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

[Junio C Hamano <gitster@pobox.com>]

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.