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// Author: Lorenz Breidenbach
pragma solidity ^0.4.18;
contract Token {
// owner -> amount
mapping(address => uint256) balances;
// owner -> spender -> max amount
mapping(address => mapping(address => uint256)) allowances;
uint256 supply;
event Transfer(address indexed _from, address indexed _to, uint256 _value);
event Approval(address indexed _owner, address indexed _spender, uint256 _value);
// Deposits ether with the contract and converts it to tokens.
// One wei is worth one token.
function deposit() payable {
// overflow check. not necessary for 1-to-1 token-to-wei peg, but
// might be good to have in case somebody decides to modify this code
// to have the owner issue tokens, etc...
require(supply <= supply + msg.value);
supply += msg.value;
balances[msg.sender] += msg.value;
Transfer(0x0, msg.sender, msg.value);
// Converts tokens to ether and withdraws the ether.
// One token is worth one wei.
function withdraw(uint256 _value) returns (bool success) {
require(_value <= balances[msg.sender]);
balances[msg.sender] -= _value;
supply -= _value;
Transfer(msg.sender, 0x0, _value);
return true;
// Spec: Get the total token supply
function totalSupply() constant returns (uint256 totalSupply) {
return supply;
// Spec: Get the account balance of another account with address _owner
// The spec is a bit surprising to me. Why should this only work for the
// balance of "another account", i.e. only if _owner != msg.sender?
// For now, I am assuming that this is just due to unclear wording and that
// anybody's balance may be queried this way.
function balanceOf(address _owner) constant returns (uint256 balance) {
return balances[_owner];
function internalTransfer(address _from, address _to, uint256 _value) internal returns (bool success) {
require(_value <= balances[_from]);
balances[_from] -= _value;
balances[_to] += _value;
Transfer(_from, _to, _value);
return true;
// Spec: Send _value amount of tokens to address _to
function transfer(address _to, uint256 _value) returns (bool success) {
address _from = msg.sender;
return internalTransfer(_from, _to, _value);
// Spec: Send _value amount of tokens from address _from to address _to
function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {
address _spender = msg.sender;
if(_value <= allowances[_from][_spender] && internalTransfer(_from, _to, _value)) {
allowances[_from][_spender] -= _value;
return true;
} else {
// Spec: Allow _spender to withdraw from your account, multiple times, up
// to the _value amount. If this function is called again it overwrites the
// current allowance with _value.
function approve(address _spender, uint256 _value) returns (bool success) {
address _owner = msg.sender;
allowances[_owner][_spender] = _value;
Approval(_owner, _spender, _value);
return true;
// Spec: Returns the amount which _spender is still allowed to withdraw
// from _owner.
// What if the allowance is higher than the balance of the owner?
// Callers should be careful to use min(allowance, balanceOf) to make sure
// that the allowance is actually present in the account!
function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
return allowances[_owner][_spender];