Mercurial > hg
view hgext/relink.py @ 29013:9a8363d23419 stable
bdiff: deal better with duplicate lines
The longest_match code compares all the possible positions in two
files to find the best match. Given a pair of sequences, it
effectively searches a grid like this:
a b b b c . d e . f
0 1 2 3 4 5 6 7 8 9
a 1 - - - - - - - - -
b - 2 1 1 - - - - - -
b - 1 3 2 - - - - - -
b - 1 2 4 - - - - - -
. - - - - - 1 - - 1 -
Here, the 4 in the middle says "the first four lines of the
file match", which it can compute be comparing the fourth lines and
then adding one to the result found when comparing the third lines in
the entry to the upper left.
We generally avoid the quadratic worst case by only looking at lines
that match, which is precomputed. We also avoid quadratic storage by
only keeping a single column vector and then keeping track of the best
match.
Unfortunately, this can get us into trouble with the sequences above.
Because we want to reuse the '3' value when calculating the '4', we
need to be careful not to overwrite it with the '2' we calculate
immediately before. If we scan left to right, top to bottom, we're
going to have a problem: we'll overwrite our 3 before we use it and
calculate a suboptimal best match.
To address this, we can either keep two column vectors and swap
between them (which significantly complicates bookkeeping), or change
our scanning order. If we instead scan from left to right, bottom to
top, we'll avoid ever overwriting values we'll need in the future.
This unfortunately needs several changes to be made simultaneously:
- change the order we build the initial hash chains for the b sequence
- change the sentinel values from INT_MAX to -1
- change the visit order in the longest_match inner loop
- add a tie-breaker preference for earlier matches
This last is needed because we previously had an implicit tie-breaker
from our visitation order that our test suite relies on. Later matches
can also trigger a bug in the normalization code in diff().
author | Matt Mackall <mpm@selenic.com> |
---|---|
date | Thu, 21 Apr 2016 21:05:26 -0500 |
parents | ad266834251b |
children | a0939666b836 |
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# Mercurial extension to provide 'hg relink' command # # Copyright (C) 2007 Brendan Cully <brendan@kublai.com> # # This software may be used and distributed according to the terms of the # GNU General Public License version 2 or any later version. """recreates hardlinks between repository clones""" from __future__ import absolute_import import os import stat from mercurial import ( cmdutil, error, hg, util, ) from mercurial.i18n import _ cmdtable = {} command = cmdutil.command(cmdtable) # Note for extension authors: ONLY specify testedwith = 'internal' for # extensions which SHIP WITH MERCURIAL. Non-mainline extensions should # be specifying the version(s) of Mercurial they are tested with, or # leave the attribute unspecified. testedwith = 'internal' @command('relink', [], _('[ORIGIN]')) def relink(ui, repo, origin=None, **opts): """recreate hardlinks between two repositories When repositories are cloned locally, their data files will be hardlinked so that they only use the space of a single repository. Unfortunately, subsequent pulls into either repository will break hardlinks for any files touched by the new changesets, even if both repositories end up pulling the same changes. Similarly, passing --rev to "hg clone" will fail to use any hardlinks, falling back to a complete copy of the source repository. This command lets you recreate those hardlinks and reclaim that wasted space. This repository will be relinked to share space with ORIGIN, which must be on the same local disk. If ORIGIN is omitted, looks for "default-relink", then "default", in [paths]. Do not attempt any read operations on this repository while the command is running. (Both repositories will be locked against writes.) """ if (not util.safehasattr(util, 'samefile') or not util.safehasattr(util, 'samedevice')): raise error.Abort(_('hardlinks are not supported on this system')) src = hg.repository(repo.baseui, ui.expandpath(origin or 'default-relink', origin or 'default')) ui.status(_('relinking %s to %s\n') % (src.store.path, repo.store.path)) if repo.root == src.root: ui.status(_('there is nothing to relink\n')) return if not util.samedevice(src.store.path, repo.store.path): # No point in continuing raise error.Abort(_('source and destination are on different devices')) locallock = repo.lock() try: remotelock = src.lock() try: candidates = sorted(collect(src, ui)) targets = prune(candidates, src.store.path, repo.store.path, ui) do_relink(src.store.path, repo.store.path, targets, ui) finally: remotelock.release() finally: locallock.release() def collect(src, ui): seplen = len(os.path.sep) candidates = [] live = len(src['tip'].manifest()) # Your average repository has some files which were deleted before # the tip revision. We account for that by assuming that there are # 3 tracked files for every 2 live files as of the tip version of # the repository. # # mozilla-central as of 2010-06-10 had a ratio of just over 7:5. total = live * 3 // 2 src = src.store.path pos = 0 ui.status(_("tip has %d files, estimated total number of files: %d\n") % (live, total)) for dirpath, dirnames, filenames in os.walk(src): dirnames.sort() relpath = dirpath[len(src) + seplen:] for filename in sorted(filenames): if filename[-2:] not in ('.d', '.i'): continue st = os.stat(os.path.join(dirpath, filename)) if not stat.S_ISREG(st.st_mode): continue pos += 1 candidates.append((os.path.join(relpath, filename), st)) ui.progress(_('collecting'), pos, filename, _('files'), total) ui.progress(_('collecting'), None) ui.status(_('collected %d candidate storage files\n') % len(candidates)) return candidates def prune(candidates, src, dst, ui): def linkfilter(src, dst, st): try: ts = os.stat(dst) except OSError: # Destination doesn't have this file? return False if util.samefile(src, dst): return False if not util.samedevice(src, dst): # No point in continuing raise error.Abort( _('source and destination are on different devices')) if st.st_size != ts.st_size: return False return st targets = [] total = len(candidates) pos = 0 for fn, st in candidates: pos += 1 srcpath = os.path.join(src, fn) tgt = os.path.join(dst, fn) ts = linkfilter(srcpath, tgt, st) if not ts: ui.debug('not linkable: %s\n' % fn) continue targets.append((fn, ts.st_size)) ui.progress(_('pruning'), pos, fn, _('files'), total) ui.progress(_('pruning'), None) ui.status(_('pruned down to %d probably relinkable files\n') % len(targets)) return targets def do_relink(src, dst, files, ui): def relinkfile(src, dst): bak = dst + '.bak' os.rename(dst, bak) try: util.oslink(src, dst) except OSError: os.rename(bak, dst) raise os.remove(bak) CHUNKLEN = 65536 relinked = 0 savedbytes = 0 pos = 0 total = len(files) for f, sz in files: pos += 1 source = os.path.join(src, f) tgt = os.path.join(dst, f) # Binary mode, so that read() works correctly, especially on Windows sfp = file(source, 'rb') dfp = file(tgt, 'rb') sin = sfp.read(CHUNKLEN) while sin: din = dfp.read(CHUNKLEN) if sin != din: break sin = sfp.read(CHUNKLEN) sfp.close() dfp.close() if sin: ui.debug('not linkable: %s\n' % f) continue try: relinkfile(source, tgt) ui.progress(_('relinking'), pos, f, _('files'), total) relinked += 1 savedbytes += sz except OSError as inst: ui.warn('%s: %s\n' % (tgt, str(inst))) ui.progress(_('relinking'), None) ui.status(_('relinked %d files (%s reclaimed)\n') % (relinked, util.bytecount(savedbytes)))