mpz_prevprime

[Niels Möller]

Seth Troisi <braintwo at gmail.com> writes:

> Trying to sum up the proposals for how this api should work,
>
> 1. add mpz_prevprime(rop, op) or mpz_precprime(rop, op)
> 2. change mpz_nextprime(-10) => -7 (instead of 2)
> 3. add mpz_nextprime_seq(a, b) => next prime a + k*b with k >= 1
>
> Considerations
> 1. on negative number return X?
> Should prevprime return "input had correct sign"? should nextprime?
> 2. this is an incompatible change for the interface (but probably not one
> many people depenedend on)
> 3. a bigger change than other two proposals.
>
> ---
>
> Personally I'm a fan of number 1. because I can clearly see the steps
> needed.

To me, 1 seems like a good improvement. It can be generalized to 3
later. For return value, I'd prefer

int mpz_prevprime(mpz_ptr p, mpz_srcptr n);

with return value 0, and p unchanged, if there is no (positive) prime <
n, otherwise 1, or possibly 2 for definitely, rather than probably,
prime.

And I think it's too unconventional to include negatives in the set of
prime numbers.

Regards,
/Niels

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