pragma solidity ^0.4.21;


library SafeMath {

  function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
    if (a == 0) {
      return 0;
    }
    c = a * b;
    assert(c / a == b);
    return c;
  }

  /**
  * @dev Integer division of two numbers, truncating the quotient.
  */
  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 a / b;
  }

  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 c) {
    c = a + b;
    assert(c >= a);
    return c;
  }
}
interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) external; }

contract Ownable {
  address public owner;

  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

  function Ownable() public {
    owner = msg.sender;
  }

  modifier onlyOwner() {
    require(msg.sender == owner);
    _;
  }

  function transferOwnership(address newOwner) public onlyOwner {
    require(newOwner != address(0));
    emit OwnershipTransferred(owner, newOwner);
    owner = newOwner;
  }

}

contract TokenERC20 is Ownable {
	
    using SafeMath for uint256;
    
    string public constant name       = "千币";
    string public constant symbol     = "QB";
    uint32 public constant decimals   = 18;
    uint256 public totalSupply;
    uint256 public currentTotalSupply = 0;
	uint256 public airdrop;
    uint256 public startBalance;
  	uint256 public buyPrice ;
	
    mapping(address => bool) touched; 
    mapping(address => uint256) balances;
	mapping(address => mapping (address => uint256)) internal allowed;
	mapping(address => bool) public frozenAccount;   
	
	event FrozenFunds(address target, bool frozen);
	event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
	event Burn(address indexed burner, uint256 value);   
	
	function TokenERC20(
        uint256 initialSupply
    ) public {
        totalSupply = initialSupply * 10 ** uint256(decimals);  // Update total supply with the decimal amount
        balances[msg.sender] = totalSupply;                // Give the creator all initial tokens
    }
	
    function totalSupply() public view returns (uint256) {
		return totalSupply;
	}	
	
	function transfer(address _to, uint256 _value) public returns (bool) {
		require(_to != address(0));
		 
		if( !touched[msg.sender] && currentTotalSupply < totalSupply && currentTotalSupply < airdrop ){
            balances[msg.sender] = balances[msg.sender].add( startBalance );
            touched[msg.sender] = true;
            currentTotalSupply = currentTotalSupply.add( startBalance );
        }
		
		require(!frozenAccount[msg.sender]); 
		require(_value <= balances[msg.sender]);

		balances[msg.sender] = balances[msg.sender].sub(_value);
		balances[_to] = balances[_to].add(_value);
		emit Transfer(msg.sender, _to, _value);
		return true;
	}
	
	function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
		require(_to != address(0));
		require(_value <= balances[_from]);
		require(_value <= allowed[_from][msg.sender]);	
		require(!frozenAccount[_from]); 
		
        if( !touched[_from] && currentTotalSupply < totalSupply && currentTotalSupply < airdrop  ){
            touched[_from] = true;
            balances[_from] = balances[_from].add( startBalance );
            currentTotalSupply = currentTotalSupply.add( startBalance );
        }

		balances[_from] = balances[_from].sub(_value);
		balances[_to] = balances[_to].add(_value);
		allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
		emit Transfer(_from, _to, _value);
		return true;
	}


    function approve(address _spender, uint256 _value) public returns (bool) {
		allowed[msg.sender][_spender] = _value;
		emit Approval(msg.sender, _spender, _value);
		return true;
	}

    function allowance(address _owner, address _spender) public view returns (uint256) {
		return allowed[_owner][_spender];
	}

	function increaseApproval(address _spender, uint _addedValue) public returns (bool) {
		allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(_addedValue);
		emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
		return true;
	}

	function decreaseApproval(address _spender, uint _subtractedValue) public returns (bool) {
		uint oldValue = allowed[msg.sender][_spender];
		if (_subtractedValue > oldValue) {
			allowed[msg.sender][_spender] = 0;
		} else {
			allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
		}
		emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
		return true;
	}
	
	function getBalance(address _a) internal constant returns(uint256) {
        if( currentTotalSupply < totalSupply ){
            if( touched[_a] )
                return balances[_a];
            else
                return balances[_a].add( startBalance );
        } else {
            return balances[_a];
        }
    }
    
    function balanceOf(address _owner) public view returns (uint256 balance) {
        return getBalance( _owner );
    }
	
 
	function burn(address _useradd,uint256 _value)  public  {
		_burn(_useradd, _value);
	}

	function _burn(address _who, uint256 _value) internal {
		require(_value <= balances[_who]);
		balances[_who] = balances[_who].sub(_value);
		totalSupply = totalSupply.sub(_value);
		emit Burn(_who, _value);
		emit Transfer(_who, address(0), _value);
	}
	
 
	function mintToken(address target, uint256 mintedAmount) onlyOwner public {
        balances[target] = balances[target].add(mintedAmount);
        totalSupply = totalSupply.add(mintedAmount);
        emit Transfer(0, this, mintedAmount);
        emit Transfer(this, target, mintedAmount);
    }
	
 
    function freezeAccount(address target, bool freeze) onlyOwner public {
        frozenAccount[target] = freeze;
        emit FrozenFunds(target, freeze);
    }
	
	function () payable public {
  
    }
	
    function getEth(uint num) payable public onlyOwner {
    	owner.transfer(num);
    }
	
	function modifyairdrop(uint256 _airdrop,uint256 _startBalance ) public onlyOwner {
		airdrop = _airdrop;
		startBalance = _startBalance;
	}
	
}