Add Custom Methods

You can easily add new methods that do additional computation and emit events.

function logTax(address token, uint expected, uint recv) private {
    uint shortfall = expected - recv;
    uint feeBips = (shortfall * 10000) / expected;
    emit TokenTax(token, feeBips);
}

And then we can use the function like this in the swap method

function swap(uint amount0Out, uint amount1Out, address to, bytes calldata data) external lock {
    uint b0 = IERC20(token0).balanceOf(to);
    uint b1 = IERC20(token1).balanceOf(to);
    swapInternal(amount0Out, amount1Out, to, data);
    if (amount0Out > 0) {
        uint delta = IERC20(token0).balanceOf(to) - b0;
        if (amount0Out > delta) {
            logTax(token0, amount0Out, delta);
        }
    }
    if (amount1Out > 0) {
        uint delta = IERC20(token1).balanceOf(to) - b1;
        if (amount1Out > delta) {
            logTax(token1, amount1Out, delta);
        }
    }
}