[PATCH] diff performance: re-establish linear runtime performance

[Pierre-Yves David]

Interresting, did you run performance benchmark on some specific diff 
for comparison ? I would like to integrat this in our benchmark suite.

On  4/30/20 3:23 PM, Elmar Bartel wrote:
> # HG changeset patch
> # User Elmar Bartel <elb_hg at leo.org>
> # Date 1588252205 -7200
> #      Thu Apr 30 15:10:05 2020 +0200
> # Node ID fd68848f8d5515f9a1afe288787f27fcdb130d42
> # Parent  5c1f356b108e07782e07b824c206a87d3a9abcff
> diff performance: re-establish linear runtime performance
> 
> The previous method with sum() and list() creates a new list object
> for every hunk. Then sum() is used to flatten out this sequence of
> lists.  The sum() function is not "lazy", but creates a new list object
> for every "+" operation and so this code had quadratic runtime behaviour.
> 
> diff -r 5c1f356b108e -r fd68848f8d55 mercurial/patch.py
> --- a/mercurial/patch.py	Fri Apr 24 12:37:43 2020 -0700
> +++ b/mercurial/patch.py	Thu Apr 30 15:10:05 2020 +0200
> @@ -2558,7 +2558,7 @@ def diff(
>                   fctx2 is not None
>               ), b'fctx2 unexpectly None in diff hunks filtering'
>               hunks = hunksfilterfn(fctx2, hunks)
> -        text = b''.join(sum((list(hlines) for hrange, hlines in hunks), []))
> +        text = b''.join((b''.join(hlines) for hrange, hlines in hunks))
>           if hdr and (text or len(hdr) > 1):
>               yield b'\n'.join(hdr) + b'\n'
>           if text:
> 
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-- 
Pierre-Yves David