Re: [PATCH] sha1_file: add slash once in for_each_file_in_obj_subdir()

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

Jeff King <peff@peff.net> writes:

> On Sat, Jul 08, 2017 at 10:59:06AM +0200, René Scharfe wrote:
>
>> Add the slash between loose object subdirectory and file name just once
>> outside the loop instead of overwriting it with each readdir call.
>> Redefine baselen as the length with that slash, and add dirlen for the
>> length without it.  The result is slightly less wasteful and can use the
>> the cheaper strbuf_addstr instead of strbuf_addf without losing clarity.
>
> This patch looks correct to me.
>
> I'm a little lukewarm on it overall, though. I'd be shocked if the
> efficiency change is measurable. What I really care about is whether the
> result is easier to read or not.
>
> On the plus side, this moves an invariant out of the loop. On the minus
> side, it has to introduce an extra variable for "length we add on to"
> versus "dir length to pass to the subdir_cb". That's not rocket science,
> but it does slightly complicate things (though I note we already have
> "origlen", so this is bumping us from 2 to 3 length variables, not 1 to
> 2).
>
> So I dunno. It's fine with me if we take it, and fine if we leave it.

Unlike origlen, base vs dir lengths are not strictly needed; we
prepare the base including '/', and we know we always have just one
'/' at the end, so anybody that uses dirlen to truncate it back to
the original before passing it down can truncate to (baselen-1), no?

In other words, something like this (not an incremental but a
replacement) to keep calling "baselen" the length of the leading
constant part we append to?

 sha1_file.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/sha1_file.c b/sha1_file.c
index 5862386cd0..d277b32bf1 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -3760,7 +3760,6 @@ int for_each_file_in_obj_subdir(unsigned int subdir_nr,
 	origlen = path->len;
 	strbuf_complete(path, '/');
 	strbuf_addf(path, "%02x", subdir_nr);
-	baselen = path->len;
 
 	dir = opendir(path->buf);
 	if (!dir) {
@@ -3770,12 +3769,15 @@ int for_each_file_in_obj_subdir(unsigned int subdir_nr,
 		return r;
 	}
 
+	strbuf_addch(path, '/');
+	baselen = path->len;
+
 	while ((de = readdir(dir))) {
 		if (is_dot_or_dotdot(de->d_name))
 			continue;
 
 		strbuf_setlen(path, baselen);
-		strbuf_addf(path, "/%s", de->d_name);
+		strbuf_addstr(path, de->d_name);
 
 		if (strlen(de->d_name) == GIT_SHA1_HEXSZ - 2)  {
 			char hex[GIT_MAX_HEXSZ+1];
@@ -3801,7 +3803,7 @@ int for_each_file_in_obj_subdir(unsigned int subdir_nr,
 	}
 	closedir(dir);
 
-	strbuf_setlen(path, baselen);
+	strbuf_setlen(path, baselen - 1); /* chomp the '/' that we added */
 	if (!r && subdir_cb)
 		r = subdir_cb(subdir_nr, path->buf, data);