Subject: Re: [PATCH] general style: replaces memcmp() with proper starts_with()

From: Junio C Hamano <[email-masked]>
Date: Wed, 12 Mar 2014 12:39:01 -0700

Jeff King <[email-masked]> writes:

>>  static inline int standard_header_field(const char *field, size_t len)
>>  {
>> -	return ((len == 4 && !memcmp(field, "tree ", 5)) ||
>> -		(len == 6 && !memcmp(field, "parent ", 7)) ||
>> -		(len == 6 && !memcmp(field, "author ", 7)) ||
>> -		(len == 9 && !memcmp(field, "committer ", 10)) ||
>> -		(len == 8 && !memcmp(field, "encoding ", 9)));
>> +	return ((len == 4 && starts_with(field, "tree ")) ||
>> +		(len == 6 && starts_with(field, "parent ")) ||
>> +		(len == 6 && starts_with(field, "author ")) ||
>> +		(len == 9 && starts_with(field, "committer ")) ||
>> +		(len == 8 && starts_with(field, "encoding ")));
>
> These extra "len" checks are interesting.  They look like an attempt to
> optimize lookup, since the caller will already have scanned forward to
> the space.

If one really wants to remove the magic constants from this, then
one must take advantage of the pattern

	len == strlen(S) - 1 && !memcmp(field, S, strlen(S))

that appears here, and come up with a simple abstraction to express
that we are only using the string S (e.g. "tree "), length len and
location field of the counted string.

Blindly replacing starts_with() with !memcmp() in the above part is
a readability regression otherwise.

> ... I
> think with a few more helpers we could really further clean up some of
> these callsites.

Yes.