Re: [PATCH v2 1/3] add strnncmp() function

[Jeremiah Mahler <jmmahler@gmail.com>]

Junio,

On Tue, Jun 17, 2014 at 10:55:18AM -0700, Junio C Hamano wrote:
> Jeremiah Mahler <jmmahler@gmail.com> writes:
> 
> > Add a strnncmp() function which behaves like strncmp() except it takes
> > the length of both strings instead of just one.  It behaves the same as
> > strncmp() up to the minimum common length between the strings.  When the
> > strings are identical up to this minimum common length, the length
> > difference is returned.
> >
> > Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
> > ---
> >  strbuf.c | 9 +++++++++
> >  strbuf.h | 2 ++
> >  2 files changed, 11 insertions(+)
> >
> > diff --git a/strbuf.c b/strbuf.c
> > index ac62982..4eb7954 100644
> > --- a/strbuf.c
> > +++ b/strbuf.c
> > @@ -600,3 +600,12 @@ char *xstrdup_tolower(const char *string)
> >  	result[i] = '\0';
> >  	return result;
> >  }
> > +
> > +int strnncmp(const char *a, int len_a, const char *b, int len_b)
> > +{
> > +	int min_len = (len_a < len_b) ? len_a : len_b;
> > +	int cmp = strncmp(a, b, min_len);
> > +	if (cmp)
> > +		return cmp;
> > +	return (len_a - len_b);
> > +}
> 
> I am not sure if the interface into this function conceptually makes
> much sense.  strncmp(entry, string, 14) was invented as the way to
> see if a NUL terminated "string" matches with the contents in an
> array of char "entry" that is up to 14 bytes long, and because the
> "entry" was allowed to fill full 14-byte space without terminated
> with a NUL, the maximum possible length is specified separately, but
> a NUL termination in "entry", if exists, is still honored.  Is there
> any case where such a pair of "maximum N bytes but could be shorter"
> strings are compared, especially with different N's defined per
> string, in our codebase (or in other people's project for that
> matter)?
> 
> Further, I do think that the interface into this function and its
> implementation are inappropriate for implementing the name_compare()
> function in tree-walk.c and unpack-trees.c.  These functions are
> designed to take counted strings; in a tuple <a, a_len> they take,
> "a_len" is the only thing that determines the length of string "a".
> There is no room for a NUL termination inside "a" come into play to
> make "a" shorter than "a_len".
> 
> In other words, "Two NUL-terminated strings can be compared with
> strcmp(a, b), but we use counted strings in many places in our
> codebase, and compare_counted_strings(a, a_len, b, b_len) function
> would help us, so let's add one and use it in name_compare()" may
> make good sense, but if we were to do so, I do not think strncmp()
> would be involved in its implementation.
> 
> 
The concept of a counted string clears up some of the confusion I was
having.  Thanks for that explanation.

-- 
Jeremiah Mahler
jmmahler@gmail.com
http://github.com/jmahler