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);
  }
}