Re: [PATCH v2] src/: ceil_prime(): Add function to get the lowest prime not smaller than n

[G. Branden Robinson]

Hi Alex,

At 2024-03-13T11:21:55+0100, Alejandro Colomar wrote:
> And use it where the same logic was being open-coded.
> 
> While at it, fix the logic, which was incorrect in the open-coded call
> sites, since for an input of 1, it produced 3, but the first prime is 2.

Are you sure?  That sounds like the bug I already fixed (but have not
yet pushed).

My working copy, complete with the following debugging output, is
attached.

$ ./build/indxbib -h 1
./build/indxbib: fatal error: argument to -h must not be less than 2
$ ./build/indxbib -h 2
./build/indxbib: debug: using hash table size of 2
./build/indxbib: fatal error: no files and no -f option
$ ./build/indxbib -h 3
./build/indxbib: debug: using hash table size of 3
./build/indxbib: fatal error: no files and no -f option
$ ./build/indxbib -h 4
./build/indxbib: warning: requested hash table size 4 is not prime: using 5 
instead
./build/indxbib: debug: using hash table size of 5
./build/indxbib: fatal error: no files and no -f option

> Also, since this is a library function, let's behave well for an input
> of 0, and return also the first prime, 2.

I'm skeptical of adding a library function for this purpose (finding the
smallest prime number equal to or greater than the input integer) when
it _has_ no other call sites.

In fact, the only other caller of is_prime() itself is in libbib, where
it's not operating on a user-supplied parameter (as in a user of the
running process).

https://git.savannah.gnu.org/cgit/groff.git/tree/src/libs/libbib/index.cpp?h=1.23.0#n603

Wouldn't it be more idiomatic C++ to migrate libbib to an STL container?

(Not that I'm saying you should do that.)

Regards,
Branden

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