I think std::floor(result * 1000000) / 1000000; should work fine. You might want to look at the result before truncating, because 1146652 is rouned up.

Thanks for your post!

Real optimization challenge, stuck on it for half a day. But it was amazing how sometimes one line of code can reduce execution time from minutes (or hours, I have never had enough patience to wait for slow version to end) to fracture of a second. Thanks to the author!