Not really sure how to fix that, any ideas?

To prevent this from going stale I'll update, but I don't really have a simple and satisfying solution yet...

A few options are

• pass the buck: submit a feature request to the decimal library to include features like constant::PI and sin(). Both are included for floats in the standard library, as well as other decimal libraries, so it's not too outlandish to request them in a numerical data-type library
• use another decimal library. For example rust_decimal offers constants and trig functions... but this option probably requires a total rewrite
• use f128 (or standard library floats) for trig and convert to d128 if possible? The drawback being the loss of precision as magnitude increases
• use lookups for well-defined angles (i.e. multiples of pi/3 and pi/4)

I probably won't work on this, but would welcome a PR. fend is a great alternative to try out, trigonometry seems to work there

One thing about other decimal libraries is some have a much smaller range of possible numbers