ERC20

pragma solidity 0.6.2;


contract ERC20 {
    string public name = "";
    string public symbol = "";
    uint8 public decimals = 18;  
    uint256 public totalSupply = 1000000000 ether; 

    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;

    event Transfer(address indexed from, address indexed to, uint256 value);

    constructor () public {
        balanceOf[msg.sender] = totalSupply; 
    }

    function _transfer(address _from, address _to, uint256 _value) internal {
        require(_to != address(0x0000000000000000000000000000000000000000));
        require(balanceOf[_from] >= _value);        
        require(balanceOf[_to] + _value > balanceOf[_to]);
        uint previousBalances = balanceOf[_from] + balanceOf[_to]; 
        balanceOf[_from] -= _value; 
        balanceOf[_to] += _value;
        emit Transfer(_from,_to, _value);  
        assert(balanceOf[_from] + balanceOf[_to] == previousBalances);  
    }

    function transfer(address _to, uint256 _value) public {
        _transfer(msg.sender, _to, _value); 
    }

    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]); 
        allowance[_from][msg.sender] -= _value;
        _transfer(_from, _to, _value);
        return true;
    }

    function approve(address _spender, uint256 _value) public returns (bool success) {
        allowance[msg.sender][_spender] = _value;  
        return true;
    }
}