contract Blackstone {
    using SafeMath for uint256;
   
    // Defining Variables & Mapping 
    string public name = "Blackstone";
    string public symbol = "BLST";
    uint256 public decimals = 0;
    uint256 public totalSupply = 40000000;
    
    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;
    
    event Transfer (address indexed _from, address indexed _to, uint256 _value);
    event Approval (address indexed _owner, address indexed _spender, uint256 _value);
    event Burn (address indexed _from, uint256 _value);
    
    // Constructor to Deploy the ERC20 Token
    constructor() public {
            name;
            symbol;
            decimals;
            balanceOf[msg.sender] = totalSupply;
    }
    
    // Transfer Function
    function _transfer(address _from, address _to, uint256 _value) internal {
    	require(_from != address(0));
    	require(_to != address(0));
        require(balanceOf[_from] >= _value);
        require(balanceOf[_to].add(_value) >= balanceOf[_to]);
        uint256 previousBalances = balanceOf[_from].add(balanceOf[_to]);
        
        balanceOf[_from] = balanceOf[_from].sub(_value);
        balanceOf[_to] = balanceOf[_to].add(_value);
        
        emit Transfer (_from, _to, _value);
        assert(balanceOf[_from].add(balanceOf[_to]) == previousBalances);
    }

    function transfer(address _to, uint256 _value) public returns (bool success) {
        _transfer(msg.sender, _to, _value);
        return true;
    }
    
    // Transfer delegated Tokens
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]);
        allowance[_from][msg.sender] = allowance[_from][msg.sender].sub(_value);
        _transfer(_from, _to, _value);
        return true;
    }
    
    // Delegate/Approve spender a certain amount of Tokens
    function approve(address _spender, uint256 _value) public returns (bool success) {
        require(_spender != address(0));
        require(balanceOf[msg.sender] >= _value);
        require(allowance[msg.sender][_spender].add(_value) >= allowance[msg.sender][_spender]);
        
        allowance[msg.sender][_spender] = _value;
        
        emit Approval (msg.sender, _spender, _value);
        return true;
    }
    
    // Increase delegated amount
    function increaseAllowance(address _spender, uint256 _value) public returns (bool success) {
        require(_spender != address(0));
        require(balanceOf[msg.sender] >= _value);
        require(balanceOf[msg.sender] >= allowance[msg.sender][_spender].add(_value));
        require(allowance[msg.sender][_spender].add(_value) >= allowance[msg.sender][_spender]);

        allowance[msg.sender][_spender] = allowance[msg.sender][_spender].add(_value);

        emit Approval(msg.sender, _spender, _value);
        return true;
    }
    
    // Decrease delegated amount
    function decreaseAllowance(address _spender, uint256 _value) public returns (bool success) {
        require(_spender != address(0));
        
        allowance[msg.sender][_spender] = allowance[msg.sender][_spender].sub(_value);
        
        emit Approval(msg.sender, _spender, _value);
        return true;
    }

    // Ping the contract about approved spender spendings
    function approveAndCall(address _spender, uint256 _value, bytes memory _extradata) public returns (bool success) {
        tokenRecipient spender = tokenRecipient(_spender);
        if(approve(_spender, _value)) {
            spender.receiveApproval(msg.sender, _value, address(this), _extradata);
            return true;
        }
    }
    
    // Tokenburn
    function burn(uint256 _value) public returns (bool success) {
        require(balanceOf[msg.sender] >= _value);
        
        balanceOf[msg.sender] = balanceOf[msg.sender].sub(_value);
        totalSupply = totalSupply.sub(_value);
        
        emit Burn (msg.sender, _value);
        return true;
    }
    
    function burnFrom(address _from, uint256 _value) public returns (bool success) {
        require(_from != address(0));
        require(balanceOf[_from] >= _value);
        require(_value <= allowance[_from][msg.sender]);
        
        allowance[_from][msg.sender] = allowance[_from][msg.sender].sub(_value);
        balanceOf[_from] = balanceOf[_from].sub(_value);
        totalSupply = totalSupply.sub(_value);
        
        emit Burn (msg.sender, _value);
        return true;
    }
    
}
function approve(address _spender, uint256 _value) public returns (bool success) {
    require(_spender != address(0));
    require(balanceOf[msg.sender] >= _value);
    require(allowance[msg.sender][_spender].add(_value) >= allowance[msg.sender][_spender]);
        
    allowance[msg.sender][_spender] = _value;
        
    emit Approval (msg.sender, _spender, _value);
    return true;
}

// Delegate/Approve spender a certain amount of Tokens