IMO, this is one of the most interesting parts of Windows Calculator: It contains an arbitrary-precision arithmetic library rather than using regular floating point numbers, with infinite precision for basic operations (addition, subtraction, multiplication, division), so that calculations never lose precision. https://blogs.msdn.microsoft.com/oldnewthing/20040525-00/?p=39193

This would be great to mention in the readme somewhere... I'm not sure where though.

Of course, there's other big integer libraries these days, but it's still very interesting historical code. Looks like it's this header: https://github.com/Microsoft/calculator/blob/master/src/CalcManager/Ratpack/ratpak.h plus the files in that directory.