I implemented a rational number library for media timestamps (think CMTime, AVRational, etc.) that uses 64-bit numerators and denominators. It uses 128-bit integers for intermediate operations when adding, subtracting, multiplying, etc. It even uses 128-bit floats (represented as 2 doubles and using double-double arithmetic[1]) for some approximation operations and even 192-bit integers in one spot (IIRC it's multiplying a 128-bit and 64-bit ints and I just want the high bits so it shifts back down to 128 bits immediately after the multiplication).

I keep meaning to see if work will let me open source it.

[1]: https://en.wikipedia.org/wiki/Quadruple-precision_floating-p...