Miller Rabin algorithm can be used to test primality for integers smaller than a max value "MaxMR" (~3e24) It can be much faster than previous implementation: ~100x faster for numbers with 13 digits, at least 5 orders of magnitude for even larger numbers (previous implementation is so slow that it requires more patience than I have for more precise estimate). Miller Rabin test becomes faster than previous implementation at somewhere in the range 1e5-1e6. It seems that the range 62000..66000 is where Miller Rabin starts being always faster, so I picked 0xffff arbitrarily; before that, or above "MaxMR", the previous implementation remains. I compared the `faster_prime` gem too. It is slower than previous implementation up to ~1e4. After that it becomes faster and faster compared to previous implementation, but is still slower than Miller Rabin starting at ~1e5 and up to MaxMR. Thus, after this commit, builtin `Integer#prime?` will be similar or faster than `faster_prime` up to "MaxMR". Adapted from of Stephen Blackstone [Feature #16468] Benchmark results and code: https://gist..com/marcandre/b263bdae488e76dabdda84daf73733b9 Co-authored-by: Stephen Blackstone <[email protected]>