pragma solidity ^0.4.18;

    interface IERC20 {
        
        function totalSupply() constant returns (uint256 totalSupply);
        function balanceOf(address _owner) constant returns (uint256 balance);
        function transfer(address _to, uint256 _value) returns (bool success);
        function transferFrom(address _from, address _to, uint256 _value) returns (bool success);
        function approve(address _spender, uint256 _value) returns (bool success);
        function allowance(address _owner, address _spender) constant returns (uint256 remaining);
        event Transfer(address indexed _from, address indexed _to, uint256 _value);
        event Approval(address indexed _owner, address indexed _spender, uint256 _value);
        
    }

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {
  function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    if (a == 0) {
      return 0;
    }
    uint256 c = a * b;
    assert(c / a == b);
    return c;
  }

  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }

  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    assert(b <= a);
    return a - b;
  }

  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}

contract BobCoin is IERC20{
    
    using SafeMath for uint256;
    
    uint256 public constant _totalSupply = 1000000000;
    
    string public constant symbol = "BOB";
    string public constant name = "BobCoin";
    uint8 public constant decimals = 3;
    
    mapping(address => uint256) balances;
    mapping(address => mapping(address => uint256)) allowed;
    
    function BobCoin(){
        balances[msg.sender] = _totalSupply;
    }
    
    function totalSupply() constant returns (uint256 totalSupply){
        return _totalSupply;
    }
    
    function balanceOf(address _owner) constant returns (uint256 balance){
        return balances[_owner];
    }
    
    function transfer(address _to, uint256 _value) returns (bool success){
        require(
            balances[msg.sender] >= _value
            && _value > 0
        );
        balances[msg.sender] = balances[msg.sender].sub(_value);
        balances[_to] = balances[_to].add(_value);
        Transfer(msg.sender, _to, _value);
        return true;
    }
    
    function transferFrom(address _from, address _to, uint256 _value) returns (bool success){
        require(
            allowed[_from][msg.sender] >= _value
            && balances[_from] >= _value
            && _value > 0
        );
        balances[_from] = balances[_from].sub(_value);
        balances[_to] = balances[_to].add(_value);
        allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
        Transfer(_from, _to, _value);
        return true;
    }
    
    function approve(address _spender, uint256 _value) returns (bool success){
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        return true;
    }
    
    function allowance(address _owner, address _spender) constant returns (uint256 remaining){
        return allowed[_owner][_spender];
    }
    
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}