pragma solidity ^0.4.16;

contract ERC20 {
  uint public totalSupply;
  function balanceOf(address who) constant public returns (uint);
  function allowance(address owner, address spender) constant public returns (uint);
  function transfer(address to, uint value) public returns (bool ok);
  function transferFrom(address from, address to, uint value) public returns (bool ok);
  function approve(address spender, uint value) public returns (bool ok);
  event Transfer(address indexed from, address indexed to, uint value);
  event Approval(address indexed owner, address indexed spender, uint value);
}

contract Ownable {
  address public owner;

  function Ownable() public {
    owner = msg.sender;
  }

  modifier onlyOwner() {
    if (msg.sender == owner)
      _;
  }

  function transferOwnership(address newOwner) onlyOwner public {
    if (newOwner != address(0)) owner = newOwner;
  }

}

contract TokenSpender {
    function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) public;
}

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
contract SafeMath {

  /**
  * @dev Multiplies two numbers, throws on overflow.
  */
  function safeMul(uint256 a, uint256 b) internal pure returns (uint256) {
    if (a == 0) {
      return 0;
    }
    uint256 c = a * b;
    assert(c / a == b);
    return c;
  }

  /**
  * @dev Integer division of two numbers, truncating the quotient.
  */
  function safeDiv(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;
  }

  /**
  * @dev Substracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
  */
  function safeSub(uint256 a, uint256 b) internal pure returns (uint256) {
    assert(b <= a);
    return a - b;
  }

  /**
  * @dev Adds two numbers, throws on overflow.
  */
  function safeAdd(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}

contract VersusToken is ERC20, SafeMath, Ownable {

  string public name;
  string public symbol;
  uint8 public decimals = 12;
  string public version = 'v0.1'; 
  uint public initialSupply;
  uint public totalSupply;

  mapping(address => uint) balances;
  mapping (address => mapping (address => uint)) allowed;

  function VersusToken() public {
    initialSupply = 700000000 * 10 ** uint256(decimals);
    totalSupply = initialSupply;
    balances[msg.sender] = totalSupply;
    name = 'VersusToken';   
    symbol = 'VSUS';
  }

  function burn(uint256 _value) public returns (bool){
    balances[msg.sender] = safeSub(balances[msg.sender], _value) ;
    totalSupply = safeSub(totalSupply, _value);
    Transfer(msg.sender, 0x0, _value);
    return true;
  }
  
    function _transfer(address _from, address _to, uint _value) internal {
        // Prevent transfer to 0x0 address. Use burn() instead
        require(_to != 0x0);
        // Check if the sender has enough
        require(balances[_from] >= _value);
        // Check for overflows
        require(balances[_to] + _value > balances[_to]);
        // Save this for an assertion in the future
        uint previousBalances = balances[_from] + balances[_to];
        // Subtract from the sender
        balances[_from] -= _value;
        // Add the same to the recipient
        balances[_to] += _value;
        Transfer(_from, _to, _value);
        // Asserts are used to use static analysis to find bugs in your code. They should never fail
        assert(balances[_from] + balances[_to] == previousBalances);
    }

  function transfer(address _to, uint _value) public returns (bool) {
    balances[msg.sender] = safeSub(balances[msg.sender], _value);
    balances[_to] = safeAdd(balances[_to], _value);
    Transfer(msg.sender, _to, _value);
    return true;
  }

  function transferFrom(address _from, address _to, uint _value) public returns (bool) {
    var _allowance = allowed[_from][msg.sender];
    
    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;
  }

  function balanceOf(address _owner) constant public returns (uint balance) {
    return balances[_owner];
  }

  function approve(address _spender, uint _value) public returns (bool) {
    allowed[msg.sender][_spender] = _value;
    Approval(msg.sender, _spender, _value);
    return true;
  }

  function approveAndCall(address _spender, uint256 _value, bytes _extraData) public{    
      TokenSpender spender = TokenSpender(_spender);
      if (approve(_spender, _value)) {
          spender.receiveApproval(msg.sender, _value, this, _extraData);
      }
  }

  function allowance(address _owner, address _spender) constant public returns (uint remaining) {
    return allowed[_owner][_spender];
  }
  
}