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

Yuya Nishihara yuya at tcha.org
Tue Aug 21 09:20:46 EDT 2018


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.

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.


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