Phillip Wood <phillip.wood123@xxxxxxxxx> 于2021年5月29日周六 下午9:23写道：
>
> On 27/05/2021 17:36, Felipe Contreras wrote:
> > ZheNing Hu via GitGitGadget wrote:
> > [...]
> >> +static int memcasecmp(const void *vs1, const void *vs2, size_t n)
> >
> > Why void *? We can delcare as char *.
>
> If you look at how this function is used you'll see
> int (*cmp_fn)(const void *, const void *, size_t);
> cmp_fn = s->sort_flags & REF_SORTING_ICASE
> ? memcasecmp : memcmp;
>
> So the signature must match memcmp to avoid undefined behavior (a
> ternary expression is undefined unless both sides evaluate to the same
> type and calling a function through a pointer a different type is
> undefined as well)
>
I agree.
> >> +{
> >> + size_t i;
> >> + const char *s1 = (const char *)vs1;
> >> + const char *s2 = (const char *)vs2;
> >
> > Then we avoid this extra step.
> >
> >> + for (i = 0; i < n; i++) {
> >> + unsigned char u1 = s1[i];
> >> + unsigned char u2 = s2[i];
> >
> > There's no need for two entirely new variables...
> >
> >> + int U1 = toupper (u1);
> >> + int U2 = toupper (u2);
> >
> > You can do toupper(s1[i]) directly (BTW, there's an extra space: `foo(x)`,
> > not `foo (x)`).
> >
> > While we are at it, why keep an extra index from s1, when s1 is never
> > used again?
> >
> > We can simply advance both s1 and s2:
> >
> > s1++, s2++
> >
> >> + int diff = (UCHAR_MAX <= INT_MAX ? U1 - U2
> >> + : U1 < U2 ? -1 : U2 < U1);
> >
> > I don't understand what this is supposed to achieve. Both U1 and U2 are
> > integers, pretty low integers actually.
> >
> > If we get rid if that complexity we don't even need U1 or U2, just do:
> >
> > diff = toupper(u1) - toupper(u2);
> >
> >> + if (diff)
> >> + return diff;
> >> + }
> >> + return 0;
> >> +}
> >
> > All we have to do is define the end point, and then we don't need i:
> >
> > static int memcasecmp(const char *s1, const char *s2, size_t n)
> > {
> > const char *end = s1 + n;
> > for (; s1 < end; s1++, s2++) {
> > int diff = tolower(*s1) - tolower(*s2);
> > if (diff)
> > return diff;
> > }
> > return 0;
> > }
> >
> > (and I personally prefer lower to upper)
>
> We should be using tolower() as that is what POSIX specifies for
> strcasecmp() [1] which we are trying to emulate and there are cases[2] where
> (tolower(c1) == tolower(c2)) != (toupper(c1) == toupper(c2))
>
I don’t know if we overlooked a fact: This static `memcasecmp()`
is not a POSIX version. `tolower()` or `toupper()` are in git-compat-util.h,
sane_istest('\0', GIT_ALPHA) == false . So in `sane_case()`, whatever
`tolower()`, `toupper()`, they just return '\0' itself.
> Best Wishes
>
> Phillip
>
> [1] https://pubs.opengroup.org/onlinepubs/9699919799/
> [2] https://en.wikipedia.org/wiki/Dotted_and_dotless_I#In_computing
>
Thanks.
--
ZhenNing Hu
