StandardToken first version

contracts/SafeMath.sol

+pragma solidity ^0.4.0;
+
+/**
+ * Math operations with safety checks
+ */
+contract SafeMath {
+  function safeMul(uint a, uint b) internal returns (uint) {
+    uint c = a * b;
+    assert(a == 0 || c / a == b);
+    return c;
+  }
+
+  function safeSub(uint a, uint b) internal returns (uint) {
+    assert(b <= a);
+    return a - b;
+  }
+
+  function safeAdd(uint a, uint b) internal returns (uint) {
+    uint c = a + b;
+    assert(c>=a && c>=b);
+    return c;
+  }
+
+  function assert(bool assertion) internal {
+    if (!assertion) throw;
+  }
+}
+

contracts/BaseToken.sol → contracts/StandardToken.sol

 pragma solidity ^0.4.0;
 
-// Everything throws instead of returning false on failure.
 import './ERC20.sol';
+import './SafeMath.sol';
 
 /**
- * ERC 20 token
+ * ERC20 token
  *
  * https://github.com/ethereum/EIPs/issues/20
  * Based on code by FirstBlood:
  * https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
  */
-contract BaseToken is ERC20 {
+contract StandardToken is ERC20, SafeMath {
 
   mapping(address => uint256) balances;
   mapping (address => mapping (address => uint256)) allowed;
   uint256 public totalSupply;
 
-
   function transfer(address _to, uint256 _value) returns (bool success) {
-    if (balances[msg.sender] >= _value &&
-        balances[_to] + _value > balances[_to]) {
-      balances[msg.sender] -= _value;
-      balances[_to] += _value;
+    if (balances[msg.sender] < _value) {
+      throw;
+    } 
+    balances[msg.sender] = safeSub(balances[msg.sender], _value);
+    balances[_to] = safeAdd(balances[_to], _value);
     Transfer(msg.sender, _to, _value);
     return true;
-    } else { return false; }
   }
 
   function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {
-    if (balances[_from] >= _value &&
-        allowed[_from][msg.sender] >= _value &&
-        balances[_to] + _value > balances[_to]) {
-      balances[_to] += _value;
-      balances[_from] -= _value;
-      allowed[_from][msg.sender] -= _value;
+    var _allowance = allowed[_from][msg.sender];
+    if (balances[_from] < _value ||
+        _allowance < _value)) {
+      throw;
+    }
+
+    balances[_to] = safeAdd(balances[_to], _value);
+    balances[_from] = safeSub(balances[_from], _value);
+    allowed[_from][msg.sender] = safeSub(_allowance, _value);
     Transfer(_from, _to, _value);
     return true;
-    } else { return false; }
   }
 
   function balanceOf(address _owner) constant returns (uint256 balance) {