Hi,
On Thu, Jun 17, 2021 at 05:42:12PM +0000, Peter Stuge wrote:
> Rolf Eike Beer wrote:
> > The previous algorithm would scan for all primes for a given number,
> > which takes needlessly long.
>
> Please also mention how this caused a problem for you in the commit
> message, to help us understand why you're proposing to change this.
>
>
> > +++ b/eclass/qmail.eclass
> > -# @FUNCTION: is_prima
> > +# @FUNCTION: is_prime
> > # @USAGE: <number>
> > # @DESCRIPTION:
> > # Checks wether a number is a prime number
>
> Maybe name the algorithm? Looks like an optimization of the sieve of
> Eratosthenes?
>
>
> > is_prime() {
> > local number=${1} i
> > - for i in $(primes ${number} ${number})
> > +
> > + if [[ $[number % 2] == 0 ]]; then
> > + return 1
> > + fi
>
> While 2 itself is prime the above returns 1 for any even number.
There's actually a much simpler solution to this:
$ is_prime() { test $(factor $1 | cut -d: -f2 | wc -w) == 1; }
$ for n in $(seq 0 10); do is_prime $n && echo $n is prime; done
2 is prime
3 is prime
5 is prime
7 is prime
$ time factor 20187319083467888113
20187319083467888113: 20187319083467888113
0.00
Cheers,
-Guilherme
