On 2014-05-19 08:16, ???? wrote:
Hi All,
Recently, I read NSS code and have a question about this line
<http://dxr.mozilla.org/mozilla-central/source/security/nss/lib/freebl/mpi/mpprime.c#380>
)
In this line, offset is "offset = prime - (rem / 2)". But I think offset is
"offset = prime - rem" because "trial + prime - rem" is multiple of prime
(I give detail later).
Why offset is "offset = prime - (rem / 2)" ?
Thanks for any help.
Please re-read the comment at the beginning of the function. Remember that
the array "siece" (into which offset is an index) does not represent the
values "trial+0", "trial+1", "trial+2", but rather represents trial+0,
trial+2, trial+4, etc. This is because trial is always odd, and we're not
interested in even values. As the comment says:
|After this function is finished,||if sieve[i] is non-zero, then
(trial + 2*i) is composite.|
But I think this computation is still flawed because of the effect it has
when rem is odd. So I might change the code to look like this:
| if (rem == 0) {|
| offset = 0;|
| }else if (rem & 1) {|
| offset = (prime -rem) / 2;|
| } else {
offset = prime - (rem / 2);
}|
This issue doesn't mean that the prime number generator generates bad primes.
It simply means the generator is less efficient than it should be because it
marks some primes as composits.
/Nelson
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