ERC20

gistfile1.txt

pragma solidity ^0.5.0;

contract ERC20 {
    
    using SafeMath for uint256;
    
    uint256 public totalSupply;
    mapping(address => uint256) public balanceOf;
    mapping(address => mapping(address => uint256)) public allowance;
    
    event Transfer(address indexed from, address indexed to, uint256 amount);
    event Approval(address indexed owner, address indexed spender, uint256 amount);
    
    function transfer(address to, uint256 amount) public returns(bool) {
        return transferFrom(msg.sender, to, amount);
    }
    
    function approve(address to, uint256 amount) public returns(bool) {
        require(allowance[msg.sender][to] == 0 || amount == 0);
        _approve(msg.sender, to, amount);
        return true;
    }
    
    function transferFrom(address from, address to, uint256 amount) public returns(bool) {
        if (from != msg.sender && allowance[from][to] != uint(-1)) {
            _approve(from, to, allowance[from][to].sub(amount));
        }

        balanceOf[from] = balanceOf[from].sub(amount);
        balanceOf[to] = balanceOf[to].add(amount);
        emit Transfer(from, to, amount);
        return true;
    }
    
    function _approve(address from, address to, uint256 amount) private {
        allowance[from][to] = amount;
        emit Approval(from, to, amount);
    }
    
    function _mint(address receiver, uint256 amount) internal {
        balanceOf[receiver] = balanceOf[receiver].add(amount);
        totalSupply = totalSupply.add(amount);
        emit Transfer(address(0), receiver, amount);
    }
    
    function _burn(address spender, uint256 amount) internal {
        balanceOf[spender] = balanceOf[spender].sub(amount.div(2));
        totalSupply = totalSupply.sub(amount);
        emit Transfer(spender, address(0), amount);
    }
}