return true;
    }
    function transferFrom( address from, address to, uint value) returns (bool ok) {
        // if you don't have enough balance, throw
        if( _balances[from] < value ) {
            throw;
        }
        // if you don't have approval, throw
        if( _approvals[from][msg.sender] < value ) {
            throw;
        }
        if( !safeToAdd(_balances[to], value) ) {
            throw;
        }
        // transfer and return true
        _approvals[from][msg.sender] -= value;
        _balances[from] -= value;
        _balances[to] += value;
        Transfer( from, to, value );
        return true;
    }
    function approve(address spender, uint value) returns (bool ok) {
        _approvals[msg.sender][spender] = value;
        Approval( msg.sender, spender, value );
        return true;
    }
    function allowance(address owner, address spender) constant returns (uint _allowance) {
        return _approvals[owner][spender];
    }
    function safeToAdd(uint a, uint b) internal returns (bool) {
        return (a + b >= a);
    }
}