Re: [PATCH 1/3] diff histogram: intern strings

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Phillip,

On Fri, 19 Nov 2021, Phillip Wood wrote:

> On 18/11/2021 15:42, Jeff King wrote:
> > On Thu, Nov 18, 2021 at 04:35:48PM +0100, Johannes Schindelin wrote:
> >
> > > I think the really important thing to point out is that
> > > `xdl_classify_record()` ensures that the `ha` attribute is different
> > > for different text. AFAIR it even "linearizes" the `ha` values, i.e.
> > > they won't be all over the place but start at 0 (or 1).
> > >
> > > So no, I'm not worried about collisions. That would be a bug in
> > > `xdl_classify_record()` and I think we would have caught this bug by
> > > now.
> >
> > Ah, thanks for that explanation. That addresses my collision concern
> > from earlier in the thread completely.
>
> Yes, thanks for clarifying I should have been clearer in my reply to
> Stolee. The reason I was waffling on about file sizes is that there can
> only be a collision if there are more than 2^32 unique lines. I think
> the minimum file size where that happens is just below 10GB when one
> side of the diff has 2^31 lines and the other has 2^31 + 1 lines and all
> the lines are unique.

Indeed, and as you pointed out, we already refuse to generate diffs for
such large amounts of data.

(For what it's worth, I totally agree with punting on such large data, it
would also take too long a time to generate diffs on such large data to be
reasonable.)

Ciao,
Dscho