Reflections on mulmod_bnm1

[bodrato at mail.dm.unipi.it]

Uhm...

>> (And talking about unusual cases, do the current tests exercise
>> borderline cases where inputs or products equal -1,0, or +1 modulo the
>> various B^n ± 1 involved?)
>
> Yes, it does.

... no, it doesn't... it only tests some borderline cases for the B^n+1 half.

Moreover, the B^n-1 side is known to give a not-normalized result if the
product equals 0.
It returns 0 if and only if one of the two operand was 0.
It returns B^n-1 if both operand are non-zero.

Unfortunately this behaviour is documented only for the reference function
ref_mulmod_bnm1 in tests/mpn/t-mulmod_bnm1.c, and not in the original
mpn/generic/mulmod_bnm1.c file...

Regards,
Marco

-- 
http://bodrato.it/