Ciao Niels,
18 feb 2024, 10:10 da ni...@lysator.liu.se:
> marco.bodr...@tutanota.com writes:
>
>> What about;
>>
>>
>> /* Arrange so that |s| < |u| / 2g */
>> mpz_add (s1, s0, s1);
>> {
>> int cmp = mpz_cmpabs (s0, s1);
>> if (cmp >= 0)
>> {
>> mpz_swap (s0, s1);
>> mpz_sub (t1, t0, t1);
>> if (cmp != 0 || mpz_cmpabs (t0, t1) > 0)
>> mpz_swap (t0, t1);
>> }
>> }
>>
>
> I think swapping of s and t must go together? I've tried out this
> similar variant
>
You are right, it's better with the two swaps together.
> mpz_add (s1, s0, s1);
> mpz_sub (t1, t0, t1);
> cmp = mpz_cmpabs (s0, s1);
> if (cmp > 0 || (cmp == 0 && mpz_cmpabs (t0, t1) > 0))
> {
> mpz_swap (s0, s1);
> mpz_swap (t0, t1);
> }
>
The only reason why I prefer my solution is: when cmp<0, there is no need to
compute
mpz_sub (t1, t0, t1);
> Seems to work fine, and it's a rather clear way to express the "lexical"
> preference that we want the cofactor pair with the smallest of |s|, |s'|, but
> in case of a tie, we choose the pair with smallest |t|, |t'|.
>
I agree.
Ĝis,
m
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