abstract contract DecimalERC20 {
    using SafeMath for uint256;
    using Decimal for Decimal.decimal;

    mapping(address => uint256) private decimalMap;

    //◥◤◥◤◥◤◥◤◥◤◥◤◥◤◥◤ add state variables below ◥◤◥◤◥◤◥◤◥◤◥◤◥◤◥◤//

    //◢◣◢◣◢◣◢◣◢◣◢◣◢◣◢◣ add state variables above ◢◣◢◣◢◣◢◣◢◣◢◣◢◣◢◣//
    uint256[50] private __gap;

    //
    // INTERNAL functions
    //

    // CAUTION: do not input _from == _to s.t. this function will always fail
    function _transfer(
        IERC20 _token,
        address _to,
        Decimal.decimal memory _value
    ) internal {
        _updateDecimal(address(_token));
        Decimal.decimal memory balanceBefore = _balanceOf(_token, _to);
        uint256 roundedDownValue = _toUint(_token, _value);

        // solhint-disable avoid-low-level-calls
        (bool success, bytes memory data) =
            address(_token).call(abi.encodeWithSelector(_token.transfer.selector, _to, roundedDownValue));

        require(success && (data.length == 0 || abi.decode(data, (bool))), "DecimalERC20: transfer failed");
        _validateBalance(_token, _to, roundedDownValue, balanceBefore);
    }

    function _transferFrom(
        IERC20 _token,
        address _from,
        address _to,
        Decimal.decimal memory _value
    ) internal {
        _updateDecimal(address(_token));
        Decimal.decimal memory balanceBefore = _balanceOf(_token, _to);
        uint256 roundedDownValue = _toUint(_token, _value);

        // solhint-disable avoid-low-level-calls
        (bool success, bytes memory data) =
            address(_token).call(abi.encodeWithSelector(_token.transferFrom.selector, _from, _to, roundedDownValue));

        require(success && (data.length == 0 || abi.decode(data, (bool))), "DecimalERC20: transferFrom failed");
        _validateBalance(_token, _to, roundedDownValue, balanceBefore);
    }

    function _approve(
        IERC20 _token,
        address _spender,
        Decimal.decimal memory _value
    ) internal {
        _updateDecimal(address(_token));
        // to be compatible with some erc20 tokens like USDT
        __approve(_token, _spender, Decimal.zero());
        __approve(_token, _spender, _value);
    }

    //
    // VIEW
    //
    function _allowance(
        IERC20 _token,
        address _owner,
        address _spender
    ) internal view returns (Decimal.decimal memory) {
        return _toDecimal(_token, _token.allowance(_owner, _spender));
    }

    function _balanceOf(IERC20 _token, address _owner) internal view returns (Decimal.decimal memory) {
        return _toDecimal(_token, _token.balanceOf(_owner));
    }

    function _totalSupply(IERC20 _token) internal view returns (Decimal.decimal memory) {
        return _toDecimal(_token, _token.totalSupply());
    }

    function _toDecimal(IERC20 _token, uint256 _number) internal view returns (Decimal.decimal memory) {
        uint256 tokenDecimals = _getTokenDecimals(address(_token));
        if (tokenDecimals >= 18) {
            return Decimal.decimal(_number.div(10**(tokenDecimals.sub(18))));
        }

        return Decimal.decimal(_number.mul(10**(uint256(18).sub(tokenDecimals))));
    }

    function _toUint(IERC20 _token, Decimal.decimal memory _decimal) internal view returns (uint256) {
        uint256 tokenDecimals = _getTokenDecimals(address(_token));
        if (tokenDecimals >= 18) {
            return _decimal.toUint().mul(10**(tokenDecimals.sub(18)));
        }
        return _decimal.toUint().div(10**(uint256(18).sub(tokenDecimals)));
    }

    function _getTokenDecimals(address _token) internal view returns (uint256) {
        uint256 tokenDecimals = decimalMap[_token];
        if (tokenDecimals == 0) {
            (bool success, bytes memory data) = _token.staticcall(abi.encodeWithSignature("decimals()"));
            require(success && data.length != 0, "DecimalERC20: get decimals failed");
            tokenDecimals = abi.decode(data, (uint256));
        }
        return tokenDecimals;
    }

    //
    // PRIVATE
    //
    function _updateDecimal(address _token) private {
        uint256 tokenDecimals = _getTokenDecimals(_token);
        if (decimalMap[_token] != tokenDecimals) {
            decimalMap[_token] = tokenDecimals;
        }
    }

    function __approve(
        IERC20 _token,
        address _spender,
        Decimal.decimal memory _value
    ) private {
        // solhint-disable avoid-low-level-calls
        (bool success, bytes memory data) =
            address(_token).call(abi.encodeWithSelector(_token.approve.selector, _spender, _toUint(_token, _value)));
        require(success && (data.length == 0 || abi.decode(data, (bool))), "DecimalERC20: approve failed");
    }

    // To prevent from deflationary token, check receiver's balance is as expectation.
    function _validateBalance(
        IERC20 _token,
        address _to,
        uint256 _roundedDownValue,
        Decimal.decimal memory _balanceBefore
    ) private view {
        require(
            _balanceOf(_token, _to).cmp(_balanceBefore.addD(_toDecimal(_token, _roundedDownValue))) == 0,
            "DecimalERC20: balance inconsistent"
        );
    }
}
function _transfer(
        IERC20 _token,
        address _to,
        Decimal.decimal memory _value
    ) internal {
        _updateDecimal(address(_token));
        Decimal.decimal memory balanceBefore = _balanceOf(_token, _to);
        uint256 roundedDownValue = _toUint(_token, _value);

        // solhint-disable avoid-low-level-calls
        (bool success, bytes memory data) =
            address(_token).call(abi.encodeWithSelector(_token.transfer.selector, _to, roundedDownValue));

        require(success && (data.length == 0 || abi.decode(data, (bool))), "DecimalERC20: transfer failed");
        _validateBalance(_token, _to, roundedDownValue, balanceBefore);
    }
//
// INTERNAL functions
//
// CAUTION: do not input _from == _to s.t. this function will always fail