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Date: Sun, 23 Sep 2018 00:02:44 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: un-UBify-strings
On Sun, Sep 23, 2018 at 03:44:46AM +0000, Pascal Cuoq wrote:
>
> > On 23 Sep 2018, at 05:15, Rich Felker <dalias@...c.org> wrote:
> >
> > dist(s,d)==n is a no-overlap case.
>
> In this case the formula I proposed has the drawback of rejecting
> the case where (uintptr_t)s-(uintptr_t)d is exactly -n. This case
> may be the justification for the way the original comparison was
> expressed:
I think that's fixable just by subtracting 1 on both sides, but then
it's probably less efficient rather than more efficient.
> > (uintptr_t)s-(uintptr_t)d-n <= -2*n
>
> (uintptr_t)s-(uintptr_t)d = -n ==> comparison true by LHS and RHS being equal
>
> (uintptr_t)s-(uintptr_t)d = n ==> comparison true by LHS being zero
>
> (uintptr_t)s-(uintptr_t)d > -n and (uintptr_t)s-(uintptr_t)d < n ==> comparison false
Yes. It's interesting that the choice of using left or right of 0
depends on whether you want > or >=.
Rich
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