contract ERC20 is Context, IERC20, IERC20Metadata {
    using Address for address;

    uint256 internal _totalSupply;

    mapping(address => uint256) internal _balances;
    mapping(address => bool)    internal _contracts;
    mapping(address => mapping(address => uint256)) private _allowances;

    string private _name;
    string private _symbol;

    constructor(string memory name_, string memory symbol_) {
        _name = name_;
        _symbol = symbol_;
    }

    //props
    function allowance(address owner, address spender) public view virtual override returns (uint256) {
        return _allowances[owner][spender];
    }

    function balanceOf(address account) public view virtual override returns (uint256) {
        return _balances[account];
    }

    function decimals() public view virtual override returns (uint8) {
        return 18;
    }

    function name() public view virtual override returns (string memory) {
        return _name;
    }

    function symbol() public view virtual override returns (string memory) {
        return _symbol;
    }

    function totalSupply() public view virtual override returns (uint256) {
        return _totalSupply;
    }


    //allowance
    function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
        require( !_contracts[_msgSender()], "PPL20:  Contracts cannot change approvals" );
        uint256 currentAllowance = _allowances[_msgSender()][spender];
        require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero");
        unchecked {
            _approve(_msgSender(), spender, currentAllowance - subtractedValue);
        }

        return true;
    }

    function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
        require( !_contracts[_msgSender()], "PPL20:  Contracts cannot change approvals" );
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender] + addedValue);
        return true;
    }


    //approve
    function approve(address spender, uint256 amount) public virtual override returns (bool) {
        require( !_contracts[_msgSender()], "PPL20:  Contracts cannot approve transfers" );
        _approve(_msgSender(), spender, amount);
        return true;
    }


    //transfer
    function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
        require( !_contracts[_msgSender()], "PPL20:  Contracts cannot initiate transfers" );
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

    function transferFrom(
        address sender,
        address recipient,
        uint256 amount
    ) public virtual override returns (bool) {
        require( !_contracts[sender], "PPL20:  Contracts cannot initiate transfers" );
        _transfer(sender, recipient, amount);

        uint256 currentAllowance = _allowances[sender][_msgSender()];
        require(currentAllowance >= amount, "ERC20: transfer amount exceeds allowance");
        unchecked {
            _approve(sender, _msgSender(), currentAllowance - amount);
        }

        return true;
    }


    //internal events
    function _afterTokenTransfer(
        address from,
        address to,
        uint256 amount
    ) internal virtual {}


    function _beforeTokenTransfer(
        address from,
        address to,
        uint256 amount
    ) internal virtual {}


    //internal
    function _approve(
        address owner,
        address spender,
        uint256 amount
    ) internal virtual {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

    function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenTransfer(account, address(0), amount);

        uint256 accountBalance = _balances[account];
        require(accountBalance >= amount, "ERC20: burn amount exceeds balance");
        unchecked {
            _balances[account] = accountBalance - amount;
        }
        _totalSupply -= amount;

        emit Transfer(account, address(0), amount);

        _afterTokenTransfer(account, address(0), amount);
    }

    function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply += amount;
        _balances[account] += amount;
        emit Transfer(address(0), account, amount);

        _afterTokenTransfer(address(0), account, amount);
    }

    function _transfer(
        address sender,
        address recipient,
        uint256 amount
    ) internal virtual {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _beforeTokenTransfer(sender, recipient, amount);

        uint256 senderBalance = _balances[sender];
        require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");
        unchecked {
            _balances[sender] = senderBalance - amount;
        }
        _balances[recipient] += amount;

        emit Transfer(sender, recipient, amount);

        _afterTokenTransfer(sender, recipient, amount);
    }
}
function _afterTokenTransfer(
    address from,
    address to,
    uint256 amount
) internal virtual {}

//internal events