From patchwork Thu Apr 30 13:23:11 2020
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Subject: diff performance: re-establish linear runtime performance
From: Elmar Bartel <elb_hg@leo.org>
X-Patchwork-Id: 46252
Message-Id: <fd68848f8d5515f9a1af.1588252991@nuc15.leo.org>
To: mercurial-devel@mercurial-scm.org
Cc: elb_hg@leo.org
Date: Thu, 30 Apr 2020 15:23:11 +0200

# HG changeset patch
# User Elmar Bartel <elb_hg@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: