Re: [PATCH] cleanup: fix possible overflow errors in binary search

On 10/6/2017 10:18 AM, Jeff King wrote:

On Fri, Oct 06, 2017 at 09:52:31AM -0400, Derrick Stolee wrote:

quoted

A common mistake when writing binary search is to allow possible
integer overflow by using the simple average:

	mid = (min + max) / 2;

Instead, use the overflow-safe version:

	mid = min + (max - min) / 2;

Great, thank you for picking this up!

quoted

The included changes were found using the following two greps:

	grep "/ 2;" *.c
	grep "/ 2;" */*.c
	grep "/2;" */*.c

You can use[1]:

   git grep '/ 2;' '*.c'

to have Git expand the wildcard. That catches a few extra cases in
compat/regex/*.c.  Even though it's imported code, it might be
nice to cover those, too (since it's a possible bug, and also as a good
example).

[1] I'd actually write:

       git grep '/ *2;' '*.c'

     to do it all in one grep. :)

Thanks for the grep lesson! I knew there would be a simpler way to do 
what I wanted.

quoted

---
  builtin/index-pack.c     | 4 ++--
  builtin/pack-objects.c   | 2 +-
  builtin/unpack-objects.c | 2 +-
  cache-tree.c             | 2 +-
  packfile.c               | 2 +-
  sha1-lookup.c            | 2 +-
  sha1_name.c              | 2 +-
  string-list.c            | 2 +-
  utf8.c                   | 2 +-
  xdiff/xpatience.c        | 2 +-
  10 files changed, 11 insertions(+), 11 deletions(-)

These all look good to me (really the only way the conversion could be
bad is if "min" was higher than "max", and each case is just inside a
loop condition which makes sure that is not the case).

-Peff

I thought this should be simple enough. When we are all happy with the 
idea of this cleanup, I'll re-roll with the missed examples.

Thanks,
-Stolee