bdiff: balance recursion to avoid quadratic behavior (issue4704)
For highly structured files like JSON or XML dumps with large numbers
of duplicate lines (eg braces) and isolated matching lines, bdiff
could find large numbers of equally good spans. Because it prefers
earlier matches, this would result in pathologically unbalance
recursion that resulted in quadratic performance.
This patch makes it prefer matches closer to the middle that tend to
balance recursion. This change improves the speed of a pathological
test case from 1100s to 9s.
Included is a smaller test that has a roughly 50x safety margin on the
performance it accepts. It's likely to fail on pure builds because
difflib also has a recursion-balancing problem.
# Extension dedicated to test patch.diff() upgrade modes
from __future__ import absolute_import
from mercurial import (
cmdutil,
error,
patch,
scmutil,
)
cmdtable = {}
command = cmdutil.command(cmdtable)
@command('autodiff',
[('', 'git', '', 'git upgrade mode (yes/no/auto/warn/abort)')],
'[OPTION]... [FILE]...')
def autodiff(ui, repo, *pats, **opts):
diffopts = patch.difffeatureopts(ui, opts)
git = opts.get('git', 'no')
brokenfiles = set()
losedatafn = None
if git in ('yes', 'no'):
diffopts.git = git == 'yes'
diffopts.upgrade = False
elif git == 'auto':
diffopts.git = False
diffopts.upgrade = True
elif git == 'warn':
diffopts.git = False
diffopts.upgrade = True
def losedatafn(fn=None, **kwargs):
brokenfiles.add(fn)
return True
elif git == 'abort':
diffopts.git = False
diffopts.upgrade = True
def losedatafn(fn=None, **kwargs):
raise error.Abort('losing data for %s' % fn)
else:
raise error.Abort('--git must be yes, no or auto')
node1, node2 = scmutil.revpair(repo, [])
m = scmutil.match(repo[node2], pats, opts)
it = patch.diff(repo, node1, node2, match=m, opts=diffopts,
losedatafn=losedatafn)
for chunk in it:
ui.write(chunk)
for fn in sorted(brokenfiles):
ui.write(('data lost for: %s\n' % fn))