# HG changeset patch
# User Elmar Bartel <elb_hg@leo.org>
# Date 2020-04-30 13:10:05
# Node ID e58422afbc74c4a2f41929705c9ecc3936b0c4a8
# Parent  a467416c493c3ab32f214b69b5d5831da24e5732

diff: 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 --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -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: