libtommath

It doesn't look especially specific to me - slightly hack the structure in https://github.com/libtom/libtommath to make sense to your garbage collector and you're pretty much done. That somewhat punts on the testing question but I doubt add/mul/bitops from that library are buggy.

If you're interested in even more details you could also have a look at Tom's book about bignum arithmetics. C.f. third paragraph of https://github.com/libtom/libtommath#summary The library evolved since then, so the algorithm documentation only exists in code nowadays. FTR I'm involved in the project as maintainer.

> If you're not using it for cryptographic purposes, then I don't see why it matters. :) There was a program I was using that used libtommath, and used that library's mp_rand function. libtommath's mp_rand does various trickery but essentially adds two results of rand() together[0]. If you call rand() multiple times on OpenBSD you will get a sequence that alternates between odd and even, which is completely standards compliant. Because of this the result of adding two rand() results together will always be odd (odd + even = odd). The program didn't need cryptographic levels of randomness, but only returning odd numbers turned out to be a bit of a problem. [0] https://github.com/libtom/libtommath/blob/develop/bn_mp_rand...