On Thu, Sep 29, 2016 at 11:37 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I'm playing with an early patch to make the default more dynamic.
> Let's see how well it works in practice, but it looks fairly
> promising. Let me test a bit more and send out an RFC patch..

Ok, this is *very* rough, and it doesn't actuall pass all the tests,
and I didn't even try to look at why. But it passes the trivial
smell-test, and in particular it actually makes mathematical sense...

I think the patch can speak for itself, but the basic core is this
section in get_short_sha1():

  +       if (len < 16 && !status && (flags & GET_SHA1_AUTOMATIC)) {
  +               unsigned int expect_collision = 1 << (len * 2);
  +               if (ds.nrobjects > expect_collision)
  +                       return SHORT_NAME_AMBIGUOUS;
  +       }

basically, what it says is that we will consider a sha1 ambiguous even
if it was *technically* unique (that's the '!status' part of the test)
if:

 - the length was 15 or less

*and*

 - the number of objects we have is larger than the expected point
where statistically we should start to expect to get one collision.

That "expect_collision" math is actually very simple: each hex
character adds four bits of range, but since we expect collisions at
the square root of the maximum number of objects, we shift by just two
bits per hex digits instead.

The rest of the patch is a trivial change to just initialize the
default short size to -1, and consider that to mean "enable the
automatic size checking with a minimum of 7". And the trivial code to
estimate the number of objects (which ignores duplicates between packs
etc _entirely_).

For the kernel, just the *math* right now actually gives 12
characters. For current git it actually seems to say that 8 is the
correct number. For small projects, you'll still see 7.

ANYWAY. This patch is on top of Jeff's patches in 'pu' (I think those
are great regardless of this patch!), and as mentioned, it fails some
tests. I suspect that the failures might be due to the abbrev_default
being -1, and some other code finds that surprising now. But as
mentioned, I didn't really even look at it.

What do you think? It's actually a fairly simple patch and I really do
think it makes sense and it seems to just DTRT automatically.

              Linus