[PATCH] diff: use a threshold on similarity index before using word-diff (issue5965)

[Yuya Nishihara]

On Tue, 21 Aug 2018 17:11:51 +0200, Denis Laxalde wrote:
> Yuya Nishihara a écrit :
> > On Tue, 21 Aug 2018 14:10:33 +0200, Denis Laxalde wrote:
> >> # HG changeset patch
> >> # User Denis Laxalde <denis.laxalde at logilab.fr>
> >> # Date 1534853203 -7200
> >> #      Tue Aug 21 14:06:43 2018 +0200
> >> # Node ID c43df6ff42d26163d19e99e15a3cf3094020d822
> >> # Parent  c62184c6299c09d2e8e7be340f9aee138229cb86
> >> # Available At http://hg.logilab.org/users/dlaxalde/hg
> >> #              hg pull http://hg.logilab.org/users/dlaxalde/hg -r c43df6ff42d2
> >> # EXP-Topic issue5965
> >> diff: use a threshold on similarity index before using word-diff (issue5965)
> >>
> >> The threshold is chosen quite arbitrarily with a value of 0.5. It does
> >> not change the results of test-diff-color.t whereas higher values (e.g.
> >> 0.6) would. Looking at what this produces on some changesets in recent
> >> history (e.g. 037debbf869c or 7acec9408e1c), this significantly improves
> >> diff readability.
> >>
> >> Similarity index is computed using difflib.SequenceMatcher's ratio()
> >> method; this is documented as being "expensive", but other faster methods
> >> (that compute an upper bound value) do not give good results.
> >> Nevertheless, since we compute this ratio on each hunk which are usually
> >> small, this might not be problematic in most cases. Also, as we'd
> >> short-circuit computation of inline colors for those hunks that are not
> >> similar enough, this "expensive" ratio computation might also be
> >> compensated.
> > 
> > Can you test this against a large BLOB-ish diff (such as machine-generated
> > 10k-line JSON, a binary in Intel HEX format, etc.)? Last time I faced that,
> > the original difflib-based algorithm was painfully slow (~100s-ish to yield
> > one hunk), which made me think the word-diff should never be turned on by
> > default.
> 
> I've set up a test repo with some JSON at
> https://bitbucket.org/dlax/hg-worddiff-tests. As far as I can tell,
> there's no significant difference when diffing the last changeset;

Thanks, but it looks cheaper to compute than the stuff I had at work. I'll
try to collect some number if I get a chance.

> it's even a bit faster with the similarity ratio patch.

The overall result may be faster, but my concern is a few sec stall with
no output.

> > Also, for me, it was a bad UX that the word-diff was disabled at an arbitrary
> > threshold. I needed to guess reason why a hunk wasn't highlighted.
> 
> I don't get this. Before this patch, word-diff is also on side of the
> hunk is empty IIUC.

Yup. I don't like it, but that's easy to spot.

> Do you mean we should make the threshold configurable?

Not really, but if I can set the threshold to 0, I'm fine with that.